Plenum Chamber Modification

[Bob Stubbs]

OK, here's a question for the airflow/physics specialists amongst you.

 

I'm currently carrying out the mod as suggested in Roger Williams book about increasing the size of the plenum chamber to increase the airflow so that cylinders 5 + 6 get their fair share of the airflow thats going.

 

I've got a spare chamber currently being modified to take the diameter out to nearly 5".  If my maths is correct that means that I'll have nearly 2.5 times the volume of air available in the new chamber compared to the old.

 

Here's the tricky bit though.  My plan is to retain a semblance of originality so I'm planning to retain the use of the original airbox assembly up near the rad(which has a K+N in it) and the original flexi pipe that connects to the plenum. To do this, there's a flange cut to the same dimension as the original plenum on the intake end of my new one.  This means that the entry diameter for incoming air is the same as the old plenum although the volume of the new plenum is as I said 2.5x bigger...........with it so far?

 

So the question is, will the fact that I've restricted the entry airflow to the original spec mean that I'm going to end up as was in air flow terms OR is the overriding factor the volume of air in the chamber.............I think I know what I mean, I just can't get my head round the physics of it all and it would be handy to know before the metal bashers finish the job. ???

[Roger H]

Bob

 

This has also been a bit of a bugbear for me as the better tuned the car is, the more difference there seems to be across the plugs. I assume a better situation is to have the inlet to the plenum in the middle, rather than one end - but space makes this impossible.

 

In a previous post, I commented that some advice given to me by a local race driver (not TR's, but straight sixes all the same) did not seem to make any difference when I applied his idea to my car. I must admit to possibly being a little premature with my observations as now, with the benefit of a few months of driving behind me (rather than just a week!) my opinion is changing.

 

His opinion is that the air movement in the chamber when the engine is running, is not just one-way, but is actually turbulent, caused by shock waves (blowback he called 'reversion') from gasses escaping  back through the inlet valves at the commencement of inlet valve opening. This happens because the exhaust stroke is not at TDC when the inlet valve starts to open and is due to the 'overlap' of the cam design. As the overlap of a performance cam gets greater to allow for higher revving, there's a drop in low end perfomance. For this reason - high overlap cams are best suited to high revs.

 

I say all this as I guess the PI TR could be considered a bit of a racer, with power coming on in the higher rev range but how many of us actually race them?  Most are daily drivers, driven at reasonably low revs with occasional bursts in the high range. With the majority of the time spent below (say) 3000rpm, the effect of the cam is more pronounced in the long airbox.

 

It is his opinion that there is a standing wave generated by the regular pulses of reversion which disrupt the inlet air. As this wave is contained within the airbox, the flat end is a lot of the problem. Drilling a small hole in the flat end allows air to escape and disrupts the formation of the wave. It may also be that a tapered airbox also inhibits the formation of this wave or that a concave or  dished end to the airbox may do the same (I'm no engineer but it's a thought!)

 

When I first tried this, I only had a week of driving before I keenly posted that there was no real benefit. Now I have to admit there is a noticeable improvement. I have also been changing plugs (varying temperatures) and also fuel additives (lead replacements). I am continuing my observations but generally I can say that there is a noticeable improvement in the burn characteristics.

 

Before you cut up an airbox, try the 'hole' trick. I drilled mine in the 11 o'clock position when viewed from the drivers seat so that I could eventually put an angled bend (to clear the bulkhead) and air cleaner on if it worked!

[Guest Neil Russell]

i have to admit to not having tuned the TR6 motor at all, but applying a bit of my tuning knowledge generally I would comment as follows:

 

What would happen if you got rid of the plenum chamber altogether, modified the throttle bodies to take some stubby ram pipes, fitted 3 K&N elements over the top or 6 pipercross filter socks, then fabricated a half round aluminium plenum sealed to the underside of the bonnet with a rubber seal and a NACA duct in the bonnet? ram air induction?

 

From memory the TR6 engine always runs on the rich side, especially around 5&6 cylinders. Is this not so much about the air flow but a result of the way that the air has to enter a one way pipe starting at the front and working toward the rear??

 

The tuscan engine I have is the same design, but the airbox has a centre inlet which probably allows the air to circulate within the airbox more evenly?

 

this should get the dialogue going!

 

Neil

[Bob Stubbs]

Thanks for the input so far guys; it's very helpful although I suspect there's more comment to come.

 

Alec; you mentioned a new Technicalities CD which sounds really useful. Can you tell us when it might be available and how you get it?

