CBR300R ECU on a CBR250R? - Honda CBR250R Forum : Honda CBR 250 Forums
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post #1 of 11 Old 04-22-2016, 01:54 PM Thread Starter
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CBR300R ECU on a CBR250R?

Hi Guys,

I'm running a 250 in some low budget local club racing and was wondering if the CBR300 ECU can be used as a straight swap on the 250?

My bike is running a very open free flow muffler, manufactured by local company here in S.Africa, along with a high flow air filter. I have concerns about the bike running too lean and possible damage in a racing environment. Did a race day two weeks back and bike ran great, no overheating, pulled spark plug yesterday and it looks good, no clear signs of a problem or overheating.

Next I'd like to have the exhaust port opened a little and a new header pipe custom made to match the port, as well as possible running a velocity stack setup and am worried that those mods would lean it to the point of being problematic.

The CBR250 and CBR300 throttle body and injector are the same part # from Honda from what I can see, so the difference seems to be the ECU, those part numbers are different. Given that the 300 is most likely also programmed to run on the lean side, and that it's actual capacity is only 286cc it's in effect only 15% larger in displacement than the 250, so it's stock fueling may be suitably rich for a lightly modifier 250?

I know the easy answer is a Bazzaz or PowerC install and proper dyno setup, but my budget honestly doesn't allow that and a CBR300 ECU can be sourced for a 1/3 or the price, so that's where I'm looking.

Is this idea worth investigating further or a waste of time? Any feedback appreciated. Thanks!
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post #2 of 11 Old 04-22-2016, 04:35 PM
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No harm in trying. They plug in the same.
Sometimes you have to be the first.
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post #3 of 11 Old 04-22-2016, 04:54 PM
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cbr300r ECU

mwlehman tried it and seemed to run but not for a long period of time. I would just get a Power Commander and put a map on it for a free flowing exhaust and K&N filter. Better yet get it dyno tuned so that map is perfect. But I know that may not be an option due to resources and money.

2012 CBR250R Black
Mods:
Racetech springs and emulators, Yamaha R6 shock, Yoshi Rearset Risers, Babyface GP Shift adapter, EBC HH Sintered Front Brake Pads, CBR600RR pegs, Mirror delete with CBR600RR block offs, Ebay Bar-end mirror, Red LED dash, The2Wheels levers, Homemade Fender Eliminator, LED Blinkers, Tank pads and grips, CBR300R seat, Spider Peak red Grips, Vortex Clip ons
Soon to be mods:
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post #4 of 11 Old 04-22-2016, 05:46 PM Thread Starter
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Thanks for feedback guys,

The available info around the stock 250 ecu's ability to regulate fuel mixture with aftermarket exhaust and air filter mods seems very limited, and down right contradictory at times.

Has anyone actually properly measured the air/fuel ratio on a bike running stock factory ecu with free flow pipe to see how what air/fuel ratio reading is, and by how much (if any) the fueling is off?

Can the stock ecu "learn" and adjust fueling enough to put the mixture into a slightly lean yet still engine safe ratio, or does it get to a point where the ecu can only bump up fueling so much and that's it's limit?
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post #5 of 11 Old 04-24-2016, 11:08 AM
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You'll have to ask people who have been dyno tuned for their AFR's, which is not many.

The ECU does adapt, to a limited degree, to mods. I believe the limit is +/- 20%, but don't quote me. I do know it is a relatively limited range. Slip-on exhausts are fine, but with an airbox mod and header change you'd need a tune to be safe, especially since the bikes are tuned lean to begin with.

In addition, the 300r ECU is taking into account changes in timing AND fueling, and I believe there is a slight cam difference as well. So not a good idea unless you've done a 300r engine swap. Not something I would risk for a $100 fuel commander on a $3000 bike.

cbr300r ECU

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post #6 of 11 Old 04-28-2016, 12:21 PM
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I did try it, it started fine, idled for a little and then shut off, repeat as offend and needed to figure out it won`t work..lol..I stuck a dynojet fuel controler on my wifes and mine and that worked much better
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post #7 of 11 Old 04-28-2016, 01:37 PM Thread Starter
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Thanks guys, interesting info, didn't factor in the cam sensor and timing etc being different on the 300... so that's the end of the swop idea.

