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Thread: Rev range

  1. #1
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    Default Rev range

    Hi all, might be a stupid question but how would i know what a safe rev range would be for my engine?
    EFI engine's have limiters but what about a carbi fed 350 SBC?
    Its a new engine that im still running in and have had it to 6500 without trying.
    Basic engine info -

    Scat crank (not forged but balanced)
    I beams with arp bolts
    Forged Flat to pistons (approx 10.5 to 1)
    Roller rockers

    Is there a safe limit to try and stay at or just let it rev to where it wants too.

    Any thoughts?

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    Senior Member JaseQ314's Avatar
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    Cam specs, heads/specs, intake, headers etc? These will help give a clue where it might make usable power to, rule of thumb approximately 10% past max power for shift point.

    Otherwise get the rings bedded in with some varied load and not ringing its neck initially then give it the what for. 6500RPM sounds like it's had a pretty reasonably rev already.

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    You could get it on a chassis dyno to finish the run in, get it tuned and then to find out where it makes peak power. Then as above, no point revving it more than 10% past there.

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    Mechanical rpm limit can be different to RPM limit due to combination.. Due to the list of engine components you have listed I would suspect a limit somewhere around 7500 rpm will be no problem.. But your combination may or may not allow this. Limiting factors are primarily piston speed and valvetrain stability

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    Cam is Comp Xtreme Energy 230/236 Hyd.
    I was looking to put it on the dyno but after run in as i was going to replace the carbi at that point and tune.

    Im not to concerned with what rev to shift at but more along the lines of is there a dangerous rev limit to try and stay under or if it wants to rev to say 7500 just let it and not worry

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    hi
    230/236 peaks about 5800--6000 for Hp influenced by carb ,Inlet manifold and headers
    A moderate set up D/plain 1 3/4 headers 750 carb modest heads on a 383 should go 6000 on a 400 cuin little less
    On a 395--400 s/plain , prefer 1 7/8 headers ,780+ carb, quality heads goes 6000rpm

    Then ... the cam duration and head port design become restrictive

    Valve springs and bottom end desiqn plus hyd roller lifter weight all reduce the limit .
    The limit for a std travel HR lifter 6000--6500 and thats using a better quality unit.

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    Quote Originally Posted by 78HZSBC View Post
    Cam is Comp Xtreme Energy 230/236 Hyd.
    I was looking to put it on the dyno but after run in as i was going to replace the carbi at that point and tune.

    Im not to concerned with what rev to shift at but more along the lines of is there a dangerous rev limit to try and stay under or if it wants to rev to say 7500 just let it and not worry

    That cam is never going to make power to 7500 in a 350 no matter how good the heads and intake are. It'll be falling on its arse long before then so keeping your foot into it even if it would get there is pointless and increasing your chances of grenading it for no good reason.

    Your short components might be good for 7000 or more safely, but I'm willing to bet that you've probably already revved it pretty close to as hard as you need to at 6500.

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    Quote Originally Posted by swampy View Post
    hi

    Valve springs and bottom end desiqn plus hyd roller lifter weight all reduce the limit .
    The limit for a std travel HR lifter 6000--6500 and thats using a better quality unit.
    Some US youtuber just took a hydro roller 5.3 to just under 9000rpm (and ran a mid 7 second pass in a 2nd gen Camaro). Hydro roller stuff is coming along nicely.

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    I think you'll find the OP's cam is a hydraulic flat tappet, not roller.

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    Default Rev range

    Quote Originally Posted by RedTaxi View Post
    You could get it on a chassis dyno to finish the run in, get it tuned and then to find out where it makes peak power. Then as above, no point revving it more than 10% past there.
    Quote Originally Posted by JaseQ314 View Post
    Cam specs, heads/specs, intake, headers etc? These will help give a clue where it might make usable power to, rule of thumb approximately 10% past max power for shift point.

    Otherwise get the rings bedded in with some varied load and not ringing its neck initially then give it the what for. 6500RPM sounds like it's had a pretty reasonably rev already.
    Quote Originally Posted by JaseQ314 View Post
    That cam is never going to make power to 7500 in a 350 no matter how good the heads and intake are. It'll be falling on its arse long before then so keeping your foot into it even if it would get there is pointless and increasing your chances of grenading it for no good reason.

    Your short components might be good for 7000 or more safely, but I'm willing to bet that you've probably already revved it pretty close to as hard as you need to at 6500.
    Agree with the above. Power would likely head south around the 6k mark. Find peak power on dyno +200rpm there's your shift light if you want one. RPM limiter a hundred or so after that for over-rev protection.

    Heads?
    Last edited by BasicQ; 05-10-2021 at 12:30 PM.

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    hi
    HR lifters also come in low travel units which rev over 6500rpm

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    hi
    If your talking 350 with F/T cam Edelbrock do performer RPM packages in Hyd F/T and Hyd Roller . Suggest u down load the catalogues and copy what they use.

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    Quote Originally Posted by JaseQ314 View Post
    That cam is never going to make power to 7500 in a 350 no matter how good the heads and intake are. It'll be falling on its arse long before then so keeping your foot into it even if it would get there is pointless and increasing your chances of grenading it for no good reason.
    Your short components might be good for 7000 or more safely, but I'm willing to bet that you've probably already revved it pretty close to as hard as you need to at 6500.
    Its not so much as keeping my foot into it but just reading the memory on the tacho, 6500rpm would have been only brief at first/ second gear as im still trying to keep the revs down for the run in period.
    I have a manual gearbox with 3.9 diff ratio so 6500 comes pretty quick.

    Quote Originally Posted by JaseQ314 View Post
    I think you'll find the OP's cam is a hydraulic flat tappet, not roller.
    Yes Hydraulic flat tappet

    Quote Originally Posted by BasicQ View Post
    Agree with the above. Power would likely head south around the 6k mark. Find peak power on dyno +200rpm there's your shift light if you want one. RPM limiter a hundred or so after that for over-rev protection.
    Heads?
    Are rev limiters available? My ignition is Mallory HEI
    What do you recommend?

    Heads are DRP alloy 196cc/ 64cc 23 deg
    Intake is Performer RPM dual plane

  14. #14
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    Default Rev range

    Quote Originally Posted by 78HZSBC View Post
    Are rev limiters available? My ignition is Mallory HEI
    What do you recommend?

    Heads are DRP alloy 196cc/ 64cc 23 deg
    Intake is Performer RPM dual plane
    A lot of options out there:

    Step up to a cdi box

    https://www.summitracing.com/parts/msd-5520

    Or a replacement module:

    https://www.summitracing.com/parts/msd-83647

    Or a stand alone rev limiter

    https://www.summitracing.com/parts/pnx-600

    Iíve used Summit as itís pi55 easy to find parts. These or similar could be sourced locally but I always check with Summit for availability and price inc delivery as it is surprisingly sometimes cheaper.

    Heads and manifold would also point to peak @6k ish. Dyno is the last word on that though of course.


    Sent from my iPad using Tapatalk
    Last edited by BasicQ; 05-10-2021 at 06:51 PM.

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    In my opinion, the conrods become the limiting factor assuming that the combo can get into high rpms. Such as a rod exiting the block....

    And it's NOT the power stroke that does the damage. It is the exh stroke, where the rod & the bolts have to arrest the piston at TDC. This becomes a load of several hundred pounds at higher rpms & increases with the rpm.

    So bolt quality & strength/material of rod are big factors.

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