The effect of tire diameter and wheel weight has been discussed on the forum many times before. I had heard this before when I was in the C5 forum and always wondered why myself.

The C5 Z06 uses a smaller diameter and lighter rear wheel.

http://forums.corvetteforum.com/show....php?t=1203527 posts #16 and #17 ttrotary explained it to me and it made sense.

http://forums.corvetteforum.com/show...highlight=dyno
he initially mentioned it here.

http://forums.corvetteforum.com/show....php?t=1363026

http://forums.corvetteforum.com/showthread.php?t=439211

http://forums.corvetteforum.com/show....php?t=1074174 post #28 by Allthrottle&nobottle. He describes a dyno change by changing rear wheels.

So consider that the transmission gearing in the C5 Z06 and the MN6 Z51 are the same in gears 1-4. They use the same differential.

It is a fact that if you go to a smaller diameter tire that the car will accelerate quicker.

This is one of the reasons why people use 17in drag radial vs an 18in drag radial or 17in ET Streets vs an 18in drag radial.

What happens if you put a car on the dyno two different times and change nothing but the rear tires to a smaller diameter? A better question is, if you were to put C6 wheels on a C5 Z06 do you think it would dyno as high as it would with it's stock wheels on it?

 

That's what I thought people were thinking. It is absolutely true that smaller diameter tire will give you better acceleration just like shorter gearing does. However, this does not produce more power. Sounds weird, but it's true. You have traded off a lower top end speed for better accelleration by using shorter gearing or smaller tires.

The dyno measures the engines rpm using a pickup off one of the plug wires. It knows the rear wheel speed due measuring the speed of the dyno drum. It then uses this overall gear ratio in caculating horsepower.

Think of it this way, if you dyno in different gears you get pretty close to the same horsepower. Try it, you'll see. By the logic you listed, the car would make more power in the lower gears, it would really accelerate the dyno drum in first gear. However, even though it will actually accelerate the drum faster, it actually makes less power in first gear versus 4th gear. This is due to the engine also having to accelerate the engine components, drive train, and rear wheels. Of course, it always has to acellerate these components and that is one reason rear wheel horsepower is less than actual engine horsepower (it's due to this and the inefficiency of the drivetrain). However, in first gear it is accelerating all of these components at a higher rate. The torque that it takes to accelerate these components is equal to the rotating inertia of these components times the acceleration rate. So at a higher acceleration rate it uses up more torque and therefore the horsepower at the rear wheels is slightly less.

Sorry, this seems confusing, I'm probably not doing a good job of explaining it.

Now the real reason why people use smaller tires on the dyno and at the track. You have a lower rotating inertia of smaller wheel/tires combos. It uses up less torque/power to accelerate these smaller tires therfore leaving more power to use for acellerating the car. Plus the shorter rear tires does act the same as shorter gearing giving better acceleration.

 

Quite confusing. About the only part of your post which I can follow and hence agree with is the last paragraph. I alluded to the same thing in my post regarding why people use smaller tires.


http://forums.corvetteforum.com/show....php?t=1373993
In post #6 Mitch Alsup describes the same thing.

You put a larger diameter, heavier wheel/tire on the same car and it will dyno lower.

The C6 is starting at a 5 BHP handicap vs the C5 Z06 to begin with.

Couple that with the disadvantage it has on the dyno using a larger heavier wheel/tire combination and the results are what you see. Which is usually around a 9-10 RWHP advantage.

About a 9-10 RWHP advantage for the C5 Z06. In other words about a 3% advantage.

Assuming 15% drivetrain losses for both cars, which use the same transmission, and the same differential, only the rear wheel/tires and the HP are different. And the HP difference is 5 BHP.

C5 Z06 405 X 0.85= 344.25

C6 Z51 400 X 0.85= 340.00

4.25 horses off the bat. Throw in the smaller diameter, lighter tires/wheels and the advantage becomes a bit more. And it is an advantage because the transmission gearing and the differential are the same in the cars. A lot easier for the Z06 to turn those than for the C6 to turn its tires.

The average C5 Z06 dynoed around 350 RWHP. I definitely think the smaller, lighter wheel combo was worth another 5.75 HP or thereabouts.

And that approx 10hp difference is what you see real world.

