Using 2.2S Pistons on a 2.4S engine...

[Wayne 962]

Using 2.2S Pistons on a 2.4S engine...

There has been a lot of questions asked to me on this issue, so I thought that I would try to clarify some details on this swap.

First of all, the 2.2 and 2.4 engines both use the same bore pistons (84mm). The difference between the two is in the crankshaft and the rods (66mm for the 2.2 and 70.4mm for the 2.4). To compensate for the longer stroke, Porsche shortened the rods as well. As a result, the deck height of the two engines remains the same. The rods were shortened by 2.2mm, which is 1/2 the increase in the stroke. A reduction in the big end rod bearing dimension also occured at this time.

When Porsche designed the 2.4 engine, they lowered the compression down to 8.5:1 from the 9.8:1 that the 2.2S had (not sure why). The formula for compression ratio is:

( pi * bore * bore/4 * stroke + combustion chamber volume ) / combustion chamber volume = compression ratio

If you look at the formula, you will see that as a rule, as displacement increases, the compression ratio increases, if you hold the combustion chamber volume the same.

In the case of using the 2.2S pistons with the 2.4 crankshaft, you have a piston that normally would generate a 9.8:1 compression ratio with the 66mm stroke. If you work backwards from the formula above, you can estimate that the 2.2S engine has a combustion chamber size of about 41cc. Using this combustion chamber size in the new formula with the stroke from the 70.4 crankshaft, you can then calculate the estimated compression ratio of the new setup with the 2.4 crank, and the 2.2S pistons. This comes out to be about 10.4:1.

In reality, supposedly, the compression doesn't actually come out to be this high. I've talked to many people on this (including Bruce Anderson), and they can only guess why.

[stuff deleted because the numbers didn't add up]

I don't have time to do a complete 'cc' and dimensional analysis of the difference crankshafts and heads, but as a brief summary, this is why this upgrade works...

-Wayne

[Henry Schmidt]

Interesting thought but rod length is measured from the crank pin center ( big end center) to wrist pin ( small end) center dimension "A" . The journal diameter does not effect rod length.

[Wayne 962]

Originally posted by Wayne at Pelican Parts

In reality, supposedly, the compression doesn't actually come out to be this high. I've talked to many people on this (including Bruce Anderson), and they can only guess why. My personal guess would be that it is related to the fact that the bearing journals on the cranshaft were reduced by 5mm, effectively bringing the rod 2.5mm closer to the crankshaft, while increasing the rod length 2.2mm. With the increase of the stroke of the engine by 4.4mm, this would produce a small deficit of about .3mm if everything else was exactly the same.

Hmm, after further thought and a few spreadsheet calculations, I'll have to squish this theory - it doesn't seems to add up with the numbers...

-Wayne

[Wayne 962]

Okay, so the question that I'm trying to answer theoretically is "is the piston pin offset in the same location on all 911 engines?" We know that the cylinder height and thus deck heights are all the same.

However, on the 3.0 and the 3.2 engines, there exists some mathematical annomalies. The 3.0 stroke is 70.4mm, meaning the the rod bearing journal is 35.2mm away from the center of the crank at TDC. The 3.2 rod bearing journal is 37.2mm away from the center at TDC. 3.0 rods are length 127.80mm and the 3.2 rods are 127mm even.

So, it would appear that the 3.0 piston pin bushing is 163mm away from the center of the crankshaft at TDC, while the 3.2 piston pin bushin is 164.2mm away. In order to maintain a consistent deck height, you would need to move the piston pin bore in the 3.2 pistons.

It is important to note that the math works out fine for the 66mm crankshafts. The rod bearing journal is 33mm away from the center of the crank at TDC, and the rod length is 130mm. This equals a total of 163mm away from the center of the crank, the same as the 3.0.

Based on this math, I would guess that all the piston pin bores are the same with the exception of the Carrere 3.2 and the Turbo 3.3 engines. I have someone measuring some pistons right now...

Any thoughts?

-Wayne

[jluetjen]

When Porsche moved from the 66 mm crank to the 70.4 mm crank, they used the same crank casting, but shifted the center of the rod journers out by half the amount that they reduced diameter of the rod journals. (They also increased the width of the journals to maintain the bearing surface area.) This would have been just a machining change (very small tooling investment) as opposed to a casting change (Big tooling $'s.) They then shortened the rod's by the corresponding amounts.

