Cheburator

Was reading my book on the greatest 928 rival in the 1980s - the BMW 635CSI - last night and stumbled upon some interesting info...

When BMW went racing, they used a 12v head with a conventional manifold and a single TB instead of the M1 inspired 24v with large airbox and 6 ITB. However, BMW increased the size of the intake valves, ported the intake tracts and fitted a really agressive cam - 324 degrees for both intake and exhaust. The exhaust manifold was also kept stock as per the Group A regulation, whilst the forged Mahle pistons increased compression to 11:1. Naturally, Bosch provided custom Motronic maps...

With the above, the 3.5 ltr engine pumped 285Bhp, up from the 218Bhp in stock form, equivalent to 81Bhp/ltr

Given that I have a Euro S2, a cheap but excellent machinist, who can work wonders with heads, what would it take to get similar specific power output from an S2? Can someone give me the dimensions for intake/exhaust valves on the S2 and also cam duration and lift.

Many thanks in advance,

Alex

mark kibort

You have to look at what the euro M3 was doing in the '90s as well. 330hp out of the 3.2. and this was a street engine! (in europe) . in the US, the same e36 M3 put out only 240hp.

the euro 928S ('80 to 85) was a 4.7 liter but put out 300 to 310hp for the later of the model year. only change was a slighly more performing cam (slightly more duration) and higher compression by a tad.

So, by the fact that the S2 4.7 euro put out more hp than the BMW race engine, comparing 285hp to 310hp, i think you are all ready there.

The easiest way to get the power way up, is to bolt on a US 5 liter bottom end. That will get you to near 350hp. the otherway is to get a more radical cam, and port the heads along with bigger valves. (currently, they are 40/45mm exh/intake) However, with the bigger valves, you might want to alter the piston valve reliefs. This would be a lot more work than just bolting on a $1000 used 5 liter bottom end from 928intl. a $100 set of rings and bearings, along with new seals and you suddenly have a new engine, in stock form will make near 300rwhp (350ish flywheel). Scots did 290rwhp with a US AFM with a 4square inch opening (basically like a 50% restrictor plate compared to a MAF or CIS inlet of near 8 sq")

Vilhuer

They are totally different engines. E36 US model M3 should really be called 330iM or something like that, not M3.

PorKen

Euro S 4.7 was '80-'86.310chp engines also have digital fuel injection and ignition.

mark kibort

Oh, i know the differences. I think the point was that the M3 3.2 liter can make 330hp. The other point is that the euro 4.7 is a pretty powerful motor too.

MK

mark kibort

There is a 10hp diff. made up of, injection, higher compression, and a better cam.
the split of what is responsible for how much of that 10hp is a question that would be tough to answer.
mk

marton

I believe Porsche were more interested in Torque than HP (Let's start that debate again )

The S2 had 310hp versus the S at 300 but the torque rose more than 10% from 263 to 295.

Marton

Cheburator

No, I am not - following BMW's engineers logic, the Euro S2 (LH-Jet) should be 380Bhp instead of 310Bhp.

It is not as simple as that - I have to get the block shipped across to Atlantic to the UK. Then there is custom duties and 17.5% VAT to pay on top. Suddenly the $1000 S3 bottom end is looking like a £1000 part. On the other hand, valves are cheap and my machinist will charge around $600 to rebuild the S2 heads with bigger valves, new guides and seats, and port them. The S2 cams are the most radical fitted in a 928, but in the grand scheme of things, they are quite tame. Moreover, I like a challenge...

Can you tell me what the overall valve dimensions are - not just their diameter please.

Thx,

Alex