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What should cruising RPMs be for SBC

GNERGY

GNERGY

1/2 ton status
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I was wondering what ideal cruising (65mph) rpms should be for a small block chevy? Mines a TBI 383 with Edelbrock performer cam and intake.
I now have 373 gears and 37in. BFG/MT tires on a 91 burb with a 700r4 trans. I was thinking of changing to 456's
4wheel parts is offering free labor up to $500 with on line coupon till the end of Dec. I would have them do the rear and do the fronts myself.
Gears are 20% off.
Tarey
 
With an OD tranny I'd say 2000-2200 rpm at 65-70 mph. Go with 5.13's...2119 rpm at 65 mph with your 37's.

Rene
 
It's not wrong, it's just for a non overdrive tranny.

speed (65 mph) x gear ratio (5.13) x overdrive ratio(.70) x 336 (for lockup tranny or manual tranny) all divided by tire diamter (37")

65 x 5.13 x .7 x 336/37 = 2119 rpm.

Going with 5.13's will be easier on the tranny, allow for OD to be useful, and provide some wiggle room should you decide on 38's or 39's later on.

Rene
 
tRustyK5 said:
With an OD tranny I'd say 2000-2200 rpm at 65-70 mph. Go with 5.13's...2119 rpm at 65 mph with your 37's.

Rene
Click to expand...

tRustyK5 said:
It's not wrong, it's just for a non overdrive tranny.

speed (65 mph) x gear ratio (5.13) x overdrive ratio(.70) x 336 (for lockup tranny or manual tranny) all divided by tire diamter (37")

65 x 5.13 x .7 x 336/37 = 2119 rpm.

Going with 5.13's will be easier on the tranny, allow for OD to be useful, and provide some wiggle room should you decide on 38's or 39's later on.

Rene
Click to expand...

x2 plus the larger tires are more rolling mass.
 
I think I'm going to order 5:13s on monday from 4 wheel parts cause they have 20% off till midnight.
Tarey
 
Don't think i would be choosing 5.13's for a 10 bolt axle. The pinion gets very small with that ratio in a 10 bolt axle then pinion gears start to break.

Your original question, cruising speedo should be within the power band of the engine to be most effiecent.
 
I'm using edelbrock performer cam, intake. What do you suggest?
Tarey
 
GNERGY said:
I'm using edelbrock performer cam, intake. What do you suggest?
Tarey
Click to expand...

Well, i would suggest 5.13s but just not in a 10 bolt axle. Since you're limited (i'm guessing) to the stock axles then i wouldn't go anything more than 4.10's for the same reason. With 4.10's you should be fairly safe on pinion size and not ripping teeth off the pinion gear. I currently run 35" BFG's with 4.56 gearing in my 1 ton axles and it really isn't enough gear, 5.13's would have been a much better gear.
 
Honestly if you are running 1/2 ton stuff I would suggest bigger axles. Find a set of axles with the gear ratio you want.
 
4X4HIGH said:
Don't think i would be choosing 5.13's for a 10 bolt axle. The pinion gets very small with that ratio in a 10 bolt axle then pinion gears start to break.

Your original question, cruising speedo should be within the power band of the engine to be most effiecent.
Click to expand...

True that, although 4.56's put's his cruise rpm at ~1800 rpm at 65 mph...which is well into lugging territory and causing the tranny to hunt between D and OD a lot. Even with a 10 bolt front I'd go 5.13's as long as the rear was 14 bolt. If he has a 10 bolt rear to match I would just chuck both diff's and start with something beefier before re-gearing. With a 10 bolt front and a 14 rear he only needs to be sane when wheeling...which he should be with 37's anyways.

It also depends on how often and what type of wheeling we're talking about. Just the fact he is talking about cruise rpm on the highway makes me think it's driven regularly more than it's wheeled regularly.

alternatively he could stay with the 3.73's and forego the OD until he can find a D60 for the front...then gear it to 5.13.

Rene
 
I did all that heavy duty stuff way back with my 80 chevy with 1 tons and 513s with detroit lockers front and rear when I used to go muddin all the time. Now I street drive my burb and use it for hunting and camping. Do you think 4:56s would be ok.
Tarey
 
10/10 diff's...leave em alone.

10/14 diff's...honestly with 37's under a 'burb a 10 front isn't a great idea no matter what gears you have. It will break.

60/14...5.13's for sure.

Rene
 
There have been many a people who have ran 4.56 and numerically larger gearing and have ripped the teeth off the pinion gear when it's a 10 bolt axle we're dealing with. Like Rene said, the front axle isn't really much concern since you're only using it when in 4WD and if something did let go you can easily just unlock the hubs and drive out with the rear axle. Now if the rear axle grenades on you it's another story so that's why Rene and i say not to go deeper than 4.10's in a 10 bolt.
 
you should be fine, bet i never recommend regearing 10 bolts.
 
i have the edelbrock performer cam, intake and roller rockers on my '89 blazer ( w/ 700r4). it did a great job of waking up the motor in the lower and mid range. if i remember correctly peak tq comes at about 2000-2200 rpm. i went up to 35s right after installing the cam. with the stock 3.73s it would not stay in od on even the slightest incline, and on some hills it would not even hold 3rd. 4.56s were a perfect match. i cruise at 65 mph at 1950 rpm, it stays in od even on steep grades now. if you have 37s i'd go w/ 4.88s.
 
runamok151 said:
i have the edelbrock performer cam, intake and roller rockers on my '89 blazer ( w/ 700r4). it did a great job of waking up the motor in the lower and mid range. if i remember correctly peak tq comes at about 2000-2200 rpm. i went up to 35s right after installing the cam. with the stock 3.73s it would not stay in od on even the slightest incline, and on some hills it would not even hold 3rd. 4.56s were a perfect match. i cruise at 65 mph at 1950 rpm, it stays in od even on steep grades now. if you have 37s i'd go w/ 4.88s.
Click to expand...

That's near enough to what we've all been saying right up until we realized he has 10 bolt front and 10 bolt rear under a Sub with 37's...

A) spending more the $50 on a 10 bolt is $50 wasted
B) See A

Having a bit of fun...

Seriously as Scott mentioned the pinion on a 10 bolt gets very small and weak when you deeper than 4.10's...and this would be on 10 bolts seeing stresses they don't normally survive with stock (stronger) gearing.

Best to leave them alone and forget about OD until something more appropriate gets swapped under it.

Rene
 
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