Twinscrew vs centrifugal
11-26-2005, 02:13 PM
#46
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Carl has proposed a shortcoming of twinscrew aftermarket systems (I hope he has sent this to Kenne Bell too!) having to do with a lack of runners keeping pulses separated. With forced induction of any kind, I don't see how the stock tubes are going to change where the air goes. Under pressure, it's going wherever it can go, and a couple of inches of open ended tube isn't going to impede flow unless the tube presents tremendous resistance (i.e., a tiny orifice). The air is no longer being sucked from a low-pressure area open to the atmosphere. The greatest resistance is at the valve opening and everything else becomes rather insignificant. The concept of shock wave affecting air distribution may be true but not relevant. Has anyone ever seen this issue documented at all with SC systems before?
Before everyone rips out their twinscrews and throws them away or cancels their orders for DR's system and switches to Carl's, it would be worth checking the hypothesis. As a start, I'm waiting to hear from Tony, as Carl says Tony has direct data and experience with this. Does anyone else? Until then, it's just good competitive marketing - FUD, as we say in the business.
Oh, Ern, my SC is remarkably unsexy. Tony looked at it one time and said something like, "Well, I guess it gets the job done."
Before everyone rips out their twinscrews and throws them away or cancels their orders for DR's system and switches to Carl's, it would be worth checking the hypothesis. As a start, I'm waiting to hear from Tony, as Carl says Tony has direct data and experience with this. Does anyone else? Until then, it's just good competitive marketing - FUD, as we say in the business.
Oh, Ern, my SC is remarkably unsexy. Tony looked at it one time and said something like, "Well, I guess it gets the job done."
Last edited by Bill Ball; 11-26-2005 at 09:57 PM.
11-26-2005, 02:28 PM
#47
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Just to keep on whipping the dead equine, primarily because I'm bored...
I don't think that harmonic intakes [my term] were implemented on street cars for the purpose of increased power. They don't really increase power per se- they increase torque. The question that occurs when you open your hood and see the beautifully formed spider, the organ pipes of the '85-'86 cars, or the muscular look of the S4 and up intakes is "what speed does this increased torque occur at".
My understanding is that the harmonic intakes can only increase torque within a fairly narrow RPM band. The runners are specifically sized in width and length so that the reversing pressure wave reaches the intake valve at the precise time it opens- and this can only occur if the air in the tubes and by extension, the speed of the engine, is correct.
-In my model airplane days, I used to compete in pylon racing, flying the little planes around three poles [and usually into the ground~]. I had a special optical tachometer that I held in front of the propellor while I adjusted the mixture....and the length of the exhaust pipe. It was a "tuned pipe", and the length had to be exactly correct or the scavenge effect wouldn't take place. This was an engine with exactly two speeds- zero or maximum. That pipe had to be adjusted to within a few millimeters! If it were off by as little as a quarter of an inch I would lose about 1000 rpm with the throttle wide open. That engine only made maximum power at one RPM.
Same can be seen with the spider intakes on the earlier cars, though not quite as severly. The torque peaks of the US two-valve engines versus the engine in my S2 are at completely different places- this is because the US intake runners are very narrow and as such the harmonic effect occurs at a much lower speed. US drivers prefer torque, and this is what this intake gives. Europeans are much more likely to use the upper RPM reaches in cruise and as such the S2 cars are tuned for higher speeds. Move the torque peak to a higher RPM and you increase peak horsepower.
-Someone at Porsche in the early 1980's got the bright idea to build a "mechanical" intake. They discovered that two separate four-cylinder manifolds actually breathed quite a bit better at low engine speeds than the all-from-one spider, but noted that this design couldn't flow properly [harmonics within the body of the spider apparently help] at higher speeds. Thus the nefarious "flappy" was born. The flappy intake seems to me like it is really an attempt at improving low RPM torque more than an attempt to make more power. I suspect if you were to "hog out" that intake and remove the flap all together, you'd see a noteable increase in power in concert with wilder cams. But you'd lose a LOT of torque at lower speeds.
N!
