I'm trying to troubleshoot a 1998 C5 vette with a LPE GT2-3 cam( 207/220 .571/.578 w 1.7 rocker 118.5 CL), ported LS1 heads, and 1.85 rockers, 918 springs - lift is around .630"
the heads have screw-in studs for adjusting the rockers, and the spring pockets have been machined down approx .090". Preload on the stock lifters is MUCH less than stock if that matters. Rocker geometry is good.

The chambers were deshrouded and polished and a tuliped exhaust valve was used, so the compression was lowered (unknown amount). Fel-Pro headgaskets were used.

the exhaust flow on these heads is as follows:
.200 -105
.300 -139
.400 -175
.500 -201
.600 -222
.650 -225

intake flow follows:
.200 -137
.300 -212
.400 -254
.500 -275
.600 -278

here's the cam card


The car has the stock 346 shortblock, A4 with stock converter and 3.15 gears with 28" tall nittos on the rear. It has full-length headers & X-pipe with Z06 mufflers.
It has been running MAFless and open loop for about a year, so that rules out many variables.

Here's the problem. It feels very lazy until about 4000 RPM, where it actually pulls harder than expected at that point. I expected it to feel a little lazy down low with the lack of gearing & converter, but not to the extent that it is, and the sudden huge surge of power comes around 4000 RPM, just like hitting a little 100-shot of juice.
The cars performance is 12.7 @111 in summer air (2000+ DA), and because of it being an automatic, this big lazy area down low is really killing it. I know that a good converter will bypass the problem completely, but I'd rather solve the problem before swapping converters.
The car is set up for a big N2O shot, so the highway gears and big rear tires must stay. I'd like to resolve this problem with the engine by looking at just the engine.

It is running on 87 octane with 30+ degrees of advance through the entire range at WOT, so obviously the burn rate has been slowed. The AFR (mid 12's) and the timing stay constant throughout the entire RPM range, so it's hard to point at either as being the cause.

It has such a small camshaft though, I'd really like to know why it behaves as it does. The delay in coming on the cam was not expected at all. Even with the lower compression I thought it should have a smoother bottom end with that small camshaft.

It looks like the cam opens the exhaust very early for blow-down correct? This would be magnified by the 1.85 rockers, and the very good low-lift flow of the heads. Lately I suspect that it may be blowing-down the exhaust way too soon and getting a positive wave in the chamber before intake opening.

All input welcome

 

Thanks for moving this. Now where are the cam gurus?

 

Have you had it dyno'd or WB tuned?


The stock converter is killing you in a bad way. The cam may not be as far off as you think.

I may not be much help, but that's my take on the situation. Good luck to you.

 

WB shows 12.2 - 12.6 rich I know, but it's a constant through the RPM band.

It dyno'd at 360hp, but the curve looks smoothe and not representative of how the car feels or performs during a run.

 

918's for that much lift with 1.8's? That seems near the limit. From what I've read not personally tested. I'm running dual springs for less lift. How did you come up with .630 from 571/578 plus 1.8's? That seems high. The flow numbers appear low compared to other heads at 400, 500 and 600. FWIW.

 

Yes my bad, they are 1.85 rockers. I will edit the above.
Yes that's a lot of lift. I had the pockets machined down .090" to increase install height.

 

The lift would be 605/612 with 1.8's assuming that your not using the new Crane vari lift 1.8's.

 

I know you've asked for the cam guys, but, wanted to post my findings. My little car has a 216/220 .525/.532 114 Comp cam with a Weiand intake & AFR 205 heads. A/F mix is 12.8-12.9 @ WOT through the RPM band & timing is about the same as yours. Runs 92 octane or more & has a 3.55 rear end.

Even with the larger rear end & a light car, it was a big fat pig up to 3,800 RPM while using the stock converter. The fact that the heavier car you are working on is running high 12's on 87 Octane is damn good. I just don't see how a cam alone will remove the soggyness of the stock converter, especially while using 3.15 gearing in the rear.

Even with just the 9.5" 2800 stall 2.0 converter change (light car w/skinny rear tires), my car became much more aggressive down low. I guess my point is that a cam isn't gonna take the soggy out while the stock converter is in, IMHO. Good-luck

Oh yah, heads wouldn't be a bad idea.

 

anyone else?

 

My vote is for the converter. Just out of curiousity how are you running 30+ deg of timing on 87 octane? Maybe it's KR?

 

You can run 29 -30* stock with 93 octane.
The ported heads lowered the compression some and the engine runs very cool. Chambers and valves were polished, and the exhaust valves are the tuliped Z06 pieces which will add a little to chamber size. Basically it's just lower compression than stock and the metal stays much cooler.
It runs faster on 87 octane by the way.

The KR is disabled.

 

ttt for me

 

The nature of these cars is that the power really feels like it is kicking in 4000 rpm. A low STR 3000 stall is the real cure. Since your post says you want to fix it through the engine you have 2 choices:

put an adjustible chain on and advance the cam 3 or 4 degrees

put in a cam that will make better low end power. I know you are spraying but a 207/220 118 cam is not the solution. You didn't say how much spray you have, but even a 216/224 114 with 2 - 4 degrees advance will do much better NA and work with a 200 shot.

 

It's a 200 shot.

I thought the LPE2-3 would be good for power adder. Could it be that this is meant to be a turbo cam?

I also thought about possibly advancing the cam, but this also opens the exhaust even sooner. If early blow-down is the problem then it would probably get worse.

If this LPE2-3 cam has pretty aggressive ramp rates, then wouldn't combining this with the 1.85 rockers maybe create a problem of the stock lifters handling this? I don't really know, so that's why I'm asking.

 

bump ttt