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  #46 (permalink)  
Old 10-19-2012, 01:40 PM
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Quote:
Originally Posted by mustang 671 View Post
cheers guys .

yep that has put my mind at rest . i will plug all vacum lines and agree i will not use the vacum off the primary metering block as intake vacum is always better .

i am going to get another top pulley to reduce the psi down to 8 and then build up at a latter stage ( if needed ) also my power brakes are off the intake .

i have spoke with the guys at the dyno shop that i am going to see a week wednesday and they stock all the bits in need for the holley .

so this weekend im going to tidy up . gap my plugs ( what do you recommend for my application ? or will stock factory set do ? they are champion )

i am going to stip the carbs down and give them a good clean out and make sure they are flowing correct etc . also i still think i have a fuel leakage . possibly fronm the power valve ? being a 6.5. will this oen and over fuel or would i need to be putting the enginbe under load for it to open ?

it is really lupmy and will not repsond to mix srcew adjustment , i am going to un hook the billet linkage and double check they are not holding open the butterflys just slightly etc .

any things to look for when im cleaning out the carbs ?

cheers guys
0.032"-0.035" max on the plug gap.

The way the PV is supposed to work is it will stay shut until the engine load causes the vacuum to drop to below the PV rating, at which point the PV opens and enrichens the air/fuel mixture to help overcome the load the engine is under.

So, check to see what the vacuum reading below the carb and above the blower is. If this reading is less than the number on the power valve, the power valve will be open and the engine will be overfueled. The engine doesn't need to be under a load for there to be fuel coming in from the power valve. If the vacuum at idle is lower than the power valve rating, there will be excess fuel, for example. This might be part of the reason you have a rough, 'lumpy' idle.

But since the carbs aren't boost referenced, the PVs should be shut- even when you want them open. That is, unless one or both PVs are ruptured from a backfire or old age. Newer Holley carbs have backfire blowout protection for the PV- if it's functioning properly. If not or if the carbs don't have blowout protection for the PV, then they could be damaged allowing fuel in when it isn't needed. If the wrong PV gasket is used, or two gaskets are accidentally used, there can be a fuel leak. Just things to look for when you take them down for cleaning.

If the initial ignition advance is too low, that will also account for a poor quality idle, and can cause the primary butterflies to be open too far. If the butterflies are open too far the engine will be pulling fuel in from the transition slot of the carb instead of running mainly on the idle circuit. If the timing is advanced at idle and the carb curb idle screw is backed out to close the primary butterflies, the idle mixture screws should become functional.

Sometimes the secondary butterflies need to be cracked open farther than "normal" to add bypass air to get the idle back to normal. Sometimes you even see where the primary throttle blades have been drilled w/small holes in them to add bypass air, although this is seldom really needed if the other methods (secondary bypass air and increasing the timing at idle) are used.

Something else to look for when you first pull the carbs is to see what the transition slot looks like. It should be between a "square" and a slot about 0.040" long. If this is too long, that's when the engine will be trying to idle on the transition circuit instead if the idle circuit. Photo below shows what I'm referring to.



If the carbs aren't boost referenced, I wouldn't want to run it on the dyno. The dyno loads the engine heavily, and w/o the PV adding fuel it can go lean. You have to be sure that the PV's were working when the blower is making boost and the vacuum drops.

This is a concern to me because of the small carbs. If you had a pair of 750 cfm carbs you might not need boost referencing at low boost levels- but this would need to be verified before loading the engine.

A 65 PV might open later than you need. It wouldn't surprise me for you to end up w/an 85, as an example.

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  #47 (permalink)  
Old 10-19-2012, 03:42 PM
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I forgot to mention that if you had to dyno the engine w/o boost referencing the carbs, you can remove the PV and use a plug made to block the PV off. Jet the carb up about 7-8 jet sizes to compensate for not having the PV installed. This will work on a race-only engine or for a wide open dyno pull. On the street it will not work because the air/fuel ratio will be too rich much of the time.
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Old 10-20-2012, 12:35 PM
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Agreed, removing the power valves (replace with a PV plug) and rejet to 78 pri and 80sec jets will work well for dyno testing and drag race running only.
Lock out the timing and set to 36deg BTDC to get the idle set up
Then reduce timing to 26deg BTDC (locked) for dyno testing @WOT. Adjust timing and jetting as required on the dyno. 26 to 32deg range.

