R&M Racing BMW M20 Adjustable Camshaft Gear

[nando]

I want to try advancing the gear and see what happens to bottom end torque. Of course the widest powerband is most desireable, but being stuck with 4.10s (or 3.91s at best), it's advantageous to me to have peak power further up, so I may just leave it. With the same gearing and a stock motor, I was on the limiter a LOT, which can't be good for anything. Being able to hold out a gear is a good thing. :)

[briansjacobs]

with my compression the engine is starving for air, I need to bore the throttle body if I want any more power. My thought process has been with the car being race only I am happy to keep the revs down so long as I make good power down low, that is where the ACG has helped, but mostly I tune for AFR not peak power. I quit counting after 6 hours on the dyno. AFR is perfect from 2500 to 6500. With 3000 rpm of power band, it is damn good on the track. At Homestead it is able to do times in 1:35's maybe a little better next time out. That is mid pack at a current Grand AM ST race!

[nando]

My power actually peaks at nearly 6500 - I originally had the limit at 7200, but after the cam gear installation it felt like it was still pulling, so I bumped it up to 7500. My exhaust may have something to do with the shape of my powerband, and I've seen a lot of variation with the same cams. a 288 should be able to make power to 7000-7500 easily, but you have to design the whole engine that way. My CR isn't much lower either, 10:1.

No billet rockers here. Just inspected Febi's and new stock springs. The head has about 35k on it now. if I were to do it again I'd go with HD springs and IE HD rockers for extra safety. having the deep valve pockets of the MM pistons probably helps - even if I completely float a valve, or break a timing belt, I won't damage the valves or rockers.

[briansjacobs]

im 2.8 288/288, with that 284/272 and the RM ACG (see back on topic) you should have great low end, I imagine power will fall off at 6000 as mine does. I will expect as mine is not on the street at all my compression is a bit higher than yours at 10.5:1, the car will go to 7000 rpm (more like 6800) but the power is gone by 6300 and you must shift so you will likely never see past 6000 other than 5th.

With this low revving motor is was almost a waste to install the billet rockers, but I broke so many in the past I was scared not to.

[nando]

well without bringing this thread too far off topic, it's a 2.8 M20 with a Schrick 284/272

and now with an RM Racing adjustable cam gear. ;)

[briansjacobs]

what motor/work do you have done?

[nando]

there we go :D

I think you just misread when I said theoretical top speed of 159mph, and thought I said I could *really* go that fast. I'm thinking I'd top out at 140-145, but I've never tried. I do rather enjoy hitting 94mph in 3rd though :)

[briansjacobs]

God damn it, you made me go out to the garage and check my log book from September 25/26 2005. That was the last time this car ran Daytona. RPM was set at 7000, we later moved the redline to 8000 RPM but could not get enough air into the motor to achieve 8000 rpms with the stock cam and 11+:1 compression.

now running 288 shrick but with the stroke can get air to run past 7000rpm, i now shift at 6200

[nando]

have you done this in real life or just playing with the calculator?

tires 225/45/15

and to quote myself, since you apparently can't read

theoretical top speed: 159mph

no way you topped out at 136mph at 8000rpm with a G260 and a 4.10 unless your tires were way undersized.

unless your clutch was slipping, in 5th gear you would have REDLINED at 160mph. So either you weren't redlining the car, you were in 4th gear, or you're lying. I'm not saying it's the last one, either.

[nando]

ugh are you not understanding what I'm saying?

You said you hit redline, which was set at 8000rpm, at 136mph with a Getrag 260 and a 4.10. That isn't possible, unless you were running 13" wheels with 19" diamter tires.

How do you hit 159 with 4.10s? 7500rpm (with 235/50-15 tires), that's how. Now, I'm not saying my car will *actually* go 159mph (I know it won't), but I'm not going to hit redline before I top the car out. *that* was my point.

You couldn't have redlined the car at 136mph, if your redline was 8000rpm. It's simple math!

tire diameter: ~24"
final drive ratio: 4.10
5th gear ratio: .83:1
redline: 8000rpm

=136mph

does not compute

[briansjacobs]

BMW's claim was 131mph with at 6500 with a 3.46, how are you going to get to 159 with a 4.10? more power does not give you more speed, it gets you to speed quicker. The extra RPM will give you some more MPH, but you still need to overcome the drag. Hoosier racing tires create some drag, would go faster on the straight with narrower tires but then could not slow for turn 1, or turn when you get there.

You can play with your silly calculator till you are blue in the face, do it in real life and tell me how it worked out for you. I am on a race track either driving or running data 2-3 weekends a month every month for the past 11 years.

[nando]

I want to move to theory, everything seems to work there!

okay, tell me what tire diameter you would require to hit the redline at 136mph in 5th gear, at 8000rpm, with a 4.10 and a getrag 260?

it's not possible. you should have shifted up from 4th, because that's probably what happened, or you never actually hit redline. ;)

edit: I'll tell you. You'd need tires almost 5" shorter than stock for that to happen. I can also tell you I can definitely hit over 130 in 4th gear.

[briansjacobs]

I want to move to theory, everything seems to work there!

[briansjacobs]

I'll tell you what, I have raced the car at Daytona and that was all I could pull out of it. at Homestead with the new motor a little short of 7000 I hit 131, and the speed is verified on GPS data, not the RPM. The old motor stopped making power at 6300 so the 1700 RPMs up to redline really did not accelerate alot. It takes alot more power to get through the air.

my new E36 style mirrors are supposed to improve top speed by 1mph. LOL

[nando]

note the disclaimer at the top of the page, it does not take into consideration rolling resistance, aero, drag....

I would venture to say if I put the car on the dyno and simulate a downhill run to reduce resistance with no aerodynamic drag I may be able to hit 160. However the E30 is shaped like a brick and creates a shit ton of drag (yes that is a unit of measure) and according to my speedometer (this was before I ran GPS data) my top speed was 136 (maybe 138 but not 140)

of course it doesn't, but you said you were on the 8000rpm limiter at 136mph, which is just not possible with a G260 and 4.10s. I know my car won't do 159, but theoretically I could if I hit the limiter in 5th. Therefore the 4.10 is not too short. Hell it might even be too tall. ;)