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Discussion Starter #1
Hello everyone, I am wondering what (if anything) the ECU needs to know the transmission's gear ratios for on a manual transmission car. I never realized that manual transmission ECUs were programmed for the gear ratios until I saw a video of someone explaining the menus in KPro and got to the gear ratios. Ever since then I have wondered why the ECU would need to know the ratios as I can't think of a reason. If someone would mind explaining that to me I would greatly appreciate it. Thank you very much in advance, any information would be greatly appreciated!
 

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Arouse the DAMPFHAMMER!
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...Ever since then I have wondered why the ECU would need to know the ratios as I can't think of a reason. If someone would mind explaining that to me I would greatly appreciate it. Thank you very much in advance, any information would be greatly appreciated!
There are some correction tables which are gear related, like the tip in ignition retard and a few others. As there is no signal saying transmission is no in gear 1, 2 or X, the software of the ECU calculate this by VSS, engine speed and gear ratio. If they are wrong, the correction tables would have a set of mismatching values for some engine speed areas. So the correct values has to be setup into the ECU SW.
 

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I always assumed that was just a Kpro feature (i.e. boost by gear). It uses the engine speed, vehicle speed, and gear ratios to determine what gear the transmission is currently in.
 

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I always assumed that was just a Kpro feature (i.e. boost by gear).
Oh, almost all standalone use an approach to calculate the gear which the transmission actually have for the same reason. Those or similar correction tables are used in all ECU's I know.
 

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Discussion Starter #6
Hmm good to know, thanks for the information! On a typical NA setup, what may I notice if the gear ratios are different than what the ECU is expecting if I have no way to correctly program it? Would performance and/or MPG suffer?
 

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From the factory? Did not know that.
Yes the OEM ECU for EURO 3 and EURO 4, roughly from 2000-2009 from Honda. As long as no gear sensor is available those calculations are necessary to have the right gear depended correction, e.g. for tip in ignition timing retard, which is an OEM feature.
E.g. VAG transmissions have in their DSG boxes gear sensor (not sure if simulated or measured) but that signal is transferred also to the ECU to work with that info. Same is duty for all other brands which are using gear related stuff.
 

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On a typical NA setup, what may I notice if the gear ratios are different than what the ECU is expecting if I have no way to correctly program it?
You mean you have no access to adjust it but changed the gear ratios, e.g. the final drive ratio? The ECU would assume e.g. the tip in retard wrong and it my not retard ign. timing, so the tip in oscillation of the crank-gear shaft-axle-system will lead to bigger oscillations and won't be dampened by the ign. retard.
 

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You mean you have no access to adjust it but changed the gear ratios, e.g. the final drive ratio? The ECU would assume e.g. the tip in retard wrong and it my not retard ign. timing, so the tip in oscillation of the crank-gear shaft-axle-system will lead to bigger oscillations and won't be dampened by the ign. retard.
Way over my head LOL. The situation I am in is I am doing an auto to 5 speed swap on my D17 powered Civic and I have a VTEC engine, so I need the VTEC 5 speed EX ECU. However, the transmission I bought is from the LX model with the non VTEC engine because I do a lot of highway driving and it's geared taller than the EX transmission and I wanted a reasonable cruising RPM for less noise and more MPG. Now I am wondering if the ECU being programmed for the short geared trans is going to be a problem with a taller geared trans. I have a stock ECU with no way to reprogram it for different ratios. What problems could this cause? Could this cause a loss of performance or MPG or some other problem with the way the car drives or would the tip in response just not be as smooth? Thank you very much for your help, I hope I made sense.
 

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Could this cause a loss of performance or MPG or some other problem with the way the car drives or would the tip in response just not be as smooth
It won't cause a loss in performance and MPG. But it CAN cause tip in issues in form of a shaking tip in, especially when you shift at lower engine speed and higher load for higher fuel efficiency. My recommendation would be, test it. If there is an issue, you may have to solve it by tune the ECU. Is that an ECU which can be KPro'd?
 

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Discussion Starter #12
It won't cause a loss in performance and MPG. But it CAN cause tip in issues in form of a shaking tip in, especially when you shift at lower engine speed and higher load for higher fuel efficiency. My recommendation would be, test it. If there is an issue, you may have to solve it by tune the ECU. Is that an ECU which can be KPro'd?
Thanks for the information, I think I will be alright then. Worst case scenario if it's really unhappy I will just get an EX trans from the junkyard for ~$100 and sell this one. I will report back once I swap, but that may be a little while because I am still waiting on parts I ordered almost month ago. USPS (n)
 

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Wouldn't tire size come into play? If the ECU uses VSS, RPM and MPH to determine gear those all can vary wildly depending on wheel/tire circumference. Just trying to understand, I put my ratios in Kpro so I'd have an easy reference.

Mak
 

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Discussion Starter #15
Wouldn't tire size come into play? If the ECU uses VSS, RPM and MPH to determine gear those all can vary wildly depending on wheel/tire circumference. Just trying to understand, I put my ratios in Kpro so I'd have an easy reference.

Mak
Great question! I believe it would since as you said the tire size changes the effective gear ratio just like changing the final drive does. Kpro may have a way to enter the tire size though, I'm not sure since I don't have it.
 

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Wouldn't tire size come into play? If the ECU uses VSS, RPM and MPH to determine gear those all can vary wildly depending on wheel/tire circumference.
That's correct, the tire circumference play into it, as KPro uses engine speed, VSS and gear ratio to calculate the actual gear. As the differences in VSS vs. engine speed and gear are still huge I am not sure how much the tolerance bandwidth allows. But there must be one. I remember a customer where the logs showed a flipping gear at lower engine speeds. This was maybe a tyre circumference which was at the max. or exceeded it already. The effect on the tune wasn't there as the gear related compensation tables were almost all zero'd.

Kpro may have a way to enter the tire size though, I'm not sure since I don't have it.
It has only the option to setup the gear ratio's. Same for FPro. What one can do is adjusting the speedometer output from the ECU (s. Misc Parameters).
 

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Discussion Starter #17
That's correct, the tire circumference play into it, as KPro uses engine speed, VSS and gear ratio to calculate the actual gear. As the differences in VSS vs. engine speed and gear are still huge I am not sure how much the tolerance bandwidth allows. But there must be one. I remember a customer where the logs showed a flipping gear at lower engine speeds. This was maybe a tyre circumference which was at the max. or exceeded it already. The effect on the tune wasn't there as the gear related compensation tables were almost all zero'd.

It has only the option to setup the gear ratio's. Same for FPro. What one can do is adjusting the speedometer output from the ECU (s. Misc Parameters).
Good to know, thank you for explaining! What tire size does Kpro assume by default?
 
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