Early 2 liter heads, detonation and piston design

[304065]

First of all, I know the early 2.0 liter head design was deemed to be inferior by Porsche and superceded. I don't have the measurements, but the earlier combustion chamber is deeper, closer to a hemisphere than the "spherical cap" of the later design.

The use of a high-compression piston in the early chamber results in the combustion chamber being split in half, causing poor flame front propogation and a pocket of "stagnation" on the side away from the spark plug. At least that is the conventional wisdom.

That being said, I'm using the early heads anyway because they are the only ones with a casting number in the center of the fin, before air injection ports, which are correct for concours purposes.

But it seems to me that in the intervening 37 years since the design change, we've learned more about piston design. Here's an idea that was posted by Grady in another thread:



Grady goes into detail about how there is an area of locally high compression at the very edge of the combustion chamber, caused by the piston dome sealing off a pocket next to the cylinder wall. Detonation here deforms the top ring groove, seizing the ring and allowing hot gas to blow by, torching the piston.

A couple of questions: in the images I have seen of JE Pistons, there appears to be a flat ring at the very edge of the piston before the dome starts. In addition to making it easier to measure deck height by giving you a place to rest the tip of your dial indicator, does this flat space provide a way for the mixture to "escape" the pocket next to the cylinder wall?

Photo Credit: www.lnengineering.com (hope you don't mind Charles)



I don't think the high compression Mahles (2.0 S and 906) have this area present, see the photo of a 906 set below. 2.0 S are similar in design.



So I know you're wondering what my question is. . . well, I have two.

1) Could the lack of a flat area at the edge of the Mahles have been responsible for the reputation for detonation of the early 2.0 liter engines? Would you recommend chamfering the cylinder head at the edge in order to create an ideal "gas escape area" as illustrated in Grady's diagram?

2) The early 2.0 heads have a smaller included angle between the valves. A piston shaped for the later cylinders has valve reliefs to correspond to a larger included angle. In less geometric terms, the top of a later piston is "flatter." Would using 84mm cylinders with 2.2 S pistons alleviate some of the detonation issue insofar as the "flatter" piston top results in more piston-to-head clearance?

What do you do to the edge of the combustion chamber when you flycut 2.0 heads for use with 84mm cylinders?

[cnavarro]

For reference, the piston you chose is an 88mm ~8.5:1 for a turbocharged Porsche 356 (1800cc), with a 10.6 cc dome and 68cc chambers. Obviously that's way different than an early 911 head, but you get your point across John.

JE also makes them without the step, and some of my early 356 pistons didn't have the step. I can't recall when they started adding that in.

Four cam Porsches (547), at least Mahle's 92mm Carrera 2 p/c's had a step in the crown, then the dome.... there's some pictures of that one on the reconditioning page on my site.

Maybe it's just simply for checking your deck height? I'll have to ask next time I call JE....

And it's no problem borrowing the picture. I looked at it first and thought, that looks familiar before seeing the credits :-)

[304065]

Charles, thanks!

I figured that was a 356 turbo piston from the title. But I can see that it has a number of features that are interesting that aren't present on the old Mahle design. For example:

1) Anti-detonation grooves. Exactly HOW these work is subject to great debate: some suggest that

a. they break up the high pressure waves caused by detonation before they reach the top ring.

b. they increase the total piston area that is exposed, allowing heat to radiate so the top ring doesn't seize up.

c. they reduce friction between the piston and bore; and that they

d. limit the formation of carbon which can lockup of the top ring, preignition and subsequent detonation.

I am somewhat skeptical of the "reduce heat" because the piston transfers its heat to the cylinder bore through the top ring (45% of the combustion chamber heat through the top ring, according to JE), not by radiation from its surface. But they are claimed to reduce damage when detonation occurs.

2) Pressure equalization groove. This groove between the first and second rings serves to distribute any pressure that makes it by the first ring that if allowed to build up could unseat the top ring.

3) Pin oilers. I'm assuming that one of the holes visible in the oil ring groove is a "forced pin oiler" that collects oil from the cylinder wall and channels it to the pin.

4) Side relief. Unlike the old mahles in my 1966 engine, the piston above has a significant cutaway of the skirt, with only a couple of support bands present at the bottom to stabilize the piston. The point of this cutaway is reduced friction and lower weight.

Here is the four-cam piston, you can see the step.



Finally, a nice diagram borrowed from a Subaru web site, actually a good technical discussion (as often occurs here).

[shbop]

enrolling..

[Steve@Rennsport]

Originally posted by Grady Clay
Damn computers!

I just wrote a multi-page anylisis of this issue (in Word) and with one errant click it dissapeared....
#24#5*
Grady

You have no idea how many times I've done the very same thing followed by some choice words,...

[Grady Clay]

Second try
John,

I think there is a lot of history (as opposed to engineering) in the 2.0 design. When the 2.0 was coming together as a design project in the very early ‘60s, the race examples the same engineers were working with were similar displacement (2.0 & 2.2) 753 & 771 8-cylinder engines. All these had very large valves compared to the bore. The 901 had two design predecessors; the 745 and 821.

This continued the combustion chamber/valve size philosophy from the 547/692/587/753/753 engines. You can look down the intake and see out the exhaust.