No ECU timing control is required. just disable it.

Once you ahve sucessfully tuned it on the dyno. (Timing-jetting) you can use a manfiold vacuum gauge on the carb base to read the vacuum created on the carb base by the blower pulling @ WOT max power rpm.

If this vacuum reading with a vacuum gauge on the carb BASE is less than 10" @WOT,,,, use a 10.5" power valve.
install 10.5" rated power valves and reduce pri jet to a 70 to 74 jet as required. (6 to 8 jets leaner than the best jet found testing without a power valve)

You will need a bunch of holley main jets and two holley power valve plugs for dyno testing.
a pair of 70 72 74 76 78 80 82. jets will cover it. Do not get greedy on the timing under boost.
Start rich on the jetting and work leaner while watching plugs, BSFC, dyno AFR and or AFR from a O2 sensor/gauge.
Use a purple or yellow holley secondary spring. Make sure the vac housing is sealed to the carb body ( little o gasket)
and the vac sec diaphram seals so the vac sec operate correctly.

This motor needs two 750cfm double pumper blower carbs. This motor does not need a ECU to congtrol timing.
All it needs is a locked distributor and a Boost retard ignition box. (Crane Cams, MSD)
real simple.

Last edited by F-BIRD'88; 10-20-2012 at 12:50 PM.
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Old 10-21-2012, 01:01 PM
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thanks for all your comments.

i have had a great day - took cobalts advise and when i stripped the carbs down the primary throttle plates where open more than the diagram you showed ( in the link ) so i reduce it to where it showed, also i turned the scre in on the base of the carb to open the secoundary plates just so slightly to allow a bit of air flow. i replaced the base gaskets (carb to supercharger ) and replace the pv with the new ones i had - although still being 6.5 ,

the dyno shop are going to measure the vacum first and and try to get it nearer ( if needed ) i also gave the carbs a through clean and blow out , seems to have sorted it as its now idling at 1135 rpm . still lumpy bit is as smotth as i could get due to the high lift cam - i didnt run it for long - prob a couple of minutes . as i dont want to wreck anything - i want the dyne shope to use their lambda sensors and heat sensors etc to see what mix im getting .

i had a few silly problems which wasnt helping - end of sprak plug lead was exposed and shorting onto the exhaust headers .

ok the specs on my carbs are .

- on the top of the carb horn - 1850-5 0209
i have converted them to have sec metering blocks although i cant fit in the rear of the front one due to them being back to back.

primary jets - 70
sec jets - 78

shooters .25
R107 booster

.65 pv ( for now )

both carbs have a orangey brown cam for the acc pump shot

i have a pair of each jets from 70 to 80 .

regarding the ecu not being needed - thats really up to personal prefernece, clearly as long as there is a spark then it will run. however when i go fuel inj the ecu will come into its own. also i can run some much more off of it. electric fans. rev limiter , wide band lambda sensors , water meth , launch control , and i can monitor any faults and the ecu will log these for fine tuning, which is a huge difference with that and a dizzy run engine. i dont want to be some where between by timing light a turening the dizzy a degree or so. with the ecu it can be exact , not a tiny bit out , thats the beauty of the ecu.

t bird , i know you keep referring to a ecu . not sure if you understand exactly it is i have. its not something i have pulled from a efi engine , i cant turn this off. i dont have a dizzy anymore due to the height of my supercharger. it wont fit - i have a dummy dizzy to run the oil pump . i particulay wanted this system due to its huge amounts of options and parameters.

its hard to explain te difference. ( i am sure you know the concept of a ecu ) but i have something that xrays the engine and reports the findings and a dizzy is like a blind massager who relies on hear and feeling, all you can do with a dizzy is stobe to time , pull plugs , and adjust approx. ( i know you can get close ) with the ecu i have a 100% safety net. once set uo this will not allow the engine to break. it will just shut off and repost a error. its what i want after spenging 10k plus on the enigne.

so if you state timing figures can you please be exact - ie. locking timing out , what does that mean ? i cant do that .

i will take a pic of my ignition chart which i can do what i want with and then i think you will be clearer.

thanks for all your help guys. nice to finally hear it idle a a sensible rpm
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Old 10-21-2012, 01:43 PM
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below is my ign table. i can put in what figures i want



below if a 3d table of the above graph . i can pull this about to change the table above.



