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EFI: larger plenum


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What is the verdict on having a larger plenum on EFI installation? The original plenum is about 3.2" and I am looking for 4.1"....

my inner fender allows for this mod. Just curious whether this dimension should be increased even more and advise who can build me this customized part.

Jochem

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Quite a few of the PI owners have done this modification to allow more air to the rearmost intake. Easy enough to do by splitting the plenum length ways underneath and a new fillet inserting.

Stuart.

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6" wouwld be more like it.

Flow depends on the fourth power of radius.     4.1" is only .4 times bigger, so will give you a flow about 3.5 times more ( (4.1 / 3)^4 )

make it twice as big (6") and flow can rise 16 times (2^4)!

There is room for it.

You may not need a bigger filter, and the ring that attaches it can stay the same.  it acts as an orifice, through which flow has merely a  square relationship.

John

Edited by john.r.davies
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Hi Jochem,

I would consider this too, re-using the original plenum. Especially if the engine will breath better/more.

Some think the sooting of the rear cylinder plugs is related to this. 
Increasing diameter (cross-section) will reduce losses by the turbulences quite a bit. 

It would be nice if someone did pressure drop measurements in the past. I didn’t, so far.

Gruesse,

Waldi

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End view of mine.   Cut off the strip of the old plenum that has the trumpets, with a rim,    Rolled a three quarter tube from steel sheet, pop-rivetted the trumpet strip in place, with seam sealer,   Welded on the back end disc and one to take the OE inlet at the front.    Painted "TRIUMPH" on it.

  Voila!

1505282624_Newplenumendview.jpg.787f6a2c48426d937a2fa316ba440b23.jpg

Edited by john.r.davies
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Hi Jochen

This would be a good idea when either PI or going to EFI if you keep the butterfly’s in position but if you use a single throttle body you would need a milder cam then the 150 one with a large amount of overlap causing the gases to come back up in to the plenum, I also enlarged mine when on Pi it did not make any difference Triumph did know what they were doing so time.

Keith

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6 hours ago, jr2 said:

I ran across this blog a few year ago.

This blog did mention how he did it, but, appears, content was removed.

Interesting read.

https://mytriumphblog.wordpress.com/2014/05/25/converting-my-tr6-to-lucas-pi/

Yes, I know him personally. I talked to him this evening. He made a cut in the bottom and opened the plenum and added a piece of sheetmetal to increase the diameter. 
I think this may be a good option. Not having to start from scratch. 

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2 hours ago, Keith Warren said:

Hi Jochen

This would be a good idea when either PI or going to EFI if you keep the butterfly’s in position but if you use a single throttle body you would need a milder cam then the 150 one with a large amount of overlap causing the gases to come back up in to the plenum, I also enlarged mine when on Pi it did not make any difference Triumph did know what they were doing so time.

Keith

Hi Keith, 

I already have EFI with butterflies but for my next generation EFI I am looking for this improvement. 
Jochem

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Posted (edited)
17 hours ago, john.r.davies said:

You may not need a bigger filter, and the ring that attaches it can stay the same.  it acts as an orifice, through which flow has merely a  square relationship.

John, actually, K&N Filters come in larger sizes such as 4" and 5"..... I do not want to create a "bottle neck" at the orifice and Filter unit. Your opinion?

Edited by JochemsTR
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Having heard the theory that the small size of the plenum chamber restricts airflow to the rear two cylinders, I bought another plenum, did the split and weld in a patch in mod, and used a BIG K&N filter. Then drove it for a number of years, including some higher altitude trips. I could not tell the difference. Nos 5 & 6 plugs were still the blackest. I got a much bigger effect on engine running by having the MU set up to run correctly and spending time to get the butterflies synchronised. Maybe for normal touring there is no effect, perhaps it is only evident at the limit. Gone back standard, looks much better.

Mick

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10 hours ago, JochemsTR said:

John, actually, K&N Filters come in larger sizes such as 4" and 5"..... I do not want to create a "bottle neck" at the orifice and Filter unit. Your opinion?

As I think I mentioned, while flow in the plenum is I hope laminar, and obeys the fourth power law (Poiseuille's), flow in the smaller entry orifice is turbulent and obeys a square law (Reynold's).    So an increase in size has a much lesser benefit.

Laminar flow - fourth power - twice as big, sixteen times more flow!

Turbulent flow - square power - twice as big, four times more flow.

I don't think there is a cone filter with a 6" flange, Jochems says there are 5" and I do not doubt him, but that would achieve only 2.8 times more flow ( (5/3)^2 ) and it didn't seem worthwhile to me!

Mick, How much bigger was your bigger plenum.    My 6"er (oooooooooooer, missus!) has equalised all the plug's appearances.

John

Edited by john.r.davies
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Has anyone calculated the effect of a tubular manifold heating the air in the plenum?

Probably irrelevant at driving speeds but tickover, is it enough to heat the air in the plenum (and more so for cylinders 5&6)?