In the late ‘60s the philosophy changed to a more “over square” engine with a larger bore compared to stroke. There was some design transition time. I think the engineers were loath to abandon prior successful principals.

The less over square (80x66 = 1.21) 901 had the (76x54.6 = 1.39) 771 8-cylinder as a successful model. The problem is the piston configuration (high pop-up) used to get 10.3-10.5:1 CR in the race engines was used to get acceptable (9.1:1 CR) with the 901 while still encompassing the high-angle “see-through” valve angle.

This design was also intended to use twin plugs. The bean counters (yes, even in ’63) axed twin plugs.

The consequence was the 901 had the high dome piston, large wide angle valves, a “see through” combustion chamber and only a single sparkplug. This all conspires to make the combustion chamber more susceptible to detonation. This is increasingly so with higher CR.



To solve your dilemma, put up with the deficiency of the 2.0 heads or use some ‘70T (’69 casting date) heads.

With a 2.0 head, I don’t think there is an issue with the “deck area” I describe (please provide a link) because there isn’t a step between the bore and the head. As the bore increased (2.0 to 2.2, 2.4 to 2.7 and 2.7 to 3.0 & 3.2) The flat I described became increasingly more significant.

An interesting comparison is to look at the combustion chamber of current (water cooled) 4-valve engines. The philosophy has changed

One of the fun things about this kind of discussion are what we might have whispered in Porsche engineers ears in 1960-’62 during the 901 gestation. I can think of many.

Best,
Grady

[304065]

Grady,

Thanks for the contribution and history! (Sorry about your Magnum Opus, that occasionally happens to me too but it gives me the opporutnity to use the approved Factory Curse Words*)

Here is a link to your prior, excellent contribution on the subject of detonation. 9.8 or 10.1?

Here is evidence of the Twin-Plug DNA of the 901 engine!



Well, as you know I will be using the original heads and accepting their limitations. I plan to utilize the benefits of modern piston design, accurate timing and 93 R+M/2 octane at all times. I was just curious as to whether extending the flat area or chamfering the combustion chamber edge would have a primary effect on reducing the likelihood of detonation damage, as opposed to the secondary effect of reducing overall compression (which it certainly would.)

* The correct factory swear is: Himmel-herrgott-sakrament-zefix halleluja-scheissklump-verreckts! Note: this is not suitable for viewing by German-speaking minor children.

[Plavan]

I have a set of those old aluminum valve covers... The 906 had those holes drilled.

[rs-vic]

I heard that the flat area on top keeps the heat from heading down to the top ring, ie having to make two bends instead of a curve.

[304065]

Chad, that's actually a photo of my own engine's covers. Gotta supress the desire to cut those holes open. . .

Vic, thank you for the information about the flat part. I think a conversation with JE is warranted now that I understand better.

Here are some head photos. The first is from Henry Schmidt at Supertec, it is a '66 head which has been machined to work with a 2,2 sealing surface.



The next is a head that has been professionally reconditioned by Anchor Atlantic Enterprises. As you can see, it is a 2,7 head. The 10-73 casting date is probably too late for a 2,4. Note the slight chamfer at the edge of the combustion chamber.

[Tom F2]

Am I correct in thinking that the machine shop added the chamfer to the 2.7 head during the reconditioning? If so, what reason did they give?

[Plavan]

I have the exact set sitting in my garage... You can drill those
I have tons of early 2.0L parts.... Cylinders etc.

[304065]

I believe that chamfer was added by the factory to expose the combustion chamber to the larger cylinder.

Chad, that's good to know, I'll contact you by PM.

[304065]

Here is a good thread with an entry by Henry Schmidt that shows the flat area on the JE Piston. Evidently this is to provide clearance if a larger piston diameter is used with an earlier, smaller-diameter head.

Piston dome volume of 3.2SS?

[304065]

Another thread on chamfering the head. It occurs to me that if you chamfer the edge of the combustion chamber to allow the larger piston to clear, you also have to watch that the deck height in that area does not get too great, which we also know promotes detonation.

Need to chamfer heads to fit RS pistons??

[ficke]

So if you twin plug the early 2.0L heads then it makes the detonation issues with high compression piston in a high doom early 2.0L ..... a none issue? and there might be some HP to gain with the flow through advantage of the heads?

[Steve@Rennsport]

Originally posted by ficke
So if you twin plug the early 2.0L heads then it makes the detonation issues with high compression piston in a high doom early 2.0L ..... a none issue? and there might be some HP to gain with the flow through advantage of the heads?

Nossir,....

These deep-chambered heads with their tall piston domes are still more detonation-prone than later engines, but they can run 10:1 on pump gas with twin-ignition.

On the flow bench, the 2.0 heads do not flow as well as the later, shallower chamber versions that permit bigger valves as well as less shrouding near TDC. Further, high-compression pistons with their tall domes block some exhaust scavenging effects from a good exhaust system.

[ficke]

Thanks Steve

[kenikh]

Here's an odd question: what if early heads were turned on a lathe, from the outside edge of the valve seats to the edge of the combustion chamber, to flatten the dome of the head? You could have JE pistons made to make good CR. Would the flame prop problems go away?

[304065]

This took a couple dozen tries but you can JUST see light if you look in one port and out the other of this 2,0 head.