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How fast is safe?

Discussion in 'The Garage' started by chriskh, Aug 21, 2006.

  1. chriskh

    chriskh Registered Member

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    After installing my tach, I notice at hwy speeds 70-80mph, I'm running over 3k RPM's. Is this safe for long distances on this motor? It's got a 350, 4bbl, 4x4, 4spd granny low tranny. I know the tach is right just from the sound of it as well as the fact that when it's idling, the tach reads right on the money.

    I will be driving this thing a lot pretty soon and I drive from Castle Rock to Colorado Springs and back all the time (approx 35 miles) where the speed limit is 75mph. What do you guys think? Anything to worry about?

    Thanks!
     
  2. 1977k5

    1977k5 3/4 ton status Vendor

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    At some point, under high RPM and light load (highway cruising with low gears), the pistons etc. start to become unstable and the piston skirts can rub the cylinder walls. If you listen you can probably hear the motor making a cyclic sound. I don't cruise with mine above 3K.
     
  3. pauly383

    pauly383 Daddy383 Staff Member Moderator

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    Your profile says you have 31's , drop some 33's or 35's on it since you have the gearing to carry the taller tire . Your cruise rpm will drop :thumb:
     
  4. chriskh

    chriskh Registered Member

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    Cool, I was thinking that might help too. It's gonna need new tires soon anyway so I'll probably grab a 4" lift and throw on some 35's at the same time. I would think that should reduce it at least a few hundred RPM's, no?
     
  5. NorthWestFab

    NorthWestFab 1/2 ton status

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  6. Pookster

    Pookster 1/2 ton status

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    its the speed limit, not the minimum speed.

    3000 RPM on a SBC is OK, from what I recall... the problem is if your doing it without any load-

    No overdrive?
     
  7. Chevy305

    Chevy305 6 Lug 14bsf Status Premium Member GMOTM Winner

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    Yup I'm in the same boat as you. I have the 465, 4.10 gears, and 35" tires. 75mph for me is about 3000rpm. I am no problem winding the thing up to 3500rpm and crusing there at about 85mph :D. I have done that 4-5 times recently for about 1 hour 15mins at that constant rpm. Afterwards just be mindful that your oil pressure drops below normal once you get back to idle because of the heat thinning the oil out a bit. That happens on all small blocks.

    The day or two afterward the engine ran smoother and more powerfull, probably because that rpm for that long must have dislodged some carbon deposits. :dunno: Who knows.


    Also if you are afraid of winding up the engine like that just think if that engine were in a boat it would see that kind of extended high rpm use just for cruising.


    Acording to that gear calculator, I guess I've taken my truck over 100mph before. :eek1: :eek1: How about that for a 305? :p:
     
  8. jonny290

    jonny290 Registered Member

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    <--noob with a noob question

    Do 4x4 guys pretty much throw speedo calibration to the wind and ballpark it when you start tweaking rear end ratios and tire sizes? I'm comin from the ricer scene (don't ask) and they can get real fidgety about their speedo gears. Wondering if you guys try to match it up or just say screw it...
     
  9. muddybuddy

    muddybuddy 3/4 ton status

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    i didnt mess with mine, but when i changed ratios, i did it based on tire size, so the speedo is only off by a little bit (i think mine reads fast by about 10%). if you how far off urs is then ur fine.
     
  10. Kyle Strong

    Kyle Strong 1/2 ton status

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    speedo? What speedometer?:rolleyes: I only use my tach. No gear in my t-case. That or just follow the flow of traffic, maybe a little faster
     
  11. tiger9297

    tiger9297 1/2 ton status

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    I know this is a rookie question but: I have a 700R4. Where this calculator asks for top gear, what would I enter here?
     
  12. K5er4Life

    K5er4Life 1/2 ton status

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    put in .70, thats for the overdrive that trans has. I like some of these calculators too..... www.4lo.com
     
  13. chriskh

    chriskh Registered Member

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    Thanks for the input guys. It's pretty helpful. :waytogo:
     
  14. Chevy305

    Chevy305 6 Lug 14bsf Status Premium Member GMOTM Winner

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    My speedo cable is way off. That is until it melted on my exhaust. So for about the past year I have been using my tach to go by. My tach once broke too so I just had to guestimate.
     
  15. pauly383

    pauly383 Daddy383 Staff Member Moderator

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    With a 700r4 , 3.73's , and 35's , I cruise around 2100 rpm @ 70 in overdrive . Was under 2000 rpm when I had 33's . I hardly , even in mountainous driving , ever see 3500 rpm at all regardless of gear . I think everyone needs a 700r4 . And the convertor makes wheeling awesome at idle speeds :D
     
  16. mudbog42

    mudbog42 1/2 ton status

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    I have a 350 with a 700R4 (86 K-5) with 4:88's and 35's and it cruises at about 3000 on the highway. I do a lot of driving with it especially from florida to tennessee and back. Is this bad? When I do downshift for grades it can get up to 4000 RPM's???
     
  17. Can Can

    Can Can Pusher Man Staff Member Super Moderator

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    Sheesh, everyone needs to relax. Some of you guys are WAY too worried about revving your motors any higher than idle. :rotfl:

    The engines in our trucks were designed to be able to run at 3000 RPMs steady for hours at a time, with no ill effects. Shorter periods at 4000 RPMs(like when passing or climbing a hill) are not unhealthy either, as long as your fluids are in good shape.

    I'd LOVE to watch some of your guys reaction when I run my 5.0 Mercruiser up to 4600 RPMs for 5 minutes at a time out on the water. :rotfl:
     
  18. beater_k20

    beater_k20 Banned

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    i used to wind my 355 to 5800-5900 all the time, cast crank, stock but played with rods. :crazy:

    never had any problem with it. i dont see why 3-4K is such a big deal to you guys.
     
  19. 1985_K5_Silverado

    1985_K5_Silverado 1/2 ton status

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    Highway cruising in a full-size isn't exactly a really light load for an engine, unless you think it only takes a couple of horsepower to push one of these sleds torough the air at 70 or 80 mph. I wish it did, because we'd all get fantastic highway mpg, wouldn't we?
     
  20. Chevy305

    Chevy305 6 Lug 14bsf Status Premium Member GMOTM Winner

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    Yeah I said it before and I'll say it again, we all drive bricks.
     

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