Race Engine Forum UK

Hello Guy,
Reading the flowbench summray thread in your results section I was wondering how does the old 8 valve TC head compare to 16 valve designs, and also how significant is the flow difference between the 8v heads with 41.8mm intakes and those with 43.5mm valves?

thank you, Tom

p.s The news site is great :D

Hi Tom!

The flow is in part function of port size and valve size, you can take two early heads, 131 TC type, both with nominally 42mm valves but one with port narrowest section approx 32mm and another with 35mm and get radically higher results from the latter, typically 89cfm and 86 cfm at 10" test pressure. And can you open out the ports to improve the low flow head, sure. The heads with 43.5 valves - same rule applies - depends on port size, but the bigger valve potentially means +10cfm.
The later 16v heads flow about 130cfm standard but can go to 170cfm at 10" even with standard size valves.

Back to back standard flows mean nothing unless you are going to swap heads round, not what I do, I have to say, but I know some like to do this. By modifying what you've got in front of you - by port, seat and combustion chamber mods you can get say - about 100cfm out of a 42mm valve head, 114 out of a 43.5mm head. If you just swap things round you never learn anything or experience the fun of developing the thing to its limit, that's not tuning.

I am talking bare port flow figures here (bpf), no valves. It is essential to start with this because the valve is always a restriction and you need to know what the flow potential of the port is in its own right. If the head flows better with the valve in you have got at battle on your hands to find out why, maybe ports too small for that valve size and air velocity too high. I've only had that happen once.

At some high lift point the flow with valve fitted should intersect the bare port flow - if it doesn't even at very high lift - you've got a shrouding problem or the valve is the wrong shape for that port layout, some heads are super sensitive to valve shape. 8v Peugeot 205 for example, where only the flattest 'penny on a stick' valve works.

The flow under valve lift is very important, particularly in the 3-10mm lift range, and you're always hoping that in raising the bpf figure, you don't mess up the lower lift regime. Provided you leave the short side radius region alone till the end you are pretty safe. The ssr and valve shape/throat region are the most important factors affecting that flow.

The 8V head below, that I have been working on Dave Beale, for his supercharged Lancia MonteCarlo was interesting, it came to me already equipped with bronze guides, 45/40 valves, inlets of a most peculiar shape, squish bands welded in like the 'reversed-port' heads, but not ported at all. To begin with the head would not flow any more than a 131 2 liter head.

GC