Exhaust port size.
Discussion
Anybody know whet the standard 4997 exhaust port size is ? My heads were worked on by one of the “specialists” a while ago but I don’t know how much if any they opened up the ports. They measure roughly 30 x 40mm and my real challenge is to find some gaskets that are large enough. Standard Landy ones are definitely too small, various people are advertising offerings suitable for Griff500 and ported heads but don’t specify the size.
Thanks for checking the size. Do you think your heads are origonal or have they been opened up slightly ?
I have what I think is a pair of origonal manifolds and they are 27.5 x 38.5 if the standard heads are bigger than the standard manifolds you would think opening them up a bit would be a regular topic.
I have what I think is a pair of origonal manifolds and they are 27.5 x 38.5 if the standard heads are bigger than the standard manifolds you would think opening them up a bit would be a regular topic.
chris52 said:
I don't think my heads have been modified as there is nothing in the history regarding this, but the exhaust manifolds are definatley smaller. Would it be an improvement to open up the manifold to match the exhaust port on the head?
Chris
Chris, i take it yours is a 500. If so the heads will be modified. I recently had the manifolds off my 430 and they were quite a bit smaller than the exhaust ports on the heads. While you have them off its definately a good idea to open the manifolds up to match the heads. Cant understand why TVR didnt do this when they went to the trouble of modifying heads.Chris
Standard gasket port size and my grubby port size template
My conclusion was it was having that much gasket sticking out into the gas flow which was giving me so much grief with gaskets blowing and rather than try to modify the gaskets myself I had a set made to my templates
My conclusion was it was having that much gasket sticking out into the gas flow which was giving me so much grief with gaskets blowing and rather than try to modify the gaskets myself I had a set made to my templates
Edited by Barreti on Sunday 30th December 15:00
40 x 30 may be too big however as TVR ported the heads to that size it is definitely not going to help gas flow if the manifold and the gasket are quite a bit smaller than that. I can’t see that the standard manifold will scavenge very well anyway as they simply welded the flange with a smaller than ideal hole on to a significantly larger pipe.
I wonder if the mismatch contributes to the Shunting some people suffer when running a plenum and slightly more lumpy cam than normal. If the gas hits a restriction as it exits and rebounds up the port there could perhaps be considerably more charge pollution at low RPM.
I wonder if the mismatch contributes to the Shunting some people suffer when running a plenum and slightly more lumpy cam than normal. If the gas hits a restriction as it exits and rebounds up the port there could perhaps be considerably more charge pollution at low RPM.
That weld bead inside the manifold needs a lot of dressing, and as the bend goes forward (unlike other RV8s where the manifold goes down) you can shorten the inner radius and widen the bend (think oxflow bends in water rather than the steady state straight flow optimium dimensions!).
Matching the heads & manifolds not only gives gains it also helps seal the gasket as long as a smooth continuous internal surface is made (you need to carefully index the manifold position as there is 1-2mm play jiggling the manifolds on bolts or studs - they don't locate accurately enough - I scribe lines in the heads at the top for that purpose).
You can actually oval the exhaust port / manifold quite a lot ~ still leaving far greater seal than that very small shoulder where the plug recesses are (don't touch that!)
My personal opinion FWIW is that a significant part of the gains Cliff got with his 8-4-2-1 trombones would have been made by the nicer head / flange work that he did. That first bend so close to the valve is very important, just look at the convoluted path the gas has to take, the main concept is to try and get smooth transitions rather than big changes in x-section / shape / direction I believe. I wouldn't worry too much about various 'gurus' opinionated optimum port dimensions
Matching the heads & manifolds not only gives gains it also helps seal the gasket as long as a smooth continuous internal surface is made (you need to carefully index the manifold position as there is 1-2mm play jiggling the manifolds on bolts or studs - they don't locate accurately enough - I scribe lines in the heads at the top for that purpose).
You can actually oval the exhaust port / manifold quite a lot ~ still leaving far greater seal than that very small shoulder where the plug recesses are (don't touch that!)
My personal opinion FWIW is that a significant part of the gains Cliff got with his 8-4-2-1 trombones would have been made by the nicer head / flange work that he did. That first bend so close to the valve is very important, just look at the convoluted path the gas has to take, the main concept is to try and get smooth transitions rather than big changes in x-section / shape / direction I believe. I wouldn't worry too much about various 'gurus' opinionated optimum port dimensions
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