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Old 02-20-2012, 07:11 PM
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blown BBC

so i have a few questions about my blown chevy motor. it's a 502 gen 6 with a roller came, 9:1 compression with a B&M 871 blower putting out 10 LBS of boost. i have dual 750 Edelbrock carbs. the guy that had this truck before was running C16. he had 56 teeth on the crank and 54 are the charger, so i swapped them so now i have overdriven. I'm having tuning problems... it doesnt want to stay running once it starts (if it does) and once it does when i go to drive it it wants to die when i stop, have to keep hitting the gas to keep it running, I'm not running 91 in it. any tuning tips would help out a lot! thanks!

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Old 02-21-2012, 12:27 PM
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Quote:
Originally Posted by rvansick123
so i have a few questions about my blown chevy motor. it's a 502 gen 6 with a roller came, 9:1 compression with a B&M 871 blower putting out 10 LBS of boost. i have dual 750 Edelbrock carbs. the guy that had this truck before was running C16. he had 56 teeth on the crank and 54 are the charger, so i swapped them so now i have overdriven. I'm having tuning problems... it doesnt want to stay running once it starts (if it does) and once it does when i go to drive it it wants to die when i stop, have to keep hitting the gas to keep it running, I'm not running 91 in it. any tuning tips would help out a lot! thanks!
When the bottom pulley ( crank) is larger then the top (Blower) it is over driven! when you swap gears and the small pulley is on the bottom your under driven and boost drops ! Usually the tooth count is more on the bigger pulley? So when you swapped pulleys you may have actually under driven you blower ! And dropped your boost ratio dropping final compression, that was at 9-1 with 10# boost came out to 15 to 1 compression on my boost chart.

Put the larger diameter pulley on the crank and the smaller on the charger and you'll be over driven!

Jester
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Old 02-21-2012, 01:15 PM
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well i guess the big reason why i did that so i could lower the boost and try to stay away from the C16 and run pump gas, so now running 91 when i get it running it seems to do great but on the first start of the day it's such a pain in the ***! sometimes it wont even start! do you know if that would be timing?
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Old 02-21-2012, 01:24 PM
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to me the big question is how did it run before you swapped the pulleys?? My SBC is blown too and with my Weiand setup I have about sixteen different driven gears (on the nose of the blower) to choose from, all of which will change the overdrive ratio. In my limited experience, swapping the large/small pulleys on the crank and blower nose is NOT going to provide a viable solution, and if it ran OK before you swapped, you should look at your pulley ratios again in greater detail before you jump to the conclusion its a timing problem.

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Old 02-21-2012, 01:54 PM
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Starting a cold supercharged motor.

Pump the gas pedal 2-3 time and TAKE YOUR FOOT OFF THE GAS.
Crank the motor with a closed throttle.
( your big foot off the gas)

When it starts, catch the throttle and run at 2000-2500rpm until the engine warms up 5-10 minutes.
Do not wing the throttle open, do not rev the motor up, do not allow the motor to idle low when warming it up.
If it stalls start it again using the same closed throttle proceedure.

Get the blower warmed up before you drive it.

If you open the throttle while cranking the motor it will probabily back fire.
If you wing the throttle when the motor is cold it will probabily backfire.
Backfireing is very hard on the blower rotors and carbs.

You need fresh non fouled spark plugs. Old plugs and or poor start up-warm up method will kill the plugs and it will start and run like *****!

Correct igniton timing is critical.
A blower motor uses a differnt spark advance curve than a normal motor does.
A stock out of the box distributor will not work.

The distributor most use a specific shortened timing curve that allows generous idle base timing but avoids excessive max spark advance. (28-31 deg at max advance)
Remeber lots if idle advance but limited max advance.
Short timing curve limit. If it ain;t right fix it.

10PSI blower boost is a lot of boost for a 9:1 blown street motor that does not see good quality octane fuel. (especially if you are inexperienced in blower motor tuning) (91 is only so so, you want better fuel 93++) I would lower the boost to a more reliable 6-7psi. or mix 110 unleaded with the your pump gas You will need to buy the correct pulley to do this.

for 10psi boost and so so fuel 91 octane you want a low less than 8:1 cr

excessive boost on crappy fuel will damage the motor

Get someone to help you that has blower motor tuning experience.

Under drive ratio has the larger pulley on the blower. (slower blower speed)

poor running, look for a leaking gasket blower to manifold or intake manifold to head. fouled spark plugs, incorrect spark advance.

