Discussion
Just out of interest. Whats the main purpose of a plenum chamber? Back in the day where i wasn't scared of taking me dolomite sprint to bits, The inlet manifold was a place for 'boom juice' and air to get together. Is the plenum the new inlet manifold ? And is a carbon fibre one any better than the shiny one ? does it matter what its made of ? Is bigger better ?
Sorry about the mechanical ignorance chaps, but does anyone know ?
Sorry about the mechanical ignorance chaps, but does anyone know ?
Aye lad. Well in these days of fuelio injectioni, yer air coming into cylinders is first measured by t'airflow meter. Thence it passes via the throttle butterfly to the plenum chamber, usually smack bang on top of... wait for it... the inlet manifold. Once the air has been measured, and taking into account throttle position, rate of change of position and other engine factors, the engine management can decide when and for how long to squirt the 'boom juice' into t'manifold (backs of the inlet valves, usually).
So, once you have an accurate measure of air, you want to calm it down after all that rushing about through ducts, throttle bodies and whatnot, so the plenum (from the Latin 'full') acts as a reservoir for the air. Then whichever cylinder is drawing, can take a big gulp of O2. This is why it is so important to eliminate any air leaks, because any air entering the plenum/ inlet through anywhere other than the throttle, is going to screw up the mixture.
Hope this helps...
Ian
So, once you have an accurate measure of air, you want to calm it down after all that rushing about through ducts, throttle bodies and whatnot, so the plenum (from the Latin 'full') acts as a reservoir for the air. Then whichever cylinder is drawing, can take a big gulp of O2. This is why it is so important to eliminate any air leaks, because any air entering the plenum/ inlet through anywhere other than the throttle, is going to screw up the mixture.
Hope this helps...
Ian
So why does it have to be a big reservoir of air. Why do you need a reservoire when air is all around us? Why stock up on it when you've got it whenever you want it for free ?? Why cant it just 'not be there' and instead have a great gaping hole to the O2 Filled atmosphere as we know it. In this instance, you can drag as much air as you want and forget the fancy aluminium or fasion item carbon fibre type thingy, along with that magic airflow gadjet in your way. Why fibre, why airflow meter, why plenum ? Is it all a scam ?
You need a plenum with lots of space because there are 8 inlet trumpets under neath and their design does influence the power. The plenum also seals the air intake so you can measure how much air is going in and the ECu will work out how much fuel to pump into it. It is big because we are talking about big boys toys with 4- 5 litres of air/mixture on every complete cycle which is a lot of air. Make the plenum too small and this starts becoming a problem .
Steve
www.tvrbooks.co.uk
Steve
www.tvrbooks.co.uk
I think the main reason is cost .. there's no instance I can think of where a plenum is preferable over individual throttle bodies other than it makes life simple and cheap. Indeed as you go wilder on cam profile a plenum is a definite liability and can cost significant low speed drivability and torque.
or, in other words...
the plenum is a contolled environment for your supply of air. the injectors can do this for the fuel, but so that the air can also be strictly monitored and measured, it needs to be controlled. the easiest way of doing this is to give the engine its own little world of air, enclosed by the plenum.
racers of the sixties had loverly chrome trumpets sticking out of the rear, but these are subject to airspeed, direction, aerodynamics etc.
so you bring in cool, dense air from the front of the car, test how cool, dense it is in the air mass meter, then control its entry ito the plenum via the butterfly.
blend with injected fuel as required and hold on !!!
the plenum is a contolled environment for your supply of air. the injectors can do this for the fuel, but so that the air can also be strictly monitored and measured, it needs to be controlled. the easiest way of doing this is to give the engine its own little world of air, enclosed by the plenum.
racers of the sixties had loverly chrome trumpets sticking out of the rear, but these are subject to airspeed, direction, aerodynamics etc.
so you bring in cool, dense air from the front of the car, test how cool, dense it is in the air mass meter, then control its entry ito the plenum via the butterfly.
blend with injected fuel as required and hold on !!!
'Why do you need a reservoire when air is all around us? Why stock up on it when you've got it whenever you want it for free ??'
Don't say that - they'll tax it. There'll be a little counter on the side of the airflow meter and every year a meter-reader will come round, calculate how much air you've used and send you a bill for it. You can pay by direct debit and save 5%, but if you don't pay up the max fine is £1,000.
(Shit, that was easy wasn't it?)
>> Edited by simpo one on Saturday 21st December 20:28
Don't say that - they'll tax it. There'll be a little counter on the side of the airflow meter and every year a meter-reader will come round, calculate how much air you've used and send you a bill for it. You can pay by direct debit and save 5%, but if you don't pay up the max fine is £1,000.
(Shit, that was easy wasn't it?)
>> Edited by simpo one on Saturday 21st December 20:28
so you bring in cool, dense air from the front of the car, test how cool, dense it is in the air mass meter, then control its entry ito the plenum via the butterfly.
blend with injected fuel as required and hold on !!!
