Plenum butterfly gap... should there be one or not?
Discussion
Just a quick question as I'm messing with the Plenum and trying to get rid of the last bit of 'hesitance' at low RPM.....
When I set my plenum up last year I think I set it with a .2mm gap or opening (top / bottom) on the butterfly... The car's been to V8D now and has come back with the butterfly closed (I know they have been trying the same thing here)... but should it have a gap or not?.. or is it subjective depending on how the engine behaves etc?
If anyone knows it would be good info to get the starting point correct.. as there are a few variables here I'm sure you can spend hours chasing your tail if you don't get a few things set right first!!... Cheers!
When I set my plenum up last year I think I set it with a .2mm gap or opening (top / bottom) on the butterfly... The car's been to V8D now and has come back with the butterfly closed (I know they have been trying the same thing here)... but should it have a gap or not?.. or is it subjective depending on how the engine behaves etc?
If anyone knows it would be good info to get the starting point correct.. as there are a few variables here I'm sure you can spend hours chasing your tail if you don't get a few things set right first!!... Cheers!
Are you adjusting it using the allen screw that pushes on the throttle linkage?
Can you remove the screw and turn it the opposite way round to experiment - so that it can be adjusted from the top side of the plenum?
I've had the 'Taraka' conversion carried out on my plenum and intake manifold at TVR Power and Dom mentioned that the grub screw would need to be removed and screwed in from the top to enable adjustment.
Can you remove the screw and turn it the opposite way round to experiment - so that it can be adjusted from the top side of the plenum?
I've had the 'Taraka' conversion carried out on my plenum and intake manifold at TVR Power and Dom mentioned that the grub screw would need to be removed and screwed in from the top to enable adjustment.
jeboa said:
Are you adjusting it using the allen screw that pushes on the throttle linkage?
Can you remove the screw and turn it the opposite way round to experiment - so that it can be adjusted from the top side of the plenum?
I've had the 'Taraka' conversion carried out on my plenum and intake manifold at TVR Power and Dom mentioned that the grub screw would need to be removed and screwed in from the top to enable adjustment.
Confused?.. yes it's the very small alen screw that stops the linkage ... it sit's on it basicaly... I'm not sure what you mean about turning it around though??... I'll have a look tomoro see if I can figure it out?..Can you remove the screw and turn it the opposite way round to experiment - so that it can be adjusted from the top side of the plenum?
I've had the 'Taraka' conversion carried out on my plenum and intake manifold at TVR Power and Dom mentioned that the grub screw would need to be removed and screwed in from the top to enable adjustment.
Will also have a dabble setting it to 'a small gap'... May smooth off the sides of the butterfly also... it's quite thick and looks like it's punched out... not nice and won't be best for air flow past it... I'm sure it will create all sorts of Eddys....
Can didn't like the hot weather today... was a bit unhappy with the hot thin air!...
Thanks Mark... It gives a measurement from the end to the butterfly top and bottom... But on a 72mm Plenum, I'm not surte if thius followes. So I think it's set using fealer gauges to .2mm... but I'm not sure... may be I'll call Power or someone who machines these out now... Cheers..
Thanks for that info... The grub screw on mine is in from the top so I can adjust it no problem... so the differance between the top and the bottom is 0.5mm is that what your saying?... I have the 72mm Plenum so I think my butterfly has more of an angle on it... do you know what gap that gives you between the butterfly and body top and bottom?.. is it about 0.2mm????.. Cheers
spend said:
I don't understand what you mean by the top & bottom measurements?
The plenums are commonly bored at an angle so that they stay within the available casting material, and the butterfly is effectively oval (in 2d) when not perpendicular to the bore.
Indeed... The measurements go back to Marks manual above... If you measure the butterfly at the top relative to the end and at the bottom relative to the end (end being the front input end)... It should be within 0.5.. according to the Rover guy's.. (This butterfly set-up will spin in the bore if you remove all the other gubbins)... it's the std RR settings...The plenums are commonly bored at an angle so that they stay within the available casting material, and the butterfly is effectively oval (in 2d) when not perpendicular to the bore.
As you say, when it's bored out to 72mm it looks like they fit the butterfly at more of an angle giving about 7.3mm differance (in my case) when measured top and bottom. (wonder why they do this and why they don't do a round 'spin in the bore' type again.. only bigger... Will have to have a look in and see if it's obvious why?).. Anyway.. with this setting you must end up with an oval butterfly so you get the seal all the way round when closed... My butterfly is also 2.4mm thick which I think is quite a bit thicker than the standard one?... So just trying to establish settings... I think it's a 0.1mm gap top (and obviously bottom)... The thing here is to watch it does not go beyond 'flat' when fully open, or it will start restricking a bit... And with all that material thickness on the butterfly, I wonder if it would benifit a bit of shaping on it's section.. to ease air flow...
Is yours 72mm and does it follow the same design??.. Cheers
Edited by TVR Beaver on Thursday 27th May 13:16
Measuring the depth of the butterfly will be completely different between std & 72mm plenums. The bore inclination, the collar, and the geometric differences due to the larger butterfly all make for an apples & pears scenario as I see it? 
