72 mm throttle plenum installed

[900T-R]

Yep, again a small job completed that makes me irrationally proud.

Plenum swapped (took the throttle assembly off and put it on the new plenum in situ; same with stepper motor housing, swapped the plate that holds the two coolant hoses to/from throttle body, too so I didn't have to mess with the hoses - they don't do anything on the new plenum as the space under the throttle has been filled with epoxy); re-set throttle pot to 0.402V (Dom specified 0.399 and this was as close as I got as it would go from .41 to .30 ish when just tightening the second screw so the procedure is a bit hit and miss... ). Had to swap AMM-TB hose too to a 76 mm inner diameter which unfortunately was shaped just that bit differently from the corresponding 70 mm part (I use a 135 degree elbow and saw it off at 90 degrees, as the legs of the 90 degree elbows available off the shelf aren't long enough) so it's a bit of a tight fit lengthwise. I may have to resort to putting an extra length of straight tubing with a coupler as the bulkhead-AMM hose is stretched to the limt due to the AMM being closer to the firewall.

Started car, blipped throttle and thought 'whoa, that's an agressive response', set off CO alarm in the underground parking, switched off engine. Found AMM still disconnected.

Test drive - OK but for high idle (1,500 crawling up to 1,750 in traffic). Called it a day - made myself dinner as I had not eaten all day (was supposed to be a small lunchtime job), found Bible.

Returned next day with Allen key to adjust grub screw in plenum. Found it's not a metric size. B*gg*r. Used Torx E30 bit. Works like a charm. Started engine. Idle settled down to reasonable level. Job done.

Sorry for the vagueness of the pic (not much light in the underground parking and holding a camera still for 0.62 sec is being asked too much I'm afraid). Still it's pretty eh (had the plenum powdercoated locally to match the rest of the engine)?

Edited by 900T-R on Thursday 25th September 07:32

[Chimjunkie]

Good job! I have one sat in my cupboard. Wasn't sure if the engine mapping would be affected by this? I have a Mark Adams Tornado tuned and chipped ECU. What does the throttle pot do exactly?

[_Dan_]

Chimjunkie said:

Good job! I have one sat in my cupboard. Wasn't sure if the engine mapping would be affected by this? I have a Mark Adams Tornado tuned and chipped ECU. What does the throttle pot do exactly?

It lets the ECU know how far you have the loud pedal down I believe.

So does the new plenum improve performance at all?

[dhf]

what size plenum does a standard 5 ltre have on it ?

[900T-R]

dhf said:

what size plenum does a standard 5 ltre have on it ?

All 65 mm LR standard. The only engines that got an oversized throttle plenum IIRC were some of the more exotic Wedges (I think 420 SEACs had a 70 mm throttle).

As for performance - I live in a city centre and I only took it out for 20 mins or so to check whether everything was OK. I'm happy to report that traffic driveability hasn't been affected.

It's safe to say though that given the state of tune of the engine itself (Richard built it originally with far more exotic intake systems and aftermarket management in mind, and put the standard intake on top for the time being) the standard sized throttle was going to be a restriction - I've blended the trumpet base before the engine was re-installed and with the smooth bore silicone 90 degree bend and the plenum insulator plate this is the best I can do while keeping things at least superficially OE... TBH it's more a 'peace of mind' thing to me than anything else.

Edited by 900T-R on Thursday 25th September 10:23

[Chimjunkie]

_Dan_ said:

Chimjunkie said:

Good job! I have one sat in my cupboard. Wasn't sure if the engine mapping would be affected by this? I have a Mark Adams Tornado tuned and chipped ECU. What does the throttle pot do exactly?

It lets the ECU know how far you have the loud pedal down I believe.

So does the new plenum improve performance at all?

So the throttle opening will effectively be greater than the signal from the pot gives? i.e the pot signal calibrated for 65mm not 72mm. Or does the throttle pot just tell the ecu that the throttle is closed, partial or fully open?

[_Dan_]

Chimjunkie said:

_Dan_ said:

Chimjunkie said:

Good job! I have one sat in my cupboard. Wasn't sure if the engine mapping would be affected by this? I have a Mark Adams Tornado tuned and chipped ECU. What does the throttle pot do exactly?

It lets the ECU know how far you have the loud pedal down I believe.

So does the new plenum improve performance at all?

So the throttle opening will effectively be greater than the signal from the pot gives? i.e the pot signal calibrated for 65mm not 72mm. Or does the throttle pot just tell the ecu that the throttle is closed, partial or fully open?

Not sure really, it depends how it's measured, it'll be measuring angle of a butterfly valve I'm guessing, so from fully open, all the bits inbetween to fully closed will still probably only be 90 degree movement so the voltage range would be the same. The fuel mapping will just need altering a little in the ECU.

All guess work so I'm sure someone more in the know will be along shortly to correct me!

[900T-R]

The way I understand it, the throttle pot voltage is used by the ECU to gauge at which point the mixture needss to be enriched for acceleration in the parts of the map that aren't lambda controlled. Get the voltage too low for a certain amount of airflow and the car will feel flat and unresponsive on acceleration. According to a letter of John Ravenscroft published in an edition of Sprint and subsequently in the Steve Heath Chim/Griff bible, cars that came from the factory with an enlarged throttle had ECU's that were calibrated to enable full load enrichment at 3,6V throttle pot output instead of ~4.6V, which of course is the other way of going about it.

I'd say Dom of TVR Power can explain it a lot better than I can though, seeing as he supplied the chip I'm running and gave the advice to re-set the throttle pot at 0.399V with ignition on and throttle butterfly closed.