Race Engine Forum UK

Guys hi

I have always been interested in the effects of simple SSR changes. Most heads I work on are of the 4v variety. On occasion I have to turn my hand to some strange 2v side draft units.

The flow in these heads starts to stall or tail off at 9.5mm and then recover. Making the short side profile wider recovered the dip by around some 8cfm but did nothing to bare port flow or high lift flow.

Laying the SSR back into the port by some 2.5mm helped bare port and high lift flow a lot and gave me the graph which I shall post here tomorrow.

The making the SSR profile wider is something i tried in a normal 4v type head last year. I made it wider instead of laying it back because the cam did not lift high enough to use the extra flow wich could have been gained by laying it back only. Making it wider and smoothing the apex gave me good gains right where the cam needed it wich also gave a good dyno result too.

There seems to be no hard and fast rules on SSR profiles so if possible is anyone willing to share thier SSR profile experiences here?

Chris S

The entire air flow testing experience is a giant can of worms (or more like a can of fleas)
Since I now have very limited flow testing experience..Now I know that I don't understand..before I just thought I knew I did not .

I think the dip is about where the port (limited) flow will make more of a difference than seat (limited) flow . This is of course a very artificial distinction as the seat does never ''live'' without the attached port and it's subsequent effects.


Widening the SSR will apparently make it easier for the air to make the approach to the seat (probably by slowing it down a bit in the regions close to the ssr).

As port speed goes up (and the valve lifts further and further of the seat) reducing the turbulence and wake effects of the SSR seems to pay off. Maybe by laying back the ssr you make more effective use of the available port area at that particular port speed/lift..

what that does on a real engine ...I don't know

SSR= short side radius

btw did the 2.5mm lay back affect the lower lift figures at all ??

The short side radius can be compared to an aerofoil in behaviour in that it has an adverse pressure gradient (pressure increasing) exactly as the trailing edge of a wing does. The SSR has common characteristics to the upper suction surface of an aerofoil and one can rightly assume that its impact on the airtream varies very much as an aerofoil does, though of course with the SSR in a cylinder head port one's concerned with variations port-to-port and engine-to-engine rather than varying angle of attack as seen in this film:

http://www.youtube.com/watch?v=Vjk9Ux2COx0

This short film (ignore the cylinder) can be taken as a demonstration of the kind of flow-robbing separation and turbulence/reverse flow that occurs on all short side radii - to an extent governed by the length of the curve, its radius and relationship to the port floor.

You can see in the film how separation caused by the pressure gradient upsets the whole flow field and grows as the angle of attack is increased. Those of you familiar with flight will recognise this. You will see readily that it's an impossible thing to define in detail as any kind of general rule for any engine without the most exhaustive flowbench testing and I spend more time trying to turn imperfect head designs into something, er 'slightly better' most days of my week. However with some heads the 'imperfect design' is so poor it can be a bit tiresome to do.

Please also read in detail my recent post at:
viewtopic.php?f=13&t=1762



GC