9 replies to this topic

[dotmatrix]

Can these valves which the text says is exhaust valves for a 1275 cylinder head be used as inlets in a 998 head (cam4810). The diameter is the same as the existing inlet valves that needs to be changed.

[nicklouse]

I don't think there is any reason they will not work as long as guide height is right. But the cross section of most exhaust valves is quite different to inlets and restrict the flow much more as they are "fatter" before the stem diameter.

I would be interested on the thoughts of the people who have looked at this. As I am only going from a poor memory.

[Spider]

While I don't have the lengths to hand, that would need to be checked, however, the stem diameters between Inlet and Exhausts are different, which Exhausts being around 0.0015" smaller, so using one as an Inlet will almost certainly result in higher oil consumption.

 

The other issue is as Nick has mentioned, with regards to the shape behind the valve head.

 

It is less than ideal.

[Turbo Phil]

The points mentioned above are all valid. The 1275 valves are longer than the small bore ones and as Nick rightly pointed out the head shape will give poor flow as an inlet.

Phil.

[neal]

Interesting...….

Edited by neal, 26 December 2018 - 06:55 PM.

[dotmatrix]

Yes very interesting. Thank you for your insightfull replys

[hhhh]

In the Yellow Bible, you can see arrows representing air flow across the back of the valve especially at lower lifts which shows that a thin valve head is advantageous vs. a tulip shape.

[Turbo Phil]

In the Yellow Bible, you can see arrows representing air flow across the back of the valve especially at lower lifts which shows that a thin valve head is advantageous vs. a tulip shape.

For a 5 port A -series intake port that is certainly true.

Phil.

[nicklouse]

In the Yellow Bible, you can see arrows representing air flow across the back of the valve especially at lower lifts which shows that a thin valve head is advantageous vs. a tulip shape.

For a 5 port A -series intake port that is certainly true.

Phil.

Well the thicker the valve head and stem the less the volume of air can be passed at lower lifts for all applications. At higher lift (not high lift) the restriction is less.

[Turbo Phil]

It doesn’t always work how you think though. On a head such as the A-series intake where the air flows across the valve because it has to make a tight 90deg turn to enter the cylinder the flatter valve design works well.
On other heads the steeper the port becomes the more the tulip shape valve can become an advantage as the air can use more of the valve area and doesn’t just tend to flow across it but rather down and around it, a sort of funnelling effect. A thicker stem obviously won’t help in any situation.

Phil.

Edited by Turbo Phil, 27 December 2018 - 09:31 AM.