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Does quench affect power?

General engine tech -- Drag Racing to Circle Track

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Bill Chase
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Re: Does quench affect power?

Post by Bill Chase »

skinny z wrote: Sat Sep 10, 2022 11:31 am
steve cowan wrote: Fri Sep 09, 2022 5:52 pm My Dart block has 0.030" piston to head,I know someone has run 0.028" without issues.
.030" ? .028" ?
To me, that's unheard of. But I guess if Pro Stock is making contact then that's the tightest of all.

There's plenty to support the effectiveness of a tight quench (from wherever that information comes from) so now the decision, with parts in hand, is whether to go for .034", which by most accounts, (but not all obviously) is tight or use the .039" and settle for .047".

Whatever one's opinion might be on quench, I think one thing that could be said is that the difference between those two options aren't likely to measurable by any metric.
Pretty common old timer thing to do with sbc around here for budget street sbc tighten squish band to .025-.030 especially for claimer, bomber stuff that doesn't run past 6500-7000
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Re: Does quench affect power?

Post by nitro2 »

F-BIRD'88 wrote: Wed Sep 14, 2022 11:06 pm The moment of max piston-rod stretch "elastic" at rpm, happens @ a bit after TDC.
True. Only happens during overlap though when at WOT.
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Re: Does quench affect power?

Post by PackardV8 »

nitro2 wrote: Wed Sep 14, 2022 8:08 pmIf you close the throttle at high rpm then the piston could touch the head at both TDCs, instead of just TDC overlap.
The moment of max piston-rod stretch "elastic" at rpm, happens @ a bit after TDC.
Those of us old enough to remember racing the '50s-'60s Jaguar XKs, the modern-for-its-day, but long stroke (4.17") DOHC straight six, if it was ever going to blow, it was usually when backing off from max RPM at the end of the longest straight. Pistons weren't touching head; usually, cause was highest inertial force uncushioned by compression.
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Re: Does quench affect power?

Post by Tom Walker »

The moment of Max connecting rod elasticity is when the big end and the small end decide they no longer want to live together as a single unit!
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Re: Does quench affect power?

Post by gunt »

RDY4WAR wrote: Wed Aug 31, 2022 6:48 pm 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.
so i have seen this statment before , peak past what ? , can you say 30deg is std or any rough ball figures
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Re: Does quench affect power?

Post by skinny z »

gunt wrote: Sat Sep 17, 2022 5:35 pm
RDY4WAR wrote: Wed Aug 31, 2022 6:48 pm 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.
so i have seen this statment before , peak past what ? , can you say 30deg is std or any rough ball figures
Isn't that to say that if a change towards a tighter quench (and all that it encompasses) without a change in compression ratio, produces the same peak HP but with less timing, demonstrates an improvement in combustion efficiency?
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Re: Does quench affect power?

Post by Tom Walker »

Yes, including possible better fuel efficiency.
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Re: Does quench affect power?

Post by skinny z »

Tom Walker wrote: Sat Sep 17, 2022 11:54 pm Yes, including possible better fuel efficiency.
Which, along with outright power, is also a real consideration in the stuff I'm building. I don't want to go broke just driving to the track for the street legal test and tune Friday nights.
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Re: Does quench affect power?

Post by LotusElise »

Sorry for interrupting the actual discussion, but I wanted to add my two cents here. I calculate squish height at the applicated engine speed range and engine temperature, means elongation of the crank drive due to thermal expansion and by acceleration. On a 86 mm stroke one should consider 0.55 mm@10,000 rpm and WOT temperature setup within a NA application. That means on a cold squish height of 1.0 mm there are only about 0.45 mm left, and what was designed to 20 m/s cold get about to 35 m/s peak squish velocity on a typical 4-valve head of a square bore-stroke engine design.

That works well and is still under the by G.P. Blair recommended peak squish velocity of 25 m/s (cold condition), which would be reached at 0.7 mm cold. But heated, this would relate at 10,000 rpm to a squish peak velocity of 49 m/s. Which is way to high and will increase knock likeliness. Why?

The radial velocity of the mixture is still profitable for combustion, but the risk introduced by both, the increased turbulent kinetic energy level triggers chemical reactions as the distance to activation energy is lowered and therefore the likelihood of a spontaneous oxidation happens as well as the kinetic pressure at squish geometry reduces the static pressure there, creating a force for the liner wall and fireland accumulated oil to be sucked into the combustion zone, causing oil droplet induced knock.

Squish induced increase of turbulent kinetic energy (TKE) is a great supporter for a fast and efficient combustion in the combustion main zone (but bad for emission), which is for 4-valves head a favorable solution. But going to far will cost engine safety and power if parameters against it are not well chosen. A swirl induced increase of TKE is a much smarter way, especially if combined with a bit weaker squish as mixture homogenization, emission and combustion duration will get improved. But as everything is a compromise, especially when it reduces VE, a compromise has to be found.

Thence, there are no recommendable squish heights in general, a better design parameter would be the squish velocity at applicated engine speed range and temperature. Of course that need some math skills to model the squish velocity correctly. Therefore a thumb rule for a collection of rod length, specific materials, bearing clearances, oil weights, stroke heights, gasket seating heights and part weights may work fine, but finally, and because testing is expensive, a dedicated calculation model is the key into the pretty complex task of getting into the optimal squish height.

