I have made this point several times when people rave about the S38. It only differs substantially from the M90 (or the M30b35 for that matter) over 4000 rpm, and if we are honest about how we drive, we spend very little time above 4000 rpm. I don't think modifying the firewall to fit an S38 into our cars is worth it. But this gets lots of admiration at car shows.
I have done driving comparisons of three different 635's with three different engines:
1986 M6 with S38 (beautiful car of a friend!)
1980 635 Euro with M90 (my car)
1988 635 with M30B35 (my son's car)
As well as road comparisons of my 1980 Euro and several Dinan M6s. The results are interesting
Starting with my venerable 1980 M90 powered car: This car has enormous low end torque.Even with a Getrag 265 trans and a 3.07 LS Diff, it is able to grunt out of tight corners with ease and the torque stays almost constant all the way to redline. That said, this car is harsh to drive. Noisy and very "mechanical".
The M30B35 635 was super smooth and quiet. The engine did not pull like the M90, but it was very balanced. From a driving perspective, it was actually the nicest of the three. Enough performance to be fun, but so well behaved that it just exuded engineering perfection.
The S38 powered M6 was smooth, and fast, but it actually accelerated a bit more slowly than the M30B35...until about 3500 RPM, when it would just take off. I found the power to be very peaky. Great on the freeway, but hard to make use of elsewhere (more on this below).
The driving experience between the S38 Dinan M6s and the M90 Euro was very interesting. I was driving the M90, and two other guys were driving the M6's. We started in Saratoga, CA, and drove up highway 9 to Skyline Dr. Then along skyline to highway 84 (Woodside road, fr the locals), and then down to coast highway. It is a bit hard to make judgments about the cars/engines with three different drivers with different skill levels and different knowledge of the road, but here is what I experienced.
The road up the hill from Saratoga to Skyline is about 5 miles. Steep and very twisty. It has numerous uphill hairpin turns. I was in the lead in the M90. With all the torque on tap, I was spinning the rear tires on every hairpin, and driving the car in what a friend called "point and squirt".. I got tot he top of the road and literally waited 5 minutes for the Dina's to show up. We then proceeded down skyline. This is a great road with a combination of sinuous rolling flat sections and tight turns. While the M90 did not have as much power as the Dinans, the road was such that they could not really sustain 3500 RPM for very long. As soon as as they got into the power band, the road would turn, they would slow down and then slog out of the turn (some of this may have been limited knowledge of how to keep the revs up on a road like this). The M90 just maintained that point and squirt attitude, brake hard into the corner, shift down and rocket out. By the time I got to Highway 84, I again waited for the Dina's to show up. We then headed down to the coast. Same basic situation as Skyline...But once they caught up with me at Coast Highway, the Dinans came alive Coast Highway has some passing lanes, and I was floored in 2nd and then 3rd as the Dinan cars roared past, as if I was driving a Yugo.. Blazing fast on the right road...
This is all anecdotal. I'd love to see some actual numbers on these engines... If I were not trying to keep a coupe more-or-less original, I'd drop in an M30B35. Solid smooth engine, lower cost and easier to maintain than the S38, and more usable power band.
YMMV.
Scott
If I did it again, I'd get a stock B35 and throw a Paul Burke cam at it to get a 40 Hp and torque bump, stay in the Motronic family and get the same (or better) result. Cheap and easy.
5150autosport.com
metricmechanic.com
