2.4 into 2.5....
 

Just quick question, i have a 1972 2.4E MFI engine, it's getting a bit tired and, if my next bonus is good enough, I'm going to rebuild it.

My question, what do i need to do to convert it into a 2.5 ST replica engine? Will everything (MFI, cams, pistons, case) need to be replaced/ modified?

Thanks in advance

hmm.. a 2.5 ST engine is a strange beast. There were 2 forms, the long stroke form and the short stroke. The long stroke used the 70.4mm crankshaft and 86.7mm pistons. The short stroke used 89mm pistons and the 66mm crankshaft. Both sets of pistons are rare or expensive. It would probably more cost/HP out of a 2.7 RS engine.

Hope this is a good start.

Thanks Tim,

This kind of explains why I couldn't find anything on this BBS with regard to building a 2.5 engine!

I don't have a 7R or 5R case so I think a 2.7 would be out of the question, maybe I'll get some S cams instead.

For what purpose is the engine to be used? Is it for a race car or for a street car?

Explain what you mean by getting tired. Are your compression numbers down?

The reason I ask these questions is that you already have argueably the best street/performance motor for a Porsche under 2.7 liters. S cams will actually rob you of the pep in the low RPM range. The E cam not only feels quicker but is quicker 0 to 60mph.

You might just try an increase in displacement from 2.4 to 2.5 by boring out your cylinders from 84mm to 86mm and installing some JE pistons to get 2.5 liters. This would not require cutting the case spigots, would be cost effective, and would be my preference for a street car rather than a 2.4S motor.

If you want some real torque then cut the spigots to 97mm so you can by a set of 93mm Mahle P+C's. This will give you a 2.9 motor but will require reworking of the MFI pump, ports, stacks and will be quite expensive when all said and done. It would probably be cheaper to buy a 3.2 and install it.

Bobby,

Yes compression figures are going down, the car is still running fine but int he next 6 to a year it will be needing some TLC. The car will be street use only, with the occasional track day/sprint etc.

I agree that the E is a sweet motor, I just thought, as I'll be ripping the engine to bits I may as well upgrade it at the same time.

Would increasing the dispalcement to 2.5ltr require an MFI retune?

Luke;
I agree with Bobby. When you have the engine apart, get some 85mm pistons from JE with ~9.5 - 9.9:1 CR. Since you have an E; I believe that your cylinders should be Byrol which should allow a 1 mm overbore for fairly small $'s.

As far as changing cams, I'm not convinced that you'll get a huge benefit by changing the cams since your intake porting is limiting the HP. Making the engine bigger will get you more torque, but most likely not much more HP. That is unless you open the ports some (say to 38 mm across). Adding a more radical cam like an S cam with your current porting will reduce your usable torque around town (lets say at rev's below <3500 RPM), but give you noticably more torque from ~3500 to ~6500 RPM after which it will tail off. The end result will be a peakier motor without much more HP and most likely not worth the trouble unless you wanted to get a close ratio gear box and dedicate the car for track use. It definitely would be a lot less fun for street driving.

Luke-
Such a small displacemment increase shoud not require much MFI work other than, perhaps, adjusting the mixture.

I recommend that people buy a core 2.7 and go from there if they are interested in displacement greater than 2.4L. If you want to stick with the current case, I would upgrade your pistons, cams, and pump to 'S' specs...

-Wayne

Thanks Guys,

you've all given me options to consider, I'll let you know what i decide!