why are there seperate ports on the intake-manifold and plenum? why not make it one big chamber of compressed air?
my phys and chem teachers will have taught me - nothing about this topic. However, im led to believe its just a design specific feature. Some plenums (particularly notable are the RB25/26 plenums and their aftermarket betters) infact are just one big chamber with a few short ports leading to the intake valves. The biggest reason for having individul runners - ie a plenum with seperate tubes for each cylinder (i think thats what they call em???) is because - dependant on the length of the runners you can tune the manifold itself so that the pulses created by the air compressing (when the valve shuts) and decompressing (when the valve opens) creates a standing wave in the air within the runner. Its somewhat difficult to explain. But, say the runners are "tuned" for 5000RPM and now imagine this. At 2000RPM (slow cruising perhaps) the valve opens, air rushes past, then the valve shuts. When it shuts, it causes air that *was* moving and about to go in to the cylinder abruptly stops, temporarily increasing the pressure at the point right near the valve. This pressure wave begins moving back up the individual runner until it hits the side of the plenum (imagine ur looking from the front of the car). THe wave then changes direction and begins heading back towards the valve. Once the pressure wave gets to the valve, it finds that it is shut, because the valve opened way after the pressure wave got back to it. So what happens? The wave just bounces back up the runner again and has probably lost 80% of its energy in those two bounces - making it unusable. Now we have the motor running at 5000RPM, and if u recall the headers are tuned for 5000RPM. Same shit happens right - valve opens, air rushes in, valve shuts, air stops. Pressure wave is once again created, it bounces off the valve and heads up towards the side of the plenum. This time, the motor is running much quicker, and as it bounces off the side of the plenum, it heads back towards the valve. However, the valve now opens JUST as the pressure wave reaches it - this action strongly supports the air in its acceleration into the cylinder The air is helped into the cylinder by the pressure wave that was created when the valve shut the cycle before. I hope u have followed, if not, query further and if i still suck at explaining ill draw some diagrams n shit
Veep, that was great, the way you explained it made me think of water hammer, is that the sort of thing you are refering too. does the term adiabatic mean anything to you? (quantities enter and leave the system so quick it has little effect) i was thinking that supply was the big thing here, perhapes there is an optimum and thats where there at.
I think i may have slightly mislead you - and im not quite sure if i have or havent. Tell me what you understood to be the reason for individual runners from my explanation and ill see if i can make it clearer for you. Its interesting though, i have never heard of an aftermarket plenum for the VG30 motor, but aftermarket plenums are plentiful for SR20s and RB series. Not quite sure about the 2JZ motors though.
the understanding i came away with was that the starting and stoping of air flow in the intake-manifold (lower plenum) and plenum causes shock-waves (pulses) within the contained air along the runners.depending on the size and shape of the runners can determine the tuning of the runners. what i was thinking was that by having a huge volume outside the inlet valve would provide more than adequate supply of pressured air.
Yesaftermarket for almost every car even vg30 (a little off topic) but the stock vg30 ones are pretty good already. The most important thing in any plenum is equal flow, if one runner flows way more then the rest then your in big trouble consider your engine blown. I was reading on another forum about this guy using aftermarket plenum on his rb30 and he killed cylinder 5 & 6. Aparently what the plenum design should have been is a diagnal bend towards the end because more air generally flows to the back of the plenum. Either way if i was to use a aftermarket plenum i would bench flow it and make sure it flowed as even as possible. This is the very reason why extra injectors or sigle jet nos are not as good as having a dedicated jet/injector per a runner, a single setup before the throttlebody will end up giving more or less fuel to certain runners. So a/f ratio might be reading rich because cylinder 6 is actually at 10.5 whereas cylinder 1 is only at 13.6. As all dyno tunning is measured by the tail pipe of the car, reading will be an average of say 12.2 dyno tuner goes perfect, but your engine blows up on your way home. Im sure theres a lot more to it but generally like exh manifolds, short runners for low rpm power, long runners for high rpm power. Detonation applies to almost every thing, even the fuel rails will have problems though i have not heard much problem about the zed ones. Wrx share the same design as the zed one and most other V engines. in which it goes through the first rail for 2 cylinders loops around and goes to the second lot of 2 cylinders then return to tank. The problem here is the first 2 cylinder will run a lot richer then the last 2, and countless WRXs have blown up because of this. stock | | | | | | --- A better design is to have Fuel in | / | | / | Fuel out
correct me here the plenum contains compressed air from the turbos? the air fuel is mixed as it enters the cylinder, so there is nothing but air in the plenum.
yes air in the plenum but more compressed air going to one runner then the other etc... Air fuel ratio is the ratio of air to fuel particles burnt after the combustion chamber, so measured after the exhaust valve not in the plenum.
once the exhaust valve opens you have a lower pressure in the cylinder than in the plenum. (air fuel measured after being burnt?) how is it measured here?