I don't think that harmonic intakes [my term] were implemented on street cars for the purpose of increased power. They don't really increase power per se- they increase torque. The question that occurs when you open your hood and see the beautifully formed spider, the organ pipes of the '85-'86 cars, or the muscular look of the S4 and up intakes is "what speed does this increased torque occur at".
My understanding is that the harmonic intakes can only increase torque within a fairly narrow RPM band. The runners are specifically sized in width and length so that the reversing pressure wave reaches the intake valve at the precise time it opens- and this can only occur if the air in the tubes and by extension, the speed of the engine, is correct.
-In my model airplane days, I used to compete in pylon racing, flying the little planes around three poles [and usually into the ground~]. I had a special optical tachometer that I held in front of the propellor while I adjusted the mixture....and the length of the exhaust pipe. It was a "tuned pipe", and the length had to be exactly correct or the scavenge effect wouldn't take place. This was an engine with exactly two speeds- zero or maximum. That pipe had to be adjusted to within a few millimeters! If it were off by as little as a quarter of an inch I would lose about 1000 rpm with the throttle wide open. That engine only made maximum power at one RPM.
Same can be seen with the spider intakes on the earlier cars, though not quite as severly. The torque peaks of the US two-valve engines versus the engine in my S2 are at completely different places- this is because the US intake runners are very narrow and as such the harmonic effect occurs at a much lower speed. US drivers prefer torque, and this is what this intake gives. Europeans are much more likely to use the upper RPM reaches in cruise and as such the S2 cars are tuned for higher speeds. Move the torque peak to a higher RPM and you increase peak horsepower.
-Someone at Porsche in the early 1980's got the bright idea to build a "mechanical" intake. They discovered that two separate four-cylinder manifolds actually breathed quite a bit better at low engine speeds than the all-from-one spider, but noted that this design couldn't flow properly [harmonics within the body of the spider apparently help] at higher speeds. Thus the nefarious "flappy" was born. The flappy intake seems to me like it is really an attempt at improving low RPM torque more than an attempt to make more power. I suspect if you were to "hog out" that intake and remove the flap all together, you'd see a noteable increase in power in concert with wilder cams. But you'd lose a LOT of torque at lower speeds.
N!
11-26-2005, 02:34 PM
#48
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I figured that the longer 16V intake tubes were designed so you could more easily remove the spark plugs. 
I would like to see a replacement 16V plenum, with internal low profile bell mouths, and a front mounted dual butterfly. Just have to deal with the oil tower, coolant crossover, fuel rail...
I would like to see a replacement 16V plenum, with internal low profile bell mouths, and a front mounted dual butterfly. Just have to deal with the oil tower, coolant crossover, fuel rail...
11-26-2005, 02:43 PM
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Oh, Ern, my SC is remarkably unsexy. Tony looked at it one time and said something like, "Well, I guess it gets the job done."
11-26-2005, 03:19 PM
#50
Three Wheelin'
Originally Posted by Carl Fausett
<snip>
The Porsche 928 further amplifies this problem because of their peculiar firing order, firing two cylinders on the same bank in succession. Look at the firing order that the 928 has; 1-3-7-2-6-5-4-8, all the cylinders are nicely staggered away from each other until we get to #6 and #5.
Here, we fire 2 cylinders back-to-back that are right next to each other. If you have an open tank manifold with no intake runners, it is more than probable that the pressure wave coming out of the head for cylinder number 5 will interfere with cylinder number 6 getting the correct charge.
The results will be some cylinders that run too rich at certain RPM’s and yet run too lean at a slightly different RPM.
The motor with no intake runners and a massive single plenum may appear to do very well at WOT but, in fact, even then some cylinders will be too lean while neighboring cylinders will be too rich. The O2 sensor installed in the collector will not show it as it is measuring combined gases at that point.
But the Twin-Screw works, you say. Yes, they do. They do start, run and drive.
A quick ride in Tony’s wonderful twin-screw 928 or one of the Jag-Eaton 928’s definitely shows that they do.
But, consider how deep Tony went into modern electronic engine management and that allows him to adjust and tune his injectors singly. The advent of modern electronic fuel systems has made it possible to tune out a lot of problems and return the car’s drive-ability. Tony is an exceptional home-mechanic and has talents (and patience) that not all 928 owners have.
