While working on my RX this weekend, I ID'd (as you might have read) the fuel injectors. I bought this car recently with the following mods:

Atkins rebuild with port & polish
Racing Beat Rev TII exhaust
Microtech LTX12
MSD Blaster SS Ign coils
SX Performance 15404 FPR
Stock fuel pump


Getting into the top of the engine, I found the previous owner had upgraded the primary injectors to RC Fuel Injection 750cc and left the secondaries stock (550cc).

I read a few of the injector threads, and it appears that the secondary injectors are larger than the primaries in the performance modifications guys are making.

So, while I have things apart, would it make sense to swap between the primary and secondary? Or, should I just upgrade the secondaries? Or perhaps just leave them as they are.

I appreciate your learned opinions


- kel -

 

If you swapped you would need a re-tune. Actually its not the NORM to have the primaries bigger then the secondaries but the Microtech can handle it just fine (harder to idle larger injectors). are you running into fueling issues?

 

I would leave it as-is if the engine is running well. You would need to totally retune the engine if you swap the injectors. If you just upgrade the secondaries, then you would pretty much only need to retune the rpms/loads in which the secondaries are currently set to operate, but that would still be an unnecessary pain if everything is working fine right now.

The stock "550" injectors flow about 600cc/min at 43.5psi fuel rail pressure. Therefore, assuming you are running that fuel pressure, a 600/750 combo would be good for about 290bhp, which is more than your stock fuel pump can handle anyway.

Speaking of which, the stock fuel pump may not be able to support your engine with the current modifications, so you may want to carefully check for fuel starvation before you attempt rapid acceleration.

 

Actually, I do have fuel issues. Part of the motivation for this project and it's disassembly is that the car has always run way rich and I get between 11 and 15mpg around town. On the highway driving it home after purchase, it got 22mpg and I was very pleased. But around town it runs poorly and I always smell raw gas out the exhaust. Once, I blew by a guy going too slow in the left lane and had him wave me down at the next light. Thought he was pissed at me, but he was excited about "the coolest thing" he ever saw; flames out my tail pipes.

The plan this week is to install a wideband sensor (Innovate LC-1), replace all the vulcanized rubber hoses and fix a few other minor things. Once that is done, I will do a complete re-do on the tuning. I also purchased the laptop dongle and hope with some advice from the gurus here I can get it running like it's supposed to.

With that in mind, it seems like overall it would be better to switch the injectors while everything is apart.

Agree?

- kel -

 

In reality, it isn't running well at all. A complete re-tune is part of this week's plan and is part of the reason I have taken stuff apart.

Your remarks make great sense. So, what would you do? I'd like to get maximum performance out of this setup. Would you upgrade the fuel pump? If so, what would you get? Would you upgrade the secondaries, or just swap what's there and run 550cc Primary/750cc secondary?

I really appreciate everyone's insight.

- kel -

 

Is a turbo upgrade planned for the near future, and if so, what is your horsepower goal? (The answer to this question will help everybody on this forum give you advice for your fuel system)

 

No plan to change the turbo. I was told by the seller that the car should be capable of 350hp. Not sure if that's reasonable, but I can be happy to get maximum performance without getting into too many dollar signs. I'm hesitant to state a horsepower goal, as I'm somewhat ignorant of what's capable by which modifications.

Still learning

- kel -

 

I do. Even better idea is to get them cleaned/flowed while they are out.

 

The link below has a pretty good list of a typical modification flow chart. Big surprise, the seller lied about your car, lol. Anyway, while I understand that you are not concerned about speculating, it is important to have a basic idea of your power goals so that you can plan accordingly.
http://fc3spro.com/TECH/FM2W/power.htm

If you are keeping the stock turbo, then I would just keep the current fuel injectors, and yes, I would swap the 550s to the primary rail if a tuning session is planned anyway. If you end up needing more fuel later on, installing larger injectors isn't much of a chore, and a new manifold gasket isn't very expensive. Also, I agree with J-Rrat in that personally, I would at least send the 550s out for cleaning and "rebuilding", and also the 750s unless they are in like-new shape. If you don't have a local shop, these two are RX-7 friendly:
http://witchhunter.com/
http://kgparts.com/index.php?page=fuelinjectclean

Sorry to ask this, but are you sure that you have correctly identified the primary and secondary fuel rails? The secondary is the long one on top, and the primary is the short one hidden under the manifiold.

Yes, I would definitely replace the stock fuel pump, although before buying anything I would take a peek just to make sure a previous owner didn't already replace it. Walbro, Bosch, and the OEM Cosmo fuel pumps are popular for in-tank upgrades. If you are considering external pumps, I like the Aeromotive and SX brands.
http://fc3spro.com/TECH/MODS/FUEL/fpump.htm

You may also want to replace the stock pressure-side fuel filter while you are at it. A new OEM filter will work just fine. Be careful if you decide to "upgrade" to one of the fancy racing type filters, as some of them have a 25-40 micron filter element that is too coarse for an EFI street car.

 

No need to apologize for asking; not a stupid question at all. But, that said, I have a stupid answer: I have indeed mis-identified them. The 750s are on the top rail (imadope). Thanks a mil for the clarification. Makes me feel a bit better that at least *that* wasn't screwed up.

The 750s should have just over 10k miles, so I'd like to think they're okay. I have an extra pair of 550's that I could send out; thanks for the shop-tip.

I am (was) pretty sure the fuel pump is stock. I lifted the access cover, and judging by the pristine state of the dirt, it appears that the screws have not been disturbed.

Much grassious for the links and explanations, I really appreciate your insight! I'll look over the link on power and come back with a (hopefully) realistic goal.

- kel

 

I think they're 550cc at the factory 2.5bar fuel pressure (36psi)

 

Very common mistake actually. We just have a lot of people on this forum who think that they should be embarrassed that they were not born with uber tuner skills. In my opinion that defies logic, but historically I have been in the minority with that opinion. I'm glad to see that at least one other forum member has that point of view.

LOL

Yes, I am just converting that value to 3bar to compare to the RC injectors. Also, it is likely that the previous owner set the fuel pressure to 3bar, and even if he didn't it would be easy for the OP to bump it up to 3bar with the SX FPR.

 

 

Okay, so I got my first GOOD surprise in this project. On your advice to get the stock injectors rebuilt; I was getting them ready to send in when I recalled a bag in the box of parts-n-stuff the seller gave me that had spare injectors. I though it'd be a good idea to get them all rebuilt at the same time so i could keep a couple of spares on the shelf.
Imagine my joy when i read "new 550cc denso" on the bag. Pulling off the wrapping, I found that they *are* actually brand new. So, no additional down time.

Reassembly begins today!

Thanks a million for the info and suggestions!

- kel -