Pi - Fuel Accumulator

[k_raven_smith]

The fuel inlet doesn't change position. The return from the prv returns straight back to the fuel tank. 

[Schnippel]

On 9.6.2026 at 18:38, michaeldavis39 said:

Wenn Mercedes mit seinem mechanischen K-Jetronic-Einspritzsystem, dessen Einspritzdüsen unter ähnlichem Druck wie die Pi-Lucas-Einspritzdüsen ansprechen, einen Kraftstoffspeicher zur Vermeidung von Kavitation/Dampfblasenbildung und zur Verbesserung des Warmstarts verbaut hat – warum hat Triumph dann nicht dasselbe in unseren TR6 mit Pi-Einspritzung eingebaut? Ich vermute, dass es damals noch keine Kraftstoffspeicher gab. Meiner Meinung nach war sich Mercedes der potenziellen Schwächen der mechanischen Einspritzung bewusst und wollte das Beste für seine Fahrzeuge und Kunden. Deshalb unternahmen sie alles, um ihre Version der mechanischen Einspritzung zuverlässig zu gestalten. Hat jemand in der Triumph-Szene schon einmal einen Kraftstoffspeicher eingebaut? Wenn ja, was war das Ergebnis? Und falls nein, habe ich vielleicht etwas entdeckt, das alle Pi-Fahrzeuge verbessern könnte und an das noch niemand gedacht hat? Für mich klingt das sehr plausibel. Erwähnenswert ist auch, dass ich kürzlich von einem TR6 Pi-Besitzer in Australien gelesen habe, der dieselben mechanischen Bosch-Einspritzdüsen eingebaut hat, die auch das K-Jetronic-System von Mercedes verwendet, und sein Auto fuhr einwandfrei. Er verwendete diese Einspritzdüsen, da sie leichter zu beschaffen und viel günstiger als neue Lucas-Einspritzdüsen waren. Sie zünden bei etwa 48 psi, also sehr ähnlich wie die Lucas Pi-Einspritzdüsen – interessant, nicht wahr?

Hello

In the 90s and early 2000s, I converted several Triumphs to Bosch injectors. Many others in Germany followed suit. The different mounting options are clearly visible in the pictures. The connection is an industry standard! However, it did happen that some 143 hp Triumphs ran a bit rough at low RPMs. Generally speaking, the Bosch injector is far superior to the Lucas. For example, it doesn't rust internally because it's made of stainless steel. The rubber mount fits snugly into the intake manifold. I got the idea for the Bosch injectors from a master mechanic in Osnabrück who serviced British soldiers and their Triumph sedans in the 80s and had also had trouble with the Lucas injectors. The workshop was an authorized dealer for both Bosch and Lucas! So, what could be more natural than to test it, and it worked! ...

Bosch No. 0 437 502 047 

Injector holder Mercedes Daimler No. 116 078 0873 

Regards

Ralf

[john.r.davies]

Vielen Dank, Schnippel!    I'm not in need of new injectors now, but I will note those valuable (and cheap!) alternatives.

John

[Spit_2.5PI]

I think the Lucas Engineers' train of thought was as follows:-

The advantage of having the PRV at the front is that petrol returning to the tank travels from the pump (heat source) to the front of the car and then back again. Haven't measured but must be around 12' round trip in free air under the car. Compare this to 2' or 3' in still air inside the boot on the standard setup.

In addition, the advantage of having the PRV mounted on the Metering Unit is that there is a full flow of relatively cool fuel (at least compared to engine bay temperatures) flowing through, cooling the MU which keeps fuel temperatures to the injectors down somewhat. This remains true even at idle (where heat soak is a problem) as fuel continues to flow through the MU at a high rate to get to the PRV and back to the tank.

Also, with the PRV on the MU, the volume between the jacket and the sleeve acts as a chamber to separate air from fuel. Small amounts may go straight through to the PRV and back to the tank without going into the rotor and on to the injectors. I've never taken the PRV Tee-Piece apart, but I imagine that there's less volume in there to perform this function. Of course, no Tee is needed with the PVR on the MU.

Cheers, Richard

[michaeldavis39]

Wow this has been such an educational, eye opening thread. In summary Bosch injectors can be used as alternative for the Lucas ones at a far cheaper price than new Lucas ones. Plus if we had simply fitted the prv onto our Metering Units and fed the returned fuel back to the tank we would have had no cavitation issues from today's poor quality ethanol added to fuel. Thank you to everyone who has contributed to this valuable information which will help anyone with a Triumph car using Pi.