Things get may get less complex once combustion effects get reduced as delay times are to big. Beyond 9000 rpm the TKE threshold to create instantaneous knock get quite high just because the self ignition delay is big enough to fail to knock, once mixture temperature at BDC is well below 40 °C, which is quite challenging. Just think of a combustion event at 10,000 rpm, which should be no longer as 0.7 ms, that means average combustion speed should be 61 m/s, which is well over 200 times of the laminar flame speed of a lambda = 1 gasoline-air-mixture combustion. Thence it is quite possible to see a well advanced ignition timing beyond 9000 rpm while the piston hit the head without any squish induced knock. Slower rotating engines are sometimes more complex :wink:.

I saw oil induced pre-ignition caused by to big squish velocities causing pressure gradients of 13 bar/°ca, which did their thing on the engine, nothing funny on almost 4000 hp engine at almost 100 psi of boost at a full-sized equipped engine test stand.
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Re: Does quench affect power?

Post by skinny z »

David Redszus wrote: Sat Sep 10, 2022 3:09 pm
Vizard writes:
I have run 0.045 progressively down to zero at 7000 rpm (piston just touched head) and yes it does affect power. However there is a lot more to quench pad design than just the clearance.
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Absolutely correct.

Squish velocity is used as representive of chamber turbulence. Squish velocity factors are Squish Area Ratio, clearance, and rpm.
Each factor will affect the resulting turbulence and resulting burn angle.

At 7000rpm
SAR = 50%
Clearance....SqV (m/s)
.025..........76.7
.028..........71.1
.030..........67.8
.034..........62.0
.039..........55.8
.047..........48.6
At any of the above clearances, the SqV would be much too high; assuming an SAR of 50%.

But if the squish area were to be reduced to SAR 25%, then a different scenario appears.

At 7000rpm
SAR = 25%
Clearance....SqV (m/s)
.025..........33.0
.028..........30.7
.030..........29.4
.034..........27.1
.039..........24.5
.047..........21.6
LotusElise wrote: Mon Sep 19, 2022 10:10 am Sorry for interrupting the actual discussion, but I wanted to add my two cents here.

That works well and is still under the by G.P. Blair recommended peak squish velocity of 25 m/s (cold condition), which would be reached at 0.7 mm cold. But heated, this would relate at 10,000 rpm to a squish peak velocity of 49 m/s. Which is way to high and will increase knock likeliness. Why?
It's not an interruption at all and all insights are welcome. At least from where I'm sitting anyway.
So, further to the above, there does seem to be a practical limit. Not including the Pro Stock example posted earlier, or any other exotic or extreme example, keeping the clearance so as to produce this aforementioned squish velocity in the recommended or safe range seems to be what one should pursue.

Case in point: a typical SBC (and all it entails) an RPM ceiling of 7000 and an SAR of 25% might see the greatest benefit from a quench of about .039". The resulting squish velocity being 24.5m/s.
Or, expanding further (no pun intended), opening up that clearance would be beneficial with the engine at operating temperature.

Perhaps oversimplified but does that seem within reason? It certainly falls into the conventional target for engines of this nature.
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Re: Does quench affect power?

Post by ptuomov »

LotusElise wrote: Mon Sep 19, 2022 10:10 am Sorry for interrupting the actual discussion, but I wanted to add my two cents here. I calculate squish height at the applicated engine speed range and engine temperature, means elongation of the crank drive due to thermal expansion and by acceleration. On a 86 mm stroke one should consider 0.55 mm@10,000 rpm and WOT temperature setup within a NA application. That means on a cold squish height of 1.0 mm there are only about 0.45 mm left, and what was designed to 20 m/s cold get about to 35 m/s peak squish velocity on a typical 4-valve head of a square bore-stroke engine design.
So counting the thermal expansion, gas pressures, and accelerations, does the squish height shrink or increase at WOT combustion and high rpms? My guess would be that the cylinder block expands some when warning up, pistons expand much more, but the rods compress even at high rpms because the gas pressure at TDC during combustion more than offsets the inertial force. As Nitro2 wrote earlier, the physical limitations of the piston touching the head bind at WOT overlap (if at all). How does your computation deal with the gas pressure compressing the rod? And will the squish velocity (only really relevant during combustion) increase or shrink compared to cold, static engine on a workbench?
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Re: Does quench affect power?

