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What is the desired1st gear ratio "1st gear x rear gear"?

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#41 ·
70GS455 said:
After having driven a ~3850 lb BB car on the street for many years and also drag raced it, i can attest that about 10:1 SLR is the sweet spot.

I had a 12:1 SLR SB and would have to shift at the end of the concrete launch pad or the other side of an intersection.

I also had a 7:1 SLR car and was horrible all around. The stock 36 lb flywheel was the only saving grace but still horrible taking off. It wore clutches often


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Those are some good points Scott. I'm thinking that camshaft choice, Stall speed RPM, with auto trans cars, vehicle weight, and even the brand of torque converter will play a major role. For instance, I suspect that if you drag raced a car that was 800 Lbs lighter, that you might have found some very different results. And I mention the torque converter in the case of cars with auto transmissions because I remember when I had a 4,200 lb car, and I switched from the factory torque converter with a 2,300 RPM stall speed and 3.55 rear gears, to a quality eight hundred dollar torque converter with a 3,000 RPM stall speed, (without changing the rear gear ratio nor anything else) it lowered my ET at the strip by .7 seconds. I found that to be very significant with a 4,200 LB car
 
#43 ·
Yep I don't see how using the SLR number will account for camshaft, torque converter, nor stall speed RPM. And of course vehicle weight too. Even the camshaft manufactures often recommend certain rear gear ratios with their more aggressive cams, regardless of transmission choice, and minimum stall speeds with auto trans cars.
 
#44 ·
71454Chevelle said:
It's been about 10 years since I did the the ST-10 to Tremec Magnum swap but if I remember correctly the Magnum was 7-8 inches longer the ST-10. The driveshaft was a custom piece built buy a local shop here in Indy.........3.5" chromoly steel with a length of 46 inches. I checked into driveshaft critical speeds at the time and I would be no where near the critical speed of my shaft. Also the deeper gears (4.56) with 30 inch rear tires, it takes a little of the effective ratio out. My setup is equivalent to running a 28" tall tire with 4.20ish ratio.
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FWIW - If the factory driveshaft for a 4 speed car is 55 1/2", the T56 magnum is 4" longer (i.e. 23" for the Muncie vs 27" for the T56), so unless you moved the engine back the driveshaft would have to be longer than 46". So if we use 52" (55 1/2" - 4" = 51 1/2") for the 3 1/2" chromemoly, you'd be hitting critical driveshat speed of 6620 at around 130 MPH with 4.56 gears and 30" tires.

All of this is to say you need to look at your driveshaft speed also when selecting rear gears. It's nice that the overdrive transmissions pull the cruise RPMs down but that doesn't mean then you can run at max RPM in top gear with your new OD transmission without looking at the driveshaft.
 
#45 ·
rpol78 said:
FWIW - If the factory driveshaft for a 4 speed car is 55 1/2", the T56 magnum is 4" longer (i.e. 23" for the Muncie vs 27" for the T56), so unless you moved the engine back the driveshaft would have to be longer than 46". So if we use 52" (55 1/2" - 4" = 51 1/2") for the 3 1/2" chromemoly, you'd be hitting critical driveshat speed of 6620 at around 130 MPH with 4.56 gears and 30" tires.

All of this is to say you need to look at your driveshaft speed also when selecting rear gears. It's nice that the overdrive transmissions pull the cruise RPMs down but that doesn't mean then you can run at max RPM in top gear with your new OD transmission without looking at the driveshaft.
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This here^ is a very good point about the drive shaft speed. I had a street/strip car that I experimented with three different rear gear ratios on. Namely 3.55, 4.10, and 4.56 gears. The transmission was an automatic with an O/D, (.67 O/D if I remember right). Has anyone here ever hit critical drive shaft speed? When I went to the 4.56 gears on that car, I either hit critical speed with the driveshaft or a I came really close two times while on the highway. And let me tell you, it's a type of sound that when you hear it, you know right away that you better let up on the go pedal immediately or something real bad is about to happen. It sounds like a freight train coming at you that's about to hit you hard, like something is going to let loose and come apart if your right foot doesn't come off of that go pedal!!

This happened the first time I took the car up on the highway and reached 90 MPH right after completing the 4.56 gear swap. Later on that day I tried it again with my buddy also in the car so that he could hear it. I then swapped out the stock driveshaft with a new aftermarket shaft that had a higher critical speed, and the problem went away for permanently.
 
#46 ·
Another factor that SLP numbers won't account for is header primary tube size. Of course this is a secondary factor since I'm speaking of the example where/IF you've chosen a header primary tube size that is too large. Many header manufactures will tell you that if you go too big with the header primary tube diameter for your particular engine combination, it will kill low end torque. This might not matter a whole lot with many trailered drag cars with high stall speed RPM's nor with any cars having 4.56 or 4.88 rear gears. But in street/strip cars it might make a significant difference. Although I must confess that I personally have not experimented a whole lot with different headers primary tube diameters on the same car/same engine. So I'm going by what the header manufactures say on that.
 