Still, Tony’s best HP and torque numbers are not as high as Tim Murphy’s best HP and torque numbers – (meaning absolutely no disrespect to Tony and his awesome installation) – but I suspect that is because Tim’s centrifugal system is benefiting from Porsche-tuned intake runners and Tony’s twin-screw system is not.
I have noticed most of the Jag-Eaton twin screw’s are running 5 psi and doing well. However, they may find that at 8 psi they cannot fuel it or tune it correctly without going to all the trouble that Tony went to. An open-box intake plenum like theirs without any runners will behave wildly different at different pressure and CFM points.
Remember that most of us are taking our air-fuel measurements at the collector – and if the air/fuel ratio is right, we think we’re good. But, a lean cylinder (like number 6 in twin-screw) will be camouflaged out if the other cylinders on that bank are rich enough. If I recall, Tony installed 8 individual EGT sensors on his 928 to specifically measure this – I just wonder how many new owners are willing to go to these lengths.
<snip
The Porsche 928 further amplifies this problem because of their peculiar firing order, firing two cylinders on the same bank in succession. Look at the firing order that the 928 has; 1-3-7-2-6-5-4-8, all the cylinders are nicely staggered away from each other until we get to #6 and #5.
Here, we fire 2 cylinders back-to-back that are right next to each other. If you have an open tank manifold with no intake runners, it is more than probable that the pressure wave coming out of the head for cylinder number 5 will interfere with cylinder number 6 getting the correct charge.
The results will be some cylinders that run too rich at certain RPM’s and yet run too lean at a slightly different RPM.
The motor with no intake runners and a massive single plenum may appear to do very well at WOT but, in fact, even then some cylinders will be too lean while neighboring cylinders will be too rich. The O2 sensor installed in the collector will not show it as it is measuring combined gases at that point.
But the Twin-Screw works, you say. Yes, they do. They do start, run and drive.
A quick ride in Tony’s wonderful twin-screw 928 or one of the Jag-Eaton 928’s definitely shows that they do.
But, consider how deep Tony went into modern electronic engine management and that allows him to adjust and tune his injectors singly. The advent of modern electronic fuel systems has made it possible to tune out a lot of problems and return the car’s drive-ability. Tony is an exceptional home-mechanic and has talents (and patience) that not all 928 owners have.
Still, Tony’s best HP and torque numbers are not as high as Tim Murphy’s best HP and torque numbers – (meaning absolutely no disrespect to Tony and his awesome installation) – but I suspect that is because Tim’s centrifugal system is benefiting from Porsche-tuned intake runners and Tony’s twin-screw system is not.
I have noticed most of the Jag-Eaton twin screw’s are running 5 psi and doing well. However, they may find that at 8 psi they cannot fuel it or tune it correctly without going to all the trouble that Tony went to. An open-box intake plenum like theirs without any runners will behave wildly different at different pressure and CFM points.
Remember that most of us are taking our air-fuel measurements at the collector – and if the air/fuel ratio is right, we think we’re good. But, a lean cylinder (like number 6 in twin-screw) will be camouflaged out if the other cylinders on that bank are rich enough. If I recall, Tony installed 8 individual EGT sensors on his 928 to specifically measure this – I just wonder how many new owners are willing to go to these lengths.
<snip
11-26-2005, 03:26 PM
#51
Developer
In aspirated engines there is a vacuum on the intake side ,as the driving force is the positive exhaust pressure. In the boosted engines you overcome this with positive pressure, and any deficits are overcome by brute force as opposed to efficiency of design.
that it is only partially right, and completely forgets the pressure pulses coming back from the valves that I wrote about above.
11-26-2005, 03:29 PM
#52
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Speking of OEM intakes and TS, does anyone have a picture or cutaway of the SLR, GT, Cobra or any other OEM TS intake manifold? I'm curious to see how they differ from the 928 designs being used.
11-26-2005, 03:36 PM
#53
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Good point....most of the OEM (think Benz) TS are intercooled and must run some type of similar intake manifold (since they are all V8's) I also would like to see a closeup of how they make the manifold!
Good point....most of the OEM (think Benz) TS are intercooled and must run some type of similar intake manifold (since they are all V8's) I also would like to see a closeup of how they make the manifold!
