Ported plenum adjustments

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gd302
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Ported plenum adjustments

Post by gd302 » Tue Apr 17, 2018 3:39 pm

Ive just fitted a 72mm ported plenum to my Tvr chimaera 3.9 with the 14cux, but am having a hard time setting it up.
The problem i have is that the throttle butterfly with the engine running at idle is quite tight, its almost impossible to drive the car as the throttle pedal is tight and hard to control. The cable itself is ok and isnt the issue.
Ive adjusted the grub screw to open the butterfly slightly but then my revs are then far too high which i dont think i can get back down.
Does anyone have any tips or has set one up before?
Thanks in advance.


1995 TVR Chimaera 400 With tweaks

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Re: Ported plenum adjustments

Post by ChrisJC » Tue Apr 17, 2018 5:09 pm

I thought that the butterfly should be fully closed at idle, and that the idle speed is controlled by the stepper motor. That being the case, if the butterfly is not perfectly centralised on the spindle, it might jam a bit; it's adjustable I think if you loosen the two screws, close it and retighten.

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gd302
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Re: Ported plenum adjustments

Post by gd302 » Tue Apr 17, 2018 6:47 pm

Thanks i will give it a try.
I think some cars have some very small clearances to help stop the initial sticking when the engine is running. Not sure either.
1995 TVR Chimaera 400 With tweaks

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Re: Ported plenum adjustments

Post by sidecar » Tue Apr 17, 2018 7:55 pm

I know nothing about EFI, Edelbrock carbs are my thing but it does sound like a similar thing that you have to do with the Edelbrock carbs, basically make sure that any throttle stop screw is backed right off, then slacken the screws that hold the butterflies just enough so that the butterflies can move with a bit of effort then shut the throttle and hold it shut whilst you re-tighten the screws. The main thing is to remember to back off the throttle stop screw when doing this.

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Re: Ported plenum adjustments

Post by DaveEFI » Wed Apr 18, 2018 8:56 am

There are mechanical ways of varying how far the throttle moves relative to the pedal. To give more pedal movement at low throttle. The Rover P6 V8 had such a system. Something like a snail cam.
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Re: Ported plenum adjustments

Post by Eliot » Thu Apr 19, 2018 11:15 am

(I make the 72mm plenums)
As described above, you need to centralise the throttle plate inside the bore. There's two screws that clamp it, the ends of the screws are flared apart to retain them - you may be able to loosen them enough without squashing the threads back together in order to be able to recentralise the throttle plate.
Once they are loose, close the throttle fully and centralise the plate - then gently nip the screws back up.

If i'm honest, a 72mm plenum is unnecessary on a 3.9 engine - unless it's making close to 300 bhp, which isn't going to happen on a n/a engine.
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Re: Ported plenum adjustments

Post by DaveEFI » Thu Apr 19, 2018 12:02 pm

Incidentally, the Land Rover manual for my flapper EFI states that the adjustable throttle stop should be set with the butterfly just slightly open at idle. About a couple of thou.
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Re: Ported plenum adjustments

Post by gd302 » Fri Apr 20, 2018 5:57 pm

Thanks, I’m on stop at the moment as I’ve sent the plenum back. I’m not happy With the work.
Great weather and no car to use :(
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Re: Ported plenum adjustments

Post by Eliot » Tue Apr 24, 2018 9:55 am

Even as someone who sells 72mm plenums, I would have to ask why you think you need a 72mm plenum?
What other supporting work have you carried out? Do you still run an air flow meter?
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Re: Ported plenum adjustments

Post by minorv8 » Wed Apr 25, 2018 11:55 am

My mate and I had some fun on rolling road 1,5 years ago. We mapped my 4.6 engine. I have a modified Rover plenum with front mounted 75 mm throttle body but no air fuel meter. With that setup it gave 314 hp at 6000 rpm. We then fitted a 34 mm restriction, made a pull and got 222 hp at 5000 rpm. The engine ran to 6200 rpm with no issues. I have since reduced the restriction to 25 mm orifice and the engine still runs nicely.

For what its worth maybe the restriction in std Rover setup is not the throttle size.

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Re: Ported plenum adjustments

Post by Gerro » Thu May 10, 2018 12:06 am

minorv8 wrote:
Wed Apr 25, 2018 11:55 am
My mate and I had some fun on rolling road 1,5 years ago. We mapped my 4.6 engine. I have a modified Rover plenum with front mounted 75 mm throttle body but no air fuel meter. With that setup it gave 314 hp at 6000 rpm. We then fitted a 34 mm restriction, made a pull and got 222 hp at 5000 rpm. The engine ran to 6200 rpm with no issues. I have since reduced the restriction to 25 mm orifice and the engine still runs nicely.

For what its worth maybe the restriction in std Rover setup is not the throttle size.
Good trials on adjustment. We may try a few settings too soon, just finishing up the installation of new rack and tonneau cover for the kayak mount on the pick up truck. Gonna start with 25mm

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