Plenum v Throttle bodies
Discussion
Please can someone explain to me in layman’s terms the advantage of using throttle bodies over a plenum ?
As I understand it running anything other than a mild cam with a plenum runs into difficulties at light throttle openings because dirty air is forced back into the plenum contaminating the next induction cycle. With individual throttle bodies the only cylinder it can contaminate is itself, so there is one obvious benefit allowing you to run a wilder cam and still have some drivability. Presumably it is also easier to accurately control the amount of fuel for each cycle and throttle response is quicker because the butterfly is not some distant point downstream of the plenum.
Are they the main points or am I missing something else ?
As I understand it running anything other than a mild cam with a plenum runs into difficulties at light throttle openings because dirty air is forced back into the plenum contaminating the next induction cycle. With individual throttle bodies the only cylinder it can contaminate is itself, so there is one obvious benefit allowing you to run a wilder cam and still have some drivability. Presumably it is also easier to accurately control the amount of fuel for each cycle and throttle response is quicker because the butterfly is not some distant point downstream of the plenum.
Are they the main points or am I missing something else ?
steve-V8s said:
Are they the main points or am I missing something else?
No, I think you have got the main points. I would also add that having the throttles closer to the valves also reduces the volume under depression which slightly reduces the amount of exhaust gas reversal (as well as preventing sharing across cylinders) and the reduced volume also means that some transient effects such as wall wetting will be reduced.steve-V8s said:
Please can someone explain to me in layman’s terms the advantage of using throttle bodies over a plenum ?
As I understand it running anything other than a mild cam with a plenum runs into difficulties at light throttle openings because dirty air is forced back into the plenum contaminating the next induction cycle. With individual throttle bodies the only cylinder it can contaminate is itself, so there is one obvious benefit allowing you to run a wilder cam and still have some drivability.
I can't see any logic in that argument. Firstly it's low rpm not light throttle openings that's the issue but that aside. What do you mean by 'dirty air' if it indeed is that? How much of it might backflow? What's the volume of each runner compared to the cylinder? Even if any got into the plenum what difference does it make which cylinder it goes back into?As I understand it running anything other than a mild cam with a plenum runs into difficulties at light throttle openings because dirty air is forced back into the plenum contaminating the next induction cycle. With individual throttle bodies the only cylinder it can contaminate is itself, so there is one obvious benefit allowing you to run a wilder cam and still have some drivability.
steve-V8s said:
Presumably it is also easier to accurately control the amount of fuel for each cycle and throttle response is quicker because the butterfly is not some distant point downstream of the plenum.
Are they the main points or am I missing something else ?
Why would it be easier to control the fuel with more butterflies? How does the control system know about them and why would it care?Are they the main points or am I missing something else ?
The butterfly is upstream not downstream of the plenum, however, what difference if any does it make to the engine if the air that has to accelerate into it on each cycle is upstream or downstream of the butterfly controlling each cylinder?
Think about the points you've made again and I'm happy to discuss them. The main difference I'm aware of between TBs and plenums is the pulse tuning but it's possible to get plenums working very efficiently if everything is the right length and diameter.
Dave Baker
I think with a plenium exhaust gasses can sucked out of one intake and into another cylinder.. reducing performance. This is only a problem with a cam with a big overlap.
I think throttle bodies will be easyer to tune as they are all seperate.
Its not easy to modify a plenium, it has to be properly designed in the first place. Where as throttle bodies can be altered more easily.
Although most of the cars ive seen running throttle bodies are not propperly tuned and much of the potential benefit is lost.
I think throttle bodies will be easyer to tune as they are all seperate.
Its not easy to modify a plenium, it has to be properly designed in the first place. Where as throttle bodies can be altered more easily.
Although most of the cars ive seen running throttle bodies are not propperly tuned and much of the potential benefit is lost.
I think the biggest advantage to a set of TB's is that invariably the connecting manifold to suit the TB's is designed for flow and not to be easy to cast / fit under the bonnet etc.
If you look at the RV8 plenum and intake manifold you see that it changes port section often constricting then expanding and with bends more than you'd like .
Whereas with a set of TB's on a well thought out manifold you should be able to see the back of the inlet valve
Matt
If you look at the RV8 plenum and intake manifold you see that it changes port section often constricting then expanding and with bends more than you'd like .
