THIS IS CONCERNING A COSWORTH ENGINE, BUT THE PRINCIPALS WILL BE THE SAME ACROSS PLATFORMS.

I want to fully understand how head porting effects BHP output. I know that for huge BHP, you need to flow as much air as you can.... I also know that bigger the ports, the more lag you will get and less lowdown response.

A standard Cossie head has 22mm ports, while a GRP A, Touring car head has 26.5mm ports.

What I want to know...........

If an entire engine (apart from head) is of 500bhp spec, will the difference in BHP from a 22mm port to 26.5mm port be a straight diaganol graph (ie, can 23mm, 24mm, 25mm be plotted against that line)? Or will it be an exponential (sp?) line (If you don't know what that means, you shouldn't try to answer the question )?

I realise the answer *may* not be that simple as Volumetric Efficiency may have a bearing on the above question as to why the line will not be a straight forward diaganol one.

Only hardcore facts please.

Regards,
Shaun.



[Edited by Shaun - 2/25/2003 1:35:27 PM]

 

BTT

 

I suppose if the port size is directly proportional to the CFM of air flowing through the port then assuming you are injecting more fuel to go with the increase air flow then I cant see why not.. (so long as VE doesnt change, which I suppose it would?)

Mark?? Pat?

David

 

I have a vague recollection that flow increases as the square of the valve seat circumference.

Not too sure, but definatly not a linear relationship.

 

Would imagine (I know you asked for hardcore facts ) that it would be proportional to the integral of the lift and the port area. Of course this is not the only restriction in the system, so for the same pressure if you increased the flow across the port by x% it does not mean that total flow across the whole system will increase by the same, especially if the other restrictions are more important. Moving the bottleneck somewhere else?

 

The shape is just as important as the size there are some improvments to be made on the cossie around the valve seat areas, you can get 150cfm with increased gas speed and no lose in drivabilty will acour std cossie head flows 120cfm. There is much room for improvment on the exhaust port. The 4wd head has a better combustion chamber and noticable gains will be found with porting. The plenum chamber is to close to the trumpits that are cast into the inlet manifold and a spacer is needed this will also give a bigger plenum area giving the charge more time to settle. I have had 162cfm out of one of these still running std valve size.
Steve.