cylinder head-update

[Digsy]

Stock exhaust seat is between 1.2-1.6mm

No, the diameter. :D

 

Surely a rough surface would have a much greater surface area than a smoth one? But even if this was the mechanism at work, I doubt the heat could conduct effectively conduct away from the peaks of the cast surface.

 

TBH I can't see how porting the head would affect EGTs one iota. I think the bigger valves and thicker head gasket and bigger intercooler are the cause. I don't even think that the bigger valve seat contact area would have that much of an effect. For conducting heat away from the valve head, yes, but the effect on EGT? Next to nothing, I reckon.

 

Incidentally, can you get sodium filled valves for the Supra?

 

...and I've worked on lots of road engine ports where porting and polishing has had next to b*gger all effect on the flow bench, too. If you absolutely have to extract every single last bit of power from an engine then its probebly worth doing, but I don't think you could attribute any huge benefits to it in a properly controlled test (i.e. on an engine dyno, A-B-A style, back to back).

[JohnA]

Intercooling has an almost linear effect on EGTs, in the sense that if you reduce inlet temps by 50C then EGTs will drop by around 50C too.

Compression ratio affects them too, especially if before cylinder pressures were way beyond stock.

 

But head porting? very low down the list.

What you do get (if the original head didn't flow properly) is the same power at slightly lower boost pressures, which is good for the engine as a whole.

To give you an idea, with mild porting my calibra made the same power (same turbo, fuelling etc) at 2-3psi less.

[Ian C]

 

Incidentally, can you get sodium filled valves for the Supra?

 

What's the big deal with these anyway?

 

-Ian

[Guest Usmann A]

Yes I agree to what your saying Darren,to a certain extent, if you look back at my statement about seats you can see the smilie I used.

 

It was a tounge in cheek type of comment.

[Digsy]

Sodium is an excellent heat transfer medium (its used in nuclear reactors, so it ought to be). A sodium filled valve stem transfers heat away from the head much more efficiently than solid metal.

 

Not used that often in road cars because they are pricey.

[Guest Terry S]

Intercooling has an almost linear effect on EGTs, in the sense that if you reduce inlet temps by 50C then EGTs will drop by around 50C too.

Compression ratio affects them too, especially if before cylinder pressures were way beyond stock.

 

But head porting? very low down the list.

What you do get (if the original head didn't flow properly) is the same power at slightly lower boost pressures, which is good for the engine as a whole.

To give you an idea, with mild porting my calibra made the same power (same turbo, fuelling etc) at 2-3psi less.

 

You were doing fine until you mentioned your Chavalier again

[Digsy]

Yes I agree to what your saying Darren,to a certain extent, if you look back at my statement about seats you can see the smilie I used.

 

It was a tounge in cheek type of comment.

We need a tounge-in-cheek smilie

[Digsy]

So....just to clear the terms up: When you guys talk about porting and polishing, just how much material do you remove from the walls of the ports?

 

I'm guessing that anything more than 1mm and you might run the risk of accidentally adding water injection to you car

[Guest Usmann A]

Darren, the +1mm oversize valves are 30mm in diameter, the stock are 29mm. IIRC

[JohnA]

Sodium is an excellent heat transfer medium (its used in nuclear reactors, so it ought to be). A sodium filled valve stem transfers heat away from the head much more efficiently than solid metal.

 

Not used that often in road cars because they are pricey.

The Calibra turbo (chavalier in drag for some perhaps ) had them as standard

[Chris Wilson]

Bigger inlet or exhaust valves, or both? How much bigger? Just curious, good luck with it.

[Guest Terry S]

Bigger inlet or exhaust valves, or both? How much bigger? Just curious, good luck with it.

 

Both circa £320 +1mm