Engine Revs at highway speed
Thread Starter
Racer
Joined: Jan 2010
Posts: 498
Likes: 77
From: DFW Texas
Engine Revs at highway speed
So I have a 330 horse crate engine in my 74, with 411 rear end and 4 speed. Recently drove down to Austin from Dallas cruising about 65-70mph. I am a little concerned about driving longer than a few hours as I am turning about 2800 rpm's on the clock. I am assuming that I should be closer to like 2000 at highway speed and have been considering adding a 5 speed but that is expensive. Would a different rear gear also be an answer and if so, what would be the best match for a mostly cruiser car which is occasionally hot rodded?
Race Director
Joined: May 2006
Posts: 18,737
Likes: 2,583
Im amazed the RPM number is that low, sounds like someone didnt install the speedometer gear to match the rear end. With a 4:11 at 65 to 70 your tach should be close to 4,000 RPM. There are online calculators to plug in tire size and gear to find matching engine speed and vice versa. Best all around gear would be 3:55.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
There is no issue at all to be cruising at 2800 rpm - this will not have any effect on engine longevity or reliability. A good rule of thumb is to keep continuous cruise rpm below 2500 feet per minute piston velocity. Do a little arithmetic to figure that one out, and you'll see that you can cruise at a sustained high rpm with no engine problems whatsoever.
Lars
Lars
Last edited by lars; Nov 5, 2012 at 05:55 PM.
Thread Starter
Racer
Joined: Jan 2010
Posts: 498
Likes: 77
From: DFW Texas
Thanks Lars. Have been tweeking my timing a bit thanks to you as well. I recently turned it back a couple of degrees as I was getting an occasional back fire thru the exhaust. Had also been running premium and now on mid level octane. NO issues. Thanks again for everyone's response.
Burning Brakes
Joined: Jun 2009
Posts: 840
Likes: 4
From: Syracuse IN
We routienly use the ZZ4 rotating assy in several Marine applications. Constant operating rpm at 4k and above. With a max rpm of 5,700. Now that is not the max the engine can turn but is max that the marinizer will allow, hence knowing some will run WOT for extended runs. 2,800 is not high at all. Al
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Keep in mind that afterfire out the exhaust is seldom a timing issue - it's a lean condition that will cause popping in the exhaust. You might want to consider richening up your mixture a tad and bringing the full timing back.
Lars
Lars
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Racer
Joined: Oct 2010
Posts: 259
Likes: 3
From: Plano TX
Speed
I happened to do a gas mileage test this past weekend on my '73 and ran the car at a steady (as I could) 2500 rpm. That was about 64 mph with 3:08 gears running 255/60 - 15 tires, a "proper" OD size.
70 mph is right at 2700 rpm, so I'd also question the 4:11 rear end, unless as has been suggested, the speedo gear is really off.
Was pretty ok with the gas mileage though, with a 454 running a Crane 13424 cam and kit, Holley 770 SA, 10.2 CR and factory manifolds, I managed 14.44 over a 130 mile distance at the steady 2500.
70 mph is right at 2700 rpm, so I'd also question the 4:11 rear end, unless as has been suggested, the speedo gear is really off.
Was pretty ok with the gas mileage though, with a 454 running a Crane 13424 cam and kit, Holley 770 SA, 10.2 CR and factory manifolds, I managed 14.44 over a 130 mile distance at the steady 2500.
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
First: Get a dwell meter or other instrument that will measure engine RPM and compare its readings with those on your tach. This is just to verify that your tach is giving you accurate RPM data.
If tach is OK, you likely have a 3.36 or 3.55 rear gear. My '71 with auto tranny (same hi range gear ratio of 1:1) ran at 2800 at 70 mph before I installed an O/D trans. With some converter 'slop' from my auto trans, you might get lower rpms with the same rear gear but a manual tranny...so, maybe 3.55.
In either case, adding a O/D tranny to those differentials will provide you with optimum flexibility for power and economy on the highway. If the money is not there yet, either wait till it is or make your choice between power (keep what you have) or economy (3.08 would be my suggestion).
If tach is OK, you likely have a 3.36 or 3.55 rear gear. My '71 with auto tranny (same hi range gear ratio of 1:1) ran at 2800 at 70 mph before I installed an O/D trans. With some converter 'slop' from my auto trans, you might get lower rpms with the same rear gear but a manual tranny...so, maybe 3.55.
In either case, adding a O/D tranny to those differentials will provide you with optimum flexibility for power and economy on the highway. If the money is not there yet, either wait till it is or make your choice between power (keep what you have) or economy (3.08 would be my suggestion).
