[HO Kenny]

My 68 350HO automatic convertible Firebird was an ok performer when I got it. It had 3.23 gears and a th350. luckily the useless 2 speed automatic was long gone.

The first rebuild I had put way too big of a cam. It was an early version of the Crane fireball. With Headers and a recurved distributor, It pulled great between 5000 and 6000 rpm. That’s when I learned about spun bearings.

On the second rebuild I installed the comp cams hi energy 268. The transition from cast bowl to machined opening was terrible. I spent some time cleaning up the ports on the heads. The result was a much wider power curve. It would pull to 6000 but I didn’t need to. I would shift around 5300. Below 5100 the tires would just spin.

I also installed a m21 tranny. The tall first gear wasn’t the best for getting off the line. For street racing it was great though. You didn’t know if the other guy was going to race so it was usually a rolling start. So slip the clutch and floor it. The Pontiacs wide power band and torque surprised a lot of “faster” cars.

We didn’t have a track. We had a stop watch and a painted 1/4 mile on the old jail road. It was in the low 14s at about 3000 ft elevation.

I don’t think the smaller valves held it back after the “cleanup”. It pulled to 6000rpm in top gear once. That worked out to about 140 with the big tires I had on the back. It actually didn’t take that long to get there. But even with a good tight suspension it was a bit floaty. I wouldn’t do it again. Times were different in 1980.

[thews]

Originally Posted by HO Kenny

My 68 350HO automatic convertible Firebird was an ok performer when I got it. It had 3.23 gears and a th350. luckily the useless 2 speed automatic was long gone.

The first rebuild I had put way too big of a cam. It was an early version of the Crane fireball. With Headers and a recurved distributor, It pulled great between 5000 and 6000 rpm. That’s when I learned about spun bearings.

On the second rebuild I installed the comp cams hi energy 268. The transition from cast bowl to machined opening was terrible. I spent some time cleaning up the ports on the heads. The result was a much wider power curve. It would pull to 6000 but I didn’t need to. I would shift around 5300. Below 5100 the tires would just spin.

I also installed a m21 tranny. The tall first gear wasn’t the best for getting off the line. For street racing it was great though. You didn’t know if the other guy was going to race so it was usually a rolling start. So slip the clutch and floor it. The Pontiacs wide power band and torque surprised a lot of “faster” cars.

We didn’t have a track. We had a stop watch and a painted 1/4 mile on the old jail road. It was in the low 14s at about 3000 ft elevation.

I don’t think the smaller valves held it back after the “cleanup”. It pulled to 6000rpm in top gear once. That worked out to about 140 with the big tires I had on the back. It actually didn’t take that long to get there. But even with a good tight suspension it was a bit floaty. I wouldn’t do it again. Times were different in 1980.

I have a 69 350HO 4-speed car with a stock 350HO. I have both an M20 and M21 transmission with a stock 3:23 posi rear. Would I want the M21 in it? How would it drive differently than the M20?

[Ponchy]

Different gear ratios, so the shift points would be at different speeds.
It might work to your advantage, it might not.
....this is where Bench Racing comes in handy.

Ponchy

[unruhjonny]

in my opinion, with 3.23:1, the M20 would be hands down the better choice...

[Ponchy]

I'm not so sure....in a heavy car, yes, but in a lighter car...he might be able to ramp up the initial acceleration with slightly less gear.
I'm betting that 350 HO has a pretty flat torque curve.

Ponchy

[thews]

Originally Posted by unruhjonny

in my opinion, with 3.23:1, the M20 would be hands down the better choice...

Thanks. This is a project car and I've got almost everything i need to put it together... except time. I looked at the build sheet and it actually is a 3:55 rear. If I remember correctly the M21 was used above 3:55. If I use the M21 (it's completely rebuilt), what would the difference be driving it vs. the M20? Is the top end the same and it just shifts at different points?

[johnta1]

4th gear is the same, so differences when driving in high gear.
1st is a higher gear than M20 (2.54 to 2.20 on M21)
It would be 'doggier' on takeoff unless taking off at a higher RPM.
2nd & 3rd or also a little higher than the M20, but once moving probably not real noticeable.

[Ponchy]

....also, the taller gears of the M21/M22 tranny's are a little stronger than the M20's (physically larger teeth), but considering the HO's power range, you'd be fine (strengthwise) with any of those of tranny's.
When figuring out ratios, I always go by the fact that the M22 / 4.10 rear end combined for 9.02:1 total first gear reduction.
Your 3.55's, with the M20's 2.54 first gear combine for an almost identical 9.017:1 total first gear reduction.
If your M20 needs rebuilding, it might be worth your while to spend the money on it.
So I might've been a bit premature in doubting unruhjonny's post about the M20 being a perfect fit for your rear end ratio (even though at that time, it was believed you had 3.23's).
...public apology to unruhjonny.

Ponchy

[unruhjonny]

no need to appologize.
we are all entitled to opinions, and i really believe it's through the exchange of differing opinions, that the community grows.

[Ponchy]

Thanks for being an adult Jonny. -b

[HO Kenny]

It may have been my imagination, but the close ratio gearboxes seemed to shift smoother.
Has anyone else experienced that as well?

[thews]

Originally Posted by Ponchy

....also, the taller gears of the M21/M22 tranny's are a little stronger than the M20's (physically larger teeth), but considering the HO's power range, you'd be fine (strengthwise) with any of those of tranny's.
When figuring out ratios, I always go by the fact that the M22 / 4.10 rear end combined for 9.02:1 total first gear reduction.
Your 3.55's, with the M20's 2.54 first gear combine for an almost identical 9.017:1 total first gear reduction.
If your M20 needs rebuilding, it might be worth your while to spend the money on it.
So I might've been a bit premature in doubting unruhjonny's post about the M20 being a perfect fit for your rear end ratio (even though at that time, it was believed you had 3.23's).
...public apology to unruhjonny.

Ponchy

Thanks for the input. The M20 looks fine inside, but I bought the M21 because it was completely rebuilt back in the 90's with all NOS parts. The M21 really is an M20, but rebuilt as an M21. When I bought it I got the receipt for the parts and it was over $900 in 1990 money. There's a stamp on top of the case (an A or B if I remember) and they re-stamped it with the M21 letter, but did it sideways over the original M20 stamp. I thought that was odd when I first got it, but after thinking about it, it would prevent anyone from selling the transmission as something it's not originally.

Thanks for the input from all. A four speed is easy enough to install, so maybe I'll install the M20 and if it works I'll sell the M21.