Swapping T pistons for S pistons, issues?

[Mr.P 911T 1970]

Forum,
Patient: 1970 911T, 2.2
Status: engine and transmission rebuild. Transmission complete.

Searched everywhere to understand the complications of swapping T pistons with S piston heads (9.8:1). I could find a few hints. In examining the S piston heads, I see the heads are recessed to allow for valve clearance with higher compression, however, what other considerations should I factor:

Do I also need S cams, swapping out my T cams?
Do I need to adjust my valves for S compression? T heads have been remachined flat and hold good vacuum.
Do I need to adjust my stroke for the increase (keeping the T crank)?

Also, I’ve changed out my zeniths for PMOs.

Engine rebuild starts next week...any comments will always be appreciated

Thanks in advance.

[BK911]

Do you have the parts yet?
If not, and your T pistons are in spec, keep them.
If they are not in spec, have your cylinders bored to 85mm and get a set of 9.x cr JE pistons.
Also get a 2.4 crank and rods.
And get an E cam, or something similar.
Keep your heads.
This will make a VERY nice streetable engine with good usable torque from idle to redline.

[yelcab1]

You are trying to make a T engine into an S engine? If so, your heads are not going to be the same.

[jpnovak]

There are a lot of confusion the post descriptions. Let's clarify.

Pistons.
T pistons and S pistons for a 2.2 are the same diameter - 84mm.
T pistons have a smaller dome volume and lower compression compared to S pistons.
S pistons have a larger dome volume and higher compression ratio compared to T pistons.
S pistons have deeper valve pockets to match the S cams that higher lift and longer duration than T cams.
S pistons are designed to go the Biral cylinders (steel sleeves, aluminum fins) to have better cooling than T Cast iron cylinders. More compression = more heat = need more cooling.

Cylinder heads.
The T heads and S heads for a 2.2-2.4 have the same physical casting.
S heads have larger intake and exhaust port dimensions.
S heads will flow more air to match the high rpm performance of the S cam.
The valve lash specs are the same between S and T engines (0.1mm)

Crank.
The 2.2T and 2.2S engine have the same crank stroke (66mm)
The S crank will be counterweighted and rotate smoother at high rpm.
The T crank will be lighter and accelerate faster up to a certain rpm.




Now,
S pistons and cylinders can be installed on a T engine.
This gives you a high compression engine with low rpm torque.
It will improve performance over an otherwise T engine.
This engine will drive like a T but make more torque.

If you are going to install S pistons make sure they match the cylinder.
I agree that an E (or solex, or mod_solex) is a great cam to install.
Your PMOs will require a jet change with the new engine configuration. More power means larger jetting to optimize the fuel requirements.

I suggest you post the entire build spec and ensure compatibility. Engines run as a system. You can't always just swap pistons and expect great things... Make sure it matches all the way through.

[Matt Monson]

Get headwork and cams.

[Mr.P 911T 1970]

Great answers, thank you to everyone. I have the S cylinders (barrels) with the new S pistons. JPNovak, glad you mentioned the solex cams...confirms my research. And thanks for the tip on the PMOs.

Head and valve work was done - took forever but glad to see them back, can't emphasize good machine work. Vacuum checks out

The original T pistons had significant galling/pitting when we pulled them (sitting water?) and we opted for the S's and new barrels.

I'll update with pictures when I get myself into a good spot. Off onto looking for E cams...

H

[911pcars]

S engines also have better connecting rods, so careful about continuous high rpm excursions.

The T heads can be opened up to match the planned Weber carbs. Likewise the exh. ports with early or SSI heat exchangers. Agree. An E cam would be a good match. Is the distributor a Bosch unit or a Marelli? If Bosch, the initial and timing advance curve can be modified to more closely match the S engine curve.

The CR bump might be marginal with our current gas. At 9.8:1 and the long duration S cam, the effective CR was lowered to offset potential detonation issues, especially with available fuel grades. With lower duration cam timing, you'll still need premium fuel.

[Trackrash]

Good info here ^. IMO your best setup would be Mod E or Mod Solex cams with S pistons. I believe you can have your T cams reground to E specs.

Open the intake ports to 34mm, or so, otherwise you will not gain much top end power. AHIK. Keep in mind that the valve size is the same for all the 2,2 motors.

Make sure to match the intake manifolds to your intake ports.

Use a sport muffler and K&N style air cleaners. Otherwise you are leaving HP on the table.

Recurve your distributor to what the S had in '70.

Buy and install an air fuel gauge. Otherwise you are shooting in the dark on your cab setup.

[tk32]

Mr. P I sent you a pm on a set of cams I have in the FS section that may... or may not be up your alley. Great T to S info on this thread

thanks

[targa72e]

I built a 2.4T MFI engine in to a S piston E cam engine. The S pistons have the advantage of more compression and being forged. Power ended up being about what you would expect, half way between the 2.4E and S. Here is a dyno of HP at the wheels from when I sold the engine.

MFI 2.4 rebuilt

A 2.4-2.7 crank would give a nice bump in displacement and compression. I would look at Mod solex or Mod s cams. Might want to open the ports a little for top end power.

I did a comparison of factory output for the 2.4 engines in link below. You can see there is not much penalty for bigger ports and cams.

The exageration about peaky S engines.

One thing to think about as you add more power is cooling. You may need to add a front cooler.

john

[BK911]

Originally Posted by targa72e

I built a 2.4T MFI engine in to a S piston E cam engine. The S pistons have the advantage of more compression and being forged. Power ended up being about what you would expect, half way between the 2.4E and S. Here is a dyno of HP at the wheels from when I sold the engine.

MFI 2.4 rebuilt

A 2.4-2.7 crank would give a nice bump in displacement and compression. I would look at Mod solex or Mod s cams. Might want to open the ports a little for top end power.

I did a comparison of factory output for the 2.4 engines in link below. You can see there is not much penalty for bigger ports and cams.

The exageration about peaky S engines.

One thing to think about as you add more power is cooling. You may need to add a front cooler.

john

I had a very similar engine in a stock 72 targa, and then in a gutted out 73 coupe track rat.
Was by far my favorite engine.
Can only imagine it in a 914.
Vroom vroom!!

[targa72e]

It worked well. The 2.4 E cam engine was good in the 914 because it did not need to have a front cooler. When I sold the 2.4 engine I installed a friends engine for a while, 2.7RS pistons with Mod S cams and webers. This was a very, very fun engine in the car. Dyno in link below (same dyno as 2.4 above). Could only drive on cool days as it would get hot with no front cooler,

911 Engine for sale

Just installed another 2.4 in the 914. This one is 9.5 to 1 JE pistons, E cams and head opened to S specs (36 I, 35 E). This engine has a set of modified Zeniths. I will dyno it once I get some more miles on it. I would have used bigger cams but trying to keep from having to run front cooler.


john

[911pcars]

Porsche finally realized that 2.7 liters was the threshold to justify an external cooler, at least for the US market. For a few years they tried to mitigate the excess heat with the Trombone "cooler" (a section of bent oil lines) until they finally relented and installed a proper Oil Cooler.

If an engine smaller than 2.7 is modified for more power (e.g. early S engines), that cooling threshold is lower.

Sherwood