RPMs

[vacuum 6]

Posted this in the drag racing section but zero replies...

An engine likes to shift at 5500-5700. If the 2-3 shift is done at ~5700 then in theory it’s 3rd gear RPMs would drop. This would move, not for the better, the engine’s 3rd gear peak HP range. Right?

 

[bison]

Posted this in the drag racing section but zero replies...

An engine likes to shift at 5500-5700. If the 2-3 shift is done at ~5700 then in theory it’s 3rd gear RPMs would drop. This would move, not for the better, the engine’s 3rd gear peak HP range. Right?

The only time that it would change the peak is if it flashed higher than the previous peak power. Theres not really a theory. The only accurate calculated part of the rpm drop is the max potential rpm drop with zero slip based on the ratio % change and the final gearing. All else has to do with the engines torque and the converters ability to transfer it to the input. Keep in mind the slip % will be very high on a ratio change and hopefully the converter will hold the engine right in the peak power/tq area of the curve for as long as possible as the converter begins to couple as driveshaft speed increases. It wont change the engines power curve though. Its possible to have a zero rpm drop if the converter is way off. You can calculate the max drop based on the ratio change % but you will never see that in a car with a viscous coupling.

 

[vacuum 6]

The only time that it would change the peak is if it flashed higher than the previous peak power. Theres not really a theory. The only accurate calculated part of the rpm drop is the max potential rpm drop with zero slip based on the ratio % change and the final gearing. All else has to do with the engines torque and the converters ability to transfer it to the input. Keep in mind the slip % will be very high on a ratio change and hopefully the converter will hold the engine right in the peak power/tq area of the curve for as long as possible as the converter begins to couple as driveshaft speed increases. It wont change the engines power curve though. Its possible to have a zero rpm drop if the converter is way off. You can calculate the max drop based on the ratio change % but you will never see that in a car with a viscous coupling.

Sorry dont know what that means?!?

Do you mean if a convertor has a 400 rpm drop, that drop should be the same regardless of the shift point RPM?

 

[vacuum 6]

Sorry dont know what that means?!?

Do you mean if a convertor has a 400 rpm drop, that drop should be the same regardless of the shift point RPM?

Bison?!?

 

[ijames]

Sorry dont know what that means?!?

Do you mean if a convertor has a 400 rpm drop, that drop should be the same regardless of the shift point RPM?

Not Bison but the rpm drop of the converter will depend on the shift point. The higher the shift point the further the converter is from the stall rpm and generally the tighter the coupling (the smaller the slip) so the greater the rpm drop. If you shift a 3500 stall converter at say 4000 rpm you may only see 100 rpm drop, but shift it at 6000 and you may see 800 rpm drop.

As for your original question, what do you mean by "If the 2-3 shift is done at ~5700 then in theory it’s 3rd gear RPMs would drop"? The rpm at which the engine makes peak hp won't change with what gear the trans is in, if that is what you were asking.

 

[bison]

Bison?!?

No. The converter will drop to its flash stall speed on a ratio change. The more torque you throw at it the higher the flash is.

 

[Hot Air]

Now that makes sense!
Conrad

 

[bison]

No. The converter will drop to its flash stall speed on a ratio change. The more torque you throw at it the higher the flash is.

This should say will drop the engine rpm to the converters flash stall speed. An example i could give is the engine in had in the black car last year had a flash stall speed of about 5500 rpm@26psi. So no matter if i shifted at 6300, 6000, or 5800 the rpm would drop to 5500. The only way it would be higher than 5500 is if it was shifted at 7000 or so since the gear ratio will not allow it to drop lower.

 

[jasjamz]

This should say will drop the engine rpm to the converters flash stall speed. An example i could give is the engine in had in the black car last year had a flash stall speed of about 5500 rpm@26psi. So no matter if i shifted at 6300, 6000, or 5800 the rpm would drop to 5500. The only way it would be higher than 5500 is if it was shifted at 7000 or so since the gear ratio will not allow it to drop lower.

So if yours was 5500, where did you shift it with best results? Did you shoot for a certain amount of rpm drop on the 1/2 and 2/3 shifts?

 

[bison]

So if yours was 5500, where did you shift it with best results? Did you shoot for a certain amount of rpm drop on the 1/2 and 2/3 shifts?

