Sustained Cruising RPMs: how low is too low?

With regards to cruising RPMs, how low is too low? With the .5:1 6th gear of a T56, the possibility for some really low cruising RPMs exists, and I’m wondering when it becomes detrimental to the engine? With the gas prices as they are, extra highway mpg is always a good thing in my book.

For example:

.5:1 6th gear / 3:42 rear end ratio / 26” tall tires

75mph =1650RPM
70mph = 1540RPM
65mph = 1430RPM
60mph = 1320RPM

The same combo with 3:23 rear end ratio:

75mph =1560RPM
70mph = 1450RPM
65mph = 1350RPM
60mph = 1250RPM

With the 3:23s, how low is too low: 1300, 1200, etc? I’ve heard people say you’re lugging the car, if cruising below 1500rpms, and I tend to agree, but GM recommends shifting into 6th at 49mph, figure they’re assuming a 55mph speed limit. With a bone stock 4th generation F-body, RPMS are at 1230. Basically I’m just wondering where you guys draw the line? 500rpms above idle, 700, 1000? I realize cars with large cams will need larger gears to keep the RPMS up around 2000. However, considering cars with mild or even stock cams, in the case of LSX motors; what is sufficient?

Just for the record, I’m not considering changing gears in my 4th gen, but rather thinking of another car, where highway mpg is not necessarily a concern, but any increase is appreciated. If you’ve got the HP (500+) and the car is mostly all street driven, I don’t really see the need for 3:73s or 4:11s, since you’ve likely got more than enough power. With the wide spread of gears, why not keep the revs really low on the highway and net 32-35mpg?

Thoughts?

Ralph,

I have 3.25's out back in my Elky with an LS1 T56 (.5:1 6th gear). I'm on 225-70/15's, so 70 mph is right around 1400 RPM's and I do not consider it too low, even with a ZZ4 with 1.6 rockers. There's still plenty of torque available to change speeds and get up hills, and 5th is never far away, either. I thought I might want to change to at least a 3.50 rear gear, but that car pulls plenty hard enough for me and I really like the practicality of the lower cruise RPM's since this will be my daily driver once I get it all sorted out.

Ryan

Ryan,

Thanks for the feedback, that's what I wanted to hear. A lot of people seem to dislike the "double" overdrive of the T56, but I really like 6th gear when I'm crusing on the highway.

While Frank and I were trying to decide what gear to order up for Eternity, these are some the the considerations and estimates he calculated...

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OK, here are some numbers. Assuming the car weighs 3400LBS done. We can make some ET predictions based on HP.
At 550FWHP (10lbs boost?), we should net around 440RWHP using a loss efficiency of about 80% through the drivetrain. Based on 440RWHP she should be able to run 11.50 @ 118MPH

At 650FWHP (14lbs boost?), we should net around 520RWHP using a loss efficiency of about 80% through the drivetrain. Based on 520RWHP she should be able to run 10.90 @ 125MPH

Now to run 118MPH in the quarter at 5500RPMs ( which is where I would want to stay RPM wise on a regular bases), and assuming a 8% converter slippage (1..08 trans ratio), we need 331 gears. To make it run 125MPH we would need to turn it 5800RPMs

The car has sufficient horsepower to weight ratio to pull overdrive for top end, so assuming a net transmission ratio of .80 (.72 + .08 converter slippage. Top speed at 5500 should be right at 160MPH

For cruising RPMs. In overdrive, assuming a net ratio of .76 (.72 + .04 converter slippage at cruise), you should turn 2250RPMs at 70MPH, 2000RPMs @ 60MPH, and 1600RPMs at 50MPH.

So we need to order a 3.30 rear

Frank

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I see it a bit different. Build the car for the purpose in mind. I have 4.11 w/ a t56. The six gears give me the ability to keep the rpm's in a certain range and then the overdrive to travel. In sixth im at 2k rpm's at about 80 mph. My 396 has a 292h cam that tends to like the upper (above 2k) rpms more. Any lower and she feels like she is waisting more gas (lugging). Soon I will be changing to a 383 – 500 hp motor intending to put it on the track (road coarse) more. To me the hp is not a determination of gear selection. The rpm band is.

6th never felt right in my 2000 Z, unless I was north of 75mph...

I'm one of those guys that is mostly unimpressed with the .5 OD in the T56. I found that to be a useful gear only when driving straight and level roads at 70+ mph. Here in the northeast, that doesn't happen much. Add a hill, A/C, slow down until Grandpa moves over for you, etc. and you're in 5th. In other words, the tranny made the car tri-modal: 1-4 are performance gears, 5th is overdrive for normal highway driving, and 6th is for driving across Kansas at 85 mph.

