K20A2 Motor Swap

[jwagner]

cooltech - would be good if you started a build thread. I'd like to follow your progress and am especially interested in a reasonable way to put a fuel cell in.

[cooltech]

Yes, for sure.... and many apologies for the hijacking of your thread. Sometimes I can be a bit dense!!!

[jwagner]

Was thinking about HP output and stress on the balance of the drivetrain.  The K20 as I was building it should be good for just over 300HP (crank) running on E85.  (12.7 compression, rods, cams, breathing mods, E85)  That's a lot for a four banger, but the motor should be capable of it and still be reliable.  But what about the balance of the stock Miata drivetrain?  Doing a bit of research confirms that, at least theoretically, a 300HP  four cylinder will put out instantaneous torque similar to a 600HP V8, and I think all the guys running LSs have upgraded the balance of the drivetrain.  This article explains my thinking: Torgue vs. Cylinders   Basically, half the number of power strokes per 4 cyl engine rotation requires twice the torque of a V8 at a given RPM for the same HP output.

I've heard that the Miata drivetrain is pretty reliable up to about 250HP, and that has me re-thinking the 300HP target.  Maybe running a stock K20 with bolt-ons will be a better bet for now since I want something reliable.  Thoughts, opinions, references are welcome.

[Trenon]

That's why I have a rx7 transmission in mine. Check out my build thread for info.

[jwagner]

The general consensus seems to be that the 6 speed trans should be OK up to about 250WHP. The rear end should be good to near that. I think those numbers might be a little optimistic for track use and want to be more conservative. Trailering broken cars home sucks.

We're going to go with the stock internals in the K20 on 93 octane pump gas, add some breathing and a tune since we already have the parts and it will put us on track by the season start. Once it's all sorted out, we'll look at building a nuts K20 with a heavy duty drivetrain, but I want to spend more time in the driver's seat than wrenching this year.

[Trenon]

6sp is a bit silly in a exocet. You're going to be shifting all the time unless you change up the gear ratio's.

[jwagner]

Mar 2, 2017 6:27:50 GMT Trenon said:

6sp is a bit silly in a exocet. You're going to be shifting all the time unless you change up the gear ratio's.

The 6 speed is the stronger transmission.  I really prefer the 5 speed on the street, but the 6 speed is faster around a track, in a Miata anyway.  Have a 4.1 now, I'll be looking for a 3.9 rear end.  That should get me to about 70mph in second with a 8400RPM redline.

[cooltech]

Yes, allegedly the 6-speeds were engineered to be a little stronger than their 5-speed counterparts. The sole reason we are using one behind the K24 - though we are tuning for much less power than you.