Thread: Hot cam and OD trans do not work together

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Most of you guys don't understand that an overdrive transmission and hot cam DO NOT GO TOGETHER. You have never seen a rump-rump cam paired with an OD trans from the factory and you never will, because the factory engineers know that a rump-rump cam has an operating range of rpm's that is much higher than the rpm's of the motor while using 4th gear overdrive. If the motor is taching 1800 at the crank, for instance, and the cam has been designed to operate efficiently from 2800 to 6000, then the crank is 1000 rpm's out of phase with the cam. Likely, the motor would not pull the hat off your head at 1800 and fuel efficiency would be in the toilet. So, what is there to gain????? If you want a hot rod that is fuel efficient ----AND FAST----, then choose a short cam like the factory would use and bolt on a turbocharger, blower or N2O so you can still go fast when you want to. BUT DO NOT BUILD THE MOTOR NATURALLY ASPIRATED WITH A HOT CAM AND OVERDRIVE. It won't work. Of course the car will still drive down the road, but it will not be efficient for speed nor mileage.

If you are building for fuel mileage, then use a cam that begins its operating range 500 rpm's before your cruise rpm's, to give the cam a chance to get "up on the tire"........ For instance, if you want to cruise at 1800, use a cam that has an operating range of maybe 1300 to 4800. By the time the crank gets to 1800, the cam will be in its range of efficiency by 500 rpm's and all will right with the world.

On the other hand, if you are building naturally aspirated to rip the tires off the vehicle and don't give a hoot about mileage, then for heaven's sake, use a 3-speed auto or a 4-speed that is 1:1 in top gear and match static compression ratio to the intake duration @0.050.....See the chart here.....
http://www.crankshaftcoalition.com/w..._compatibility
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Good advice, Tech.

Likewise, I have seen folks run OD with as low as 2.75 gears. That puts final drive ratios lower than 2 to 1. That ain't gonna work either, even with a stock cam. My current project car came with a T5 OD and 3.00 gears. Even with a stock cam, it was a dog in OD. Felt pretty good in 4th, though. Even with stock cams, most factory cars are running at least 3.42's.

Some OD transmissions, like the 700R4 have a low 1st gear and can efficiently get away with 3.25's or so, depending on tire size. I've seen factory setups in that range.

Back in the "old days" when OD's were electric shifted add-ons to a 3 speed manual, nearly every factory setup came with 4.11 gears. This gets you about a 2.85 to 1 ratio with typical 30% OD.

I ran a test with my car (the one with the T5 and 3.00's) once when I didn't have anything else to do. I ran a 75 mile loop twice on the same day. Once using 5th gear and once keeping it in 4th. Filled up at the same station, same gas pump. I picked up maybe 1 mile per gallon in 5th, but it's hard to say for sure. I put the fact that it ran OK in 5th down to a well tuned Q-jet (yes those do exist) with it's very efficient boosters that still meter well at fairly low air speed. For no more than I gained (if I actually gained anything), I would have been just as well off with a 4 speed.

My Healey project has a close ratio T56. It has a 2.97 first gear and a .64 6th. My cam will have an advertised operating range of 1200-6000 rpm. Based on that, I could go as low as 3.27 in the rear (which would be about 450 rpms above minimum at 60), but will most likely start out with 3.55's (about 580 above minimum at 60), since I already have a set.

Hope you don't consider this a hijack.

The more the merrier, Mike. Thank you.

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yep it's a fine line when running an aod . i never go beyond a 262 on cams. my 54 truck has a 256 comp cam in it . not a rumper but it is a red light bandit . 302/aod/342 gear with tall tires .

Just picked up a Ford 302 with a lopey cam so it looks like no o/d for me.

You can use an AOD with that 302, I do.
You just have to plan ahead. It takes gears and convertor working together with the cam / heads / intake / carb...

My cam is the ford motorsport X303 alphabet, meant for a 5 speed too.. the rear has 3.90 ratio, and the tires are relatively short ( 28" if I remember) .
The trans is modified (of course) some B&M some off the shelf and others, the Hi-Rev kits make the biggest changes. by shimming the springs in the governor
you can tailor the shift points, not always easy to do but it is possible.

If you have a cam with a huge overlap between I / E, then I wouldn't bother without installing a manual shift valve body and a higher stall convertor, the motor will need to rev a lot higher to make use-able power. Effective? No, but it sounds so cool right!?!?

I was planning on using a 4 speed with that 302 when i find a car to put it in.

i have a 5.0 roller motor , x300 cam and a set of 9.2:1 289 heads, tunnel ram 2x4's . not sure if this will suit me yet . looking for a set of the 10.5:1 heads. before it is over i may just go ahead and and bump it up to a 12:1 motor , will be running a 4 speed with 4:11 gears and tall tires. not looking for drive-ability or millage .

Sounds like it'll be a hoot to drive there Shine!

Go for it Jeff. What kind of car you searching for? How about an ol International truck? Shine has one!?!?! LOL..

not any more !! i sold the international and the 41 ford commercial truck. getting rid of projects i will never get built. keeping my falcon . as soon as the shop is empty i will be on the hunt for a 56-60 corvette to build for auction . still hunting a fleetline. found on in hays kansas but the web site it's on is kaka . cant contact the guy .

I liked that I H truck. Wish we were closer and then I'd own it!

The Falcon Gasser will be down right cool!

Reading this gave me a flashback to our '64 Corvette: no OD, of course, but the 365HP solid lifter carbureted engine, the cam was a bout a 2500 to 7500 item, and even with 4.11 geared rearend, it was a dog off idle until it got above 2000; by 3000, it had you pushed back in the seat, and wouldn't let go.

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i've thought of putting my old 454 bracket in it but i want to keep the sbf. already rounded up a bunch of parts. looking for a set of pistons for it to bump it up on compression .