Alternative Induction Systems

[John Maher]

Digging through some old photo albums I found a couple of plenum intake system pics I took during a trip to the US in 1990.
The early days of PRA Pro Stock saw a few racers experimenting with this style of induction:

Adam Wik's Pro Stock engine. Plenum with removable top plate, plywood carb spacers and tubular runners. 48 IDAs.


Jim Larsen's Pro Stock bug. Welded rectangular runners. Pretty sure the carbs were 40 IDFs.

[Shag55]

Jimmy's 1st runners were too small and to much carb. Second was better.

[modnrod]

Tunnel rams work.
Real well.

The 2nd one looks OK, but both of them looks like they're tuned more for 6500 than 8000, length-wise.
How to get them to work on a flat motor? Did anyone ever make the usual I.R. IDA crowd second-guess themselves?

I must confess a love for centremounts, and have fed them with everything from a Mikuni TM42 through to Weber 40DCNF and Holley 350 (isolated), but have also run twin Mikuni BST40 (CV carbs off KTM/Suzuki singles). Only street stuff though, not race.

Perhaps you can ask JPM nicely please for a couple of pics of his 4 x FCR Keihin set-ups?

This thread shows promise........

[John Maher]

Tunnel rams work.
Real well.

As I understand it, the plenum systems appearing on various VW Pro Stock cars in the early '90s came about as a result of engine capacity increases and airflow requirement beginning to exceed the capability of traditional IR/individual runner/one choke per cylinder intake systems i.e. 48 IDAs. Sticking a pair of 40 IDFs on a plenum (as per the Jim Larsen pic above), allowed each intake valve to draw from ALL FOUR CHOKES instead of just one.

Remember this was in the days before 62mm Terminators and Berg 58 IDAs.
The plenum design wasn't new - it's been used on V8 drag racers since forever but it WAS a whole new thing seeing this technology being applied to the aircooled flat four. Traditionally, drag race VWs ue one choke per cylinder, whereas V8s would typically have one (sometimes two), four barrel carb(s) mounted on a plenum.

Thing is, on a V8 the path from plenum to cylinder head is straight and relatively short. On a flat four the runners are relatively long and far from straight.... they're forced to execute an abrupt turn to direct the air/fuel mix into the cylinder head. Without getting into the science of wave tuning and optimum intake length etc, it's the length, volume and change in direction that works against the centrally mounted plenum in a VW horizontally opposed application. The arrival of larger capacity carbs (eg 62mm Terminators, Berg 58s etc), effectively saw the end of development of plenum based intake systems on naturally aspirated VW race engines. The IR approach works just fine as long as the individual choke sizes are sufficient to satisfy all of the individual cylinders' demand. It works in F1 ;-)

Nevertheless, you've got to admire the ingenuity and efforts of the racers who experimented with plenum intake systems - they were searching for an advantage you couldn't buy on a shrink-wrapped piece of cardboard with a fancy logo and a barcode.

Maybe the plenum approach still has potential in a mega high rpm, narrow power band, short stroke motors (i.e. short runners), but from a theoretical standpoint I can't see it competing efficiency-wise against a one throttle per cylinder (of adequate size), straight shot, EFI intake system ().

[modnrod]

Tunnel rams work.
Real well.

On a flat four the runners are relatively long and far from straight.... they're forced to execute an abrupt turn to direct the air/fuel mix into the cylinder head. Without getting into the science of wave tuning and optimum intake length etc



Maybe the plenum approach still has potential in a mega high rpm, narrow power band, short stroke motors (i.e. short runners), but from a theoretical standpoint I can't see it competing efficiency-wise against a one throttle per cylinder (of adequate size), straight shot, EFI intake system ().

I don't know what revs the current NA Pro's are running, apart from a guess at "probably lots", but yes, the tuning length is hard to fit the package.
Not getting into the science of it?!@?! Why not? Haven't we got 28 hours in the day? 

