[enkeivette]

PS. I think pulling timing can help with detonation, but detonation is basically dieseling isn't it? So at a certain point, it shouldn't matter how much timing you pull, or even if there is a spark at all. Or am I missing something?

[94cobra69ss396]

That means that you are still running about 22-24 degrees total which is too much to be be able to run 91. I'm also running 9:1 and at 17 degrees my car will ping above 5000rpms on 91. At 15 degrees I have no issues.

You need one of these. It will allow you to run more timing at lower boost levels and then pull timing as boost increases. It makes a huge difference in the low and mid range power. I wish I would have kept the one that came with my kit because I would have given it to you but I sold it years ago. I didn't need it because Steve controls it in my tune.

http://www.summitracing.com/parts/MSD-8762/

[94cobra69ss396]

Originally Posted by enkeivette

PS. I think pulling timing can help with detonation, but detonation is basically dieseling isn't it? So at a certain point, it shouldn't matter how much timing you pull, or even if there is a spark at all. Or am I missing something?

Detonation is caused by the fuel igniting before the piston has reach TDC. The pinging sound you hear is from the air/fuel mixture igniting while the piston is still traveling up. The pre-ignition is trying to force the piston back down before it has reach TDC. This can be controlled by having the mixture ignited at a later time (less ignitial timing).

[big_G]

Originally Posted by enkeivette

Vette had too much blowby. Did a compression test, zero in the number 7 cylinder! So I pulled the head off and this is what I found.



Oopsie Daisie. Guess 13lbs of boost is just too much on 91 octane! Regardless of how much timing I pull and how rich I tune it under boost. Time to order that intercooler and mandrel pipe kit!!! I'll clean off the melted aluminum tomorrow, and decide whether or not the cylinder is ok. If so, I'll just order another piston and throw it in there. Cylinder head was fine this time. If not, I might bore it 60 over and go with some low compression pistons. I really don't feel like pulling the motor though.

Are those SRP pistons?...When I talked to SRP/JE, they said the SRP would not hold up under the boost I was planning, and that their JE blower piston would be much more durable. I am running 18-20 lbs boost (intercooled) at 32* with no retard. (8.3 scr and 92/93 octane). I did see some specks on the plugs after a hard dyno session, and have since switched to a colder plug. 11:1 afr also helps prevent detonation at the upper end (5,000-6,500 rpm).

[94cobra69ss396]

One more thing to add is you are not going to be able to just throw another piston in that block. Those scratches look bad.

[enkeivette]

Big G. Yes SRP, can a piston really crack just from too much boost? That would explain why I never hear detonation when this happens.

Ron, I know... but the wall looked bad last time too, and it was just streaked aluminum. Well see. If I clean it and my fingernail catches, I might pull it out to bore. Might get a new set of pistons too if the SRPs won't handle boost. They are forged though, strange. I don't see how one forging could be weaker than another.

[Ryridesmotox]

Have you tried running it on E85. There are guys running up to 30 degrees of advance on stock ecotec LSJ blocks with over 25 psi with a turbo. E85 is great for a boosted motor. It runs cooler, has an O2 molecule attached to it, takes longer to burn, is harder to burn, and allows for a ton of advance. With an intercooler and E85 you could probably run easily into the 20psi range.

[Ryridesmotox]

Originally Posted by enkeivette

PS. I think pulling timing can help with detonation, but detonation is basically dieseling isn't it? So at a certain point, it shouldn't matter how much timing you pull, or even if there is a spark at all. Or am I missing something?

Diesel's use the injection of the fuel to initiate fuel combustion. The direct injection diesels can see like 30,000 psi, or some crazy shit like that, in the injectors. The extreme heat and violent injection makes the instant explosion and it sounds like knock but its not. The air is literally so darn hot and compressed that the diesel ignites. Diesel also burns EXTREMELY SLOW. The "detonation" sound in the diesel has more to do with the length of stroke and high cylinder pressure than actual pre-detonation. Pre-det will still destroy a diesel like anything else really. But since the fuel isn't injected until combustion is desired, there isn't much of an issue with pre-detonation really. I hope that makes sense.

[SkunkLookingCar]

Originally Posted by enkeivette

Big G. Yes SRP, can a piston really crack just from too much boost? That would explain why I never hear detonation when this happens.

Ron, I know... but the wall looked bad last time too, and it was just streaked aluminum. Well see. If I clean it and my fingernail catches, I might pull it out to bore. Might get a new set of pistons too if the SRPs won't handle boost. They are forged though, strange. I don't see how one forging could be weaker than another.

It depends on the alloy used. Most SRP pistons are 4032 which is a higher silicon content aluminum that is strong and has good wear characteristics but is very brittle. Pistons forged from 2618 are generally considered a better choice for higher performance pistons since it is a much tougher alloy and doesn't allow crack propogation as easily as 4032. 2618 does wear more quickly than 4032.

[enkeivette]

No that's what I was thinking. Diesels create the explosion with the high cylinder pressure, and control timing by waiting to inject the fuel.

But with a gas engine, when the fuel is already in there, and the cylinder pressure is too high, it detonates or explodes from the high pressure, like a diesel would. At least that's how I was thinking.

Still don't quite understand how timing can prevent that, need to read what Ron wrote again.