Air/fuel ratio doesnt have to do with pressure The air fuel ratio is measured any where long the exhaust system, usually after the muffler for convenience. Though by the time it gets to the muffler the reading will apear to have more air particles then fuel. So as close up in the exhaust system will give a more accurate reading. Air/fuel ratio is not related to air flow etc.. it is the outcome of intake air flow, put simple its a very accurate way of measuring if your car is running rich or lean. It is a simple electrical sensor the two most accurate is the lamda sensor and the wide band o2 sensor. Our cars come stock with narrow band o2 sensors but these are only good for minor corrections as the computer uses and are pretty much useless for tunning. I did say it was off topic remember , do you get what i mean now.
so Pat, what your saying is that the Air / Fuel mixture can only be determined after combustion. OK, I can live with that, thanks for sharing your knowledge Pat.what i was suggesting with the plenums was machining the throats out completely, that is from the inlet port where you can see the valves, you have free space made up of the lower plenum and plenum all pressured, just one big chamber. johno
Intake Runners are a set lenght for tourqe charcstics. If you change the lenghts you will get diffrent results. Not to important when on full boost, but when getting the engine to speed you will have problems. try and look under a VB supercar bonnet. they have big covers to prevent anyone looking at the throats of the runners and the hight. plays a big roll in power Just my 2 cents anthony
The Volume of the manifold is far less significant to performance than the shape, design and construction. Why? Well, imagine u have an ordinary sized manifold. In an NA vehicle, the HIGHEST the ABSOLUTE pressure can be is a touch over 1 Bar (14.7psi) which will read as o bar (o PSI) on a boost gauge. This means it dont matter how big the plenum is in theory, since the maximum pressure cant be more than the outside pressure anyway - so the air has no tendency to go anywhere in particular anyway - until the intake valve opens up and air is SUCKED into the motor by the piston). So the important feature of the intake manifold here is not its size. In a turbocharged car, size is still irelavent - actually, a very large plenum is a disadvantage. Air is being forced into the manifold right, giving a positive pressure reading in the manifold since the pressure is higher than the ambient external pressure (9PSI on a stock Zed). A small manifold will be quickly filled up, judging by the speed at which a boost gauge moves up i imagine it would take less than 100ms to fill a stock manifold even with stock turbos. A large manifold would have a larger volume, and therefore would take longer to pressurized to 9psi. Whether or not the size difference would make a significant difference to the amount of time it took to pressurize the chamber im not sure - though i could calculate it if i knew the flow rates for the turbos. However, my point is that the stock manifold DOES supply enough air to the motor. When you set boost to 9psi, boost stays at 9psi (with the occasional spike, which is turbo or piping related). This indicates that the turbos are pushing enough air into the plenum to keep it filled at 9psi while the engine churns away. Anyway, the supply of air is dependant upon the turbos, the hosing and design of the manifold much more than its volume - i hope iv been able to show u this. And some off topic - the ECU can (provided the correct maps are there) create an accurate 14.7:1 mixture of air/fuel with the data coming in to it from the CAS and the MAF.
Thanks anthony thats way more informative then my "long runner for top end, short runner for low down power".
do you HAVE to have runners for every cylinder what would be wrong with just one runner then a chamber, also what if you could adjust the length of the runner, as the engine was running? ie shorten at low rpm then lengthen at high rpm, i don't mean to sound stupid, i am just trying to be a little creative innovative. if it cannot work then thats that.i appreciate the discussion /input.
YOu cant not have Runners Think of the valve rescess as part of the runner. it is impossible not to have some sort of runner. The runners on the Z are about 90mm Lower plenium. and around 150mm for the upper. the in head section would only be 45mm from memory. I have some pics if you want of what im talking about. And as for adjusting the lenght to the runner as to RPM. Put it stright into the two hard basket. Even on a two stroke where the expansion chamber is running on the inverse laws (physics) they dont adjust it. Spend a grand on water injection, you will get far more power that what you are looking to gain here. Forced induction removes alot of the problems associated with airspeed in the heads. 1 more pound of boost is worth 1000's of dollars in head porting. When i had my heads done the guy spent minimal time on the intake. most time on the exhaust. Cheers Anthony
Hey anthony you were running a waterinjection on your car correct? what kind of results did you find, being able to run more timming at high boost im guessing? wouldnt more timming also require more fuel, and suppose you are maxing out the 370cc injectors would the water injection still offer any advantage. Did you run one of those expensive aquamist systems or home made, if home made waht did you use as a pump, what sort of nozel did you use and wehre did you locate it. Cheers Pat
