So as some of you know, for the last year or so I've been rebuilding my Z after having an engine failure at Vmax back in 2012.

 

The aim of this rebuild has been to make the setup more efficient and rectifying what I believed contributed to the engine failure. The physical engine rebuild was completed late last year and it's been on and off the dyno (over 1000 dyno miles and 280 litres of fuel) since the beginning of the year. The main source of my issues has been around fuel delivery and maintaining fuel pressure. After going through a couple of fuel pumps and fuel pressure regulators (all of reputable brands like AEM, Tomei and SARD) and testing my Nismo 740's to find no issues there, I decided to further investigate other aspects of the fuel delivery system.

 

At peak boost I required a fuel pressure of 66.5 psi (43.5psi base plus 23psi boost) at a minimum of 240lph

 

The main symptoms I was suffering with were:

 

Sudden fuel pressure drop (down to 1 bar at times) when fuel level was below 1/4 tank

Fuel pressure differential not holding a consistent 3bar

Fuel temp hitting 50c

higher VE numbers than expected on the VE map

 

The OE fuel lines on the Z of a 8mm bore with a lot of 90 and 45 degree bends in the system an is near as dam it 4.5 meters from the exit pipe of the tank lid to the FPR.

I spoke to a friend of mine who works in the motorsport industry who did a fuel calculations for me around the delivery capability of the OE fuel lines. The maths showed that at 67psi and attempting to flow 240lph through an 8mm bore pipe along a LEVEL AND STRAIGHT (important point) there would be a 15.2 psi pressure drop from end to end of that section of pipe. That pressure drop is then increased for every bend that is put into the pipe and any changes of elevation in that pipe. Realistically there is potential of a 20psi pressure drop along stock fuel lines at the fuelling requirements I needed.

 

This means that for the FPR to maintain 66.5psi at full boost, the fuel pump has to generate a minimum of 81psi of pressure and still deliver a minimum of 240lph (which if you look at the AEM 320lph pump, it can't deliver that much fuel at that pressure!)

 

But the problems don't stop there. Heat is the next issue. None of the stock fuel lines are insulated, so heat soak can become an issue. But that's not the only source of heat. At that pressure, the friction of the fuel being forced down a skinny pipe heats up the fuel. On top of that you then have a fuel pump that is working absolutely flat out, so the fuel heat soaks as it passes through the pump.

 

This is where a viscous cycle becomes apparent. As your fuel is heated up as its pushed through the pump, down the fuel lines and into the engine bay, only a small amount of it is used in all but WOT situations. What fuel isn't used is returned to the tank via the FPR. No while this isn't a huge issue when you have a near full tank, as the volume of fuel in the tank is able to absorb that heat with very little increase in temp to the rest of the fuel in the tank. However, when fuel levels get lower, there is less volume to soak up the heat of the fuel, causing the remaining fuel to slowly get warmer and warmer and warmer.

 

Based on the data I have collected from my data logs there is a direct correlation between fuel pressure and fuel temps, in that when the fuel gets above 40c fuel pressure becomes more and more unstable. Once into the 50c area is where sudden pressure drops appear.

 

This ONLY happens at fuel levels below 1/3 tank.

 

My theory is that at those fuel temps, the fuel pump begins to cavitate, which is what causes the sudden drop in fuel delivery and pressure.

 

There appears to only be two viable options to solve this problem:

 

1. fit a fuel cooler

2. Increase the size of the fuel lines.

 

Which leads me to my chosen solution for the problem.....

That's a really interesting read, thanks for posting,

 

Been trying to work out fueling for my zed that's about to have its rebuild, again with 740cc injectors,

 

Sounds like the fuel pump I just bought would be ideal for you, walbro 450lph, should still fit in OE place, at 80psi flows 273lph according to the chart.

 

Thought of a cryo2 system? One configuration is to use the co2 to cool the fuel lines...

