Which is better, the SOHC or DOHC 2.5RS?
In stock to medium modified vehicles, it's my opinion that the SOHC is a better motor. It makes more power earlier in the RPM range, and is more reliable since it lacks the infamous "head gasket failure" of the DOHC. In a heavily modded vehicle, however, the extra control of DOHC can be a big advantage. Here is a dyno comparison between a stock SOHC and a stock DOHC 2.5RS:
Obviously by looking you can see the SOHC has better midrange torque and power, but the DOHC holds power to redline a little better.
A little background on this graph:
Quote:
A little while back a group of us NESIC types got together for a dyno day at Adrenalin Motorsports in Massachusetts. I dyno'd my '97 Legacy 2.5GT 5-speed sedan in stock configuration (well, stock + K&N drop-in filter and synthetic fluids), and Marc Sawaya dyno'd his '00 2.5RS 5-speed also in stock configuration.
We all know that the SOHC is supposed to have better mid-range than the DOHC, and I wanted to see how true that was. So, I finally got around to reading the numbers off of my graph and off of Marc's graph to create a combined DOHC vs. SOHC graph. I did actually cheat a bit on my graph - the day that Marc dyno'd his car (and hit 104.x hp) my car dyno'd at 100.6hp or something like that. On a previous day, also in stock configuration, and on the same dyno, I had dyno'd at 104.x hp. Knowing that (theoretically) the peak power should be the same on the two engines, I used my graph from an earlier day so that the graphs would "match". Who knows, maybe in real life the SOHC cars actually do make a few more HP than the DOHC engines, as was indicated on the day we dyno'd at the same time. Or, perhaps the few extra HP that Marc's car laid to the rollers was due to the fact that my engine has twice (?) the mileage and hence is a bit tired? I dunno. I just figured for a "neater" comparison I'd use the graphs that had virtually matching peak power outputs.
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