Throttle body question
Discussion
Can someone out there tell me the advantage of multiple throttle bodies over single. I know that race engines normally go for a plenum with multi throttle bodies where as production cars tend to be single into a plenum.
I have also had cars with both. The Rover v8 had a single body yet the AJP8 was multiple but I cant seem to work out the advantages of one over the other.
With carbs I can get my head around it as you are fueling a cylinder at a time and the metering of fuel which is relativly heavy is better performed this way. But with just the air to meter im a bit lost as to why the complexity of having 4 or 8 objects in the airflow as opposed to just the one.
Im sure the answer is obvious but cant work it out.
Along the same lines what is the advantage of individual air filters rather than having one feeding a common inlet.
With expense no object what would you use and why?
I have also had cars with both. The Rover v8 had a single body yet the AJP8 was multiple but I cant seem to work out the advantages of one over the other.
With carbs I can get my head around it as you are fueling a cylinder at a time and the metering of fuel which is relativly heavy is better performed this way. But with just the air to meter im a bit lost as to why the complexity of having 4 or 8 objects in the airflow as opposed to just the one.
Im sure the answer is obvious but cant work it out.
Along the same lines what is the advantage of individual air filters rather than having one feeding a common inlet.
With expense no object what would you use and why?
The main benefit is that it tames the cam at low rpm. This is because the lower volume downstream of the throttle means you gt less EGR at low speed, and having each cylinder separated means that EGR from one cylinder can't affect its neighbours. With a plenum you get a far bigger volume under depression so you get far more EGR at low speed, and you get a significant cross talk between cylinders where one cylinder gets a lung full of fresh air and burns properly, this increases the EGR for the next cylinder so it doesn't fire properly and so on. This is what causes the hunting effect you hear on plenum systems at low rpm.
As a secondary benefit, you can usually get a much bigger total cross section area on the intake which means less restriction and less pressure drop.
As a secondary benefit, you can usually get a much bigger total cross section area on the intake which means less restriction and less pressure drop.
GreenV8S said:
The main benefit is that it tames the cam at low rpm. This is because the lower volume downstream of the throttle means you gt less EGR at low speed, and having each cylinder separated means that EGR from one cylinder can't affect its neighbours. With a plenum you get a far bigger volume under depression so you get far more EGR at low speed, and you get a significant cross talk between cylinders where one cylinder gets a lung full of fresh air and burns properly, this increases the EGR for the next cylinder so it doesn't fire properly and so on. This is what causes the hunting effect you hear on plenum systems at low rpm.
As a secondary benefit, you can usually get a much bigger total cross section area on the intake which means less restriction and less pressure drop.
I think Green is talking here about bodies without plenum whereas Jet Fixer is talking single or multiple bodies into a plenum. I'm with him in not understanding why one would use multiple into a single plenum.
Steve
You might feed all the throttle bodies from a common air intake duct if you are using air metering, or need to bring the air in from a remote filter. I'm not sure if it would be strictly correct to call that a plenum. To me, plenum implies a space which is connected to multiple cylinders and is downstream of the throttle. I haven't seen the term defined formally though, so I could be wrong here.
Many thanks,
all is starting to become clear now.
the idea of having an airbox feeding the throttle bodies only came from my cerbera which uses the system due to having a remote filter feeding both banks. The presure does rise in this slightly when traveling a a fair rate similar to the effects now employed on many motorcycles.
anyone know what dictates the size of the throttle body???
Does the length of both inlet track pre butterfly and manifold make a huge difference????
SHPUB what inlet and throttle bodies have you used on your RV8.
>> Edited by jet fixer on Friday 1st July 11:58
all is starting to become clear now.
the idea of having an airbox feeding the throttle bodies only came from my cerbera which uses the system due to having a remote filter feeding both banks. The presure does rise in this slightly when traveling a a fair rate similar to the effects now employed on many motorcycles.
anyone know what dictates the size of the throttle body???
Does the length of both inlet track pre butterfly and manifold make a huge difference????
SHPUB what inlet and throttle bodies have you used on your RV8.
>> Edited by jet fixer on Friday 1st July 11:58
Its the overall distance from the back of the valve head to the mouth of the trumpet that determines the rpm tuning of the set-up, the position of the butterfly dosn't affect that but the closer to the valve head the faster the throtle response.
Have a look at the FAQ page on this link :
www.jenvey.co.uk/
Have a look at the FAQ page on this link :
www.jenvey.co.uk/
I'm a big fan of port throttles.
These are the benefits that I see (There's probably some overlap (no pun intended) with what GreenV8S said):
They allow design freedom for a bigger volume upstream of the port throttles. This allows, for example- a bigger plenum; With a conventional throttle (located perhaps downstream of the MAF) throttle response must be taken into account- and this means there is a upper constraint on how large you can make the volume from back of intake valve to the throttle butterfly itself. However a bigger plenum volume can liberate more top end power (BMW M3 straight six has a 14 litre plenum volume- compare that to my M20 BMW engine which has been forced to a smaller 1.75 litre plenum volume). Individual port throttles allow you to effectively remove the plenum volume constraint upstream of the port throttles so you could potentially reap power if designed correctly.