below is my main start and run chart whchi i view what the engine is doing, i can also add other section when i need them like lambda sansors. fuel inj etc.



below is a line chart of the first 2x ign charts. again i can pull this about to change the ign curve etc



lastly a short video of it diling at 1135 rpm. ( dont worry about the smoke - its old oil on the exhaust wraps that i spilt just burning off ) i paniked and turned the engine off. all ok



what your thoughts on exhaust header wraps ? always get mixed opinions on them. if you speak with exhaust guys they like them because they flow the wate gas better ? but others say not good as they kill headers ?

cheers shaun
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Old 10-21-2012, 03:08 PM
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what your thoughts on exhaust header wraps ? always get mixed opinions on them. if you speak with exhaust guys they like them because they flow the wate gas better ? but others say not good as they kill headers ?

cheers shaun
Your engine control unit sounds like it should work fine once the other areas are covered.

Have you used a vacuum gauge to take a reading at the base of the carb yet? You could use one of the manifold vacuum ports for this if you wanted. I'm just curious- but this can wait until the guys at the dyno shop get tuning on it.

As you know the 1850-5 Holley was originally a 4160 and it had a secondary plate instead of a block until you converted it to a 4150-type set up. For reference, the original 134-9 secondary plate your carbs came with had a secondary orifice equivalent to a #64 jet. So if you are now using a 78 secondary jet, this should be plenty- it would prolly be enough even w/o the power valve. If you added the power valve to it, you'd have the equivalent of about an 85 jet in the secondaries!

Also for reference, here's a Holley numerical listing showing the factory specs, etc. And here's a chart showing the relationship between plates and jets.

Header wraps are OK, if you were to use stainless steel headers, the wrap wouldn't cause rust. If you use mild steel I'd prefer ceramic coating inside and out. But anything that'll keep the underhood temps down and the exhaust gas hot inside the header (and the gas velocity up) will be a good thing, overall.
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Old 10-21-2012, 04:02 PM
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I haven't read the vacum pressure yet , will do next weekend , I have called off the dyno shop for another week so I can get cleaned up , will let you know what it is , thanks a lot for your help , the other thing is even though I have cleaned the carbs out the new needle valves let by every now and then , with a tap on the adjutment nut it stops it , my regulator is set for 6 psi , so you think I should drop it ? If so what to ? ,

I also was pumping too much fuel in , it was spitting out and blowing unburnt fuel back , so I closed the choke half way and left it for 30 mins , then just cranked with not touching the pedal and it started staright away , but it does like the choke off , I presume these blown motors like a lot of air flow ,

Its all new to me
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Old 10-21-2012, 06:55 PM
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I haven't read the vacum pressure yet , will do next weekend , I have called off the dyno shop for another week so I can get cleaned up , will let you know what it is , thanks a lot for your help , the other thing is even though I have cleaned the carbs out the new needle valves let by every now and then , with a tap on the adjutment nut it stops it , my regulator is set for 6 psi , so you think I should drop it ? If so what to ? ,

I also was pumping too much fuel in , it was spitting out and blowing unburnt fuel back , so I closed the choke half way and left it for 30 mins , then just cranked with not touching the pedal and it started staright away , but it does like the choke off , I presume these blown motors like a lot of air flow ,

Its all new to me
It should hold 6-1/2 psi w/o overpowering the needle and seat. You might need bigger, higher flow needle and seats. What you don't want to do is lower the fuel pressure too much so the WOT pressure stays as high as possible. It's not unusual to see 6-1/2 to 7 psi under WOT conditions on an engine making boost. The last thing you want is for it to go lean.

You probably won't need to use a choke. Cold, two or three pumps, foot off the pedal and crank. Hot, no pumps, foot off the pedal, and crank.
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Old 10-21-2012, 07:13 PM
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If you cannot under stand locked fixed timing that does not move 36deg BTDC for start up. How the fug do you think you will understand and program the computer timing tables???

