Race Engine Forum UK

I've read guys article here, but I'm still a little confused here. Down stream matching (inlet is smaller then head port??) how important is it? I have various schools of though here.

1) Its a good thing in general do it. All sharp edges are bad
2) Don't do it especially on the short side. Since the fuel probably is running down this edge at low rpm. This "step" creates much needed turbulance which aids in the mixing of the fuel and air.
3) I think generally Guy thinks this should be done..(but we are talking raod engines here too that don't always run at high rpm)..(correct me if I am wrong here guy )
4) You don't have a flowbench or a clue. Don't go there


Reason being is last time I had my carbs off I decided to match the inlet to the head since there was at least 2mm. Most of this work was done to "short side radius" of these ports. Since it was this inner curve that showed most mismatch. Roof of upper curve was ok. They basically now match perfect.

It drives much the same (who know maybe some more HP who knows) but it is noticibly harder to start without lots of choke. But when warn runs fine...even at low rpm

Hi Chris

I cannot give a quantitative answer in this case, only in principle. If the inlet manifold flows the same or more than the port it does not matter, and the step - well it doesn't matter either, the air will just flow straight past it. Well it affect inlet tract pressure wave, well, there are degrees of mismatch but here - not in my experience.

What would be critical with your setup is to examine the behaviour of the head and manifold together, after examining how they behave as separate items. There may well be an issue with the main flow vector of the air into the head. This is a common problem with inlet manifold with acute curves like yours. It may be that in trying to make it better by matching you have made it a bit worse, easily done without a flowbench.

Some guys like to dowel manifolds to heads (see below) but this is a total waste of time unless the head-manifold pair have been flow tested, which they clearly had not in this case. GC extreme limit flow development on this Peugeot 205GTI 8v 1600 head took the flow from an as received, (previously prepared) 100cfm with to 117 cfm with manifold and from 155 bhp to over 170bhp. Plus the head as it came in had inlet valves fitted that were too large for the seats (..a pseudo 'big valve conversion'..) and I actually reduced them in size and reshaped them to get it to fly.

GC

Thanks for the reply Guy, Appeciated. Just getting my head round the principles.

inlet manifold flows the same or more than the port it does not matter

Very hard for an unexperienced layman like me to know this I suppose. The port is larger then the manifold but obviously has a much larger short side radius then my curvy maifold. Sure its impossible to know without a flow bench...

What would be critical with your setup is to examine the behaviour of the head and manifold together, after examining how they behave as separate items. There may well be an issue with the main flow vector of the air into the head. This is a common problem with inlet manifold with acute curves like yours. It may be that in trying to make it better by matching you have made it a bit worse, easily done without a flowbench.

I like way you use a piece of string in your example..I understand what you mean now

Yes of course you are so right its an dangerous thing on the eye porting without a flowbench. Thats why I only cleaned up the ports in the head and not rework the shape.

On the manifold maybe I went a bit too far...(I must remember to put a superflow on christmas list :roll: ) but of course it must depend n who is using it :?

We all have to learn somewhere :oops: Good thing I have an unmolested inlet and head as spare just in case

Thanks

Christopher

Hi Chris,

don't feel bad, this website is going to be a learning curve for a lot of people, I hope. Sure it's easy for make to say 'u have to do this & that', but I/you never stop learning... I didn't know any of this 20 years ago. And, in fact it took me 3 weeks to unravel the Peugeot head secrets - when if came in I first thought 'I won't be able to better that'.. but I did and it was useful exp.

GC

:D

Yep I am learning.

It's looks fab Guy!! :D I 'm sure it will run as fab too!!

Bit off topic but...

A few pics of my unlcleaned and unmolested spare TU24 head so you have an idea. Looks pretty different to the XU engines.

Better let you get too it

Many thanks again for the explanations..