I was on the ross pistion web page and tay say to leave the piston in the hole to lower the CR more or to where you want...can you do this or is this a godd idea...

 

IMHO, really not a good idea. Nor is a thicker head gasket.

It will work, but you are lowering the CR to make it less sensitive to detonation, and then you are making it more sensitive to detonation by eliminating quench.

The best way to do it, is a d-shaped dish (mirror of head quench pad), with the correct quench distance and CC to get the desired CR.

 

agreed...

 

I don't understand how playing with the quench will lead to detonation...you want to hit 39ish is realy going to hurt it and how ifd i would goto .45, .50...that kinda thing....

Hoe dose detonation become more sensitive

 

On a stock LT1 the pistons are .025 in the hole, and the head gaskets are .039 (Felpro 1074's) your looking at .064 for quench which is a little on the high side. Decking the block to bring quench down in the .040-.045 area will raise compression some, but you can counter that with a dished piston or larger cc head.

 

My stock LT1 block with TRW blower pistons ended up at .015 down the hole, which I think is acceptable. So far so good, no detonation issues even at 16 psi on pump gas. But every combo is different (disclaimer!!)

 

The difference between relief sizes on the heads or piston is like stuffing a fat chick in an cavalier or an escalade. Less compression...less heat. The heat of compression is how diesels run. If you ran 20something:1 compression with a turbo...atomized coca-cola would ignite. (alright...slight exaggeration.)

 

because a tighter quench helps fight detonation...do a search on it.

 

Ok guys, Lets ASSUME that dishing the piston and increasing the combustion chamber size can't happen. How much would you have to lower the compression to make up for the lack of quench? When would the lowering of the compression outweigh the quench loss? Does that question make sense?

 

Here are a few things to consider.
Many have jumped on the bandwagon of quench vs. detonation resistance, but I haven't seen anyone able to quantify the actual difference that it makes. Yes it helps, but how much?

Unfortunately, I cannot answer your question, but at least I'll give you some ideas.

I doubt anyone here can actually answer your question..... especially in a blower application w/ dished pistions.

Here are some facts.
A "D" shaped dish will still offer quench while lowering compression. If possible use this type.

Keep in mind that the max quench is less than 1/2 of the piston top due to the combustion chamber shape even with a flat top.

The commonly referred "turbo" piston is a total dished piston with only a small flat ring (less than 1/2" wide) around the perimeter to offer virtually no quench. Remember quench can only be obtained on 1/2 the piston. Here is one on LJ's site.... look at the 1000hp tab & there is a picture of one http://www.noid.org/~lj/ I do not feel the quench will matter much here. The actual area is minimal (likely less than 15% of the chamber). I can't see how extra deck clearance would matter much here. If someone has tested this in particular, I would like to know as well.

I have seen some serious turbo people run large deck clearances (1/8" in the hole) to control compression. Bennett racing has claimed this on some of their turbo motors & they do build some of the fastest pro 5.0 Ford motors around. Yes, this contradicts common thoughts & practices, but I would consider them very knowledgable.

Are they wrong..... I doubt it.

Is quench overplayed in low compression applications..... I'd like to know as well. Just keep this in mind in your decision.

 

Yes i think you have very good point as NO one has give WHY we need this perfect quench of .035-.040 some will say you can go as low as .030 but i think that is N/A...

I am going to leave my piston in the hole when the time comes to deck the block....

I would like to see some one prove this as well if i leave my piston in the hole .010 am i missing out on free power if i don't 0 deck the block, Like you said these D shape pistons how do they afect quench.....

VERY GOOD POINTS....Lonnie

 

The reason that I'm asking is that I know a couple of people running the .074" thick Cometic head gaskets and they say that it doesn't cause any problems with the lack of quench. I've also called Cometic an they say that its not an issue. Its not exactly the same as dropping the piston, but it poses the same basic new combustion chamber shape.

 

There are reasons to use a 'salad bowl' type dished piston with no quench in a -RACE- motor.

BUT, if you want to maximize detonation resistance in a -STREET- motor, a D-shaped dish piston, and tight quench distance, will make a lot of difference.


Everything is a compromise. In this case, power vs resistance to detonation/octane requirement.

 

INTMD8 these are 2 good points...BUT no one is saying WHY, just don't do it ...What is happing inside the chamber that you need this tight quench .....

 

I THINK the reasoning is because you need some kind of turbulance in the outer areas to get a good burn in the chamber. The piston moving up through this area causes enough of the molecules to move to help with this. Some people think that since the air on a forced induction engine is being pushed into the chamber instead of pulled that the momentum of the particles will give this area enough turbulance for a proper burn.

I'm not exactly sure why you need turbulance in these areas, but its what I've gathered from my searching. The FI part may make this less essential, but I'm not sure how much, I don't have enough experience in it. I'm in the same boat you are. My rebuild starts next month and I'm not exactly sure what I'm gonna do.