too much compression vs. cam timing, what happens?

[Warpspeed]

There can be other combustion problems beside just the fairly obvious detonation.

A very oversized and badly shaped piston crown can seriously disrupt flame travel to the point where power is seriously diminished.
If it does not burn cleanly and fast, you are wasting your time trying to get power out of it.
What the piston tops look like might be trying to tell you something. Very deep valve reliefs are never good for combustion.

Another clue to slow, incomplete, and very erratic combustion is that it probably requires excessive ignition advance, a lot more than it should. And often if the flame is sputtering out where there should be peak cylinder pressure, just after TDC, it will not rev. Long con rods and large cylinder bores potentially make all this much worse.
It feels like it ought to go, but the power is just not there.

As for valve timing versus static compression ratio, the measured cranking pressure should be a reasonably good clue to that.

[pdq67]

Triptane along with some TEL and go!

pdq67

[nastydollar]

There can be other combustion problems beside just the fairly obvious detonation.

A very oversized and badly shaped piston crown can seriously disrupt flame travel to the point where power is seriously diminished.
If it does not burn cleanly and fast, you are wasting your time trying to get power out of it.
What the piston tops look like might be trying to tell you something. Very deep valve reliefs are never good for combustion.

Another clue to slow, incomplete, and very erratic combustion is that it probably requires excessive ignition advance, a lot more than it should. And often if the flame is sputtering out where there should be peak cylinder pressure, just after TDC, it will not rev. Long con rods and large cylinder bores potentially make all this much worse.
It feels like it ought to go, but the power is just not there.

As for valve timing versus static compression ratio, the measured cranking pressure should be a reasonably good clue to that.

ok so if i got what you said correctly, badly shaped piston / excessive dome can affect combustion travel thru the chamber thus delivering poor detonation .
and the fact that if there isnt enough advance in ignition because compression is so high that ignition cannot be triggered any earlier , then combustion cannot be complete and can lead to hp drop . i still have 20° ignition advance running with E85.
that could be my problem and i dont know if anybody here could confirm that an engine with a compression ratio above 15:1 can rev up to 7700 RPM ?
this way i could drop my ratio knowing it would fix the problem.
Thank you for your help ;

[Warpspeed]

Pull the head and look at the piston crowns and combustion chamber.
If this is your problem you should see some evidence of very uneven combustion.

[wyrmrider]

have you tried retarding that cam a bunch- check pv clearance first

[nastydollar]

hi tony,
i pulled the head 4 days ago to install some stiffer valve springs and i didnt notice any uneven combustion marks really. running E85 actually keeps the internals very clean.
i show you the combustion chamber i machined to have a very even squish facing the spark, and the dome of the piston.
tell me what you guys think about it. it looks aright shaped to me.

2015-05-27 13.49.12.jpg

[groberts101]

Pull the head and look at the piston crowns and combustion chamber.
If this is your problem you should see some evidence of very uneven combustion.

Tony, I can't remember if I've ever read your opinion on this topic. Do you consider very clean areas in a combustion chamber to be good or bad?

Obviously I could ask that another way but this one answered should make that one become pretty obvious.

[nastydollar]

have you tried retarding that cam a bunch- check pv clearance first

there is not much movement i can give to the camshaft as my clearance is quite limited at TDC. peak is at 110° which is late already. and the thing is that i used to run another engine with a little less displacement and a comp ratio of 12,5:1 with that same camshaft and it was just running perfect, going to the rev limiter (7700rpm) with no struggle. im thinking small exhaust line that could cause to much back pressure and limit engine from revving .

[Warpspeed]

Tony, I can't remember if I've ever read your opinion on this topic. Do you consider very clean areas in a combustion chamber to be good or bad?
Obviously I could ask that another way but this one answered should make that one become pretty obvious.

From what I have seen, a very clean shiny area after many hours of engine use, usually means an area constantly scoured by fresh entering high velocity air and fuel. Any combustion deposits never get a chance to build up.
Its the overall general coverage of any combustion deposits, which is highly fuel dependant.
If something is stopping the flame front, you should be able to see that.
Its all much easier to see with gasoline than with propane or alcohol which burn more cleanly.

I was rather surprised to see you were able to get the compression as high as you have, without having deep valve reliefs.

[pdq67]

hi tony,
i pulled the head 4 days ago to install some stiffer valve springs and i didnt notice any uneven combustion marks really. running E85 actually keeps the internals very clean.
i show you the combustion chamber i machined to have a very even squish facing the spark, and the dome of the piston.
tell me what you guys think about it. it looks aright shaped to me.

2015-05-27 13.49.12.jpg

LSx engine head and piston that have been modified. I ask because I flat don't know? And I don't want to read the complete thread here is all.

Lazy SOB I have become.

pdq67

[Adger Smith]

sure is dirty above the top ring with burnt oil deposits. since it is a Hemi type chamber with that spark plug location I would throw a bunch of timing at it.
That chamber, plug location & compression might not like the fuel type. I ran into that with the splayed valve V-6 on a D Track combination. Didn't like high compression and methanol. Ran well on C-14

[Warpspeed]

Thinking about this a bit more, there are a few things you could try.

If you can source a thicker head gasket to try, it might be worth a go. Its not the complete answer, because it changes squish as well as lower the compression, but its either going to do nothing or maybe give us a further clue. Anyhow, its a fairly quick and painless thing to try.

The other thing might be to run all your engine dimensions through Pipemax, everything, induction and exhaust lengths and diameters, valve sizes and valve timing. You might find something there that is killing Ve at high rpm.
But I still feel this is more likely to be a combustion related problem, or it could be a combination of several things.

Even less likely. Are you absolutely certain your ignition system up to the job ?

A last resort might be to put it on a dyno and monitor cylinder pressures. That will pretty much tell you if its a Ve problem or a combustion/ignition related problem.

[MadBill]

Ignition was my first thought; often systems see too little voltage from bad connections, undersized wiring, etc. The quick and easy check it to run it up to the RPM 'wall' at wot, then start backing off the throttle to see if it comes back to life at say 85% throttle. (Alternatively, try closing the plug gaps down at least 0.005" from whatever they are now.)

[Adger Smith]

Bill I agree most people overlook the wires, esp the coil wire. Remember it handles all the electrical pulses where the plug wires only handle their portion of the electrical pulses. 8-1 on a V-8, 6-1 on a 6 cylinder, 4-1 on a 4 cyl. so on...
Another overlooked area is grounds. The head needs to be grounded to the ignition system.

[user-23911]

sure is dirty above the top ring with burnt oil deposits.

It's been burning lots of oil.
That lowers the octane and makes it knock.