SBC 350 cruising RPM
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  1. #1
    1972 Formula 350 owner Marc's Avatar
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    SBC 350 cruising RPM

    Hello again everybody!

    Many thanks to those who have helped me with a previous issue.

    Now, onto another issue, and relating to a new build which will entail the use of a standard SBC 5.7 & TH350 trans along with a volvo 240 rear axle (3.31-1 ratio), as well as using 255/60 15" rear tyres.

    I am figuring out the approx cruising speed etc with the help of the online calculators elsewhere on the forum (engine revs, trans, axle ratios and tyre sizes etc), just wanted to know on average what rpms your 350 does in any modified car that you have put together.

    Using the ratios etc that i have on the vehicle so far, at approx 70mph i am looking at a tad over the 3000rpm mark. Ideally i was hoping to have it lower than this as it will be an everyday car and not a drag strip goer if you see what i mean!?

    So, in essence, i was wondering if approx 3000rpm is acceptable or unbearable with this motor & set-up etc. What rpms do your own vehicles do at say 60, or 70mph?

    Many thanks to anyone who could help me out with this!

    Marc
    Last edited by Marc; 01-07-2010 at 09:37.

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    Dragging not Bragging vince-305's Avatar
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    Hi Marc

    Have a look here mate you can see a similar question asked by me

    https://www.rodsnsods.co.uk/forum/tec...s-per-mph-2915

    Im using 2.75:1
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    Odd Rod junkie
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    A 3000 rpm small block will run alright, but suck the fuel pretty bad. If you want the short gearing, but still be able to cruise, have you looked at installing a torque splitter? A decent rpm for a mild build performance will cruise around just smoothly at 1750-2500rpm. Mind you, that is an expensive way to get the overdrive. The only ones I have seen are for manual transmisisons, also would need a different driveshaft since it adds a bit of length to the trans.

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    Exceeded sell by date Plumpcars's Avatar
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    My 55 has a 350. Probably only about 300hp. TH400. 2.7 gears and on the current tyres it's cruising comfortably at app. 2400 at 70mph. Returns low twenties mpg at that pace.

  7. #5
    Full Bloody Rebel! Mad Mark's Avatar
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    Mine is doing 2000rpm @ 40mph, 3000 @ 60, etc. Sounds like mine is a bit short on rear ratio!

    Mind, on a run I am getting about 24mpg. (As long as I can keep my bloody foot off the 'GO' pedal!!!)

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    I'm Not Jed Clampett stueeee's Avatar
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    3000 RPM @ 70 MPH will be OK for an SB Chevy , but unless your ride is incredibly heavy with the aerodynamics of a housebrick, you should be able to run higher gearing than that without any problems.

    My 1950 Chevy truck has a warmed over 350 motor and runs 2100 RPM @ 70 MPH in fifth gear, never had to change down for hills or such.
    Stuart.
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    Assuming you want decent mpg and nice relaxed cruising why not use a 200r4 overdrive auto instead of the th350.
    It has a low 1st ratio to give some punch off the line but in overdrive top it will drop your revs and save you on mechanical wear and give much better economy. The 200r4 is similar in size and shape to the th350 so you can use the same gearbox mounting.

  10. #8
    1972 Formula 350 owner Marc's Avatar
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    Quote Originally Posted by firebird68 View Post
    Assuming you want decent mpg and nice relaxed cruising why not use a 200r4 overdrive auto instead of the th350.
    It has a low 1st ratio to give some punch off the line but in overdrive top it will drop your revs and save you on mechanical wear and give much better economy. The 200r4 is similar in size and shape to the th350 so you can use the same gearbox mounting.
    Whats the 200r4? I originally bought my chevy lump with what i was told to have a 700r4 on it before i picked it up, but unfortunately turned out to be a 4l60e box instead so was useless for my purpose as the 4l60e has a fully electronic valve gearchanging unit which is all wired into the original wiring harness.

    Might be an idea for me to maybe post a wanted ad in the forum for one of the 200r4 / 700r4 boxes then to gain the overdrive!?
    Last edited by Marc; 04-07-2010 at 14:45.

  11. #9
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    200r4,700r4 and4l60e are all GM automatic overdrive boxes , 200r4 is closest to a th350 ,the 700 and4l60e are basically similar.There is a guy on here who ships in cut-outs from the States [ Adey?] , never used him myself but might be worth a try.
    The thing to remember is if you want street performance any of these boxes will work but for serious strip performance they are not as strong.

  12. #10
    1972 Formula 350 owner Marc's Avatar
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    Ok, thanks for everyones help, I dont suppose by any chance somebody has a 200r4 box kicking about that they wish to sell me!? :o)

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