I'm looking around and I see everyone shooting for a .035 to .045 quench, but all the forums I'm reading the guys are NA or using nitrous.

And then on other forums with guys talking about droping the compression ratio a tiny bit, they are saying throw on a thick head gasket, but mention nothing about quench. This leads me to believe going over .045 isnt a big deal.

I'm working on boosting my 408 and I could use a tiny bit of compression drop. AFR recommends using a cometic 4.160 head gasket, and it comes in three different thicknesses to choose from. Do I try to keep it around .040 quench or can I just get the thickest one they make in 4.160

 

What's you deck height (pistion out of the hole) ?

 

No idea. Still have to check. But if going over .045 is fine then I'm just going to order the .051 gasket. If not then i guess I'll measure. The choices are 040, 045, and 051

 

With boost guys tend not to worry about it. Is that where you were seeing guys just slap on a thicker gasket?

 

Yep. Seeing that on boosted threads. That's why i was thinking too much quench isn't bad for boost and maybe the na and nitrous guys care so much about it because they're trying to squeeze every hp they can.

So if going over .040 quench is fine then I'd like to order the .051 thick gasket. Unless someone tells me everyone should shoot for a .040 quench

 

Quench is not about compression at all. People just slap on a thicker head gasket hoping to raise compression but don't really understand what quench is or does.

If you do the calculations on static compression, you'll see that increasing the head gasket thickness by .020" really doesn't change much.

There's a lot of info available for the reading. But in a nut shell, the tighter quench you get the more efficient the burn will be, and the less chance of detonation. You can sacrifice static compression, thinking the engine will run better with less detonation. But you're actually increasing the risk, and making less power.

0.040" is generally considered minimum for an average high performance engine. During high rpm use, parts stretch and change shapes at varying rates, dependent on rpm's, heat, stretching, etc. If you were building a low rpm engine, like for a tow truck, you could use less if you're brave enough to do some destructive testing for that last little bit of torque or efficiency.

 

There's the reasoning i was looking for. Thank you. Well hopefully i can find a damn valve spring compressor tool that will work with my afr heads and it won't come to having to pull the heads. I just wanted this info for future reference

 

To answer your question, yes you can have too much quench. On something max effort and set on "kill", clearances are so tight that its common to have to index your spark plugs to avoid the piston actually tapping the spark plug and closing the gap. That can still happen even when the piston is machined to allow PTV clearance. My last car had a BB Mopar assembled by Shafiroff Racing. It was a max effort NA 446" Nostalgia Super Stock engine that required me to index the plugs.

 

I am no engine builder but my take on this is completely different than your's. I suspect that with boost it matters less due to the denser charge being more turbulent without having to be squeezed so tightly.

 

How much of a "tiny drop" do you want/need ?

What about getting the chambers CNC'd a bit larger, rather than a thicker head gasket ?

 

I'm not very worried about the drop on compression ratio so much. I know it wouldnt even be enough to make a difference. I have 70cc chambers now so I'd just put new pistons in, make it around 9.5:1 and call it a day.

The real reason why I was asking is because,
A. Too much quinch doesnt matter, if thats the case then I would just go for the .051 gasket and not have to do all the measurements and chance screwing something up.
or B. Quinch does matter and I would do everything possible to make it right.

 

The correct answer is B.

The purpose of the quench area is two fold. First, to take all that air:fuel mixture outside of the actual combustion chamber and and push it back into the combustion chamber where it belongs. This will increase combustion efficiency, make more power, and less yuckey emissions.

Second, by squeezing all that that stuff together, you create a really turbulent, fast moving, and homogeneous mix of air and fuel. This actually keeps the combustion chamber a little cooler and helps to prevent detonation. And, it keeps the flame front out of the ring lands. Or so the theory goes.

In short, quench matters very much. Not only in power production, but also in maintaining efficiency and preventing detonation.

This is all part of blueprinting an engine. Making sure everything is right, and the parts are assembled correctly to ensure best operation.

I'm in the slow process of overhauling a used LS1 for a track car. I just measured my deck height a few hours ago. It is a minumum of 0.010". I'll try to use a 0.050" head gasket to get that perfect 0.040" of quench.

If I was building turbo or nitrous car, I might go 0.045- 0.050" of quench, just to be on the safe side. Power adders can sometimes add additional heat if they're not tuned perfectly. An additional 0.005-0.010" would give a safety margin if needed on a bad day.

 

Good explanation. And you even answered my next question of what is the perfect quench

 

Quench isn't really important in boosted applications, but it is in NA applications, especially with lower compression ratios.

If you were to imagine lighting a fire in a desert versus a dense forest, which one would spread faster. A boosted engine is similar in that the air and fuel is packed in tighter and closer, so the effects of an "ideal" quench aren't actually very effective. An NA engine needs that mixture motion, but I would just go with the .051" gasket and call it done.

 

I recommend that you do not select your head gasket thickness(es) until you measure the deck height of all 8 installed pistons.