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How fast is safe?

chriskh

chriskh

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Joined
Jul 10, 2005
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Location
Castle Rock, CO
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!
 
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.
 
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:
 
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?
 
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?
 
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:
 
<--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...
 
jonny290 said:
<--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...
Click to expand...

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