Porsche 911 2.2->2.5 Conversion

[Leopoldo]

Hi there,
Sadly my engine (2.2e) turned to a habit of losing a lot of oil recently so i figured it is time for a reversion.
I was thinking of pepping it up a little and converting it into a 2.5 short stroke build.
So far I would use Mahle 90mm cylinders, Carrillo rods and would like to put 90mm 2.7 twin plug heads on it (I guess they should fit?) together with s cams and the bigger oil pump. I would also use about 10.5:1 pistons.
I am currently using 40mm Weber’s on it and was wondering if they are big enough for street use and maybe some slight track use.
Has anyone built up a similar setup or can help me out if that configuration is possible?
I was told there could be some vibration problems
Thanks in advance

[stownsen914]

I think the vibration problems are more with the 2.7 motors. The early short stroke motors are known to be better in that respect. 2.7 heads (or have your 2.2E heads machined) are good as you note. Keep in mind that with the shorter stroke, you won't get the full listed compression ratio from the 90mm pistons since they are rated for the 70.4 stroke crank. I think you lose about a point of compression with the 66mm stoke. 40mm carbs will be good for mixed use on this size engine. Have the crankcase checked when you send it for machining for the larger cylinders. It will need case savers for the head studs, and have the oiling updates done too. Should make a nice motor.

[Trackrash]

Originally Posted by stownsen914

I think the vibration problems are more with the 2.7 motors. The early short stroke motors are known to be better in that respect. 2.7 heads (or have your 2.2E heads machined) are good as you note. Keep in mind that with the shorter stroke, you won't get the full listed compression ratio from the 90mm pistons since they are rated for the 70.4 stroke crank. I think you lose about a point of compression with the 66mm stoke. 40mm carbs will be good for mixed use on this size engine. Have the crankcase checked when you send it for machining for the larger cylinders. It will need case savers for the head studs, and have the oiling updates done too. Should make a nice motor.

+ 1

Totally agree.

A couple of thoughts. The 66mm stroke lowers the CR about 1/2 a point. So 10.5 would be about 10 -1. Perhaps using 10.5 pistons with the 66mm crank and thick base gaskets could get you around 9.8 -1 which would be the max for single plug, may be a consideration.

IMO, unless you absolutely ,want to go with the twin plugs $$$$, 9.5 pistons with E cams and 40mm Webers will be a great running motor.

With twin plugs and a higher CR, you would probably want more cam, like an S.
Also I think carrillo rods would be nice, but probably overkill, unless you plan on running constantly at a high RPM, which may not be the best compromise for a street motor.

[Walt Fricke]

I think your 40mm Webers are large enough - you can increase the venturi size as needed. Worked fine on my 2.3, and my 2.5 which Grady Clay originally built. Anderson, in his book, goes over carb sizing - worth your while to have it in your library.

I'd not want to use more than 9.5 for the CR unless I twin plugged it.

I assume you plan to reuse your 66mm crank to avoid the expense of another one? The 2.7 crank can be made to work in high RPM conditions by using 150 lbs/ft of torque on the flywheel bolts, installed with a very careful application of some red Loctite. Porsche never figured this out, but some mechanic in Florida did back when Porsche was racing the 2.5s and having the flywheel bolts back out. The street 2.7s don't have this issue - no reason to rev above about 6,500 rpm (and little reason to do that either with stock cams and intake).

[Leopoldo]

Firstly thanks for the quick responses!

@stownsen914 glad to hear that the vibration issue is not such a problem with the short stroke. Oddly a friend of mine once told me that the long stroke version of the 2.5 litre engine would be better in that perspective which made no sense to me since the pistons have a longer distance to go at the same rpms.
My mechanic suggested looking for a Aluminium case since they are more reliable but I think especially with the oiling updates and case savers the magnesium case should work fine.

Should have stated that in the beginning but I was thinking of using (with the 70.4mm crank) 11.5:1 compression pistons so believed that they should come around with a compression ratio of around 10.5:1 with the 66mm crank, since I want a very responsive engine. So I guess I have to stick to twin plugging the engine.

About the rods: I think @trackrash you are right about the Carillo rods but I do not have any experience on how reliable the stock rods are on such a build or which other aftermarket rods are available so I preferred Carrillo’s to make sure. But I would be very happy about any advice on wether buying different aftermarket rods or reusing the stock rods with an 2.5 conversion makes more sense.

@walt Fricke bought the handbook yesterday should hopefully arrive soon
And I indeed want to reuse the 66mm crank which is installed right now because I would prefer a short stroke setup. Should I use lock title for the flywheel with the 66mm crank also?

Also I am glad to hear that the 40mm Weber’s should work fine!
Do you think for a mostly street engine the s cams would work fine?
I have read something about using 904 cams but figured since that was a race car that they might be too aggressive.

[Trackrash]

A couple of thoughts. I think the vibration issue MAY stem from the fact that the 66mm T cranks are non counterweighted and not well suited for high RPMs. Not your worry.

As far as cams go. You have to understand that cams will determine what RPM range your motor will produce power. Cams also need to be matched to the size of the motor and the compression ratio in order to be most effective.

You should talk to a cam grinder about your plans and get their opinion.

In your case, IMO, in a 2,5 you should not go past a mod S cam if you plan on driving it on the street. S cams will have very little power in a 2,5 below around 5K rpm. You have to ask yourself, do you want power and response NOW when I hit the gas, or wait until the motor starts screaming in order to start pulling.

My last motor was a SS 2,5 with E cams. With those there was not much power until I hit around 4K rpm. I think I could have been able to use S cams, but anything beyond that would have been frustrating on the street.

[jpnovak]

Just like any engine there are many choices to be made depending on what the usage is. First I will have to mention engine character. Don't build a hot cam race engine for the street - Its just the wrong tool for the job.

The 2.5 is a great engine. The short stroke crank, optimal rod length to stroke ratio, piston dwell times all make for a perfect early car combination. Consider your choices for 90mm pistons, twin plug, nominal 10.5:1 CR standard fare.

The Al case is great if you can find (and afford) one. Best to use a 7R mag case. The 2.2 mag case will also work. Make sure to have all oil mods and other updates done to the case. This includes , but is not limited to, oil bypass, case savers on head and middle stud, piston squirters, and (optional) cross drill port.

The 66mm crank is great. The early T non-counterweighted crank works great and has noticeable reduction in mass. I don't think you will spin fast enough to worry about the vibration harmonics. These will show up north of 7500rpm.

Cams. Choose the cam. I am a fan of the mod_Solex grind for street engines. If you run DEs or see track time then step up to a DC40. Mostly track time would take more cam. The higher the rpm the more you should consider to cross drill the crank. An oil pump upgrade would match well here.

Size your cylinder head ports for the cam rpm range. This is very important to get right. Too small and the engine chokes in the power band. Too large and the port velocity will stall at low rpm and leave you with large holes in the power band.

Don't forget to chamfer the sealing surface of the heads (2.2/2.4) when stepping up from 84mm to 90mm pistons. This is already done if you use 2.7 heads.

I think the 40mm webers will be fine but this depends on the rpm range you choose.

[Walt Fricke]

Pauter rods should receive some consideration.

An SC or 3.2 oil pump would be a nice upgrade - you don't need a Turbo, 964, or GT3 pump.