[Z06] Relationship: RPMs and MPH
Thread Starter
Race Director
Joined: Jul 2007
Posts: 10,961
Likes: 2,643
From: Texas Hill Country
Hi Higgs -
Perhaps, but....
Automatic, on a hill, holding the car from rolling backwards by modulating the throttle to hold the car steady. Capture the RPM's necessary to hold the car steady.
Now, repeat that same rpm on flat level ground.
Did the rpm/mph ratio change?
On a viscous-coupled drivetrain, possible.
Not on a hard-coupled drivetrain IMO.
best regards -
mqqn
Perhaps, but....
Automatic, on a hill, holding the car from rolling backwards by modulating the throttle to hold the car steady. Capture the RPM's necessary to hold the car steady.
Now, repeat that same rpm on flat level ground.
Did the rpm/mph ratio change?
On a viscous-coupled drivetrain, possible.
Not on a hard-coupled drivetrain IMO.
best regards -
mqqn
Thread Starter
Race Director
Joined: Jul 2007
Posts: 10,961
Likes: 2,643
From: Texas Hill Country
I have noticed that if I raise the rear ride height, the car does go faster. I think it is because the center of mass of the car is going more downhill when I raise the rear ride height, and that allows gravity to provide more of a "forward pull" on the car, instead of just straight down. This weekend, I will try lowering the rear ride height and raising the front ride height and see if that slows the car down. (Center of mass would be going more uphill in this case).
Mr. Wizard is turning over in his grave seeing this thread; didnt anybody other than me actually watch the show?!?!?!?!?!?!?!
Mr. Wizard is turning over in his grave seeing this thread; didnt anybody other than me actually watch the show?!?!?!?!?!?!?!
Melting Slicks
Joined: Nov 2007
Posts: 2,780
Likes: 934
From: Indianapolis IN
My old Mustang = old, busted hotness (my 2003 Cobra was nothing more than a fancy 1979 Fairmont)
My (new to me) 2002 Z06 = new hotness
OK, before everyone labels me as the dumbest person on the face of the earth, I do hope ya'll realize I was kidding about the whole ride height affecting vehicle speed. I forgot the
and the 
and the 
Race Director
Joined: Jun 2003
Posts: 14,051
Likes: 9
From: Houston TX
I was always 100% sure and confident of the constant relationship between rpm and speed in a manual transmission automobile - but that other topic had such convincing sounding mensa manucia I began to wonder - for about 8.9s...
How can one even try to begin to 'prove' that a given rpm would produce a different speed with a manual tranny?
I mean without tyre slippage, slutch slippage or gear mangling how would it be even possible for "the wind" to alter the mechanical relationship of the conrods, crank, clutch, input shaft, gears, outputshaft, propshaft, diff, half shafts, hubs, wheels and tyres?
Even taking into account drive line slop, clutch slippage and tyre growth/shrinkage surely the difference in rpm for a given speed will be minimal.
I dont even think mashing the gas would increase the HP at that point, as HP is a function of RPM - you need the revs to produce HP.
That other thread was almost as exciting as the QM/LG Cam thread earlier :P hehehe
How can one even try to begin to 'prove' that a given rpm would produce a different speed with a manual tranny?
I mean without tyre slippage, slutch slippage or gear mangling how would it be even possible for "the wind" to alter the mechanical relationship of the conrods, crank, clutch, input shaft, gears, outputshaft, propshaft, diff, half shafts, hubs, wheels and tyres?
Even taking into account drive line slop, clutch slippage and tyre growth/shrinkage surely the difference in rpm for a given speed will be minimal.
I dont even think mashing the gas would increase the HP at that point, as HP is a function of RPM - you need the revs to produce HP.