For a high-performance engine with enough cam to get to 500+ hp, I can't imagine cruising at 1700 rpm. I would think the engine would surge, buck, and be downright unhappy. I think you should think in terms of 2000-2400 rpm for cruising.

jp

Similar question came up a year or so ago (okay, I confess, I remember because I was the one that asked it).

https://www.pro-touring.com/forum/showthread.php?t=915

Might be worth a read on this subject.

How you would figure it out if you had all the information (equations simplified):

Your first “known” is your desired speed, V_car
From this, you can compute losses due to aero and rolling losses in the tire. You can also make a first estimate at drivetrain loss (this is an iterative problem)

Road Load Power = P_tire+P_drivetrain+P_aero

At constant speed, the engine produces just enough power to match all the losses. If it were producing more, you would accelerate. If it were producing less, you would decelerate.

P_engine = RLP

So, you know how much power you need to produce. Now what you need (and probably don’t have) is a complete BSFC map for your engine. This gives you the engine’s efficiency in fuel consumtion per horsepower (lbs_fuel/[hp*hr]) versus torque and engine speed. Since horsepower = T*N_engine/5252, you can draw a constant power equal to RLP, and use it to find the highest efficiency point ALONG that line. Follow the intersection point down to the RPM axis to give you the engine rpm.

KEY POINT
You will NOT be at the engine’s peak VE/BSFC. If this were the best for economy, GM would gear its V8s to cruise 70 mph at 3000 rpm.

Now that you have the desired engine speed and desired vehicle speed, gearing can be calculated:

N_engine = V_car*gearing factor (NOT just rear end gearing, also has tire size)

Using the engine speed and gearing, you can now check you estimated drivetrain losses using engine, transmission, and rear end data, and re-calculate using more precise info for road load power.

General guidelines: every single component of the road load power is speed-sensitive. In other words, the best way to save fuel is to slow down! Assuming you don’t want to slow down, you can minimize the drivetrain losses by at least slowing IT down with tall gearing.

You engine’s BSFC map will have it’s highest values at high torques, in other words closer to WOT. To have a high torque without having high power, you need low RPMs, so here is another reason that tall gears help increase efficiency. More throttle means less pumping losses, greater efficiency.

As pointed out, an engine’s efficiency will fall off if the rpms get TOO low, but to say precisely where would require the BSFC map

Isn't lugging also a consideration for the health of the transmission? I do know it's not recomended to tow in overdrive. Think about how hard it is to pedal a bike in it's top gear. You are straining, pushing really hard on the pedals, and pulling up on the handlbars. Everything gets easier as your speed increases. Seems to me the gears in the trans. would try to climb away from each other and pressure the shafts. Maybe it's not even an issue, just think it may be. Anyone have an opinion on this?
Brett

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Originally Posted by Jagarang

While Frank and I were trying to decide what gear to order up for Eternity, these are some the the considerations and estimates he calculated...

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I understand the consideration when trying to figure out what gears will be most efficient for the car in terms of a performance stand point, but I'm mostly concerned with how low is too low.

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I see it a bit different. Build the car for the purpose in mind.

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My '68 is being built as my race car. 600rwhp, .82 overdrive and 3:25 gears. I'll be right there with you crusing 70mph at 2400rpms. The car I'm wondering for, would be being built as more of a dialy driver, so gas mileage is part of the purpose in mind. I'm not too concerned about performance and the RPM band, as I think the car will still be plenty fast. My 3600lbs '01 SS is good for High 12s with 360hp and 3:42s. This car will have another 150hp, and weigh 200lbs or so less. Even with 3:23s, the car should run 11s. However, the car will also need to commute 60 miles per day once or twice a week, and it'd be nice to pull the 30mpg my SS gets.

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6th never felt right in my 2000 Z, unless I was north of 75mph...

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I really must be one of the few, heh. I shift into 6th around 60mph if I know I'm going to be sustaining that speed for atleast 2 or three miles. It feels fine to me anywhere above 60-65. Ya gotta love crusing 2,000 rpms at 90, too.

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For a high-performance engine with enough cam to get to 500+ hp, I can't imagine cruising at 1700 rpm. I would think the engine would surge, buck, and be downright unhappy. I think you should think in terms of 2000-2400 rpm for cruising.

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The cam in this car would be fairly small .224/.228 w. AFR 205s and Fast 90/90. Should make 440-460rwhp and idle like stock. My '68 will be crusing around 2000-2400, but for the purpose of this car, unless absolutely necessary, that would be unacceptable for me.

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Similar question came up a year or so ago (okay, I confess, I remember because I was the one that asked it).

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Thanks, Derek. Lots of good info in there.

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How you would figure it out if you had all the information (equations simplified):

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Whoa, that's pretty in-depth. Sounds like I need to do some research for more numbers on the motor (map).

