I'm in the process of changing my exhaust so my bike will be going back on the dyno. The last dyno run showed the 4.9 gps injectors at over 95% duty cycle at wot. Since the bike will be dynoed again I would like to go up with the injectors, especially since I should be picking up some top end with the stepped pipe. So should I go with the HPI 5.3 or SE 6.2 injectors? The build is an 08 FXD, 124" 10.6:1, 585 cams, stg 4 VEETWIN heads, 58mm HPI, Sawicki stepped 2-1. It made 129/147 last dyno run. Id like to leave some room for improvement for down the road if it's feasible

 

If you don't pick up too much more HP, a 5.3 will get it done, they're good for right around 67 hp per cylinder.

 

HPI injectors are matched, I'd go that route.

 

Is there a downside to going with the bigger injectors? Can't the tune be altered to accommodate? What do you mean by the injectors are "matched"?

 

I have HPI 5.3's in a 107 that is making 120/123. I don't know what my pulse width (duty cycle) is for this tune. I have a set of 6.2's with the thought of using them the next time i go to the dyno next month. trying to squeeze a little more out! I'm using a SERT. The 5.3's have 4 holes in them and the 6.2's have 1 big hole it looks like. I would assume that you can tune out a larger injector to a certain point. In your case it shouldn't be a problem. The research i have done it seems that 5.3's are good to about 135 hp. You have a lot of volume to fill so i would imagine the 6.2s would be fine.
I followed your build last year as i doing my 107 at the same time. I'm jealous.

 

At 95% seems the tuner would have made a call alerting to the need for a tick larger.
Scott

 

Would going to a bigger injector increase the hp at all or just leave you room to make more power.. Does that make sense? I'm waiting on HPI to call me back and see what they recommend, their website says 6.2s recommended for 120hp+ But it seems 5.3 will fit the bill. I dunno if rather go with the bigger injectors if im buying them anyway, just don't want to use the wrong one if it can't be adjusted at the tune

He did tell me that it was unsafe at wot and recommended using bigger injectors. I've kept that in my mind while riding and don't stay at wot for long because of that. What size would you recommend Scott?

 

5.3 injectors is fine for a 107 and 124, if you were going to go to a 66TB I would move up in size but 62TB with 6.2s is as high as I would go personally.. I haven't seen any Benefit from the 66tb on a 124..

On a 107 58Tb and 124 62MM both with 5.3 injectors and we do very well with that combination. (again personal opinion and experience)

 

The 5.3's (HPI-LMINJ-1) is the only injector that is a straight fit on their website. They are a bit longer and have the EV-6 plug that you bike has stock. To use the yellow band 6.2's you will need a different fuel rail or extensions and EV1 to EV6 adaptors. If Jimmy doesn't have what you are looking for. I would suggest some 5.7's matched from five-O.


There is a tolerance with injectors. Matching them means you flow a bunch and match up two that flow close to the same. It's nice to have because they have been flowed and will do what they say, but we can tune both cylinders separately so.....


A bigger injector isn't going to make anymore power than a smaller injector if the smaller injector can supply enough fuel. It's really about what an injector can support. So much HP is going to need a certain amount of air and fuel. Bigger injectors can support more HP because they can pass more fuel for the time aloud. Basically you have 20ms to spray fuel at 6000rpm. So the injector doesn't stay on full time and to obey the law of 80%. That gives us 18ms to spray fuel. A bigger injector will spray more fuel in this allotted time over a smaller injector. Pretty simple.

 

The throttle body should not be the choke point. The heads should. Back to back testing on a 124R showed me that there was no top end power gain between the 55/58, 58/58, and a 66. All with 6.2's. Getting enough air is getting enough air.