2.5 liter engine build - am I on the right track? (Cross-post from Rebuilding Forum)

[Willem Fick]

Hi folks,

I have a matching numbers '70T that I would like to add a little "fun" to. Though probably not too valuable in the grander scheme of things I want to leave the car as unmolested as possible so that I can revert to original at some point down the line. I therefore do not want to do any mods to the engine, but rather put that into long term storage and get something nicer to substitute for a while. In this light I also want to keep the 911 transaxle, which limits me to the amount of power the engine puts out (180hp?)

I have looked into the options of a 2.7 and even a US spec 3.0SC, but deep down I really want a 2.5 "stroker".

So;

I happen to have enough good parts lying around to build an engine around the following:

• 2.7l Alusil P&C
• 2.2l S crankshaft
• 2.7l Case complete
• 2.7l Heads

I have heard that it is possible to put these together as a 2.5l stroker, but see that this is mostly done with Nikasil cylinders and special LN high compression pistons.

So a couple of questions:

• Can what I want to do be done with what I have?
• If it does work, what effect will the lower compression ratio (I would guess around 8.x:1) have?
• What sort of power output range can I expect?

Thanks,

Willem

[Jack Stands]

Can't help too much, but I've always understood that the combination you're contemplating has a very low compression ratio and does limit the horsepower considerably.

You might want to contact LN Engineering to see what they have available as I think the proper pistons would make a big difference. When they were racing these 2.5 liter engines back in the 70s, they were screamers.

[tharbert]

I'm not sure this is totally accurate... I played with the summit racing compression calculator with 90mm pistons, 70.4 crank, 1mm deck, 1mm gasket and then not knowing the exact numbers, I just played with ballpark numbers for piston dome (28.5cc) and head volume (75cc) for a c/r of 8.52:1. Then, I simply changed the crank to 66mm. Compression dropped to 8.05:1. So, your estimate is close.

Are you planning CIS? If stock, the pistons should be for CIS? That would also complicate the equation.

[Willem Fick]

Thank you very much for your feedback! The link is great and gives me a good starting point!

As for your question; No, I will not be running CIS. I will either be running carburetors, or custom throttle bodies with EFI.

Willem

[Trackrash]

Your parts you have will make a SS2,5. Short stroke. If your pistons are for a CIS motor you will be limited on your cam choice.

To make it work you will need pistons that will give you decent compression and work with old school cams.

Do you have rods to go with the crank?

My motor is based on a 2,2 T. It has 90mm Mahle RS pistons and cylinders. With the 66mm crank it is 2515 CC. I manged to get 8.4 to 1 CR by leaving out the cylinder base gaskets and had the heads flycut 1 mm. Back in the 70s that wasn't so bad, but higher would be better for sure. On the positive side I can run regular gas.

With ported heads, a sport muffler, E cams and Webers it makes almost 200 HP.

I wouldn't be too concerned about the 901 trans, as long as you don't make a habit of doing burn-outs.

I autocrossed mine for years with no ill affects on the trans. I NEVER popped the clutch, however. 4 grand, feather the clutch, then floor it and take foot off clutch. Never had a problem.

[Willem Fick]

Hi Gordon,

What you have is exactly what I was thinking of building save for the fact that I have an "S" crank which though heavier than the "T" will give me nice revability. I want an engine that is reliable and streetable, but that will allow me to vent my midlife crisis when I feel like it. My car will be as lighweight as I can get it without blowing the budget, and assuming 900kg 200hp should do just fine!

Oh yes, I have 2.2 rods.

Given that the 2.2 has a shorter stroke than the 2.7, did you shorten the cylinders at all?

Warm regards,

Willem

[Trackrash]

I did not shorten the cylinders. I did leave out the cylinder base gaskets, and the heads were flycut 1mm (.040"). That is as far as I could go. At that point my piston to head clearance was at a minimum and I had to fudge the cam housings to get them to fit, since the motor was essentially narrower than stock. The other problem when you move the heads closer together is the cam chains gain more slack.

Your best course of action is to find or have made pistons that will give you the compression you want with plenty of valve clearance for your cams, and that will stand up to your highest RPM.

My motor has Mahle RS pistons and 8.4 to 1 compression ratio, E cams, Webers, and 35mm intake ports. My power band is from 4500 to 6500 rpm. It still has some torque left at the limiter at 7300 rpm.

Before porting the heads the motor really didn't come alive at higher revs. After I opened up the intake ports it really ran great. I ran T cams for many years and it ran fine. With that combination it came alive at 4000 rpm and pulled like mad till 6000.

I switched to E cams a few years ago and it really woke it up at around 5000 rpm. I've been tempted to switch to S cams, but have another motor in the works so that will be where my future efforts go. My 2,5 has been a great motor. I built it in 1984 and it has only needed routine maintenance to date.

[Willem Fick]

Thanks for that Gordon!