Tighter Quench = Horsepower Loss NA?

[CGT]

I know this is unlikely to have been verified by anyone here. But was just kind of thinking of this lately. Has anyone actually seen a power loss from too tight of quench on a NA application?

[PackardV8]

No.

Theoretically, what could cause such a power loss?

In a perfect engine design, there would be zero dead space above the rings and zero dead space between the head and the piston. All of the mixture would be concentrated into the combustion chamber space.

Tight quench = lots of squish/mixture movement toward the combustion chamber. It's very helpful in resisting ping. Those of us who grew up trying to make compression in hemi and 13.5:1 popup 301" SBCs and 426" Mopars; all we had to work with were OEM heads with large chambers. The result was all that surface area, with the mixture spread all over the cylinder in a thin layer a long way from the spark plug.

Look at the combustion chamber and piston design of today's LS direct injection pistons to see what the OEMs have learned.

[CGT]

No.

Theoretically, what could cause such a power loss?

In a perfect engine design, there would be zero dead space above the rings and zero dead space between the head and the piston. All of the mixture would be concentrated into the combustion chamber space.

Tight quench = lots of squish/mixture movement toward the combustion chamber. It's very helpful in resisting ping. Those of us who grew up trying to make compression in hemi and 13.5:1 popup 301" SBCs and 426" Mopars; all we had to work with were OEM heads with large chambers. The result was all that surface area, with the mixture spread all over the cylinder in a thin layer a long way from the spark plug.

Look at the combustion chamber and piston design of today's LS direct injection pistons to see what the OEMs have learned.

Most LS heads have very little quench, LS3 and LS7 especially, they tend to run well and be good enough on detonation resistance. I've also seen NA engines run really well with full dish pistons in them and virtually 0 quench. Just thinking out loud.

[Ken_Parkman]

Grumpy wrote in his book there was power to be gained by ensuring there was some quench clearance, minimum .010" running up to .030" running even with the loss of compression. His theory was pumping loss.

[Stan Weiss]

What really needs to be looked is squish velocity. There are a number of factors to look at other than quench clearance. Quench bore ratio is another, as well is CR. If the other 2 are held constant as you increase CR you decrease squish velocity, so as you decrease CR you increase squish velocity.

Stan

[Scotthatch]

I have run it so tight that the .030 stamped on the piston imprints it self on to the head and never seen a ill effect on HP ..


The other part is fuel timing and how well the burn is ...

A Vortec head will do a lot better then a old double hump .... so with the old stuff I push the quench lower then I would the new stuff ... this all gets into swirl and tumble and fuel droplet size and distribution ... and why OE pushes more control like direct injection

[groberts101]

Lots of good answers here so far. I'd guess it's possible to lose power when that is the only variable changed but also believe that if other factors, both good and bad, are considered and the engine is retailored to help offset those negatives?.. it would be unlikely on a 4 stroke engine.

Possible variables and component changes might be..

spark plug gap/spark output(spark blowout/lack of molecules between gap)
cylinder head port design(existing tumble or swirl causing motion related issues)
piston crown design
cam timing
fuel droplet size/homogeneity/centrifuged-fuel pocketing related issues
exhaust size/shape design parameters

and probably a dozen others

[David Redszus]

What really needs to be looked is squish velocity. There are a number of factors to look at other than quench clearance. Quench bore ratio is another, as well is CR. If the other 2 are held constant as you increase CR you decrease squish velocity, so as you decrease CR you increase squish velocity.

Stan

Stan, as usual, has it right.
Squish velocity, for a given bore, stroke and conrod, will vary with vertical clearance, squish area ratio, and piston speed.

In the process of raising compression, it is often the case that the squish clearance has been reduced which will result in a much higher squish velocity.

If squish velocity is increased, the ignition timing must be adjusted to prevent an unwanted shift in the combustion pressure curve.

Be aware that a squish velocity peak will occur on both sides of TDC; but in opposite directions.

[GARY C]

I always wondered if a directed quench would be better then a flat quench but never tested it.

[PackardV8]

piston crown design

As previously mentioned, those big domes pushed a lot of the mixture away from the spark plug.

Been covered in other posts; counter-intuitively, some tests indicate a spherical dish to be superior to a D-dish which mirrors the combustion chamber. It's still not obvious to me all the reasons why, but it means I can save a lot on custom pistons.

[Keith Morganstein]

DV wrote about testing losing power when tightening quench in certain engines, possibly the Mini Cooper.

[ptuomov]

Isn’t the logic of tight squish clearance and high squish velocity to get the engine run well at part throttle when there’s little energy in the charge? With a lot squish velocity, the low density charge at part throttle cruise burns a lot more consistently and completely. At least that’s what I’ve understood.

[PackardV8]

DV wrote about testing losing power when tightening quench in certain engines, possibly the Mini Cooper.

For true, DV got his start writing on BMC A-series engines; bores from 58mm (2.28") to 70.6mm (2.78"). With that tiny bore and a 2-port intake, somewhat of a law unto themselves.

[MadBill]

I always wondered if a directed quench would be better then a flat quench but never tested it.

Please tell us you're not referring to Somender grooves! https://www.speed-talk.com/forum/viewtopic.php?t=4069

(The Toyota 1.8 L. in my Vibe has angled squish, aimed up into the middle of the chamber rather than across a flat crown.)

[pcnsd]

I always wondered if a directed quench would be better then a flat quench but never tested it.

Toyota published a paper on the design development of the Toyota 1ZZ-FE engine including a review of flat verses angle quench areas. Angled wins. I have the pdf but not a link. If you haven't seen it pm me an email address.

Whoops... It is a SAE paper (981087)