Thread: New Member - 1988 IROC

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"pretty beefy hydraulic cam"

You probably will not like using a hot rod cam with an overdrive transmission.

The idea of an overdrive trans is to reduce the rpm's of the motor at cruise speeds, for an increase in fuel economy. Problem with a hot rod cam is that it does not want to make power in the range of rpm's where the auto trans wants to run......and if the cam is not efficient in the cruise range, then you'll likely be using more fuel than you would with a standard 3-speed transmission.

There is a range of about 500 rpm's from where a cam begins making power to where it is "up on the cam" and contributing to increased power in the motor. The efficiency range of most cams is about 3000 rpm's. In other words, a cam will make power from 800 (idle) to 3800 or from 1000 to 4000 or from 1500 to 4500 or from 2000 to 5000 or from 2500 to 5500 or from 3000 to 6000. Let's say you wanted to make your power at 2000 rpm's with an overdrive transmission. OK then, you'd want to begin making power with the cam at about 1500 rpm's, so that at 2000, the motor would be up on the cam and everything would be working as it should be. That cam would be rated to make power from 1500 to about 4500. Some cam grinders will try to cheat and tell you that their cam will be effective in a 4000 or 4500 rpm range. Bull Hockey. It will not. A 3000 rpm range is realistic, with a leap to 3250 if you are enchanted with the cam grinder.

Now, on the other hand, you decide that I'm full of the aforementioned Bull Hockey and decide to use the cam of your choice. After talking it over with the experts down at the Sonic Drive-In, you decide that you will purchase a stick with a range of 2500 to 6500, because this grinder really likes his products and claims a longer range of power for his products, a 4000 rpm range. OK, fair enough.

Now, using the aforementioned lead of 500 rpm's, remember if you want power at 3000, you use a cam that begins making power at 2500, you slide the killer stick into the motor and take off on a trip. What do you discover? Well, you probably discover that the motor doesn't have the power to pull the hat off your head at your assigned cruising rpm of 2000 and further, fuel mileage is in the toilet. Not quite what you expected, huh?

I have talked on forums to fellows who use cams that begin making power at 4000 and swear that their results are good with an overdrive transmission. I just smile and go to the next thread.

OK, you have some facts and figures to chew on. Make up your own mind as to what to do.

If you're ever in doubt about why something is one way or the other, take a look at the OEM equivalent of what you want to do. They are in business to make money with a product that works for everyone in the car market. Check out any new car with an overdrive transmission and listen to the idle quality. You are likely NOT to hear any cam lope from the motor because of what I have just taught you and because the intake valve duration is no more than 204 degrees, making for a smooth idle.
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