Number crunching

[Firebird Man]

Ok guys, pull off your shoes, you made need fingers and toes to figure this one.

Ok, so maybe it's really not that bad.

 

I run a 68 Firebird in the SE region, with running various tracks I can't afford to swap gears every time I change tracks.

A track like CMP and RRR are fairly tight and technical, while Road Atlanta and VIR are high speed tracks.

Been running an M-20 and a weak 10 bolt 3.08 since I already had them, but I'm changing the tranny and getting a 9" over the winter. Running a 365 Pontiac, solid flat tappet with HRE heads, great engine up to 6500, then runs out of breath by 6800, which is my rev limit anyway.

The sweet spot is about 4000-6300 it seems.

 

Looking at switching to an M-22X tranny, 2.199-1.506-1.174-1:1, out back I'm having a tough call between the 3.50 and 3.70, using 26.7 OD tires. With the 3.70 I'll loose top end at RA, but face it, over 120 and the areo on that car sucks anyway, I'd rather get out in front off the corners, if someone has the kahunas to chase me down at 130-140 and out brake me in another 40 year old car, (think Vintage class) more power to them.

So far I'm leaning toward the 3.70 but the tight 2nd-3rd of the X might work better with the 3.50 where I could pull them a little longer, and 3rd-4th is nothing.

 

Another option is the std M-22, 2.199-1.64-1.274-1:1, little wider gear spread and I am dealing with a Pontiac that has a decently wide power band.

Got any opinions on this? I have to stay with a Muncie 4 speed as this car is NASA and HSR legal.

 

 

[nape]

I'd say probably the 3.50 ratio and the ratio in the transmissions depends on how many slow 2nd gear corners are on the tracks you run. If the X ratio puts you too low in the power band, it would be hard to get a good run out of the slow corners.

 

I've got a 3.64 with a T-10 (2.64, 1.75, 1.33, 1) and only 310RWHP and I almost run out of gear on the back straight at Mid-O (~6700RPM w/ 7000RPM redline). If it hadn't rained at Road America earlier this year, I think I would've had to lift before the end of the straight.

 

If I put another center section together, it'll either be a 3.4x or 3.50 for the big tracks and maybe change the 3.64 to a 3.8x for the smaller ones.

 

PS- If you have the head flow to support those HP numbers, the power falling off abruptly is probably valve float from those RPM with big SS valves. FWIW, I was talking to some NHRA guys (Super Stock?) and they said that Ti valves were the key to higher RPMs on an old Pontiac.

[Firebird Man]

Yep, we think my drop off is from the intake Valve (SS 2.07) bouncing at high RPM with the solid flat tappet. Pretty steep ramp on it.

But HSR has a 6800 rev limit anyway so it was built around that number. Ti valves would be nice, next time it's down maybe, plus I'll change the cam slightly. I'd like to get a solid 7000 out of it, but it is a Pontiac and a solid roller can be murder on Pontiac lifter bores.

Intake flow at max lift, (.565) is around 270 cfm.

 

This gear selection is tougher than I thought it'd be, lots of compromise unless I get two pumkins.

With what I have I can run CMP in 2nd-3rd only. If I moved everything up with the M-22X and 3.70 I'd get into 4th 3 times, the 3-4 RPM drop using the M-20 and 3.08 makes 4th useless there. To get about the same spread at RA I think I'd need the 3.50

[Jim P.]

Since you are going to put a 9" in you should probably get both sets... every year in the late fall in the Mooresville/Charlotte area the local NASCAR teems sell off all of their "used" race parts and 9" center sections are plentiful. You could possibly get a gear set for every track

 

PS. they also sell a bunch of used (rebuildable) tranmissions...