Whereas with a set of TB's on a well thought out manifold you should be able to see the back of the inlet valve
Matt
We went from a single Plenum to TB's on the 5ltr RV8 in the tuscan racer.
If you look at the power graph we actually lost a little bit of tourque in the mid range, but made slightly more power higher up that didnt drop off when we got to our rev limit.
With a stel crank and a higher rev limit 400 bhp looks easily in sight. something that would never be achieved with the plenum.
In terms of driving and lap times the car was transformed... The throttle response was much much sharper and the car more eager to bring the revs out, with a corresponding drop in lap times.. not something you can see from the power graphs...
G
If you look at the power graph we actually lost a little bit of tourque in the mid range, but made slightly more power higher up that didnt drop off when we got to our rev limit.
With a stel crank and a higher rev limit 400 bhp looks easily in sight. something that would never be achieved with the plenum.
In terms of driving and lap times the car was transformed... The throttle response was much much sharper and the car more eager to bring the revs out, with a corresponding drop in lap times.. not something you can see from the power graphs...
G
Graham said:
If you look at the power graph we actually lost a little bit of tourque in the mid range, but made slightly more power higher up that didnt drop off when we got to our rev limit.
What else changed? Did you end up with identical intake runner lengths with the TBs? Did you change the cam? Did you optimise the fuelling to suit the new air flow characteristics?Graham said:
With a steel crank and a higher rev limit 400 bhp looks easily in sight. something that would never be achieved with the plenum.
I tend to agree with Matt, the problem is more with the inlet manifold than the plenum itself. It looked to me like even LR were on the same track in redesigning the Thor inlet manifold. (3,4,5 & 6 crossover and 1,2,7&8 are direct - but most people don't seem to look below the curly plenum & 'trumpet' base difference..) IMHO we have seen historically that the key to big gains are shorter equal inlet tracts. I fail to see how the engine can work efficiently when there is such a large discrepancy of flow across the 8 ports due to the crass efi inlet manifold, your mapping at best has to be an average of what is in the chamber?.GreenV8S said:
Graham said:
If you look at the power graph we actually lost a little bit of tourque in the mid range, but made slightly more power higher up that didnt drop off when we got to our rev limit.
What else changed? Did you end up with identical intake runner lengths with the TBs? Did you change the cam? Did you optimise the fuelling to suit the new air flow characteristics?engine remapped on roller by same person that did the previous map
That would be me then. I can confirm that the car was much more civilised on the TBs than on the plenum .. the advantage then being that you could if wanted run a more radical cam without ruining the low speed manners completely. Plenums have their place, but not downstream of throttle on big cammed RV8s. The difference in driving terms on a 360plus hp RV8 between plenum and TBs is quite significant, for whatever reasons that may be.
Graham said:
JE inlet manifold, and 8* je throttle bodies, intake lengths matched,everything else the same,
In that case if the overall intake length was kept the same it's puzzling that you lost mid range power. Possibly indicating that there was a benefit from the plenum/trumpet interaction which was lost after the change?GreenV8S said:
Graham said:
JE inlet manifold, and 8* je throttle bodies, intake lengths matched,everything else the same,
In that case if the overall intake length was kept the same it's puzzling that you lost mid range power. Possibly indicating that there was a benefit from the plenum/trumpet interaction which was lost after the change?
GreenV8S said:
According to this theory, if some brave soul with a plenum setup was to take the top off while it was on the rollers, we'd expect to see a similar loss of mid-range torque?
Quite possibly. I'm only expressing my professional opinion though (which doesn't extend to dynos). It's more likely though, that the engine management (assuming it's factory) would s
t itself and go into limp home mode until it had adapted to the new found air. Plenums are probably like exhaust headers in that they give a greater average in an 'as is' state. Once you tune the lengths ie TB's you'll get higher figure but only in one place where you only rev to some of the time. Ideal for a race car but not so good for a street car where averages apply.
Intake tuning is a very knotty subject as you get pressure waves and reflections occurring off everything inside the airbox / plenum etc. So at some points in the rev range you're getting a positive reflection occurring at the right time to help mid-range and everywhere else in the rev range its destructive.
The only thing I can say which works repeatably is to make the airbox big.
Matt
The only thing I can say which works repeatably is to make the airbox big.
Matt
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