optimal freeway RPM's for a small block chevy??

lets say yer gonna go 200 miles on the freeway... what RPM would be the max you'd care to run for that long??

my last chevy (350) would spin 3000 at freeway speed, which I guess was around 65 -70. going with the flow of traffic. I didn't like to go above that for prolonged periods.

my new car will have a 305, 700R4 and 3.08's. even with smallish tires at 24" tall I'll be spinning 2000 @ 70 MPH. somthing like that... (I lost my link to the RPM / MPH formula)

what do you think?? at what point to yo start putting excessive strain on your motor??

 

I don't like running over 2,500 rpm max for extended periods of time. Most engines are not efficient above that range, and if you're going hundreds of miles at 3,000 plus you had better have a ****** cooler as well. For mileage (and peace-of-mind) 1,800 to 2,200 seems about right...

 

i would think that on a good engine 2000 rpm would be good.. good fuel economy, good for the life of the engine. some where around between 2000 and 2500.

 

Depends more on the cam profile, to get in the sweet spot. anything below 3000 would be safe, as long as you're cooling system is up to it. I wonder if 2000 is enough to get into a 305's torque band?

 

I wonder if 2000 is enough to get into a 305's torque band?


that's kind of what I was wondering too.... I might have to do 85 - 90.

 

2500-3000...

 

Seems someone on this list just told me about carefully monitoring manifold va***e for optimum running - and how it is directly related to fuel consumtion and the proper running of the carb. And metering fuel air ratios? I've never done that but certainly will on my next build. I image if you are "lugging" it, either or both of these measurements will be effected, along with mileage, eh? Or not? Gary

 

Are you building the engine? If so, there are a few ways to get the torque band down. If you want low end torque out of a 305 you need to keep the cam small...larger cam shifts the rpm up. With 3.08's you will have a bit of a time getting where you want to be. Raising the compression ration would help, and headers would help too, but just keep the cam small.

 

absolutly true!

 

Like 53sled said, your cam will dictate where the sweet spot is in the RPM range. You want to be cruising where you are just starting to 'climb up' on the torque/power range of your cam.
Either find a cam that will fir your rear gears and final drive ratio of your ******, or find a cam that suits you and change the other two to match your cam.
r

 

im working on this very issue right now with my 53 chevy with a 327 in it.
I just got done putting larger valve heads on it with a bit larger cam in it. my trans is a th350 and rear gear is 3:73 and a 26" tall tire in the back..im thinking with all the math that i did my 65 to 70 mph cruzin mode should be right in the sweet spot of this combo..night before last on my way down the e-way she felt so much better. and if i stuck my foot in it at 70 she would pull like a beast.
Im hoping i nailed it. just got to do a bit more tweeking on the timing and carb adjustments .

 

the 305 came from a wrecked Trans Am. I regasketed it, hi output oil pump, new timing chain and a "RV Grind" cam... don't recall the specs, my machine shop guy picked it out. edelbrock performer and a Holley 390. no headers, but 2" duals.

I think I may have gone from one extreme to the other... too many freeway RPMs in the last one, and not enough in this one.

 

Decades ago Jeff Smith and super chevy did a SBC efficiency series that went fairly in depth. Car was a 70-72 monte. Moral of the story was cruise RPM in the range of 1800-2200 worked best for most p***enger car builds.

I have OT cars with overdrive and drag race gears that do road trips at 3100ish. Was nerve racking at first, tens of thousands later no biggie. But I also change oil every 3k with full synthetic. Also I have a non OD pickup with a 454 that cruises 32-3300 all day long with a loaded car trailer and 1500 lbs of junk in the bed. Chevy geared it that way, not me. Barely whimpers with tall hills and huge loads.

You should see some of the RPM punishment 350s take in ramp/tow trucks and duallys. Some of those guys turn nearly 4k going down the highway.

 

Not to be a post hog, but 3.08s and OD is right on the edge of acceptable IMO. GM loved to do that combo on G bodies, and they just don't use OD unless it's on clear highway, and even then it's kicking out the converter for everything. MCSS used 305/OD/3.73 with great success, I put 100K on one and it's a very acceptable setup, and much easier on the engine and trans too. RPM and speeds just match up and rarely does either component struggle against the other. Good luck

 

All I know is my 62 C10 with a 4.10 does 3000 rpm @ 65mph and it is NOT a comfortable ride At least with the manual transmission I'm not worried about a cooler.

