Below are performance results (3 graphics) resulting from a change I made to the intake runners on my BMW R1100S sports bike.

There are close similarities between R1100 engine and the 4.5L 928 engine, viz:

- 4V wedge combustion chambers.
- 540cc vs 570cc per cylinder.
- both over square, with the R1100 more so.
- low overlap cam timing
- 80+ RWHP @ 6500+


Equates to 320 RWHP from a 4.4L V8.

(OK, ready to be shot down in flames by the wizards about detail, but read on).


The R11 engine had a number of variants, and two are relevant here:

- the 'S' - oriented to top end sport performance.
- the 'GS' - oriented to off-road mid-range performance.


A graphic shows the considerable ( & perplexing to me) spec differences between the two (cam timing and valve sizes: 2mm smaller on the 'S' ??? ).


Anyway, people being what they are, someone wondered what would happen if they put the long, torque-orientated GS intake tubes on other R11
models ...


Fast forward to now, with my 'S' bike fitted with GS intake runners:

WOW....!! IN YOUR FACE better! Can only be the result of better cylinder filling via the smaller dia. GS runners.

The GS runners are twice the length and 2mm smaller diameter that the 'S' runners. Not that you would call the 'S' item a runner, as it's more like a lily than a tube.

I have heard that the 32V port and intake runner dimensions are more suited to 6L motors, and in the light of this very obvious result from downsizing and lengthening of the intake tract, I'm inclined to agree.

So 'll leave you to look through the graphics and draw your own conclusions re application to the 928 engine. (Note the absence of a variable resonance manifold on the R11 engine).

In the end, unless you're looking for a great ET or you're at Bonneville, which of the two torque curves would you like to have available in the real world? (Pssst ... want to know which torque curve is more fun to ride ... ? hee hee ):-]


Cheers.

UpFixen.

 

That's intake tube above the throttle plates, right? ASSume fuel injection rather than carburetors, so the 928 corollary would be the GS pipe dimensions on an ITB setup. Good stuff! There's still a lot of lumps in the torque curve that desrve attention, sufferable with the net increases.

 

Motorcycles (and other wheel driven thingies that have a high power to weight ratio) are intentionally engineered to get rid of excess midrange torque. That's why motorcycle headers are weird as well. When traction is at a premium, it's a lot more pleasant to drive/ride anything with completely flat torque curve.

In contrast, when the power to weight is low, like almost always with street driven cars, that extra torque hump in the mid range is most pleasant.

 

Yes Dr Bob. Forgot to explain that bit.

Above the throttle butterfly back into a plenum airbox. Fuel injection. ITB's.

Re lumps in the torque curve: very sufferable ... ahem ... I go through them so quickly they just don't matter ... . (not hitting the rev limiter is the problem).

Given this was a lucky find and not a researched mod, I agree. They're most likely caused by 3rd or 4th harmonics in the runners or by airbox resonance.

As it happens BMW uses those intake 'lilys' in their M5 V10 VANOS engine (which doesn't have a torque curve, but a torque 'flat').


.

 

This is nothing new, we made the same thing with the old BMW airhead flat twins back in the 70´s. Many people did not believe it worked but thought fitting a short air horn directly on to the carburetor was the right way to go, which is totally wrong. Like to know the total length of the intake duct from the opening of the air horn down to the intake valve for your both setups.

I am still surprised people in this forum are running very short intake ducts for their V8 engines having ITB´s. Understand they like everything under the hood but still?

Åke