Port Flows More With The Valve In.

[christopher 5 1 Cars]

One for our expert tuners here:-)

 

Has anyone seen situations where the flow of the inlet is greater with the valve inserted then with just a bare port?

 

What could be the possible reasons for this?

[PumaRacing 1]

It's not uncommon. It just means the valve head is actually helping turn the air out of the port and into the chamber. Flow will increase up to a certain lift and then fall again as the valve lifts further. The trick is to get the point of peak flow to coincide with the cam lift in that particular engine. Whether it then decreases at higher lifts is of no concern. Not all heads do this and it also seems to vary depending on the type of flowbench which by definition means not all flowbenches measure flow accurately. On my bench which is designed according to the British Standard on flow measurement and has long pipes to let the flow stabilise properly I hardly ever see this. Flow usually flattens off eventually or just continues to increase very slightly at ever increasing lifts. Other people's benches, usually the compact type designed to fit into a small box on a worktop, seem to show it more often.

 

It's also the reason why you can't develop port shapes without fitting a valve and doing a complete flow test at different lifts. It's much quicker to put a head on the flowbench without valves of course and just take the one reading but the port shapes you might develop from that won't usually be right.

[christopher 5 1 Cars]

It's not uncommon. It just means the valve head is actually helping turn the air out of the port and into the chamber. Flow will increase up to a certain lift and then fall again as the valve lifts further. The trick is to get the point of peak flow to coincide with the cam lift in that particular engine. Whether it then decreases at higher lifts is of no concern. Not all heads do this and it also seems to vary depending on the type of flowbench which by definition means not all flowbenches measure flow accurately. On my bench which is designed according to the British Standard on flow measurement and has long pipes to let the flow stabilise properly I hardly ever see this. Flow usually flattens off eventually or just continues to increase very slightly at ever increasing lifts. Other people's benches, usually the compact type designed to fit into a small box on a worktop, seem to show it more often.

 

It's also the reason why you can't develop port shapes without fitting a valve and doing a complete flow test at different lifts. It's much quicker to put a head on the flowbench without valves of course and just take the one reading but the port shapes you might develop from that won't usually be right.

 

Thanks for your description Puma, that was very interesting. I see the concept. If I understand you right you say the modification to the inlet should be done so that peak flow should be at the full valve lift. But what if the flow is still increasing after the valve is lifted more then the existing lift. Do you lift it a bit more on the bench and then try to increase flow and again in increments at this lift? How to determine how much max lift should be...?

[Rippthrough 98]

Thanks for your description Puma, that was very interesting. I see the concept. If I understand you right you say the modification to the inlet should be done so that peak flow should be at the full valve lift. But what if the flow is still increasing after the valve is lifted more then the existing lift. Do you lift it a bit more on the bench and then try to increase flow and again in increments at this lift? How to determine how much max lift should be...?

 

Depends upon your camshaft spec, the clearance to the pistons and the weight of the valve gear I would have thought.

[christopher 5 1 Cars]

Depends upon your camshaft spec, the clearance to the pistons and the weight of the valve gear I would have thought.

 

Hi Rippthrough

 

The camshaft has not been chosen yet so in theory the lift cam be as high as it needs to be within reason. I'm talking anout the flow dynamics here so lets ignore the piston valve clearance for a second it is not related to this discussion (and there is loads anyway)

[PumaRacing 1]

Thanks for your description Puma, that was very interesting. I see the concept. If I understand you right you say the modification to the inlet should be done so that peak flow should be at the full valve lift.

 

Well, only if the flow is already starting to fall before the valve has reached full lift do you have a problem. Then you have to try and find out why and change the port shapes. Anyway, like I said it's very rare for flow to drop at some point. It usually just keeps increasing gradually until it levels off.

 

But what if the flow is still increasing after the valve is lifted more then the existing lift.

 

Then you don't have a problem. That doesn't mean the port is optimised though. How you determine what your flow targets should be is a whole different story and not one I could possibly explain on here. It's a bit like the old joke "How do you get to Carnegie Hall?"...................Practice!

[christopher 5 1 Cars]

Well, only if the flow is already starting to fall before the valve has reached full lift do you have a problem. Then you have to try and find out why and change the port shapes. Anyway, like I said it's very rare for flow to drop at some point. It usually just keeps increasing gradually until it levels off.

Then you don't have a problem. That doesn't mean the port is optimised though. How you determine what your flow targets should be is a whole different story and not one I could possibly explain on here. It's a bit like the old joke "How do you get to Carnegie Hall?"...................Practice!

 

Indeed!! Thanks Puma