Running .035" Squish In 8V Cylinder Head

[Vigo]

Well if you assume that heat and pressure 'travel' faster than the flame front, then distance has to be some kind of factor. If you put the spark plug all the way to one side of the cylinder, wouldnt that make the engine more likely to detonate at the other end of that cylinder?

[OmniLuvr]

The idea of mirroring the combustion chamber in the piston kind of seems like an oversimplification

yes, but to a point, it really prob depends on the shape of the combustion chamber, id like a g-head piston that matched the combustion chamber, not just the small circle dish they came with.

and so far im liking how my t3 flat tops are performing with my g-head (they had about .013 pop out when installed?) and failpro headgasket @ 20 psi, just need a little more ball$ to use premium fuel instead of e-85 and see where it takes me...

[zin]

The heat generated by the increasing pressure of combustion does effect the charge/chamber as a whole, and all at once, so the flame front isn't as dominate as one tends to expect. This is why a hot spot (plug tip, exhaust valve, etc) can be both a point of pre-ignition and detonation. Both tend to make the "local" mixture less stable, one before ignition and the associated pressure rise, one just by itself, both with similar outcomes.

Since detonation is the uncontrolled combustion of the end gasses, which by definition are at the "end" of the combustion chamber, you'd be correct that distance can make a difference, both in terms of detonation (as in how long will the "far end" be subjected to rising temps/pressures), and whether or not the hot/unstable portion is consumed before it can "go off".

An example would be the ubiquitous SBC angled plug head. While both the straight and angled plug heads have the plug in roughly the center of the chamber, the angled plug reduced detonation dramatically buy angling the plug towards the hot exhaust valve area, thereby burning the hottest/most unstable portion of the charge first, leaving the portion that could best endure the heat and pressure as they increase.

You'd also be correct in that the mirroring of the chamber in the piston is an over-simplification. If one knew of nothing else to do, mirroring would have a good chance at "fixing" a detonation problem and/or increasing combustion efficiency, but would likely be less than optimal, though the "shot gun" approach might hit some of the targets.

Mike

[glhs875]

yes, but to a point, it really prob depends on the shape of the combustion chamber, id like a g-head piston that matched the combustion chamber, not just the small circle dish they came with.

and so far im liking how my t3 flat tops are performing with my g-head (they had about .013 pop out when installed?) and failpro headgasket @ 20 psi, just need a little more ball$ to use premium fuel instead of e-85 and see where it takes me...

I have a set of those T3 pistons that I plan on using on an 8v engine as well! But I will be using a Cometic headgasket.

[zin]

yes, but to a point, it really prob depends on the shape of the combustion chamber, id like a g-head piston that matched the combustion chamber, not just the small circle dish they came with.

That arrangement would actually be pretty good! It's close to a fully spherical chamber, which has excellent surface to volume ratio and would effectively force the out laying gases into the combustion space, aiding turbulence.

A couple of downsides, off the top of my head, would be that the hottest part of the mix wouldn't be lit first, due to the plug placement, though I'm not sure just how much that would effect things...

Second would be that timing could be tricky. If started too soon, it could "set off" the mix before it gets quenched by the piston and chamber. This has more to do with quench than squish, kind of like snuffing out the fuse on the dynamite before it gets to the stick.

This also assumes a reasonable compression ratio could be obtained between the piston and chamber without ridiculously shaped/configured pistons.

Mike

[Reaper1]

Most well designed 2-valve combustion chambers (with the few exceptions) will typically aim the spark plug toward the exhaust valve. This is done for the exact reasons Mike pointed out.

Now, lets think about the chamber design as a whole (which includes the piston face). If you simply mirror the upper part of the chamber in the head onto the piston, yes, you will get more squish, more quench, and probably better combustion efficiency as a whole. BUT, the charge is still over most of the face of the piston, which means a good portion of it isn't near where the combustion event starts at all. This is where we start to NOT mirror the upper part of the chamber. You want to actually force the charge into the area where the plug is, and bias it towards the exhaust valve.

