Therein lies my problem. During my recent buildup, it seems I missed this piece of data. My quench with a .025 deck and a .039 compressed gastket thickness is .064. My engine pings like a son of a you know what. Is it possible that a .004 difference can cause that much havoc?

I intend on calling KB tomorrow morning. I was just curious to see what the greater engine gurus over here might think as applies to this subject. This is on a budget 383. Specs are:

1994 coupe
Eagle 15034L internal balance assembly.
KB197 -12cc D-Cup Hyper pistons
GMPP Hot Cam kit (cam, springs, 1.6rr)
Opti-II conversion
P&P heads.
ported manifold
stock injectors (getting swapped out to 30# this morning)
OE exhaust manifolds
OE cats and catback
Centerforce neutral balance SM flywheel
Centerforce DF clutch
pcmforless tune

Thanks in advance for looking at my thread. All opinions are welcome and appreciated.

-DarkMatter

 

I think optimum quench should be around .040". You can reach that figure by replacing your current head gaskets with FelPro .015" thick shims.

Good luck!

 

I think optimum quench should be zero clearance between the piston and the head. Because of bearing clearances, pin clearances, and metal (rod) stretch, that isn't possible. Chevrolet recommends a minimum of .035" . The benefits of a tight quench area gradually decreases as the piston to head clearance increases, until approximately .060", where the benefits have all but disappeared. There is nothing "magic" about the .060" number and a change of .004" to your clearance would make no noticeable difference.

A tight quench area helps the engine to be detonation resistant. However, it cannot clear up problems caused by "hot spots", excessive cylinder pressures, or other build up flaws.

RACE ON!!!

 

Thanks you gentleman, your input is much appreciated.

CFI-EFI, I understand the specs I posted here are very minimal. Can you recommend any specifics that I might look to inspect/change simply from this?

Thanks again.

-DM

 

This interests me as i have an Gen II LT1 in my '94 camaro. BTW what is you cyl compression pressure? I suspect too-hi compression as the cause here but don't know how u calculated it nor have time to estimate it or cam overlap for you.

So to lower compression we can list the options in order of cost:

- cheapest option is to change head gaskets. Now 2 ways to go here. Thinner head gasket will increase comp but can produce quence to maybe stop the detonation - maybe not. I called FelPro myself and they really do recommend thier rubber coated steel head gasket for use with aluminum heads - others have posted using it will good initial resuts though no follow-up info. Using the FelPro rubber coated steel head gaskets on my iron head (GEN I sb) car with great results - heck even overheated once already and held fine. BTW i don't know if you can use or modify the FelPro gasket for use on a GEN II/reverse flow head. A company called "Cometic" (i think) sells custom (thin) composite head gaskets though i have no experience with these.
Second method would be a much thicker head gasket. U already passed the majic .060" quench hieght and futher clearance should only reduce compression without changing quench effects - u have none anyways. This could get u a reduction of possibly .2 compression points with very little cost (sorry but u need to crunch your own compression numbers). At the same time polishing the cyl head chambers will help to reduce detonation and open the chamber/reduce compression even if ever so slightly. This is a low effort method at the lowest cost to u.

- next of course would be install heads with more open chambers to reduce comp. Trick Flow, Edelbrock, and AFR sell'em. But your spending closer to $2k when done.

- change cam to smaller lobe sep angle or with more overlap. But a cam change is not fun, a lot of work - more work than a head change IMHO.

- deck block and change the pistons. Not where u want to go i'm sure.

Hope this helps ya.
cardo0

 

I haven't done a cylinder pressure/compression test yet. My math was done with various engine calculation programs.

It seems my original math was flawed due to errors in specs on my part. My understanding was that LT1 heads were 54cc chambers. I also went with a deck calculation of .025. Using these numbers and gasket dimensions of .039 comp thickness, 4.125 bore, a cylinder bore of 4.030, and -12 cc dish I wound up with 10.8-ish SCR. The cam should have a 58.5* overlap.