What is the gap for? AIUI you need a small clearance so the butterfly doesn't hammer the bore, allowing for any heat expansion. When closed it wants to let no more air through than needed in consideration of the range of the idle air by pass screw. You also need to check that the butterfly spindle doesn't 'overcrank' so that the throttle mechanism is jerky - you can feel that.
With different sized engines breathing differently on top of quite different implementations of '72mm' plenums I cannot see that there is anything like a std amount of air going past the closed butterfly from car to car. Even if the bypass valves are clear/clean, copying someone elses number of turns would seem pointless to me.
Getting the car up to temp, opening the idle valve whilst blocking off the stepper pipe, and then screwing down the idle screw to suit seems a simple process to me. Providing air within a reasonable range is escaping around the 'closed' butterfly you should be able to adjust the idle bypass valve to fine tune the air supplied at hot tickover. Bear in mind the advance and stepper may be fighting this.. if so they are the problem IMHO.
I just try and close the butterfly down so that a sheet of paper will not be trapped at the top or bottom. Then make sure the throttle fully opens, surprising how many only get to @80degs.. Profiling the butterfly edges does give you more clearance and less chance of jamming?, but you have to set the closure more accurately and I learned the hard way which edges to profile best in respect of this
You may also find there is some small play in aligning the butterfly on the spindle, as well as a little L-R float of the spindle, so you have to check these by feel as you assemble IMHO.
In short I really cannot see that trying to compare measurements, even if you can measure such small tolerances accurately, is of any benefit whatsoever? Trust the force Lewk
As mentioned IIRC by Rob in the past the air disruption around the butterfly and vac take off for the dizzy advance can be a problem... have you tried isolating this yet?
What is the gap for? AIUI you need a small clearance so the butterfly doesn't hammer the bore, allowing for any heat expansion. When closed it wants to let no more air through than needed in consideration of the range of the idle air by pass screw. You also need to check that the butterfly spindle doesn't 'overcrank' so that the throttle mechanism is jerky - you can feel that.
With different sized engines breathing differently on top of quite different implementations of '72mm' plenums I cannot see that there is anything like a std amount of air going past the closed butterfly from car to car. Even if the bypass valves are clear/clean, copying someone elses number of turns would seem pointless to me.
Getting the car up to temp, opening the idle valve whilst blocking off the stepper pipe, and then screwing down the idle screw to suit seems a simple process to me. Providing air within a reasonable range is escaping around the 'closed' butterfly you should be able to adjust the idle bypass valve to fine tune the air supplied at hot tickover. Bear in mind the advance and stepper may be fighting this.. if so they are the problem IMHO.
I just try and close the butterfly down so that a sheet of paper will not be trapped at the top or bottom. Then make sure the throttle fully opens, surprising how many only get to @80degs.. Profiling the butterfly edges does give you more clearance and less chance of jamming?, but you have to set the closure more accurately and I learned the hard way which edges to profile best in respect of this
You may also find there is some small play in aligning the butterfly on the spindle, as well as a little L-R float of the spindle, so you have to check these by feel as you assemble IMHO.In short I really cannot see that trying to compare measurements, even if you can measure such small tolerances accurately, is of any benefit whatsoever? Trust the force Lewk
As mentioned IIRC by Rob in the past the air disruption around the butterfly and vac take off for the dizzy advance can be a problem... have you tried isolating this yet?
spend said:
Measuring the depth of the butterfly will be completely different between std & 72mm plenums. The bore inclination, the collar, and the geometric differences due to the larger butterfly all make for an apples & pears scenario as I see it?
What is the gap for? AIUI you need a small clearance so the butterfly doesn't hammer the bore, allowing for any heat expansion. When closed it wants to let no more air through than needed in consideration of the range of the idle air by pass screw. You also need to check that the butterfly spindle doesn't 'overcrank' so that the throttle mechanism is jerky - you can feel that.
With different sized engines breathing differently on top of quite different implementations of '72mm' plenums I cannot see that there is anything like a std amount of air going past the closed butterfly from car to car. Even if the bypass valves are clear/clean, copying someone elses number of turns would seem pointless to me.
Getting the car up to temp, opening the idle valve whilst blocking off the stepper pipe, and then screwing down the idle screw to suit seems a simple process to me. Providing air within a reasonable range is escaping around the 'closed' butterfly you should be able to adjust the idle bypass valve to fine tune the air supplied at hot tickover. Bear in mind the advance and stepper may be fighting this.. if so they are the problem IMHO.
I just try and close the butterfly down so that a sheet of paper will not be trapped at the top or bottom. Then make sure the throttle fully opens, surprising how many only get to @80degs.. Profiling the butterfly edges does give you more clearance and less chance of jamming?, but you have to set the closure more accurately and I learned the hard way which edges to profile best in respect of this You may also find there is some small play in aligning the butterfly on the spindle, as well as a little L-R float of the spindle, so you have to check these by feel as you assemble IMHO.
