Quarter mile times for bone stock LS1/A4

[1g1yy]

Oh, and I did not mean to imply that it would take less fuel during around town driving than at highway speeds, just that 3.15 gearing could (depending on how it is driven) result in a higher MPG figure around town relative to a car with 2.73 gearing. This because there is less "external load" on the engine with greater torque multiplication, therefore less need to get into the throttle as much. Obviously highway driving with appropriate gearing uses the least fuel.

 

[C5Richard]

Come on guys -- this is not rocket science!! At highway speeds the 2.73s use less fuel -- not because of less engine friction from lower RPMs, but because it has enough torque at that lower RPM to maintain the cars speed while the accelerator pedal is not having to be depressed as far (in other words the injectors are giving it less fuel). It takes very little power to maintain an aerodynamic C5s speed at seventy. At highway speeds with 3.15s the throttle is open a bit further because RPMs are higher, so it uses a bit more fuel -- though only about 2 or 3 MPGs.

At low "around town" speeds the 3.15s can get slightly better MPG because it takes less effort (less depression of the accelerator, therefore less fuel) for the engine to get the car moving.

Why do you think the general fixed the M6 tranny as he did!!? He gave you a low gear to takeoff, and high gears for cruising. Trust me -- he did some MPG testing!

As to the original question -- GET 3.15s AND A MILD T/C! The difference will amaze you, and you won't notice much difference in MPG at all! It will still feel stock in normal driving.

C5 isn't tuned to get the best gas mileage at 70 mph which is claimed to be 28 mpgs. My rpms are too low at 70 mph to get the best mpg. So I get 27 at 70 mph. At 80 mph I get about 32 mpgs. Thus lowering the ratio increases mpg at 70 mph. Which is the more likely speed most of us will drive at. Potencially increasing gas mileage in city and highway driving at legal speeds.

Richard.

 

[DRTH VTR]

But, in a sense, engine friction might be considered a constant apart from any external load.

My point is that it is NOT a constant. An intuitive equivalent would be this thought experiment: Place your hand against your pants and slowly move the hand back and forth. Note any warm sensation. Now repeat, but move the hand much faster. I suspect that it will be warmer, no? The heating comes from the energy necessary to overcome friction, and it is not independent of speed. The engine equivalent would be the movement of pistons (rings), bearings, etc. against their mating surfaces.

I won't post again on this topic. It really doesn't matter who is right.

 

[tipcapman]

I would like to have a professional engineer or a knowledgeable mechanic (c4c5?) address this issue.

OK, here I am!!!

I am a licensed professional engineer in Connecticut.

Low RPMs generally yield better gasoline mileage for several reasons, among them...

Lower engine speed means less internal mechanical engine friction.

At the lower RPMs, you actually have the throttle open more than for a higher numerical gear ratio, hence the engine also suffers less "pumping losses"... less work done pumping air past the throttle plates at the high manifold vacuum inherent with small throttle opening.

L

 

[SLICKMAN]

2500 rpm with 3:15?? How fast were you going at that RPM? You had to be doing no less than 100mph... At 1500rpm I'm doing 70mph (6 speed) and a friend of mine with an auto at the same speed is at 1900rpm..

For what it is worth in this discussion regarding mileage, I have the 3.15 rear end in my coupe, and at 2,500 rpm on level ground with no head or tail wind, I have been getting 34 instantaneous mpg. The first time I checked, I though I was benefiting from a tail wind. I then exited the freeway and retraced my path in the opposite direction and still came up with 34 mpg instantaneous at 2,500 rpm. I would tend to agree with C5Richard that the 3.15 rear end puts less load on the engine which results in higher gas mileage at a given speed. I would be curious what MagikDraggin (or anyone else with an A4 and 2.73 rear end) is observing for instantaneous mileage at 2,500 rpm on level ground with no wind.

In any event, if you are looking for faster quarter mile times, you are going to get better performance by looking at the rear end first.

 

[C5Richard]

OK, here I am!!!

I am a licensed professional engineer in Connecticut.

Low RPMs generally yield better gasoline mileage for several reasons, among them...

Lower engine speed means less internal mechanical engine friction.

At the lower RPMs, you actually have the throttle open more than for a lower numerical gear ratio, hence the engine also suffers less "pumping losses"... less work done pumping air past the throttle plates at the high manifold vacuum inherent with small throttle opening.

L

A licensed profesional engineer would know he had to factor in exactly what speed produces the best efficiency in that equation. And he would know that once the engine was at the desired rpms's engine friction is so nominal if the proper oil is used that it wouldn't be a detectable factor. And off course if at the test speed the gearing is too tall there's a greater loss of efficiency.

