Quench and Detonation

[The Badger]

Hi, first time posting but I have been reading on this forum awhile just trying to see what everyone else is doing and getting good results and have been doing some research and have not been able to find what I have needed and wanted to see if someone could point me in the right direction. I have been trying to study up on detonation(And ways to suppress it) And I got into quench and how it will raise the detonation threshold, however I was reading thru some old posts on this forum and I was reading were a few members that are way above my knowledge were stating that any tighter quench than .050 and there are no gains any tighter you go. I was wondering if this is from just tightening the quench were no gains are being seen power wise or if there was no help there or in helping to prevent detonation. Would really just like to know if there are gains to be had or detonation less likely if I were to run say .025 quench if I were to find a way to make it that tight without risking anything.

Thank you to everyone in advance for your opinions and advice.

[PackardV8]

Would really just like to know if there are gains to be had or detonation less likely if I were to run say .025 quench if I were to find a way to make it that tight without risking anything.

Yes, depending on the build and where you're looking for them, there are gains to be had. They're just not always at max horsepower.

No, there's no way to run .025" without risking anything. Inertial loads go up with the square of RPM. Higher is exponentially riskier.

Maybe, if you have new aftermarket rods and pistons which are identical, or a rod boring machine, a really good crank grinder or a top-quality aftermarket crankshaft which has equal stroke and a square-deck fixture to correct the block, you can start at .035" and see what actually happens when you assemble and try to hold that tight. Hypereutectics are probably a good place to start, as they can be run really tight; less rocking in a large diameter bore.

Just be aware, you can't start building tight quench with average used OEM junk. Typical used rods can vary .005", especially if they've been reconned. A typical reground OEM crank will vary unknowably in strokes, as most grinders don't look at stroke and indexing; just get 'er on undersize and reach for the next one. FWIW, trying to equalize stroke and correct the index on older pre-CNC OEM crankshafts would make two out of three go junk. The newer CNC-produced crankshafts are much more likely to be on spec.

FWIW, I've built a low-RPM, cast dished piston, tight wall clearance, down to .025" and it's working well. Definitely less pinging with iron heads.

[wyrmrider]

I've got .025 on a chevy 250
crank and rods are perfect- new bolts- forged 307 pistons with a "d" dish
hit 6500 rpm on the dyno HEI 2bbl marine carb and flame arrestor, wet marine exhaust manifold through the prop
runs on any gas at the marina- which is not saying much sometimes
It just worked out that way
I usually try for .030-.035

[bigjoe1]

I have never seen the real tight quinch worth anything- 040 to 050 is just as good



JOE SHERMAN RACING

[wyrmrider]

i think it then depends on the size of the quench area and maybe bore size
I've seen .050 + not work well on 460 Fords running bad gas

[modok]

I was reading thru some old posts on this forum and I was reading were a few members that are way above my knowledge were stating that any tighter quench than .050 and there are no gains any tighter you go.

I believe it, but you have to ask what they are working on.
It's going to be different for every engine and application tho you can,.... well, I can make a reasonable guess.

Scientifically designed tests tend to show quench STARTS at about .040" and above that not much happens.
But how close IS your piston to your head? How will you measure that?
There is piston rock, all the parts are at different temps, and under forces, and often there is carbon build up.

A high revving, NA, big V8 might need .040 just to keep the pistons from hitting the head.
High compression and boost and low rpms will compress the piston and rod, so many diesels are set up at HALF that much. Depends on the case.

[digger]

isn't it more important to have tighter squish when you dont have a high squish area to bore area ratio?

is the consensus that suppression of knock would be most beneficial at lower rpm when mixture in the combustion space has less motion and there more time for whatever motion there is to decay?

[MadBill]

Squish does more than just working towards suppressing detonation. Its basic function is to increase mixture turbulence and thus speed combustion, which can mean less spark advance requirement and reduced 'negative work' BTDC.

[wyrmrider]

two things going on that you have to keep in mind
the two cold surfaces which suppress pre-ignition
the jet of gas for turbulence
you can get away with none (or less)- (what I think Big Joe is saying) with race fuel like with a BBM if you have to, but even with alcohol quench helps even in a open chamber
otherwise I go with the.040 ( unless you hve big loose pistons etc)

[groberts101]

I'm a firm believer in making quench as tight as possible for the specific application. Especially for low-mid rpm light throttle conditions. Pretty hard to change combustion chamber designs or know where that limit is without taking some chances though. As others have already pointed out, engine architecture/piston skirt length and skirt/bore protrusions present some added difficulty in achieving perfection. All one big balancing act.

Maybe not as much to be gained, but I would also try to move the top ring up as high as the applications durability requirements would allow. All cumulative affect to improve overall gains.

[jacksoni]

Using aluminum rods which stretched 0.050", Gumpy Jenkins set static clearance at 0.060" to end with 0.010"quench clearance for carbon build up and said there was definite power in doing so. Up to .020" didn't lose though. Everything has to be right or Mr Piston meets Mr Head pretty hard. As stated by many others, .035 to .050" is regarded as reasonable and likely safe in a less precise setting.

[David Redszus]

Given a constant squish area ratio and rpm, a change in squish clearance will produce the following results in squish velocity. But changes in squish velocity may or may not be beneficial to power production and detonation avoidance depending on the needs of the engine.

Clearance........velocity m/s
.020.........66
.030.........49
.040.........39
.050.........33
.060.........28

Even with uniform step changes, the velocity becomes less sensitive to larger clearance values.
Peak squish velocity angle is always after 10 deg BTC and ATC.

[statsystems]

Given a constant squish area ratio and rpm, a change in squish clearance will produce the following results in squish velocity. But changes in squish velocity may or may not be beneficial to power production and detonation avoidance depending on the needs of the engine.

Clearance........velocity m/s
.020.........66
.030.........49
.040.........39
.050.........33
.060.........28

Even with uniform step changes, the velocity becomes less sensitive to larger clearance values.
Peak squish velocity angle is always after 10 deg BTC and ATC.

Since the rod to stroke ratio does affect the time the piston is near TDC, is it fair to say the engine "sees" a smaller chamber longer longer with a higher rod ratio?

If so, can an engine with a high rod to stroke ratio use less quench and make it up by the piston being near TDC for a greater period of time?

Also considering we are dealing very small element of time that the piston is near and around TDC.

[David Redszus]

Since the rod to stroke ratio does affect the time the piston is near TDC, is it fair to say the engine "sees" a smaller chamber longer longer with a higher rod ratio?

If so, can an engine with a high rod to stroke ratio use less quench and make it up by the piston being near TDC for a greater period of time?

Also considering we are dealing very small element of time that the piston is near and around TDC.

Let's see what effect a substantial change in rod length has on squish velocity and peak velocity angle.
Clearance....velocity m/s....peak angle
.020..............66.3.............5.8
.030..............49.4.............6.8
.040..............39.4.............7.5
.050..............32.7.............8.2
.060..............27.9.............8.7
Rod ratio........1.62

.020..............67.6.............5.7
.030..............50.4.............6.6
.040..............40.2.............7.4
.050..............33.4.............8.0
.060..............28.4.............8.6
Rod ratio........1.38

Even a rod length change of over one inch produces very little change in squish velocity or velocity peak angle.
In a real engine the cycle to cycle variances will dwarf the differences shown above.

This demonstrates once again the relative insignificance of rod ratio when the piston is near TDC.

[wwmtlineman]

I had a friend build a 350/255 69 Chevy engine for NHRA stock class to the legal blueprint specs at the time of .018 head gasket & .002 deck. Didnt work out too well smashed all the top ring lands closed. Most have been corrected but some are still the same