That other thread was almost as exciting as the QM/LG Cam thread earlier :P hehehe
Thread Starter
Race Director
Joined: Jul 2007
Posts: 10,961
Likes: 2,643
From: Texas Hill Country
Le Mans Master
Joined: Dec 2001
Posts: 7,788
Likes: 6
From: Winchester TN
Cruise-In III Veteran
Cruise-In IV Veteran
Cruise-In V Veteran
St. Jude Donor '03-'04-'05-'06-'07-'11
Yes, if you are allowing the car to rest on the transmission the TC will be resting on the movement point and if you are sitting still you have to flash to the stall speed (approx) to get moving. This takes a split second to accomplish when you press the gas.... This is a little out of the realm of aerodynamic resistance, lol, which is part of the discussion.
The topic, as you stated it, is:
Originally Posted by Higgs
The debate is whether aerodynamics, friction, or engine load can affect the engine RPMs at the same MPH...
post here.
I never thought about this until I read your post, but interesting discussion.
best regards -
mqqn
Burning Brakes
Joined: Oct 2001
Posts: 983
Likes: 23
From: Northeast Ohio
Remember HP/TQ curves are just one measure of output (WOT). More rpm if it's still flowing well will make more hp, But so will part throttle cruise to wot.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
Burning Brakes
Joined: Oct 2001
Posts: 983
Likes: 23
From: Northeast Ohio
In the original topic manuals were mentioned a few times, But as Higgs mentioned earlier even with a automatic at cruise the tc should be locked. In the case of using the throttle to hold position on the side of a hill the tc will be slipping mph will not equal rpm x gear ratio.
Burning Brakes
Joined: Apr 2007
Posts: 1,030
Likes: 1
From: Douglasville Ga
Ok...so all emotion aside and just looking at the facts. Everyone has made very good points, but I have a situation to suggest. You are all saying that in a certain gear at a certain rpm you will be going a certain mph...correct?? Why then can I get in the car and put it in first/second gear, then step on the brake (and/or pull the e-brake) and step on the gas a little (not floor it) and the car doesn't move at all but the rpm's go up. The engine is trying to go, but there is an opposing force applied. Besides....has anyone actually gotten in their car and did the experiment. I was going to yesterday, after reading the other post, but it was raining.
Just puttin' that out there.
Just puttin' that out there.
Burning Brakes
Joined: Oct 2001
Posts: 983
Likes: 23
From: Northeast Ohio
Ok...so all emotion aside and just looking at the facts. Everyone has made very good points, but I have a situation to suggest. You are all saying that in a certain gear at a certain rpm you will be going a certain mph...correct?? Why then can I get in the car and put it in first/second gear, then step on the brake (and/or pull the e-brake) and step on the gas a little (not floor it) and the car doesn't move at all but the rpm's go up. The engine is trying to go, but there is an opposing force applied. Besides....has anyone actually gotten in their car and did the experiment. I was going to yesterday, after reading the other post, but it was raining.
Just puttin' that out there.
Just puttin' that out there.
When you're stopped with an automatic in gear, and step on the gas, the converter is slipping.
With a manual there's no slipping (or shouldn't be any), once you step on the gas, the rpm will start to go up. Any jump is probably instrumentation. And our cars are quite powerful, the rpm will go up quickly depending on the gear you're in.
Also if you're measuring speed after some distance at wot, the mph/rpm you measure will differ depending on grade, wind, starting speed (drive out of a corner), weight, towing or not, etc. Same with top speed, it is dependant on those effect, but at 60 mph@rpm cruise it's not.
Oh, as for doing the test, the issue will be maintaining the proper throttle to maintain a constant rpm. I will also point out you can use your cruise control to show the 60 mph=x rpm, As long as you realize the control loop isn't designed to maintain exact rpm, only regulate the mph to within a few mph.
Last edited by QKSLVRZ; Jan 9, 2008 at 09:20 AM.
Burning Brakes
Joined: Apr 2007
Posts: 1,030
Likes: 1
From: Douglasville Ga
When you're stopped with an automatic in gear, and step on the gas, the converter is slipping.
