Importance of quench and overlap in a forced induction app. - Motorgen - Automotive Events, Meets, Cruises and Forums

enkeivette · 10-20-2014, 12:48 AM

I was reading another engine building article, and it made me wonder if what I've heard from fellow forum shit talkers is true.

Quench. The importance is turbulance in the cylinder for fuel and air mixing. To promote better combustion and avoid hotspots. I had many shit talkers warn me not to decrease my CR with a thicker HG for fear of fucking up my quench. But I did it anyways and my motor has never run better (it has also stopped melting pistons, knock on wood. It seems to me that my gigantic belt driven snail that blows more air than a leaf blower at 650 rpm would have to do a pretty damn good job of forcing the air and fuel to mix anywhere at or above idle speeds, prior to it being quenched. So, with forced induction does it really matter?

Overlap. Ill keep it simple. With NA, the higher velocity air jamming in to the cylinder compensates for what leaks out. So, ya, with more overlap, more boost will leak out. But who is to say that isnt more than compensated by the boost being rammed in those last few nano seconds? Thoughts? Science? Shit?

enkeivette · 10-20-2014, 12:59 AM

Although, it does make sense that with NA you need velocity to get the air in there, but with boost you dont, so it would be better to have it sealed up to increase the pressure for a longer period.

Vettezuki · 10-20-2014, 07:44 PM

Quench is pretty complex, and was probably more necessarry on older NA motors that didn't have nearly as sophisticated air flow through the heads. My impression is that modern heads alone make quench less of an issue, but I could be wrong. I'd also tend to agree/assume FI mitigates some of the mixing issues, thought that's probably heavily related to design. Yours is a blow through setup right? The air coming out of the blower is probably turbulent as all hell anyway and continues to "mix" the fuel into the fuel air mixture through the intake. If the fuel air mixture is mixed coming in, quench is not so important or even irrelevant?

I'd be curious to know what the pro opinion on the overlap in a blower application is. I dunno. My "hunch" is you get all things equal, the marginal gain from a blower cam with less overlap is measurable, but not extreme? There are a lot of factors in the system. I'm not an engineer and just spit balling.

94cobra69ss396 · 10-21-2014, 07:07 PM

Do you even know what your quench is? How deep in the hole is the piston and what is the compressed thickness of the gasket? I know on my 347 the piston is .015 in the hole and I use a .041 gasket which gives me .056 quench and the compression comes in approximately 9.18:1. The old 302 had the same .041 head gasket but was .030 in the hole and the compression was approximately 8.8:1 if I remember correctly. With that said, the 347 hasn't pinged at all but the 302 would ping often when running pump gas. The only other thing that changed besides the longer stroke and bigger bore is that the cam went from a 110 LSA to a 114.

Vettezuki · 10-21-2014, 09:19 PM

Wouldn't that put your old quench at .071? From what I've read (at least on a Mopar site) that would NOT be terribly effective (too big). It appears to be a pretty narrow range where the quench works well.

Quote:

the general accepted standard is between .035 and .055 but the numbers that i would recommend are anywhere from .040 to .045
IMO .035 is too close and .055 is too much to very effective.

http://www.moparts.org/Tech/Archive/motor/15.html

enkeivette · 10-21-2014, 11:51 PM

Ya quench is supposed to be like .039-.041?? My piston is .020 down and my new thicker HG is like .060. Mine is the Cold Play of quench, very very far from acceptable.

enkeivette · 10-21-2014, 11:54 PM

Also, Ron, if your engine is now a different size I wouldn't feel safe pin pointing anything on just a cam. But if you went to 114 from 110, according to the shit talk consensus, that would mean you are trapping in more boost, and should be even closer to detonating, ya?

Damian · 10-22-2014, 10:06 AM

114 is a good LSA for a boosted motor.

94cobra69ss396 · 10-22-2014, 11:30 AM

I don't know enough about cams to answer that but yes it would mean that it is bleeding off less boost out the exhaust. I agree that too many things changed to pin point why the new engine doesn't ping like the old one but I did try to make sure I had the quench as tight as possible due to the pinging of the old combo.

Now on the opposite side, a bunch of the knowledgeable N2O guys say that with a BBC on N2O you want a bigger quench like around .080 because it works better with N2O. I actually don't have a clue what the Chevelle is at because I didn't check how far in the hole the piston is. I do run a .039 head gasket though.

Shaolin Crane · 10-22-2014, 02:20 PM

There way too many variable involved in why an engine wont "ping" When Ron and I first went in for our tunes (my first engine) his torque was considerably higher, but HP was nearly the same. I was running nearly 17lbs, to his 12(?) 308 to a 347 his 9.18 to one to my 10.5:1 (verified AFTER the demise of the engine) and neither engine was "pinging" on pump gas, this was when mind you, we were running the EXACT same cam shaft. Metal composition, ring gap, fuel brand, etc can all play in why an engine wont ping. Doesn't work the same way with actual factual measures. Just because your engine is running better with the thicker gasket, doesn't mean it wouldn't run even better further so with a thinner one and a piston further in the deck.