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Discussion Starter · #1 ·
I have a 69' 396 in my 70 chevelle. I have 3:31 gears. At 60 miles per hour I am turning about 3,000 rpms.I am not worried about engine noise or mileage. Is 3,000 rpms for a steady period bad for the motor? It was home built by a friend, not a high tech machine shop. The rpms don't bother me. I just don't want to damage it. What rpm is considered too high for cruising.

[This message has been edited by monkeyboy (edited 08-18-2001).]
 

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Hey Monkeyboy, My '69 SS(w/ 396)also has 3:31
gears. I was told by the previous owner that the motor was built back in 1989. Since then
about 20,000 miles has been put on it. I haven't had any problems. The motor just hums
right along. Seems like the only long term solution to drop the rpms is to install a
700R4 trans. bye
 

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I don't think that 3000 rpms for extended periods will hurt your engine, it may not be too good for your gas mileage, but it shouldn't hurt your engine. Maybe BillK will jump in here and give us his view.

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Leo Paugh
Maryland Chevelle Club #017
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there is only one way to fail, and thats to quit.
Maryland Chevelle Club
 

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O.k.....here's whgere I get to scare people.........

I have cruised mine at 4K yes 4k, but typically cruise her at about 3400 if on a long distance ride......4.10 gears a distance driving arent fun, so I cruise the car at points where she runs smooth and the car hums along.

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"Kickin ass on the wild side"

**Dave Armstrong**
'67 Chevelle
12.45 @ 107 mph. 1.73(best) 60ft
SOON TO BE 454 BSE!!!
'95 Firebird Formula LT1(2FST4LOV)
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Mine runs right at 3000 at 70-75mph. After two/three hours of steady driving I do notice my oil pressure begin to fluctuate about 5lbs. If I back down to 65 (about 2700), I don't have any problems. I've been to Nashville the last two years with the car driving 7-8 hours a day only stopping for fuel at the 3000 range but I really prefer 2600-2800 for long distances.

Whether it 'hurts' the engine or not is a matter of how it's built. If you can go a taller tire, overdrive, etc and get the rpms down in the 2400-2600 range for cruising, common sense would dictate the engine would last longer.


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Dale McIntosh
TC Gold #92
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www.dalesplace.com
Restore it or customize it, but drive it and enjoy it!
 

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<BLOCKQUOTE>quote:</font><HR>Originally posted by Big Block Dave:
O.k.....here's whgere I get to scare people.........

<HR></BLOCKQUOTE>


I once made the trip back home from college one evening in teh winter months. cruised about 4500RPM... Thinking it was 90 give or take a few. I figure, I built the damn thing, it better take it! Alas, the one thing I didn't build (the heads) wher ethe demise of the motor.



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71' 3880# with me. Big Block 402, Merlin oval heads, 10.2:1CR, TH400, 3.73 posi,
BEST 1/8th: 8.252 @ 84.56mph (1.855 sixty foot)
Last years previous best 1/4: 13.1sec @ 105 MPH (2.10 sixty foot) to be corrected soon.
Picture of me roasting the tires and other guy stuff
Video of me staging (smoke of course)
 

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3 G's @ 60 mph is just about purrrfect.
You young guys don't remember the days before overdrive!

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Myles Turner, the "odometer rollback specialist"

"The difference between Men and Boys are the price of their toys"

1970 SS 396 TH400
 

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Here's a solution for you to consider. Get some really tall tires and mount them on a spare set of wheels for your long distance trips.

I recently took my 70 SS on its first road trip since being rebuilt. It was a 400 mile round trip. I went to a salvage yard and got a pair of used 30" tall pickup tires and plain old 15" steel wheels for the highway portion of this trip. This lowered my rpm from 3350 @ 60 mph to 2900, (3.73 gear). When I got to the car show I had to swap back to my stock 14" SS wheels and then swap them back again 2 days later to cruise home. But, that's still better than turning the engine that fast for that long,IMO. It also helped the gas mileage. It DID look kinda funny, but only for a few hours.

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Bob Von Kaenel
70 SS 402ci TH400 (Yes, it's the dreaded Cranberry Red, and I like it!)

We've not been called upon to hold the fort, but rather, to storm the heights!
 