#47 · (Edited)
70GS455 said:
After having driven a ~3850 lb BB car on the street for many years and also drag raced it, i can attest that about 10:1 SLR is the sweet spot.

I had a 12:1 SLR SB and would have to shift at the end of the concrete launch pad or the other side of an intersection.

I also had a 7:1 SLR car and was horrible all around. The stock 36 lb flywheel was the only saving grace but still horrible taking off. It wore clutches often


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I just AM NOT seeing this as a problem with the street driver 70 conv. With a healthy SB400, M22W(2.56 1st gear), stock 11in diaphragm clutch, 2.73 12bolt posi, 27in tires, which translates to a 6.9888 for 1st gear.
From a dead stop clutch slippage is virtually none.
No, I do not drag race, and in town at slow speeds, I simply down shift into a lower gear (2nd or 3rd---------------------that's what a manual tranny is for). Interstate driving is just great and it even pulls a jet boat up the boat ramp just fine.
But, many of you already know our 70.
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Oh ya, and on top of that, the 76 Cutlass with a W30spec Olds 455(468), stock 11in diaphragm clutch, Richmond Street 5sp(3.27 1st gear and 1:1 5th), stock Olds 2.41 posi (translates to 7.888 1st gear) 27in tires, does just great with that combination and again, virtually zero clutch slippage from a dead stop------------------------------ OR--------------------pulling a trailer.
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#48 ·
DZAUTO said:
I just AM NOT seeing this as a problem with the street driver 70 conv. With a healthy SB400, M22W(2.56 1st gear), stock 11in diaphragm clutch, 2.73 12bolt posi, 27in tires, which translates to a 6.9888 for 1st gear.
From a dead stop clutch slippage is virtually none.
No, I do not drag race, and in town at slow speeds, I simply down shift into a lower gear (2nd or 3rd---------------------that's what a manual tranny is for). Interstate driving is just great and it even pulls a jet boat up the boat ramp just fine.
But, many of you already know our 70.
View attachment 726161


View attachment 726162


View attachment 726163


Oh ya, and on top of that, the 76 Cutlass with a W30spec Olds 455(468), stock 11in diaphragm clutch, Richmond Street 5sp(3.27 1st gear and 1:1 5th), stock Olds 2.41 posi (translates to 7.888 1st gear) 27in tires, does just great with that combination and again, virtually zero clutch slippage from a dead stop------------------------------ OR--------------------pulling a trailer.
View attachment 726164


View attachment 726165
Click to expand...
I'm just curious what diameter those header primary tubes in the pic are. 1 3/4"? And do you remember what the camshaft specs are? I'm guessing you have cams that are good for making low end grunt in both the Olds and the Chevelle.
 
#49 · (Edited)
BillyGman said:
I'm just curious what diameter those header primary tubes in the pic are. 1 3/4"? And do you remember what the camshaft specs are? I'm guessing you have cams that are good for making low end grunt in both the Olds and the Chevelle.
Click to expand...
The cam in the Olds engine is a reproduction 1970 W30 spec cam. I don't remember the profile specs, but I really like it. For those who are not familiar with hi-perf Olds engines, the 1970 Olds W30 and the Buick GSX 455 were Olds and Buick counterparts to Chevy's LS6. So if you are not familiar with those engines/cars, in well tuned condition, they are NOT to be messed with!
To begin with, the Olds 455 is a long stroke, long rod engine with a bigger stroke(4.25) than bore(4.125), thus, plenty of low end and mid range torque.
The headers ended up being replaced by reproduction W30 iron manifolds, W and Z (which have individual internal passages) and 2 1/2in outlets. It was such a frustrating dog fight to get the engine installed with the headers bolted to the engine vs laying them in and the dropping in the engine. So, I went with stock type iron manifolds and have been very pleased with the results (I put the headers up for sale, and fortunately they sold for almost what I paid for them).

I built the SB400 in the Chevelle over 20yrs ago, and I have no clue exactly what cam profile I installed, but it seems it was in the range of .500 to .512 lift and around 235 duration at .050in lift.

Here are the iron manifolds I went with on the Olds 455. Since there was NEVER such a thing as a 1976 Cutlass with these manifolds, nor a true dual exhaust, I had my muffler guy fabricate a FULL 2 1/2in system with generic turbo mufflers.
Also, I had to roundup a 73-74 transmission cross member that had the 2 humps for 2 pipes. Found a cross member in a salvage yard for $10.

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#50 ·
rpol78 said:
What driveshaft are you running? At 120mph it looks like driveshaft would be spinning at about 6150 rpm. I know the driveshaft on my '67 with a T56 is over 50" long and if I remember correctly 54-55".

