In my case it's a 557 bbc 8.36 to 1 with reverse dome pistons. The motor is assambled and my quench is .065. I'm getting a lot of conflicting info, some say .040 boosted or not, others are saying reduced quench to .060 or so is o.k. I'm really not looking to run on the ragged edge here, but I also don't want a motor thats going to be a ping magnet either. I guess I don't have enougph conflicting opinons yet so I need more.lol

 

https://www.theturboforums.com/smf/index.php?topic=120797.0

 

That was good. I wanted to bump that thread but wasn't sure if that would be the best place for my questions. I got out of that(linked thread) that there is no magic answer. How much does engine temp, and fuel type have effect on this. In my case I have an endless supply of cool lake water(temp shouldn't be an issue). I recall,there was a post or 2 that said as you throw boost at it, a little less quench(looser) might even help??? For a little more info, pump gas, aluminum brodix bb2plus heads. 6k redline. Planing on 2- 475's if that matters. If I'm missing info let me know. Fwiw I boat at 4500 ft. elevation.

 

I don't think I gave a number in the other thread but we like to have .040" minimum from the head surface to the piston. This is for a steel rod as at high rpm the rod can stretch that much and the piston will run into the head otherwise. On an aluminum rod engine, you need at least .060" piston to head as the rod will "grow" more.

That being said the MINIMUM QUENCH DISTANCE IS EITHER .040" OR .060"
when you build the engine on the stand. In a dynamic mode at high rpm, the piston might be only a couple of thousandths from the head deck. I am a conservative guy so I use .050" and .070" numbers.

Tom Vaught

 

Gotcha, I'm at .065 with steel rods. I guess that's less than ideal. Or more. lol

 

.015" dynamic quench is not a bad number and the engine will run fine with that. When people use zero deck or .005" deck (piston to top of the block) they are trying for maximum compression (with-in the rules) typically. Not much air can get trapped between the head and the piston with .015" quench. The vast majority of the gasses will be forced toward the combustion chamber.

JMO

Tom Vaught

 

What about .350 in the hole?

 

The only thing the engine sees is a bigger combustion chamber, Tim. You get past .125" down and there is basically no quench in the system. A piston .125" down has enough gap for the spark to completely ignite the mixture in that location (if it had a quench head).

Tom Vaught

 

Maybe I'm way off here, but the more I read, it seems the benefit of a good quench # is 2 fold. #1 it helps cool the piston. #2 it helps the mixture to be more easily combusted(for lack of a better way to put it), it gets it good and stirred up.lol It sounds like boosting a engine helps #2. I'm running an open chamber head 115cc, does that make things worse or ??? I'm not sure what a quench head is. I've heard closed chamber heads have less detonation issues. Is there even such a thing as an aftermarket, closed chamber head for a bbc??? Sorry maybe that was 1 or 2 too many questions.

 

Quench head is typically a "closed Chamber" style head with a flat surface exposed in the cylinder bore. A hemi or a "Pent Roof" chamber would have no quench.

Biggest disadvantage of deep combustion chamber heads is that the valves have to move farther away from the seat to get away from the curtain area of the head.

For example a Traditional Pontiac head from Edelbrock will have two cc volumes available: 72ccs and 87ccs. The main difference is that the valve seats are deeper in the head due to the larger cc chamber. The 72 cc head might not be affected (as far as flow goes) by the combustion chamber wall after .400 camshaft lift. The 87cc head might need .500 lift to reach the same flow number as the seat was .100 deeper.

Hope that makes sense.

Tom Vaught

 

Does fuel type effect quench at all? Fast burning 88 octain vs some 110 for example? In other words is it more crucial to have a good quench # with one fuel vs the other?

 

I have wondered this myself. I know different fuels have different chemical makeups. Also, cylinder pressure effects the rate of burn.

 

It's amazing those Hemi's run so well

 

Hemi engines use a different strategy for cylinder filling, Typically with a large dome piston to get mixture motion as well as high compression.

We had preformed a lot of testing where a flat top piston with a proper chamber works very well on a boosted engine. This would be a 4-valve direct injection type
cylinder head.

Hemi engines have a well founded reputation for making power but there is more to it than just the chamber design, in my mind.

Tom Vaught

 

A Hemi's design differences is just one of many different details that could have been discussed.

 

Hopefully they will be discussed in this thread or a new thread started. Hemis basically have no quench so this may or may not be the proper thread for that discussion.

Tom Vaught

 

It's funny. I used to be a tight quench person. But the more I read, the more I find that the major engine builders say quench don't mean anything on a blown application. In fact, there are many who recommend a looser quench.

 

I built a tight quench engine because my motor is 95% street driven. I think that the quench effect on that has been proven. Racing with high octanr fuel I think is a whole other ball park.

 

There is a lot of info out there on the Hemi Combustion Chamber, This link covers things pretty well in a short overview:

http://www.ptdoityourself.net/tl/info_combustionchamber.html

Tom Vaught

 

The action of squish is to significantly reduce the octane requirement. Ideal squish clearance is defined by C. F. Taylor as a maximum of .005' per inch of bore diameter in operating conditions of speed and temperature. The tighter the squish clearance and the greater percentage of bore area dedicated to squish, the less octane is required for normal combustion at a given power output. In other words, as the action of squish goes up, the octane requirement goes down, all else equal, load, boost, RPM, temperature, etc.

Two of the factors of greatest influence in the effect of increasing resistance to detonation are turbulence and a more compact combustion space at TDC. Each of these act in different ways but bring about the same result, which is combustion of the same mass of working fluid (A/F) in less time, therefore fewer crank degrees. When the combustion is accomplished in fewer crank degrees less ignition advance is required.

The controversy enters this because many people are unable or unwilling to grasp the concept that using the least amount of spark advance for best power is the correct technique. I don't know what to call it but superstition. People are so fixated on a particular spark timing number that they have modified the mechanical details of an engine so it will run with more advance (and require higher octane fuel) instead of simply using less ignition lead.

I think it's in Obert's book that squish behavior is euphemistically defined as 'mechanical octane'. In other words, mechanical details can give an engine the ability to operate with a given fuel as if that fuel were higher octane.

I've seen this scenario played out so many times I've lost count. Otherwise fairly intelligent people will blame everything except their reluctance to understand some engines in some circumstances require less advance than prior experience with a similar engine suggests. This is called 'something new' and I guess some of us humans fear that most. If that ain't superstition, what is it?

Again (and again and again'¦'¦..ad. infinitum'¦..), there is a no-fly zone between about .050' and .180' squish clearance that should be avoided because it encourages formation of radicals that encourage self-ignition and therefore detonation.