The timing required will stay near 0 change right up untill boost is add at which point the motor needs LESS timing

same at idle as at WOT but must retard as blower boost comes in to avoid detonation on pump gas.
What is frigginhard to understand.???? the computer has to have a reference point.
It has to be somewhere inorder to move from that somewhere.
That somewhere is 36deg BTDC. Since you do not have a pressure sensor to sence blowermanifold boost to retard the timing in reference too, the computer is as dumb as you are.
So then RESET the ignition timing (fixed) at 26deg BTDC to start testing under boost @WOT on the dyno and creep up on the tune up.. 26 to 32deg BTDC

You have something that you don;t need and have no concept on how it needs to be programed.
The only processor you need is a spark retard box. Go buy the jets nd the power valve plugs I listed before testing. Get someone wit a clue to set them ignition timng. set all tables to 0.
No change. Run the engine at 36deg BTDC. Then move the crank mag pickup so the timing is 26deg BTDC. (using a timing light) Now you are reay to start WOT testing. The computer tables do no tneed to be set up because YOu don;t have a clue what the tables should be anyways.
A roots blower motor is not like other motors.
You wil make tmore power and it will run better with blower carbs and a simple ignition system with boost retard.
FOR A fraction of the money. and a fraction of the bother. YOU DON:T HAVE MONTHS of engine dyno time and chassis dyno time AND $10000000 's to spend on dyno ,drive mode testing to create a fuel map or a timing map etc etc etc.
You are wasting your timing and money for nothing and are hell bound to end up witha damged burned up motor. Find someone who has a clue as to how these roots blown motors actually run.

I don't have the time or patience or the 10pages of text to explain how the world works, how engines run, how roots blower motors run and why they need 36deg timing at idle. ( Stick you head near the carbs and wack the throttles open when the blower is cold with the timing set as you think it should be and you will get lesson #1 on blower motors.
You will have all that time till your hair and skin grows back to think about what just happended. (if you don;t damage the blower rotors from blower back fire. You will then run out of money and buying pistons trying to figure out why I say to start timing at 26deg BTDC under boost on pump gas. @ some 14lbs boost. ( a really dumb boost level to start tuning at.
GET SOME RACE GAS. Consider yourself for---warned ...

36deg BTDC for start up/warm up 26 to 32 dg BTDC for testing under BOOST.

Get a distributor ....lock out its mechanical advance curve and stick it in the motor.
A computer is just a dumb machine unless you have a clue how and why to program its.

Last edited by F-BIRD'88; 10-21-2012 at 07:28 PM.
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Old 10-21-2012, 07:40 PM
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Holley sec metering plates can be modified to accept holley main jets.
the 134-9 plate is way too lean. Drill Hole A to .081" at least.
or get a #21 and #27 plate. ( #21 plates are found in 3310 holleys)
Or drill and tap for holley screw in main jets and block Holes A

http://www.holley.com/data/Catalogs/Holley/90.pdf

start rich.... work towards lean

Start at 80 pri and 78 sec with power valve plugs installed.

An educated guess suggests you will end up with best power jetting
right around 74 to 76 jets square ( with no power valve)
That means the Hole A in the metering plate will want to be +/- .080"

Start tuning rich big jets and work leaner noting power and spark plug appearance.

Do not go to the dyno test with just 4 jets. and no power valve plugs or carb bowl/metering block gaskets.
Bring extra spark plugs.

Last edited by F-BIRD'88; 10-21-2012 at 07:58 PM.
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  #56 (permalink)  
Old 10-22-2012, 02:28 PM
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Quote:
Originally Posted by F-BIRD'88 View Post
If you cannot under stand locked fixed timing that does not move 36deg BTDC for start up. How the fug do you think you will understand and program the computer timing tables???

The timing required will stay near 0 change right up untill boost is add at which point the motor needs LESS timing

same at idle as at WOT but must retard as blower boost comes in to avoid detonation on pump gas.
What is frigginhard to understand.???? the computer has to have a reference point.
It has to be somewhere inorder to move from that somewhere.
That somewhere is 36deg BTDC. Since you do not have a pressure sensor to sence blowermanifold boost to retard the timing in reference too, the computer is as dumb as you are.
So then RESET the ignition timing (fixed) at 26deg BTDC to start testing under boost @WOT on the dyno and creep up on the tune up.. 26 to 32deg BTDC

You have something that you don;t need and have no concept on how it needs to be programed.
The only processor you need is a spark retard box. Go buy the jets nd the power valve plugs I listed before testing. Get someone wit a clue to set them ignition timng. set all tables to 0.
No change. Run the engine at 36deg BTDC. Then move the crank mag pickup so the timing is 26deg BTDC. (using a timing light) Now you are reay to start WOT testing. The computer tables do no tneed to be set up because YOu don;t have a clue what the tables should be anyways.
A roots blower motor is not like other motors.
You wil make tmore power and it will run better with blower carbs and a simple ignition system with boost retard.
FOR A fraction of the money. and a fraction of the bother. YOU DON:T HAVE MONTHS of engine dyno time and chassis dyno time AND $10000000 's to spend on dyno ,drive mode testing to create a fuel map or a timing map etc etc etc.
You are wasting your timing and money for nothing and are hell bound to end up witha damged burned up motor. Find someone who has a clue as to how these roots blown motors actually run.