 

Sorry I couldn't explain it better, but just be assured, the extra horsepower is due to the lower rotating inertia of the smaller wheel/tires, not due to the advantage you get from having an overall shorter gearing. My point is the dyno accounts for the shorter gearing by getting an overall ratio by dividing the engine speed by the wheel speed. Also rest assured that your car will show less power on a wheel dyno in first gear than it will in 4th gear.

 

OK. I thought I said that earlier, but if I did not, I agree with what you are saying here. Thats also what MitchAlsup was saying in the post I linked to.

I went back and read my posts and I don't find where I said that "the extra horsepower is due to the advantage you get from having an overall shorter gearing."

The improvement in acceleration is, but not the increase in HP.

My point was that AFVETTE was musing that his '03 Z06 dynoed at 354 RWHP bone stock and his C6 dynoed at 346.7 RWHP. About an 8 RWHP difference. He mentioned that his C5 Z06 (with a smaller diameter tire) "felt stronger" (improved acceleration over his C6).

I pointed out that his C5 Z will dyno about 3% higher. Thats all.

So to sum it up, his acceleration difference was in part to the slight gearing advantage, and his observed dyno difference was due to the lighter smaller diameter wheel tire combo on the Z06, (and of course the 5 BHP advantage)

 

I agree with you on your statements.

Also, as far as trying to explain the differences of dynoing in first gear rather than 4th gear, this is from http://www.superstang.com/horsepower.htm#What

2. In first gear, because of the additional torque multiplication produced by the gearing, the applied torque to the dyno drum increases proportionally to the gear ratio (less the additional losses from the gear meshes), which causes the dyno drum to accelerate much faster. Now you might think this would indicate higher HP, but the drum speed is similarly proportionally reduced by the added trans ratio, so neglecting friction and inertia effects, the power would remain the same. But the power measured at the drum DOES decrease, because all the drivetrain components (engine rotating and reciprocating masses, flywheel, trans gears and shafts, driveshaft, rear gears, axles, wheels and tires) all accelerate faster now (but in first gear, the starting and finishing speeds are much less compared to high gear). Since it takes power to accelerate mass, the higher driveline component accelerations use up more power in first gear than high gear, and thus the available power at the dyno drum is decreased by the HP "robbed" in the vehicle drivetrain due to the higher driveline component accelerations.

 

As a (kind of) aside, which is really back to the air flow point: I installed my K&N Aircharger. I had a good chance to compare the new one to OEM. There is a huge difference in area to draw air in, and capacity of the tube to move air to the MAF. It is a simple fact that the K&N style (Halltech, SLP et al) is far superior to the stock system. The stock lets air in at the ends only. The huge filter on the new system/s lets air in 360 degrees. Without cutting the shroud--which I chose not to do, the potential volumn has to increase by a factor of at least .6. The higher the speed--and more venturi affect of the air--meaning a dyno cannot capture this measurement, the greater the potential increase in airflow and therefor, fuel allowed by the ECM. I can't quantify the number, of course, but I have no doubt this type of filter will add substantial, let's say 5+, hp at highway speeds. I also think the computer must learn the new airflow, so the actual improvement won't come until several hundred (?) miles of use. I have an old friend who was a principal engineer for Corvette at Chevy years ago. He agrees. The bottom breathers are probably better in this though process, but as I said, I don't want to cut and perhaps harm the shroud's intended purpose--certainly not as hot and as high as Colorado Springs is.

Mike

 

You may be right, I made 333.9hp bone stock in my A6.

 

That's true of drag racing, but true of circuit racing also? I thought large wheels, low profile corners better?

 

The Halltech or the K&N Aircharger are not ram air, so testing on a Dyno should catch all the benefits. You are right, there is a huge difference in area of the filter. So what does that translate to? It translates to a lower pressure drop across the air filter of 2 inches of water. And this translates to 2 to 2.5 hp (even admitted by the filter manufacturer, Donaldson). I'm sorry, but I'm a skeptic. I bought the K&N AirCharger and it's very pretty and it is probably worth 2 to 3 hp, but that's it. My opinion is the Halltech would be the same and some people have posted that only got 2 hp from the Halltech.

 

Formula One cars use 13 inch wheels with 45 profile racing tires with a maximum width of 14 inches. NASCAR runs a tire with a 11.6 inch section width on a 15 inch wheel, which works out to a 77 profile. You won't find large wheels with very low profile tires on professional race cars. They wouldn't be competitive, nor would they be safe.

