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Discussion Starter · #1 ·
okay, i have another post in the drivetrain section because i am trying to figure out what gears i need out back. that discussion has brought up a question.

my car is a 70 chevelle ss clone. i am currently building a 396 bbc to put in. it has a comp hyd. roller cam, with an advertised rpm range of 2400-5500 rpm. it it running an aluminum intake with a road demon 750 4-barrel. its running a standard high volume oil pump, a stock desplacement water pump, a 3 row copper and brass rad and a 165 degree thermostat. it is expected to make about 410 hp, but thats just a guess.

i want to drive the car as a daily driver for the summers, be able to drive thecar to shows and on long trips at 70mph, and maybe take it on the strip a few weekends.

my question is what would be a good rpm for the engine to run at when driving on the highway at 70 mph for extended periods of time?
 

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Lower RPM is better unless you're to the point where you're below the stall speed of the torque converter. How you achieve that is up to you--rear gears or an overdrive transmission; or some combination.

Ditch the 165 (??? Never saw a 165 before!) thermostat for a 180; or even a 190-195 if you don't have detonation.
 

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Recently drove a car back from Chicago: 4.10s w/a 28" diameter tire.

Speedo did not work, but 95% of the trip was @ 3200-3500 rpm.

1100 miles, only stopping for fuel & food. Loved it, no prob. :thumbsup:


But, I'm somewhat of a grizzled jaded hardcore ol' road-dawg. :yes:


Everyone's "tolerance" level for such differs. You might have been

miserable, nervous, tense...sure we were going to spin a bearing,

drop a valve, overheat, fry a piston...on the road to ruination.

The car used not quite a pint of oil. ;)


I did think during that trip how miserable it would've been for

me if the car used "certain brands" of mufflers...I'd go Greyhound! :eek:


"Most" folks probably aim for no more than 2500 rpm @ 70 mph.
 

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Like Schurkey said, less is better but, unless you go to an OD trans, you'll need to compromise some. 2.73's will provide the lowest rpms but also the laziest accelleration. I've been around long before OD transmissions were the norm so I don't sweat 3k at 70mph. That was just the norm. If I were replacing my rear end, 3.42's would be my first choice as the best compromise.
 

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I had 4.10's in the rear and it was a little to steep for me. I would get to 4th gear and start searching for 5th which a M-21 don't have. Now I have 3.31's and like them much better. Huge improvement on gas also so I would not hesitate to make the same swap again. I do have a 454 that nets me decent torque at 2500 rpm. I think 3.42's for your combo would also be a good choice for cruising.
 

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for a 28" or taller tire, something in the 3.73- 4.10 range.. shorter than 28" tires go with 3.42-3.73. if it's an over drive trans, then go 3.73- 4.10 regardless of tire diameter.
if you run a 700r4 overdrive trans, you could even go with 3.08 gears and really knock the cruise rpm down because the deeper first gear would help it get off the line like a TH400 with 3.42ish gears.
 

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Discussion Starter · #8 ·
The 165 thermostat is what the engine specialists gave me. They say that they use them in all their carb engines. Says it keeps engine nice and cool at about 180 degrees driving. And I am just wondering about a good cruising rpm to ask for. I was thinking about 2500, am I way off? Would a car geared for this have poor acceleration? Top speed?
 

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My 427 (bored and stroked to 460) cruises along happily at 2500 hooked to a Tremec 5 speed. I also run a vacuum gauge in the car, at 1900-2100 rpm I am actually pulling 22" of vacuum at cruise. My current cam is modest, the previous, not so much. Try whatever you got, run it where it has the best vacuum down the road.

Devin
 

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okay, i have another post in the drivetrain section because i am trying to figure out what gears i need out back. that discussion has brought up a question.

my car is a 70 chevelle ss clone. i am currently building a 396 bbc to put in. it has a comp hyd. roller cam, with an advertised rpm range of 2400-5500 rpm. it it running an aluminum intake with a road demon 750 4-barrel. its running a standard high volume oil pump, a stock desplacement water pump, a 3 row copper and brass rad and a 165 degree thermostat. it is expected to make about 410 hp, but thats just a guess.

i want to drive the car as a daily driver for the summers, be able to drive thecar to shows and on long trips at 70mph, and maybe take it on the strip a few weekends.

my question is what would be a good rpm for the engine to run at when driving on the highway at 70 mph for extended periods of time?
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Randy,bottom line is you and only you have to be happy/comfortable in the car with the cruise rpm you choose to run when you choose a ring & pinion and nobody else so remember that.

Some guys dont mind 3k-3,200rpm cruise rpm all day long and some guys hate higher cruise rpm so you cant go by other peoples opions when it comes to the subject of what cruise rpm to run. I personally dont hink 3,200cruise rpm for extended driving time is a good place to be.

