I have been reading up on cylinder head design, as im trying to spec what will be essentailly an 8v race head. The common method of tuning 8v heads is the 35mm exhaust valve and then the 40.5mm inlets. Mr H's 8v being one of the exceptions. I have been reading up and its generally seen that for engines need an exhaust valve between 82 to 84 percent the size of the inlet. which doesnt quite add up when looking at the traditional modified VW head specs. However this is complicated more by the fact that a cylinder with a capacity of 500cc needs an exhaust valve of about 35mm. this is even complicated further by the port shape and manifold design etc, however we are limited to an extent on port shape due the head design. then there is cam lift. its suggested that a cam only has to open the valves 25% of the valve diameter to acheive max flow. so a 40mm valve only has to open 10mm where as my schrick 288 has a lift of 11.8mm. is this overkill or just another myth when it comes to engine building? if you have any further suggestions on books or net articles it would be appreciated.
vw engines for whatever reason respond very well to high lift cams - I have 13mm+ lifts in my valver for eg which goes against all theoretical manuals on lift/valve size ratios I can only assume it has something to do with the overall design of the head
There are MANY different opinions as to the ideal valve size ratios, but on our (8v) type design, 0.8-0.83 (80-83%) is more than up to the job on the exhaust size. The "fabled" 35mm exhaust valve is around 87% of a 40-40.5mm inlet, so I've no idea where that one came from... and I've asked here many times before. Just to add to the confusion, the higher the compression ratio goes, the smaller the exhaust valve can be... dont ask! As for the 25% lift bit... After the valve has been lifted 25% of it's diameter (the "curtain" area), flow tends not to increase numerically due to the valve no longer being a restriction, but that does'nt mean increasing lift wont gain power past that point... there are other restrictions to be taken into account, i.e valve shrouding via the chamber wall, and to a lesser extent (though it still needs to be considered) the bore wall, so lifting the vlave past the curtain area can still be beneficial.
I have just read that a cylinder of 500cc needs a 36mm exhaust valve??? that would mean a inlet of around 43 or 44mm, that just not going to fit in a VW head with the original valve centres is it
Ok, as a rough rule of thumb, what you want is a ratio of about 75% of valve area for exhaust / inlet ratio. 36mm exhaust valve area = 1,018 mm2 Therefore you would want an inlet area of 1,357 mm2 = 41.5 / 42 mm I would go for a 36mm exhaust 42 mm inlet if that will fit. New seats, more than likely needed.
Further to this, I am not entirely sure where the VW obsession with exhaust valve sizes and flow comes from, but in general terms your power will come from increasing inlet flow, not the reverse.
Me too, and if you take a look at the cnc head site, they show a gain of around 2cfm at peak and a bit more off the seat (no bad thing) when going from standard size exhausts to 35mm items, and thats with the extra air flow from their 100mm bore. There is however a much bigger increase when going up on inlet size, as would be expected.... Alan, take note ! 8v flow development to follow after I've finished the current 16v project.
Quick update... I ran the ex "slick 50" head on the flow bench last week with some surprising results. I would have expected the 41mm inlet valve that it's fitted with to have helped flow up to around high 90's-100cfm. Well, (your dying to hear this Alan arnt you!) it made 87.8cfm @ .450 lift. I understand that a standard 40mm inlet GTi head is around 75-76 cfm, and when ported can be brought up to around 90cfm. On investigation, the port, throat & seat sizing is a bit small in relation to the valve size, and the guide is'nt tappered off... well it all was all like that until I fired up the die grinder yesterday! It's going back on the flow bench in the next couple of days so will report back soon. Nothing to do with this thread really but (and I'm not sure where else to put it)... flow testing on another head revealed that a 45 Weber loose's around 4cfm (well 4.5 if you wanna be picky!), it's bound to loose something due to the butterfly/Aux vent/choke so I'd say thats in the ball park, tho it'll vary depending on the choke size (fitted with a 38mm choke in this case). But the big one is... sock filters. A sock filter dropped 3.1 cfm off the open trumpet flow of the Weber when held at the edge... but when allowed to be sucked onto the end of the trumpet as they so often do ("but it'll still flow cuz the air flows into the front of the filter"... as some so called "pro's" claim) flow dropped from 88.3 to 53.3... 35 cfm!
Mr H thats some nice work, I have some sock filters for my ITB's ill post up some pics which include a 'RobT' mod to stop the filter being sucked in, im not sure whether to use the sock or not, and get some proper filters.
lots were tsr 'pack-d' heads. The really good ones had extra work done by Dave Baker at Puma Race engines.
Sock filters are pants! I was loosing over 10 bhp with them on my trumpets, cant beat proper filter or even better complete air box for resonance
It a strange one really... As far as I know it was tsr sourced originally, so I'd imagine it was possibly a "shed" head, but the previous owner quoted it as a pack "E" spec (something I've "never" heard of other than in this application) as it had bigger than normal (normal 40.5 pack D) inlets at 41mm with 7mm stems... which it has. The exhaust is the fabled 35mm job but on standard 8mm stems. Maybe it was an experiment or a different source than normal ? The original inlet porting was all but standard size before I opened them out both to suit the valve size and when converting from K-jet to carbs. It also does'nt have tapered guides, but they could have been replaced at some point. The original engine was allegedly built by Paul Rose, though the bottom end was stock apart from a bit of flywheel lightening and the pistons topped off to reduce the bowl size & raise the c.r... maybe he used a different head supplier on customer engines, or maybe its a Dave Baker head like his (Paul Rose's) own engine.
My own are 41.91mm & 35.56mm (1.65 & 1.4, hence the odd metric size), and 42 & 35mm's definitely go together. I dont feel the exhaust's need to be that big so want to try a 34mm exhaust next to a 43mm inlet.