The other thing when inspecting plugs as a guide to mixture richness is to do a plug chop. Drive the car, cut the ignition, dip the clutch  and then stop. Rather than park up and switch off which tells you about tickover rather than running).

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Ooooooooooooooookay!  Let's give it go!

At idle, an engine may consume 0.6L of petrol/Liter of capacity/ hour.   So a TR engine will use 1.5L/hour, or, at 750gms/L =1.13kgs

Petrol has an energy content (assuming a perfect mixture) of 32.6megaJoules/litre.   And it may convert 40% of that into movement, the rest into noise and heat.   

So the energy radiated as heat will be 60% of 32.6 x 1.13 mJ/hour = 22.1mJ, or 0.37 megaJ per minute, or 370kiloJoules

If the engine idles at 1000rpm, then it will draw in 2.5 x 500L/min = 1250L

Air has a density of 1.218gms/L, so that air will have a mass of 1522.5gms, say 1.5kilograms

The Specific Heat of Air is 1kiloJoule/Kilogram/degree Kelvin [Klevin degrees are the same as Celsius, just with a different zero] so if all that heat were to get into the plenum it would raise the temperature of the air in one minute  by  1.5/370 =0.004K.   In other words, the manifold will not heat the plenum air significantly. 

Or at all of course, as much engine heat is removed by the coolant, and most of the heat in the exhaust gas goes out of the exhaust pipe.   What does heat the manifold is radiated in all directions, so only a tiny fraction will heat the plenum.     An idling engine may be even less efficient than the figure used above, but even if 100% of its energy became heat, the effect on plenum air would still be insignificant.

And anyway, far more heat is exuded into the engine compartment by the radiator, which is designed by having an air-to-surface ratio several orders of magnitude higher than the plenum to be as highly efficient a heat exchanger as the plenum is not.       For the same reason, that heat will not raise the temperature of the air inside the plenum, but if the air filter is allowed to draw this hot air from behind the radiator, then inside the plenum will be heated air, just not air heated by the manifold.

I'll be glad if someone would check my calculations!

John

Edited by john.r.davies
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On 10/2/2020 at 2:20 PM, Waldi said:

Some think the sooting of the rear cylinder plugs is related to this. 
Increasing diameter (cross-section) will reduce losses by the turbulences quite a bit.

I would expect to see a progressive enrichment - hence sooting - towards the rear cylinder if this was the case. But if it's only the rear cylinder that gets sooty then I think it's the inlet trumpet's proximity to the rear wall of the plenum that's causing the problem. It means that part of that trumpet is "obscured", so it behaves differently and does not draw as well as all the others which are well clear of the back (and there is no front wall).

Cheers, Richard

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As to heating of the air in the plenum, install a Inlet Air Temp sensor (IAT) in the back of the plenum. They are cheap and all they are is a thermistor. Easy to devise simple circuit to measure. Then you can compare ambient temperature with the actual temperature inside the plenum. My manifold is a bit different, it does have a heat shield, but I live in a high temperature environment.  I have seen IAT’s 40-50f over ambient when stationary, cooling the IAT down took 50-60 mph for 5 minutes with, most importantly, 1/2 throttle or more. My air filter takes air from in front of/alongside the radiator.

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1 hour ago, CK's TR6 said:

As to heating of the air in the plenum, install a Inlet Air Temp sensor (IAT) in the back of the plenum. They are cheap and all they are is a thermistor. Easy to devise simple circuit to measure. Then you can compare ambient temperature with the actual temperature inside the plenum. My manifold is a bit different, it does have a heat shield, but I live in a high temperature environment.  I have seen IAT’s 40-50f over ambient when stationary, cooling the IAT down took 50-60 mph for 5 minutes with, most importantly, 1/2 throttle or more. My air filter takes air from in front of/alongside the radiator.

Good practical information. I'm surprised  to see it takes  relatively long before the AIT reduces.
 

Waldi

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I make my own from scratch. Personally I wouldn't use a standard one as a start point as they are often rotten and loads of leaks. You'd be better off selling that on ebay and using the money for the rest of the build.

Exhaust tube and a bit of time will get you a decent plenum.

 

 

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WhatsApp Image 2020-08-16 at 16.45.33 (1).jpeg

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Nice work, mtrehy!    Beautiful, even!

But why did you you stick with a 3" plenum, when you mounted the trumpets right out into it, not agaianst the wall?   You're going to get a lot of turbulence and flow restriction down the tube!

John

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Even so!

4" over 3" is a ten times better flow, when 6" is SIXTEEN times better!

And with al thos trumpets in the way, IMHO you need all of that and more!

 

Above, I tried to calc the effect of manifold heat on intake air temp, and asked if anyone would check and correct it.  Can't belive it's perfect!

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12 hours ago, mtrehy said:

You'd be better off selling that on ebay and using the money for the rest of the build.

I would love to...

Besides the discussion regarding diameter and flow, I am still looking for a sheetmetal guy willing to fabricate such custom plenum. 
Jochem

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