 

I don't follow NASCAR, but have been following F1 since the days of the Vanwalls !

You probably aren't aware of the 2008 rule changes under consideration.

"When asked what diameter Formula One wheels would go to if the current 13-inch mandate were removed, the head of Michelin's F1 program predicted 17 to 19 inches, adding "certainly not 20 or beyond."

May not count much as it appears that Bridgestone will be the only tire supplier in 2008.

This is right from a FIA press release:

"WHEELS AND TYRES
- Tyres will be supplied by one manufacturer appointed by the FIA after an invitation to tender. Such an appointment will be conditional upon :
- a suitable supplier being available ;
- a suitable system to ensure tyre testing is carried out in an equitable manner ;
- no team being disadvantaged by the appointment of a single supplier (detailed regulations will be written to ensure this would not be the case) ;
- there being no legal impediments during the process of appointing a supplier
- Slick tyres will be introduced for use in dry weather
- Lower profile tyres will be introduced
- Significantly larger wheels with minimum and maximum sizes stipulated for front and rear will be permitted
- Tyre blankets and other heating devices will be prohibited
- All tyre regulations will reside in the Technical Regulations

Reasons- a single supplier would allow a bigger safety margin
- the absence of competitive tyre testing would reduce costs
- as relatively small differences in tyre compound and construction can have a significant effect on lap times, a single tyre supplier would simply ensure that no team would be adversely affected by being contracted to the “wrong” supplier
- slick tyres would be re-introduced as a part of the low-downforce and high-mechanical-grip package
- lower profile tyres would be introduced in order to give the wheels and tyres a more modern look and also permit more freedom on brakes and suspension
- a ban on tyre heating devices would eliminate this significant but unnecessary expenditure"

F1 is about global politics, the 13" wheels with approx 26.4" max OD, 14.2" front width and 15.2" rear width was an attempt to slow the cars down and make the lesser teams more competitve. same reason for using a treaded tire. As you can see above they are going back to slicks and using larger tires for reasons of safety...less downforce required and bigger brakes can be utilized. So much for your theory !!


No disrespect intended,all in fun, but best to be informed with the direction of technology !! Who knows you might even warm up a bit to the dreaded "Why do we need so many gears A6" ?

 

Sure do....smaller sidewall = stiffer sidewall = better response and handling

 

it really doesn't need to learn, that is what maf does is to tell ecm how much air exactly is coming in. there will be a very small st/lt fuel trim adjustment to keep it at 0%.

 

nice, so much for that theory.

 

Really, that's interesting. Almost every change to the mapped expected parameters in the ECM requires 'learning,' at least it did in my F-bodies. If it was too far out of parameter, the SES light would come on. If it was reasonably near the mapped values, the ECM would learn to accept it and adjust other inputs accordingly. Perhaps the Vette is different.

 

the function of mass air meter is to measure the exact amount of air that is coming. if more air is coming it will see it (within reason)...you are right about the check engine light if it is too far out of the parameter like force induction or nos and yes ecm will accept the value and adjust injector pulse width to stay within the correct a/f ratio, over time it will adjust the short & long term fuel trim to be at 0% if possible (again it is a very small adjustment, assuming you are not too far off the "parameters")

 

I have to say, this explanation is pure gibberish. It's true that cars will dyno slightly lower in a lower tranny gear, or with a shorter (higher numerically) axle rartio. This is caused by parasitic losses in the drivetrain. There is more internal friction involved with the lower gears (more gears in the tranny, more teeth).

This is also why Vettes typically dyno at a slightly less power than F-bodies with the same HP engine. The independent rear of the Vette is not as efficient as the solid rear in the F-body.

It's also not an absolute given that a shorter gear will always result in faster acceleration. If will always result in the engine revving more quickly, but the acceleration may stay the same, or even be slower.

 

Torque equals Inertia times Rotational acceleration. So if the acceleration is higher when you are in a lower gear, then the torque required to accelerate the moving parts in your car is higher and thus less engine torque is available to accelerate your vehicle.

It may sound confusing, but it's just physics. Come to think of it, that's why it sounds confusing!

 

??? You'll have to explain this one to me. The engine will rev more quickly but the car won't? Are you saying that your car will accelerate more quickly in second gear than in first?