1MeanZ

iTrader: (9)

383, Profiler 185 aluminum heads, 10:1 CR, XFI268 cam, Super Ram intake, 30lb injectors. Stock 89 GTA intake ducting, de-screened MAF.

I would think this combination would idle smoothly at 750rpm or maybe lower. Since I've had this engine built I've never been able to get it to like to idle below 850. I put a new BBK throttle body on the car, so I've been taking a fresh look at the tune since I had to touch up cold and warm starts and get rid of the stall saver correction. The car now idles and transitions better than it ever has, but not below 850 rpm.

Throttle follower is set at 25steps, 24* SA at warm idle, WBO2 shows about 15.0 AFR +/- .3. pulse width is about 1.5ms or so. TPS=.55v. IAC is at 20 steps at 750 (can't remember what it was at 850). These settings with an 850 idle rpm the car is happy. no stalling after a quick rev, stable smooth idle. 800 rpm starts to see some light surge and prone to stalling after a quick rev. 750 rpm has more surging, idle starts to get rough, wide band starts swinging further, stalls nearly every time after a quick rev.

I just wanted to post up because I think I'm missing something but not sure where to go now. I won't be unhappy if I can't get it to idle lower than 850, it starts and transitions nearly stock now, I'm just being picky. What have some of you guys done in this situation?

RBob

iTrader: (1)

The engine may not be happy at a much lower idle. Sounds like 825 RPM may be a good compromise.

The other item that comes into play is the injector compensation values. With the idle PW being so small they play a large part of having a stable idle.

RBob.

1MeanZ

iTrader: (9)

Yah the pulsewidth being that short is not helping I'm sure. It's the funny in-between compromise with 30lb injectors on an engine with a real mild cam.

RBob

iTrader: (1)

One thing that I did was to do a 'net search on that cam. One guy has a u-tube video of his Dad's '89 Corvette with that cam in it:


850 RPM idle speed...

RBob.

anesthes

iTrader: (12)

How much power does it make? Those injectors might be a little big for that combination.

-- Joe

1MeanZ

iTrader: (9)

Somewhat disappointingly low power compared to what I thought it would do. I made one pass down the 1/4 mile on the Power Tour last year. It was about 85* and super humid, it had just rained so I'm sure the barometer was real low. Full weight on stock wheels and street tires with WOT AFR of 12.3:1 and PE timing at 30* max, it went 13.40@108.5mph with a 2.3 60ft. I was expecting high 12s at 111mph. For reference, during the same session, stock 5th gens and stock 4th gen LS cars were trapping 100-104mph as I recall.

I was expecting about 440hp and 500tq at the flywheel. If I assume the car weighs 3500lbs and 15% driveline loss, 108.5mph comes out to about 420fwhp.

Car has a '93 T56 and a 3.27 gear. Recently changed PE timing at WOT to 32* which it really seemed to like on the street.

I chose 30lb injectors because my Camaro with essentially the same engine (old school LPE 383) seems to need them. I have to run 50psi fuel pressure to keep that motor at 12.5:1 AFR at WOT, it's very thirsty and timing sensitive compared to the 383 in the GTA. But the camaro went 12.48@114 on 315-35-17 street tires repeatedly.

anesthes

iTrader: (12)

Hrmm. Injectors sound right then. I'd use 30's up to around 450 fwhp.

That cam is somewhat similar to an LT4. It's got slightly faster ramps as it gets to .050" in 50 degrees where the LT4 cam takes 61 degrees, has a little more lift, though it's got a little wider lobe separation.

Anyway, the LT1 guys report idle around 850 seems to work best.

I would expect your displacement to swallow a little bit of that cam. I think if you keep at the tune you could probably get a smooth 800rpm out of it. I'd probably lean out your idle and add more SA.

You running closed loop idle? What is the vac like?

-- Joe

1MeanZ

iTrader: (9)

yah closed loop idle. no idea what vacuum is at idle, should probably check it. It currently idles at 15:1 afr, timing has been anywhere from 29 to 24 degrees advanced and it seemed to like 24 a little better, it was smoother and more stable.

As you and Rbob have been so helpful, I'm starting to realize maybe an 850 idle rpm is just fine. I just have this idea in my head that this is a dump truck motor and it should idle at 600rpm. That factually may not be true. I should probably stop worrying about it and just enjoy the car. It does a pretty good job otherwise.