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kenikh's Avatar
 
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Question Hey engine experts, explain me this...

I have been re-re-re-re-re-reading Wayne's engine picks in his book and I keep noticing this motor:

3.5L Upgrade - 74.4mm crank, 100mm pistons, S/GE60/GE80 cams, twin plugs, 11-12:1 CR.

OK, color me confused. Why do I keep reading that you shouldn't go above 10.5:1 with twin plugs on pump gas on an S-cammed (or 906 or GE60 cammed) 2.0, but a motor with pistons 2cm larger in diameter can go to 12:1? Intuitively speaking, you'd expect the smaller pistoned motor to be able to handle higher CRs than the bigger one.

So, given the same cams and twin plugs, what gives?
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Old 02-16-2006, 09:52 AM
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For one thing the 2.0 pistons have a much bigger dome, and the combustion chamber in the 2.0 is also deeper. So while the diameter is smaller, the distance the flame needs to travel is still substantial and contourted.

Starting with the 2.2's, Porsche made the chamber flatter, and the piston domes became smaller. So even though the cylinder diameter became larger, the flame path was shorter then (or maybe closer to equal to) the 2.0, but also less contourted. As the cylinder capacity increased, piston domes became smaller since the combustion chamber capacity stayed pretty much the same. So it was possible to get a higher CR with a smaller piston dome. The result is that compared to the cylinder capacity, the combustion chamber became more compact (relatively speaking). If you look at 3.5 liter pistons and 2.0 pistons with the same CR, you'll see that the dome is much more pronounced on the 2.0 pistons.
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'69 911E

"It's a poor craftsman who blames their tools" -- Unknown
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Old 02-16-2006, 10:20 AM
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Let me make sure I understand you correctly: because of head design, smaller cylinders give up their diameter advantage in terms of detonation resistance. Thus, even though the 2.0 pistons are smaller in diameter, the linear distance of flame travel due to the dome is roughly equal to that of a larger piston, which has a much more direct path for flame travel between the twin plugs. Correct?

One more thought to clear up: the earlier pistons had to be domed to accomodate the large valve sizes required, where the larger heads have the real estate to be flatter in design. This is why 4 valve heads can be flat, since they are more efficient in real estate usage due to smaller valves fitting the heads more efficiently (and don't need multiple plugs due to a centered plug).
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Old 02-16-2006, 11:19 AM
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Yup. Except 4-valve heads are not flat (unless you're talking about Huron heads (ich!) llike the 924 had. But yes, their combustion chamber is much more compact with the spark plug in the middle and little if any piston dome (aka: intruder) needed to achieve the desired CR. This is one reason why my wife's Mazda6 is perfectly happy with a 10+:1 CR ratio and regular fuel compared to my 2.0E with S pistons which can just squeak by with premium fuel and a 9.8:1 CR. (Although the engine management system helps!)
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'69 911E

"It's a poor craftsman who blames their tools" -- Unknown
"Any suspension -- no matter how poorly designed -- can be made to work reasonably well if you just stop it from moving." -- Colin Chapman
Old 02-16-2006, 11:39 AM
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So is flatter a cure for detonation? If 4 valve heads can be flat, yet aren't; what is ideal? What about radial valve orientation? VW heads seem to be very flat and are 2 valve/air-cooled, yet typically VW builders won't go higher than 7.5:1. I am still confused.
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Old 02-16-2006, 11:58 AM
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What about swirl?

But wouldn't you think that a >10:1 piston, like the 906 shape and its deep valve pockets, would have much better turbulance at faster (>5k rpm) given that there is almost no squish or swirl in the 'lazy' 911 head as compared to the later large bore 'clean' chambers?

This is the only reason I can come up with to explain why detonation resistance would be greater at higher revs than lower revs.

As for the increased linear travel argument, don't both the larger motors and the smaller motors use the same chamber shape just a touch bigger (>2.2L)? If we assume the plugs to be point sources, then both motors still have the same distance along the hemisphere chamber to burn that fuel-air charge. Even with the 906 dome shape the bulk of the charge is still hiding close to the plugs in the valve pockets.

