High fuel pressure effect on injectors

[TurboBob]

Question: Will higher than “normal” fuel pressure harm the fuel injectors.

The car is a 95 Talon TSi AWD with a 16G and the usual mods. I installed an AutoMeter electric fuel pressure gauge about two years ago with the stock fuel pump still in the car. At idle the gauge read 37 psi with a normal rise with boost. After installing a Walbro fuel pump and re-wire I have 50 psi at idle with rising pressure with boost. I know this is high. I have had HRC (Denso?) 625 cc injectors installed for about a year and have put 20,000 miles on the car in this condition.

The injectors are controlled by a S-AFC with excellent idle and tip-in characteristics. I recently passed the Missouri emissions test, done on rollers, with read outs well below maximums.

So given that I am happy with the way the car is running, should I be concerned about the life of the injectors with the higher pressure? Thanks.

Bob

[Peter 92TSI]

The added extra fuel pressure should be fine, but too be honest you have a lot of fuel for the level of mods you have. But to answer your question, it won't harm the injectors. In theory it will actually increase their size, from the added fuel pressure. I don't know the equation off hand. ;)

But the problem you might encounter with a higher base fuel pressure is at high rpms running high boost. The stock fuel pressure regulator, adds X amount of fuel for each psi of boost. Since you are running 50 psi base, and the FPR can only adjust about up to 70 psi, you won't really be able to run more than 19 psi without trouble, because after that you might start running into problems with fuel starvation.

Peter

[Nick 92 TSi AWD]

It's 1psi of fuel pressure for each pound of boost. The FPR does this to make sure that you have linear fuel delivery. Say you're at part throtle, a little bit of vacuum, 50psi fuel pressure. Then you start boosting, say 2psi manifold pressure, but still at 50psi. Before fuel pressure was 50psi higher than the pressure in the ports, but now under boost it is only 48psi higher. So now less fuel for a given injector on time will be entering the ports while under boost. Then all of a sudden at 13psi boost the fuel pressure regulator will kick in and start turning up the fuel pressure. This wreaks havoc on tuning properly.

It's time to get a real fuel pressure regulator. Go to vfaq.com and look up the fpr vfaq.

DO NOT get a sx or paxton regulator though. They are rather cheaply constructed and fall apart after time. I recomend an aeromotive or a aem fpr.


Oh and to answer your question, no it won't hurt the injectors at all. At 80psi fuel pressure injectors start getting sticky, but that's about it.

[Peter 92TSI]

Good info, thanks Nick.

Peter

[jasen]

Depends on the injectors as well. In my experience with other cars, after about 70psi stock injectors start to leak and can sometimes burn out due to excessive duty cycle time. That's when you know it's time to get bigger injectors and turn the pressure down. DSM injectors may be a little more tolerant? Guess I'll find out soon enough when I start modifying my fuel system later on.

[Nick 92 TSi AWD]

Not a problem. It does depend on the type of injector, type of fuel used, quality of fuel filter, etc...

Also most fuel pumps start to drop off flow at pressure that high.
I'd never go over 75psi fuel pressure on any car, personally.

[TurboBob]

Thanks you all for the information. I am sending off to get an HRC Super 20G and their new high flow exhaust manifold so I hope to take advantage of the bigger injectors soon. I got the 625 cc injectors instead of the cheaper 550 cc units because I was told the cheaper 550 units (don’t know the brand) were not as reliable. I have been very pleased with these 625 cc units which are low impedance and seem to work very well with the S-AFC. My part throttle response is better than with the stock injectors, and my idle has been fine.

I am looking at getting a different FPR but I may try to thread the fuel rail for an AN fitting rather than go the welding route. Thanks again for the info.

Bob

[Nick 92 TSi AWD]

Personally I tapped it into the fuel rail and then had someone tig weld around the an fitting....I'm really anal :)

You can kind of see my sx in this picture when I was putting on my gc camber/caster plates:
http://www.pics.dsm.org/files/DSCF0229.JPG

My engine bay looks like crap in that one. It was a pre de-japanesing pic :)

[lx88aod]

Nick 92 TSi AWD,

What size of fitting did you tap into the fuel rail? Did you have the fitting welded to the fuel rail after you screwed it in because you really thought it would leak, or just because you wanted to? Do you think that gooping the fitting's threads with JB Weld before screwing the fitting in would have been effective in sealing the fitting adequately, as an alternative to welding??

I've been looking at the FPR end of the fuel log, and it looks to me as if you could have tapped for 3/8 NPT or 1/2 NPT, then screwed in an NPT to AN adapter, but the hole looks like a "tweener" to me - a bit too big for 3/8 NPT and a bit too small for 1/2 NPT. The hole is also tapered and a bit off center from the centerline of the fuel log, and there's a bit of a shoulder at the bottom of the taper that looks like it might cause interference with the end of the tap during the tapping process. How did you deal with that?

Of course, maybe you didn't tap for NPT at all, so I thought I'd just ask.

Thanks.

[99gst]

50psi base fuel pressure seems like a lot to me at idle. I'd think you are running very rich at idle and part throttle. How many miles are you getting per tank?

[Nick 92 TSi AWD]

They're 8an fittings on both ends. It's just taped and welded for added safety. That is definetly NOT somewhere you want leaks coming out, or fittings backing off.

I highly highly recomend against JB weld in this area. It is not compatable with fuel and will break down. The fuel literaly just eats it away.

Tapping for npt is probably a smatter way to go though. NPT fittings have fine threads and seal via the actual threads. AN fittings have wide coarse threads and actually seal with a rubber o ring. It's not really the ideal thing for a high pressure application like this. That is not something I knew when I did my install. If I were doing it again I would tap with NPT.

I didn't actually do the work on the fuel rail myself. I sent it to extreme and had them do it. They charged $90 for tapping/welding both sides. I'd definetly have the tapping done by a good machine shop, it's very easy to screw up. Of course most of us don't have tig welders at our disposal either....

hope that helps

[lx88aod]

Nick 92 TSi AWD,

Thanks for the reply.

One last set of questions about welding the fitting - did Extreme think the fitting would seal successfully without being welding, or did they recommend the welding because they were afraid it would leak, or did you even ask their opinion about whether welding was necessary?

Thanks again.

[Nick 92 TSi AWD]

I just told them I wanted 8an fittings put on. Didn't expect the welding. I would do it anyway though, considering how extremely important that is....