 

Roger, your idea about a hole at the bulkhead end of the chamber is interesting and I'd be really tempted to give that a go if its worked for you. Might even add an interesting turbo like whine if the hole was "tuned" appropriately.  I presume your idea of fitting a filter to it is based on the concern that some air would enter through it (at low revs?) whereas normally there would be air leaving the plenum through it.

 

Neil; I did think about converting to K+Ns and simply pointing the flexi hose from the airfilter back up so that there's some cold air flow heading in the general right direction.  I was put off the idea on 3 counts though, one I'm trying to keep some vestige of originaility - tenuous as it is on my car - so not a great argument I admit; two was the issue I'd read of earlier which was keeping a cool airflow over the filters was a problem and performance often suffered as a result and finally three; what do you do with the crankcase breather pipe and idle adjustment pipe which currently fit on the plenum?  If anyone could clarify the last points I might go that route.  Anyone done that successfully on a PI?

 

Ram induction..............now that really does sound interesting.

[john.r.davies]

All,

If I may offer my comments:

Advantage No.1  of a plenum is, as Bob says, cool air.   No amount of directed air from a filter is going to cool intake from the engine compartment without a tin box around it.

 

Advantage 2 is raised pressure.   Air passing through ducts of any sort has a velocity, and as Bernouille says, pressure is inverse to velocity.    A plenum allows the air to slow down as it enters from the ducts, so that it has a higher pressure for ease of induction.

So make the plenum as large as possible.

 

  The standard TR plenum is a tube, whose resistance to flow is inversely related to the fourth power of the radius, so double the radius, the resistance falls to one sixteenth.    And you get a nice big plenum.

Ram-air is a bit of a myth at less than 100mph, and the intake size/expansion geometry has to be carefully matched to gain an advantage.  NACA ducts are great for enclosing nice smooth laminar flow - I'm not confident of that on a TR's bonnet.

Roger's racing friend is right about pulses from the inlet valve, and the standing wave in the inlet manifold.   This can be used to tune the inlet, just like an organ pipe, to raise the pressure at the inlet valve, while reducing it at the inlet.   This needs longer than usual inlet ducts, and again, there is an inverse relationship.  To resonate, the inlet length required is inversely proprotional to the input frequency, or in this case, revs (look at organ pipes - the higher the shorter).  Revs 4-6K happen to need pipes 600-400mm long!

 

On the affect on mixture to the back cylinders, I can only report work in progress.   I have turned the plenum around, to accept air from the high pressure pocket in front of the windscreen, to minimise air flow into the engine compartment.  So No.6 cylinder is first in line for air. I'll report back.

 

John

[Guest Neil Russell]

The cheap fix.

 

if we just want more performance via achieving a clean burn on all six cylinders what about the following:

 

drill a largish hole on the rearmost wall of the plenum chamber. weld/solder or braize a nut on the end covering the hole to create a threaded boss. use a brass air type union of the type that has a ferruled end. then secure some suitably sized rubber pipe onto the boss and secure with a jubilee clip. The other end can be secured to one of those nifty little K&N crank case breathers and clipped to the bulkhead or other suitable location.

 

In my opinion this will solve the rich running on 5 & 6 pistons, since the air will be drawn in all directions and any pulsing or shock effect due to sealed plenum design will be eliminated.

 

it will also look sexy and not spoil the aesthetics of the original installation.

 

The other option is to chuck it all in the bin and fit a Weber alpha programmable set up with modified throttle bodies.

 

Has anyone ever thought of using an intercooler in a non forced induction application? would this work??

[Peter Vernon]

I would have thought that the size of the chamber would be of little relevance and it is the airflow that was the important point.

 

Why not take the guesswork out of it and fit the filters directly on the manifold? K&N do a very nice filter set of three to do just this.

 

:cool:

[Bob Stubbs]

John

 

Thanks for your very comprehensive reply which I think I understand.

 

I can follow the logic in making the plenum as large as possible which I have done (its now about 4.75" dia) although as I mentioned, I've retained an entry diameter at the front end of the plenum to the original 3.1ish" thus allowing the original pipe and airfilter arrangement. So will I have shot myself in the foot doing that?

 

It would be relatively easy if need be just to slice the front end off to give an air entry of 4.75" but then how would I handle the filtering to ensure cool air flow?

 

I would be very interested to know more about your idea of reversing the infeed to the rear. Have you any photos of that mod at all? How have you handled the idle adjustment screw and suchlike?