I don't have the time or patience or the 10pages of text to explain how the world works, how engines run, how roots blower motors run and why they need 36deg timing at idle. ( Stick you head near the carbs and wack the throttles open when the blower is cold with the timing set as you think it should be and you will get lesson #1 on blower motors.
You will have all that time till your hair and skin grows back to think about what just happended. (if you don;t damage the blower rotors from blower back fire. You will then run out of money and buying pistons trying to figure out why I say to start timing at 26deg BTDC under boost on pump gas. @ some 14lbs boost. ( a really dumb boost level to start tuning at.
GET SOME RACE GAS. Consider yourself for---warned ...

36deg BTDC for start up/warm up 26 to 32 dg BTDC for testing under BOOST.

Get a distributor ....lock out its mechanical advance curve and stick it in the motor.
A computer is just a dumb machine unless you have a clue how and why to program its.
cheers t bird , think we will have to agree to disagree on some of that . from what you have posted i do thank you for the figures and timing numbers. having not known each of our back grounds or knowing each other personally its hard to just read something off of a post and take it the way its meant . ( or how you mean it ) i feel i may have angered you . sorry if thats the case , i did ask you for some exact figures on timing. i do know how to progrmme the ecu and if you knew my back ground you would know i could. i do have a boost gauge in the manifold ( thanks for asking before presuming i didnt ) currently the ecu reads firstly from the tps and then the boost. this may be changed when the dyno shop tune it . these guys at the dyno shop do know how to tune blowers - they tune the drag cars for the uk santa pod market . the reason why i am asking is so I know and not some shop where i spend lots of money.

today i visited a friend who is building a ex formula 1 engine for a swiss client that was run at moanco and lemand this year. he also had a v12 aston martin engine on the bench for a rebuild. and numerous methanol tunning engines to service for next season. he also backed up all of yours and others timing figures.

if you can give me some exact figures like others are trying to do that would be most appreciated. as i said before i will not run this until its set up correctly and something the ecu will not allow and will turn off ,

the below is a rough guide as each engine will require what it wants

i have been told to crank from 0 to 450 rpm ( start up programme ) @ 10d then 10d to 20d up to 1000 rpm being the start of my ign curve- then a sharp curve from 1000rpm to 1500rpm @ around 28d then from 28d @ 1500 rpm to 2000rpm at 32 d then a flat curve ( which you refer to as locked out ? ) the reason why i am reading tps first is so when the boost builds we can dial in the boost paramter reading to scale down the ign to compinsate for the given amount of boost we have , also sometimes engines will be under boost but no throttle ( load ) how does a engine ign deal with that ? so thats why the tps is read first then backed up by the boost guage under the blower.

this all may change but its how its running now.

cheers shaun
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Old 10-22-2012, 02:34 PM
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Quote:
Originally Posted by F-BIRD'88 View Post
Holley sec metering plates can be modified to accept holley main jets.
the 134-9 plate is way too lean. Drill Hole A to .081" at least.
or get a #21 and #27 plate. ( #21 plates are found in 3310 holleys)
Or drill and tap for holley screw in main jets and block Holes A

http://www.holley.com/data/Catalogs/Holley/90.pdf

start rich.... work towards lean

Start at 80 pri and 78 sec with power valve plugs installed.

An educated guess suggests you will end up with best power jetting
right around 74 to 76 jets square ( with no power valve)
That means the Hole A in the metering plate will want to be +/- .080"

Start tuning rich big jets and work leaner noting power and spark plug appearance.

