Individual Throttle Bodies / 951
03-24-2017, 09:12 PM
#18
Rennlist Member
Very nice!!
03-25-2017, 09:56 AM
#19
Nordschleife Master
ITB logic
I think that ITB system really shines on a car that has a lot of camshaft overlap and that needs to also be driven at low rpms and low loads. I believe that the reason for this is that with a long valve overlap period, the engine tends to be unstable at part throttle and low rpms. Higher cylinder filling gives a stronger exhaust suction pulse, which then in turn gives a higher cylinder filling, etc. When the cylinder filling is poor, the spiral runs in the other direction. With a single-throttle plenum manifold that has a large plenum, this process is basically unchecked as there is a lot of air in the plenum for each cylinder to draw from. ITB system with the throttle close to the intake valve stabilizes this process, as with the throttles just cracked open there's limited volume of air between the throttle and intake valve. If the camshafts don't have much overlap, the benefits of the ITB system are in my opinion limited. If the camshafts have big overlap, the ITB system can produce a car that has nice manners when with the same camshafts the car would be unstreetable with a single-throttle plenum manifold.
This begs the question when would a turbo engine benefit from ITB system. By my logic, when the turbo engine has big camshafts with large valve overlap period. Such engines are relatively rare. For one, the exhaust has to work perfectly for big overlap to make sense. The 180-degree exhaust blowdown interference needs to be solved one way or another. The turbo-back exhaust needs to have a very low restriction. And the turbine needs to be sized in a way that the exhaust-to-intake (cycle average) pressure ratio is close to 1:1. If the system is otherwise built like that, using high overlap camshafts makes sense and then ITB system would improve drivability.
I can't get the exhaust pulses sufficiently separated in my project (not 951), can't figure out a way, so I can't use big overlap cams, and have therefore abandoned the turbo-ITB path.
This begs the question when would a turbo engine benefit from ITB system. By my logic, when the turbo engine has big camshafts with large valve overlap period. Such engines are relatively rare. For one, the exhaust has to work perfectly for big overlap to make sense. The 180-degree exhaust blowdown interference needs to be solved one way or another. The turbo-back exhaust needs to have a very low restriction. And the turbine needs to be sized in a way that the exhaust-to-intake (cycle average) pressure ratio is close to 1:1. If the system is otherwise built like that, using high overlap camshafts makes sense and then ITB system would improve drivability.
I can't get the exhaust pulses sufficiently separated in my project (not 951), can't figure out a way, so I can't use big overlap cams, and have therefore abandoned the turbo-ITB path.
03-25-2017, 01:23 PM
#20
Wouldn't the twin scroll turbo and exhaust setup like Patrick has help with the exhaust pulse issue you are referring too?
I think that ITB system really shines on a car that has a lot of camshaft overlap and that needs to also be driven at low rpms and low loads. I believe that the reason for this is that with a long valve overlap period, the engine tends to be unstable at part throttle and low rpms. Higher cylinder filling gives a stronger exhaust suction pulse, which then in turn gives a higher cylinder filling, etc. When the cylinder filling is poor, the spiral runs in the other direction. With a single-throttle plenum manifold that has a large plenum, this process is basically unchecked as there is a lot of air in the plenum for each cylinder to draw from. ITB system with the throttle close to the intake valve stabilizes this process, as with the throttles just cracked open there's limited volume of air between the throttle and intake valve. If the camshafts don't have much overlap, the benefits of the ITB system are in my opinion limited. If the camshafts have big overlap, the ITB system can produce a car that has nice manners when with the same camshafts the car would be unstreetable with a single-throttle plenum manifold.
This begs the question when would a turbo engine benefit from ITB system. By my logic, when the turbo engine has big camshafts with large valve overlap period. Such engines are relatively rare. For one, the exhaust has to work perfectly for big overlap to make sense. The 180-degree exhaust blowdown interference needs to be solved one way or another. The turbo-back exhaust needs to have a very low restriction. And the turbine needs to be sized in a way that the exhaust-to-intake (cycle average) pressure ratio is close to 1:1. If the system is otherwise built like that, using high overlap camshafts makes sense and then ITB system would improve drivability.
I can't get the exhaust pulses sufficiently separated in my project (not 951), can't figure out a way, so I can't use big overlap cams, and have therefore abandoned the turbo-ITB path.
This begs the question when would a turbo engine benefit from ITB system. By my logic, when the turbo engine has big camshafts with large valve overlap period. Such engines are relatively rare. For one, the exhaust has to work perfectly for big overlap to make sense. The 180-degree exhaust blowdown interference needs to be solved one way or another. The turbo-back exhaust needs to have a very low restriction. And the turbine needs to be sized in a way that the exhaust-to-intake (cycle average) pressure ratio is close to 1:1. If the system is otherwise built like that, using high overlap camshafts makes sense and then ITB system would improve drivability.
I can't get the exhaust pulses sufficiently separated in my project (not 951), can't figure out a way, so I can't use big overlap cams, and have therefore abandoned the turbo-ITB path.
03-25-2017, 01:38 PM
#21
Three Wheelin'
That ITB+turbo (or just ITB) question arises from time to time and there are always people who don't think that they are worth the effort and there are always people who swear by them.
IMO (after owning both, turbo+ITB and turbo+single TB), I would probably not build a track car without ones but for a street car may not be worth the effort though the engine behaviour changes so drastically that few people who have driven my race-car said they had hard time believing that it was nearly 40 year old 2valve engine that was originally designed as a diesel. Yet I use factory rods, pistons and crank which are heavy.