Edited May 14, 20169 yr by (adl) phoenix

I decided to uprate the whole fuel line setup with braided hoses and AN fittings. This cost me a total of around £400, which to be honest is a drop in the ocean compared to the rest of the engine build!

 

Having conducted many hours researching chose to run AN8 fuel lines main feed from the tank, split into AN6 lines to feed the fuel rail in parallel, back into an8 to the fuel pressure reg and an8 back to the tank. Being braided, they're more insulated than stock lines. Further more, as they have a large cross section, less heat can be absorbed by the fuel. It was best described to me as this. If you pass X amount of fluid through a thin walled small diameter pipe over a heat source, the temp of the fluid will be higher to that of the same volume of fluid passed through a larger walled pipe with a larger cross section.

 

Over the same distance using the above sized piping, I will only get a 0.7psi pressure drop across my system.

 

This option will kill two birds with one stone. Less heat soak from outside heat sources, less pressure drop meaning the pump doesn't hawork as hard to create the volume and pressure I require

 

ve to

 

The first part I modified was the fuel tank lid. I had an AN8 boss welded on to the existing exit pipe on the tank lid.

 

 

I also drilled out the return pipe and fitted an 90 degree an8 bulkhead fitting.

 

 

I then fed an8 lines down the underside of the car, fitting a slimline 30micron fuel filter inline under the car.

 

 

For the fuel rail, I cut the ends and OE feeds off the OE fuel rails (my car is a 1995) and had an6 bosses welded on the end of each rail. The reason I chose to do this vs aftermarket rails was after speaking to some of the big guys in the states (ep racing, specialty z) there is very little if any flow benefit with aftermarket side feed rails. The main reason people fit then is to end able the fitment of later style injectors. Being a late car, I don't have this issue, so modifying the stock rail for significantly less £££ made more sense to me.

 

 

This is the system all complete (return was plumbed in after this pic was taken)

 

 

I've got a few more things to sort on the car before it goes back on the dyno, so I will be able to report back with the final results, but I'm confident in my theory, which should result in a consistent fuel pressure, lower fuel temps and I can use more of the fuel in my tank without fear of sudden pressure drops

Nice work. Be interested in your results, Mines also a 95 but I have chosen to replace the OE rail, I wouldn't trust my welding, seemed like an unneccessary weak point,

 

How does it look under the zed, any trouble routing the larger pipework?

That's a really interesting read, thanks for posting,

 

Been trying to work out fueling for my zed that's about to have its rebuild, again with 740cc injectors,

 

Sounds like the fuel pump I just bought would be ideal for you, walbro 450lph, should still fit in OE place, at 80psi flows 273lph according to the chart.

 

Thought of a cryo2 system? One configuration is to use the co2 to cool the fuel lines...

 

Unless your planning on running 35+psi of boost, you shouldn't need such a pump. I still have an AEM pump in my car and I'm confident it's capable of delivering the amount of fuel I need.

 

The main point that I'm trying to make with this thread is this. While there are plenty of example of high (500rwhp plus) cars running stock fuel lines, it isn't with undesirable side effect and potentially serious consequences. At best they suffer pressure drop at low fuel levels, at worst they eventually run lean and blow an engine.... Like me.

 

In the grand scheme, when those of us are spending thousand of pounds on speccing high horsepower engines, to the not consider auxillery parts like fuel systems may prove naive. Another £400 quid on a £6000 bill is nothing. I'm guilty of it, and it cost me a forged engine.

 

I'm hoping that those of you that potentially high hp builds, or are considering them in the future, will consider this advice and take a moment to consider their entire setup not just the physical engine.

