Anything that happens at the bottom of 4th gear will have negligable effect on ET. MPH, yes; ET, no. Concentrate on the things that improve your 60' time.
I did some math on the M-20 vs M-21/22 thing... assumming 3.70 rear axle (which is what I have) and 27 inch tires. M-20 1st gear (2.56) 6483 rpm @ 55mph 2nd gear (1.91) 4836 rpm @ 55mph rev to 6600rpm @ 75 mph 3rd gear (1.50) 5100rpm @75mph rev to 6480 rpm @ 95mph 4th gear (1.00) 95 mph 4374 revs to 6446 rpm @ 140 mph so assuming you loose 0mph between shifts, and 6500 is the redline, you loose about 2050 between 1st and 2nd, about 1500 between 2nd and 3rd and about 2100 between 3rd and 4th if my math is correct. most motors with a 110 or 112 LSA should easily have a powerband of more than 2000 rpm so this gearing should be good on both the track and certainly good on the street. M-21/22 1st gear (2.20) 6584 rpm @ 65 mph 2nd gear (1.64) 4908 @ 65 mph rev to 6418 rpm @ 85 mph 3rd gear (1.28) 5000 rpm @ 85 mph to 6488 rpm @ 110 mph 4th gear (1.00) 5000 rpm @ 110 mph rev to 6446 rpm @ 140 here its dificult to draw a dead on comparison, because to achieve the same rpm with the M21, you are going faster becasue of the ratios. Assuming all is else is equal, with the M-20 trans, you'd get up to about 90 MPH faster, and by then, you are illegal on the street LOL< but the M-21 would be cranking out at the top end of 3rd gear. I' don't know how much this helps but the formula I used was 336x(gear ratio)x(rear end ratio)x (speed) divided by tire size. any thoughts?
Aaron, The math is fine. Now divide 2.56 by 2.20 and you come up with approx 1.16. Multiply that times your 3.70 rear gear and you get 4.31. Crane Corvette just happens to sell a 4.33 ring and pinion that will bolt in where your 3.70s are right now. That will get you the same overall off the line multiplication you have with the M20 with less rpm drop between shifts. Watch ya think about that? :Brow: :3gears:
I recall that vehicle hp reqmt is primarily overcoming inertia and rolling resistance at low speeds (under 30) and primarily to overcome air resistance from 60 MPH on up. I would guess that above 75 or 80 with our old "boxes" that would probably equate to 80-90%or more of the hp reqmt at those speeds (like where the 3-4 shift happens). The drop in RPM from 3-4 at high speed would certainly be more critical than at a lower vehicle speeed. I noticed in my car at Martin with the M20, that when I shifted from 3 to 4 (about 6000), the RPM dropped way down into the 4600 range and only barely got up to 5800 going through the traps. The M21's gotta be better. I run a 4.56 so it shouldn't be a problem with the low 2.20 low gear. The M20 came with the original 3.91 in the car. Olds offered 2 combinations on the 68 Ramrods: M20 (2.56 low) with a 3.91, or M21 (2.20 low) with the 4.33. 4.66 and 5.00 were available at the dealer over the counter. Most big time serious Ramrod drag cars ran the 5.00's.
Considering ones going to be beating on it, I'd pull the original numbers matching "325" case Muncie M20 & put it on the shelf. Then due to large gear drop issue with M20, go back with either a well built mid '65-70 M21, OR have a "325" case Muncie built up with mid '65 & later production M21 gear set. Sourcing... Many many parts "325" casting Muncies are at swappers all over the country with problems, as these earliast 7/8 pin style were the weakest of all Muncie 4spds. Occasionally one will find a deal on good used one, but usually the cluster bore in the case on these 7/8 pin Muncies will be stretched, & often cluster & a gear or two destroyed Front inside of case can be boogered up too, so have to be careful. One can upgrade a "correct" 325 case Muncie by having the front & rear cluster holes machined for later 1" diam cluster. The rear hole will have to be machined on a taper. Have built several such 325 case Muncies for '64 & early '65 GTO buddys, & its a good way to move good 325 cases that always seem to be piling up. Hope this is helpful, hate to see original #'s matching small pin Muncies scattered. :3gears: Roger
the old m20 vs.m21 ratio aaron ,i have to agree with Mark and John .I have 68 nova ss 396 automatic car but I also have a 68nova ss 4speed car I have owned for 20 years..It currently has a warmed over 350 in the car now with a m20 tranny and a 3;73 gear.i didnt notice the the drop in rpm as much with a 4;56 gear;but with the 3;73 gear it has alittle trouble gaining the rpms up in high gear.the m20 runs just fine with some gear;but this is with a car that has headers and good size camshafti.I am curently gathering the parts for a factory stock 396/375 motor to put behind the 4speed car and i think I will try a 4;10 gear with the m21 first.chris Teed AKA mrbigdaddynova
Aaron, I think you should do the exhaust, get a good mandrel bent kit with an X-pipe. It seems the trans has gotton a lot of attention, in which I wouldnt worry about it. It might cost you some time but there are bigger fish to fry in the case of that exhaust. I would try to get in with the "bass boat" guys and see what they do to the suspension to get them to hook. Donny is a guy with an L79 vette who was very nice to me over the internet. I think you have plenty of engine to get a 12.9 is you do all the suspension tricks and get a good exhaust. Also, I would take this advice from all these guys, they are all really good at pure stock racing. It just seems you are disagreeing with alot of the advice, and that is not a good mentality to have with folks that are trying to help you. Good Luck,