However, this is where I could kick myself, it now seems that with the 1.433 compression height I instead end up with a .017 deck. That puts me at right about 11 cr flat. The cam specs are :

advertised 279/286
@50 218/228
112 LSA
109 ICL
.525/.525 total lift

My calculator says that I should have a 68.5* seat time ABDC. That should end me up with 8.5DCR. I honestly thought this motor should have been a more than happy pump gas runner.

-DM

 

I can confirm your overlap numbers and that is a bit small for your engine combo. And i made the similar mistake with estimating quench but in the other direction as i had figured .045" quench until i later discovered my actual piston compressed hieght - now put it right at .060". But my C3 engine is only a 8.8 c.r. wheezer and not a real problem. Now my stock '94 camaro LT1 is somewhere over 10.5 c.r. and is very sensitive to octane. Knock sensor bucks any pwr bursts on 87 octane but ok for fwy cruise.

Late advice now but for future use is to measure piston volume at TDC using oil/liquid from calibrated beaker/burret for accurate - with the engine out of car of course (unless you can roll the car 45* ). Just pack a little grease over the top piston ring.

Yea i think a cam change to add overlap may get you out of detonation but while cheaper than replaing heads is a lot of work. Sounds now like you actualy do have a good quench and new head gaskets not the solution now. With small stock vlvs u gonn'a find more mid-range torque with 110* or even 108* LSA. But remember your gonn'a lose idle vac too. But u can buy electric vac pumps.

For now check your cyl press. I bet your over 200psi when hot and need to get below 200psi - 180psi would be just fine. Yea, check the cyl press and if you pull the heads remmember to polish the cyl chambers - u can remove metal to nail cyl chamber vol equal in each cyl too.

Good luck.
cardo0

 

First, I want to thank you a bunch Cardo. Your input has been priceless.

I've been working on some other problems with the car as well. I was definitely pretty sure I was leaning out. I replaced the stock injectors with some 30#ers this morning. Took probablyt half the ping out. By which I mean it was still detonating, but the intensity of the knock was much, much less.

One further question, if you don't mind. If I keep the 112 LSA, but add duration to delay the intake valve seating, will that help? I'm looking at a cam right now by Comp Cams. It has a 112 LSA, but has an advertised duration of 290/307. @50 it ends up with 230/244 with
74.5* overlap, and a seat time of 74* ABDC. Do you think this will help out??

This should give me a dynamic stroke of 2.684" and a DCR of right around 8:1.

Opinions??

Thanks again

-DM

 

I don't think a flat tappet cam that large would live long on the street - 30k mi maybe. Now a roller i would expect to last longer and a billit core cam even more so. I just don't expect a cam over 280* duration to live long using required spring pressures - race use only.

Well your 58.5* overlap is really more like 63* using the 1.6 rockers that increase the through area in overlap. So 74.5* overlap is more like 80.5* with 1.6 rockers - too much for the street - race only. Now your original 279/286 cam on a 110 LSA would be a nice bump to 67.5 using 1.6 rockers. And 72* overlap for a 108 LSA cam. Myself i would stay less than 70* overlap on the street but i'm a bit conservative.

One more consideration i should have mentioned before is the computer may not work well with a low manifold vac signal. You might need custom programing if vac gets too low.

Ya know for some reason used GEN II LT1 reverse flow heads are cheap. Seen plenty for $300/pair. Myself i would remove the heads and try to polish out a few cc from each chamber or pay a good performance machine shop if i could afford it. If you grind though a chamber $300 gets you back in the game. Yea, hog out 5cc or more, balance each cyl vol and polish each chamber can be done on your own or may be only a few hundred bucks by a pro - shop around. And not an exact science itself as long as long u know where to use caution - plenty of good books on head porting.

But heck if your detonation is improving already make that compression test first. If below 200psi a thick head gasket and cyl polish might be enough of a fix. Leave the big azz cams for the track.

cardo0

 

Overlap has nothing to do with cranking compression. It occurs on the OTHER end of the valve events that create compression.

If you tighten LSA and keep same lobes you WILL increase overlap, but you will also increase cylinder pressure because you are essentially advancing intake lobe and closing it earlier. Going to a wider LSA will do just the opposite..it closes intake later, decreases cylinder pressure and decreases overlap.

