Fuel Line Plumbing on Twin SUs

primer · Post by **primer** » Wed Aug 18, 2010 6:59 pm

Hello folks, Im after a bit of advice regarding the twin-SU setup on my new motor.

Its a 3.5 RV8 from a P5B (the hi-compression 10.5:1 version), thats been modernised with stuff like proper lip seals on the crank, P38 heads & gaskets, a warmer cam and a high-volume oil pump kit.

Its still running the twin SUs but Im hoping for around 180bhp. These motors are supposed to produce about that in stock form (I assume mostly due to the CR), and its had a few extras, although I suspect the inlet will hold it back.

I've got a Facet solid state pump, number #40106, which is supposed to fuel motors upto 150bhp. It produces 5~6psi.

My concern is this; the carbs (and the lines to the tank) are the 'feed/return' type. Ie. a continuous loop. The pump's 6psi is twice what the SUs can handle, but would this matter on a feed/return system?
Surely even considering the pressure required to push the fuel along the lines, it shouldnt be any higher than a couple of psi?

Also, if there is a continuous flow of fuel past the carbs (I believe the pump is rated at 34gph unrestricted), will it still be limited to 150bhp? Im guessing the flow will be pretty unrestricted, so even if the pump is pumping 25gph (albeit at only 1 or 2 psi in the lines), will this increase its potencial fueling ability? I cant imagine it burning 25 gallons of fuel in an hour, although this figure must be when the engine is churning out maximum bhp?

If I've really got the wrong end of the stick I'd greatly appreciate it if anyone could point out where. I'd like to avoid buying a new pump if I can, although I also would like to avoid nasty lean mixtures at full whack aswell!

Cheers,
-Tom.

topcatcustom · Post by **topcatcustom** » Wed Aug 18, 2010 7:22 pm

Hi, firstly, unfortunately- I'd be slightly suprised if one of those little facets could keep up with your engine at WOT, but I guess you could try it first- not a big job to change fuel pumps if not good enough as you already have it.

Secondly, I dont think it's worth having a return pipe, just make sure they dont flood by putting a regulator in line, a filter king filter with regulator built in should handle it quite well.

Be careful about revving it when cold and it make sure your oil isn't too thick as the high volume oil pumps put more stress on the cam/dissy drive gears, which dont take long to wear out as it is!

primer · Post by **primer** » Wed Aug 18, 2010 8:22 pm

The only reason I've hooked up the return line as it was already on the car, and I was hoping it would allow me to not use a regulator and not worry about flooding my floats. Does this not sound feasible?

Its looking like a bigger pump is in order. I'm gather the next stage up is the 'Red Top'? These are pretty pricey, although if thier the right part for the job....

I've just found a nice little formula for calculating fuel flow (CC per minute = BHP X 4.7), so I'll rig up the pump and see what it manages. Following the formula, a 200bhp motor should need 940cc of fuel per minute. Results shortly!

primer · Post by **primer** » Wed Aug 18, 2010 9:21 pm

primer wrote: I've just found a nice little formula for calculating fuel flow (CC per minute = BHP X 4.7), so I'll rig up the pump and see what it manages. Following the formula, a 200bhp motor should need 940cc of fuel per minute. Results shortly!

Running zero lift (fuel can and pump both sat on the bench), it pumped the 940cc in about 20 seconds! This seemed a bit quick so....

Max lift is supposed to be 30cm, so I sat the can about 50cm below the pump (with the outlet hose terminating a further 30cm up), and tried again. 25 seconds!

Its looking very good, but I have no idea how the long runs of fuel line on the car will affect it so no doubt the best plan will be to rig it up on the car and run the test again.

Any ideas regarding the return line? I'd like to leave it in place if possible?

RoverP6B · Post by **RoverP6B** » Thu Aug 19, 2010 1:53 am

Hello primer,

Leave your return line in place. The fuel pump supplies each carburettor, filling the float bowls to the level set by each float, with excess returned to the tank.

Provided your fuel pump can maintain the maximum level in each carburettor at full throttle, then the pump is satisfactory. You will know if it cannot as the fuel level in each carburettor will drop, the mixture will thus become too lean the the engine will likely hesitate.

Ron.

primer · Post by **primer** » Thu Aug 19, 2010 7:06 am

RoverP6B wrote:Hello primer,

Leave your return line in place. The fuel pump supplies each carburettor, filling the float bowls to the level set by each float, with excess returned to the tank.

Provided your fuel pump can maintain the maximum level in each carburettor at full throttle, then the pump is satisfactory. You will know if it cannot as the fuel level in each carburettor will drop, the mixture will thus become too lean the the engine will likely hesitate.

Ron.

Hi Ron,

Thats pretty much what I was hoping.

What do you think about the pressure issue? The SUs can handle 3psi max, but the pump can (apperently) make 6psi. Im assuming it'll make 6psi stalled (ie. without a return line, and excess not returned to the tank). How are regulators for impeding flow? If I fit one, I may garentee safe pressure, but no longer have sufficient flow!

Apologies for all the questions. This is my first V8 and you guys no a lot more than I do!

Ian Anderson · Post by **Ian Anderson** » Thu Aug 19, 2010 7:27 am

Put the regulator on the return line (after the carbs)

Everything before it will be at your set pressure

Ian

RoverP6B · Post by **RoverP6B** » Thu Aug 19, 2010 7:36 am

Primer wrote,...

