problems with running my supercharged 351w
hi all , have been posting for a while as i have been finishing my 67 eleanor mustang . i have a 351w supercharged engine with a 6/71 weiand super charger , spec below .
high volume canton sump with windage tray and scraper
high volume oil pump
re-ground balanced crank
crane supercharger crank specs are -- fair to moderate ride,3pm 3.400-3.800 cruise rates at 8.5-1 max
degrees duration 226d @ .05"
degrees advertised 286d @ .05"
lobe seperation 112d
open and close intake 6d and exhaust 40d
gross lift .502"
hydraulic lifters on roller rockers
forged rods with hyperutectic pistons ( keith blacks)
edelbrock performer heads (need to find the spec but are good for a blower)
high volume engine run fuel pump
weiand 6/71 blower with 2x holley 600 vac secs carbs
what i need help on now is your opinions on what jets i should be using as a starting point in the primarys and secoundaries ?
currently i have .72 in the primary metering blocks and .78 in the secoundaries .
i have adjusted the timming to be 28d advanced with a idle of 22d advanced ,
the problem i am having is the carbs are spitting and when the engine finally runs its lumpy as hell ( i know its got a lumpy cam but its more so than it should be )
it will not idle and i have to keep the revs high ( say 1500 to 1800 ) but it is still rough at that rpm
do you have any idea on if my jets are too big ? it doesnt change when i adjust the air mix screws . they are currently out 1 turn .
i have a omex ecu and can adjust the timing very easily ,
i have got to 2 x boses ready for lambar sensors for when i get it dynoed and have also fitted 2x heat sensors in the exhaust manifold ,
but before that i just want to get it running on idle , also i cannot start it with the choke on . i have to have the flaps on the carbs open, which tells me it needs lots of air -which i have heard most supecharged engines and high lift cam engines never use the choke ??
am getting really frustrated as the car is 99% finished and i just want to get it on the road for 6 weeks or so to get some miles on it before the snow and ice come in december . then i can store it up till spring and adjust and fix any problems.
The carbs need to be boost referenced. If there's no response from the idle mixture screws the primary throttle blades are open too far. Adding initial timing can help this just don't let the total timing get too high when you add initial. You might need to go richer from baseline if the carbs are stock. Baseline jetting can be found here.
To start it give it a couple pumps of the throttle (cold) and w/the throttle CLOSED, crank it up. Do not pump the throttle while cranking, do not partially open the throttle while cranking.
thanks for the info - much appreciated. i take your points and will apply them and double check the timing and also the primary throttle blades are closed .
thanks for the link to the carb spec sheets.
its hard for me to work out what jets i should have as they dont list it for a application but only for a specific carb model or part number.
so with a 351w with 30 thou over bore and this supercharger and with 2x 600 vac sec holleys. is there a close idea of what jets i should be using ?
i have seen around .62 for fronts and i have .72 but also have 2 carbs. so do i go lower as i have 2x carbs ? ie 2x lower than .62
very confusing and i want to get it somewhere there before going to dyno etc. i am sure with a bit of help and advise i should be able to get it to idle and run fairly normal. under small load before going on the road
also you say to lock the timing out etc. i havent got a dizzy. i have a ecu with a chart that i am able to enter actual timing numbers into. so for example what timing would you advise on the following rpm's ?
as i can put any figure against those rpm's but i can also go above the scale and add differnt figures for engine load. so ie 10% engine load could be 28 btdc but at the same rpm with engine load at 70% that wouldnt be the same figure ?
What you'll want to do is to tee off at either of the carb inlets and run a small copper line back to the firewall, then up past the hood lip and to the cowl. On the cowl, temporarily mount a mechanical 0-15 psi liquid-filled fuel pressure gauge so that you can monitor the pressure going into the carbs by reading the gauge through the windshield as you drive. The better the quality of the gauge, the better the resolution you will get from it. The mounting system for the gauge won't have to be fancy unless you intend to make it permanent. You can make do with duct tape and tie wraps just for getting the pressure under control, then removing the line and gauge. Personally, I would want to keep it there permanently, but that's just me.
Cobalt327 offered up valuable information in his post #2. He may have gotten busy and can't come back to follow up right now. Here is the information from the Holley engineers about boost-referencing the carbs. He gave you a link, but I don't know if you saw it.
"QUESTION What is a Manifold Referenced Power Valve?