So with that said if your car is on the road that has a t400 in it try driving on open road with it in 2nd gear taking it up 2500 rpm and cruse for a while.

Then take it up to higher rpm in 2nd gear like 2800/3k/3,200 and emagine that rpm for long period of time to see what cruise rpm you can tollerate.

But also keep in my more cruise rpm means more fuel used ,more engine noise in car,more wear on motor-drive accessories/more underhood heat generated by motor/etc.

If you car isnt on the road yet you could also ask friends with muscle cars with gear ratios your interested in running to take you out for a cruise esp on the open road to see what you think of it.

If you dont know people running gear you want to try then go to local car cruises /shows this comming season and ask guys that own muscle car what gear they are running . Then if some people have the same gear ratios your interetsed in you may want offer them a few $ for fuel to take you for a cruise sometime on open road to see what you think of it before you buy new gear.

In worst case sit in your car with the motor running and hold throttle for a while at 2,500/2,600/3krpm while thinking which rpm lvl can i tollerate for an extended period of time when doing open road cruising? And also listen to the motor to see how happy it seems to sound when holding it variuos rpm lvls .

Also keep in mind your TH400 auto trans with mild stall with a NON lockup converter will likely turn 250-300 rpm more then a manual trans setup does on avg which you also need to consider when choosing a ring & pinion too.

As i said before my m20/3.31 gear/26" tires turns approx 3k rpm at 70mph and i gotta tell ya i would not want any more rpm then that meaning i would not consider even 3.42 gear that only add maybe 150-200rpm at cruise,when is enough enough /LOL!!!!!

A bbc thats built well & maint well that's in good cond with a good cooling system should be able to handle open road cruise rpm of up to 3k IMHO.

But i also remember reading someplace (in GM litertature ?) a while back that 2,400-2,500 rpm is the MAX rec open road cruise rpm for a BBC which was sliuthgly higher for a SBC.

The optional 3.07 economy gear in the 12 bolt that came in my 69 chevelle ss 396 when new from GM turned approx 2400-2450 rpm @ 60mph and 2,750 maybe 2,800ish rpm @ 70mph.

But since your sensitive to keeping cruise rpm reasonable (which is smart!) with your setup i'd stay somewhere in the range of 3.08 (avail new) to 3.31 (only avail used or NOS on ebay/here in T/C/ etc), but my 2nd choice would be 2.73 if you want less cruise rpm.

I would not consider 3.42 or higher numerical gear because thats just getting into too much cruise RPM IMHO and that to me is when enough is enough and you gotta stop somewhere./LOL!!!!.

Hope this helps you out.

Scott
 

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Discussion Starter · #11 ·
Just so every body knows, I can't do the trial and error method. As of today, I have the body off the chassis, the small block is out, the 396 going in is at my school waiting to be built, and I'm still looking for a t400 tranny. Obviously not going anywhere for a bit.
 

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Like Schurkey said, less is better but, unless you go to an OD trans, you'll need to compromise some. 2.73's will provide the lowest rpms but also the laziest accelleration. I've been around long before OD transmissions were the norm so I don't sweat 3k at 70mph. That was just the norm. If I were replacing my rear end, 3.42's would be my first choice as the best compromise.

I agree. But one thing to note, if the OP has a 12 bolt (like a good SS clone should) he may either have to choose gears based on what series carrier he has or come off the wallet and change carriers.

I have a 4 series carrier, so unless I decide to cough up the money to get a different posi unit, I am limited to 3.73 (or numerically higher) gears.
 

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Just so every body knows, I can't do the trial and error method. As of today, I have the body off the chassis, the small block is out, the 396 going in is at my school waiting to be built, and I'm still looking for a t400 tranny. Obviously not going anywhere for a bit.
========================

But you can still go for rides in other people muscle cars running gear ratios your thinking of using.

Since your car is no where near completion that should give you plenty of time to find guys at car shows/cruises that are willing to take you for a spin to see what you think of the different gear ratios.

I think most guys would be willling to help you out talking you for a quick spin if you tell them what your doing/trying to decide on what gear ration to run and need to experience it 1st hand to make best/right choice that your comfortable with.

But i'd stay under 3.42 (2.73/3.08/3.31 MAX!) with your auto trans with stall and non lockup converter and you should be ok @ 70mph.

But if you go with 3.42 or more gear (3.55 etc)i bet you will be sorry you did so when trying to cruise at 70mph for any length of time with no OD/no lockup/& additional stall over stock with your TH400 which will collectively increase cruise by at least 250-300rpm VS a manual trans setup.

FYI,3.42's with the setup you plan to run will turn more then 3k rpm @ 70mph which can get old pretty fast.

