2.5 l motor

[steveW]

As ever, I have been thinking about a 2.5 ltr motor for my 2.2T.

The obvious answer is to fit 2.7 pistons etc, but in this part of the world we do not have a pile of bits like you lot.

Now, would it be possible to maybe start with a 2.7 motor and fit a short 2.2 crank in order to make it 2.5.

This would obviously cut out all the machining work, but what are the cons?

[71T Targa]

I could understand making your 2.2 a 2.5, but why would you take a 2.7 and make it smaller? Why not just run the 2.7?

[TerryBPP]

Destroking is usually done when turbocahrging. I think the the Porsche Performance Hanbook has some info on the 2.2-2.5 stroker.

[steveW]

The reason why I don't want a 2.7 is because everyone and his frog has one and in 1971, the biggest engine was 2.5 litre.

The PPH book only tells you how to go from 2.2 to 2.5, but I'm wondering how to get from 2.7 to 2.5.

[Tyson Schmidt]

Yes you can put a 66mm crank in a 2.7 and get a 2.5. You'll need a counterweighted crank though, so your T crank is out. You'll also want the 2.0L rods, since the 2.2 rods didn't hold up well in racing.

This is an awesome motor, and will rev to the moon without trouble as long as you do the above. You'll need custom pistons to up the compression, and to allow the use of some S cams or equivelent.

[71T Targa]

Originally posted by Tyson Schmidt
This is an awesome motor, and will rev to the moon without trouble as long as you do the above. You'll need custom pistons to up the compression, and to allow the use of some S cams or equivelent.

Is there any advantage to this over a 2.7?

[steveW]

Yes - I will be different!!!

My thinking is that it should hold up better than a 2.0ltr or 2.2 being made bigger.

[Wayne 962]

This combo will work (with the counterweighted crank). Uniqueness is indeed something to be desired...

-Wayne

[racea911]

I think a 2.5 will rev up to 8000 rpm. That's both unique, period correct and bad a$$! Why can't you use a regular 2.2 T crank? All the VARA/HSR guys are using the non counterweighted cranks in their race motors because they are lighter and rev faster.

[cstreit]

Wouldn't you also have change out the valve springs, etc... or is the existing 2.0L or 2.7: valve train sufficient for revving "to the moon [alice]"

What is "too the moon" anyway? 8k, 9k?

[MT912RS]

EBS has Mahle 2.5 P+C on sale right now.

[jluetjen]

While the 66 mm crank makes the bottom end of the motor better able to survive high rev's, the top end will still need to be built to match. If you want to rev it "to the moon" (I'm guessing 8000 RPM), you'll need full race valve springs and retainers. You'll also most likely want S valves since at 8000 RPM, the valves are closed for a shorter period of time and I'd be afraid of too much heat building up in the exhaust valves -- thus the sodium filled S valves. You'll also need to port the intake ports out to something like 38 mm's to ensure enough air at 8000 RPM. Furthermore you'll also want to be very careful with your assembly tolerences. Finally you'll need to pick a cam which moves the rev range up to 8000 RPM -- something like a GE80, 906 or RSR cam. The problem that all of these cams are going to offer is that they have noticibly less torque then an E or S cam below 4000 RPM where most people spend most of their time driving.

I'd say build your short stroke 2.5 engine, port the heads to S spec and use either an E or an S cam. This engine will rev to over 7000 RPM all day long. The rest of the top end can be built to normal E or S spec. The E cam will be a little happier driving around town while the S cam will give you a little more peak HP above 4500 RPM. If you're going to be racing the engine, then I'd certainly recommend one of the full-race cams and chances are that your bottom end will be more reliable then a 2.7's in the long run, but you'll also have less HP. Either way sounds like fun to me!

[jpnovak]

Ask Curt about the 2.5. He seems to love it.

If the 2.2 ever gives it up I will be building a 2.5.

I think that destroking of a 2.7 is a great way to build one.

To quote the Duff man...

"[pelvic thrust] Oh yeah!!! [/pelvic thrust]"