Safe low-RPM range

[Mark06GT]

I’ve recently switched my stock muffler for a set of Accufab x-pipes. I have mixed feelings about the sound. On one hand, I think they sound more like the GT should sound (aggressive, grumbly, etc.), but between 2,000 and 2,800 rpm there’s an unpleasant resonance. If I drive above 3,000 rpm the resonance goes away, but I feel like I’m being loud enough that I might irritate my neighbors. The sound below 2,000 rpm is very tolerable.

My question is: What RPM is too low?

When in 6th gear on the highway, I’m usually below 2,000 rpm, so I know it’s okay to run the engine that slowly, but I’m a little concerned about lugging the engine if I consistently run it too low. Does anybody have any guidelines I can use to help me prevent damage?

 

[TEXAS GT]

Yes, 4000 rpm and screw the neighbors.:biggrin

 

[Empty Pockets]

When in 6th gear on the highway, I’m usually below 2,000 rpm, so I know it’s okay to run the engine that slowly, but I’m a little concerned about lugging the engine if I consistently run it too low. Does anybody have any guidelines I can use to help me prevent damage?

I'm not aware of any guidelines regarding your concern - or any resulting damage you'd need to worry about. My own rule of thumb for any hi-pro car has always been to chose a gear that keeps the engine revs at or above 2K PRETTY MUCH all the time (under some kind of load condition, that is). 'Nothing "scientific" behind that though! :biggrin

But, this subject/concern has never come up here on The 'Forum. And God knows if there was anything to worry about with regard to this issue - one of the techno/mechanical wizards on this board would have already thought of it, analyzed it, and suggested the solution long before now!:lol

 

[Shadowman]

I’ve recently switched my stock muffler for a set of Accufab x-pipes. I have mixed feelings about the sound. On one hand, I think they sound more like the GT should sound (aggressive, grumbly, etc.), but between 2,000 and 2,800 rpm there’s an unpleasant resonance. If I drive above 3,000 rpm the resonance goes away, but I feel like I’m being loud enough that I might irritate my neighbors. The sound below 2,000 rpm is very tolerable.

My question is: What RPM is too low?

When in 6th gear on the highway, I’m usually below 2,000 rpm, so I know it’s okay to run the engine that slowly, but I’m a little concerned about lugging the engine if I consistently run it too low. Does anybody have any guidelines I can use to help me prevent damage?

I understand your concerns however rest assured that your gal will be just fine in the 1800-2000 RPM under typical light load driving conditions and then drop a gear or two as needed for more spirited driving.

Shadowman

 

[BlackICE]

My own rule of thumb for any hi-pro car has always been to chose a gear that keeps the engine revs at or above 2K PRETTY MUCH all the time (under some kind of load condition, that is). 'Nothing "scientific" behind that though! :biggrin

I guess sitting in the garage comes under the non-load condition. :lol

 

[RALPHIE]

I guess sitting in the garage comes under the non-load condition. :lol

I'm surprised 2K in the garage didn't really bother the neighbors - along with re-ramping the driveway for a car that never moved in or out!

 

[Empty Pockets]

I guess sitting in the garage comes under the non-load condition. :lol

:skep NOW, WHUDDID I TELL YEW YESDERDEE? HUH? :mad

An' th' same goes fer YEW, RALPHIE!







...'couple uh no good, mizzerbull, dadgum, no account...

'Dissin' an OLD pursun. Whutz this Forum comin' too...

 

[DWR46]

With modern engines and lubricants, I would have no qualms about steady speeds as low as 1400-1500 rpm. What you do not want to do is accelerate hard and use large throttle openings from those low revs. Large throttle openings at low revs create significant loads on both the main and rod bearings that can potentially lead to eventual bearing failure. In reality, this was much more of a problem with the engines of 40 years ago, and rarely affects today's motors, but why take a chance. As Shadowman said, when you want to go fast, just drop down a couple of gears and get on it. However, light steady accelaration from 1400 would be just fine.

 

[roketman]

The beauty of the Gt guys xpipe is no drone at any rpm .Plus it is a true cross over x pipe.The note is musical.

 

[BlackICE]

Large throttle openings at low revs create significant loads on both the main and rod bearings that can potentially lead to eventual bearing failure.

I guess this is because of the marginal oil pressure and flow at low RPMs. The loads are no greater and probably less than at the peak torque RPM. I will have to remember to look at my oil pressure gauge when pulling from 1500 to see what the pressure is.

