What's the best 3.5L block + head combo?

BimRLuvR said:
Scott,

Thanks for sharing your impressions on the M90. I too have an M90B34 with the L block, Schrick 282 Cam, D-Jet with Autonnics DME, and headers (with Pauls cool dual exhaust). This is a torque monster for sure, and with a 265/6 and 3.45.1 diff, I find it smooth and powerful. I love revving it to about 5800, and after that it just falls off. No point in revving more. I also find I drive primarily as a 4 speed, and only use 5th as OD on the highway. I find this a good set up that is a blast to drive. I agree on an S38, looks cool, its a revver, and every one I have seen is always the talk of car shows! Hope this helps add to the discussion! -jim
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Yeah, I have an M90 powered 635 (1980 Euro) that runs a 265/6 and a 3.07 diff. Very long legged car, but the M90 has the torque to make it work. I plan to run a 3.45 in my M90 powered E9. The difference will be running triple DCOE Webers instead of L-Jet. Should be interesting!!
 
ScottAndrews said:
Yeah, I have an M90 powered 635 (1980 Euro) that runs a 265/6 and a 3.07 diff. Very long legged car, but the M90 has the torque to make it work. I plan to run a 3.45 in my M90 powered E9. The difference will be running triple DCOE Webers instead of L-Jet. Should be interesting!!
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Yeah, your build sounds great. I admit I am little jelly on the triple webers. The induction music from them is intoxicating! I wonder what the performance difference between webs and injection (if any) will be?! Send a video when its rolls! :)
 
BimRLuvR said:
Yeah, your build sounds great. I admit I am little jelly on the triple webers. The induction music from them is intoxicating! I wonder what the performance difference between webs and injection (if any) will be?! Send a video when its rolls! :)
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Will do! When I asked @Stevehose what he thought of the idea, his reply was “massive intake noise!” Can’t wait! Good question on performance. The L-Jet CSi system on my 635 doesn’t allow for much tuning, and I think the tunability of the Webers and the fact that this setup basically creates ITBs should beat the L-Jet. Probably not as good as using a Megasquirt with ITBs, but much sexier!
 
I do not know 5150, but Emenegger in Switzerland built 4.2 liter S38s already during the time the M5 E34 was still produced. They are still in business, you may want to have a look at their homepage. There is no English version, but have a look at the "zu verkaufen = for sale" section, you will see a B2 and a Group 2 racecar there:
Emenegger CH
If I would spend the money on such an engine, I would do it with them.
That said, remember the S38 was intended to be a high-revving engine and the B38 is already quite on the edge with its stroke and the resulting median piston speed.
When comparing M90 and M88/S38 also remember that the DOHC engines had different crankhafts and always used the longer connecting rods also in the 3.5/3.6 liter versions (my B11/3 Alpina engine which is M30B35 based also uses the S38B36 crankshaft and connecting rods, combined with the custom pistons for the hemispheric Alpina combustion chambers).
I absolutely agree that triple webers are superior to L-Jet on the M90. But I do not see a rationale to use them on a high-performance 24-valve head... ITBs with EFI will be superior and the sound of pretty open ITBs should not be that less dramatic :)

Edit:
Quick example:
Kolb Motorsport
 
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Clubsport_CSi said:
When comparing M90 and M88/S38 also remember that the DOHC engines had different crankhafts and always used the longer connecting rods also in the 3.5/3.6 liter versions (my B11/3 Alpina engine which is M30B35 based also uses the S38B36 crankshaft and connecting rods, combined with the custom pistons for the hemispheric Alpina combustion chambers).
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Are you sure about the different crank and rods? Certainly the B36 would be different, but the stock M88/S38 have the same bore and stroke as the M90. The stroke is determined by the crank, and for a given stroke, the rod length will be determined by the distance from the crank centerline to the block deck. AFAIK the M88/S38 have the same dimensions as the M90. The head is obviously different, and the pistons may be. It is possible the crank has slightly different oiling passages, but the same dimensions.
 
ScottAndrews said:
Are you sure about the different crank and rods? Certainly the B36 would be different, but the stock M88/S38 have the same bore and stroke as the M90. The stroke is determined by the crank, and for a given stroke, the rod length will be determined by the distance from the crank centerline to the block deck
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Hi Scott - yes, I am sure. I can provide details later the day. In your equation on rod length, the height of the piston is missing :)
You can also check part numbers, they are different.
Disclaimer: I am used to the Euro versions of the engine; US specs may vary...
Kind regards, Dan
 
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For the rods:
Relevant is the length from crank to piston eye - less height of the piston top enables longer rods.
The standard M30 engine has 135mm rods, the M88/S38 has 144mm rods. Part no for the 144mm rods:
11241307653
I do not quickly find my side-by-side pics but surely you can find some via Google.
The 86mm S38 crankshaft is ligther and differs by the shape of the balancing weights from the M30 variant, as far as I remember. But you are right, I have to revise: The M88 should have the same 84mm crankshaft as the M90... Sorry.
 
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Very interesting.

Good point about the piston height. I was musing on this while riding the car rental bus in Seattle yesterday. I had not before thought about the key dimensions that define any given engine architecture.
In terms of stroke, we have:
Crank throw (distance from the crane centerline to the rod journal centerline) Lc
Rod length (distance from the rod journal centerline to the wrist pin center line) Lr
Piston height (distance from the wrist pin centerline tot he top edge of the piston) Lp
Block Deck Height (Distance from crank centerline to the top of the cylinder bores) Ld

And I suppose that Ld=Lr+Lp (unless the piston does not stop at the top of the bore (short or long), which would seem unusual)

The engine stroke is governed by Lc, but as you point out, there are many choices for Lr and Lp.

The smaller crank throw would seem to allow for a faster rate of RPM change because the crank has a lower angular momentum, and I suppose lower piston speed at any given RPM, since the piston moves less distance each cycle.

On the other hand, a longer stroke crank should theoretically have more leverage, and thus higher torque, but oddly in the M90 this is not the case. Must be other fat tors at play here. (shorter stroke means more efficient and uniform air fuel vapor density in the cylinder?

I would also imagine that piston height would have some relationship to wrist pin cooling (taller piston making it easier to cool the wrist pin), and piston weight (taller piston being somewhat heavier (but mated with a shorter rod..so maybe a wash).

I'll have to go study engine design now!
 
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Scott, this is my favorite book about engines:
20260126_214333.jpg

Not sure if it is available in English...

Don't go too far down that rabbit hole - you may end up only thinking about the perfect engine but never build one :)

However, if you want to build a high-performance M90 or M30, another nice feature of the S38 is that it has oil jets in the crankcase for piston undercrown cooling. I can send part numbers.
Also used on the M106 turbo engines, not on standard M30.

For longer rods in sport engines, I cannot explain better than AI:
Longer connecting rods improve engine efficiency and performance by reducing piston side-loading, which lowers friction and heat, and allowing for higher RPM capability. They enable the use of shorter, lighter pistons to reduce reciprocating mass, improve piston stability, and optimize the torque curve for mid-to-high RPM, ideal for naturally aspirated, high-performance engines.
 
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