In short I really cannot see that trying to compare measurements, even if you can measure such small tolerances accurately, is of any benefit whatsoever? Trust the force Lewk
As mentioned IIRC by Rob in the past the air disruption around the butterfly and vac take off for the dizzy advance can be a problem... have you tried isolating this yet?
No.. I agree.. Having now seen the angle I have measuring others is no help... One thing I have found is that closing the butterfly right up gives a shudder as you put your foot down.. where as setting it open a tad appears to cover this... I was thinking this is why it's done and not for expansion.. but you have a point here...(BTW.. paper is about 0.1mm so that ties up What is the gap for? AIUI you need a small clearance so the butterfly doesn't hammer the bore, allowing for any heat expansion. When closed it wants to let no more air through than needed in consideration of the range of the idle air by pass screw. You also need to check that the butterfly spindle doesn't 'overcrank' so that the throttle mechanism is jerky - you can feel that.
With different sized engines breathing differently on top of quite different implementations of '72mm' plenums I cannot see that there is anything like a std amount of air going past the closed butterfly from car to car. Even if the bypass valves are clear/clean, copying someone elses number of turns would seem pointless to me.
Getting the car up to temp, opening the idle valve whilst blocking off the stepper pipe, and then screwing down the idle screw to suit seems a simple process to me. Providing air within a reasonable range is escaping around the 'closed' butterfly you should be able to adjust the idle bypass valve to fine tune the air supplied at hot tickover. Bear in mind the advance and stepper may be fighting this.. if so they are the problem IMHO.
I just try and close the butterfly down so that a sheet of paper will not be trapped at the top or bottom. Then make sure the throttle fully opens, surprising how many only get to @80degs.. Profiling the butterfly edges does give you more clearance and less chance of jamming?, but you have to set the closure more accurately and I learned the hard way which edges to profile best in respect of this
You may also find there is some small play in aligning the butterfly on the spindle, as well as a little L-R float of the spindle, so you have to check these by feel as you assemble IMHO.In short I really cannot see that trying to compare measurements, even if you can measure such small tolerances accurately, is of any benefit whatsoever? Trust the force Lewk
As mentioned IIRC by Rob in the past the air disruption around the butterfly and vac take off for the dizzy advance can be a problem... have you tried isolating this yet?
) Is butterfly profiling common practice then... I've not seen this before but thought it might be something that would help... I understand the edge thing... It reverses sides top to bottom.. and yes, getting it wrong will need a re-make of the valve (or reduction and re-set at a lesser angle.....
I'll set the base idle blocking the stepper tube and vac tube (is this 500 RPM??)_... (although I've already moved this forward so it don't come into play until 2K).. and see what we get... It's also quite rough on the casting in there... I'm sure it could do with an amout of polishing to help air flow and may be thinning down of the centre screw hole that passes through the inlet.
And also checking it goes flat... and not short of this or even past this... Something to play at there then... Give me something to do tonight... It's raining here in Yorkshire!... Cheers
Edited by TVR Beaver on Thursday 27th May 17:28
TVR Beaver said:
Is butterfly profiling common practice then... I've not seen this before but thought it might be something that would help... I understand the edge thing... It reverses sides top to bottom.. and yes, getting it wrong will need a re-make of the valve (or reduction and re-set at a lesser angle.....
I don't think it's very common, AFIK I'm the only one that's spent a lot of time profiling the butterfly and spindle (common on the cerbs BTW) and even more effort porting out the plenum. The tricks are knowing which shapes & profiles aid flow - knife-edging can be very misleading It's pretty easy to make pigs ear of things, best to try on a spare component - that way you also have a base to compare performance against.TBH I believe you could prepare (with a lot of effort) a 65mm plenum to work easily as well as a 72mm plenum on a Lucas ECU ~ kindly accelerating the air over the butterfly is the key IMHO. At the end of the day its probably easier / cheaper to hack out to 72mm for quick&dirty results IYSWIM.
Simple analogy is rough casting gives greater surface area - thats good on the outside of the engine, but detrimental where you are trying to increase flow which is very basically ~ surface area/cross section?
spend said:
TVR Beaver said:
Is butterfly profiling common practice then... I've not seen this before but thought it might be something that would help... I understand the edge thing... It reverses sides top to bottom.. and yes, getting it wrong will need a re-make of the valve (or reduction and re-set at a lesser angle.....
I don't think it's very common, AFIK I'm the only one that's spent a lot of time profiling the butterfly and spindle (common on the cerbs BTW) and even more effort porting out the plenum. The tricks are knowing which shapes & profiles aid flow - knife-edging can be very misleading It's pretty easy to make pigs ear of things, best to try on a spare component - that way you also have a base to compare performance against.TBH I believe you could prepare (with a lot of effort) a 65mm plenum to work easily as well as a 72mm plenum on a Lucas ECU ~ kindly accelerating the air over the butterfly is the key IMHO. At the end of the day its probably easier / cheaper to hack out to 72mm for quick&dirty results IYSWIM.
Simple analogy is rough casting gives greater surface area - thats good on the outside of the engine, but detrimental where you are trying to increase flow which is very basically ~ surface area/cross section?
by this time tomoro I should have scrapped it off! 
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