ANSWER Nothing will kill a blower or Nitrous engine quicker that a lean condition. You want plenty of fuel available for the engine to use .There is a thing you need to know about the power valves on a roots style blower engine. The power valve is installed to keep the engine from loading up and running rich at an idle. On a naturally-aspirated engine the engine vacuum at idle will hold the power valve closed. When you step on the gas the throttle plates open and the engine vacuum drops as you accelerate. When the vacuum drops below the rating of the power valve, it snaps open and richens up the main system. On a blower with the carb mounted above the rotors there is constant vacuum all the time even under wide open throttle. The power valve will never open and you will have a lean condition. To remedy this there is a modification you can have done that is called manifold referencing the power valve. You plug the vacuum feed hole in the baseplate for the power valve. Then you drill a hole in the side of the main body into the hollowed out vacuum chamber for the power valve. You then insert a vacuum nipple in this hole. You will run a vacuum line to the lower intake manifold from the new vacuum nipple. Now you will have vacuum on the power valve at an idle, and when you hit the gas as the boost builds, it will force the power valve to open and richen up the main system. This can be done by most carb modifiers or even yourself. We offer quite a few different size blower carbs with this already done. Consult you local Holley dealer or our Techline for the correct application."
Here's what Holley has to say about ignition timing.....
"Blower engines like timing advance. If the initial timing advance is not enough it will cause issues. Most blower engines will run between 12-20 degrees of intial timing and a total of 30-32 degrees. You do want a fairly fast timing curve. All the timing should be in by 2500-2800RPM.This is just a guideline. All engines are different. The other main cause is a lean running engine. Make sure the carbs are tuned correctly for the setup and that there are no vacuum leaks. Remember the blower moves a lot more air through the engine so it needs more fuel as well!"
I went through the Holley site and found this technical info as well. I highly recommend reading through it a few times.....
If this were my project and I had no means of finding out what jets to run in the carbs, I would run the ones that came in the carbs out of the box, then I'd be on top of reading the spark plugs if I had not equipped the exhaust system with oxygen sensor(s). But just let me say here that if I were you, I'd be on the phone to the Holley engineers and I'd have every last detail of your combination written down and in front of me before I made the call.....
Holley Performance Products
You have far too much money invested here to be making guesses.
One last thought regarding air supply. When I'm asked about air cleaner requirements, I generally suggest 14" diameter by 4" tall for a naturally-aspirated street motor up to 406 cubic inches and 6500 rpm's. That gives the motor 175 square inches of filter area and insures that the air cleaner will not be causing excessive vacuum at the carb inlet. The motor has to inhale easily through a well-designed intake system and the motor has to exhale easily via a well-designed exhaust system if it is to make good power.
Your motor, depending on boost, could easily pass 1.7 times the air that a naturally-aspirated motor of the same displacement would pass, so if it were my project, I'd opt for air cleaner element area of 1.7 times 175, or 300 square inches. There will be others who will disagree with this, but that's the way I'd do it. Actually, I'd look for a large oval element to enclose both carbs and make my own base and top lid. Maybe a diesel element of some kind.
thanks alot for this info. it really helps . i have booked it in with a company to have it rolling road as is, i havent got the time to try to fix something that they will know excatly what it is. they stock all the parts required to boost ref the carbes . i will take the notes with me to re-assure myself of what they are doing - will keep you posted and also post a video of when i get it running correctly. thanks again shaun
AFA giving you numbers to go w/each rpm, the numbers can be plotted on a graph. It might start out at about 18-20 degrees at idle, ramping linearly up to about 30-ish degrees at 2600-2800 rpm. This is w/the engine under a light load. Now, this is not written in stone. While these numbers should be safe (no detonation), still be on guard for detonation, and increase the amount and/or rate of advance slowly, evaluating each change as you go. I would strongly recommend you start a tuning log or journal to keep tabs on each change and the result. Don't rely on memory!
You need to compensate for engine load by pulling out advance as the load increases- just like a vacuum advance would. Hopefully your ignition is vacuum referenced so it can sense the engine load.
I looked around and found the graph below which can give you an idea of how engine load should affect timing. Remember, load is estimated by the amount of vacuum the engine makes at whatever rpm it is running.
The numbers on the graph are just a representation, your engine will likely have different amounts for how much vacuum you have at idle and at what vacuum the advance is all removed. Also the amount of advance change may need to be different than the ~2.5 degrees per in/Hg vacuum shown on the graph:
How much boost are you planning on using? What bolwer drive ratio?