What do you think about the pressure issue? The SUs can handle 3psi max, but the pump can (apperently) make 6psi. Im assuming it'll make 6psi stalled (ie. without a return line, and excess not returned to the tank). How are regulators for impeding flow? If I fit one, I may garentee safe pressure, but no longer have sufficient flow!

Hello Primer,

The standard AC mechanical pump on the P6B is rated at 3.5 to 5 psi and that is running twin SU with spill return.

You won't have a problem with a possible 6psi maximum, and as such a fuel regulator is not necessary.

Ron.

DaveEFI · Post by **DaveEFI** » Thu Aug 19, 2010 8:00 am

topcatproduction wrote:Hi, firstly, unfortunately- I'd be slightly suprised if one of those little facets could keep up with your engine at WOT, but I guess you could try it first- not a big job to change fuel pumps if not good enough as you already have it.

Secondly, I dont think it's worth having a return pipe, just make sure they dont flood by putting a regulator in line, a filter king filter with regulator built in should handle it quite well.

Be careful about revving it when cold and it make sure your oil isn't too thick as the high volume oil pumps put more stress on the cam/dissy drive gears, which dont take long to wear out as it is!

It will depend on the installation to some extent and how the car is used, but the purpose of the return line is to try and combat fuel evaporation with high under bonnet temperature like in a traffic jam etc - and on a P6 it didn't quite work.

Of course the P6 had a mechanical pump which would have low flow at idle - the later SD1 used an in tank electric one.
It *might* help if you have a powerful electric cooling fan - but I'd not guarantee it. FWIW, it's one of the reasons I changed my P6 for an EFI SD1 - and that too can suffer fuel evaporation under arduous circumstances. But not like the P6 which was a nightmare.

topcatcustom · Post by **topcatcustom** » Thu Aug 19, 2010 8:07 am

Didn't think of evap! Just I haven't had a car on SU's with an active return before and never had any probs- but none were kicking out V8 heat! Time to get some NASA space foil stuff for insulation!

primer · Post by **primer** » Thu Aug 19, 2010 6:24 pm

Thanks for all the replies folks!

So its looking like Im ok pressure wise, as long as I keep the return line? This is exactly what I'd hoped for, but just wanted confirmation from someone with more experiance than myself.

I never new regulators went in the return line. Is this to 'bleed off' excess pressure? -So I guess they wont impede flow to the carbs?

I'll install the pump and check the flow rate again.

This 'fuel evaporation' thats mentioned; what sort of symptons does it show?

DaveEFI · Post by **DaveEFI** » Thu Aug 19, 2010 7:05 pm

topcatproduction wrote:Didn't think of evap! Just I haven't had a car on SU's with an active return before and never had any probs- but none were kicking out V8 heat! Time to get some NASA space foil stuff for insulation!

That's an idea - but I'm not certain just where the evaporation occurs. Insulating the carbs might be tricky.

I had an early XJ6 on carbs, with air-con. It had an air-con cooler for the fuel supply. And I reckon unleaded petrol makes things worse. But can't prove it.

unstable load · Post by **unstable load** » Thu Aug 19, 2010 7:35 pm

I believe unleaded does make things worse for vapour lock. It has to do with the additives and lack thereof in modern fuels.

As for your flow rate, primer, connect it to the pipe and then see if it does the flow numbers.

BTW, have a Huco pump on mine that does the job admirably and does not use the return line. It was specced for an XJ6s'motor requirements, I believe.

RoverP6B · Post by **RoverP6B** » Thu Aug 19, 2010 8:40 pm

primer wrote,...

This 'fuel evaporation' thats mentioned; what sort of symptons does it show?

When it happens, it does so very quickly,...shuddering,..jerking, combined with a considerable loss of power and possibly a complete shutdown altogether.

However, this will only happen if you are running a mechanical fuel pump. As you are running an electric fuel pump and assuming you place it beneath the car close to the fuel tank, you will never experience fuel vapourisation.

unstable load wrote,...

I believe unleaded does make things worse for vapour lock. It has to do with the additives and lack thereof in modern fuels

My P6B here in Australia went through a terrible time with fuel vapourisation in the early 1990s and that was on leaded fuel. On modern unleaded fuel, I have never had a problem.

Ron.

topcatcustom · Post by **topcatcustom** » Thu Aug 19, 2010 8:42 pm

primer wrote: I never new regulators went in the return line. Is this to 'bleed off' excess pressure? -So I guess they wont impede flow to the carbs?

With the regulator in the return line, it keeps the pressure constant through the feed pipe and carb and lets any extra off straight into the return to tank. With the more common position of it in the feed line just before the carb, it lets any amount of fuel through up to 3psi (or whatever you set it) but once it hits 3psi pressure behind it (in the carb when the float chamber is full and the float shuts the feed valve) it will close and stop any more going through. In practice though this doesn't happen (when running) as it will be constantly fluctuating as your revs change or find a position when it lets a steady trickle through when sitting at constant revs inc idle. The problem with this one is that in theory, half the fuel let through by the regulator into the carb could go straight back down the return line into the tank thus not actually keeping good pressure in the float chamber.

The problem with Ian's suggestion is that if you get a cheapy regulator it may not regulate the pressure infront of it- only behind, so it could let all or most of the fuel through as it will never see a pressure behind it as the return to tank is open, and can require a dead head to get some back pressure.

Sorry bit of a waffle but hope it makes sense!