Non Lockup RPM Loss

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73Swinger18

73Swinger18

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I have a 904 and was wondering what approximate RPM you loose to the rear end in a non lockup setup. The reason I ask is, when I'm cruising at 3,000 RPM in third gear, I'm doing 75 MPH. When I do the math below, it shows more like 2,750.
gear.jpg
 
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Tire height can be a deceptive thing??? You have to compensate for the "flat spot" at the bottom of the tire where it meets the asphalt. If you measure from the center of the axle to the ground it will give you a better reading on tire height (with the car sitting on the ground). Of course, tire pressure plays a big role in tire height!!
The no lock converter will slip so you'll lose some there as well.

treblig
 
I measured my actual tire height (24.5") and punched that in. Looks like I'm loosing around 160 RPM after factoring in the tire squat. Thanks everyone.
gear.jpg
 
The tire has a certain roll-out; that is when installed on the vehicle, and you roll the car out for one exact complete DRIVE-tire revolution, you will get a number. That number then goes into the formula and it spits out a number. If your tach or speedo does does not agree, then one of three things is happening; The tach is wrong, the speedo is wrong, or the TC is slipping.

A 24.5"tire has a rollout of 77 inches. The formula says

(MPHx1056xrear gear)/tire roll out= rpm and in your case this is 2840 rpm. That would be zero slip.

I usually see less rpm than the zero slip while PartThrottle cruising at say 55MPH. But as the speed goes up and the engine has to work against the wind, then the slip begins to climb.It can't go too far tho cuz the TC will reach a point where slip is maxed out, usually around 5%.
At your statement of 75=3000, you are already at 3000/2840=plus5.67% slip. This sounds excessive to me. If you were at full power, I could see it, but at PT, it sounds excessive to me.
I would question the instruments.
 
AJ/FormS said:
The tire has a certain roll-out; that is when installed on the vehicle, and you roll the car out for one exact complete DRIVE-tire revolution, you will get a number. That number then goes into the formula and it spits out a number. If your tach or speedo does does not agree, then one of three things is happening; The tach is wrong, the speedo is wrong, or the TC is slipping.

A 24.5"tire has a rollout of 77 inches. The formula says

(MPHx1056xrear gear)/tire roll out= rpm and in your case this is 2840 rpm. That would be zero slip.

I usually see less rpm than the zero slip while PartThrottle cruising at say 55MPH. But as the speed goes up and the engine has to work against the wind, then the slip begins to climb.It can't go too far tho cuz the TC will reach a point where slip is maxed out, usually around 5%.
At your statement of 75=3000, you are already at 3000/2840=plus5.67% slip. This sounds excessive to me. If you were at full power, I could see it, but at PT, it sounds excessive to me.
I would question the instruments.
Click to expand...
The 205/70R14 tire I have is 25.3", but when measured from the top of the tire to ground, it's 24.5 with the weight on it. Not sure if that is what you were calculating. Kinda lost me.
 
73Swinger18 said:
The 205/70R14 tire I have is 25.3", but when measured from the top of the tire to ground, it's 24.5 with the weight on it. Not sure if that is what you were calculating. Kinda lost me.
Click to expand...
No, it was just an example. The diameter does not factor into accuracy, only the roll out. The diameter changes with loading and tire pressure. The roll out may change with tire rpm, but modern street tires limit that to almost insignificant at street speeds.
The 205/70-14 maths out to 25.3 all right. And that times Pi (3.1416) maths out to 79.48 inches. But what is the actual roll out?
See, you have to remember that when the tire is manufactured it is installed on a checking rim,and inflated to a checking pressure, and then the section width is measured to the nearest 5mm. Then the tire is branded. If you install it on a different width rim than the checking rim was,or inflate it to a pressure other than the checking pressure, then the section may not be the same as the branded size.In any event, your 205 could be anything between 203 and 207,even on the checking rim! This means the diameter could vary between 25.2 and 25.4, and therefore the perimeter,not roll-out, could vary between 79.17 and 79.8. This is a variation of 0.79% or about 22rpm at 75mph with zero-slip. That error is just in the roll-out error.And that is why the roll-out needs to be measured exactly.
This is why the math is not always right, and why the roll-out is the only way to get accuracy.
 
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