Richard.

 

[tipcapman]

A licensed profesional engineer would know he had to factor in exactly what speed produces the best efficiency in that equation. And he would know that once the engine was at the desired rpms's engine friction is so nominal if the proper oil is used that it wouldn't be a detectable factor. And off course if at the test speed the gearing is too tall there's a greater loss of efficiency.

Richard.

Richard...

DRTH asked for some facts from a PE, so I obliged, not sure why I get the attitude.:confused

Engine mechanical friction horsepwer increases as the square of the engine speed, i.e. if you double the engine speed, the friction increases by a factor of four.

As to your comment that "friction is so nominal.... it wouldn't be a detectable factor", friction is real, it's measurable, and it is significant, especially at steady state speeds, where road load horsepower relatovely low (compared to say acceleration or hill climbing.) It's the reason engined designers come up with ideas like low tension piston rings, roller tip rocker arms, etc.

Regards,

Len

 

[MsSchroder]

C5Richard,

It looks like you've recently joined, and may not realize that the CAC is a little different from some other Corvette-related web sites. We like to maintain a standard of being the friendliest Corvette related site on the web.

Thinly-veiled insults and condescension are not regarded favorably here, particularly in the technical forums.

Thanks for your cooperation,
Tammy

 

[C5Richard]

C5Richard,

It looks like you've recently joined, and may not realize that the CAC is a little different from some other Corvette-related web sites. We like to maintain a standard of being the friendliest Corvette related site on the web.

Thinly-veiled insults and condescension are not regarded favorably here, particularly in the technical forums.

Thanks for your cooperation,
Tammy

I wasn't trying to be rude in any way. Just pointing he was incorrect. I don't really belong to any other corvette forum that discuss much of anything so I wouldn't know. IRL this is easily proven by driving a couple cars around. Which I did to double check.

Richard.

 

[Patrick]

A licensed profesional engineer would know... And he would know that... And off course...

Richard.

Richard,
There's a way of phrasing things (even when responding to a post you believe may be inaccurate in its facts) that doesn't sound quite so combative and confrontational. Just the opening to the three sentences you posted in message No. 26 makes it sound like you were questioning tipcapman's professional competence. Tipcapman noticed it, and so did I.

That's what I think Tammy was pointing out.

-Patrick

 

[DRTH VTR]

If lower gears help fuel economy, the use of overdrive gears must cost us extra fuel on the freeway. Why did all those professional automotive engineers choose to cost us all extra money?

If lower gearing increases fuel economy, then I wonder why that large staff of engineers at GM decided to use the tall gears that they selected. They must not know what they are doing, either, I suppose...

 

[SLICKMAN]

I don't know if GM was wrong or not but, I will tell you this. I was on a long 550 mile trip about a month ago and I tried using 5th and not 6th and my gas milage did not change much at all. If there was a 1 mpg change, that was it. So, I think going to say a 4:10 from a 3:42 in a 6sp will not change the MPG much at all based on what Ii did..

 

[C5Richard]

If lower gears help fuel economy, the use of overdrive gears must cost us extra fuel on the freeway. Why did all those professional automotive engineers choose to cost us all extra money?

If lower gearing increases fuel economy, then I wonder why that large staff of engineers at GM decided to use the tall gears that they selected. They must not know what they are doing, either, I suppose...

Those engineers built us a performance car not an economy car. They can't post mileage figures to the feds that were achieved above the highest legal speed limit. They attempted to get reasonable mileage at highway speeds not optimum mileage at legal speed.
All things considered the car gets the best gas mileage at 80 mph give or take a few. That is not a legal speed. I know I don't get a save chance to often to run at 80 for a prolonged time. The reason you will get better mileage with a slightly shorter gear is because it brings the "optimum" speed for the best mpg down a bit to around 70. One gets a lot more chances to run at the legal limit of 70 than one gets to run at 80 mph. Thus based on the premise you would spend a reasonable amount of time driving at 70 mph your gas mileage will improve. If the legal limit were 80 mph and we could drive around at 80 all this would be a moot point.

Richard.

 

[donnyonee]

ok, with all that.

and now back to the questions.
i got a 5.08 time with my passport gt2 0-60 .
99 vert, auto, traction control off, let it shift by itself. 3.42s
magnaflow x and back.

 

[Edmond]

ok, with all that.

and now back to the questions.
i got a 5.08 time with my passport gt2 0-60 .
99 vert, auto, traction control off, let it shift by itself. 3.42s
magnaflow x and back.

Hello donnyonee and welcome to the Forum.