11-26-2005, 03:44 PM
#54
Developer
As long as my axe is sharp, I'll give you another urban legend I am tired of hearing....
"If I want more boost, I'll just put on a smaller pulley"
Not so much if you're a twin-screw install. The specs I have read on the Eaton say that the max RPM it is designed for is 10,000 RPM at the impeller. They are meant for bottom-end, not top.
Think: a 6" crankshaft pulley diameter, a 3" supercharger pulley, 6,000 RPM on the tach = 12,000 RPM at the screw. IF your twin screw has a 10,000 RPM design, then you are over-clocking the twin-screw, and cavitation of the rotors plus rapid bearing wear and premature failure will be the result.
If your twin-screw pulley is smaller than 3", or your crank pulley is larger than 6" - the over-clocking problem is even worse.
Nobody gets a free lunch - Centrifugals also have their limits. They just are designed for the other end. They start later, but can go higher. For example: The Powerdyne in standard build can spin to 30,000 rpm before failure, and the Powerdyne with a racing rebuild can go to 60,000 rpm, and the Vortec gear driven can go to 46,000 rpm.
Of course - NOBODY gets to just add a smaller pulley and go. Usually you need to now adjust your FMU, tweak your pressures, and possibly go to the next level on your injectors. Sometimes you also have to change your fuel pump (or add another one) or even replace your 7.5 mm fuel lines on your 928 CIS with larger ones.
Its just another supercharger urban legend.
"If I want more boost, I'll just put on a smaller pulley"
Not so much if you're a twin-screw install. The specs I have read on the Eaton say that the max RPM it is designed for is 10,000 RPM at the impeller. They are meant for bottom-end, not top.
Think: a 6" crankshaft pulley diameter, a 3" supercharger pulley, 6,000 RPM on the tach = 12,000 RPM at the screw. IF your twin screw has a 10,000 RPM design, then you are over-clocking the twin-screw, and cavitation of the rotors plus rapid bearing wear and premature failure will be the result.
If your twin-screw pulley is smaller than 3", or your crank pulley is larger than 6" - the over-clocking problem is even worse.
Nobody gets a free lunch - Centrifugals also have their limits. They just are designed for the other end. They start later, but can go higher. For example: The Powerdyne in standard build can spin to 30,000 rpm before failure, and the Powerdyne with a racing rebuild can go to 60,000 rpm, and the Vortec gear driven can go to 46,000 rpm.
Of course - NOBODY gets to just add a smaller pulley and go. Usually you need to now adjust your FMU, tweak your pressures, and possibly go to the next level on your injectors. Sometimes you also have to change your fuel pump (or add another one) or even replace your 7.5 mm fuel lines on your 928 CIS with larger ones.
Its just another supercharger urban legend.
11-26-2005, 04:58 PM
#55
Drifting
We are running a Vortech V1-T at 55000rpm (it's max!) on a basically stock 5 litre engine...
Without moving up to a larger capacity supercharger, we are at the limit. We encountered the same thing on the Vortech V2-S which only developed 7psi after the intercooler on the same engine. It's harder to develop boost at altitude we are finding.
Without moving up to a larger capacity supercharger, we are at the limit. We encountered the same thing on the Vortech V2-S which only developed 7psi after the intercooler on the same engine. It's harder to develop boost at altitude we are finding.
11-26-2005, 04:58 PM
#56
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Carl has a very good point with respect to the intake runners. It is one of the main reasons Herr Kuhns' twin turbo setups have such magnificient torque bands. Also take a look at the torque numbers on Marks' turbo setup, once again quite beautiful torque numbers. My twinscrew had a nice torque band that was flat but it was considerable less than what Mark is getting from his turbo at near the same boost.
11-26-2005, 05:17 PM
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Originally Posted by Bill Ball
Very interesting. If the whole scenario changes with higher pressures, then either system needs tuning. The Porsche manifold was not designed with boost in mind.
I knew Tony was collecting a lot of data but I didn't know he was tuning per cylinder.
Tony, have you found issues with lean/rich cylinders that you have had to tune individually?
I knew Tony was collecting a lot of data but I didn't know he was tuning per cylinder.