Now, the reasons for doing that don't just lie in the combustion event. Think about what happens AFTER the charge is spent. Now we have the exhaust stroke in a 4-stroke engine. By not only mirroring the upper part of the chamber, but also biasing the volume towards the exhaust valve, now we are forcing the spent charge out of the open exhaust valve and making sure the chamber is as purged as it can be of the spent charge. THAT is the part of the trick they aren't telling you!

Now, add in a tiny bit of valve overlap along with swirl and you can really clean the chamber out for the next cycle!

[glhs875]

(Now, add in a tiny bit of valve overlap along with swirl and you can really clean the chamber out for the next cycle![/QUOTE])



A little overlap can help to cool the exhaust valve during that cycle as well. I like the idea of using a piston/chamber design to force things where they need to go. This is a good thread!

[OmniLuvr]

i feel as if the swirl pistons with a g-head doesnt have a good quench area (because the dish is so large), but they seam to work good, not sure if its the low comp ratio or?

i ran a swirl head on g-head pistons and e85 with no intercooler (14.7psi) and made enough power to blow up the stock 525, which isnt saying much i guess? but it still felt pretty good, i wanted to take it farther, but went with the g-head/t3 piston motor while swapping in a 523 instead.

i think i might even try the swirl head on the t3 piston motor next, prob a little easier to get a tune for it i think... just worried about valve to piston issues...

[johnl]

A good friend of mine told me something a number of years ago that was also quoted by John Kaase in so many words after another win in the Engine Masters Challenge. The piston dish should mirror the combustion chamber. I have seen some very tight quench gaps. Personally, from what I have researched on it, we cannot go wrong with improvements in it. But as Kaase stated, most people are not willing to go through the effort and expense to do it.

As I understand it, the perfect chamber has the absolute minimum surface area, the idea being that chamber/piston area exposed to heat pulls heat away from combustion and therefore from application of its energy to moving the piston. On this aspect alone, leaving other complications and necessities aside, the perfect chamber would be a sphere.

[Reaper1]

i feel as if the swirl pistons with a g-head doesnt have a good quench area (because the dish is so large), but they seam to work good, not sure if its the low comp ratio or?

i ran a swirl head on g-head pistons and e85 with no intercooler (14.7psi) and made enough power to blow up the stock 525, which isnt saying much i guess? but it still felt pretty good, i wanted to take it farther, but went with the g-head/t3 piston motor while swapping in a 523 instead.

i think i might even try the swirl head on the t3 piston motor next, prob a little easier to get a tune for it i think... just worried about valve to piston issues...

I personally think the swirl head on g-pistons having good results is more of the lowered compression ratio (I got all messed up here and confused. This actually yields a HIGHER compression ratio, but the rest of my comment here still stands.) than any kind of increase in combustion efficiency. The only way to really tell would be to use EGT probes and monitor the A/F and the amount of fuel being used (either by a fuel flow meter or by calculating it using the pressure and pulse width at the injectors with the flow capacity of the injectors being a known). This would have to be done with a back-to-back test using both set-ups in each test, preferably on the same day. Increase in power or detonation resistance doesn't really tell the whole story.

As I understand it, the perfect chamber has the absolute minimum surface area, the idea being that chamber/piston area exposed to heat pulls heat away from combustion and therefore from application of its energy to moving the piston. On this aspect alone, leaving other complications and necessities aside, the perfect chamber would be a sphere.

This is where ceramic coating comes into play.

[zin]

On this aspect alone, leaving other complications and necessities aside, the perfect chamber would be a sphere.

You'd be correct John, but there are other complications, some become big deals, others not so much.

An interesting mental exercise is to imagine this mirrored chamber, or spherical chamber being filled with high pressure gas, and how that force is acting on the pieces when they are "sealed up" at TDC, then imagine them pulling apart, and going back together.

It starts to become clear how timing, both physical positions and flame front become important factors.

For example, it is possible to have a chamber with lots of quench and the associated "squish"/tumble/swirl/turbulence that helps combustion and staves off detonation (the quench part), but if the timing is too early the end gasses could start to burn before the two halves get together to perform the quench, resulting in lots of HCs (from effective, but late quench) at best, and detonation at worst.