Here's a couple of graphical illustrations from Pano in January '72.





Maybe they will help some.

[Henry Schmidt]

The difference between 2.2 and 2.4 pistons is the dome height not wrist pin location. In the early engines they shortened the connecting rod to make up the difference in stroke. I believe the measurement we are talking about is called compression distance and the compression distance on 2.0, 2.2, 2.4, 2.7, and 3.0 are all 1.335 in. or about 32 mm as I stated in the earlier response.
One of the problems with Porsche engine is that they keep changing the rod length to stroke ratio.
In the early cars it was almost 2 to 1. As the stroke grew the ratio got smaller causing some very bad rod angularity. I believe in the later engines the ratio is down to around 1.63 to 1.

[arrivederci]

Based on rumors from previous owners, my '72 has 2.2S pistons mated to a 2.4T motor. I don't have any paperwork to support it other than a 145 rwhp dyno sheet and the knowledge that the motor has T cams, T distributor (w/ T redline), and a T MFI pump.

Just another data point...

Dyno Sheet
Dyno Video

[Wayne 962]

Good info everyone. I'll stipulate to the facts in this thread. All piston pin locations were the same from for 2.0 thru 3.0 liter engines. It appeared to change on the 3.2 and the 3.3 engines though?

I'm having someone measure some pistons (or if I'm feeling motivated, I know there's a box of old pistons somewhere in my garage)...

-Wayne

[KobaltBlau]

Thanks, everyone. I really learned something from this thread.

[dtw]

I've got some trashed 2.2S, 2.2T, and 2.4E pistons lying around, if this would contribute to the cause at all...

[NevenM]

Wayne

You making an assumption that the Head Vol for the 2.2 and 2.4 is the same, It would only take a slight machining change to add a couple of CC's. I cc'd my 2.4 heads recently and it was 65.7cc. It might be a resource you could consider adding to this site a database of cc values (heads and pistons)

Neven

[jluetjen]

I've got some 2.2 and 2.4 heads and they all cc'd the same at 70 cc's. Neven, I wonder if your heads have been shaved?

Curiously Josh, the significant increase in CR didn't seem to do anything for your engine. Based on the factory HP charts, here's what I show a stock 2.4T generates for HP.

45 @ 2000
60 @ 2500
77 @ 3000
94 @ 3500
110@ 4000
125@ 4500
135@ 5000
139@ 5500
139@ 6000
135@ 6500

[Wayne 962]

Originally posted by NevenM
Wayne

You making an assumption that the Head Vol for the 2.2 and 2.4 is the same, It would only take a slight machining change to add a couple of CC's. I cc'd my 2.4 heads recently and it was 65.7cc. It might be a resource you could consider adding to this site a database of cc values (heads and pistons)

Neven

Right, I am assuming that. Primarily because the head volume doesn't change if you use the same heads on your motor (i.e. swapping in 2.2 pistons into a 2.4 engine).

Also, I'm pretty sure that the combustion chamber size is the same on the 2.2 through 2.7 heads, but I'm not 100% sure...

Whopps - just saw John's post. Looks like the measurement data backs up my assumptions on the combustion chamber size.

-Wayne

[NevenM]

Wayne

Yes my heads were skimmed, but another point taking the 70cc head and I cc'd the dome volume of my new 2.4E pistons at 12cc then I would get a 'theoretical' comp ratio of 7.15 (with 1mm deck height) up to
7.75 (with no deck height), So what gives, have my pistons got unusually flat domes (I may clean and cc the old ones) or were Porsche stated comp ratio figures (8 to 1 for an E) high?

Neven

[arrivederci]

Originally posted by jluetjen
I've got some 2.2 and 2.4 heads and they all cc'd the same at 70 cc's. Neven, I wonder if your heads have been shaved?

Curiously Josh, the significant increase in CR didn't seem to do anything for your engine. Based on the factory HP charts, here's what I show a stock 2.4T generates for HP.

45 @ 2000
60 @ 2500
77 @ 3000
94 @ 3500
110@ 4000
125@ 4500
135@ 5000
139@ 5500
139@ 6000
135@ 6500

It would appear that there's no change, until you figure that the dyno numbers are RWHP. Therefore 144 rwhp / 0.85 = 169 fwhp. The curve will remain the same with the compression ratio bump because the cams and timing remained the same.

