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FPR vacuum with ITBs

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Hi all - I have a high compression dedicated race engine recently built. 4 cyl NA with large cams. Everything not needed for racing is gone.

Given it will spend most of its life between 5000 and 8500rpm, and will never be driven on the road, do I need to make up a vacuum manifold and connect that to the FPR? Or can I let the FPR reference atmosphere? I doubt there is much vacuum at idle or much below 15% TPS.

I ask as I have had conflicting opinions and it’s a long job to create the manifold - mainly as the engine builders blanked off the runners vacuum ports with blanking inserts and locktite. They will not budge.

also, with this setup, if I slightly altered the static injector flow in the ecu (ie we are not sure we set it correctly prior to tune) would we need to do the entire tune again? I’m talking about static flow currently set at 462cc vs what it probably should be - 495cc

an idle test is showing running rich when sweat to 462cc and bang on at 495cc without changing any other parameters. I’m concerned of course about it effecting higher RPMs (leaning it out at the top)


Can't see any point in running referenced FPR, as the vehicle is not going to be under forced induction which is where referencing is very useful for balancing the pressure differential between runner and fuel rail.

However, if you're running a cold air box, especially if there's any ram affect, I'd plumb the FPR to that. I would certainly be wary of plumbing reference into the runners, as with 'big' cam's you will have a lot of reversion at lower rpm, which may screw up the signal references - any others prepared to comment with experience of this?

As for the rest, not something I can really comment on, other than to suggest checking exactly what the injector spec's are and at what fuel pressure?

Not needed. Many modern OEMs run returnless systems with static fuel pressure just fine.

I generally connect it to the air box. Whatever you do, dont block the reference port as differential fuel pressure will then vary with temperature and altitude, either let it vent to atmosphere via a small filter or some other clean relevant source such as the airbox.

As for changing injector flow rate, yes in most ecu's this would influence the tune all over.

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