Post by LotusElise »

skinny z wrote: Mon Sep 19, 2022 10:44 amCase in point: a typical SBC (and all it entails) an RPM ceiling of 7000 and an SAR of 25% might see the greatest benefit from a quench of about .039". The resulting squish velocity being 24.5m/s.
Or, expanding further (no pun intended), opening up that clearance would be beneficial with the engine at operating temperature.
My simple approach on a 4-valve head would be design for the highest possible squish which just cause no issues on the combustion side (e.g. squish induced knock). On a simple 2-valve head I would mix a dedicated swirl with a more moderate squish. On the latter case it much depends on the external mixture homogenization, the better the higher swirl can be. But be aware of, swirl cost VE potential, especially if it need to be enforced by port geometry.
ptuomov wrote: Mon Sep 19, 2022 8:57 pmSo counting the thermal expansion, gas pressures, and accelerations, does the squish height shrink or increase at WOT combustion and high rpms? My guess would be that the cylinder block expands some when warning up, pistons expand much more, but the rods compress even at high rpms because the gas pressure at TDC during combustion more than offsets the inertial force. As Nitro2 wrote earlier, the physical limitations of the piston touching the head bind at WOT overlap (if at all). How does your computation deal with the gas pressure compressing the rod? And will the squish velocity (only really relevant during combustion) increase or shrink compared to cold, static engine on a workbench?
I am not sure if I get your question as my English maybe is not well developed enough yet. But sounds like a good question, nailing a crucial point: thermal expansion of the block and liner system.

The block is for most cooling concepts sheltered or insulated by a coolant wall/space, so that the expansion is limited in a first order assumption to ECT. The liners are different, beside topological boundary conditions like open or closed deck and a higher in-homogeneity of the vertical temperature distribution of the liner a part of the thermal expansion is converted into liner forces as the head bolts pull against it. Finding how much compensation of expansion of the crankdrive by the liner system is expensive. Just an example of Larry Widmer's B20-300 whp attempt engine, the squish height was according his post 0.81 mm cold and got zero'd at 9500 rpm. That fits quite well to my assumptions, as the B20 has a unfortunate rod-stroke ratio of 1.57:1 at a 28 mm compression height, causing a bit higher mechanical stretch as within the K20 engine of Honda.
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Re: Does quench affect power?

Post by ptuomov »

LotusElise wrote: Tue Sep 20, 2022 6:54 amJust an example of Larry Widmer's B20-300 whp attempt engine, the squish height was according his post 0.81 mm cold and got zero'd at 9500 rpm. That fits quite well to my assumptions, as the B20 has a unfortunate rod-stroke ratio of 1.57:1 at a 28 mm compression height, causing a bit higher mechanical stretch as within the K20 engine of Honda.
Importantly, the piston to head clearance almost certainly got zeroed during the overlap TDC, not during the combustion TDC, in that engine.

There are two times when the piston is at TDC: Overlap TDC and combustion TDC. At WOT, overlap TDC has a bit of a vacuum and combustion TDC has a massive pressure in the combustion chamber. Because of this, the piston to head clearance shrinks much more during the WOT overlap TDC than during the WOT combustion TDC. If the piston starts hitting the head at high rpms and WOT, it is almost certainly during the overlap and not during the combustion.

Squish height is only relevant to combustion quality during the combustion TDC. I don't think one can infer much anything about the squish height during the combustion TDC from the piston to head clearance during overlap TDC. Inferring the squish height at combustion TDC from piston to head clearance at overlap TDC will give you the wrong squish height and consequently the wrong squish velocity.

(There's a separate question of what role squish plays in a high-performance four-valve engine. My opinion is that one is better off prioritizing tumble and that leads to basically no squish, beyond what is incidentally created by the need for a high compression ratio. My opinion is that well-guided pure tumble that is broken down into small eddies at the combustion TDC is the best solution for high rpm, WOT operation of a performance engine. But that's just an opinion.)
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Re: Does quench affect power?

Post by skinny z »

ptuomov wrote: Tue Sep 20, 2022 7:43 am Squish height is only relevant to combustion quality during the combustion TDC. I don't think one can infer much anything about the squish height during the combustion TDC from the piston to head clearance during overlap TDC. Inferring the squish height at combustion TDC from piston to head clearance at overlap TDC will give you the wrong squish height and consequently the wrong squish velocity.
Also, wouldn't the velocity be affected by the open valves at TDC overlap?
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Re: Does quench affect power?

Post by LotusElise »

ptuomov wrote: Tue Sep 20, 2022 7:43 amI don't think one can infer much anything about the squish height during the combustion TDC from the piston to head clearance during overlap TDC. Inferring the squish height at combustion TDC from piston to head clearance at overlap TDC will give you the wrong squish height and consequently the wrong squish velocity.
That's a very good point, I have a simple model of that in my rod bolt force model which tells me with respect to IGT and VE as well as some Vibe parameters how much that is, just not integrated into my squish design model. To give you a feeling how much we talk about on a 2-Liter NA race engine at a VE of 120 % at redline: 0.35/1 (gas force/rod bolt forces), so only a partial compensation.
ptuomov wrote: Tue Sep 20, 2022 7:43 amThere's a separate question of what role squish plays in a high-performance four-valve engine. My opinion is that one is better off prioritizing tumble and that leads to basically no squish, beyond what is incidentally created by the need for a high compression ratio. My opinion is that well-guided pure tumble that is broken down into small eddies at the combustion TDC is the best solution for high rpm, WOT operation of a performance engine. But that's just an opinion.
Tumble is the most inefficient kind of combustion processing. Formula 1 had long time issues to exceed the 12,000 rpm. With introducing of proper squish they found the way up to 20,000 rpm on 90+ mm bores.
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310 hp@8200 rpm
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