My buddy has an air / fuel ratio meter that we are going to use next week to setup jetting on his carb race bike. Sensor for meter screws into a welded bung in the header pipe, gets used by many of the racers running carb setups to get their tune spot on.

We are going to give my 250 a few runs while we at it and see what kind of readings we get. Hopefully it'll screw right into the existing lambda sensor hole, if not we'll make up an adaptor in his machine shop. Will report back with any info. My bike is running a very open muffler and a high flow filter so lets see how the ECU compensates to accommodate.

Incidentally... what is a "good" performance bias air/fuel ratio to be running for racing?

Thanks!
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post #8 of 11 Old 04-28-2016, 03:52 PM
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Quote:
Originally Posted by capetowndave View Post
Thanks guys, interesting info, didn't factor in the cam sensor and timing etc being different on the 300... so that's the end of the swop idea.

My buddy has an air / fuel ratio meter that we are going to use next week to setup jetting on his carb race bike. Sensor for meter screws into a welded bung in the header pipe, gets used by many of the racers running carb setups to get their tune spot on.

We are going to give my 250 a few runs while we at it and see what kind of readings we get. Hopefully it'll screw right into the existing lambda sensor hole, if not we'll make up an adaptor in his machine shop. Will report back with any info. My bike is running a very open muffler and a high flow filter so lets see how the ECU compensates to accommodate.

Incidentally... what is a "good" performance bias air/fuel ratio to be running for racing?

Thanks!
If you remove the O2 sensor in the header pipe to fit your A/F ratio meter, the ratio will be off. The CBR's ECU uses the single O2 sensor in the header for fuel trim compensation. Best way in that scenario is to use a sniffer probe down in through the back of the muffler up into the header pipe.

Usually a richer ratio at about 12-12.5:1 provides the best power, while 14.7:1 is the cleanest burning. Kfox and a few other racers on here can probably chime in, but IIRC, most racers in my area shoot for the 12:1 range for power.

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post #9 of 11 Old 04-29-2016, 02:51 AM Thread Starter
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Quote:
Originally Posted by cbrlocal View Post
If you remove the O2 sensor in the header pipe to fit your A/F ratio meter, the ratio will be off. The CBR's ECU uses the single O2 sensor in the header for fuel trim compensation. Best way in that scenario is to use a sniffer probe down in through the back of the muffler up into the header pipe.

Usually a richer ratio at about 12-12.5:1 provides the best power, while 14.7:1 is the cleanest burning. Kfox and a few other racers on here can probably chime in, but IIRC, most racers in my area shoot for the 12:1 range for power.
Aah of course, thanks cbrlocal... ECU needs the O2 sensor input to do it's calculations ... geez I didn't give that a lot of thought!

Will go the long probe route as suggested and shove it through to the header pipe... thanks!

Last edited by capetowndave; 04-29-2016 at 04:41 PM.
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post #10 of 11 Old 06-05-2016, 02:00 PM Thread Starter
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Sorry for the long delay in giving feedback on this issue, but finally got around to testing the Air Fuel Ratio (AFR) on the 250 using my buddy's meter.
Just a quick recap... my bike has a bone stock motor, with a very open muffler (stock header) and high flow air filter, no other mods done.
Bike was first warmed to operating temp and then AFR tested at idle and under load at around 8,500 RPM, also with a few different throttle positions.
At idle the AFR read consistently around 12,5 - 12,8.
At 8,500 RPM reading was between 13.8 - 14.3 on average, slightly more towards the 14+ side though... so pretty lean!
We didn't have a stock exhaust system we could use to determine a factory baseline, however as an experiment we installed the dB killer into my muffler, which restricts the exhaust flow significantly and the AFR readings stayed fairly consistent with what we saw earlier.
So it would seem that the ECU does have the ability to "learn" on the fly and make adjustments to the AFR as exhaust flow changes, but it is always gonna fuel to a very lean pre-programmed (emissions friendly) condition.
My bike is strictly for racing and to date I've not had any overheating issue from running lean, but I think a Power Commander is now a "must get" on my shopping list, to get the engine AFR running closer to 12.5, cooler combustion and potentially more HP.
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