Or you spend the GRP of a small African nation and have three throttle bodies with machined trumpets which are covered by an expensive air filter to calm the air and two 80mm diameter air feeds from the high pressure zone at the bottom of the windscreen which provide enough air to exploit the twin fuel pumps that deliver around 12 litres of fuel per minute to the injectors....
You do get a bit more power
simpo one said: While I'm on, what would happen if you had a little bottle of compressed/liquid oxygen that you could squirt into the system when you needed a bit more power? Gases cool as they expand, which might be handy too.
(If you make it work, 10% to me please!)
I think you will find it is called a NOS system....
Guillotine said: or, in other words...
the plenum is a contolled environment for your supply of air. the injectors can do this for the fuel, but so that the air can also be strictly monitored and measured, it needs to be controlled. the easiest way of doing this is to give the engine its own little world of air, enclosed by the plenum.
racers of the sixties had loverly chrome trumpets sticking out of the rear, but these are subject to airspeed, direction, aerodynamics etc.
so you bring in cool, dense air from the front of the car, test how cool, dense it is in the air mass meter, then control its entry ito the plenum via the butterfly.
blend with injected fuel as required and hold on !!!
Aha, Now i Get it
So whats the carbon fibre advantage then ? Apart from looking nice that is ?
mainly temperature/airflow.
the ally plenum is attached to the inlet, which gets 'kin ot! so all that nice cold dense air goes into an oven! try touching one when you've warmed up the engine!!
the main advantage is that carbon fibre does not transmit the heat from the inlet / surrounding air to the incoming air anywhere near as much as an ally one, however, the gains would be 5 - 10 bhp and on a 300 bhp engine thats not alot for the bucks.
other advantages will be better/smoother airflow across all the trumpets giving greater and more even power across the cylinders. if you look at the pics on ACTs site, you can see the improved shape and therefore airflow.
you could do with a chat with Tim at ACT. he is the man for these. and i would just say that if you have spent alot of money upgrading brakes suspension and engine, a rolling road set up with an induction upgrade from ACT are pretty much essential! the pure figures would not reflect the improved throttle response and racing improvments that would be shown with this kit.
I WOULD HAVE ONE, I JUST CAN'T AFFORD ONE!
the ally plenum is attached to the inlet, which gets 'kin ot! so all that nice cold dense air goes into an oven! try touching one when you've warmed up the engine!!
the main advantage is that carbon fibre does not transmit the heat from the inlet / surrounding air to the incoming air anywhere near as much as an ally one, however, the gains would be 5 - 10 bhp and on a 300 bhp engine thats not alot for the bucks.
other advantages will be better/smoother airflow across all the trumpets giving greater and more even power across the cylinders. if you look at the pics on ACTs site, you can see the improved shape and therefore airflow.
you could do with a chat with Tim at ACT. he is the man for these. and i would just say that if you have spent alot of money upgrading brakes suspension and engine, a rolling road set up with an induction upgrade from ACT are pretty much essential! the pure figures would not reflect the improved throttle response and racing improvments that would be shown with this kit.
I WOULD HAVE ONE, I JUST CAN'T AFFORD ONE!
Cheers guilotine.
The most important/ best value upgrade is the trumpet base, worth about 5-10 lb ft depending on application,
The Plenum shape and size plays a big part in performance increase, with Carbon fibre we can "play around" with the specification very easily and achieve sexy shapes.
The reduction in temperature plays a smaller part, but still important.
I agree with making sure your car can stop effectively before upgrading the engine. this doesn't need to cost a fortune though.
Tim
tim
The most important/ best value upgrade is the trumpet base, worth about 5-10 lb ft depending on application,
The Plenum shape and size plays a big part in performance increase, with Carbon fibre we can "play around" with the specification very easily and achieve sexy shapes.
The reduction in temperature plays a smaller part, but still important.
I agree with making sure your car can stop effectively before upgrading the engine. this doesn't need to cost a fortune though.
Tim
tim
The gain on mine was 15bhp and 10lbsft, however the response is much more instant and the torque hold up much better at higher revs. Bob, e-mail me if you want more detailed info or search archives over last few months. (or take a look at my web site, url is in my profile) I'm well pleased!
Guillotine said:...the gains would be 5 - 10 bhp and on a 300 bhp engine
Rich... Lots of the above is correct but I reckon the main reason for the plenum chamber lies in the Helmholtz resonator principle. The idea is to contain a measured amount of air in a container (plenum) which is proportional to the volume of one cylinder. There are sound waves or pulses which are reflected up and down the inlet ports/tracts. At a certain frequency these agitate the air molecules in the plenum and have the effect of forcing some extra air down the inlet tract before the valve closes. The longer the column of air is, in the tract the more of it will be squeezed into the cylinder thus increasing torque. Long runners make even more torque. Its a bit like tunnel ram tuning but in a box and with resonation. Most production cars have a plenum as this torque gain is pretty useful. Tuning is done with the valves and cam etc. The agitation occurs only at one rpm predetermined by the manufacturer/designer probably about 3-4000rpm, depends on the engine type. I think I'm on the right track with this.
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