I only had enough spring to go to 6100-6200 so thats where i shifted. 6200 in 3rd was 140+mph. I wasnt concerned with rpm drop as long as the converter coupled by the end of the pass. The engine was the strongest around 5500 and thats where it spent most of its time during a quarter pass.

 

[vacuum 6]

Not Bison but the rpm drop of the converter will depend on the shift point. The higher the shift point the further the converter is from the stall rpm and generally the tighter the coupling (the smaller the slip) so the greater the rpm drop. If you shift a 3500 stall converter at say 4000 rpm you may only see 100 rpm drop, but shift it at 6000 and you may see 800 rpm drop.

As for your original question, what do you mean by "If the 2-3 shift is done at ~5700 then in theory it’s 3rd gear RPMs would drop"? The rpm at which the engine makes peak hp won't change with what gear the trans is in, if that is what you were asking.

This should say will drop the engine rpm to the converters flash stall speed. An example i could give is the engine in had in the black car last year had a flash stall speed of about 5500 rpm@26psi. So no matter if i shifted at 6300, 6000, or 5800 the rpm would drop to 5500. The only way it would be higher than 5500 is if it was shifted at 7000 or so since the gear ratio will not allow it to drop lower.

Bison & ijames,
Example: a 5000 2-3 shift is at the 600' mark with a 5700 3rd gear trap RPM, now shift the 2-3 @ 5700 you are at the 660' mark my trap RPMs should be less since you are 60’ further down the track. Right? If so, how do you balance this (engine likes 5700)?

 

[ijames]

You are saying since the 2-3 shift happened further down the track you have less time to accelerate in 3rd so you think the final rpm will be lower? Don't forget that you will be starting at a higher rpm in 3rd gear because the rpm was higher in 2nd before the shift. I don't really think there is a good theoretical way to get an accurate answer, you just have to try it both ways and see what your car likes.

 

[vacuum 6]

You are saying since the 2-3 shift happened further down the track you have less time to accelerate in 3rd so you think the final rpm will be lower?

Yes.

I don't really think there is a good theoretical way to get an accurate answer, you just have to try it both ways and see what your car likes.

I have tired it. I see an 1/8 MPH increase but lose it in the bigend.

Anyway to correct it? Maybe is just my car...

 

[bison]

Bison & ijames,
Example: a 5000 2-3 shift is at the 600' mark with a 5700 3rd gear trap RPM, now shift the 2-3 @ 5700 you are at the 660' mark my trap RPMs should be less since you are 60’ further down the track. Right? If so, how do you balance this (engine likes 5700)?

The best way to figure out hte exact shift point is by logging g force and mph throughout the quarter pass. You can nail down the shift point very well with that data. I doubt you would be at 5000 and making a 2-3 shift @ 600'. If i remember your combo correctly the engine should be spending as long as possible between 4600 and 4900 rpm throughout the quarter pass. Therefore id want the flash stall to be right around 4600. 5700 seems a little steep for a 120mph car. Probably need a different converter or an adjustment.

 

[vacuum 6]

The best way to figure out hte exact shift point is by logging g force and mph throughout the quarter pass. You can nail down the shift point very well with that data. I doubt you would be at 5000 and making a 2-3 shift @ 600'.

It was just an example.

If i remember your combo correctly the engine should be spending as long as possible between 4600 and 4900 rpm throughout the quarter pass. Therefore id want the flash stall to be right around 4600.

GN1s with 212/212. It likes 5500+.

5700 seems a little steep for a 120mph car. Probably need a different converter or an adjustment.

I use 26" tires. Its around 5600-5700.

 

[NM Tom]

Your sig say you have a 12'' converter, at your performance level (120mph in 1/4) I would think you would get some gains with a 9 or 10" converter.

 

[WSLN 6]

28" tires and a PTC 9.5" would do wonders...

 

[bison]

GN1s with 212/212. It likes 5500+.



I use 26" tires. Its around 5600-5700.

You would be better off keeping it around 5000-5200 with the 212-212. It will be a rocket if you can get the boost up to 25+ and keep it in that range. I agree with WSLN 6 100% about the converter and tires. I ran the small 212-212 (under .500 lift) with 27.3" MT drag radials and a PTC 9.5" and it went low 10's. The highest cylinder pressures occurred around 5100 based on the fuel consumption vs boost/rpm on my tired iron headed combo. It went to 5750 at the traps.