S4 intake manifolds and side cover spacers.
07-06-2013, 09:06 AM
#1
Pro
Thread Starter
S4 intake manifolds and side cover spacers.
For some years I've been attempting to better my understanding of the going's on in intake manifolds and ports.
Along the way I've formed the view, that as lovely and tecchie as the S4/gt/gts manifold looks and putting it politely, performance wise it's is the south end of a north-bound horse.
I had thought about using spacers to give more breathing space around the intake bellmouths, but then saw the dyno results with the spacers from 928 motorsports.
Until now I could not understand the loss in low end torque. If the plenum resonance now occurred at a lower frequency surely that would just move the torque peak, not reduce it?
While seemingly unrelated, a 'whoo hoo' intake runner experience with my BMW R110S sports bike led me to this:
Underlying principle: firing order such that the 'valve closed' reflected +ve pressure wave returns to its bellmouth in time to reinforce the induction wave on the other cylinder. Bellmouth separation not greater than 0.75 tract diameter.
Lost the URL in a disk failure.
Viola! When this concept is matched with the firing order of the cylinders whose bellmouths appear in the L & R plenums, the low speed resonance mechanism becomes clear.
Also, the loss of low speed torque 'with spacers' is explained: while at first glance the close proximity of the side covers over the intakes appears to shroud the airflow around the intake bellmouths, it otherwise improves the 'pressure wave' coupling between the bellmouths, which is what produces the low-end torque.
The use of 'spacers' has two effects:
- it decreases the pressure wave coupling, but -
- it un-shrouds the bellmouths and allows better high rpm breathing.
Verified by the dyno tests:
Dyno results with 'spacers':
All I have time for at the moment.
Be back soon with more thoughts on the S4 manifold.
John C.
aka UpFixenDerPorsche.
'90 GT
'89 S4
'03 BMW R1100S
Along the way I've formed the view, that as lovely and tecchie as the S4/gt/gts manifold looks and putting it politely, performance wise it's is the south end of a north-bound horse.
I had thought about using spacers to give more breathing space around the intake bellmouths, but then saw the dyno results with the spacers from 928 motorsports.
Until now I could not understand the loss in low end torque. If the plenum resonance now occurred at a lower frequency surely that would just move the torque peak, not reduce it?
While seemingly unrelated, a 'whoo hoo' intake runner experience with my BMW R110S sports bike led me to this:
Underlying principle: firing order such that the 'valve closed' reflected +ve pressure wave returns to its bellmouth in time to reinforce the induction wave on the other cylinder. Bellmouth separation not greater than 0.75 tract diameter.
Lost the URL in a disk failure.
Viola! When this concept is matched with the firing order of the cylinders whose bellmouths appear in the L & R plenums, the low speed resonance mechanism becomes clear.
Also, the loss of low speed torque 'with spacers' is explained: while at first glance the close proximity of the side covers over the intakes appears to shroud the airflow around the intake bellmouths, it otherwise improves the 'pressure wave' coupling between the bellmouths, which is what produces the low-end torque.
The use of 'spacers' has two effects:
- it decreases the pressure wave coupling, but -
- it un-shrouds the bellmouths and allows better high rpm breathing.
Verified by the dyno tests:
Dyno results with 'spacers':
All I have time for at the moment.
Be back soon with more thoughts on the S4 manifold.
John C.
aka UpFixenDerPorsche.
'90 GT
'89 S4
'03 BMW R1100S
Last edited by UpFixenDerPorsche; 07-06-2013 at 09:15 AM. Reason: improved grammar.
07-06-2013, 01:39 PM
#2
Nordschleife Master
Your dyno test doesn't show up, the link is broken.
Any dyno test that claims increased peak hp from spacers comes from that end of the horse you referenced above. I am not hedging this statement.
The greatest impact from the plenum spacer is that additional plenum volume moves the two flappy-closed torque peaks down to a lower rpm level. One could in theory create the opposite effect by fillling the cavities behind the trumpets with epoxy.
Finally, BTW try searching for threads on the intake manifold.
Any dyno test that claims increased peak hp from spacers comes from that end of the horse you referenced above. I am not hedging this statement.
The greatest impact from the plenum spacer is that additional plenum volume moves the two flappy-closed torque peaks down to a lower rpm level. One could in theory create the opposite effect by fillling the cavities behind the trumpets with epoxy.
Finally, BTW try searching for threads on the intake manifold.
07-06-2013, 10:54 PM
#3
Nordschleife Master
