14 replies to this topic

[hydra]

A friend and I had a protracted discussion about the effects of intake (manifold) tuning on the performance of a Turbocharged engine, and the importance of selecting the right runner length. For some reason, my engine simulation program seems to favor very short length runners, around 200-250mm long overall for 3-7500rpm operation. What happens is that it shows a very mild loss at the lower-rpms, and a decent gain at high-rpms, above 6.5-7k or so, and the combination resulting in the greatest area under the power curve usually falls in the 200-250mm range. This holds true for most engines I've modelled... Now the problem is that this doesn't make much intuitive sense to me (or my friend), what is it about turbocharging an engine that would destroy, or radically alter intake tuning effects? Here are some factors/explanations I could think of. Am I on the right track, or is this complete nonsense?

- The compressed charge is usually at a higher temperature than ambient, increasing the speed of sound, but wouldn't this INCREASE the optimum runner length over an NA engine?

- turbo-charged engines are very knock-limited, moreso at low-rpms, so the penalty of going with shorter runners is offset by the increased timing

- The inertia of the dense, viscous air-column, is significantly greater than that in the N/A, giving more of a VE drop-off for a given intake length. This effect would get stronger with higher manifold pressures/temperatures.

- Perhaps the high-rpm rotation of the compressor wheel somehow dampens intake pulses/reflections, or the high-density intake charge attenuates pulses/reflections a little more than in NA engines?

I'd love to hear a better explanation than the one I could muster, so let's hear em!

[Greg Locock]

FWIW I agree, not much to be gained from tuned pipes on a turbo, from hearsay.

I think your last guess has a lot to do with it, somehow the turbo destroys the acoustic termination at the end of the duct, so the reflected wave doesn't.

.

[NTSOS]

Since the compressor is driven by a turbine, as opposed to a belt drive, the best way to quickly spool up the compressor is to increase the mass flow and pressure available at the turbine inlet.

So, three foot long/small diameter runners.......whatever damn manifold configuration it takes to increase the maximum possible VE of the engine from off idle up to the transition point between normal aspiration and boost pressure in the shortest time as possible is the goal.

Once the compressor is up to speed and the transition point has been achieved, rip off the off idle/transition speed manifold and replace it with a large plenum chamber that simply connects the cylinder head ports to the pressurized air........only minimal inlet restrictions need apply.

A variable geometry induction system that provides low and high end VE would be ideal for a turbosupercharged application. With such a system, slightly increased camshaft durations and larger exhaust turbine A/R housings would be a side benefit which in turn would allow for the widest possible power band.

IMO.......a killer street motor could be made using a Ford normally aspirated/variable geometry racing manifold:





........and a Aluminator supercharged crate motor with twin turbos.




John

[cheapracer]

I'm a little confused here, Turbo simply offers pressure negating the need for sonics to do it.

Low rev off boost you would think that long runners with some volume may assist depending on where the butterflys are.

[hydra]

NTSOS,
No manifold (well not one you can package under a hood anyway) will give you a tuned design rpm of 2000-2500rpm, so your idea, while good in principle, isn't really practical..

[Greg Locock]

How do you work that out? Not every tuned pipe works on the first wave, and lots work on other cylinders.

[hydra]

Greg,
At 2500rpm & assuming a VE of 85%, a runner length of 440mm works out to the 5th harmonic, not insignificant, but not all that strong either...

[Stefan_VTi]

Originally posted by hydra
Greg,
At 2500rpm & assuming a VE of 85%, a runner length of 440mm works out to the 5th harmonic, not insignificant, but not all that strong either...

Wouldn't you need to have cam timing to work out the length? With multi-profile systems (for instance VTEC) you could seriously shorten the needed manifold length.

[hydra]

Originally posted by Stefan_VTi

Wouldn't you need to have cam timing to work out the length? With multi-profile systems (for instance VTEC) you could seriously shorten the needed manifold length.

For my calculations I assumed a typical OEM-sized camshaft of about 200 degrees @ 0.050" , but optimal length isn't very sensitive to camshaft duration. A 24 degree increase in duration only necessitates a 30-40mm INCREASE in overall runner length...

[zac510]

Not to mention that most people with an unrestricted turbo engine get greedy and want more more MORE top end!

[Greg Locock]

Well, the reason I was suspicious is that we have a broadband manifold in production, and I can't imagine that the low speed setting is only effective above 3000 rpm, since nobody drives up there.

[cheapracer]

Its all in the volume.

Just as an aside, I liked Yamaha's idea to gain down low on their turbo'ed bike. Normal turbo system except it also had a bypass inlet manifold (bypassing the turbo) with a 1 way reed valve in it. At lower revs when the turbo would cause restriction, it drew thru the bypass manifold (of course on boost, the reed valve came into play).

[Canuck]

Reed valve intake on a turbo system sounds familiar...John?

[NTSOS]

Originally posted by Canuck
Reed valve intake on a turbo system sounds familiar...John?

Hi Canuck,

Yes sir, I think maybe....1987 or so!

John

[hydra]

So, any input?