 

Peter; yes I have considered K+Ns direct onto the inlets but as John has commented and I had read on earlier posts and in other reading on the issue that you need a cool airflow for optimum performance and open filters sitting over a hot exhaust manifold are a problem I believe.

[john.r.davies]

Bob,

There's always a compromise, between the ideal and what can be done.   The ideal is a carefully crafted intake up in undisturbed flow, with a matched plenum right underneath - see F1.  With your larger plenum, I doubt if you will blast your rivals into the weeds (!), but it is likely to make a difference.

None of us here (I suspect) is in the position of making microchanges to our engines or cars, testing the result on dyno or windtunnel and then reiterating the process, so we must do what seems right, by eye or seat of pants.

Try a bigger duct as your next iteration, perhaps junking the filter box and sticking one of K&Ns wide range of filters out in front of the radiator?

John

[Roger H]

Bob

 

The hole in the rear of the plenum has made a positive difference in my case.

 

I started with a 'small' hole as it's easy to make it bigger. If the hole is too large (there is a limit if you want to fit a filter), hot air may become an issue.

I also have the 3 K&N setup with ram tubes as an alternative for racing (yet to be track tested), but this is not comfortable on hot days or sitting in traffic where there is no air flow through the engine bay.

 

There are neat aluminium hose connectors (bends) available from racing suppliers to which you can fit a small PCV filter. When you make the hole, have a hose connector handy  - if you drill too close to the edge of the box (to clear the bulkhead), you may not be able to fit the securing nut to the inside of the airbox. My hole is currently 1/2" Ø but will need to be bigger to accommodate a bend of equivalent I.D.

[Bob Stubbs]

Guys

 

Thanks very much for all your advice.  A lot of ideas here for me to play about with; I'll report back once I've experimented a bit.

[Andy Moltu]

As you have made the plenum larger in diameter you really need a bigger air filter and pipework to feed the plenum chamber.

As Alec say the volume of the plenum isn't hugely important it's the ability of the plenum to deliver the volume of air to all the cylinders. Having the bigger diameter enables greater airflow only if you increase the feed to it. You can acheive this by making a hole at the rear but at the expense of sucking in hot unfiltered air. So a bigger filter and ducting from the front is the better solution.

 

Individual filters are fine however they need to be fed with cool air ducted in from the front otherwise they will be sucking in hot air from around the red hot exhaust manifold! Hotter air is less dense & so contains less oxygen per volume so theoretically less power.

[Schnippel]

Hello, the diameter from the plenum is completely sufficient.  My TR 6 was already equipped before 20 years ago with central butterfly in front of the plenum. All spark plugs has the same color.  The individual butterfly valves are distant.  The change took place in connection with Bosch k-Jetronic and already several times on EFI.

By a central butterfly valve also the synchronisation problems are void.

It should not be difficult to adjust the lucas metering unit on a large butterfly valve (64 mm from BMW 2,5/2,8/3,2 L )

It works very nice. and you get more power. 130 PS/5100 U/min  on the whells !

the machine breathe deeply freely !

You want pictures, please mail !

Regards from Germany

Schnippel

[jonlar]

Just a thought, has anyone worked out what volume of is drawn through the engine per 1000 rpm (multiples up to max revs easily worked out) {I assume for a 2.5 L engine it would be 1250Lpm?} and tried using an electric fan capable of providing more than max. required?   As I say just a thought, or am I blowing out of a certain orifice!

[john.r.davies]

Jonlar,

Worth a try, however someone was offering "electric turbo"  - a fan - on a US website last year and got thoroughly rubbished.   But don't knock it, etc etc.

John

[Andy Moltu]

The problem with using a fan to force more air in is that it will confuse the Lucas PI system which delivers the fuel according to the inlet manifold pressure compared to atmospheric. The lower the inlet manifold pressure (wide throttle openings) result in a larger quantity of fuel being delivered. Pressurising the inlet side of the system to deliver more air will fool the metering unit to deliver less fuel.

 

Later PI systems such as the Bosch meter the fuel delivery relative to air flow which allows greater ability to compensate for different inlet (atmospheric pressures). This in turn makes it far easier to then use forced induction systems and maintain an accurate fuel air mix.

 

In the USA they were probably using SU/Stromberg carbs which are more able to maintain the fuel air mix at differing inlet (atmospheric or fan induced) pressures than the Lucas PI system.