Last edited by F-BIRD'88; 02-21-2012 at 02:02 PM.
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Old 02-21-2012, 01:57 PM
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yeah it ran great before, i tore the motor apart to figure out what the compression was (the guy i bought it from had no idea) so when i put it back together fired it up and drove it home it was great. then when i got home i started messsing with the timing a little bit lowering it to 18 at idle. thats when i started having problems with it. now it's very hard to start for the first start of the day and when i go to stop at a light it'll want to die. this is my first blower motor so I'm new to tuning this.

also switching from c16 to pump gas should i have done anything different with the carbs?
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Old 02-21-2012, 02:13 PM
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wow i did not know all of that! that will help out a lot! thanks!
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Old 02-21-2012, 02:13 PM
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Quote:
Originally Posted by rvansick123
yeah it ran great before, i tore the motor apart to figure out what the compression was (the guy i bought it from had no idea) so when i put it back together fired it up and drove it home it was great. then when i got home i started messsing with the timing a little bit lowering it to 18 at idle. thats when i started having problems with it. now it's very hard to start for the first start of the day and when i go to stop at a light it'll want to die. this is my first blower motor so I'm new to tuning this.

also switching from c16 to pump gas should i have done anything different with the carbs?
Get the timing right 24 to 30 at idle , 28 to 32 at max advance, under boost
low idle timing ='s poor idle and start up issues.

Excessive max advance under boost on pump gas ='s damaged motor.

You have modify the distributor curve to get this. Or lock it out.

Any carb rejetting from that fuel switch will be minor assumeing the carbs were tuned correctly before. I would not assume that at all.

Check the intake and blower gaskets.

A setup-tune up on a chassis dyno will save you a lot of greef. and money.

9:1 on 91 octane go easy on the boost and the timing under boost.

6-7 psi when the motor is tuned correctly ( jetting timing) makes huge street power and avoids detonation $$$

Get a AFR gauge to help dial in the carbs.
get a good timing light and a timing tape to allow you to check the idle and max advance timing. A electronic boost retard ignition box is real nice to have.

reme3mber generous initial at idle but modest at max advance under boost.
Often its best to just lock out the distributor and run fixed 28-30deg timing.
If you got no retard box.

Locked out timing and GM starter motor. Get the required $10 OEM rear starter mounting bracket that is supposed to be on your starter.
Install a ignition power interupt switch to aid hot engine start up.

ALL ways crank start a blower motor with a closed throttle.

WOT jetting under boost you want a slightly rich AFR ratio but not pig rich.
12.5:1 to 11:1 AFR ratio under boost .
The extra fuel cools and helps avoid detonation.

idle afr 13:1 to 12:1 use a vacuum gauge to set the idle. Warmed engine first

part throttle cruise around 14.7:1 at no boost light throttle, the AFR must progress richer as the boost comes in. ( edelbrock carb metering rod and step spring choice) carb vacuum under WOT boost cannot exceed the vacuum step point of the spring. use as high a vacuum value spring, as possible

start jetted rich for WOT under boost then tune it leaner, to the correct safe ratio under boost . a bit Rich is safe. Pig rich is not. lean @WOT is always bad.

vacuum advance must run off the intake manifold below the blower, not on the carb. A stock vacuum advacne won't work a stock out fo the bex mechanical advacne curve will not work.

yank the distributor out and fix it.

The carbs may need larger accelerator pump shooters.
Edelbrock carb fuel pressure: you need a constant 5.5psi at idle that dose not drop below 3psi at WOT high rpm. The fuel pump must be able to keep up and feed the motor. Excessive fuel pressure is not good and won;t fix it if the pump cannot keep up..

You need fuel flow volume, not pressure.

lower the blower boost to 6-7psi ( new pulleys) for pump gas.

Last edited by F-BIRD'88; 02-21-2012 at 02:42 PM.
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Old 02-21-2012, 02:21 PM
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I agree with Firebird. Get someone with good experience at tuning blown motors to help you. I don't think you want to get too deep into tweaking carburetion and swapping the pulleys on your blower and changing the general tune of the engine too much. I know I can get in waaaay over my head on my setup, and there's no substitute for experience on this. Also I can confirm that backfiring into a blower can cause problems. It isn't hard to end up with a blower fire as a result, and that will usually run into $$$ before its over. Seek help and ask whoever is working on it if they'll let you watch and learn.......


