Asymmetric intakes - damage engine?
Asymmetric intakes - damage engine?
(OP)
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#Techni al_Informa tion_and_E ngine_Spec ifications
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
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





RE: Asymmetric intakes - damage engine?
RE: Asymmetric intakes - damage engine?
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.
RE: Asymmetric intakes - damage engine?
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
RE: Asymmetric intakes - damage engine?
>> 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
RE: Asymmetric intakes - damage engine?
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Asymmetric intakes - damage engine?
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
RE: Asymmetric intakes - damage engine?
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.