Compression test - pressure 2 high???

[njsteve]

This is in reference to my 455HO Firebird thread:
http://www.yenko.net/ubbthreads/ubbthreads.php/topics/503488/Re_The_New_Project_Part_Deux#Post503488

I just warmed the car up and did a compression check. (Throttle propped at WOT, all plugs out, cranked 7 revolutions for each cylinder- gauge stopped climbing at 6 revolutions).

I would say the cylinder pressure is just a little bit high for an 8.4 to 1 compression engine?

Cyl 1: 230 psi
Cyl 2: 220 psi
Cyl 3: 225 psi
Cyl 4: 230 psi
Cyl 5: 227 psi
Cyl 6: 230 psi
Cyl 7: 220 psi
Cyl 8: 235 psi

For you engine gurus out there, would that confirm that the camshaft in this thing is WAY WRONG for this application? It does have stock looking flat top pistons, as I can see the valve reliefs through the spark plug hole (no dome). The car pings at just about any timing setting. Even on 100 octane low lead avgas mixed with 93 unleaded.

I guess this could also be the cause of the oil leaking past the main seal under full throttle. A whole lot of overpressure down there.

[PxTx]

What heads are on this thing? Volume of combustion chamber?

[Chevy454]

Not necessarily the wrong cam, simply building cylinder pressure sooner, so my guess is low rpmn short duration...but I'm no cam wizard.

As for mixing your fuel, thats a no no...run the straight 100ll and try it, otherwise id say youre closer to 93 than the 100 you're shooting for...mixing doesn't work linearly like you'd think it should. I assume you've checked your timing/dizzy to make sure it's no doing anything wonky like adding too much advance?

[njsteve]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: pxtx</div><div class="ubbcode-body">What heads are on this thing? Volume of combustion chamber? </div></div>

The original low compression 7F6 heads with 111 CCs.

[njsteve]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Chevy454</div><div class="ubbcode-body">I assume you've checked your timing/dizzy to make sure it's no doing anything wonky like adding too much advance? </div></div>

Yes. It wont even tolerate the factory spec of 8 BTDC. These engines were designed to run on regular unleaded. My prior 1972 455HO in the Trans Am with all the same pieces, except for the unknown mystery cam, would run any gas at all without complant. I had the distrubutor redone and the vac advance limited to 10 degrees and the mechanical set as well, by a very good Pontiac tuner. I have even run it with vac advance blocked. The result was more sluggish but still pinged. The distributor tune improved it dramaticly but it still rattles under full power.

[larry_csa]

I would say the cam is to far forward in the motor. Looks like about 104 to me. did you degree the cam in?? where was it?? do you have a crank gear that could be advanced??
Larry

[njsteve]

The engine was rebuilt by the prior owner 15 years ago so I dont know whats inside. Judging by the other generic parts used in the car during its prior restoration, I would imagine there is some poorly selected camshaft in there. Because of that, I am pullng the engine to check the internals and replace the cam with the orignal spec cam this winter.

[RichSchmidt]

Stock smog engines had some pretty high cranking compression,but that seems too high. The very very weak cam timing and late intake centerline gave them more cranking compression then the old school muscle engines. They also required an EGR valve to run at cruise speed,and the distributors with the low compression EGR equippend engines would cruise at about 55 degrees total timing at 2000 rpm. That was only with the vacum advance hooked up though. If you dont have an EGR valve and are running a smog era distributor with the vacum advance hooked up,it will almost always ping,and no amount of octane in the world will fix it. The cylinder pressure is peaking way before the piston reaches TDC and thats what you are hearing. By introducing exhaust gases into the intake charge it slows down the combustion process a lot.