Jeep Strokers

Wow, thats really high. You could get a cam with more duration to bleed off some of the compression. Or have the engine builder replace the pistons with ones with a larger dish. Either way I think a conversation with your engine builder is on order.

Would a bad cam grind / wrong camshaft cause these high numbers? or the cam couldn't make it that high and it has to be to much taken off head and block when decked ?

Example- the shop ordered the correct cam, company sent wrong one, they didn't measure/check it before they installed it and here I am with 235 cylinder compression?

I found on a turbo site some numbers. It does depend on your cam, more overlap will decrease the compression test psi.

7:1--141
8:1--162
9:1--183
10:1-204
11:1-225
12:1-246

Yours at 235 seems to be about 11.5:1 !! Yikes!

http://www.hotrod.com/techarticles/113_ ... z228lMDw74

SPECIAL CONSIDERATIONS
High Compression: Cam companies have long preached the gospel of conservative cam specs for a true street car, meaning that you're always better off to go one step smaller on the cam instead of one step bigger. Most of the time that's true, but there are instances where a street car should run an aggressive cam, and sometimes bigger is indeed better.

Overlap (lots of duration and tight lobe-separation angles) decreases cylinder pressure, especially at low rpm, which allows an engine to run a higher compression ratio and still work on pump gas. High cylinder pressure, which is caused partly by a high compression ratio, is what makes an engine detonate on pump gas. Decreasing the cylinder pressure by adding duration is just like taking compression out of the engine, but mostly only at low rpm. A compression ratio of 11.5:1 running on pump gas is not unheard of when the cam has enough overlap and duration to bleed off the low-speed cylinder pressure.



Read more: http://www.hotrod.com/techarticles/113_ ... z228lMDw74

Ok I have the manufacturer piston and cam model #'s

Piston - H802cp
Camshaft - CS1585

This is a stock replacement camshaft. Will it work?

You have 235psi cranking compression, do you need to ask if the cam is working in the combo?

I found this on another post- Is that a small amount of dish?

...could've gotten me h802cp20s and the premium moly metric rings for ~$200. Thus far, the majority of the specs I've seen for these are 15.1cc dish...

Using the calc here with 3.895" bore, 4.2l crank and rods, 15.5cc piston dish, guessing on the deck clearance at .030" from previous experience, late model cam grind, 57cc combustion chamber since milled, you have a SCR of 9.72 and DCR of 8.41. Given that info it should run on 93 octane w/o ping especially since you had the ignition timing retarded. I had one with 9.83 SCR and 8.42 DCR and it ran fine on 93. Was the cam degreed in? If it wasn't and it is actually advanced that can increase your dynamic compression. Given your compression numbers that could be the case.

fixed

gradon wrote:You have the DCR and SCR switched(you know better but others don't).

Thanx. Fixed. I don't know how the heck I did that. Trying to work on 10 things at the time.

SilverXJ wrote:Using the calc here with 3.895" bore, 4.2l crank and rods, 15.5cc piston dish, guessing on the deck clearance at .030" from previous experience, late model cam grind, 57cc combustion chamber since milled, you have a SCR of 9.72 and DCR of 8.41. Given that info it should run on 93 octane w/o ping especially since you had the ignition timing retarded. I had one with 9.83 SCR and 8.42 DCR and it ran fine on 93. Was the cam degreed in? If it wasn't and it is actually advanced that can increase your dynamic compression. Given your compression numbers that could be the case.

The builder said yes, it was degreed in. He said it called for 0* and that is what they set it at. He said it is exactly at 0* with the timing.
Its a stock cam, i don't know.

If you will tell me how to degree it in, I will check to make sure they did it correctly.

Compcam's instructions: http://www.compcams.com/technical/instr ... es/145.pdf around page 8

Isky's with some really good info on cams: http://www.iskycams.com/degreeing.html

Crane's cam writeup is pretty good as well: http://cranecams.com/uploads/instructions/803.pdf

You may be able to check it with just pulling the valve cover off, rockers and spark plugs out. Install the degree wheel with the harmonic balancer bolt. Use a positive stop down the #1 cylinder spark plug hole to find TDC. Finding exact TDC is the most important first step.

If the cam is degreed in correctly I am at a loss why the compression ratio is so high unless the head or deck has been cut more than we think or those pistons have a smaller dish then known.

SilverXJ wrote:Compcam's instructions: http://www.compcams.com/technical/instr ... es/145.pdf around page 8

Isky's with some really good info on cams: http://www.iskycams.com/degreeing.html

Crane's cam writeup is pretty good as well: http://cranecams.com/uploads/instructions/803.pdf

You may be able to check it with just pulling the valve cover off, rockers and spark plugs out. Install the degree wheel with the harmonic balancer bolt. Use a positive stop down the #1 cylinder spark plug hole to find TDC. Finding exact TDC is the most important first step.

If the cam is degreed in correctly I am at a loss why the compression ratio is so high unless the head or deck has been cut more than we think or those pistons have a smaller dish then known.

Thank you! I had to read each a few times to get the process down in my head, and i will print the directions out when i do it.

Quick question while I am tearing it down to change out the camshaft.

Does anyone know the reason why it only detonates when the engine gets up to operating temperature?
and even at operating temp, it doesn't detonate if its in neutral or park???? only in gear.

Possibly because the engine is hotter. Pistons, combustion chamber, oil, coolant, fuel, etc. Hotter it is the more chance of detonation. It doesn't do it in neutral or park because the engine is not under load.