Race Director
Joined: Nov 2002
Posts: 12,142
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From: Taylor Michigan
with the proper size tires {stock size} a 4.11 gear will have a 2-1 ratio RPM to speed......or for example 70 MPH will register 3500 RPM's.....so 2800 would be reading in the 55 MPH area if it has or with a 4.11.....
Melting Slicks
Joined: Aug 2012
Posts: 3,487
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From: Titusville Florida
Here's a calculator I found with a quick google search
http://www.csgnetwork.com/multirpmtirecalc.html
http://www.csgnetwork.com/multirpmtirecalc.html
Melting Slicks
Joined: Oct 2002
Posts: 2,317
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From: St Louis MO
Better than guessing at your rear end ratio, or using a calculator which assumes that the mph or rpm is correct, test it!
Jack the car up on jack stands; put it into neutral and position one rear tire to have the valve stem full down and mark a spot on the drive shaft. With posi-traction, rotate the tire one full turn while counting the number of turns of the drive shaft - this is your rear end ratio; if non posi-traction and only one wheel turns, multiply the driveshaft turns by 2 to determine the rear end ratio. Alternately, it may be easier to turn the driveshaft until the wheel(s) make(s) one full turn.
Jack the car up on jack stands; put it into neutral and position one rear tire to have the valve stem full down and mark a spot on the drive shaft. With posi-traction, rotate the tire one full turn while counting the number of turns of the drive shaft - this is your rear end ratio; if non posi-traction and only one wheel turns, multiply the driveshaft turns by 2 to determine the rear end ratio. Alternately, it may be easier to turn the driveshaft until the wheel(s) make(s) one full turn.
Drifting
Joined: Jun 2008
Posts: 1,495
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From: Blanco County, TX
65-70 @ 2800 is not 4.11's unless you have really big tires. With 245/60/15's and 4.11's 70mph is 3627rpm. 3.73's are 3292 rpm. 3.55 is 3140, 3.36 is 2965, 3.08 is 2718.
Since you probably have a 3.36 rear, you're rpm's about as low as you can go without a transmission swap. Assuming you have an M20 transmission (2.56 first), I personally would't go lower than you are, especially if you are occasionally hotrodding. In fact, I might consider going to a a bigger gear. I'm learning quickly you can't have everything.
Good to know. But why, if I may ask?
That would be 4310 rpms with a 3.48" stroke.
Since you probably have a 3.36 rear, you're rpm's about as low as you can go without a transmission swap. Assuming you have an M20 transmission (2.56 first), I personally would't go lower than you are, especially if you are occasionally hotrodding. In fact, I might consider going to a a bigger gear. I'm learning quickly you can't have everything.
That would be 4310 rpms with a 3.48" stroke.
Last edited by Kevin68; Nov 6, 2012 at 06:20 PM.
Melting Slicks
Joined: Jul 2009
Posts: 2,091
Likes: 40
I'll bet it is 3.55 rear. 3.70 runs about 3350-3400 RPMs , haa LOL... if you had a 4:11 you'd be in the 4000RPM + you'd really be very unhappy , if 3:55's worry you.
For average use 3:36 -3:70 gives the most fun without dreading a 100 mile trip over a 70 mph highway.
I think 3:70 is perfect , instant power.
Either install a 5spd or put a 3.08 sissy gear in it....
For average use 3:36 -3:70 gives the most fun without dreading a 100 mile trip over a 70 mph highway.
I think 3:70 is perfect , instant power.
Either install a 5spd or put a 3.08 sissy gear in it....
Melting Slicks
Joined: Aug 2012
Posts: 3,487
Likes: 83
From: Titusville Florida
I have a 76 with 4 speed and 3:55 gears, I have been pondering this very issue. My tach is wacky so I'm not sure what RPM's I'm turning at highway speeds but I know it's more than I like. I had thought about going the easy route and put 3:08's in but a buddy talked me out of that so I will stick with current gears and WHEN I CAN I will put in a 5 speed but I really don't want to mess with all the hydraulics and bell housing so I will be looking for the best/least fab work I have to do to drop in a 5 speed... just my 2 cent's for what it's worth 
Le Mans Master
Joined: Aug 2010
Posts: 5,339
Likes: 546
From: Central Texas
The overdrive transmissions are the ticket when you have higher numerical gearing. The best of acceleration and economy. Imo I'd keep the gearing and put an overdrive in if you truly have the 4.11s.