Port throttles allow wilder cams with more overlap to be used without detriment to idle speed quality. This is because there is less tendency of EGR or Exhaust gas residual to flow back up the -now smaller- volume upstream of the closed port throttles. Typically there is less residual content in the cylinder during part load operation. Too much EGR content is what effects combustion stability at idle speed and otherwise- worse still is uneven EGR distribution cylinder to cylinder but that's another story.
Port throttles can give a very quick throttle response: as touched upon already the reduced volume between the back of valve to throttle means that the engine will see the intended driver throttle angle change much quicker. The trick of calibrating fueling on a MAF based system is to match the quick "air charge" response. In steady state operation, the fuel puddle mass is filled at the same rate that it drains into the cylinder, but during transients (changing throttle position) the air has no problem with acheiving a newer steady state while the fuel puddle takes much longer. The result is lean mixture while transferring to acceleration and rich mixture while transferring to deceleration. Reducing this puddle volume helps this, and with large intake air/plenum volume from the injectors to the air flow sensor there is some compensation but not enough that no acceleration enrichment is necessary with a port throttled system. With this mind it's not hard to see why with a throttle angle system is used to calibrate fueling with port throttles or a manifold pressure sensor.
It's been proven in several experiments that port throttles benefits fuel economy at part load throttled condition over a conventional single throttle engine. Bascially there is quciker pressure recovery in the port. The theory is that the pumping loop in a PV-diagram is cut or "lopped off" thus saving work. There are papers by Roger Duckworth and others with experimental data backing this up. Economy gains can be in the region of 3-8% depending on how throttled you are.
There are a few factors in determing the size of the throttle body. One is air leakage- a bigger throttle size area for a given engine size will give more leakage and you have to be sure you can throttle your engine enough to get down to your target idle speed.
Then there is throttle progression: the rate of change or area of this throttle butterfly as you "tip-in" will dictate this- although this can be altered by the the mechanics of the throttle-linkage fulcrum arrangement. SOme cars have a smaller throttle that opens first followed by a bigger one (vauxhall XE Astra GTE type engine or BMW 318is engine) which ensures a more progressive response at small openings. If the single throttle is also being used for idle speed control (typically on electronic throttle arrangements) then a larger size of throttle will make it harder to control the idle speed.
And obviously for performance you want to make your throttle big enough such that it is no longer a restriction.
These are the benefits that I see (There's probably some overlap (no pun intended) with what GreenV8S said):
They allow design freedom for a bigger volume upstream of the port throttles. This allows, for example- a bigger plenum; With a conventional throttle (located perhaps downstream of the MAF) throttle response must be taken into account- and this means there is a upper constraint on how large you can make the volume from back of intake valve to the throttle butterfly itself. However a bigger plenum volume can liberate more top end power (BMW M3 straight six has a 14 litre plenum volume- compare that to my M20 BMW engine which has been forced to a smaller 1.75 litre plenum volume). Individual port throttles allow you to effectively remove the plenum volume constraint upstream of the port throttles so you could potentially reap power if designed correctly.
Port throttles allow wilder cams with more overlap to be used without detriment to idle speed quality. This is because there is less tendency of EGR or Exhaust gas residual to flow back up the -now smaller- volume upstream of the closed port throttles. Typically there is less residual content in the cylinder during part load operation. Too much EGR content is what effects combustion stability at idle speed and otherwise- worse still is uneven EGR distribution cylinder to cylinder but that's another story.
Port throttles can give a very quick throttle response: as touched upon already the reduced volume between the back of valve to throttle means that the engine will see the intended driver throttle angle change much quicker. The trick of calibrating fueling on a MAF based system is to match the quick "air charge" response. In steady state operation, the fuel puddle mass is filled at the same rate that it drains into the cylinder, but during transients (changing throttle position) the air has no problem with acheiving a newer steady state while the fuel puddle takes much longer. The result is lean mixture while transferring to acceleration and rich mixture while transferring to deceleration. Reducing this puddle volume helps this, and with large intake air/plenum volume from the injectors to the air flow sensor there is some compensation but not enough that no acceleration enrichment is necessary with a port throttled system. With this mind it's not hard to see why with a throttle angle system is used to calibrate fueling with port throttles or a manifold pressure sensor.
It's been proven in several experiments that port throttles benefits fuel economy at part load throttled condition over a conventional single throttle engine. Bascially there is quciker pressure recovery in the port. The theory is that the pumping loop in a PV-diagram is cut or "lopped off" thus saving work. There are papers by Roger Duckworth and others with experimental data backing this up. Economy gains can be in the region of 3-8% depending on how throttled you are.
There are a few factors in determing the size of the throttle body. One is air leakage- a bigger throttle size area for a given engine size will give more leakage and you have to be sure you can throttle your engine enough to get down to your target idle speed.
Then there is throttle progression: the rate of change or area of this throttle butterfly as you "tip-in" will dictate this- although this can be altered by the the mechanics of the throttle-linkage fulcrum arrangement. SOme cars have a smaller throttle that opens first followed by a bigger one (vauxhall XE Astra GTE type engine or BMW 318is engine) which ensures a more progressive response at small openings. If the single throttle is also being used for idle speed control (typically on electronic throttle arrangements) then a larger size of throttle will make it harder to control the idle speed.
And obviously for performance you want to make your throttle big enough such that it is no longer a restriction.
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