 

There were one or two lucas PI metering units with atmospheric pressure compensation facility but these were considered too expensive to fit to production TRs. So Triumph did a cheapie & simply specified a leaner fuel curve for those cars supplied to high altitude markets (Switzerland)

[jemgee]

Desperately trying to remember O level Physics - would there be any merit in surrounding the bulkhead end of the manifold with a 'cone' shaped stub duct to create a venturi effect ie to help suck air from the front of the manifold through to the rear and exit via a hole in the end of the manifold?

[john.r.davies]

Andy,

The metering unit has two pressure connections - the top one for manifold pressure, the lower for atmospheric.  In effect the meter compares these two.

If you connected the 'atmospheric' to the downstream side of the fan, well upstream of the manifold, then that would be the standard for comparision with the manifold pressure.   Wouldn't that allow the metering unit to do its job?

 

Jemgee,

O-level physics spot on, but you would need a fast flow of air around the outside of the plenum to drive the venturi.  A pipe connection to a point on the car where lift is generated would work just the same, and by Sr.Venturi's effect as well!   But, as I understand it, the air starvation at the rear cylinders is due to the flow from the front being robbed by the front cylinders.   A further extraction of air from the rear of the plenum is going to make matters worse!

John

[Andy Moltu]

<font color='#000000'>Andy,

The metering unit has two pressure connections - the top one for manifold pressure, the lower for atmospheric.  In effect the meter compares these two.

If you connected the 'atmospheric' to the downstream side of the fan, well upstream of the manifold, then that would be the standard for comparision with the manifold pressure.   Wouldn't that allow the metering unit to do its job?

 

Not quite - the flaw is the Lucas metering unit relies on the assumption that atmospheric pressure is constant and that the air temperature is constant too. Naturally neither of these occur outside of the Lucas test bed! The volume of fuel delivered suits the relative difference between atmospheric pressure and manifold pressure. If the inlet air is less dense (from heat or altitude) the relative pressure difference caused by the pistons going down is less than at normal temp & pressure & the metering unit interprets this as wide open throttles & squirts in more fuel.

 

Modern PI systems measure air flow & temperature so that the mass of air can be given an accurate volume of fuel to match.

[trimprop]

I read this forum yesterday, and had a sleepless night pondering how to alter the air intake to give the back two cylinders more air.

 

Has anyone considered, or tried inserting a half-moon shaped length of tube, that could be secured to the side of the plenum that would sit flush with the openning of the chamber, and end just after the hole for cylinder 4?

 

My thinking is that by adding in something like this, a certain volume of air is directly tunneled to the back cylinders, thus avoiding the problem of the first four taking all the air - but without having to cut into the chamber with extra pipes, or open the chamber up and allow hot air in.

 

The link below is a picture of how i pictured it (the red bit is the half-moon insert)

 

http://www.trimpropeller.com/images/plenum.jpg

 

Any thoughts? It is probably something quite easy to test - which, unless there are good reasons not to, I will at some point soon.

Edited September 21, 2007 by trimprop

[Red 6]

I read this forum yesterday, and had a sleepless night pondering how to alter the air intake to give the back two cylinders more air.

 

Has anyone considered, or tried inserting a half-moon shaped length of tube, that could be secured to the side of the plenum that would sit flush with the openning of the chamber, and end just after the hole for cylinder 4?

 

My thinking is that by adding in something like this, a certain volume of air is directly tunneled to the back cylinders, thus avoiding the problem of the first four taking all the air - but without having to cut into the chamber with extra pipes, or open the chamber up and allow hot air in.

 

The link below is a picture of how i pictured it (the red bit is the half-moon insert)

 

http://www.trimpropeller.com/images/plenum.jpg

 

Any thoughts? It is probably something quite easy to test - which, unless there are good reasons not to, I will at some point soon.

 

From my experience with road going PI 6's, if the problem exists at all, it is negligible and therfore an intangible benefit would be achieved by fiddling with the airbox.

 

I think there is an element of sympton and cause, where the focus is on the symptom. If there is evidence of richness ie black plugs or noticibly different coloured plugs after a long run then the cause is unlikely to be air starvation at 5+6 but a fueling problem, i.e. metering unit, injectors, prv etc etc. The metering unit is the automotive equivalent of a gattling gun with some diagphrams to control the out flow (obviously i have simplified this).

 

I can relate to the sleepless nights bit and i think your design is good because it directs air to the rear inlets so well done on that.

 

On my race car i had long trumpets and a big plenum chamber, and i mean big! This worked fine, but i bet that if i put the std plenum back on it would still have worked fine, probably because i had an enormous air filter, providing almost unrestricted cold air to the engine.