[Steve-B]

Agreed 1,000%

——

@TorontoTim Any chance this thread might be worthy of a pinned topic?

[PodOne]

55 minutes ago, Steve-B said:

Agreed 1,000%

——

@TorontoTim Any chance this thread might be worthy of a pinned topic?

+1 a winter project to compare results.

[TorontoTim]

3 hours ago, Steve-B said:

Agreed 1,000%

——

@TorontoTim Any chance this thread might be worthy of a pinned topic?

Your wish is granted!!

[Andy Moltu]

On 6/10/2026 at 11:23 AM, Drewmotty said:

Any heat taken out the fuel prior to getting back to the tank is a bonus so a nice uninsulated 6ft of copper will help. I’d consider a large diameter return of 15 or even 22mm alongside the chassis worth it to maximise cooling. 
These cars would really benefit from a compensating pump so that the fuel doesn’t get heated up in the first place. 
Is anyone aware of a suitable pump which doesn’t come off an aircraft and cost as much as the car is worth?🥴

Running fuel back next to a hot exhaust pipe might not be that effective for cooling the fuel.

 

[JohnC]

My spare MU is one of those very early ones with the PRV outlet. Maybe I should try it. But I don't suffer fuel vaporisation problems, so maybe not. But I love to fiddle...

John C

[Steve-B]

After reading Andy's work on how he insulated to remove heat from under the boot/exhaust, etc I've secure some of the same heat shielding material. Besides adding the fuel cooler as a winter project, hopefully before then I'll get 'her back on the jacks and do the shielding. I am also seriously thinking now about a new fuel return line and moving the PRV to attach it to the firewall near the MU.

[john.r.davies]

18 hours ago, Andy Moltu said:

Running fuel back next to a hot exhaust pipe might not be that effective for cooling the fuel.

 

Depends how close the two pipes are.    In motion, air flow will minimise heat transfer by convection.      A small fuel pipe will have only a small surface area to receive it by radiation, and  if they are more than a inch or two apart, the inverse square law comes into effect.  Are they close by on a TR6?   I run Pi on my Vitesse, where the exhaust pipe goes down the middle and the fuel pipe  (PRV in boot) goes along the outside of the chassis rail - no direct line of sight between them so non radiative heat transfer.

And heat inside the car, is never from the exhaust pipe!    I once built a car with the radiator in the back, for aerodynamic reasons - it was a very cold car!   Your highly inefficient internal combustion engine chucks out at least 60% of the energy supplied as heat!   Thus you need that great radiator, which filles the engine bay and car underside with an enormous amount of hot air.     The exhaust pipe contributes almost none.

John

[Steve-B]

I went searching today for fuel volatility as vapour locking/fuel starvation clearly is an issue for our older cars.  The FBHVC  has some updated their useful info on this page: https://www.fbhvc.co.uk/fuels#vapourisation that others may want to take a look at…

in particular: 

* Reduce the flow of (exhaust) heat to the liquid fuel supply to the engine

* Route fuel lines away from heat sources

* If possible arrange for a smaller diameter return fuel pipe from the carburettor feed to the fuel tank. This will help to ensure cool fuel from the tank is always available at the carburettor. This is particularly important after the vehicle has stopped after a run, as heat soak from the engine will warm the stagnant fuel in the fuel lines. An electric pump is helpful, in that turning on the fuel pump will flush the warm fuel back to the tank prior to starting the engine.

* Locate the fuel pump (eg electric pump) away from heat sources and if possible below the fuel tank to ensure it runs with a positive head on the suction side to limit vapour build up and avoid cavitation.

* Use a thermal break (eg plastic spacer) where possible for mechanical (engine-mounted) pumps

* Shield carburettor(s) from radiant exhaust heat (especially where inlet and exhaust are on the same side of the engine)

Edited June 13 by Steve-B

[Andy Moltu]

On 6/12/2026 at 5:51 PM, john.r.davies said:

Depends how close the two pipes are.    In motion, air flow will minimise heat transfer by convection.      A small fuel pipe will have only a small surface area to receive it by radiation, and  if they are more than a inch or two apart, the inverse square law comes into effect.  Are they close by on a TR6?   I run Pi on my Vitesse, where the exhaust pipe goes down the middle and the fuel pipe  (PRV in boot) goes along the outside of the chassis rail - no direct line of sight between them so non radiative heat transfer.