Most of the high-rev high-end bike stuff is effectively more tunnelram than I.R. in reality, at least for the tuning at peak power and the rev range. It uses individual throttles, but they all join into the airbox, with length/volume/taper mixing with volume/shape to form an effective TR at WFO really anyway.

I've often thought that I.R. big bore carbs with the throttle near the head for good mixing, but joining all trumpets tuned to 2nd order into a large TR-style airbox would be an excellent mix of the two different setups, and would follow the current bike designs.
But I'll leave that for you John, it's not my day job. 

[Eddie DVK]

Very cool stuff.
I think I saw a version comming by, a few years ago, with a plenum for each bank (with a carb on top of it) wasn t that working properly.

Regards Edgar

[wph]

Very cool stuff.
I think I saw a version comming by, a few years ago, with a plenum for each bank (with a carb on top of it) wasn t that working properly.

Regards Edgar

It should work ok, allthough a lot of work is required to shape the inside of the plenum to work well in regards of
fuel distribution and suspension in airflow. Carb throats will size up very differently as per IR setup, your emulsion tubes
may need changing also. You´ll have to tune the intake tract for 3rd order to make it short enough. Plenum volume
and carb/stack height will affect the tune and horsepower output, a lot of variables to get "right".     

[morkrieger]

Friend of mine and i tried an 'alternative' way of making his 2.4 typ4 breathe better a while ago on his track car.
The idea was perfect, convergent-divergent inlet flow-path, billet CNC manifolds, as straight as possible into the chambers....
 execution...well....if he hadn't shoved his entire drivetrain forward into the car through the torsion tube for a better weight distribution since the uniball conversion.... .

More or less an oppertunity to either start drinking or give up on it  

It was a nice attempt though, perhaps shorter ITB's will make it fit right in this specific situation (although it would upset the tuned length...)
It was supposed to breathe through airboxes in the wheelwells, possibly with ducting through the fenders.

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[Jon]

Cool thread. I know of a couple of different guys walking down the Helmholtz plenum volume route on our boxer.
Has anyone tried this? In short it's to have a tuned plenum above the stacks, and a tuned intake to this plenum.
Seems like one would have to treat it like two 2 cyl engines.

[John Maher]

Cool thread. I know of a couple of different guys walking down the Helmholtz plenum volume route on our boxer.
Has anyone tried this? In short it's to have a tuned plenum above the stacks, and a tuned intake to this plenum.
Seems like one would have to treat it like two 2 cyl engines.

I'm not sure how well it would work on a flat four .... the left and right banks can't really be treated as two separate engines. Each pair of side by side cylinders doesn't fire at equally spaced intervals. You would need to pair 1 & 3 and 2 & 4 to have balanced pulses.

Both sides of the Porsche 911 engine have equally spaced firing, so more can be done with Helmholtz resonator tuning e.g. check the Varioram intake:

[modnrod]

Although the following is not A/C VW, it does relate precisely to what you said John.
Honda have a new motorcycle engine out, the NC700. Parallel twin, but with a 270* crank to make it sound like a Ducati! LOL Anyway, they have simplified everything down to the smallest tune for this 7000rpm unbalanced twin, using "shed spares" to make it and so take a profit (the pistons/valves come from the Auto Honda Jazz). They have only 1 throttle body for ease of tune and packaging, but to allow the second cylinder to fire with the same power as the first (normally the second out-of-phase cylinder would be intake charge robbed, and therefore fire weakly), they have put different cam specs and different timing specs on the second cylinder to boost it up to the same level. I've ridden the bike, and it is a lovely smooth machine to cruise on.
I only mentioned this to show that it IS POSSIBLE to get two unevenly firing cylinders to work together and have both providing even leverage on the crank, but it does take a bit of left-field application sometimes. For example, this thinking with the charge-robbed second cylinder on a VW Single Port head could be nullified by cam timing and exhaust tuning on the second firing cylinders?

Not really a picture of an alternative intake system, sorry, but an idea for someone maybe to follow?