Do not go to the dyno test with just 4 jets. and no power valve plugs or carb bowl/metering block gaskets.
Bring extra spark plugs.
cheers - the dyno shop stock loads of accesories for holleys , so they have everything i need , thats great help on my sec plate as i did want to jet it . will have a look at the weekend .

also just to confirm i cannot use a distributer , i dont have the room so i cant stick one in,

i also have a ranges of jets from 70 to 80 ,they have all pv sizes ,

will get another set of plugs aswell .

cheers
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Old 10-22-2012, 03:27 PM
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You were told wrong. It will backfire when cold when set like that.

You need fixed timing that does not move with rpm. 36deg BTDC. Then take timing out as boost comes in.
"boost retard". back to 26deg (take 10 deg timing out for 10psi boost@ WOT) (26 to 32 depending on boost and fuel octane)

For your initial dyno testing all you need to do is used fixed locked timing. real simple No computer....Start at 26deg bTDC
and work up Just like I said. Real simple. Get someone to show you how to set the timing at 36deg BTDC
for start up. then when all warmed up and idle mix is set up, reduce the Fixed locked timing to 26degBTDC and start testing. Its just that simple. Roots Blower motors do not run like a N/A motor or a turbo motor.
Or a blownmotor that has direct port EFI. wher fuel is injected blow the blower.

Do it you way and you will find out for yourself. Stick you head near the carb and wack the throttle open.
with only 10deg of timing at idle ha ha ha. Some people have to learn the hard way.

I gave you all the info you need to get it dialed in with the carbs ( the best you are going to achieve with those carbs that are not intended for a roots blower motor.)

Your way will backfire and foul plugs. risk blower rotor damage from blower back fire.
Consider your self warned. pump throttle 1 or 2 times start up with throttle closed, when it startt bring to 2000-2500 rpm and hold while it warms up 36dg BTDC. do not wing the throttle when cold. If it stalls restart with throttle closed.
warm it up. When its warm set idle mix for best idle ( manifold vacuum at idle) 1000 rpm ish at idle 36deg BTDC fixed timing)

Now you will see how nice a blower motor can idle. YA IT NEEDS 36deg BTDC at idle.

reduce the timing to 26deg to test @WOTunder boost. Get the main jetting right then add a bit of timing,while looking for pinging/detonation. (, over heated peppered plugs, huffing valve cover breathers) Too much timing. too lean, lack of octane for boost used.

higher fule octane allows more spark advance @ WOT under boost. AFR under boost should be 10.5:1 to 12:1
12.5:1 is too lean.

14.7:1 at part throttle cruise ( what a 02 sensor/computer ECU will try and trim the AFR to) is too lean for a Roots blower motor. Thats why you ECU contoled roots blower motor will not work like you think.

You must cruise at something richer than 14.7:1 or one of the cylinder will burn up ( too lean)
idle must be what ever AFR the motor needs to idle correcty (12:1+/-)
Not all the cylinder get the same fuel. when fuel is intoducted above the blower on that motor.
correct blower Carbs will work way better than EFI...... save your self the $$$$$ and pain. and disapointment.

When your carbs are all dialed in for best power and you see what the motor needs for timing under boost,,,,then you can map a boost retard curve 's of retard/# of boost (psi) using a simple boost retard box as sold by MSD and holley.

14PSI of boost on pump gas will require a tune that is dead nuts on the money or you will toast that motor.
do not use junk fuel. do not get greedy with the WOT timing under boost @14psi and pump gas.

At part throttle cruise (when the blower is not making boost (vacuum in the intake manifold) and the engien load is light
the motor wil want more than 36deg timing. +10. When manifold vacuum is 0 ZERO ( before boost starts building) timing will want to be 36deg BTDC. Timing mut reduce as boost builts. stating around 4 ---5psi.
both map sensors (manifold pressure and manifold vacuum) read off the intake manifold below the blower.

Again a roots blower motor need a lot of idle timing at idle that does not drop off at low idle rpm
therfore the fixed 36deg BTDC basic timing. Your incorrect curve will cause poor motor warm up, stalling, blower back fire and FOULED plugs.
and generall poor drivability.

Last edited by F-BIRD'88; 10-22-2012 at 03:55 PM.
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Old 10-22-2012, 04:05 PM
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You were told wrong. It will backfire when cold when set like that.