[g3ngs]

Originally Posted by arrivederci

Based on rumors from previous owners, my '72 has 2.2S pistons mated to a 2.4T motor. I don't have any paperwork to support it other than a 145 rwhp dyno sheet and the knowledge that the motor has T cams, T distributor (w/ T redline), and a T MFI pump.

Just another data point...

Dyno Sheet
Dyno Video

@arrivederci / Josh

Going through old threads regular trying to work out how to get a bit more out of my 2.4T with MFI (and 123 Bluetooth tuneable ignition/ distributor) and came across something you mentioned a few years ago in the quote.

Very interested in going down the same route of just changing to the 2.2S pistons but changing nothing else. Were you able to ever confirm that the cams, distributor advance and MFI remained all 2.4T? Also did the higher compression of what I'm guessing must have been over 10:1: cause any engine life or other issues.

Thanks in advance

[arrivederci]

Never confirmed on my motor by teardown but based on the paperwork I have and the dyno performance of my motor, all the moons are aligned. It was reliable, torquey, and solid. And I brought it up to altitude (5200+ ft) and it didn't have issues. Full disclosure I have pulled that motor and replaced with a '96 3.6L. So I have that original 2.4L motor pickled under a bench in my garage...not in use any more.

[Henry Schmidt]

A couple of data [points to glean from this thread.
First: Porsche published compression ratios for all early air-cooled 6 cylinder motors is very optimistic. Generally about .5points of compression.
2.2 S claims 9.8:1 is actually closer to 9.3:1.
Head chambers vary about 2 CCs.
Second: compression ration alone offers very little performance enhancement. You need, port size, exhaust, ignition and fuel to truly see a difference.
Engine specifications have a symbiotic relationship. Changing one thing is generally a fools errand.
Third thing is perhaps the most important: measure to verify specs. Don't believe the published data. Check volumes, cam specs and dimensions. These engines are so old that you rarely know what happened before you got there.

[g3ngs]

Thanks for the reply.

Going from that then if the 2.2S pistons are 9.8:1 with the 2.2 crank, with the 70.4 of the 2.4 that would go up to 10.3:1

My reading of that is that would be considered high and need attention. E or S cams then become desirable, changing the space cam to suit required and then twin plugging. Which is a hefty amount of project creep and £$£$

Personally I'm not interested in the benefits or performance of the E or S cam as my predominant use will be street, some motorway (which might be two hour run at 70mph +/- ) and a rare track. For me most of what I want to achieve is all done sub 5,500 rpm.

Keeping the T cams I guess negates changes to the space cam.

And performance aside of the twin plugging my read the benefit is a more complete burn of low RON petrol.

The flip side is that with tunable distributors and the availability of >97 RON everywhere is the received wisdom from the early 2000s still accurate.Though excess heat or a warm day stuck in traffic would be a concern

It doesn't look like you had problems with that set up so I'm very tempted to head down the 2.2S barrels and piston route (particularly as I have a set of Mahles in the garage)

[Henry Schmidt]

Originally Posted by g3ngs

Thanks for the reply.

Going from that then if the 2.2S pistons are 9.8:1 with the 2.2 crank, with the 70.4 of the 2.4 that would go up to 10.3:1

My reading of that is that would be considered high and need attention. E or S cams then become desirable, changing the space cam to suit required and then twin plugging. Which is a hefty amount of project creep and £$£$

Personally I'm not interested in the benefits or performance of the E or S cam as my predominant use will be street, some motorway (which might be two hour run at 70mph +/- ) and a rare track. For me most of what I want to achieve is all done sub 5,500 rpm.

Keeping the T cams I guess negates changes to the space cam.

And performance aside of the twin plugging my read the benefit is a more complete burn of low RON petrol.

The flip side is that with tunable distributors and the availability of >97 RON everywhere is the received wisdom from the early 2000s still accurate.Though excess heat or a warm day stuck in traffic would be a concern

It doesn't look like you had problems with that set up so I'm very tempted to head down the 2.2S barrels and piston route (particularly as I have a set of Mahles in the garage)

Apparently you're not listening. The 2.2S pistons do not produce 9.8:1, period.
Everything you're reading about 2.2S pistons on a 2.4 crank creating 10.3:1 is inaccurate.