Almost forgot; Swimming up to a blown BBC and then looking to run ecogas may not be an easy combination to make. If you are really committed to running lower octane gas and not Avgas, you may not want to have a blower assembly at all. You can get a ton of horsepower out of an engine without having a blower. I know that you can get good performance (and low operating temps too) out of E85 if your carbs are set up for it, but I'm not sure you can get away with E85 on any kind of a blower application. Firebird, how about it?

Last edited by Moose2; 02-21-2012 at 02:26 PM.
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Old 02-21-2012, 02:24 PM
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#1 You probably have to re-jet the carbs too! I cant give you an exact timing But I would back it off about 3 degrees at a time on initial.but it wont do you any good in my opinion! Even if you drop your boost down to 6# your compression will be 12.7 to 1! C16 is 114 octane with lead, for high boost or large nitrous shots. Your still going to need 110 octane at least! Premium gas and off the shelf boosters will do nothing! You bought a monster built engine and are trying to fight the built in running characteristics with little cures for normally aspirated street & strip engines. Timing will only go so far!! If you drop your boost to 2# you'll still be at 10.2 to 1! At 0 boost and re-timing you can run pump gas with 9-1

No offense at all meant to you! but its like giving a wild wounded wolverine an aspirin and then trying to pet it without getting bit! That engine you have is a man eater!! Most guys on here would give their eye teeth to have it C16 included! I pay almost 7 dollars a gallon for 110! and am retired now so I know how expensive it is! But I would have to almost rebuild my engine to run pump gas and have it run right. I can back off my timing and cruise around but if I shower down on it the power isn't there.

I have seen the new water alkie injection systems work well on high compression engines including turbos & superchargers and they can run pump gas with minor changes ! You could probably put your engine back the way it was and run one of these systems

Post a picture Im interest in seeing it

Someone else on here may have some Ideas for you that's a cheep way out though good luck! Ill be following these posts Im interested in your progress

Jester

LOL When I started typing this there was only one post this things filling up fast you have a lot of help and good people

Last edited by painted jester; 02-21-2012 at 02:30 PM.
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Old 02-21-2012, 04:36 PM
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thanks a lot guys! all this info is GREAT !!! looks like i have my work cut out for me! i do have a lot of C16 left over i can mix in with the 91. taking the blower off has crossed my mind but after having it on there it seems to kinda complete the truck! ill start working on her this weekend and let you know how things work out! ill see if i can figure out how to post a pic of it on here! it's a 1986 chevy 4x4 one ton, dana 60's, 513 gears, lockers front and rear, sitting on 44's!
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Old 02-21-2012, 04:43 PM
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here's some more pic of when i took it out and cleaned it up
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Old 02-21-2012, 06:09 PM
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You said that it ran fine after you put it back together and before you mesed with the timing. So it seems that the timing is the issue. I personally would lock it out at 32* or so. I would def go back to underdriven, there really is no reason to overdrive a 8-71 for the street. A 8-71 will put out more than enough boost for a street driven truck.

Lock out the timing, underdrive it, get a boost timing master (BTM) if you don't have one. There is not much more you can do without lowering the compression, running underdrive on 93 octane. With 9-1 you are pretty much stuck with c16, unless you get a water/meth injection and if you want to run any measureable amount of boost.

Just my opinion, but I am no means an expert.

I have a BBC with 7-1 comp and I can run more boost than I can handle on the street with 93 (10% underdriven)


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Old 02-27-2012, 04:39 AM
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running great not temp question!

ok so i did alllll that work this weekend, timing, checked for leaks, installed a tach, had to fix some wiring. i found the sweet spot for the timing, mixed some C16 i have with the 91 that was in the tanks and took her for a 45 min drive, motor did GREAT! didn't pop so spit or anything! thanks a lot everyone for the help! now even on a cold start it fires up like a dream! but i have one more question for you guys. for the most part of driving around the temp stayed at 180, but tords the end of the drive i saw the temp creeping up to 200 then 205 then 210 then i looked at the boost gauge and at an idle it was holding at 20 pounds (psi)... so was it building up presser making it get warm????
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Old 02-27-2012, 09:04 AM
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You don't have 20 #'s boost at idle, a blower doesn't make boost at idle, and at 56 top 54 bottom teeth, I show that as under driven on Weiand's charts.

I assume this is an 8mm pitch, not 1/2"? 55/54 is -1.8 under driven, 57/54 is -5.3 % under. Also, if you are making 10 pounds of boost with 9:1 static c.r., your effective c .r. is 15:1, definitely not pump gas friendly. A water/meth injection system similar to the AIS, and timing retard system would help.
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