 

So that is where to start, big cold air, and if there is a plug issue look at the MU and associated components.

 

ps my engine was optimised for 7.5k rpm with bursts to 8250.

[Richard Crawley]

From my experience with road going PI 6's, if the problem exists at all, it is negligible and therfore an intangible benefit would be achieved by fiddling with the airbox.

 

I think there is an element of sympton and cause, where the focus is on the symptom. If there is evidence of richness ie black plugs or noticibly different coloured plugs after a long run then the cause is unlikely to be air starvation at 5+6 but a fueling problem, i.e. metering unit, injectors, prv etc etc. The metering unit is the automotive equivalent of a gattling gun with some diagphrams to control the out flow (obviously i have simplified this).

 

I can relate to the sleepless nights bit and i think your design is good because it directs air to the rear inlets so well done on that.

 

On my race car i had long trumpets and a big plenum chamber, and i mean big! This worked fine, but i bet that if i put the std plenum back on it would still have worked fine, probably because i had an enormous air filter, providing almost unrestricted cold air to the engine.

 

So that is where to start, big cold air, and if there is a plug issue look at the MU and associated components.

 

ps my engine was optimised for 7.5k rpm with bursts to 8250.

 

Got to say I totally agree with you; I think this is a perceived problem compounded by theorisation from compulsive tinkerers. I've never had a problem or even noticed any difference either in the first 15 years I thrashed the car to its ultimate death or in the last 3 years since its restoration with up-rated engine; lead free, low octane & all! If there is a problem, my advice would also be to look for the cause elsewhere!

 

My understanding is that Triumph realised the plenum design was marginal early on but concluded the effects were ‘insignificant’ in reality. No doubt modifying the plenum would benefit a highly tuned or race engine with much higher air flow requirements but for even moderately tuned road cars, I can’t see the point; it seems a lot of effort to go to for very little gain! If it ain’t broke, don’t try & fix it or it could well end up broke!

[Andy Moltu]

<font color='#000000'>Andy,

The metering unit has two pressure connections - the top one for manifold pressure, the lower for atmospheric.  In effect the meter compares these two.

If you connected the 'atmospheric' to the downstream side of the fan, well upstream of the manifold, then that would be the standard for comparision with the manifold pressure.   Wouldn't that allow the metering unit to do its job?

</font>

 

I don't think it's quite that sophisticated - the metering unit compares manifold pressure against a spring. It doesn't have the capacity to adjust for air density (which is ultimately determined by atmospheric pressure & temperature) which is why the PI cars are tricked into running excessively rich at altitude. Some metering units were calibrated for altitude (set leaner) hower one or two special metering units did have a proper mechanism for pressure compensation but these were never standard fitments. Likewise forced induction or even a ram effect can fool the metering unit into running excessively lean.

 

Modern injection systems use air flow meters - usually achieved by using a heated wire in the air flow & a temperature sensor. The rate of cooling of the wire reflects the mass (rather than volume) of air entering the engine relative to the ambient air temperature and the fueling adjusted to suit.

[Red 6]

Got to say I totally agree with you; I think this is a perceived problem compounded by theorisation from compulsive tinkerers. I've never had a problem or even noticed any difference either in the first 15 years I thrashed the car to its ultimate death or in the last 3 years since its restoration with up-rated engine; lead free, low octane & all! If there is a problem, my advice would also be to look for the cause elsewhere!

 

My understanding is that Triumph realised the plenum design was marginal early on but concluded the effects were ‘insignificant’ in reality. No doubt modifying the plenum would benefit a highly tuned or race engine with much higher air flow requirements but for even moderately tuned road cars, I can’t see the point; it seems a lot of effort to go to for very little gain! If it ain’t broke, don’t try & fix it or it could well end up broke!

 

Richard,

 

15 years!!! you were not trying hard enough!

[Richard Crawley]

Richard,

 

15 years!!! you were not trying hard enough!

 

But I did pampered its every whim & service it every 3k miles. It started to look quiet tatty towards the end but still went like stink until the twice rebuilt box spectacularly destroyed itself with a bang that almost lifted the car off the tarmac. Perhaps the noise it was making should have given the game away but in those days, I didn’t really care! The box was totalled &, alongside other interests, is why it spent the next 7 years in languishing in 2 garages; when I finally got around to restoring the chassis, I was shocked at the amount of damage all that clutch dumping had done to the diff mountings; but the original Laycock clutch was still serviceable & is still lurking around in the loft somewhere. That was just the start though as it took me another 10 years to finish the car!