Das Beast: My E30 track / street build

[Old525i]

You mean it slips???

[digger]

boost comes on late?

[ForcedFirebird]

I was thinking the same thing. Typically when a turbo is over sized, the more boost you add, the earlier it will come in. Looks to me like the exhaust housing is a bit large for this engine.

[dvallis]

Yeah, the clutch is slipping in 4th gear at 19 psi. We're running a Clutch Master FX400 which is supposed to 550 ft-lb. It doesn't. SPEC stage 5 should solve it.

Talk to me more about exhaust housing size.

[ForcedFirebird]

Smaller exhaust will spool earlier. Would have to read back and see what your turbo specs are, but usually you can purchase a direct bolt-on exhaust housing to change the pressure ratios. For instance if you have a t3 .83 housing and go to a .63, the heat and pressure will build sooner, in turn lowering the RPM where full boost comes in. The trade off is you may run out of flow for the exhaust to exit at high RPM, but looking at your sheet, it looks to have plenty of flow up top. Basically it will make the power more linear (you have a large "dip" in the lower RPM). This is specially important in a road course car as you don't want big power jumps like that when driving the car to the limit of the tires - imagine exiting a corner and all of a sudden your car has 200 more HP on tap when you are trying to get traction. You have 150hp at 3500rpm and 350hp just 1000rpm later.

[dvallis]

'Bird, thanks for the explanation. Makes sense. We're running a Precision 6262 with T4 divided twin scroll turbine, 0.84 A/R exhaust housing, 62 mm wheel, 3-5/8" V-band flange. Talking to Precision, they offer a 0.68 or 0.58 T4 direct bolt-on replacement (see below) I think we'll go with the 0.58 to get this thing spooling fast as possible. The compressor is rated to 700 HP so will not run out of flow for ~400 HP. Precision gave me these MPNs which are not on the web site:

K-TH5862A: T4 tangential 0.58 A/R 62 mm wheel 3-5/8" flange
K-TH6862A: T4 tangential 0.68 A/R 62 mm wheel 3-5/8" flange

Here's KAMotors dyno for the Canyon Crusher V2, which this whole build was based on from waaaayyyy back when.

[ForcedFirebird]

The layout on that graph is deceiving at first, but when you look at the RPM on the X-axis, you can see all the power comes in at ~5k. Not much fun on a circuit when you really only have ~1000rpm to work with.

The hairpin at Sebring would be a good example. By exiting in 2nd, you don't have much time before the limiter, or use 3rd and the RPM drops to about 3200 when hitting the accelerator, the car wouldn't even really be straight when all of a sudden you have 2x the power. Took me a long time to master that corner when I was in HPDE (one of the other guys used to call my car "the m20 lawnmower" lol).

When I mentioned flow, it's on the exhaust side. Going to a smaller turbine decreases max flow capability - on the exhaust side - so you get in a situation where the power level will flatten out up top since the smaller turbine can only allow so much through it. This is the reason a lot of BMW guys complain about boost creep - and they are using a tiny t3 .58 with a 62mm compressor which should go on a small displacement high revving engine. What you have is the opposite.

[Digitalwave]

Great numbers. Bummer about the clutch. It's always something.

[dvallis]

"M20 Lawnmower". LoL. I'm familiar with the concept.

I think a 0.58 turbine with the power flattened out on top will be fine, as long as it spools faster. 375 HP puts us at the top of ST2 in NASA including a 0.9 penalty for <2600 lb and non DOT tires. I'd rather be there than the bottom of ST1.

What RPM would you expect the power to start coming on for a 0.58 turbine housing?

[ForcedFirebird]

Yeah, I was gonna mention power/weight ratio, but figured you were already figuring stuff out - much less than 9:1 and the cars are a bear to drive. Also, that curve is going to hurt you competitively. You will need average power for your dyno sheets at NASA, and your avg will be high, but it's all up top. Ideally sacrifice upper end power and fluff the curve lower, it will be a better performing car.

You might be familiar with Epic Motorsports. Randy caused the TT rules to change as he put a v10 in his e46 and de-tuned it so it has the max power from like 2000-8000rpm. They used to use peak power until he was sweeping every event he entered, and adjusted his tune to whatever class he wanted to be in (he is also a phenomenal driver to boot, not discrediting that).

Not sure how much faster is will spool, there's so many dynamics in a build, but anything will help your numbers right now. At 4000rpm, your car is only making 18hp more than my n/a 9.4:1 2.7, but your doubles that at in about 1000rpm lol.

[dvallis]

Now we're finally getting back to racing. My favorite book is Mark Donohue "The Unfair Advantage". He was one of the first racer / engineers. I suspect Randy's detuned V10 in the E46 is something Donohue would have loved.

Sounds like I need to get my act together on the power to weight and power band width. Going from 6:1 to 9:1 on every turn would truly suck :-)

[Mushi]

Clutch options now is DKM twin disk for the m5x and do m5x starter.

[dvallis]

PM me a link. DKM says it doesn't fit M20

[zaq123]

Why SPEC? Southband is a better option IMHO.

[dvallis]

I'm still open to options. Have you seen a southbend clutch hold over 400 ft-lbs? If so, which one for M20?