5.5 or 6.2 injectors on 124 Build.
Road Master
Joined: Mar 2017
Posts: 894
Likes: 224
From: Lynden, WA
The stock M8 injectors are 4.38gm and will support in the upper teens for HP. The only current option from H-D is their 5.5gm injectors. In most mild to moderate 124/128" combination's we use 5.5gm injectors, if you are expecting the build to be above about the 150Hp range we would step you up to the next size.
I am building an S&S 124 using the 550 cam with FM level A heads, 64mm TB and the SE cast manifold and a billet cat. Is the 5.5 gonna be enough? I’m expecting to be at about 140whp.
HD EFI Guru
Joined: Jan 2008
Posts: 4,462
Likes: 4,718
I just updated my 2017 RGS 107 to the S&S 124 Kit with 475C cams, stock heads, 6.2 Screaming eagle injectors, stock throttle body, and the Power vision Tuner. Fuel moto told me I absolutely needed to upgrade to the 6.2 injectors to support the 124. They indicate problems with those who have stayed down with the 5.5.. I don't have the exact details other than knowing you may starve the bike of fule, causing it to run lean and hot.. The fuel moto guys , which I highly respect, could shed more light on the subjects. My thoughts, if it's torn down, do it now. Good Luck!
Yes, rear wheel HP. The 5.5 injectors will work great with your combination. We've tuned quite a few 124" S&S 550 cam, Level A head combinations with them.
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Jamie Long / Fuel Moto USA
The USA's Leader V-Twin EFI & Performance www.fuelmotousa.com
Contact 920-423-3309
Email jamie@fuelmotousa.com
Jamie Long / Fuel Moto USA
The USA's Leader V-Twin EFI & Performance www.fuelmotousa.com
Contact 920-423-3309
Email jamie@fuelmotousa.com
Road Master
Joined: Mar 2017
Posts: 894
Likes: 224
From: Lynden, WA
to get a base tune. Sounds like you might have one for this combo.
Happy to help Kyle, we will have a good base map for your combination and will assist with any necessary tuning support.
HD EFI Guru
Joined: Jan 2008
Posts: 4,462
Likes: 4,718
The above chart shows what you can expect to get when using stock M8 injectors. Like I stated before you may not get these exact limits but it will be very damn close. So your looking at about 112 HP max from the stock injectors on a M8.
This chart shows the SE 5.5 injectors and there limit is about 140 HP. So the math works, as this is just where we see the 5.5 injectors hitting there limits in testing. The problem start to come in based that many injectors do not get the fuel properly atomized and into the ports properly. the spray pattern, position and delivery quantity is very important because as the engine RPM increases you have less and less time to get the job done. If you look at the above charts you will see the Duty Cycle is 90 % with a pulse width of 18 Ms @ 6000 RPM. Most injector manufactures (not resellers) recommend to not run the injectors above 85% Duty Cycle. So lets see what happen if we turn the RPM up to 7000 RPM..........................
So now the injector would be trying to operate at 105 %Duty Cycle and that's never going to happen. Hope this help some of you to have a better understanding of what's really going on.
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Cruiser
Joined: Mar 2012
Posts: 161
Likes: 92
From: Connecticut
I have the H-D 5.5 injectors in my 124” build for over 5000 miles. It has made 154hp with one cam and just under 150hp with the current cam. I do hit the rev limiter but don’t cruise around at 6,000 rpm. I’m curious as to how these 5.5 injectors are too small for my build as it seems to state above?
Banned
Joined: Jan 2019
Posts: 3,036
Likes: 1,013
From: Nunya.
Steve if you swap injectors does the bikes ECU know what type they are or do you say with the SEPST(non EPA) tuner have to program that in to the lap top you are using for the tune?
Like my PC knows what any mouse or keyboard etc what it is when plugged in.
Like my PC knows what any mouse or keyboard etc what it is when plugged in.