My man trans setup turns 3k @ 70mph with 3.31 gear ,now swap in a th400 with stall/non lockup/no OD with a smidge moire gear @ 3.42 and you will see a minimum of 3,300rpm if not more relisitcally 3,400rpm or so @ 70mph. That's unless you run abnormally high rear tires that may reduce cruise rpm by 100-150 rpm depending on tire height still leaving you at approx 3,250-3,300rpm @ 70mph which is too much for most people,even 3k rpm @ 70mph for long period cruising is on the edge of being too much cruise for many people me
included.

I think best case would be approx 2,500 - 2,700 rpm (MAX) for long term more comfortable cruising @ 70mph for a BBC (IMHO) in which case 2.73 or 3.08 would be best choice to get even close to that goal with you'r proposed setup.

Since your still in the build stage you could go with the speedomotive storker kit that will take your 396 out to 434cubes,the additional 38cubes will increase trq/pwr a bit that can be helpfull when running gear in 2.73-3.08 range where you will liklely end up.

Good luck.

Scott
 

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The 165 thermostat is what the engine specialists gave me. They say that they use them in all their carb engines. Says it keeps engine nice and cool at about 180 degrees driving. And I am just wondering about a good cruising rpm to ask for. I was thinking about 2500, am I way off? Would a car geared for this have poor acceleration? Top speed?
==============================

Exacty the same resullts i got when swapped to a std flow 160 deg stat in my bbc setup which runs rock steady 175-178ish deg @ cruise & 180 in traffic in 90= deg temps.

Thats with my mild 396 with hi flow w-pump/stock shroud/stock 772 7 blade clutch fan/HD fan clutch/newly recored 4 core harrison radiator/50/50 coolant mix/15-16lb repop RC15 rad cap.

With the 180 hi flow stat i was running prior to trying out the 160deg std flow stat it ran 190-195ish deg @ cruise and 200 -205deg (210 if still for 5 mins or mope) in trafffic all in 90deg + temps. Thats wasnt all that bad but not the greatist either esp considerting the cooling system setup i had & swapping to the 15356 autozone durlast 160 std flow sta got it running " consistantly 15-20 deg cooler" at all times.

The motor really responded well to running 15-20 deg cooler not loosing as much pwr/trq in hotter temps,oil stayed cooler too so oil pressure was 5-6psi higher at all times too.

I tried that same stat out in a couple other bbc's i was working on with same results with those motors running consistantly cooler, but they never ran too cool like 160-165 deg in above 55-60 deg temps.

Maybe if you were going to drive the car a lot in 30-50 deg f temps you would need to move up to a 180 stat to ensure motor heats up enough for proper oil temp but not for when most of us drive these cars in warmer months of the yr.

Scott
 

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Just so every body knows, I can't do the trial and error method. As of today, I have the body off the chassis, the small block is out, the 396 going in is at my school waiting to be built, and I'm still looking for a t400 tranny. Obviously not going anywhere for a bit.

So, with the advice you've gotten, what are you thinking?

Does the car have a 12 bolt? If so, do you know what series carrier it has?
 

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Discussion Starter · #16 ·
No. My car was originally a 307 car, and had only a t350 and a 10 bolt, which I have no idea what it has for gearing. I never got to drive the car. Due to a burnt head gasket from a cramping modified 267, a leaking tranny pan gasket, and a rust hole in the frame, I never got to even drive the car on the highway since I bought it. I am looking for a 12 bolt rear, and I think I found my transmission
 

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Not sure if this is right or wrong, But it has worked for me since 2003.
I built a 70 SS 454 Sta. Wgn. (4300 Lbs.) It was a 307 with a 350T and a 3:08-10 bolt.
Installed the 468 Very mild cam, Stock Q-Jet, ( engine is close to a stock LS-5 ) 700R4= 3:06 first gear, V01 cooling system with Fac. A/C. & a late 80/90s Fan clutch, For a 3/4 t. Burb with dual A/Cs. The clutch fits a 772 blade just like the OEM Eaton. I left the 3:08 -10 bolt in just to try it. Drove it two years with the 10 bolt 3:08. Then went to a 12 bolt with a 3:07 gear set. For me, Its hog heaven, Why?
At 70-73 MPH. the engine is turning 1950 RPMs, Temp. 180. Milage, Always 18-20 Mpg. I use to run a ton of timing, Now, A lot less. ( timing is factory LS-5 setting )
On a very hot day with the A/C on, It will climb to 215 in traffic and stay while you are sitting, Moving it goes down to around 195.
I do believe, If you have ALL OF THE FACTORY items in place under the hood it WILL help your cooling system. My wagon is a Big time driver, This combo has been driven from Pitts, Pa. to- Fla, Tenn, Ind, Canada, Oh,NY,Mich or 34000 miles and NO problems. I plan on NO changes. 99% of the time on the interstate it is in cruise, ( Electronic Servo Cruise.)
Bob:hurray:
 
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