You need fixed timing that does not move with rpm. 36deg BTDC. Then take timing out as boost comes in.
"boost retard". back to 26deg (take 10 deg timing out for 10psi boost@ WOT) (26 to 32 depending on boost and fuel octane)

For your initial dyno testing all you need to do is used fixed locked timing. real simple No computer....Start at 26deg bTDC
and work up Just like I said. Real simple. Get someone to show you how to set the timing at 36deg BTDC
for start up. then when all warmed up and idle mix is set up, reduce the Fixed locked timing to 26degBTDC and start testing. Its just that simple. Roots Blower motors do not run like a N/A motor or a turbo motor.
Or a blownmotor that has direct port EFI. wher fuel is injected blow the blower.

Do it you way and you will find out for yourself. Stick you head near the carb and wack the throttle open.
with only 10deg of timing at idle ha ha ha. Some people have to learn the hard way.

I gave you all the info you need to get it dialed in with the carbs ( the best you are going to achieve with those carbs that are not intended for a roots blower motor.)

Your way will backfire and foul plugs. risk blower rotor damage from blower back fire.
Consider your self warned. pump throttle 1 or 2 times start up with throttle closed, when it startt bring to 2000-2500 rpm and hold while it warms up 36dg BTDC. do not wing the throttle when cold. If it stalls restart with throttle closed.
warm it up. When its warm set idle mix for best idle ( manifold vacuum at idle) 1000 rpm ish at idle 36deg BTDC fixed timing)

Now you will see how nice a blower motor can idle. YA IT NEEDS 36deg BTDC at idle.

reduce the timing to 26deg to test @WOTunder boost. Get the main jetting right then add a bit of timing,while looking for pinging/detonation. (, over heated peppered plugs, huffing valve cover breathers) Too much timing. too lean, lack of octane for boost used.

higher fule octane allows more spark advance @ WOT under boost. AFR under boost should be 10.5:1 to 12:1
12.5:1 is too lean.

14.7:1 at part throttle cruise ( what a 02 sensor/computer ECU will try and trim the AFR to "closed loop") is too lean for a Roots blower motor. Thats why you ECU contoled roots blower motor will not work like you think.

You must cruise at something richer than 14.7:1 or one of the cylinder will burn up ( too lean)
idle must be what ever AFR the motor needs to idle correcty (12:1+/-)
Not all the cylinder get the same fuel. when fuel is intoducted above the blower on that motor.
correct blower Carbs will work way better than EFI...... save your self the $$$$$ and pain. and disapointment.

When your carbs are all dialed in for best power and you see what the motor needs for timing under boost,,,,then you can map a boost retard curve 's of retard/# of boost (psi) using a simple boost retard box as sold by MSD and holley.

14PSI of boost on pump gas will require a tune that is dead nuts on the money or you will toast that motor.
do not use junk fuel. do not get greedy with the WOT timing under boost @14psi and pump gas.
A knock detector will not save your ***.

At part throttle light cruise light engine load (when the blower is not making any boost (vacuum in the intake manifold) and the engien load is light
the motor wil want more than 36deg timing. +10. When manifold vacuum is 0 ZERO (before boost starts building) timing will want to be 36deg BTDC. +/- 4deg .....Timing must reduce/retard as boost builds. starting around 4 ---5psi.
both map sensors (manifold pressure and manifold vacuum) read off the intake manifold below the blower.

Again a roots blower motor need a lot of idle timing at idle that does not drop off at low idle rpm
therfore the fixed 36deg BTDC basic timing. Your incorrect curve will cause poor motor warm up, stalling, blower back fire and FOULED plugs.
and generall poor drivability. It will run like doo dooo..... and eat plugs, your way.....

you will not get those 600cfm carbs dialed in 100% sharp with a good oerall fuel curve because the emusion circuit and air bleeds will and the PVCR's will not be right for the blower motor. Keep it a bit rich. Watch for fuel curve drift at the top end ...
keep it rich. Lean is bad...@WOT

buy two 750cfm double pumper holley blower carbs They will be very close out of the box and allow fine tuning the fuel curve (hi speed air bleeds and main jets and PVCR size.) Make sure the fuel system can feed the carbs @ max power.
flow volume not pressure. GPH

Last edited by F-BIRD'88; 10-22-2012 at 04:35 PM.
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Old 10-22-2012, 04:09 PM
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Water/methanol injection will allow more timing under boost (little or no boost retard) on pump gas and or more boost
without detonation on pump gas. That needs to be dialed in too.
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