2.5l Motor - What is it?

[Kevin G.]

I've seen several references lately to a 2.5l motor, apparently based on a smaller, early case. Just wanted to find out what is done to make one of these, and why it is desirable. Is this an application that can use MFI, or do you need to go to carbs?

On another track, I recently checked the compression on my 2.2S motor, and had readings averaging 145 psi. Total difference was within the 10% recommended specification, so I wasn't concerned, but reading that 3.0l compressions seem to be in the 160 range I have gained concern that mine were too low for a 9.5:1 motor. Rebuild was done about 10K miles ago, and I used new Mahle nikasil cylinders with JE pistons. I took great care in making sure that the ring gaps were properly offset on installation. Do I have good reason for concern?

Thanks in advance for the information!

[Scott Clarke]

Kevin-
There are several ways of arriving at 2.5. Porsche made 2.5 motors for race applications in the early '70s. There was a 85x70.4mm motor using the 2.4 crank and rods, and a 89x66mm version that utilized parts from the 2.2. 2.5s are often created by using 66mm cranks (from 2.0s or 2.2s) and 90mm pistons and cylinders from 2.7 motors. Often these tend to be fairly low compression, as the reduction in stroke decreases compression ratio. It is possible to buy high compression pistons (rated 10.5:1 for 70.4 stroke) and install them on a 2.2. This will result in a short-stroke 2.5 with a favorable compression ratio (9.5:1 or therabouts). This is exactly what I'm currently doing with my 2.2 E. For me, it allowed the use of a good crank and rods, while providing new pistons and cylinders to replace mine, which are worn. See previous threads concerning bore vs stroke (search using "bore vs storke." The advantage of a short stroke motor is that the piston speed is slower for a given rpm than for a comparably sized long stroke configfuration. This allows a (theoretically) higher redline, and hence more power.

Don't worry about your compression figures. Compression tests are contingent on many factors, including valve overlap. Assuming you have an S cam, valve overlap is likely responsible for your low figures. 3 liter motors have cams with very little overlap. Wayne's new book details this issue well.

[Kevin G.]

Thanks for the reply, Scott.

Now I wish I had known that when I rebuilt the motor 2 years ago and replaced the P/C's with the 2.2S equivalents. What kind of horsepower would this have given me?

OTOH I imagine that something would have to be done to the MFI pump as well to address the change in volume, so maybe the opportunity wasn't totally missed. As it was, the rebuild taxed my bank account even though I did as much of it as I could myself.

Are you going to have to change your stacks and throttle bodies on your 'E' to do your mod? Just curious, because I have a complete 2.2E MFI setup in my spares package, and if I decide to do this change later in life, I can do any needed modifications using this system piecemeal before I get involved with tearing the motor down.

[Scott Clarke]

Kevin-
The hypothetical 2.5 would fall between a 2.4S and a 2.7RS, but would have higher compression than either. Since the 2.4S is good for 190hp, and the RS 210hp, it sems that the 2.5 would be good for 200-205hp. As for the pump, I have been told that my 2.2 pump (PO had it rebuit by Pacific) should work with the 2.5. I don't think that there was a difference in MFI pump space cams between the 2.2 and the 2.4, so it should be ok.

My car is a '70, so it had no piston squirters. The only replacement 2.2 cylinders now available are nikasil. This meant that I needed squirters even if I stuck with the stock displacement (nikasil cylinders should only be used in conjunction with squirters). As a result, I elected to use a 7r case, as this provided not only squirters but also the right diameter cylinder spigots. As you may imagine, all this means $$$$, and as a result, I will be using E spec injection (with throttle bodies currently being restored by Eurometrix). If I win the lottery, I will likely be changing injection, ports, and cams to S spec. It will be interesting to see how this motor is. I would guess that it will be very tractable, with good power across the rev range.