 

[Neilda]

With modern engines and lubricants, I would have no qualms about steady speeds as low as 1400-1500 rpm. What you do not want to do is accelerate hard and use large throttle openings from those low revs. Large throttle openings at low revs create significant loads on both the main and rod bearings that can potentially lead to eventual bearing failure.

I tend to follow this advice...

Generally I like to keep mine north of 2k and as DW says, don't stamp on it at low revs.

Also make sure the engine is warm too!

Whilst the GT is tractable in any gear, I think we all like to treat them with care as most intend hanging on to them for a while.....

 

[DWR46]

BlackICE: I do not pretend to be an automotive engineer, but I suspect another aspect of this is that the time between firing impulses applied to the crankshaft is longer at low rpm. At higher revs, the load on the crank is smoothed out by a higher number of impulses over a given time. Thus high load-low rpm operation creates more cyclical load on the bearings than high load at the torque peak. Your comments about oil pressure and film strength are right on the money.

 

[t32b]

I'll add that it's the resistance to these big power impulses that create stress. I have no idea if it's above the margin for an engine like our 5.4, but the combination of a tall gear (which requires more 'leverage' to overcome inertia and accelerate), and large throttle settings (creating larger combustion pressures) would place more stress on the piston crown, connecting rod, crank and beyond; compared to the opposite set of conditions. Exacerbated by lower oil pressure as a function of low RPM's and you're creating some tough conditions for the drivetrain.
Of course like EP, my GT sees extremely low RPM conditions when in my darkened garage. I can't afford to stress it by starting it up!

BlackICE: I do not pretend to be an automotive engineer, but I suspect another aspect of this is that the time between firing impulses applied to the crankshaft is longer at low rpm. At higher revs, the load on the crank is smoothed out by a higher number of impulses over a given time. Thus high load-low rpm operation creates more cyclical load on the bearings than high load at the torque peak. Your comments about oil pressure and film strength are right on the money.

 

[Mark06GT]

Thanks all for the advice. Very helpful. :thumbsup

 

[FM99]

I have followed this thread with much interest. I took my annual drive from Iowa to N.C. in little red. On the interstate I usually shift into 6th at 65 mph and I will then be running at 1,500 RPM. This has not seemed to be lugging on most of the interstate. However, I note in my owners manual (2005 FGT - completely stock) that Ford recommends shifting from 5th to 6th at 50 MPH for "best fuel economy". Now I do not care much about fuel economy, though it is fun to tell folks that I get about 24 MPG on my trips. However, shifting into 6th at 50 MPH would put the RPM at less than 1,200. This seems to me to be lugging and a possible source for engine fatigue. I would be intersted in what the experts have to say about freeway driving and RPM range. Thanks to all. This is a wonderful forum.

 

[BlackICE]

All auto makers suggest that you drive to maximize gas mileage to increase the corp fuel economy averages. I don't think running at 1200 RPM at a constant speed on level ground would do any harm to the engine. However driving this way leaves the car non-responsive to the gas pedal leaving you one less option in emergencies. Also it is not consistent with the character of the car and plain not fun. Of course EP always ran his even lower, at ZERO rpm while sitting in the garage.

 

[Empty Pockets]

Of course EP always ran his even lower, at ZERO rpm while sitting in the garage.

Anybody know of a reliable "hit man" in the 'Bay area of N. Cal...?

Anyone...?

 

[Mark06GT]

PM sent :secret:

 

[BlackICE]

Anybody know of a reliable "hit man" in the 'Bay area of N. Cal...?

Anyone...?

I guess I should clean and oil my tools, before Guido visits. I will have some 230gr JHPs waiting along with some 12g buck.

 

[FM99]

Dear BlackICE,
I agree that stepping on the right pedal in 6th at 65mph is about as responsive as stepping on a plum. I also enjoy running through the gears on short outings. However, when I have 600 miles ahead driving perhaps the most eye-catching machine on the highway, it is sometimes nice to put it in 6th and travel along where a very slight press of the pedal takes one effortlessly from 65 to 70. My inquiry was regarding potential engine fatigue mentioned by others. The owner’s manual states that for best fuel economy shift from 5th to 6th at 50mph. That is at about 1025 rpm, which seems to me to be lugging the engine and not very good or economical. I do not think the government bases CAFE standards on how people drive cars but on what the government has determined under test conditions. Thus, Ford would have no interest in how we drive our cars regarding fuel economy, other than to give information that would look good to the feds. But, would they be telling us to lug the car if they expected it to damage the engine? Previous posts suggested that one should stay above 1800 or 2000 rpm. I was interested in what the more mechanical members thought would be best for our babies on long trips.