Trap RPM

[ont_guy_38]

I am considering putting 4.56 gears in my truck but am worried about the rpm at the end of track and if my motor could handle the rpm.The motor is a 350 standard bore,flat tops,air gap intake and vortec heads with a fair amount of porting done to them.The cam is a Comp , duration 228 int ,234 ext, lift int 480 ext 499 with 4/7 swap and 1.5 ,1.6 rocker combo and holley 700 carb . I don't spend much time at all on the highway mostly just cruise nights and the odd trip to the track which is about an 40 minute drive .I have 29.5 inch tires but might go to 28 inchers eventually . I do have a set of 3.90 gears right now to put in it but wondering if i should just go for it and go deeper .The motor came out of a car with 4.11 and a 26 tire so I'm thinking this combo would be about the same . I really wanna do it but don;t want to be sorry I did

[djracer]

I would lean toward about a 4.30 with that short of a tire.

[cableguy0]

I wouldnt go 4.56 with that combo either. Try a shorter tire first and go from there. With your combo I dont see a benefit to being at higher rpm for longer. Its not going to make the power up high to justify coming through the traps at 7krpm. It probably wont stay together for long anyhow especially if the engine isnt balanced

[team39763]

I had that combo on my truck when I first bought it...28" tires with 4.56 gears running a 350 and TH350 trans. It turns a lot of rpm at the end of the track. I remember at that time I had just did my engine swap and I was still into street racing back then. One race we lined up a few yards too far from the 1/4mile finish line and my truck didn't make it to the finish line before it was on the rev limiter(around 6800-7200).

[ont_guy_38]

Thanks guys ..ya I don't want it reving that high , it's not built for it

[68 C-10 KID]

I also like the 4:30's. I want to run a set. If your running the truck 12 bolt you with have to get the conversion bearings to run the 12 bolt car 4:30's. If ya have a 9" your fine.

[chevytruck1970]

conversion bearings?? please explain

[Captainfab]

You can run passenger car 12 bolt gears in a truck 12 bolt housing by running different pinion bearings. Timken #26882 and #26923

Originally Posted by chevytruck1970

conversion bearings?? please explain

[Super73]

Here is a calculator I put together for everyone to use http://67-72chevytrucks.com/vboard/s...d.php?t=274314

What did the car run that the motor out of. (Mostly concerned with mph)
What did said car weigh?

These two things are really going to help you take an educated guess of what you will run if:

You know your weight?

[ont_guy_38]

the car weighed 3300 and went 103 mph 27 inch tire though , i thought he said 26 , I would guess the truck weighs 4500

[ont_guy_38]

So found out they make 4.30 gears for a 8.5 but only a few manufactors anybody have any guessses why that would be , not enough call for them? I ended up selling the 3.90 gears so now I have a 12 bolt posi carrier that i can use to build a 12 bolt rear end or just put these 4.30's and a posi unit in my 10 bolt .