With a manual there's no slipping (or shouldn't be any), once you step on the gas, the rpm will start to go up. Any jump is probably instrumentation. And our cars are quite powerful, the rpm will go up quickly depending on the gear you're in.
Also if you're measuring speed after some distance at wot, the mph/rpm you measure will differ depending on grade, wind, starting speed (drive out of a corner), weight, towing or not, etc. Same with top speed, it is dependant on those effect, but at 60 mph@rpm cruise it's not.
Oh, as for doing the test, the issue will be maintaining the proper throttle to maintain a constant rpm. I will also point out you can use your cruise control to show the 60 mph=x rpm, As long as you realize the control loop isn't designed to maintain exact rpm, only regulate the mph to within a few mph.
With a manual there's no slipping (or shouldn't be any), once you step on the gas, the rpm will start to go up. Any jump is probably instrumentation. And our cars are quite powerful, the rpm will go up quickly depending on the gear you're in.
Also if you're measuring speed after some distance at wot, the mph/rpm you measure will differ depending on grade, wind, starting speed (drive out of a corner), weight, towing or not, etc. Same with top speed, it is dependant on those effect, but at 60 mph@rpm cruise it's not.
Oh, as for doing the test, the issue will be maintaining the proper throttle to maintain a constant rpm. I will also point out you can use your cruise control to show the 60 mph=x rpm, As long as you realize the control loop isn't designed to maintain exact rpm, only regulate the mph to within a few mph.
Thread Starter
Race Director
Joined: Jul 2007
Posts: 10,961
Likes: 2,643
From: Texas Hill Country
I agree with all that, and thats why I brought up the point that if I have the e-brake on and step on the gas a little. Just enough to make the car try and accelerate against an opposing force. Obviously laying tha hammer down will not do much good as the tires will spin no matter what.
Thread Starter
Race Director
Joined: Jul 2007
Posts: 10,961
Likes: 2,643
From: Texas Hill Country
You're right. My intention was engine load at WOT (top speed) as opposed to part throttle/from a stop. There are a lot of variables at part throttle activities and autos.
Burning Brakes
Joined: Apr 2007
Posts: 1,030
Likes: 1
From: Douglasville Ga
Well, of course. That's what drag racers call "Stalling it up" for a stronger launch. When you put in a bigger stall torque converter, you increase the RPM capacity that the converter will hold before allowing power to the wheels. You can stall it up higher (into a more powerful area of the RPM range). This is also why higher stall converters get worse mileage. They slip much longer and you don't go forward as much for each given RPM up to the stall point.
Burning Brakes
Joined: Oct 2001
Posts: 983
Likes: 23
From: Northeast Ohio
As long as there's no slip, it will be mph = x rpm, with automatic's the torque converters are made to slip under certain conditions, with manuals, with the clutch working properly, and the tires hooked up, there is no slipping.
Race Director
Joined: Jul 2003
Posts: 12,742
Likes: 43
From: Color my life with the chaos of trouble.
But doesn't that go against everything everyone has been saying. Basically....in a given gear, at a given rpm, my tires should be moving at a given mpm; but applying a load (i.e. the brakes) will change the mph at a given rpm, and if at low rpm, why can't an applied load change the mph at a high rpm. Granted the effect will not be as pronounced, but it will be there. But I plan on giving it as close to a real world test as I can this evening, and with an open mind as well.
Race Director
Joined: Jun 2003
Posts: 14,051
Likes: 9
From: Houston TX
I guess my point was that given the fixed mechanical relationship of the driveline any increase in rpm must result in an increase of speed - but that an increase in throttle opening does not always result in an increase in rpm and thus not always an increase in speed.
Last edited by RC45; Jan 9, 2008 at 12:53 PM.
Le Mans Master
Joined: Mar 2002
Posts: 5,955
Likes: 161
From: PNW
... it's not rocket science.