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Variable Length Intake Runners
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ideola  



Joined: 01 Oct 2004
Posts: 15318
Location: Woodstock IL

PostPosted: Thu Feb 05, 2009 12:56 am    Post subject: Variable Length Intake Runners Reply with quote

I've been ruminating on an idea for a rudimentary, mechanical variable length intake manifold. I think I've solved the basic design challenges relative runner configuration, placement, plenum configuration, and how to control transition from the short to long (at least in principal, if not in practice).

The question I have not been able to find an answer to is whether variable length runners serve any benefit on forced induction. It seems like most of the applications I've seen on modern cars are for normally aspired engines. I'm just wondering if forced induction renders the benefits moot. Anybody know anything about this subject?
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Rich H  



Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK

PostPosted: Thu Feb 05, 2009 1:34 am    Post subject: Reply with quote

I would imagine they will have some effect but it will be very limited.

The wave tuning is all about getting the high pressure bits in the right places, clearly under boost your density is higher (That is the whole point) and your temp is higher (Probably) so most of the standard atmospheric assumptions don't apply.

Thinking aloud (Please challenge!):

The speed of sound will be different (Higher as density is higher) so your waves will probably be longer (The waves travel further between intake openings) so to get advantage you will either have very very long runners or tuning on the 10th wave (Harmonics) or something so the effect is unmeasurably small.

Or I'm talking sh*t, please let me know.

Rich
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ideola  



Joined: 01 Oct 2004
Posts: 15318
Location: Woodstock IL

PostPosted: Thu Feb 05, 2009 2:23 am    Post subject: Reply with quote

Well, I don't think your talking sh*t, Rich...

There are some online calculators that I think could be used to calculate a "close-enough" runner length for long and short runners, and IIRC, those calculators allow you to account for the differences in speed of sound for different air densities.

My question wasn't so much on the "tuning" aspect of the runners themselves, but more on the general principals involved. Specifically, that longer runners help low-end torque, but shorter runners help with high end power. The theory behind variable length is that you can get the best of both worlds, which would seem to be a benefit on our torque-challenged cars.

The reading I've done on custom manifold design seems to indicate that it is probably sufficient to come up with a design that is "close enough" in terms of runner length...in other words, if you get the runner length within 10% of the optimum length, any additional gains you get from perfecting the runner length further are so negligible that it's probably not worth the effort (the proverbial point of diminishing returns). For now, I'm accepting that as a valid assumption. But that still leaves the question of whether or not the same benefits would accrue in forced induction that seem to accrue in normally aspired applications.

To clarify: at lower RPM you want longer runners to help improve velocity of the charge air, which aids in low-end torque, whereas at high RPM, you want shorter runners to aid in faster cylinder filling to aid in high-end power. Conventional single-length intake designs are presumably based on the best acceptable tradeoff between the two extremes based on the OEM's desired performance/economy characteristics and development budget.

What I can't figure out is if the much higher volume and velocity of forced induction renders these general principals moot, or if they still apply equally, or perhaps maybe they're even magnified? What I find fascinating is that on the 931, the intake runners are at least an inch or two longer than on the NA. Why??? Was it because the denser air required longer pulses for optimum intake tuning? Was it because the longer runners could be now deployed to improve lower torque because the boosted air at higher RPM overcame the need for shorter runners for top-end power?

I can't seem to find any empirical references that talk about the applicability on FI vs NA applications, only the overall general principals.
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Last edited by ideola on Mon Feb 23, 2009 10:47 pm; edited 1 time in total
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bass gt  



Joined: 02 Dec 2004
Posts: 971
Location: Johannesburg for now!!

PostPosted: Thu Feb 05, 2009 2:48 am    Post subject: Reply with quote

Dan,

For a force inducted engine, i don't think there is any discernable benefit to be gained. if you make the runners optimally sized for the low end, off boost rpm, the high end will be catered for by the boost.
As you know, the idea of variable runners is to increase the engines VE by using harmonics/resonance to "force feed" more air than would be normally possible into the cylinder. However, the turbo is performing this function, hence negating the usefullness of such runners.
You would be better served by reducing the stock intake to head angle, giving the air a smoother path to the cylinder. Such as i have done on my lump. This will have a massive increase in the VE capabilities of the head.
I read some interesting info re the 80's Lotus Sunbeam rally car. Lotus raised the engine angle(it sits at 45deg like ours) and had to lower the intake system. This resulted in a reduced angle between carb and valve, and they accidentally discovered a significant HP gain. I think there is a lesson to be learnt for us all here. The 135deg stock angle is appaling performance wise.

Steve
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Last edited by bass gt on Thu Feb 05, 2009 6:36 am; edited 1 time in total
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ideola  



Joined: 01 Oct 2004
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Location: Woodstock IL

PostPosted: Thu Feb 05, 2009 2:54 am    Post subject: Reply with quote

Exactly the confirmation I was looking for.