What is the static compression ratio? Plan on using the very best pump gas available (or E85, but that's a whole 'nuther ball o' wax). You may need to blend race gas w/pump gas.
Do you have a boost timing retard box? to reduce the spark advance as the boost increases?
If not lock out eh advance curve and set the locked timing to 28 to 32deg BTDC.
If you have a boost retard box, lock out the timing and set the locked timing at 36deg BTDC.
Then set the retard box to retard the spark by 1deg per 1 psi boost. ( 8psi boost needs 8deg pretard) ( aprox.)
This is a initial base line tune up.
High boost will require boost referenceed power valves. If yours re not ,, keep the blower boost low
Blower drive speed ... 5psi max. Use the best gas you can. 92+++octane.
carb jetting 68 pri 78sec. Primary power valves 10.5"
Remove both carbs flip over and look at the pri and sec throttle opeing at idle.
preadjust all 8 barrels even so about .020 to .030" of the idle transfer slot is showing at idle.
reinstall. timing locked at 36deg for initial start up. Let the blower warm up. adjsut idle mix screws for best idle only once the blower is warmed up. Do not wack the throttle when cold.
start and hold rpm at 2500-3000 untill some heat getsin the blower case. set timing (Locked)
set idle mixture evenly 1000+/- idle speed.
spark plug gap .033" to .035"
start power testing at 26deg BTDC. ( either locked timing alone or 36deg - the boost retard (1deg/#boost)
start modest boost and work up on the tune. What is the engine compression ratio?
If 8.5:1 or higher do no go beyond 8psi boost on pump gas.
Watch the plugs for over heat/detonation/...
If you want to lean on the boost hard you will need to boost reference the carbs power valves so the blower does not pull the carb lean (close the power valve under high boost) @ WOT under boost.
The 600Vac sec holley street carbs don't make very good blower carbs.
You need some 650-750 DP HP carbs (with blower motor fuel calibration. and replaceable air bleeds.
The 600's will need more fuel curve work than they are worth, to get them dialed in.
Blower motors like big carbs.
Get some .035" .037" accelerator pump shooters as a start. (or drill yours)
you need a lot of accel pump volume shot for a blower motor.
blower motors love timing at idle. Don;t be shy. More is better.
Blower motos want modest WOT timing under boost, on pump gas.
less is better. Thus the locked timing and or locked timing - progressive boost spark retard relative to boost.
Race gas can handle higher boot and higher timing under boost (little or no boost retard)
Don;t get gretty on pump gas. 6 to 8psi is plenty.
What is the model list number stamped on the carb air horn?
stock metering blocks...or.
Pri and sec idle and high speed air bleed size? Primary and sec idle feed restriction size.?
Primary power valve restriction size.?
Use small precision drills or wire gauges to measure and gauge that with a dial caliper.
Think about getting some blower carbs. 650-750DPers
guys this is great thanks so much . i have this weekend to get all of these tips in place for next wednesday .
ok compression ration is 7.48 to1
i have a 14 psi max ratio on the pulleys . ( so weiand say )
i had to have a new pulley machined to fit over my rattler harmolnic balancer to save space so thats why the top one it larger than normal . bottom pulley is 45 and top is 51 teeth so its 0.88 so its being underdriven.
i can regulary get hold of 98 octane gas .
thanks t bird for the tech stuff . just what i needed. i have 6.5 power valves . so they must be getting pulled open all the time ?
so my .72 in the primary also seem to rich ?
will check my plug gaps . i have just been sold a cooler plug from champion and i havent gapped them . i wonder what they are factory set at ?
i have larger headers and also nice elderbrok victor heads . so feel confident on them . what about my air filters and scoop . i hope i am not restricting air flow ?
i have a ecu so will have to speak with them regarding boost and timing retard box and how that will work with it ?
what about colour for fuel squirt cams ? and colour of spring on sec's ?
also the vac sec's have 2x air tubes coming out of them and are fixed to the manifold "below " the supercharger !! is this where it should be plumbed in to read vacum to open the vac sec's?
thanks guys . good job !!
600carb vac sec spring. use the yellow, purple or plain steel (default) springs.
Do not use a black or brown stiff sec vac spring.
Where are these "air tubes"? on the side of carb body? or on the metering block? or on the carb baseplate??
72 is fine to start. You will need some jets to work with.
DO NOT START testing AT 14psi blower boost. 14psi is way too much for pump gas ...start at 5psi and work up.
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