Some figures here taken from development test work on the Superflow rig with a gasflowed 1438 Twin-Cam head.
I'd like to be able to say the tests were conducted on a 'fully developed' inlet seat, but that would be a completely unsubstantiable claim, because in reality to produce an absolutely optimum result the seat would have to be reshaped and optimised for every valve, a quite impossible task really, it would take dozens of heads and weeks to do. The tests were done on an optimised seat that was certainly optimised for the early rather 'tulip' shaped valves, and all I was doing really was seeing how well the seats flowed with other - later - designs.
So, in the meantime this will have to do, and the purpose is to show how the flow can vary according to valve shape, note which valve gives better lower lift flow and which gives better high lift flow. It's interesting how the late generation inlet valve used on all the post 1973 TCs is actually a very compromise between the two.
I've done similar tests on several other heads too downdrafted ports - sidedrafted ports etc etc - sometimes making dummy valves from aluminium - and they all have their particular 'preferences'.
porting, development, valve and seat work, combustion chambers, cams, head construction, etc
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- flow curves: a case of judging from dyno experience which lift regime is going to be best - mid lift or peak lift. Lift below 2mm you can almost discount.
- Valve shape tests Fiat 8v TC 1438.PNG (16.69 KiB) Viewed 13663 times
- 20deg x 45deg x 2mm seat, 70deg throat bored out with lower parallel section and about 1.5mm bigger there.
- 1438 seat fully developed for its (early) OE valve.jpg (115.77 KiB) Viewed 13648 times
- some of the valves used, L-R they are 1438 OE, late OE, GC SCCA, 'Cosworth' style. Back grind was tried on the OE ones too.
- Valves used for test.jpg (114.85 KiB) Viewed 13665 times
- Penny-on-a-stick type, worse than even the OE 1438 valve except at about 4mm lift, and although a different seat form might work better I've never found this shape any good on the TC, they do need a generous head angle & radius like the late OE one.
- POS valve in situ.jpg (107.23 KiB) Viewed 13635 times
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