If the quench/squish is too aggressive, it could "blow out" the flame front as well, as the flame front will (within a certain degree) always burn at the same speed/rate, which is one of the reasons why big bores are difficult to make anywhere near as efficient as smaller bores, even though their larger surface area would tend to make them more desirable in most cases.

I'm not quite sure how I feel about the idea of holding the mix around the exhaust valve, I'm concerned that being in proximity to that much heat might destabilize the mix, resulting in detonation.

I do like the idea of pushing the spent gases out the exhaust with the chamber and piston shape, just not sure I'd work to concentrate the mixture in the hot spot... maybe if it were a very lean stratified charge...

Mike

[Shadow]

I personally think the swirl head on g-pistons having good results is more of the lowered compression ratio than any kind of increase in combustion efficiency.

Swirl head on G-pistons = Higher comp...........No? I did this years ago and the car went like a BOOH at 14 psi, everything else stock TII.

G head on Swirl pistons = Lower comp AFAIK.

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If the quench/squish is too aggressive, it could "blow out" the flame front as well, as the flame front will (within a certain degree) always burn at the same speed/rate, which is one of the reasons why big bores are difficult to make anywhere near as efficient as smaller bores, even though their larger surface area would tend to make them more desirable in most cases.

Mike

This is a Good point, and one to keep in mind when building a boosted mtr.

[Vigo]

Swirl head on G-pistons = Higher comp...........No? I did this years ago and the car went like a BOOH at 14 psi, everything else stock TII.

I dont know what BOOH means but i assume it's good. My spirit has that combo and seemed pretty strong at 12psi for, like you said, stock tII everything else.

[OmniLuvr]

BAT OUT OF HELL :-)

and yes they do...

[Reaper1]

I think I actually tried the opposite combination, swirl pistons with a moderate port job and the chambers had been modified a little (laid the back wall by the valves back a bit to help unshroud the valves). That combination felt SO much faster at 10# than the original did at 14#. I will admit that I believe I made some other changes between set-ups that would most likely have that kind of effect as well.

Yes, I think the G-pistons with the swirl head would yield a higher static compression ratio. Please refresh my memory here. If IIRC the G-pistons have a dish that is smaller in diameter than the swirl pistons, and the dish is deeper, right? If this is the case, then that combination might be doing the altered squish area and charge volume control on a very basic, but what seems to be effective level. If so, that would be pretty sweet and kinda shows that the theory is sound.

[Shadow]

The biggest - for the swirl head vs the G-head IS how shrouded the valves are, so simply unshrouding them with no other work done will yield gains.

How much did the dynamic comp change from 85 (G-head) to 86+ (swirl)? We Know that the G-head has a significantly larger volume (cc's) chamber vs the swirl. So placing a stock swirl head on a G-head bottom end Will Increase the dynamic comp of That mtr.

What relation it will have to a regular swirl mtr we would need to bring more info into the fray, but I would be willing to bet that the swirl head + G-bottom end will still be Higher.

[Vigo]

I think the reason it isn't more common is that the g-pistons dont have proper valve reliefs for swirl head, which makes it interference. I have not tested this theory by breaking a belt yet.

[Shadow]

Common info on the 84-85 2.2l mtr is that Chrysler dropped the CR to 8.5:1 for turbocharging. When we bolted the swirl head on I'm pretty sure I cc'd the chambers and figured we would be up in the 8.8-9.0:1 range.

So, Chrysler would have had to drastically Increase comp on the swirl mtrs in order for them to be higher and I'm pretty sure (already having det. problems if proper octain not used) they Didn't.

Also, I seem to remember looking at the pistons and it seemed apparent (at the time) that the swirl mtr had the lower comp. pistons to make up the diff.

Someone jump in here and correct me if I'm Wrong!

[turbovanman²]

Swirl head on G-pistons = Higher comp...........No? I did this years ago and the car went like a BOOH at 14 psi, everything else stock TII.

G head on Swirl pistons = Lower comp AFAIK.

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This is a Good point, and one to keep in mind when building a boosted mtr.

Booh as in crap or good? How can lower compression and a lazy chamber go better than a properly setup up TII? Everyone who has run a G-head on swirl pistons has noted, including me, they run like crap until the spark timing is changed.

