DDK - Die Deutschen Klassiker

I'm looking for a set of six good usable circa 90mm pistons and cylinders to drop into the 2.2E engine I have, in anticipation of the original Biral cylinders being corroded when I get into them. I understand widening the stud holes for the 2.2 studs can be hit and miss so I'm reluctant to buy new when they may potentially be weakened by the machining.

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Do you realise that if fitting 90mm pistons you'll end up with quite a low compression ratio ?


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Lightweight_911 wrote: ↑Thu Apr 18, 2024 1:01 pm .

Do you realise that if fitting 90mm pistons you'll end up with quite a low compression ratio ?

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No, I was expecting it to be higher. This is on a 66mm crank. Am I being daft (as usual)?

Not sure where I can I see the rod length and pin height for the 2.2E vs a 2.7. I presume it depends which 90mm pistons I use too?

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The 2.7 litre 911 engines have a stroke of 70.4mm so if using the 90mm pistons with a 66mm stroke, the swept volume - & hence the compression ratio - will be (significantly) lower.

For instance, if using std 2.7RS pistons with a 66mm crank you'll end up with (from memory) a CR of ~ 7:1

Thus to get any benefit from using 90mm pistons in a 2.2 litre engine you'd need higher compression versions (eg JE pistons) ...

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Lightweight_911 wrote:.
The 2.7 litre 911 engines have a stroke of 70.4mm so if using the 90mm pistons with a 66mm stroke, the swept volume - & hence the compression ratio - will be (significantly) lower.

For instance, if using std 2.7RS pistons with a 66mm crank you'll end up with (from memory) a CR of ~ 7:1

Thus to get any benefit from using 90mm pistons in a 2.2 litre engine you'd need higher compression versions (eg JE pistons) ...

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Ah cool, thanks. So used barrels/cylinders are of use to me but I'd need modern pistons.

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Why not stick with barrels that don’t need the cases and heads machining ? A high compression 2.2/2.3 with let flywheel etc is a wonderful engine

Agree, that ^^^ is a much easier engine and risk-free Rob.
I thought a '2.4' conversion to a 2.2 engine was a bit meccarno? Could be wrong, been a very long time since I thought about 2.4's as opposed to 3.2's.

I think the Dempsey book goes through a lot of the common tune-up configurations?

911hillclimber wrote: ↑Fri Apr 19, 2024 9:32 am Agree, that ^^^ is a much easier engine and risk-free Rob.
I thought a '2.4' conversion to a 2.2 engine was a bit meccarno? Could be wrong, been a very long time since I thought about 2.4's as opposed to 3.2's.

I think the Dempsey book goes through a lot of the common tune-up configurations?

The bigger crank is plug and play I think, but a bigger bore engine is more what I'm looking for.

Lightweight_911 wrote: ↑Thu Apr 18, 2024 1:01 pm
Do you realise that if fitting 90mm pistons you'll end up with quite a low compression ratio ?

ok I did some calcs and notionally the 2.7 pistons must be 1.908mm shorter than the 2.2 pistons. That assumes the cylinder heads are the same (anybody know the truth of that?) If the heads are indeed the same then the "average unswept length" of the cylinder will all go with the pistons. If the heads are different then all bets are off in terms of making a theoretical calculation.

Retaining that assumption that the heads are the same, the "unswept length" of the 2.2E engine would increase from 8.15mm to 10.06mm, decreasing the compression ratio from 9.1:1 to 7.5625:1, a little lower than the 2.7, as you said. Of course, practice trumps theory.

Based on that calculation, taking 2.75mm out of the deck height would convert that into a 10:1 compression 2.5 engine. Can the 911 engine take that kind of modification? I'm not really seriously considering it, just curious really. I'd expect to run into problems with the pistons coinciding with the valves, since they're not designed to fit that tightly.

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I thought you were originally asking for a set of 90mm pistons & barrels for a 2.2L not a 2.4 ?


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Lightweight_911 wrote: ↑Mon Apr 22, 2024 9:08 am
I thought you were originally asking for a set of 90mm pistons & barrels for a 2.2L not a 2.4 ?

Yes, sorry my bad now edited. (Same pistons tho I think;)

Here are the calcs