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PolyQuad - The Future of Four Valve Engines
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PolyQuad Revealed - is it the future of four valve heads?
wedge chamber, PolyQuad, two Valve head, head, David Vizard, engine Builder, low torque Speed, Cylnder Motion, head design, ignition, ffour valve per cylinder, GFN readers, engines, Honda engines, speed, AFR
Written by David Vizard on 29 Jun 2010
  A two valve versus four valve debate is certainly nothing new for me. Anyone who has spent time talking with me on this subject soon discovers I am not really a fan of four valve engines for the true street power plant. But they are out there in the millions and rather than fight the situation I thought it might just be more practical to look for ways to make a four valve engine that much better. So here is the patented PolyQuad head design that David Martin and I worked on for a number of years before taking out the patents in 1999 - 2003.

Granted there are plenty of variations on 4 valves per cylinder and this begs the question as to what a PolyQuad head does better than a current and typical four valve per cylinder head?

Answer - everything

Compared to Honda's VVT and other such variable valve timing system costs almost nothing and can be produced in a typical head shop! To best appreciate how the PolyQuad concept came into being making a start at the two versus four valve debate would be as good a place as any.

There have been some serious debates concerning combustion efficiency going on in serious hi-tech forums and from these it seems that it is well understood that there is more to producing torque and power than just filling cylinders with air and fuel. At low speed the engines ability to breath is hardly a problem as relatively speaking, there is plenty of time to fill the cylinder as full as it is ever going to get. The key to making low speed torque is two-fold a) how much charge is trapped at valve closure and b) how effectively the charge is burned.

Aspect a) is governed by the reverse flow properties of the intake valve seats and to a lesser extent the port area and the cam event timing. Better anti-reversion at low lift and higher port velocities are good here but at the end of the day it's still largely down to the intake valves closing soon after BDC.

AS for point b) we find that how effectively a charge burns is down to mixture preparation and in-cylinder motion at the time of ignition. Assuming that mixture preparation is good then, at this point, best results would be achieved by the cylinder with the best combustion dynamics stemming from appropriate in-cylinder charge motion. The bottom line here is that a parallel valve two-valve bath-tub or wedge chamber design has inherent swirl properties. This means a head designer would have to work hard to eliminate the heads natural advantageous tendency to swirl the mixture (and believe me some have managed to do that). For a two valve head then the natural tendency is to swirl the mixture and this, in part, contributes to a two valve heads normally superior low speed torque.

On the other hand a conventional four valve head has no natural tendency to swirl - but it does have a tendency to 'tumble' the charge. The following illustration shows the difference between the two.

Not unreasonably a first inclination would be to assume that both types of mixture motion would work equally well. Up to a point they do but as the piston nears the top of it's stroke these two forms of mixture motion diverge in characteristics. The tumbling charge motion breaks down to small tumbling eddies where as the swirl, as it gets compacted in the chamber smaller than the cylinder bore can speed up (conservation of angular momentum) while still retaining it's swirling motion form. This swirling motion in conjunction with a well placed spark does a good job of effectively and rapidly burning the charge. On the other hand the tumbling action seen most often in four valve engines tends not to produce enough charge motion at low speed to produce as good results as the two valve engine swirl. Also tumble can be almost cancelled out when compression ratio's much above 11/1 are used.

In addition to the lower combustion speed the inclined valve four valve heads tendency to allow the fresh incoming charge to cross flow out of the exhaust during overlap also cuts the low speed torque as well as increasing fuel consumption and emissions. With the appropriate proportioning of valves and bore/stroke, a two valve engine, with a well designed cylinder head, can out-perform a four valve engine to about 4000 rpm.

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Response to: David Vizard's article
posted: 29 Jun 2010 12:58:13 PM
Interesting article, I can't see the pictures yet but I guess you are still editing?

You may want to approach the motorcycle community with this concept. Bikes typically don't need a whole lot of torque but it would allow them to use less gear to achieve their goals. It would be a real test on the PQ system as far as not sacrificing top end power when you convert a motor that redlines at 14k rpm.
Response to: David Vizard's article
posted: 29 Jun 2010 12:58:22 PM
Gears turning... I smell smoke.

Very interesting!

Concerning the biasing of flow in the side of the port with the larger valve, in general, was the port volume altered based on throat area for each valve, or more on desired ratio of flow/velocity bias required to achieve the desired swirl?
On a bore where the top of the cylinder is already notched for valve clearance, will the reduction in valve size unshroud the smaller valve enough to upset the flow bias in the larger valve's side of the port? Or is this small unshrouding irrelevant?
reply to: Devious's response
posted: 29 Jun 2010 1:01:22 PM
Here is an SRT-4 head I did about a year and 1/2 ago, before ever hearing about your PQ head. The exhausts were the same size, but the intakes were staggered as per your article. I also did port biasing work to augment the swirl/tumble effect. And, yes there are Somender Grooves there.
Response to: David Vizard's article
posted: 29 Jun 2010 1:02:14 PM
Very interesting

David, I am impressed as usual with your common sense approach to problems.

Thank you very much for sharing this information with the community!


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