Common info on the 84-85 2.2l mtr is that Chrysler dropped the CR to 8.5:1 for turbocharging. When we bolted the swirl head on I'm pretty sure I cc'd the chambers and figured we would be up in the 8.8-9.0:1 range.

So, Chrysler would have had to drastically Increase comp on the swirl mtrs in order for them to be higher and I'm pretty sure (already having det. problems if proper octain not used) they Didn't.

Also, I seem to remember looking at the pistons and it seemed apparent (at the time) that the swirl mtr had the lower comp. pistons to make up the diff.

Someone jump in here and correct me if I'm Wrong!

G head pistons have a very small dish, swirl head have a large dish. We had all the chamber CC's in the old KC but I guess we need to redo it again since its gone. G head on swirl pistons puts the compression down 1/2 a point.

I came up with this a few years ago-

Ok, these are averages and not exact, I worked out that its about 1 point using a TIII head on 2.5 pistons, TBI pistons are close to TIII pistons in dish volume and TBI's are 8.9:1 so based on that-

8 valve pistons, swirl head, 8.0:1
8 valve pistons, G-head, 7.5:1
8 valve pistons, TIII head, 7.0:1
8 valve pistons, Neon head, 8.0:1

TIII pistons, TIII head, 8.0:1
TIII pistons, swirl head, 9.0:1
TIII pistons, G-head, 8.5:1
TIII pistons, Neon head, 9.0:1

[Shadow]

Booh as in crap or good? How can lower compression and a lazy chamber go better than a properly setup up TII? Everyone who has run a G-head on swirl pistons has noted, including me, they run like crap until the spark timing is changed.

Not sure who your talking to here? No one is talking about a G-head on a swirl bottom end, the conversation is about the Opposite; Swirl head on a G-bottom end!

G head pistons have a very small dish, swirl head have a large dish. We had all the chamber CC's in the old KC but I guess we need to redo it again since its gone. G head on swirl pistons puts the compression down 1/2 a point.

I came up with this a few years ago-

Well, if that's true it would Support what I'm saying; IF the G-head Drops comp by 1/2 point, then the reverse would also be true; that the swirl would Increase it by 1/2 point making the Swirl + G bottom end 9.0:1 comp. (I guestimated 8.8-9.0:1 with that combo)

So your Agreeing with me

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Yes, I think the G-pistons with the swirl head would yield a higher static compression ratio. Please refresh my memory here. If IIRC the G-pistons have a dish that is smaller in diameter than the swirl pistons, and the dish is deeper, right? If this is the case, then that combination might be doing the altered squish area and charge volume control on a very basic, but what seems to be effective level. If so, that would be pretty sweet and kinda shows that the theory is sound.

You confused me earlier by saying the the Swirl head on G-bottom end = Lower comp. So now your saying the opposite, that it would yield Higher comp? (so agreed?)

That being said, and if it holds true, how could we possible know that Any benefit from squish is being had? Don't you think the simple answer to the gained "feel" of performance would be the raised comp?

Don't get me wrong, I would love to agree with you guys about this, but I believe it's a waisted venture in a highly boosted engine. Your making the mtr Work Harder, and at 2-3 x atmospheric pressure the Squish is no longer "as relevant".

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Warren Stramer is running his pistons down in the hole by a bit (similar to the NSRT 2.4 Turbo) and when asked about it he responded that he has noticed a current trend for high performance FI engines to not be using squish much, if at all. While I can't argue with his results, knowledge, or skills, I can't agree with that thinking simply because it doesn't make much sense if you look at the "problem" from an engineering physics point of view. Maybe I'm all "starry eyed", and have fallen prey to an elaborate gimmick, but I've seen the results from using this train of thought in practice, and I've had other very experienced engine people agree with it as they'd also messed with portions of the idea and seen results themselves. Also, given that new, high efficiency engines are employing this type of design (which Larry and his company have been contracted by auto makers to develop products for them)...I can't help but to believe this is the way to go.

When you say "down in the hole a bit", what are we talking here? Stock height, lower, or is this simply a reference to anything run "under deck height"?

Not being a smart a$$, just trying to get some clarafication, cause I'm Not aware of Warren running his pistons any lower than one normally would.