78mm crank in S2 engine what’s new compression ratio

[944m3]

Hi everyone, long time reader first time poster.

I have an S2 engine that I’m planning to turbo. I want a fun street car and of course on a budget.

I’ve tried to research but can’t seem to find the answer. What would the compression ratio be if I replaced the 88mm crank with a stock 951 78mm crank?

Thanks in advance.

[944m3]

Forgot to mention, using the stock length rods and stock S2 pistons. In other words, everything being equal except crank.

[J-Dub]

That would leave you with 5mm extra deck height, I don't think this is a good idea. Typically deck height is around 1 mm, or 0.040". To effectively lower your compression with this method you would want to get new pistons that are designed to compensate for this or longer connecting rods. Not knowing the S2 engines I don't know what is commonly done or what parts are easily sourced but I do hope you still found this helpful.

Jeremy

[Latitude48]

Isn't an S2 already a "fun street car?" Turbocharging an S2 engine is no easy task to do it right, and the way you are thinking is much less than ideal. Based on some calcs I did your compression ratio will be somewhere around 6.6, which is much too low, and you'd have 5 mm of cylinder wall exposed to combustion gases, which won't be good. Forget this idea! In fact, forget the whole thing IMO unless you are prepared to spend some real money.

[Tom]

I got to about 6.6 - 6.7:1 too if you just swap in a 2.5 (78.9mm) crank. Probably not going to work well.

I essentially turbocharged an S2 block, but used an 88mm crank and lower compression pistons. There have been a few guys who added a turbo to a 3L block on the cheap over the years, with varying levels of success, but for the most part, building a 3 liter turbo required some custom parts and a relatively big budget. Not sure the S2 rods are forged, but don't think so, and not sure about the pistons either. You'd want forged rods and pistons at a minimum.

[944m3]

Thanks for the responses.

I never said this was a good idea haha. But I realize that is way to low. Will need custom pistons.

I’ve owned an 86 NA, pulled and resealed engine new bearings and rebuilt head. I’ve rebuilt a 951 that sat for 10 yrs, spun a bearing and swapped with a used 951 engine and rebuilt turbo. So I’ve played around with these cars for a minute.

I purchased a complete S2 engine really cheap that turned out to be in great condition. Just trying to figure what to do with it. I’ve read a million posts. I guess I have that itch lol. I’ll figure something out. It’s my therapy

[blueline]

Thanks for the responses.

I never said this was a good idea haha. But I realize that is way to low. Will need custom pistons.

I’ve owned an 86 NA, pulled and resealed engine new bearings and rebuilt head. I’ve rebuilt a 951 that sat for 10 yrs, spun a bearing and swapped with a used 951 engine and rebuilt turbo. So I’ve played around with these cars for a minute.

I purchased a complete S2 engine really cheap that turned out to be in great condition. Just trying to figure what to do with it. I’ve read a million posts. I guess I have that itch lol. I’ll figure something out. It’s my therapy

By the way, welcome to Carpokes!! You're fitting right in!

[Bergerac]

Thanks for the responses.

I never said this was a good idea haha. But I realize that is way to low. Will need custom pistons.

I’ve owned an 86 NA, pulled and resealed engine new bearings and rebuilt head. I’ve rebuilt a 951 that sat for 10 yrs, spun a bearing and swapped with a used 951 engine and rebuilt turbo. So I’ve played around with these cars for a minute.

I purchased a complete S2 engine really cheap that turned out to be in great condition. Just trying to figure what to do with it. I’ve read a million posts. I guess I have that itch lol. I’ll figure something out. It’s my therapy

I'm doing this with an 968 engine right now, so very expensive to do it right, hopefully it's worth it. The quality secondhand parts/aftermarket parts are few and far between, nothing like 10-15years ago.

[944m3]

That’s what I’m working out in my head, what can I afford to do right. That being said, I do take inspiration from Peep and others I’ve read posts from. You can do a lot with just stock parts. Won’t win Le Mans but will work just fine for your grocery run and occasional track day.

At the end of the day, it’s just a hobby though, so I’m willing to make a mistake or two.

The other thought that has crossed my mind was selling the S2 engine and just going with a 944S 16v head that bolts on to a 2.5 block. (Yes I know I will still need custom intake and crossover). I can keep the internals I have now, just flycut the pistons for valve clearance, maybe upgrade rods with lighter Molnar rods.

If anyone has one laying around, let me know. I might be interested.

[Latitude48]

You didn't say if you have a car for this modified engine.

Another option would be to sell the S2 head and try and find a 2.7 NA head so you can make an 8V 3.0 (which is what Tom did, but with a complete 2.7 I believe). Mahle makes pistons for this configuration, but they are 0.5mm oversize, so the block will need to be reworked. The 16V head will make more top end power, but the 8V is great for the street where torque is king.