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Discussion Starter · #1 ·
I have a 69 camaro with a 540 dart big block chevy with a 8-71 blower,2 holley 850cfm double pumpers,aluminum heads .782 roller cam.my question is i have seen other cars with blowers and carburated that have a distintive idle or blower roll idle.i have talked to alot of people that own these cars and also some companys and machine shops and get different sujestions and ive tried everything still cant get it always that smooth idle. ive tried many different cams , rocker ratio's and street rodder mag said a few months ago its caused by having the wrong power valves so tried them all.i have tried many different pulley setups over driven/under driven. does anybody own anything that idles that way or know what causes that type of idle have any idea's.right now the engine is on a dyno and i can change things easily.could it be in the advance curve in the distributor?? thanks
 

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Another indication of this problem is what we call supercharger roll; while idling, the engine speeds up, then slows down, and does it repeatedly. This is caused by the supercharger's vacuum signal to the power valve not being the actual engine vacuum, but the vacuum created by the supercharger. The power valve reads the higher vacuum created by the supercharger, this vacuum closes the power valve, causing the fuel mixture to go lean, the engine then slows down due to the lean air/fuel mixture, so the vacuum signal from the supercharger causes the power valve to open as the supercharger's vacuum drops, so again the engine speeds up due to the richer air/fuel mixture, rolling from rich to lean, causing the engine to speed up and then slow down. The cure for all these ills is boost reference.

http://www.streetrodderweb.com/tech/0607sr_blown_chrysler_hemi_engine/
 

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This is something many people can't agree on. What causes it. I don't think it has to do with the power valves (well maybe). Injected blower cars "hunt" for idle too. I can tune mine in and out with just the idle mixture screws. 8-71 with 2-750 Holley blower carbs on a small block. It actually runs it's best when it is tuned to surge or "hunt" (starts the best and is most responsive). Your cam has a lot to do with it. The surge itself is caused by a lean/rich condition alternating back and forth. The reason I doubt it is just the power valve is because injected cars don't have PV's. When I was talking to the cam grinder about getting my custom cam set up, I told him exactly what I wanted and he said, "you will probably have some blower surge at idle" that tells me that the cam has a lot to do with it. He was right on the money and it performs exactly how I asked.

Blowers tend to smooth out a choppy cam which is why most blower cars idle smooth. You didn't say what you have for a cam so it's hard to say if that's why your's don't surge. I bet that's why.

It's not all about the surge though as long as it is making the power you want, I wouldn't worry about the surging.
 

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My 6-71 355 with 2 Edel 600s does it too, so I would agree that it isn't necessarily the PVs, although I can see how they could contribute. I can tune the surge out but I kinda like it. Kind of adds to my T-bucket's bad boy image ;)
 

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Discussion Starter · #6 ·
Ok ,thanks guys for the response,i had the cam ground by comp cams and told him what i was looking for (the surge)and he said this will do it,intake .782 exhaust .765 duration @.050 282/290 LOBE SEPERATION 112.0 we own 6 blown camaros small and big blocks and from a B&M mega blower,pro charger F2 ,6-71 to 8-71. we have tried everything from what the machine shops and different car owners have said caused the surge/roll.i know most injected cars do it and i think you can make it do it on that application.i have 2 holley 850cfm 4150's that are NOT blower carbs . so there NOT referenced.i have tried all the power valves sizes to see what would happen and same idle. compression is 9.5. with fel pro mls gaskets, i have tried running 3lbs to 19lbs boost.i have 8 different roller cams and same thing.also had this blower on a 468 and same idle.this is not an everyday car just for fun and experimenting.since i had this motor in and out of car many many times i figured its on a dyno now and its easier to make changes.alot of people want to get rid of that idle ,but im looking for it.if any of you guys reply ,i am going to the PRI show in florida leaving tonight and will be back next thursday, ill be asking ?'s there also. I e-mailed DYERS BLOWERS and there reply was this. THANKS

We see that a lot with a large duration cam and having a distributor set with the start point on the advance at the same rpm as the idle point.

If you set the advance curve to start at 800-1000 rpm and idle it there, you should see a surge when it picks up the timing.

Bill [email protected]
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On Fri, 8 Dec 2006 22:10:23 EST, [email protected] wrote:
 
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