Been reading on quench , I am putting together my short block now and have not odered a head gasket as of yet until I decide on a thinkness . I am getting varying ideas depending on were I am reading . Kinda appears as though in a perfect world you would want 0 clearance between the piston and the head ,In the real world on the street it appears as though about .045 is good with a max at .060 . Any thoughts on the subject of quench on a street driven supercharged engine ?

Zero deck clearance isn't necessary you can use a thinner gasket to achieve a good quench. You will want a max of .044" and some say as low as .035".

I use 0.035" to 0.040" for a tight fitting piston like cast or hypereutectic and 0.040" to 0.045" for a looser fitting forged piston because it has more room to rock on the pin. Blown or unblown wouldn't make any difference to me.

I've read about some serious blown engines using very wide quench heights so I know it is done. I don't know the details about the pros and cons though.

There is always alot of talk about perfect quench... 100's of internet experts will give you "ideal" numbers... The reality is very few people have the time or money to do all the R&D to prove out any of these theorys...


I was a big quench expert at one time, then i bought a big block chevy short block that was built by a person that is known for very fast, very dependable blower engines. The pistons were .100 down in the hole, and it had a .040 gasket on it. SOOOO, that is like .140 quench. That engine ran low 9's high eights with zero signs of "any" detonation. Oh yea that was on 100 octane fuel. Now i am no longer a quench expert...

Keith

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K-Star Automotive
Performance Engines
The crash was easy: rehab is killing me-"John Force"

A blower motor is something of a different problem. Since the blower provides a lot of mixture turbulence, even when cruising along not providing much pressure, this reduces the need for the step to provide so much squish as needed in a "normally aspirated" motor.

For quench, while a blower motor can make good use of quench, compromises typically need to be made to get the compression ratio down to a livable number so quench steps tend to have wider spacing. To a large extent under competition loads where the blower is spun up and engine speed high, this isn't too much of a problem as again the blower's action is making plenty of turbulence and the high initial cylinder pressures from the overfill cause the flame front to move so quickly as to preclude the opportunity for detonation to occur ahead of the flame front.

The dangerous point with a blower motor is dumping a lot of throttle at low RPMS with a lot of load on the engine. This is a point where detonation can suddenly appear with disastrous consequences. Conditions for this would be a heavy car, with an undersized engine with a blower of some sort at low engine speeds in a high gear suddenly seeing a lot of throttle without a down shift. Another would be a heavy boat with a fixed pitched prop or one in low pitch in which the throttle is suddenly opened.

It's the old story "how can you have your pudding, when you haven't eaten your porridge."

Bogie

Yea, a tight quench on any engine is a good thing, but sometimes you need to increase it to lower the static compression ratio- which is very important on blown engines. The best of both world would to be have high quench along with a reverse dome type of piston. Though even that may not be enough to get your static compression ratio low enough.

I agree with Bogie.The supercharger homegenizes the mix prior to induction.This effect offsets the need for tight squish like in a n/a engine.Quench and squish are the same and different,depending who you talk to.I always thought that squish shoved the a/f into the usable chamber vs quench that cooled far off a/f that could fire off and collide with the initial burn.