Don Terrill’s Speed-Talk

Assuming 2 equal-ish builds, the only difference being a .040" quench and an .080" quench (different piston types to give the same compression ratio, and timing curves to give the best performance in each scenario), and assuming detonation isn't an issue in either case...would you expect to see any difference in power?

On a related note...does chamber shape really have an effect on detonation resistance once the quench distance gets bigger than the "no man's land" of .060-.080"?

I am going to say yes to both your questions.
It would be like comparing a open chamber head to a closed chamber head having the advantage in my opinion

Tighter quench will have better on off throttle response.

Looser may not fall off as abruptly past peak HP

I do not have any dyno data, but just from my reading a tight quench creates turbulence in the combustion chamber causing a better burn, and reducing detonation.

Tighter quench gives a higher squish velocity which means a higher turbulent flame speed. This improves throttle response and power with less timing required. Anytime you have to advance the timing for peak power, it indicates less efficient combustion.

I suspect that at higher engine rpms WOT the realized anti knock benefit of squish will deminish as the air flow has plenty of swirl. tumble energy.. But quench head cooling benefit may be a different story.
Aparently on (street) supercharged engines the ease of being able to simply lower the compression ratio (by say just adding a extra head gasket) out weights the (loss of) squish benefit, on roots street strip pump gas engines.
(Relative modest boost) (low cr)

The position of the spark plug has a big effect also.
More central is always better.

IMO most of the benefit is realized at lower rpms.

If emissions and driving fuel mileage efficientcy was a priority you'd think the OEM's would employ this on every OEM passenger car engine...

travis wrote: ↑Wed Aug 31, 2022 6:04 pm Assuming 2 equal-ish builds, the only difference being a .040" quench and an .080" quench (different piston types to give the same compression ratio, and timing curves to give the best performance in each scenario), and assuming detonation isn't an issue in either case...would you expect to see any difference in power?

On a related note...does chamber shape really have an effect on detonation resistance once the quench distance gets bigger than the "no man's land" of .060-.080"?

Take a read through this. It may not give direct answers and quote dimensions as in your questions but enlighten nonetheless.

https://uomustansiriyah.edu.iq/media/le ... _25_AM.pdf

Turbulence – tearing of the air and fuel molecules results in better homogenization ( mixing of the gas makes the gas burn faster )
faster burn > less time for spontaneous combustion > less time available for detonation to take place.
Faster burn > less ignition spark advance required > less burn pressure before reaching TDC > reduces breaking action to the piston

While the terms “quench” and “squish” are often used interchangeably by many manufacturers, quench and squish are not the same thing, nor are they produced by the same set of conditions. 

Quench is the ability to lower temperature of the end gases trapped between the piston dome and head by conduction. This prevents a second flame front from igniting the air/fuel mix prematurely.
Quench – creates a boundary layer of cooler gas ( a few molecules thick ) attached to metal surface ( piston crown, & cylinder head surface ) compared to flame temp ( 3000 degrees F )


Squish ---- portion of piston crown ( not the dome ) within about .040” from the cylinder head ( squish region ) and the close proximity of boundary layers quenches any attempt for gas in that region to burn >reducing chamber temperature > less heat to cause detonation during the time from TDC to 16 degrees ATDC ( after the squish zone )

 define squish as the gases trapped between the piston and head that are ejected across the combustion chamber at high speed by the near-collision of the piston and head, causing turbulence and mixture homogenization.

This cooling affect is referred to as “ Virtual Octane “

But it has little affect if the squish clearance is .060” or greater.

My interpretation is: Squish and turbulence create quench .

Dr. Randolph at ECR has stated they run tight enough p-h clearance that they just start to kiss in the squish pads as they get some carbon.

More than just power, NASCAR builders, other endurance type racing, and OEMs are after increasing efficiency. In racing world, more efficient engines use less fuel and that means less pit stops or starting with less weight.

This is a very timely thread in that this is a decision I'll have to make as I put together this latest SBC.
Maybe a maximum effort drag racing engine (with the parts I have) or maybe what I've always built and that's a street/strip deal.
Pistons .008" down (Wiseco flat top forgings) and a .026 gasket I've always used. That's pretty tight at .034". Heads are a Vortec style chamber.
Or, another head gasket I have that's .039".
If .034" isn't manageable in terms of clearance, what am I giving up with a .047" piston to head clearance.
Or is .047" in that "sweet spot" where it doesn't make a lick of difference?
I've been successfully using the .026" gasket in the last short block with the piston .014" down. That was a street/strip combination that also saw plenty of highway miles. Fuel economy had some real relevance there and it was pretty good considering the 288/236@.050" hydraulic roller and 10.2:1. (Easily would get 24 miles per Imperial gallon.)

FWIW, I'm still on the fence with which direction I'll go when I get a cam spec'd for this 357. For the sake of the valvetrain and some longevity, I'm looking to keep RPMs reasonable at around 6500 peak.

288/236@.050" hydraulic roller ?

288 advertised/236@.050". (Old cam spec)
Thought that might be confusing.

I prefer seat timing. Some like the .050" spec. That was both.

Based on your previous posts the pistons are the pro tru street series 4032 material which mean they don’t expand as much as 2618 pistons forgings so the piston to wall clearance is tighter and therefore meaning less rock at tdc. I would run the tighter 0.034” quench if this was my engine. However if you are going to play with nitrous on the engine i would consider the larger 0.047” quench distance.

Monza355 wrote: ↑Thu Sep 01, 2022 10:39 am Based on your previous posts the pistons are the pro tru street series 4032 material which mean they don’t expand as much as 2618 pistons forgings so the piston to wall clearance is tighter and therefore meaning less rock at tdc. I would run the tighter 0.034” quench if this was my engine. However if you are going to play with nitrous on the engine i would consider the larger 0.047” quench distance.

I was thinking that particular forging would be just as you have said. Thanks for that.
I've read of too many benefits when running a tight quench that it's hard get past giving up on the opportunity.
No nitrous that I can foresee but maybe a few open road events with sustained WOT.
Yet to be determined.

My take is- indirectly:

- improves octane tolerance which changes tune which in turn affects power.