I have done quite a bit of searching and have yet to find a definitive answer to this question:

What is the difference between the SL4-550 and the SL9-550?

I know the following is true about both:

Part Number Type Flow Rate

SL4-550 Saturated 550 cc/minute
SL9-550 Saturated 550 cc/minute

I found a site that listed this:

SL9-550 SATURATED INJECTORS-HIGH RESISTANCE For HONDA S2000
Fits Any Honda S2000
Connectors And Spacers are Needed

SL9-550 SATURATED INJECTORS-HIGH RESISTANCE For HONDA ACURA
Fits Any Honda / Acura OBD1 Motors
92-95 Civic All
94-95 Acura Integra All
For 96+ OBD2 Honda / Acura - Connectors Needed

SL4-550 SATURATED INJECTORS-HIGH RESISTANCE For Acura RSX
Fits:
Any Acura RSX With K20 Motor
Any Honda Civic 01+ with K20 Motor (2.0 Liter)
Connectors And Spacers are Needed

Here is a reference picture of a SL4 injector:



Here is a reference picture of a SL9 injector:



The only difference I really see is the end of the injector where one is a consistent diameter and the other isn't.

 

I take it no one knows the difference. I will e-mail RC.

 

Damn I figured RC would take awhile to reply. Only took 10 minutes, haha. Here is my response from Carol Chung [Carol@rceng.com]:

Thank you for choosing R.C. Engineering. Regarding your previous e-mail, you requested RC part number information. The SL4-550, is a custom modified injector to fit any application. Since, we have alot of Honda/Acura B-series customers, the SL9-550 were designed to fit and have ready in stock. These may also fit some Mitsubishi applications. If you have anymore questions, please contact us at your convenience.

Thank you,
R.C. Engineering

Sounds like I will go with the SL9-550s since they are meant for Honda/Acura B-series engines.

 

so which one of those is actually the same size as a stock injector?

 

One is the newer version. I forgot which one.