That's a pretty good time, sure would win some unsanctioned races.

 

[tipcapman]

Those engineers built us a performance car not an economy car. They can't post mileage figures to the feds that were achieved above the highest legal speed limit. They attempted to get reasonable mileage at highway speeds not optimum mileage at legal speed.
All things considered the car gets the best gas mileage at 80 mph give or take a few. That is not a legal speed. I know I don't get a save chance to often to run at 80 for a prolonged time. The reason you will get better mileage with a slightly shorter gear is because it brings the "optimum" speed for the best mpg down a bit to around 70. One gets a lot more chances to run at the legal limit of 70 than one gets to run at 80 mph. Thus based on the premise you would spend a reasonable amount of time driving at 70 mph your gas mileage will improve. If the legal limit were 80 mph and we could drive around at 80 all this would be a moot point.

Richard.

Richard....

Your statement about best gas mileage being achieved at 80 MPH is possible, however I don't think it is likely. As I mentioned before, engine internal friction increases as the square of the engine speed, and road load horsepower increases as approximately the cube of the vehicle speed (aero drag goes up as the square, rolling friction is flat or slighty increasing with speed, so it is probably a little less than a cube relationship). I believe the EPA highway mileage test is performed at an equivalent vehicle speed of 60 miles per hour, therefore the road load horsepower is 77% higher at 80 compared to 60.

I believe the engineers were doing their best to obtain the highest possible mileage rating at the EPA test speed to avoid the gas guzzler tax. Any impact on the EPA CAFE rating would be insignificant, if the quantity of vehicles produced in included in that calculation (not sure about that).

Most passenger cars get best mileage at 30-40 MPH, for the reason I mentioned above, the C5 may bump that a bit higher because of its unusually long gearing.

One thing we do agree on... they built a heck of a perfomance car that can get great mileage as well!!!

Regards..

L

 

[C5Richard]

Richard....

Your statement about best gas mileage being achieved at 80 MPH is possible, however I don't think it is likely. As I mentioned before, engine internal friction increases as the square of the engine speed, and road load horsepower increases as approximately the cube of the vehicle speed (aero drag goes up as the square, rolling friction is flat or slighty increasing with speed, so it is probably a little less than a cube relationship). I believe the EPA highway mileage test is performed at an equivalent vehicle speed of 60 miles per hour, therefore the road load horsepower is 77% higher at 80 compared to 60.

I believe the engineers were doing their best to obtain the highest possible mileage rating at the EPA test speed to avoid the gas guzzler tax. Any impact on the EPA CAFE rating would be insignificant, if the quantity of vehicles produced in included in that calculation (not sure about that).

Most passenger cars get best mileage at 30-40 MPH, for the reason I mentioned above, the C5 may bump that a bit higher because of its unusually long gearing.

One thing we do agree on... they built a heck of a perfomance car that can get great mileage as well!!!

Regards..

L

I still believe you're not consider other efficientcy factors. My vette gets better mileage near 80 mph. I can't break anything over 27 to 28 mpg anywhere before slightly before 80 mph. I don't see that you're including the factors that make that possible.

Richard.

 

[tipcapman]

I don't know if GM was wrong or not but, I will tell you this. I was on a long 550 mile trip about a month ago and I tried using 5th and not 6th and my gas milage did not change much at all. If there was a 1 mpg change, that was it. So, I think going to say a 4:10 from a 3:42 in a 6sp will not change the MPG much at all based on what Ii did..

Not sure exactly how you made the comparison, but 1 MPG improvement in 6th compared to 5th would be about 3-5%, so it is probably about right.

When was the last time GM was wrong?

L

 

[DRTH VTR]

Tipcapman

I ordered that book on internal combustion engines that you recommended. I need a little light reading for bedtime! ZZZ
Jim

 

[tipcapman]

I still believe you're not consider other efficientcy factors. My vette gets better mileage near 80 mph. I can't break anything over 27 to 28 mpg anywhere before slightly before 80 mph. I don't see that you're including the factors that make that possible.

Richard.

Richard...

If you tell me which factors I should consider, I will discuss them with you.

Engine friction and road load horsepower are the major drivers, increasing at the square and cube of engine speed and vehicle speed, respectively. The influence of these factors is so powerful that it overcomes all other factors at vehicle speeds over about 30 MPH.

Most vehicles get best fuel economy at about 30 MPH. The M6 C5 has the advantage of having a lot of gear ratios, so it can get extremely good mileage at all moderate speeds, provided you are in the highest gear possible for that speed.

Engine efficiency doesn't improve with increased speed, in fact it decreases above a certain moderate speed.

Regards...

Len