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Isn't lugging also a consideration for the health of the transmission? I do know it's not recomended to tow in overdrive.

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I'm sure it can't be good for the tranmission, either. I just can't get over the fact that GM recommends shifting into 6th at 49mph. That's extremely low, and I'm wondering why they're not concerned with lugging. 70% of the people who bought 4th gens probably had no idea they could harm their motor by driving with the RPMS so low. CAGS is another thing. It would shift the car into 4th at 20mph or so, and REALLY lug it. You'd be at idle when the clutch engaged, well maybe not idle, but 1000 rpms or so.

Thanks for all the replies.

Gear selection in these Pro Tour highway cruisers is a very important consideration, especially when combined with high HP, highly camed motors. Long story short, you do not want the engine RPMs so low where there is little horsepower. One thing to avoid as JP mentioned, is avoiding cam overlap. Even the widest lobe center cams, big enough to make any real HP, let's say north of 450HP, at low RPMs can have a fair amount of overlap. In other words, they are still cutting a lick, choppy, hitting, or whatever other term you want to use.

Frank,

Thanks for the info, that is more the point I'm getting at. My engine knowledge is ill so please bear with me here. How does one determine where (what RPM range) cam overlap will become detrimental? Can it just be determined from the cams specs? Or is it a vary-by-use type of thing that will be different with each car because of the tuning, etc?

Well, it could be as simple as looking at the RPM range of the cam card. There are so many variables, I do not know if I am capable of exlaining it all. But a few things I can tell you.

1 It's all about duration, well not all about it, but lift means nothing here.

2 Lobe centers are important, tighter lobe center, like 106 / 108, make lots of peak power, but are radical miserable beotchs down low. Wider lobe centers like 112 /115 are smoother, and make more low end torque, but not as much peak horsepower. So for our purposes in the PT community, we should look for wider LCs.

3 FI systems, blowers, and nitrous all like wider LCs.

4 The larger the engine CI, the smaller a cam will act in it. So in other words, a 235 duration cam in a 350 on a 110 LC will sound pretty choppy, like it has a lot of cam in it. Put that same cam in a 427 small block, and it will sound like a station wagon motor. So it is hard to say how much is too much

5 Fuel injection can compensate for some overlap and give a better AF were a carb will not be as good at providing perfect AF at all RPMs

6 Another thing about luging under the HP range of a motor, it tends to instigate detonation. And detonation is DEATH!

Your in the LSX world, and I am the first to admit I am not the expert on which cams act what way. I would suspect Tony is the man for that.

Thanks for the info, Frank. There's some good stuff in there.

The combo I would probably use, as it's proven would be

AFR 205 CC heads milled for 10.5:1
Ported FAST 90/90
Cam specs are: 224 228 .581 .588 114

It's a fairly small cam for a 346ci, and has a wide LCA, so it seems cam overlap probably wouldn't be too bad, especially with the EFI of the LS1. The combo should make 450rwhp or so (through a manual) and have real good street manners. Tony Mamo of AFR made this combo famous. He made 480rwhp with extensive tuning. Most guys were going with huge cams on stock cube motors to make that number.

I'd likely spray a 100 shot on it too, for when the 500hp just wasn't enough.

The more I think about it, when the time comes I'll probably just run 3:42s. I tend to cruise around 70-75mph, and 1600-1700rpms should still net me 26-30mpg.

Thanks for the help,

When I installed my T56,my car had 3.50 cogs. Got 23-24mpg on the Power Tour but always found myself clicking back up to 5th every time a small hill came along.
Installed some 4.11's last Summer and the car likes them better. Mileage is still up around 22mpg.
The cam timing/gearing interface can't be overstated.

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The combo I would probably use, as it's proven would be

AFR 205 CC heads milled for 10.5:1
Ported FAST 90/90
Cam specs are: 224 228 .581 .588 114

It's a fairly small cam for a 346ci, and has a wide LCA, so it seems cam overlap probably wouldn't be too bad, especially with the EFI of the LS1. The combo should make 450rwhp or so (through a manual) and have real good street manners. Tony Mamo of AFR made this combo famous. He made 480rwhp with extensive tuning. Most guys were going with huge cams on stock cube motors to make that number.

I'd likely spray a 100 shot on it too, for when the 500hp just wasn't enough.

The more I think about it, when the time comes I'll probably just run 3:42s. I tend to cruise around 70-75mph, and 1600-1700rpms should still net me 26-30mpg.

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We are talking about project FANTOM RIGHT?!?!?! :box:

No Vince. This is for his "other" f-body project.

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Originally Posted by MrQuick

We are talking about project FANTOM RIGHT?!?!?! :box:

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Fantom's getting a 402 :) Gas mileage is of no concern there.