Asymmetric intakes - damage engine?

[apex944]

I did a search and didnt find anything along these lines...I'm working on developing a high output race motor based on a 3L VW VR6. If you are not familiar with the VR6 here is a link:

http://en.wikipedia.org/wiki/VR6#Technial_Information_and_Engine_Specifications

Basically its a V6 with 1 cylinder head.

So in developing a short runner intake we start to be faced with the reality that the asymmetric runner lengths IN the head (3 short, 3 long) are not going to be truly equalized. It's not that a manifold with 3 short and 3 12cm longer runners cant be made to equalize it, we can, the issue is that while porting we are finding that the 3 short intakes are flowing a lot more than is possible on the long runners. And here is the kicker... All 3 long runners fire sequntially, and then a 3 short ones!

So my concern is that if I maximize output at the expense of equality I'll have basically 2 three cylinder engines firing in sequence. I'm concerned that the 'deceleration' I'll see when the lower output bank fires vs the high output bank will cause 'vibrations' that could tear it apart.

But I have no real idea where to start my investigation into how far apart I can make these 2 banks before I run into problems. Need data and resources to research.

Any guidance would be helpful.

- Bill

 

[r2800]

They may look unequal lengths but are in fact compensated for within the cylinder head casting and inlet tracts.

 

[apex944]

No.

We've been building VW engines for 20 years and know what we are talking about. We've just never taken the VR6 to this extreme.
I'm not talking about OE manifolds here, just the head.
The head is asymmetric. Also as an FYI the newer intakes do not compensate. The compensation is done totally with asymmetic cams (front vs rear) since 1999.

So my questions still stand.

 

[IceStationZebra]

Every engine runs differing amounts of power from each piston, but how much different? If you think of the typical carb'ed straight 6 from years past the middle cylinders will flow much better than the end two. I wouldn't be surprised with 10-20% variance between pistons (especially pre-OBDII), but then again these engines aren't being pushed to the edge. So I doubt you will find any kind of "rule-of-thumb". But in your application I would think that the strength of the crank will be the limiting factor. You should also watch for harmonics in the cam drive - chain if I remember correctly.

As for the short runner manifold, how long (or short) are the runners? You could make up some of the length difference by extending the short runners into the plenum, say 6cm?

ISZ

 

[apex944]

Wow, I came back to this thread after a few weeks and when I re-read it, boy did I come off as a pompous as$! Sorry about that. Must have got up on the wrong side that day...

>> 20% variance between pistons

20%. OK I can see that on some of the old straight sixes. But with the firing order they used I would think the loads imparted to the crank would still be more evenly distributed. (That's a pretty much uninformed guess however)

With this crazy VR6, I potentially have 3 cylinders firing in a row with a VE of 110-122% and then 3 firing in a row with a VE of ~0.8-1.1. (The diff is due to port length and race porting)

So instead of alternating cylinder pressures I'm going to have 3 low in sequence and 3 high. It's this 4th ignition transition point that has me worried. Do you think the acceleration & jerk of the crank shaft at lower RPMs (3k-4k) will be so abrupt during the transition from 3rd to 4th cylinder firing that I'll have serious problems? I'm picturing a pretty nasty twisting moment on the crank during this transition on every full rev.

Also I've made the argument to myself both ways that a relatively lighter flywheel or a heavier flywheel will help this situation... what would the forums' wisdom be in this case?

Thanks for your time guys.
- Bill

 

[GregLocock]

What you'll get is a strong(er) first order excitation of TVs. Given the short crank I don't think you'll hurt anything, unless you are revving>10000 rpm.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[apex944]

>> I don't think you'll hurt anything

Thanks Greg, now I can proceed with a bit more confidence. I had almost convinced myself I was going to destroy the crank or valvetrain if the disparity between cyl. banks really winds up being this large.

>> strong(er) first order excitation of TVs

Perhaps I'll investigate what my crank dampening options are.

- Bill

 

[GregLocock]

The standard TV damper is probably correctly tuned, it doesn't care what the excitation order is, it is tuned to the crankshaft resonances.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.