[Benno]

I have some general questions about engine modes, larger pistons, stroker cranks, and the interplay between cams, valves, and intake. I am in the middle of an S4 motor rebuild. It's my first motor rebuild and I have lots to learn and even more questions. My plan is to rebuild the motor stock-ish because I am mostly scared of getting in too deep. However, as I learn more about mods I am feeling more comfortable with them.

I need to repair one cylinder. my options are to oversize that piston (1st oversize). Alternatively, I can have the all 8 cylinders bored out 5mm larger and use the 968 pistons for a larger displacement motor.

Here are some of my questions:
1. Will this block (1987/Squirter) support remove 2.5mm from the cylinder walls? I understand the S3 blocks have thicker cylinder walls and are ideal for this. But what about the later blocks?

2. Is a stroker crank required can I use the stock crank (78.9mm stroke) which would yield a 5.4L motor? Is there a downside to expanding the bore without expanding the stroke?

3. Do I need different valve sizes? The S4 and GTS share the same valve size between the 5.0L and 5.4L so I am thinking the valve can remain.

4. Do I need different cams? I would assume I would need different cams to take advantage of the increased displacement. But not sure which direction to go. The GTS cams may be a good option but my understanding is those are optimized for torque which is a byproduct of longer-stroke engines. I could use some education on this topic.

5. Is the stock intake going to be the restrictor here and is adding displacement even worth it with known restrictions in feeding air into the engine?

Thanks for letting me ask these very rudimentary questions, I appreciate whatever knowledge you all can share.

[worf]

Note that I do not consider myself an expert on these topics. What's below are "general" answers I've accumulated over the years.

I need to repair one cylinder. my options are to oversize that piston (1st oversize). Alternatively, I can have the all 8 cylinders bored out 5mm larger and use the 968 pistons for a larger displacement motor.

My understanding is that oversized pistons are effectively impossible to find.

968 pistons are 104 mm so your cylinders would be bored out by 2mm (for a total bore of 104.)

1. Will this block (1987/Squirter) support remove 2.5mm from the cylinder walls? I understand the S3 blocks have thicker cylinder walls and are ideal for this. But what about the later blocks?

There are more than a few 104mm 87+ engines running around. Mostly these are, of course, GB's motors.

I have a 104mm motor on a stand, which I assembled, waiting to go into a 928. So, I've seen these cylinders with my own eyes.

I have discussed concepts with my "machinist" who spends the vast majority of his time pondering these things for high-performance applications. In short: as long as the compression ratio remains more-or-less stock (9.5 to 10.5), it's not a problem. Although there's no way to opine on ultimate longevity, one should assume that the "thinner" walls won't "last" as long as the thicker walls. But, the thicker walls seem to last "forever" as long as they aren't damaged. It takes a long, long time for the AluSil walls to wear unless they are abused. I have a 230k-mile motor on a stand with very good compression numbers. I haven't taken it apart to see what the cylinders look like. I have seen a number of 100k+ mile 928 cylinder bores and none of them were worn enough to worry about.

2. Is a stroker crank required can I use the stock crank (78.9mm stroke) which would yield a 5.4L motor? Is there a downside to expanding the bore without expanding the stroke?

The 968 pistons are different enough that you can't use stock rods and the stock crank. You'd need one or the other if not both. IIRC the 968 pistons are a lot shorter than the S4 pistons, so you end up with a stupid low compression ratio. Wrist pins may be different, etc. But, I stopped paying attention at "stupid low compression."

3. Do I need different valve sizes? The S4 and GTS share the same valve size between the 5.0L and 5.4L so I am thinking the valve can remain.

Everything I've read or heard about the S4 intake tells me that it is the intake that is the limitation (for a normally aspirated engine.) Mark Anderson had a lot to say on this about 20 years ago when he was sinking money into his 928 racing habit. I'm pretty sure Sterling came to the same conclusion around the same-ish time (that was pre-Web when all this was discussion on the PorscheFans 928 (e-)mailing list.) Both of them (MA for sure, pretty sure about Sterling) spent a lot of money with big valves, etc. All to naught more-or-less.

4. Do I need different cams? I would assume I would need different cams to take advantage of the increased displacement. But not sure which direction to go. The GTS cams may be a good option but my understanding is those are optimized for torque which is a byproduct of longer-stroke engines. I could use some education on this topic.

You don't *need* different cams, but it would be a good idea to have different cams or you'll leave a lot on the table. My understanding is that the GTS cams just suck. Always.

5. Is the stock intake going to be the restrictor here and is adding displacement even worth it with known restrictions in feeding air into the engine?

See above with regard to valves. The S4 intake has an upper limit to air flow and it's right about where the GT motor is. You can get more air flow from more displacement until velocity through the intake kills you. So, you can fatten the torque curve 'down-low' in the RPM range where intake air velocity is low, but gains up high are hard to be proud of.

I don't think a bore increase without significant effort on the intake is worthwhile. This, for an NA application.

Taking a look at Sterling's pictures. He's done as much as anyone else has to get more horses out of the 928.

[worf]

I need to repair one cylinder. my options are to oversize that piston (1st oversize). Alternatively, I can have the all 8 cylinders bored out 5mm larger and use the 968 pistons for a larger displacement motor.

I see I didn't really address your room elephant.

You may be able to find one oversize piston. Everyone's looking for them as far as I know though. Strike fast. Strike hard.

One thing you cannot do is use any piston with a 'coating' that's "compatible" with AluSil.

Everyone that I have talked to has told me that that's a recipe for disaster.

"My guy" tells me that the only pistons that work in a Porsche AluSil block are the ones you get from Mahle A.G. (via PAG.) Mahle motorsport pistons' coating comes off and kills the engine. Nothing other than Mahle Germany's Porsche pistons work. This from my guy that works with AluSil extensively.

Last time I was at my guy's shop picking up heads, I had the conversation of "one of these days I will bring you a 928 block with cylinder wall damage." The answer in terms of cost-effectiveness was NikaSil. And at that point you can have any displacement you want because crank, rods, pistons, rings are all non-Porsche.

If you find, or if anyone else has, a different answer I'm all ears.

Hmmm...

I guess I really need to take apart the 230k-mile good compression motor I have and see if it's got good walls. I was hoping to sell it in "as is" condition. I'd like it to be gone. It's a great candidate for a stroker build. It's a better candidate for someone that just needs a good block. But, I don't know if it's really "good" on the inside.

[Benno]

Thanks for the awesome feedback, I really appreciate the insights and opinions. It all makes good sense.

Miraculously, I have been able to secure a single 1st oversized piston. It is currently en route to me. Based on the insights above, I will likely continue with the original path of a mostly stock S4 motor.

[hans14914]

Not sure if it helps, but pretty sure I have a set of eight 944S2 pistons in one of my odd boxes of stuff.

[Benno]

Hi Han's thanks for the message. I saw your personal email so I'll follow up there directly to your questions.

For everyone else: The 944 S2 piston is a 104mm piston. The S2 motor is identical in bore (105) and stroke (87.8) to the 968. The S2 piston has a full piston skirt lip. The 968 does not. They are both the same weight. Using S2 pistons would require boring out the block and replacing the crank with an 87.8mm stroke crank.

[maddog2020]

tis a slippery slope