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EFI: size throttle bodies

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How do I decide the size of the throttle bodies?

There are various systems out there with 40mm - 42mm - 45mm - 50mm ....

Can it be assumed coming from Weber 40 DCOE using 40mm throttle bodies is the same?



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Hi Jochem,


40mm Jenveys should be enough for a street car, as most streetcars on DCOEs are 40mm and they are restricted by the venturies, which isn't the case on a comparable EFI throttle body.

They need also to fit the Weber inlet manifold, which are often for 40mm carbs, thus 40mm Jenveys. Could possibly open them up to 42mm though...

The original PI manifolds were 45mm, which is rather big for a 2,5l. But it works, obviously.


I use original PI manifolds with 6 butterflies on EFI and put some effort into the linkage to make the butterflies open progressively, in order to tame everything at small throttle openings.





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Hi Jochem,


I intend going EFI this winter and have done some research online, if as Harry has done using single throttle body then the 52mm size is favoured, they are fitted to MGTF which go up to a genuine 160BHP, enough for most TR's. If you go larger then little gain and less drivable around town etc.

Some people use existing throttle bodies with throttle flaps, the advantage of a bit more power, snappier response but but more in depth setting up required with throttle balancing and TPS sensor fitting etc and more expense to achieve an excellent result.

As I intend taking Harry's route using modified TR6 throttle bodies with throttle flaps removed I know very little about EFI using aftermarket multiple throttle bodies. If you used DCOE inlet manifolds I would have thought that 40mm bodies would be OK.

I did find it odd that you could get approx standard power outputs from a single 52mm throttle body when you have 6 massive throttle bodies existing, (if I am honest I still find it odd) but quite a few people have done it and have dyno maps appear to back it up.




Edited by lightningburns
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I am not really into this (1) single throttle body but rather the 3 x 2 Jenvey heritage or 3 x 2 Jenvey TBs 90mm Long...

With this said, the 40mm versions seem to be the right one....with Weber Manifold from Webcon.


Jenvey has a rule of thumb:


"As a general rule of thumb a large bore results in lower flow resistance, a small bore results in better throttle response and more precise air/fuel ration mix. There are a few guidelines to be followed when selecting bore size and assumed BHP/cylinder using an assumed rev range up to 9000rpm.

BHP Millimeters

35 40

40 42

45 45

55 48

65 50




Edited by JochemsTR
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People routinely put over large carbs or throttle bodies on and actually destroy horsepower and/or flexibility. 40 mm is plenty unless you've got a real road burner. So if going for an EFI conversion, there's lot's of alternatives out there that are based on that diam. I've got some that are EFI versions of 40 DCOE's. I think they were originally intended for a BMW 6 conversion.






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Agree that 40mm is plenty.


Currently building up a bike-based install.




(pics towards the end)


TBs on this are 34mm. There is still some taper involved as the ports are only 30mm. This is for a 2L not 2.5, but it does rev to 7k.


Mildly surprised that 52mm is considered enough for the 160 hp MGF as I'm currently running a 54mm single TB (which is similar flow area to 2 x 1.5") and running out of real puff at 5,500 rpm, apparently due to lack of air. I don't have anything like 160 hp. Maybe 130.....



Edited by Nick Jones
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As tuned by Dave Walker (Emerald) with engine rebuild and chocolate cam 180 bhp

2ND rebuild Newman cam PH1 and ran out of puff at 3500 rpm

Could not figure out the huge loss but drove like an automatic.

3rd rebuild with CP forged pistons, Fardon rods,Newman cam PH2. All then machined to close tolerances by some one who cares. 1st and 2nd was caused by faulty workmanship.

3rd rebuild by me and I know what I rebuilt using top quality parts. Same rolling road 170 bhp and Dave Walker has this calibrated regularly.

Dave Walker says BHP is for talking down the pub, it is how it drives that counts and it drives well.

Figures at the flywheel and not counting the losses through the drive train.


Regards Harry

Edited by harrytr5
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  • 9 months later...

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