 

In my '57 Chevy truck with a 350, T-5, and 3.08 geared twelve bolt with 30" tall tires I usually cruise about 2200 rpm, I'll go 2500 rpm if I need to get somewhere in a hurry, or 3000 in 4th going up hills.

 

Damn I had a 92 chevy PU [ 5 speed standard ] 5.7 and it would cruise about 75 right around 1800 rpm . very smooth. It was geared tall. It seemed to be at the lower end of the power range.

 

who's got the RPM's x trans X tires = speed formula?

I have it onmy computer at home...not here where I'm supposed to be working.

 

I run @ 2500rpm at 60 mph in my hotrod pu, 350/th400/3.73/33" tires....seems to like it just fine.
My OT car w/440 runs @ 3500 rpm.....@ 65mph.....been that way for years....

 

I'd suggest you not use the 390 cfm Holley on a 305, especially if you have a non-stock cam.
It'l starve the engine.
USe a Holley 600 vac*** secondary and tune the secondary spring.
r

 

I think you guys are under-shooting it a bit. For 2 years I drove Odessa to Lubbock, then returned every weekend. I drove a 56 Chevy pickup with a 30 over 350, M22 4 spd, and a set of 388s in a '69 12 bolt. Thats about 300 miles each weekend counting in the town driving.

At 75 - 80 I averaged 3800 - 4000 RPMs. When I finally wore the motor out it had over 100,000 miles on it.

 

Imo, lugging the engine is harder on it than spinning it a bit.

That said, doubling the rpm squares the stresses on the engine from what I understand.

But, lugging it creates stresses of another kind, detonation is a possibility and it's harder on bearings.

Good mileage is not all about slow rpms on the highway, but it helps.

As you can see, every aspect of running an engine is a two-edged sword that cuts both ways.

Monitoring a vacuum gauge can help quite a bit.
I note that my 88 Mustang GT with 5.4 liter and 5-speed would pull a higher vacuum in 4th on long desert highways with their subtle - in some cases - grades.
Slide the trans up into 5th, vacuum dropped 3"-4".

Mileage running from point A to point B was 24.5 mpg using 5th during all the moderate grades and flatland running.
Same route using 4th on the grades returned 25.5 mpg.


Couple of weeks back, ran the roadster at 3300 rpm - 80 per - and vacuum was about 16"-17" during most of the 60 mile trip.
Too much rpm though, don't know what the mpg was, we were more interested in getting home after a long hot day than we were in mileage.

One thing to think about when running elevated rpms for a while is that there's a lot of oil tossed up into the engine valley and under some conditions it can be hard to control.
You'll know how your oil control system is doing if you nail the throttle after extended high rpm running and get a whole lot of blue smoke.
Any other time no smoke probs when running through the gears.

An improvement in oil control will help here, but popping in a set of 3.00 gears in place of the 3.70s will help the most.

 

Here's a link i found while searching the net for formulas.
Lot's of good formulas here.
The gear ratio one is what i was searching for.
The ******** Mustang Drag Racers Forum - Drag Race Calculators and Math Formulas

I used this when i set up my 53..
My last 'daily' hot rod ran a slightly warmed over, high mileage 305 and it would cruise all day long right around 2600..that was about 65-70 mph.


Tony

 

I'm building my 59pu with a 30 over 327 running a t56 and 355s but I dont think I can use 6th (.50 overdrive) even with 25 inch tires. Looking for a set of 411s for my dana 60 then hope to use both overdrives.

 

use the carb formula in the link above^^. this is a 305 which will never see the other side of 4,500 RPM. at 5500 max the formula calls for 412.58 CFM. plus I got the carb with a K&N filter in almost new condition for $25.00.

 

...that's a great link. 23" tires I'll be doing 73 @2200. 26" =79mph..

 

My Chevy pickup has a 327 running 10.5:1 with 3:2X something rearend and a M22....and IMO I would like to jump up to 3:73's or so. it's hard to get that 327 up on the pipe with such a shallow rearend.

And the fact that I want to jump up to 29" tires doesn't help too much either...

 

mph = rpm x tire diameter divided by gear ratio x 336
rpm = mph x gear ratio x 336 divided by tire diameter
tire diameter = mph x gear ratio x 336 divided by rpm
gear ratio = rpm x tire diameter divided by mph x 336
The FOGGER

 

This is basically the combo that I have in my 62 Bel Air Wagon. I'm about 60 at 2800, 65/3k, 70/3200. As far as I know, it's stock 327. I feel like an O/D would help to bring the RPMs down on the highway. I keep thinking about changing the rear gearing, but the around town acceleration is good so I hate to get too low and kill the performance when not on the highway.