So based on that, it seems reasonable to assume that the reason for the longer runners on the 931 is to provide better low-end torque...the runners could now be extended as compared to the NA runners since less of a compromise was required to accommodate high-end power, which is aided by on-boost induction at higher RPM.
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bass gt  



Joined: 02 Dec 2004
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Location: Johannesburg for now!!

PostPosted: Thu Feb 05, 2009 3:06 am    Post subject: Reply with quote

Dan,

Exactly. i am thinking of modifing my system to make the runners very short and dead straight. Although this may affect the low end characteristics, as my car is a dedicated racer, driveability is not a key concern, it'll be revved to within an inch of it's life!!

Steve
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Martijnus  



Joined: 29 Dec 2006
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PostPosted: Thu Feb 05, 2009 3:16 am    Post subject: Reply with quote

Besides the effect... you have another (possibly much influencing) factor to tune

I'm not saying anything about any theory if it will have effect...don't have the proper knowledge for that but I like the idea (thought about it when I started rebuilding my engine, just before yamaha put it on their newer bikes)
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ideola  



Joined: 01 Oct 2004
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Location: Woodstock IL

PostPosted: Thu Feb 05, 2009 4:14 am    Post subject: Reply with quote

Martijnus wrote:
Besides the effect... you have another (possibly much influencing) factor to tune

Yeah, I wasn't thinking about this for the UWB...it was really more just ruminations and wondering about the possibilities for some future development. But as I'm hopelessly addicted to the boost-drug, it looks like this development will be shelved in some dark recess, probably to be forgotten

The intake runner orientation is definitely a new path to investigate and contemplate, although again, not for the UWB, but for what is to follow
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flosho  



Joined: 01 Jul 2004
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PostPosted: Thu Feb 05, 2009 5:06 am    Post subject: Reply with quote

Yeah, just to echo what Steve said...

In my experience with another car, that used "Secondaries" or in other words the variable intake runner length/diameter, once guys started going towards forced induction like turbos and superchargers they removed the butterflies that controlled the secondaries, in effect it was just one big intake then. So I would also say there is no concernable advantage to using this system.
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Peter_in_AU  



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PostPosted: Thu Feb 05, 2009 12:00 pm    Post subject: Reply with quote

Ford also agrees that the difference on a forced-induction engine is not worth the expense.

The 6-cylinder engine they used in the Falcon down here had variable runners on the NA version and fixed runners on the turbo version.
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flosho  



Joined: 01 Jul 2004
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Location: Eau Claire, Wisconsin

PostPosted: Thu Feb 05, 2009 12:36 pm    Post subject: Reply with quote

Peter_in_AU wrote:
Ford also agrees that the difference on a forced-induction engine is not worth the expense.

The 6-cylinder engine they used in the Falcon down here had variable runners on the NA version and fixed runners on the turbo version.


Interesting, the vehicle I was referring to was a Ford too. Ford Taurus SHO.. Used the 3.0 and 3.2 L Yamaha built v6 with two different intake runners.
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Lizard  



Joined: 03 Nov 2002
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Location: Abbotsford BC. Canada

PostPosted: Thu Feb 05, 2009 4:08 pm    Post subject: Reply with quote

To comment on the talking sh*t, you arent. You are actually pretty damn close.

And with regards to the runner lengths, for the turbos you WILL see a benifit for street driving esp if lower CR.
The longer runners will aid in the low speed drivability, but they will definetly hurt top end power.

How much is really hard to say. But I would imagine the difference between 4" runners, and 18" runners to be somewhere around 35-40HP at around 20psi of boost. But keep in mind that is strictly a rough guestimate.

As this is going to be mainly a track car, I would opt for the SHORTEST physically possible, and I would put a phenolic spacer between the manifold and the head. You should only need roughly .8" between the intake plenum wall and the edge of the bellmouth (if you are using that).
I would make the injector runs straight out following the port angle as steve is indicating. I would have JUST enough room between the base of the manifold and the flange to be able to bolt it onto the car. Then the placement of the injectors, I would not leave them in the head. But instead put them in the plenum pointed straight down the bellmouth.
That would lend itself for the MOST HP, driveability will be hurt a little for sure.

BUT this is more a track car than anything else.

If you are not in a hurry for this piece I might even be able to make it for you.
But leaving the country tomorrow for two months will make it abit slow for you to recieve.
On the other hand it would be tig welded lol.
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Raceboy  



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Location: Estonia, Europe

PostPosted: Thu Feb 05, 2009 6:29 pm    Post subject: Reply with quote

Variable profile camshafts though will broaden the powerband greatly.
Too bad it's not easily doable.
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flosho  



Joined: 01 Jul 2004
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PostPosted: Mon Feb 09, 2009 11:40 am    Post subject: Reply with quote

Lizard, you're welcome to make me one. I have 2 months.
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ideola  



Joined: 01 Oct 2004
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PostPosted: Mon Feb 09, 2009 12:11 pm    Post subject: Reply with quote

Steve, I'd be interested to hear what you reckon the ideal angle of the runners would be for the 2.0L head...
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