2.2 to 2.4 conversion. What pistons?

[whackit]

Hello all. I'm converting my 2.2T to a 2.4. I happen to have a 2.7 crank and rods so that part is a no brainer. I want to build a performance street engine with good reliability and nice even curves, I don't want an S motor. I also have a good set of Biral cylinders. I have always preferred to use mahle pistons in my rebuilds, but I'm not sure what to go with here. I would buy a new set of P/C's if that was the best way to go. I have heard mixed review of JE as they are generally a race piston. It seems that using say a set of 2.2 E pistons is going to give me a CR of around 10:1. I'm worried that that's a little high for a street driver and 92 octane but not sure. I'm open to cams as well but have a decent set of E cams. Any suggestions?

[Grady Clay]

whackit,

Good project but a lot of issues.

First make sure your case is in good condition. If not, it is prudent to find a later good 7R case.

Your ’71 case may need the piston oil squirters installed, the oil bypass mod performed, case savers installed and more. To use the 70.4 mm stroke crankshaft, you will need to machine case clearance for the piston skirt at BDC. If you are going to all this effort, I would modify the case for 90 mm Nikasil cylinders. The 2.7RS are a good choice and can be bumped to about 9:1 CR with some head mods.

Twin plug is always a good option for octane limited street engines.

The 911E cams or “Solex” cams are very suitable.

A new set of 40 mm PMOs will give good performance and be adequately adjustable. Of course I like to do this with MFI.

You will want a front oil cooler and the 1.82:1 engine fan.

You also want to freshen your Type 911 transmission and clutch.

Lots to plan.

Best,
Grady

[kucharskimb]

i guess i did something wrong, then, because i simply replaced the crank and rod set in my 2.2E with a 2.7 set and rebuilt it...

nice even torque curve. dyno'd twice at 172 and 165 RWHP.

webers
crane ignition
when in california, 91 octane...
single plug

[Grady Clay]

Matt,

Count your lucky stars. The entire 4.4 mm longer stroke is at BDC. Porsche did this by making the rod journal smaller and offset to gain the stroke. At the same time the connecting rod is shortened from 130 mm to 127.8 mm. The 2.2 mm change is half the stroke change.

This lets the piston come out of the cylinder spigot by an additional 4.4 mm. The good news is there isn’t an issue with crankshaft counterweight clearance to the piston skirt.

Many (most?) conversions to the long stroke crank find interference between the piston skirt and the case at BDC. I suspect yours is VERY close.

Look at page 136 of Bruce Anderson’s 2nd Ed. Jerry’s knife is pointing at the area of potential contact. This is either close or interference with a 2.0 and 2.2 case. It is almost always interference with a 90-93 mm piston.

Some aftermarket pistons have this clearance built in. The trade-off is they aren’t as well supported in the cylinder.

Best,
Grady