450F 2014 yz450f fuel injector sizing for mod motor

Short story: 2014 yz450f with a ported head, long duration cam, and 500cc big bore, how big injector? TFL 460cc or 500cc or maybe 550cc? or maybe geisick 450cc "+20%" shorty injector with 4 fat fuel holes instead of 12 small.

Long story: bought a 2014 yz450f a few years ago, wanted power for sand dunes in utah and idaho, got a gytr ported head from ebay and a hotcams intake cam. Took me a year to figure out i needed to polish the long duration camshaft to get it running, so was running the stock cam for a while, so with only a ported head, i decided to get a stock fuel flow rate geisick injector, was the only aftermarket injector i had found and i was interested in the short nozzle venturi-effect. But i'm thinking that it might add top end hp but isn't really worth the poor performance down low. stock injector has i think 12 little holes and the geisick has 4 fat holes on a shorter injector so the space the normal injector nozzle takes up is supposed to act as a venturi tunnel? not great at low rpm. very excited to have found the TFL rebuilt stock style injectors? with increased flow rate. I seized the piston ring to the cylinder wall at the idaho dunes this summer, dunno why a piston/cylinder kit can't come with piston rings sized properly, my engine builder i hired cause i couldn't figure out how to put the tranny together must not have gapped the rings. so the bike is now athena 500cc big bore piston and high comp, bike runs fantastic but doesn't start. GYTR tuner doesn't effect fueling on the stock computer from idle to 1/8th throttle, i'm thinking that now that i've got a cam, ported head, and big bore, and that i can't increase fueling in the stock computer until 1/8th throttle, that i'm not getting enough fuel for my leg to kick the bike running. so i'm shopping for a fatter injector, assumed i was going with the geisick 120% but now found the TFL option. Hoping the stock style rebuilt injector will help in starting and low rpm atomization, even if a little top end hp is lost, definitely worth the trade as i don't have a trail bike and try to rock crawl the 450 when i'm not at the dunes haha. stock fuel flow rate is 387cc? 460cc is plus 20 percent, 500cc is plus 30% and 550cc is 42% more. Might need 550 or 600 cc if i convert to e85 but i don't think im gonna go that route with utah being high altitude and low atmospheric pressure.

sorry i'm rambling and confusing y'all, thanks so much for your time. Really just need to decide between 460cc and 500cc for my size of engine. 10% displacement increase and maybe 9% from the ported head, maybe 6% from the cam, long duration will add air at rpm but not much at idle? I'm thinking i'm up 25% air, but am unsure how much air is added by the ported head and cam. Undecided between the 20% and 30% injector. Thinking i should go with the 500cc injector particularly in case there's compounding effects meaning i'm up more than the 25% estimate. But i don't wanna overshoot injector sizes and end up being too rich to start. Any bikes on the dyno with a sniffer and similar mods to my motor have a figure for idle air increase? I know the answer is break out a grand for an athena ECU and a dyno tune.

Thanks again for y'alls time.


Staff member
I know the answer is break out a grand for an athena ECU and a dyno tune.
Unfortunately, You know the answer.
A butt-dyno can only get you so far. My streetbike dyno tunes have been worth their weight in gold.
Starting becomes instant, almost no crank. off idle throttle applications become so smooth and useable.

Your starting issue is not with the size of the injector, but how it's being asked to operate at low rpm and starting, by the ECU.
I imagine the stock fuel injectors are not tapped out in stock form and have plenty of head room for you to tune with.
They need to be told to open more often or for longer, if you're needing more fuel to start.
-BIG DAN:thumb: