S50 crank, m20 stroker questions

[downforce22]

So I am looking at stroking the m20 I have. I have forged pistons and rods and am really just double checking what needs to be done to make this work.


I have found a few things that need to be done:

-Rods+Pistons changed
-Oil Seal on the front of the crankshaft
-Machined Intermediate Shaft
-Longer Crankshaft bolt.

The pistons were designed to be a 3.0L 9.75:1 with a 89.6mm crank stroke. I have turbo plans down the road and have been looking at possibilities that change the CR and displacement. It should currently have the 75mm stock crankshaft.

My plan is to use the S50 crank with these pistons and rods which will lower compression while still increasing displacement.

Here is the current motor plan:
s50 crank
8.16:1 CR
135mm rods
Ross Racing pistons
2.86 Liters Displacement

http://www.zealautowerks.com/index.html?setup=6,m84.00,m206.7,m86,42,-5,m26.9,m135,m1.727,m0,7000,6500,0

I worry that the compression ratio will be too low at altitude and without any boost. That is an effective CR of just above 7:1 here in denver, where a stock m20b25 is at about 7.7:1 in denver. Is this too low?

About the crankshaft I have found the oil seal can be machined out of an m50 hub with a thickness of 0.720 inches:






Can I just pull this piece and a longer crankbolt from an m50 to make it work? Does this piece come on the e36 318?

If that will work are there any other notches that need to be made? And how does it make a good seal, or is it just a spacer?

If that is all good the only other question I have is about the intermediate shaft. What needs to be done to it? Does it need to be machined smaller for the bigger crank, and will it matter that this will be put on a 325iX as I guess the intermediate shaft rotates the opposite direction?

[supernaughtIX]

so you already have the ross pistons? Could you use M54B30 flat-top pistons (~28mm CH?) which will raise your CR a bit?

When using that calculator, you might notice that the background changes colour. Green is good, Black is not so good.

[downforce22]

I already have the Ross Pistons/Rods. I am worried that a CR of 9.75:1 before a turbo will be too much and cause detonation, even at ~6000 ft altitude. Even at 5000 ft the effective compression ratio of the 3.0L ross setup is 13.5:1 (showing green in that calculator) and only 11.1:1 effective compression ratio in the s50 crank setup (showing black in the calculator).

I have been reading that the 885 head's combustion chamber is a little less than ideal due to the hemispherical shape. Would the extra 0.5 Liter (or 0.25 liter in the case of the s50)of displacement work toward or against me?

[downforce22]

What do you guys think of the 9.75:1 setup with boost at altitude?

A stock us m20b25 that most people run for turbo has an 8.8:1 CR and an effective CR of 13.59:1 at sea level with 8 psi without problems.

http://www.zealautowerks.com/index.html?setup=6,m84.00,m206.7,m75,42,-1.721,m34.2,m135,m1.727,m0,6500,0,8

If I were to run the 9.75:1 CR at altitude I would have an effective CR of 13.56:1 at 5000 ft with the same 8 psi boost, and I am higher than that altitude usually.

http://www.zealautowerks.com/index.html?setup=6,m84.00,m206.7,m89.6,42,-5,m26.9,m135,m1.727,m0,6500,5000,8

Obviously changes would need to be made if the car ever did see below 5000 feet, but would you reckon this is a good comparison and safe assumptions about the combustion chamber efficiency, fuel octane, altitude, and Compression Ratio effects?

Keep in mind I have a stock 885 head, no cam or porting done and this setup would be close to 3 liters.

[tinkwithanr]

I would think you should be okay keeping it limited to 8psi or lower due to your altitude. That being said I would be a little concerned in the instance you travel to a lower altitude. It may be worth installing a cutoff switch/electronic BOV that you could activate from the cabin to drop to 0-1psi total boost pressure. Could save you from blowing a hole in those nice shiny pistons.

Also I do think that the 8.0 CR the S50 crank would give you would be way to low off boost, especially at your altitude.

[downforce22]

Thanks tink that is what I was thinking. I don't think it would be hard to have an altitude adjustment, possibly some sort of open dump wastegate valve at 1 psi and an altimeter.

What about the increase in pressure ratios? (12.7+8/12.7) where denver's pressure is 12.7 psi versus the pressure ratio of sea level (14.7+8/14.7)?

1.63 vs 1.54 respectively. Will have to look through maximum boost again and do some calculations of sea level and altitude differences.

[downforce22]

I did some more digging as far as the crankshaft is concerned and found some answers, this may help anybody else who is looking to do this in the future.


The crankshaft bolt does need to be longer, I read 133mm for the longer bolt, here is a picture:


S52 bolt is on the right and stock m20b25 bolt on the left.

The crank nose is slightly different so the oil seal spacer needs to be made:


I read some people welding them and others tolerencing them to fit. What would be best? It seems as a spacer as well as a seal for the oil, tightest fit is probably best. The only thing to look at is whether or not this needs to have some sort of keyway to line up with the crank gear and get the timing right.


I read one guy who needed to machine his intermediate shaft and one guy who didn't need to. Does anybody have any input on this?

Thanks, about to make a decision on which crank and ultimately CR to use.

[tinkerputzer]

The crankshaft bolt does need to be longer

That is correct. I beleive any 24v crank bolt will work. You need to machine down the step on the crank bolt washer too. How much depends on the thickness of your spacer. I had to take .055" off on mine.

I read some people welding them and others tolerencing them to fit. What would be best? It seems as a spacer as well as a seal for the oil, tightest fit is probably best. The only thing to look at is whether or not this needs to have some sort of keyway to line up with the crank gear and get the timing right.

I don't think welding is necessary. I nice precise fit is all you need. I installed mine with locktite for retention and sealing purposes. A key way in the spacer isn't necessary either. Just remove the keys from the crankshaft, install the spacer then put the keys back on.

I read one guy who needed to machine his intermediate shaft and one guy who didn't need to. Does anybody have any input on this?

I had to machine mine down slightly but, I'm using an s52 89.6 mm crank. Only the shoulder of the rod hits the raised text on the shaft and that only happens every couple of rotations. If you're using a s50 86 mm crank it might not even be an issue. You'll just have to see when you mock eveything up.

[tinkerputzer]

Here are a couple of well documented builds with great pictures.

http://www.msportster.co.uk/m20b28-engine-build/

http://www.flickr.com/photos/peters-bmwmarlin/sets/72157625008707209/

[downforce22]

Very nice pictures tink, thank you!

This one sums up the spacer, but page 8 of the first link has more good info and pics


"To fit the 328i M50 crankshaft iin an M20 engine you need to make a spacer. Apparantly this can be made from the M52 vibration damper mounting flange, shown here."

[AlexHale]

Where can you buy the spacer needed?

[SkiFree]

I have them.

[AlexHale]

Do you have the oil seal too?

[downforce22]

I think I just used some kind of sealant when mounting it.

[digger]

i used the loctite for bearing housings. it needs to be sealed but not really structurally (maybe aginst fretting) since the big bolt squahes it between the gear and crank with alot of force