When i got my head from the shed, he did say that he had built a few heads using 41mm inlets but they were few and far between, and due to the extra cost most were happy with 40.5mm, which seems to be a fairly proven combination 43mm Inlets !! I would love to see that head!!
Interesting... possibly one of those then. Well, I've just come back from the flow bench with some interesting results all round. I flow tested a totally standard '84 vintage MK2 GTi head, which the standard appears to be around 75-76cfm when tested on Superflow benches. On ours today I saw a peak of 71.5 cfm... possible the difference between two head castings or two different benches, but I think it's close enough for comparrison, and at least our bench is low rather than giving a false high reading, so I'm happy with that. The second test was on the ex slick 50 head. I did some measuring up over the weekend and felt an improvement could be had in a few area's... the top cut of the seat was a bit sharp blending into the chamber, the guides were square sholdered rather than a nice taper (both of which could have been caused by guide replacement and seat re-cutting in it's life) so I tapered off the guide as best I could in place (it wont stay like that, it's just for this test) and radiused off the top cut of the seat into the chamber. The lower part of the seat insert where it joins the head casting and the main throat/bowl area were both a bit small for my calculations for the valve size being used so I opened them out, along with raising the roof of the port around the guide. On measuring I found the roof to be around the standard height, with only the guide boss removed and the area flattened off... these heads are prone to the airflow tailing off early in the lift range and it has been proved in the past that raising the roof allows a better turn for the air... it's a trend the slick 50 head was showing a little, and something the standard head shows big style. Well the above mods worked ok, I had done some calculations and on that valve size (41mm) I rekoned around 97cfm could be possible and that was my upper aim. The figures are... (original slick 50 port in brackets)... .050 17.0 cfm (14.1), .100 30.5 cfm (27.2), .150 44.8 cfm (41.6), .200 58.1 cfm (55.8), .250 70.2 cfm (66.9), .300 79.6 cfm (76.5), .350 87.4 cfm (81.0), .400 92.2 cfm (85.1), .450 95.9 cfm (87.8) .500 93.9 cfm (86.3) A touch off my target (though I think I can allow myself a 1 cfm error), but just goes to show what can be done with a bit of detailing, even over a pro head. If anyone wants to put the above into a chart then feel free, or hold on until I do it with the standard figures as well. I dont think a 40.5mm valve would be far behind as long as the rest of the poritng was correct, and would certainly save on new seats... something I'll be looking into in the next few months. The 43mm valve could be good, though will need a lot of work. The ports and the throat area will break through, but thats nothing my compitant alloy welder cant fix, and the chamber will need opening back a fair bit, so will only probably work on a big bore short stroke 1800 hillclimb engine.
Are those 43mm exhaust valves Mr H? Does anyone know how to upload excel graphs on here? I have the both the ABF and 8 valve plotted up...
It is also interesting to note that even the best modified 8 valve heads do not get anywhere near the flow of the 16 valve, right across the whole valve event. Now think of it this way, if you get your inlet, exhaust, cams and compression correct on the 16 valve, you will produce more power than even the best 8 valve cars, without so much as farting on the head...
43mm exhausts ! ... I'm sure you meant to say inlets really I have loads of graph's on our, just need to get the printer working, or maybe a memory stick to transfer them. Either way feel free to post up your own version.
I pretty much think the same.. I'm quite sure a standard 16v head can support 185-190hp, possibly even more, but everything else would have to be on the money... but thats for the 16v section for another day. Nothing wrong with a good 8v though..
