bunker

I have a custom cam, its pretty big in size, 24X/25X & on a 109LSA, thats all I can say about it for the purpose of advice & still respect the grinder's descisions on keeping this secret.

Anywho, I'm in the process in making a tune, I have a ton of timing in that cam, I'm mostly concentrating on numbers for 90-100 for MAP. I've dealt with part throttle pretty well & I think I have sufficient timing there, but now I'm wondering, is 40* advance too much at 2000rpm for example? here is what I'm thinking:

revs ex. 2000-40* 3000 38* 4000 36* 5000 35* 6000 34* 7000 33*

reason being for that high of timing is that with the lobe separation & durration on a stock cubed LT1 I'm building barely any dynamic compresion down low & figured I'll need more lead timing to build compression down low & taper off with RPM with compression will build & I won't need as much lead timing.

Note below 2krpm I'm tapering off to a minimum of 24* at 1000 rpm aswell, so above & below 2000rpm I'm tapering off, as in at 2000rpm I want to have the highest timing. At the moment I'm at about 34* at 2000rpm.

Matt

Chris 96 WS6

If anything I would think you need less timing at lower RPMs and more as it goes higher, because the flame front from the spark travels at a fixed speed, so at higher RPMS you need to get combustion started earlier to reach peak cyl pressure at the same piston position vs. a lower RPM.

I've never heard of an LT1 making best power with more than 38* of advance.

Ultimately you are going to want to get the car on a dyno, but from about 4500-6200 I have 38* all the way across in my 90-100kPa cells.

bunker

See it it made sense to me aswell that the higher the RPM the more advance extactly for that reason being that the flame burn rate stays the same yet the piston travels much faster, yet aparently the higher the speed of the piston (rpm) the less timing needed because you don't need to build cylinder compression early & or don't need it, I searched around & aparently some of the oldschool guys would wire in a switch to retard about 4* at higher RPMs, so I asked Joe Overton on this & he told me that I don't want as much timing advance at top end even though I explained to him the whole concept of piston traveling faster & yet the burn rate is the same, he's also dyno tuned several cars & saw an increase in power by lowering the timing. On my buddie's LT1 we lowered the timing by 2* going from 38* to 36* at 6krpm & gained 6rwhp so go figure.

As for timing being 40* at 2k, I got 109*LSA & 24X & 25X durration so I figure I need as much cylider pressure as I can squeez down low.

And now, I assure you, lowring the timing downlow ruins the drivability & car becomes a slug, I've tried it with many setups.

Let me know any more opinions, thanx guys.

Matt

OldSStroker

I'd listen to Joe O., and/or whoever spec'd the cam for you concerning timing (especially if it was Joe).

Don't overthink what MAY be happening; start with mild advance and let the engine tell you what it wants during your dyno runs.

Good luck and let us know the results.

Schurters LT1

I tryed the same thing with the timing taking it out as the rpm went up....all said in done made less power....

The only way to fig out timing is on the dyno, i started out at 20 and went to 34* for a base tune then add from there....DO this on the dyno....

YOu never tune What YOU think the car needs you give it what it wants

bunker

true enough, just wanted a baseline before hitting the dyno, but I've been tunning some more tunning, the car behaves real nice, reason I'm asking is cuz Joe told me to start high & end low which didn't make much sense to me, I was hoping someone would clear that up.

SStrokerAce

Me either, you think he would know better. Hell last thing I want to do is hole a piston because I put too much advance in at the bottom end, start low and go up till the motor talks to you. With Knock sensors the power curve will show it anyways.

Bret

bunker

good things make sense again

bunker

Thinking about it, maybe the reason he says this is cause the cam has no cylinder pressure down low so he tells you to dumb timing in & as cylinder pressure builds up you taper off cuz it would be too much at 6k rpm where a huge cam would make a ton of cylinder pressure.

With normal cam you go low to high because the cam builds peak TQ early meaning high cylinder pressure = less timing to save the engine? While this thing prolly peaks way high & has no cylinder pressure down low.

I dunno we'll see what dyno says, still making little sensor to me going high to low.