2005 GSX R750 Rev Limiter 13,600

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tqmidget88

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I have a 2005 GSX R750 installed in a midget race car. Can anybody tell my why I am hitting the rev limiter at 13,600 RPM? Everything I read says the limiter is 13,900 RPM. I have tried more than one ECU and I still get a max. of 13,600. I don't particularly want to run the engie at 13,900, but I would much prefer that it not hit the chip before the end of the straightaway and stop pulling. Our rules mandate that we run carbs and not FI, and somebody else modified the wiring harness to work for our purposes. The engine runs strong and clean, but only turns 13,600 before the limiter kicks in. Another racer told me that some ECU's have the rev limiter set for 13,600 while others are set for 13,900, but I have not been able to find anything anywhere to corroborate that.
Any assistance anyone could provide would be appreciated.
Thank you
 
tqmidget88 said:
I have a 2005 GSX R750 installed in a midget race car. Can anybody tell my why I am hitting the rev limiter at 13,600 RPM? Everything I read says the limiter is 13,900 RPM. I have tried more than one ECU and I still get a max. of 13,600. I don't particularly want to run the engie at 13,900, but I would much prefer that it not hit the chip before the end of the straightaway and stop pulling. Our rules mandate that we run carbs and not FI, and somebody else modified the wiring harness to work for our purposes. The engine runs strong and clean, but only turns 13,600 before the limiter kicks in. Another racer told me that some ECU's have the rev limiter set for 13,600 while others are set for 13,900, but I have not been able to find anything anywhere to corroborate that.
Any assistance anyone could provide would be appreciated.
Thank you
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What are you measuring it with?
 
Soft fuel cut off at 13600 hard fuel cut off at 13900 iirc, flashed ecu's or kit ones to go up to 14.2. Peak power & torque is stockish trim is around 12k mark.
 
Measuring RPM with a racing tach. I am sure it is accurate because others are running the same final drive gear ratio and getting basically the same RPM.
 
skip said:
Soft fuel cut off at 13600 hard fuel cut off at 13900 iirc, flashed ecu's or kit ones to go up to 14.2. Peak power & torque is stockish trim is around 12k mark.
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At first thought, your response seemed to make perfect sense. But after thinking about it a little, the theory is somewhat flawed. Given that we are running 100% mechanically fed carbs and not the normal FI, I can't see how the ECU could limit fuel to initiate either the soft or hard limiter. There is no mechanism to make that happen. Perhaps the limiters are initiated by reducing spark to one or more cylinders and not by reducing fuel.
 
It's most likely cut by spark or when you hit the limiter it would cause a lean condition. Call Z1 and talk to Zaid, he can probably lead you in the right direction without blowing up your motor.
 
There is an ignition cut off in the 1k's but I wasn't sure about the 750's. Been a while since I played with one.

Give a read through ecuhacking.activeboard.com/f435697/gsxr-750/
 
T.O. Rider said:
300 rpms are useless. If you are hitting the limiter before the end of the straight ou have to change your gearing.
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Yep, you're correct. I changed the gear and it dropped the RPM by 300. Unfortunately it also made the car a little slower on starts and restarts. We have to start the race in our racing gear, usually 6th gear; i.e. no shifting. I really don't want to turn it any higher than 13.6 to 13.7 or so, but I would prefer it pull all the way down without hitting the limiter.
 
tqmidget88 said:
Yep, you're correct. I changed the gear and it dropped the RPM by 300. Unfortunately it also made the car a little slower on starts and restarts. We have to start the race in our racing gear, usually 6th gear; i.e. no shifting. I really don't want to turn it any higher than 13.6 to 13.7 or so, but I would prefer it pull all the way down without hitting the limiter.
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You could find a solution to this with tire selection.

Even tires of the same brand/model will vary in size, how much is a matter of manufacturing tolerances and you'll have to do some ground work to figure this out(i'm gonna make a guess that you're running hoosier's?) I know there tends to be a fairly large difference in outside diameter size with hoosier's.

When you're purchasing, bring a measuring tape and measure several different rear tires for OD.

with the difference in size, you may be able to get the extra RPM you're looking for without any mechanical changes.

I've seen this done with legends and pro challenge cars with great results.
 
reciprocity said:
You could find a solution to this with tire selection.

Even tires of the same brand/model will vary in size, how much is a matter of manufacturing tolerances and you'll have to do some ground work to figure this out(i'm gonna make a guess that you're running hoosier's?) I know there tends to be a fairly large difference in outside diameter size with hoosier's.

When you're purchasing, bring a measuring tape and measure several different rear tires for OD.

with the difference in size, you may be able to get the extra RPM you're looking for without any mechanical changes.

I've seen this done with legends and pro challenge cars with great results.
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Tires can make a difference for sure. We measure them before we buy them, after we put nitrogen in them, and before AND after every track session to see how much they grow, But our choices are limited. In addition, our primary focus with tires is to get the correct stagger, usually about 2 inches for most tracks we race at. Tire stagger is much more important to lap times than gearing and RPM. We are required to run an American Racer on the RR and Hoosiers on the other 3 corners. The American Racers usually come in at 67 1/2 inches and we size the Hoosier LR to 65 1/2 inches, giving us the 2 inches we need to get through the corners. The correct stagger is an absolute requirement. The engine RPM is less important, but still worth trying to maximize.
 
reciprocity said:
Would it be worth searching for the taller Right rear and adjusting up to match?
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I have a couple larger RR's (69") but it is virtually impossible to grow a Hoosier big enough to give me the correct stagger (67"). We ran earlier kn the year with 2 1/2" to 3" of stagger, and the car was way to loose to be fast. Coming off the corners it was a handful and wanted to spin and go around.
 
Ohh, those silly 4 wheel things.

My street car does that on ramps.

Well, ya got me stumped, not much experience with 4 wheel racing(other than road course trackdays) Lots of experience with sliding the rear of a bike but we have the ability to affect the slide with things like body position, rear brake application, throttle control etc because of the relatively small weights involved.

you might be stuck with an ECU flash unfortunately.
 
reciprocity said:
Ohh, those silly 4 wheel things.

My street car does that on ramps.

Well, ya got me stumped, not much experience with 4 wheel racing(other than road course trackdays) Lots of experience with sliding the rear of a bike but we have the ability to affect the slide with things like body position, rear brake application, throttle control etc because of the relatively small weights involved.

you might be stuck with an ECU flash unfortunately.
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I'm thinking the same thing. But at least I have a solution available that will give me what I want.
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