Hi all, if any of you have read the general board, you know I am currently unemployed. This means my stroker with over 11 to 1 is going to sit for awhile. Premium fuel spiked with AV gas is just not in the budget. I am looking for what are your personal experiences with octane 87 fuel, what compression can be run and how did you achieve it. My two wheeler is currently powered by a 390 with a 222* and 234* @050 cam, iron dual plane CJ manifold, and standard valves in ported heads that have the heat risers welded shut. Unfortunately this engine also was equipped with 410 pistons, so the compression is 8 to 1 or less, and the clearance at where the quench should be is .270 or so. I do have some four eyebrow flattops from a 67 390 that I could swap in if it is worth the effort, and if it will still live on 87 when done. What are the rest of you running and how well does it work? Yes, I am trying to be cheap here, but that is what my current situation calls for. Do you feel the FE series is more or less tolerant of cheap fuel than other engines?

 

 

Come on now guys, surely someone out there has put a 67-71 390 from a car with flattop pistons into their truck, how is it running? I have always been a fan of 10 to 1 or more, adding AV gas to the mixture as needed to stop the rattles, but that is just not in my budget right now. DF

 

This might not help much, but I have 205# cranking compression and I think it's 10.25~10.5 and I run reg cheapo gas for now. I haven't noticed any pinging, but I've only put about 50 miles on it and haven't run it hard yet. I have an edelbrock alum intake and welded exhaust crossovers and polished chambers, custom curved dist. The pistons are the L2291F custom cut to use a 428 crank and Felpro head gaskets so I'm getting pretty good squish.

On the other side, it's not summer yet and who knows how she'll do then.

I'd say you can to 9.5 to 10 if you work it right.

 

I run 10.5 pistons in my '67 LTD and 89 octane gas. Timing retarded 4 degrees. 18 mpg on the highway. Plenty of power. No pinging. You can always rejet the thing if it pings a little. Richer air-fuel ratios in the power circuit can help curtail pinging if you have any, since pinging usually occurs when you have your foot in it.

 

also dont forget that a engine with poor quench will have more detonation problems than a engine with proper quench

 

Someone please define "quench"

 

Rat, the quench or squish pad is the part of the head where the piston sees the flat deck of the head, as opposed to the part where the dish of the combustion chamber is. The ideal clearance between the piston and the head is about .035. What happens is that as the piston gets close to TDC the mixture gets squished out of the tight space and pushed into the combustion chamber, causing the mixture to tumble around. This is supposed to be easier for the plug to light and burn more completely. Once quench clearance gets opened up past 060 it rapidly begins to lose its effectiveness. DF

 

There are a couple of terms that come into play here.

Quench in this context is cooling by a metal surface, and absorption of heat during a combustion event. You are lowering the temperature of the event to a desirable level by having a metal of a certain metallergical quenching value and having a certain surface to volume ratio that promotes a predictable even burn. An aluminum cylinder head quenches much better than an iron one, thus their tolerance to higher compression on the same fuel. The trade-off is power. If the iron head was not into detonation, an aluminum head will bleed off enoungh thermal energy to drop the power level.

Something that enhances quench is a concept called squish. When you make a quench style combustion chamber of sufficient surface to volume ratio you have to make a curtain area over some part of the bore. It has been found that if the gap between the crown of the piston and the curtain area of the combustion chamber is wider than .060 there is enough mixture trapped in there to combust spontaniously when the residual chamber heat gets high enough. Closing that distance not only reduces the amount of charge there, it also by virtue of the close collision of the two surfaces will purge the mixture out and if the chamber is designed like it should be will push the whole chage toward the plug. This promotes a good even burn that will turn the crank and not melt the pistons.

So, we have quench, the cooling effect of the metal surface of the chamber. And we have squish, the close collision of the "squish pad" and the crown of the piston. The end result is a lively well mixed charge in the middle of the chamber moving toward the plug to be consumed like a branch going through a wood-chipper.

 

Thanks guys, I was thinking something close to that but you had a much better definition.