camaro7826

Ok, this may be the dumbest question ever but im not too bright myself so here goes.

I hear all these high peformance cars at drag strips, and when they rev up, when they let off the gas, the car goes immediately back to idle. They dont rev. back down slowly like normal cars do like my ss. They rev up quick and immediately go back to idle. to me it sounds badass and i would like to know what makes them do this. Also, how could i make an ss with an ls1 do this.

Thanks for the help

Brad

squirrels

I'd guess it has something to do with a lighter powertrain...lighter means less rotating mass and less inertia. Revs up quick, revs down quick.

Or it could have something to do with tuning.

Maybe a lighter flywheel/rotating assembly?

Just a guess.

BigTex

iTrader: (16)

My guess is less exhaust backpressure is part of the answer.

sited12

If you rev to 9500 and idle at 1500 RPMs you should get the same effect. Its probably alot to do with the tuning. Our cars come down from a rev with pops and sounds like that from running rich. It might be the valve train or it might be the tuning. Those are my 2 ideas on it.

LS1x2

iTrader: (11)

It has more to do with the converter. Mine does this with the TH350.

camaro7826

Im not sure if you all are talking about the same thing I am. When the dragsters at the track are staging, they rev up and its like as soon as they let off the gas, it is idling again. It just revs up and immediately goes to idle again instead of reving back down slowly. I was thinking it could be a cam with high lift and low duration. The reason for low duration causing this is possibly because the valves shut much more quicker than with a high duration.??? Any other ideas. Thanks

LS171Malibu

Well, its cause those are billy badass cars man...oh yeah and carburated <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />

LS1derfull

Our cars have a IAC valve that soften the closing rate of snapping throttle closed.Idle speed is brought down slowly to clean emissions and prevent backfires.If you removed its function you could get some of the results of a carbureted race car.Cam profile and well sprung valvetrain also have a say in this quick rev and down rev situation. <img border="0" title="" alt="[Cool]" src="gr_images/icons/cool.gif" />

TransAmaniac

I thought it was directly related to the compression ration.
Like a 12:1 car will De-Rev quicker than a 10:1 car.
Am I right?

GeorgeC

LS1derfull hit the nail on the head... A carbed car returns directly back to idle when you rev it and let off the gas. Our IAC motors/PCM keep our late model EFI cars from working this same way.

Orange Krush

LS1derful is a very smart GM tech ! <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />

Jake99SS

So if we could defeat the IAC motor and control that valve manually we could get an LS1 to sound like that? I've been thinking about doing this mod since I have a cam and it surges on warm starts. Didn't Steve Harmon do this mod a while back? I remember seeing something posted on Tech-LS1.com

Colonel

Yes, the IAC has alot to do with it slowly drifting back to idle but if you take that out of the picture (simply disabling the IAC will not make your engines RPMS come down as quickly as a race engine) it also has ALOT to do with the C/R of the engine. The compression is the biggest part of what is slowing the engine back down (along with friction, oil windage, and valve spring pressures [this point is sometimes confused with the spring pressure itself actually slowing the engine but this is not so since for every action there is an opposite and equal reaction. At any given time you pretty much have just as many springs pushing the engine forward as you have pulling it back. Really, the extra spring pressure just makes for more friction which does of course drag on the engine.]) Inertia from heavy parts helps to make the RPMs fall slower.

So...

Combine EXTREMELY high C/Rs (by far the biggest factor here), superlightweight moving parts (less inertia), and super high valvespring pressures and you can understand why the revs come back down so quickly.

My 12.2:1 422 would back down ALOT quicker than my stock LS1s (10.1:1). And yes, it did have a functioning IAC.

<small>[ September 26, 2002, 05:16 PM: Message edited by: Colonel ]</small>

Colonel

Quick experiment...

Go unplug your IAC while the car is at idle. It's now out of the equation. Rev the engine to 6,000 and let off. Did it return quicker? I bet it didn't from high RPMs (6000 down to 3000) like you're looking for but the rate of descent probably didn't slow down at 1500 or so and then drift back to idle (it may have even stalled out) like it did before.

Point being, getting rid of the IAC is not going to give you that quick pop up and then back down like the race cars have. <img border="0" title="" alt="[Sad]" src="gr_sad.gif" /> Bump that C/R up to 16:1 and see what happens (might as well run some methanol while you're at it!) <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />

camaro7826

So in other words its prolly not possible with a lightly modified LS1 camaro.

sited12

Ha i just finished putting the 16 CR engine in my car. I mean comeone 125 octane is so easy to get a hold of to put in my 3 gallon fuel cell every 2 blocks. Ha i think 11.25 compression is what i am gonna shoot for. Maybe less, gotta keep that 93 octane.

CAT3

iTrader: (9)

As well as compression ratio, doesn't the Lobe Separation Angle (LSA) have a lot to do with the snappy rev's. If you install a cam with 110 LSA and compare it to the same engine with a factory 118 LSA, the 110 will snap, quick fast and in a hurry up and down the rpm range. During this process it helps to lower the internal rotating mass which free up horsepower to a point, but tuning will either make your sh*t run tight or like sh*t.

SJH

My IAC motor is removed and the throttle body cavity completely plugged and blocked off.Yes I controlled it manually for a while via cable. Now it's totally gone.
I really don't find it necessary to have.
But don't expect doing that will make your motor sound much different.I just don't like the damn lazy step motor in that thing <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />

Steve