Tony, have you found issues with lean/rich cylinders that you have had to tune individually?
But, consider how deep Tony went into modern electronic engine management and that allows him to adjust and tune his injectors singly.
If I recall, Tony installed 8 individual EGT sensors on his 928 to specifically measure this – I just wonder how many new owners are willing to go to these lengths.
Im not "tuning" at all.
I havent turned a screw
on my engine in nearly 10 months...then it was to only install my 24lbs injectors and seal any leaks i had from the original install. I havent done any "tuning" whatsover...messed with my fuel prssures abit +- a few psi. Thats it. Im just educating myself on what is making it tick with all the data logging. I have stock managment and a Boost sensitive regualtor. No different than what you have running Bill. Not sure how all that came out?
No egts.
no individual tuning of the injectors.
The injectors all fire at the same time with X amount of fuel pressure behind them and X+Y pressure under boost.
No Piggy back....nunca...nada..nyet..
And my 403rwhp while pretty darn good isnt the highest HP level yet on the 928 TS set up...far from it. Jim hit 465 IRRC?
Last edited by Tony; 11-26-2005 at 06:30 PM.
11-26-2005, 05:31 PM
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Originally Posted by Hacker-Pschorr
Speking of OEM intakes and TS, does anyone have a picture or cutaway of the SLR, GT, Cobra or any other OEM TS intake manifold? I'm curious to see how they differ from the 928 designs being used.
Last edited by Tony; 11-26-2005 at 06:48 PM.
11-26-2005, 05:52 PM
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Originally Posted by Carl Fausett
As long as my axe is sharp, I'll give you another urban legend I am tired of hearing....
"If I want more boost, I'll just put on a smaller pulley"
Not so much if you're a twin-screw install. The specs I have read on the Eaton say that the max RPM it is designed for is 10,000 RPM at the impeller. They are meant for bottom-end, not top.
Think: a 6" crankshaft pulley diameter, a 3" supercharger pulley, 6,000 RPM on the tach = 12,000 RPM at the screw.
IF your twin screw has a 10,000 RPM design,
then you are over-clocking the twin-screw, and cavitation of the rotors plus rapid bearing wear and premature failure will be the result.
.
"If I want more boost, I'll just put on a smaller pulley"
Not so much if you're a twin-screw install. The specs I have read on the Eaton say that the max RPM it is designed for is 10,000 RPM at the impeller. They are meant for bottom-end, not top.
Think: a 6" crankshaft pulley diameter, a 3" supercharger pulley, 6,000 RPM on the tach = 12,000 RPM at the screw.
IF your twin screw has a 10,000 RPM design,
then you are over-clocking the twin-screw, and cavitation of the rotors plus rapid bearing wear and premature failure will be the result.
.
In the 928 applications using the Autorotor we have A LONG way to go before we come close to the redline on the blower. LONG WAY!\
Last edited by Tony; 11-26-2005 at 06:12 PM.
11-26-2005, 06:14 PM
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Individual runners to the intake ports isnt that hard. Just would take some time and some talented mold making. Is it worth it on our cars...i highly doubt it.
flow chart for 422 at 2 bar. (im sure this will add some discussion) Yeah it consumes power...quiet already John.
But according to my **** it makes more than it consumes! 
And it makes it where i want it most in my 2.2 auto.
Again...IMHO, it comes down to personal choice in what you want and what you want to do and what you like the looks off. Simple really.
You can tell the population all the unhealthy stats and nutritional tidbits about big macs all day long...but they still eat them becasue they like them.
Each his own...im just adding what I hope is constructive stuff along the way.
flow chart for 422 at 2 bar. (im sure this will add some discussion) Yeah it consumes power...quiet already John.
But according to my **** it makes more than it consumes! And it makes it where i want it most in my 2.2 auto.Again...IMHO, it comes down to personal choice in what you want and what you want to do and what you like the looks off. Simple really.
You can tell the population all the unhealthy stats and nutritional tidbits about big macs all day long...but they still eat them becasue they like them.
Each his own...im just adding what I hope is constructive stuff along the way.
Last edited by Tony; 11-26-2005 at 06:49 PM.