What confuses folks is that usually we all use larger lobes that create more overlap by nature. But all those larger lobes do is close intake valve later and reduce pressure. Since that happens, people assume that increased overlap is what did it. You can easily spec a cam with a LOT of overlap that closes valves very early..you just need small lobes. I have several buddies using some solid flat tappet oval track cams in their street small blocks and we picked them to compliment the rest of the combo. One of the cams we like is a 238/244 @.050 (268/274 adv)but we spec'd it with 107* LSA. Since lobes are pretty small we had to use a tight LSA to get enough overlap to allow it to run strong. They idle clean, mild lope and will pull 7000-7500 rpm in the 327 and is still pulling strong at 6400 in a 383. For example, I lowered compression to 10.2 in the 383 with chamber work, but also used .002 OUT of the hole pistons and a .038 gasket with the iron heads for .036 quench.

That 218* cam is awful short for an 11.0 motor. I'm sure you have some pretty serious cylinder pressure even with a 112 LSA. I'm assuming this is a Hyd roller? Those cams are relatively mild looking with respect to lobe intensity as far as I can tell looking at the figures. Pretty good bit of adv duration for a small .050 figure. But we all know the HOT cam runs very well overall in EFI deals.

Cardo is right..the EFI setup will want a lot of vacuum unless you have a stand alone setup to run it or something.

I think there ARE folks out there with the HOT cam and 11.0 compression that get by on pump gas though. How confident are you in the ECM tuning? What is the timing map looking like?

One other thing...find out what it will take to fix it SOON. Those hypereutectic pistons are NOT going to like being detonated and you will soon be able to rebuild it anyway you want when one of them shatters.


JIM

 

:o Yea, i forgot overlap kills the vacuum - not compression. Probly since most large duration cams have low dynamic compression i overlook the cause. I bet i remember that now.

cardo0

 

Alright, I just want to make sure I understand here. I can keep the 112 LSA, or even go 114. What I should be looking at is a steeper ramp rate/more lift, and longer duration. Correct?

Would a 230/242 @ .050 with 536/544 lift be overkill for a strong street motor? This cam is the Comp Cams 503. It has more lift and duration, with the same LSA. BUT, it has a shorter seat time, and less overlap. Wouldn't that put me right back into a high cylinder pressure situation? Or will the extra lift and duration help keep the pressures out of the detonation region?

I'm trying to get a firm grip on this before I run headlong into the motor, put her back together, and still be popping like a can full of gravel.


Thanks again gentleman. You have my gratitude. (and a few beers wouldn't be bad either I bet )

-DM

 

What's the whole part number on that cam? The 503 I see ain't that one.

JIM

 

Oh, sorry. I was thinking onto the page. Put the wrong specs down for the cam I was listing. After actually looking at the cam through one of my calculators it gives me an earlier intake closing angle. So, I'm back to looking again. Do you have any recommendations 427?

-Lou

P.S. the car doesn't get driven at all right now. Just a small buz after various attempts at diagnosis/repair. Then she gets parked again. Thanks again.

 

Before I tore down the motor I would sure look at anything else that might contribute to the problem. Timing, AFR, cooling system?????

 

I use the Hotcam, and my last engine (350) had a 11.3 SCR and ran on 93 octane w/ no problems. There are folks running 12:1 on pump gas w/ a well designed LTx package. Hell, the LT4 is 10.8:1 stock! My new engine worked out to just under 11:1 but it wasn't because I was afraid of the CR. That's just what I ended up with to achieve a good but safe quench. (.041")

 

CorvetteKid.....that's what I was thinking, I knew there were several folks out there. That's why I'm questioning that ECM tune in there.Maybe it's for a lot different combo?

JIM

 

Cometic makes a .027" and SCE makes an .021"

Cometic is a Silcone covered steel gasket

SCE is a Copper gasket.

I have used both and either will be an easy fix for your quench issue.

 

Going to go out and do a compression test now. Will report back with the results.

-DM

 

Just look at your specs! You have a very low compression motor

You have a timing problem and or fuel delivery. hot cam, dished pistons and lots of quench space should make your car run fine on 87 octane. You have a very low D.C.R. a thin head gasket is not going to fix your problems