And heat inside the car, is never from the exhaust pipe!    I once built a car with the radiator in the back, for aerodynamic reasons - it was a very cold car!   Your highly inefficient internal combustion engine chucks out at least 60% of the energy supplied as heat!   Thus you need that great radiator, which filles the engine bay and car underside with an enormous amount of hot air.     The exhaust pipe contributes almost none.

John

I agree that the heat from the exhaust is mitigated by airflow but much of that engine heat goes down past the fuel lines in the airflow?

[john.r.davies]

Yes, but without major mods to direct airflow or move the rad .....

[michaeldavis39]

I have seen shielding added on tr chassis at the point where the copper fuel lines are very near the exhaust which would help after engine is switched off obviously.

 

Edited June 13 by michaeldavis39

[john.r.davies]

Surely it's better to run the fuel lines outside the chassis rail?   The exhaust cannot heat them then. That's what I did.

Less possibilty of damage in an accident, too.

John

[stuart]

16 hours ago, john.r.davies said:

Surely it's better to run the fuel lines outside the chassis rail?   The exhaust cannot heat them then. That's what I did.

Less possibilty of damage in an accident, too.

John

That would be difficult on an IRS chassis and would involve drilling holes through the chassis box plus at the rear the trailing arms would be in the way.

Stuart.

[TR NIALL]

While doing a few upgrades to the Fuel System I’m considering moving the PRV to Tank return to where the Metering Unit return is in the top of the Tank in other words swopping them around,I’ve heard the PRV return has a long pipe running down into the Swirl Pot area,would I be safe to shorten (cut off)this as it will no longer reach the Swirl Pot.

Edited June 15 by TR NIALL

[stuart]

2 minutes ago, TR NIALL said:

While doing a few upgrades to the Fuel System I’m considering moving the PRV to Tank return to where the Metering Unit return is in the top of the Tank in other words swopping them around,I’ve heard the PRV return has a long pipe running down into the Swirl Pot area,would I be safe to shorten (cut off)this as it will no longer reach the Swirl Pot.

No point TBH, the metering unit return is only a dribble anyway. Its built into the tank anyway so I dont think you could remove it.

Stuart.

[PodOne]

 

4 hours ago, TR NIALL said:

While doing a few upgrades to the Fuel System I’m considering moving the PRV to Tank return to where the Metering Unit return is in the top of the Tank in other words swopping them around,I’ve heard the PRV return has a long pipe running down into the Swirl Pot area,would I be safe to shorten (cut off)this as it will no longer reach the Swirl Pot.

I've swapped my around as have others on the basis that the "hot" PRV return enters the tank as far away as possible from the swirl pot/outlet to avoid hotter fuel been sucked directly back into the fuel pump.

Unfortunately in my case the MU return is in the centre of the tank rather than at the far end where the PRV would be best relocated to. The PRV feeds an external fuel cooler before returning to the tank.  

[john.r.davies]

6 hours ago, stuart said:

That would be difficult on an IRS chassis and would involve drilling holes through the chassis box plus at the rear the trailing arms would be in the way.

Stuart.

Thank you, Stuart.   Different models, different ways.  On the Vitesse, some wiring is run along the outside of the chassis rail, so that there is a cut-out at the outrigger's base:

(picture taken from an eBay item)  Very convenient!

John

[TR NIALL]

3 hours ago, PodOne said:

 

I've swapped my around as have others on the basis that the "hot" PRV return enters the tank as far away as possible from the swirl pot/outlet to avoid hotter fuel been sucked directly back into the fuel pump.

Unfortunately in my case the MU return is in the centre of the tank rather than at the far end where the PRV would be best relocated to. The PRV feeds an external fuel cooler before returning to the tank.  

Can you remember if there’s a metal pipe on the PRV return where it enters the Tank.

[PodOne]

1 hour ago, TR NIALL said:

Can you remember if there’s a metal pipe on the PRV return where it enters the Tank.

Yes on both the PRV and MU returns to accept 5/16" rubber pipe.

[TR NIALL]

53 minutes ago, PodOne said:

Yes on both the PRV and MU returns to accept 5/16" rubber pipe.

Sorry your misunderstanding me,I mean when the PRV fitting enters the top of the Tank is there a metal pipe on it that goes down into the Tank to allow the fuel re enter the the Swirl Pot area or does it just dribble back into the Tank like the MU return.