Theory is one thing, reality is another. I would just suggest to test-drive someones car and then decide if it is worth it or not.
IMO (after owning both, turbo+ITB and turbo+single TB), I would probably not build a track car without ones but for a street car may not be worth the effort though the engine behaviour changes so drastically that few people who have driven my race-car said they had hard time believing that it was nearly 40 year old 2valve engine that was originally designed as a diesel. Yet I use factory rods, pistons and crank which are heavy.
Theory is one thing, reality is another. I would just suggest to test-drive someones car and then decide if it is worth it or not.
03-25-2017, 02:01 PM
#22
Rennlist Member
John Milledge produced a carbon fiber intake with individual throttle bodies. It is some the best engine **** out there. He only produce a few due to the expense. From my understanding the short run is not necessarily beneficial for engines that use MAP to tune their engines. Then again I take my car down to have it tuned as I have no skill at tuning.
03-25-2017, 02:01 PM
#23
Nordschleife Master
Yes it would. Also, the 951 does have the room to get some pulse separation from just the runner length. So the turbo-ITB may in fact be well suited for the intake-side turbo placement of the 951.
03-25-2017, 02:03 PM
#24
Nordschleife Master
That ITB+turbo (or just ITB) question arises from time to time and there are always people who don't think that they are worth the effort and there are always people who swear by them.
IMO (after owning both, turbo+ITB and turbo+single TB), I would probably not build a track car without ones but for a street car may not be worth the effort though the engine behaviour changes so drastically that few people who have driven my race-car said they had hard time believing that it was nearly 40 year old 2valve engine that was originally designed as a diesel. Yet I use factory rods, pistons and crank which are heavy.
Theory is one thing, reality is another. I would just suggest to test-drive someones car and then decide if it is worth it or not.
IMO (after owning both, turbo+ITB and turbo+single TB), I would probably not build a track car without ones but for a street car may not be worth the effort though the engine behaviour changes so drastically that few people who have driven my race-car said they had hard time believing that it was nearly 40 year old 2valve engine that was originally designed as a diesel. Yet I use factory rods, pistons and crank which are heavy.
Theory is one thing, reality is another. I would just suggest to test-drive someones car and then decide if it is worth it or not.
03-25-2017, 05:43 PM
#26
Rennlist Member
That ITB+turbo (or just ITB) question arises from time to time and there are always people who don't think that they are worth the effort and there are always people who swear by them.
IMO (after owning both, turbo+ITB and turbo+single TB), I would probably not build a track car without ones but for a street car may not be worth the effort though the engine behaviour changes so drastically that few people who have driven my race-car said they had hard time believing that it was nearly 40 year old 2valve engine that was originally designed as a diesel. Yet I use factory rods, pistons and crank which are heavy.
Theory is one thing, reality is another. I would just suggest to test-drive someones car and then decide if it is worth it or not.
IMO (after owning both, turbo+ITB and turbo+single TB), I would probably not build a track car without ones but for a street car may not be worth the effort though the engine behaviour changes so drastically that few people who have driven my race-car said they had hard time believing that it was nearly 40 year old 2valve engine that was originally designed as a diesel. Yet I use factory rods, pistons and crank which are heavy.
Theory is one thing, reality is another. I would just suggest to test-drive someones car and then decide if it is worth it or not.
03-26-2017, 04:38 AM
#27
Three Wheelin'
Thank you Michael, much appreciated!
03-26-2017, 02:17 PM
#28
Nordschleife Master
I use standard 924 Turbo camshaft, here's info from Dan's site: http://garage.ideola.com/top-DL-Inte...4Profiles.html
Just to be clear, i'm not trying to pretend to be an expert. Just trying to learn. Obviously, hearing first hand from someone actually running a turbo-ITB engine is a great learning opportunity for me. I've spoken to Hayabusa turbo builders and they all vouch for it. My previous understanding was that ITB system really starts helping and earning some return on the higher cost and complexity when the camshaft overlap gets big. Yet, your cams are quite conservative for a two-valve engine at the following overlap:
At .015" hot lash:
Intake opens 22.0° BTDC
Exhaust closes 18.0° ATDC
Do you have some huge valves and very high flow in your head?
More importantly, for the benefit of my education, why do you think ITBs make such a big difference in your engine?
03-26-2017, 03:19 PM
#29
Rennlist Member
Certainly no expert in this area but I have logged pressure at different places in the intake track and when the TB is opened/closed a lot of dynamic activity takes place in the plenum and in fact the whole intake track. Pressure appears to bounce all over the place do to pressure waves. After doing this logging it became clear why the factory chose the location it did for the wastegate input. It seems to me that with ITB's the plenum would be in a much more stable state ready to deliver air more efficiently leading to improved responsiveness.
03-26-2017, 03:40 PM
#30
Nordschleife Master
Certainly no expert in this area but I have logged pressure at different places in the intake track and when the TB is opened/closed a lot of dynamic activity takes place in the plenum and in fact the whole intake track. Pressure appears to bounce all over the place. After doing this logging is was clear to me why the factory chose the location it did for the wastegate input. It seems to me that with ITB's the plenum would be in a much more stable state ready to deliver air more efficiently leading to improved responsiveness.
Out of curiosity, what system are you using to log the pressure at high enough sampling rate to get the resolution there? I've got one of those full TFX single cylinder systems, but we haven't yet installed it because it's much harder than it looks to mount on the 928 engine while not burning the sensors and still getting meaningful results. I am guessing just getting one of those bad boys reading from the plenum would be much easier.
Again, not telling anyone how it is, just asking questions about how ITBs work.