 

The above is why I went to a Haltech. If something goes wrong, I now have the right sensors in the right places to identify these problems and an Ecu that is capable of saving the engine should I encounter issues. Those of us who ran a mapped stock Ecu wouldn't know about these problems without a fuel pressure sensor and a capability of data logging. Fuel pressure gauges just don't cut it, as evidenced by this article

http://www.adaptronic.com.au/fuel-pressure-regs-is-there-much-of-a-difference/

Ive not finalised the underside yet. The pipes run along the carbon cannister pipes (I still use mine as I was getting noticeable fumes after hard runs) but it's all nicely protected like the stock lines

Just suggested the pump as yours wasn't up to it with OE pipes. But you've found a much more efficient solution to the problem then just a bigger pump

 

Unfortunately I'm using a remapped OE ecu so won't have the same safe guards as you,

 

If you don't mind me asking how many meters of pipe did you need? Will probably do mine if you get the results you were after.

Just suggested the pump as yours wasn't up to it with OE pipes. But you've found a much more efficient solution to the problem then just a bigger pump

 

Unfortunately I'm using a remapped OE ecu so won't have the same safe guards as you,

 

If you don't mind me asking how many meters of pipe did you need? Will probably do mine if you get the results you were after.

 

I ordered 3 meters of AN6 and 9 meters of AN8. I've probably got half a meter of AN6 left and maybe 1.5 meters of AN8 left over. It was the fittings that were the most expensive part, about double what the piping costs.

Would it not be far simpler/cheaper to reroute the stock pipes with larger diameter hose and delete/modify where appropriate restrictive bends. I'm assuming that given your prefix of the hose attatchments to the pump diameter isn't the factor per say. By this I mean the take offs are bottle necks, the collective resistance over a straight length of pipe is relatively low comparatively to a 90' bend, not to say it's not a significant factor over a long run, if the fuel lines had no bends then I suspect the problem would be far less pronounced.

If we instead changed to say 10mm hardlines avoiding harsh bends, long sweeps etc then I recon that you would save a good deal on hosing costs with an acceptable outcome. Obviously I haven't done the math and you have. I'm merely saying I recon this potentially worth while route could be made far more pocket friendly.

Be interesting to see at what point this becomes problematic. Obviously it stands to reason that at WOT a car with 370cc injectors will suffer far less than one with 1000cc injectors. For example it would be interesting to see the effect on a 400-500fwhp car on 550cc injectors as this is fairly common, comparatively.

I think you've stumbled across something really worth more thought for zed tuning.

 

The main symptoms I was suffering with were:

 

Sudden fuel pressure drop (down to 1 bar at times) when fuel level was below 1/4 tank

Fuel pressure differential not holding a consistent 3bar

Fuel temp hitting 50c

higher VE numbers than expected on the VE map

 

This ONLY happens at fuel levels below 1/3 tank.

 

 

How are you driving the car when this occurs?

How are you driving the car when this occurs?

 

Wide open throttle.

 

Now while it could be suggested that I simply don't boot it with less than a third of a tank of fuel, that simply just isnt acceptable to me. if I'm driving down a country road and want to overtake a car, I don't feel that having to glance at the fuel gauge first before making the manoeuvre is something that I even contemplate having to consider in such s situation. It wouldn't be acceptable to a company that releases a 500hp car, so it isn't acceptable to me.

Would it not be far simpler/cheaper to reroute the stock pipes with larger diameter hose and delete/modify where appropriate restrictive bends. I'm assuming that given your prefix of the hose attatchments to the pump diameter isn't the factor per say. By this I mean the take offs are bottle necks, the collective resistance over a straight length of pipe is relatively low comparatively to a 90' bend, not to say it's not a significant factor over a long run, if the fuel lines had no bends then I suspect the problem would be far less pronounced.

If we instead changed to say 10mm hardlines avoiding harsh bends, long sweeps etc then I recon that you would save a good deal on hosing costs with an acceptable outcome. Obviously I haven't done the math and you have. I'm merely saying I recon this potentially worth while route could be made far more pocket friendly.

Be interesting to see at what point this becomes problematic. Obviously it stands to reason that at WOT a car with 370cc injectors will suffer far less than one with 1000cc injectors. For example it would be interesting to see the effect on a 400-500fwhp car on 550cc injectors as this is fairly common, comparatively.