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Think about this......
My 69 Malibu has a 2:56 12 bolt (original).
193,000 miles on bone stock original 300hp 350.
Except for a new chain and an upper gear, IT HAS NEVER BEEN APART AND STILL RUNS LIKE A.... YOU KNOW WHAT !!!!!!!

I bought the car with 139,000 in 1985.
The guy I bought it from said he changed the oil every 2000 miles religiously since he bought the car in 1975.
If you wanna run your engine at sustained speeds at 3000 RPM and above, and you want it to last, you better keep good oil in it. (change it often. It's cheap insurance).
My 69 SS396 is built to stock specs (350hp), TH400, and 3:31 posi. It runs about 3200 rpm at around 70 mph. I've put 20,000 mi. on it in 20 months since the build. I have a 2:73 built and ready to go in for better MPG. The rpm's don't bother me, but the fuel mileage could definitely be better.


Nate


[This message has been edited by SS3964N8 (edited 08-18-2001).]
 

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The only problem I ever had was with a 69SS that I did a lot of highway driving with at around 3000 rpm. When I took the engine apart for a check out, the exhaust valves were hammered into the heads so bad they were almost unfixable. If you plan on a lot of highway driving I would get hardened exhaust seats installed.

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Dave
70 chevelle ss396 conv
66 chevelle ss396 hdp/conv
72 chevelle
TC member #493
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<BLOCKQUOTE>quote:</font><HR>Originally posted by drptop70ss:
If you plan on a lot of highway driving I would get hardened exhaust seats installed.

<HR></BLOCKQUOTE>

Good advice in any event. With unleaded gas, you'll need hardened exhaust seats...now or later


Side note: big block plugs sure last a lot longer with unleaded fuel.



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Dale McIntosh
TC Gold #92
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www.dalesplace.com
Restore it or customize it, but drive it and enjoy it!
 

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did you check your speedo?? my 331's run 60mph at 2500rpm. that's with very small dia. tires. check an rpm calculator, personally i don't run over 2500rpm during cruise. mine is a daily driver, though when i bought the car, i drove it at 70-75 at near 3000rpm from Minnesota to Texas.
but i like to keep the cruise rpms down.

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1970 SS396 AKA 454/4spd
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If the noise bothers you 3000 rpm might be a problem. It is not hard on the engine though. We have many engines that have run for days at a time at 3000 rpms with fairly heavy load. Many have lasted the equivalent of 200,000 miles or more.
 

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drptop70ss and Dale McIntosh.

I installed Martin/Wells hardened exhaust seats in my heads myself. I also machined and built and installed my 396 ! ALL, by myself.
(20,000 mi. ago !). Whoops, I better think about that some more. My speedo is off about 10 mph @ 70 mph !!

Nate
 

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I've posted this before and I'll do it again. The wear rate of a motor is proportional to rpm^3. That is a cube power.

Example:

Say a motor will last 5 years constantly running at 3000 rpm. That same motor would have lasted 8.6 years if the rpm was 2500!

5 * (3000/2500)^3 = 8.64

If this motor see's mainly 1/4 mile racing at an average of 5000 rpm, it would only last 1.08 years.

5 * (3000/5000)^3 = 1.08


RLK
 

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The biggest danger with higher rpm cruising is for those without hardened valve seats. High rpm and or high loads tend to eat away the valve seat. Our '67 with the L30 327 has run over 50k miles with 3.08 gears with no problems. Our L78 with 4.10 gears cruises in the 3000-4000 range on the highways. The engine cruises its smoothest and quietest at 4k. With over 15k miles with unleaded gas this engine has seen no ill effects so far. Both engines had quite a few years of unleaded gas before this so the seats have probably soaked in enough carbon to be partially hardened any way. Just some personal experience.
 

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3000 rpm @ 60 mph; you say you're running 3.31s? Perhaps your speedo gears are off a touch.
Even if you have 4.10s and your peak torque happen at 3600 rpm, you can still get some great rubber at 60 mph!

It is worth noting that the police specials (generally the big cars) used 3.07 gears for an acceptable compromise between decent acceleration, economy and engine life. 3.31 gears should be a great combo, although 3.55 is pretty acceptable. 3.73s are fairly busy and are probably the limit for all but diehards. The late 70's Z28s with 4 speeds and no a/c came stock with 3.73s (GR70-15 ~ 235 tires), and the engines they used weren't anything special.
 
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