I should think that the total timing would be good indicator as to how well the combustion chamber burns since at a given rpm the required advance should be how long the fuel burns. So our little 2.0L has a single plug value of 30* BTDC, I have seen recomendations to change that to ~22-24* BTDC with twin plugs and the 81 SC recomends 25* at 4k so I would guess that it would actually be a larger value at 6k than our 30* 2.0L (anyone actually know what a bigger bore 2 valve motor runs for advance at 6k?).

tadd
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Old 02-16-2006, 12:19 PM
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Flat is not the issue. Combustion chamber shape is (along with squash, swirl and turbulance.

Here's an image of a Cosworth XB combustion chamber.


For comparison, here's Toyota 4AG combustion chamber.


1) It's got more valve curtain area compared to a 2 valve head.
2) Note the distance from the spark plug to the edge of the combustion chamber.

Here's a picture of the XB pistons.

Note how small the domes are.

Here's a drawing of a recent Mazda head in relation to the piston at TDC. Note that it doesn't even have a piston dome, but rather a small bowl, but still the CR is most likely at least 9.5 to one, most likely more like 10.3:1. The valve curtain area is ample
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Last edited by jluetjen; 02-16-2006 at 12:34 PM..
Old 02-16-2006, 12:31 PM
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But that is the point

Both of those chambers are more lens or disk like than hemispherical. This seems to put the 'fat' part of the charge right under the plug and although you still have to go a long way to get to the edges, the meat and potatos is close by.

Now if we take your cosworth disk and stretch it over a spherical surface, then we get our porsche 2.0L. The valve pockets in the 906 piston put the bulk of the charge close to the plug(s) then there is a thinner remaining charge to be finished off at the edges of the cylinder. A topologial view says that they are not that far appart in design.

Maybe what we need is a high dome that is 'dimpled' in the middle for the bulk of the charge?

One last thought: If the clearences are tight, I forget how tight, that portion of the charge will not burn. For example, what is left between the piston and the head in the squish area of a chevy does not burn.

tadd
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Old 02-16-2006, 12:44 PM
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Here's an automated cross-section of a 2.7RS engine which is I believe made by a fellow Pelicanhead.



Note that the piston crowns are nowhere near as extreme in a 2.0. Especially an S or 906. Here's a couple of pictures from BA's book with the 2.0 pistons in the center. Note how much further the dome intrudes into the combustion chamber on the 2.0 pistons.


Yes, a lot of the combustion chamber is under the valves, but that's not a very compact shape since it is shaped like a figure 8 to allow space for the valves. All of those edges are prone to hot-spots and detonation. Unburnt gasses can also collect in the perimeter which promotes fouling at low rev's. The lack of squash and swirl/tumble also means that the mixture tends to be more uneven at low rev's.
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"It's a poor craftsman who blames their tools" -- Unknown
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Old 02-16-2006, 01:06 PM
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Re: Hey engine experts, explain me this...

Quote:
Originally posted by kenikh
I have been re-re-re-re-re-reading Wayne's engine picks in his book and I keep noticing this motor:

3.5L Upgrade - 74.4mm crank, 100mm pistons, S/GE60/GE80 cams, twin plugs, 11-12:1 CR.

OK, color me confused. Why do I keep reading that you shouldn't go above 10.5:1 with twin plugs on pump gas on an S-cammed (or 906 or GE60 cammed) 2.0, but a motor with pistons 2cm larger in diameter can go to 12:1? Intuitively speaking, you'd expect the smaller pistoned motor to be able to handle higher CRs than the bigger one.

So, given the same cams and twin plugs, what gives?
Fact is, you CAN'T use pump gas on an 11-12:1 CR 911 of any kind, especially these big bore engines. You'll buy new P/C's, bearings and more, if you do,....

Given the variations in cylinder head temperatures, I'd not exceed 10.25:1 on any pump gasolines (with twin-ignition, of course).
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Old 02-17-2006, 09:52 AM
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Hows about a long stroke?

Ok, if the dome is the problem, what if we went with the 70.4 mm crank. This should give an additional 22 ml displacement per cylinder (assuming my maths skills are still valid). Going with the 9.8:1 of the S pistons that gives a chamber volume of 33.9 ml when used with the longer stoke we end up with a nice 10.4:1 CR.

So since the dome on the S pistons is less than the dome of a 10.3 906 piston is now a 'safer' ratio?

tadd
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Old 02-18-2006, 11:43 AM
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