I think you've stumbled across something really worth more thought for zed tuning.

 

Cheaper, no. Larger hard pipe may be cheaper to buy but requires a more expensive type of fitting. Easier: deffinatly not! Routing the braided lines was a simple task that took very little time. Having helped my motorsport friend fit 12mm hard pipe fuel lines to one of his race cars, it's a ROYAL pita, very time consuming and you make one wrong bend it potentially ruins the whole section of pipe.

Wide open throttle.

 

Now while it could be suggested that I simply don't boot it with less than a third of a tank of fuel, that simply just isnt acceptable to me. if I'm driving down a country road and want to overtake a car, I don't feel that having to glance at the fuel gauge first before making the manoeuvre is something that I even contemplate having to consider in such s situation. It wouldn't be acceptable to a company that releases a 500hp car, so it isn't acceptable to me.

 

Have you considered that air is getting into the fuel system when you WOT with the remaining fuel in the tank being pushed up the

back of the tank exposing the pump intake to air.

Because our fuel tanks are designed to pressurise and the pump is submerged it's highly highly unlikely. I suppose you could say that if it was occurring on track with rapid changes of direction that it could be fuel starve from the fuel sloshing around in the tank, but all my testing and data is from dyno time where the car is effectively motionless. I feel it makes more sense with the data that's available that the fuel is vapourising as its drawn into the pump itself, causing the cavitation and loss of delivery

So the car is not being driven - I read your comment "if I'm driving down a country road and want to overtake a car" at WOT

as the event trigger. That's why I asked.

 

Have you spoken to Greg@SZ?

Yeah spoke to Greg and Mitch at EPRacing, both agree that what I'm saying is entirely feasible.

 

There do seem to be some exceptions to which I've found, which I can't explain. However I would say that it's not an issue that's directly linked to power levels, it's more around pressure requirement. Obviously a larger turbo will make more power at a set psi compared to a smaller turbo. It's also worth pointing out that in the vast majority of cases with in tank pumps, as pressure requirement goes up actual fuel flow reduces from the pump.

Well it seems (as you have spent the dosh) to fit the new braided hoses and see what happens.

 

Have you looked on TT.net? I came across this which may (or may not) help as it ended with the same conclusion

as you have, but did cover other possibilities.

 

http://www.twinturbo.net/nissan/300zx/forums/technical/view/766092/Fuel-pressure-drops-under-boost--Any-thoughts-detailed-gtgt.html

Yeah I went through all those changes aswell as a part of the diagnostics sessions I had on the dyno.

 

Changed the wiring to the pump to thicker gauge (hence that extra connection on my tank lid in one of the pics) and bypassed the FP controller, changed where we took the vac feed from, different sized vac lines, different pumps, different fprs

 

Trust me when I say I've spent much much much more money trying to diagnose the issue than I have on replacing the fuel lines!

I'm sure you have Nick, however sometimes the most intractable problem can be down to the simplest cause

which can be overlooked time and time again. Let's see what happens with those new lines.

With fuel temps being a massive factor in this upgrade would it not be wise to insulate those lines under the bonnet to avoid heat sink from the motor...maybe coming in from the front of engine bay to avoid running them over the top of a very hot engine. Great read btw and a lot of interesting facts there Nick:)

With fuel temps being a massive factor in this upgrade would it not be wise to insulate those lines under the bonnet to avoid heat sink from the motor...maybe coming in from the front of engine bay to avoid running them over the top of a very hot engine. Great read btw and a lot of interesting facts there Nick:)

 

It's deffinatly something I'm considering. My supply and return lines have extra insulation where they come up the from the underside of the car and past the down pipe and turbo.

 

The routing of the lines has been the biggest challenge with the new setup, aswell as deciding on the location of the FPR. I could relocate the battery to the boot and use that space for the FPR, but frankly I couldn't be arsed with the ball ache!