Driveshaft critical speeds from Mark Williams website.
Image
Click to expand...
I got my driveshaft cut down to 52.5” which was about 1-3/4” shorter than with my Richmond Super T10 4 speed.
I also forgot to mention I am running 235/60 R 15 tires on 8 inch rims on the back which are 26.1” tall. I used the Tremec calculator and got 9.82 So I think everyone saying around 10 is a very good goal for a street cruiser. Good luck!
Billy
 
#51 ·
DZAUTO said:
I just AM NOT seeing this as a problem with the street driver 70 conv. With a healthy SB400, M22W(2.56 1st gear), stock 11in diaphragm clutch, 2.73 12bolt posi, 27in tires, which translates to a 6.9888 for 1st gear.
From a dead stop clutch slippage is virtually none.
No, I do not drag race, and in town at slow speeds, I simply down shift into a lower gear (2nd or 3rd---------------------that's what a manual tranny is for). Interstate driving is just great and it even pulls a jet boat up the boat ramp just fine.
But, many of you already know our 70.
View attachment 726161


View attachment 726162


View attachment 726163


Oh ya, and on top of that, the 76 Cutlass with a W30spec Olds 455(468), stock 11in diaphragm clutch, Richmond Street 5sp(3.27 1st gear and 1:1 5th), stock Olds 2.41 posi (translates to 7.888 1st gear) 27in tires, does just great with that combination and again, virtually zero clutch slippage from a dead stop------------------------------ OR--------------------pulling a trailer.
View attachment 726164


View attachment 726165
Click to expand...
Drag racing is a much different story. With 2 identical cars with the same weight, power and traction, the car with 10:1 SLR has 500 engine ft lb times 10 or 5000 ft lb axle torque starting out. The 7:1 SLR car has 500 times 7 or 3500 ft lb axle tq. So the 7:1 car starts the race off at a deficit, which continues and starts to catch up but the damage has been done. Spirited street driving is the same principle. Just easing around town, meh not such a big deal.

Sent from my SM-G991U using Tapatalk
 
#52 · (Edited)
ToocoolZ28 said:
What should the 1st gear ratio be multiplying "1st gear x rear gear" ratio. Right now I have a 3.31 rear gear with 2.20 1st gear which gives me 7.28 which isn't optimum for driving in traffic. I'm going to be buying a new Tremec TKX and am trying to figure out which gear ratios to order. I don't want a super steep ratio in 1st, I had a trans with 3.27 1st gear with 3.73 rear gear and I didn't like it, too steep for me. So now I'm running 3.31s in the rear and am trying to decide which way to go with the new trans. What's the right thing here? The car isn't a race car but it does have 600hp and 640 lbft.
I'm thinking somewhere around 10:1 with 28" tall rear tires.
Click to expand...
Though great conversation for drag racing, the OP already settled on a final gear of 3.31's and has 600hp/640tq. The original post relates to street driving. He's going to have a fun time anyway. Now he's looking for 5th gear.

Mike
 
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#53 ·
70GS455 said:
Drag racing is a much different story. With 2 identical cars with the same weight, power and traction, the car with 10:1 SLR has 500 engine ft lb times 10 or 5000 ft lb axle torque starting out. The 7:1 SLR car has 500 times 7 or 3500 ft lb axle tq. So the 7:1 car starts the race off at a deficit, which continues and starts to catch up but the damage has been done. Spirited street driving is the same principle. Just easing around town, meh not such a big deal.
Click to expand...
Just something to think about- acceleration rate affects the engine's torque numbers. Basically, some of the engine's torque gets absorbed by its rotating assy as it gains momentum. Engine dyno accel rates are typically either 300 or 600 rpm/sec, but 1st gear acceleration in your actual car is going to be at a much faster rate than that. Here's some rough comparisons of calculated torque output numbers for a "steady state" 500ftlb sbc, at different acceleration rates thru the heart of its torque curve...
...At a typical 300rpm/sec engine dyno accel rate, about 18ftlbs is absorbed by the rotating assy, which means about 482 of that steady state 500ftlbs actually gets applied to the input shaft.
...At a typical 600rpm/sec engine dyno accel rate, about 35ftlbs is absorbed by the rotating assy, which means about 465 of that steady state 500ftlbs actually gets applied to the input shaft.
...At a 1975rpm/sec accel rate, which is more representative of an actual 1st gear engine accel rate, about 116ftlbs is absorbed by the rotating assy, which means only about 384 of that steady state 500ftlbs actually gets applied to the input shaft.

Kind of illustrates why changing flywheel weight doesn't seem to make a huge power difference when tested at the typical engine dyno test rates, while the same change in an actual car at real world acceleration rates can net a much more impressive result.

I've never gathered numbers on a typical BBC 496 like the op, but expect it's heavier rotating assy will absorb more torque than my sbc example.

Grant
 
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