Fiat 8v T/C Valve seat angles

[sumplug]

I would like to ask you Guy, after many hours of you flow testing different angles, you say that a 30 degree seat top grind is prefered, but recent experiments by other tuners suggests 38 degrees. Have you tried 38 degrees? Does this effect standard sized valves, or bigger valves like 130tc type?
I know on a flow bench this is done on a bare head with the valve moved up and down manually, but what about in the real world with a gas mixture of air and fuel? The fuel mixture will be slower and denser then pure air, so will a different valve angle make a difference?
What iam trying to get at is, fueling can make a big difference on power and torque, so does different fueling and fuels for that matter need different valve seat angles and timing? And what about combustion chamber shape? Pent roof type seem to make most power. Why is this?

Andy.

[Guy Croft]

Andy,

For myself I found this top grind angle influence by trial and error a while back while doing a SOHC 8v Fiat heat, I was getting rather poor low lift flow results (elsewhere good) and so I wondered if the top grind on the seat might be related to the back grind on the inlet valves, changing it from 20 deg to 30 deg (same as the back grind) worked instantly, and that really got me thinking.

First thought of course was 'is it generally true?' and of course it's not and anyhow sometimes you cannot copy the valve head profile onto the top of the seat. And the steeper the top grind the deeper the valve is going to go in the head unless, like a Vauxhall XE it is already recessed below a say, 40 degree section anyway.

And I wondered what the effect of sharp edges vs blended might be,a nd in the case of the SOHC radiusing them immediately lost the gain I'd just generated which is pretty annoying. One has to be aware too that unless oyu recess the valve way deep into the head that 20/30.whatever angle top grind is going to be of very small radial section. In ahead where the top region is pretty big - measured along the diagonal - my own feeling is that sharp edges are bad news for combustion and shouls be radiused - check out the power curves in GC V/W I recently posted for the Vauxhall XE units - all the combustion chambers were fully blended and I think on engines doing over 8000rpm fast burn is critical.

Now as far as I can tell the absolute optimum inlet valve angle on the TC is 20deg top grind x 45 seat (1.5-2mm matching valve and seat face) with 70 deg throat grind with sharp edges, and 30 deg back grind (sharp) on the OE valve. However that said, I don't think any of the OE inlet valves are anywhere near perfect for any of the TC heads early or late and I really need to design some new race valves specifically for the gasflowed unit. They will be unlikely to be effective on the standard head. That is the unfortunate fact about head dev - what works with one setup will rarely work with another - the best result is no more than a properly evaluated compromise.

The central plug in the pent roof gives equal travel for the flame front (looking in plan view) and this is important for maximum utilisation of the fuel, it also tends to minimise the risk of detonation. In that plane anyway. With an intruder dome on the piston the view from side elevation is a different matter and I am firmly of the view that close attention should be paid to relief of valve relief edges and dome to give the flame front every chance to expand with a minimum of intrusions. You have to pay attention to things like that to get enough top end torque to develop high power. A HC 16v head will generate peak power way higher than an HC 8V because of superior burn alone.

As for the effect of gasoline-air mixture vs just air on flow test the gasoline (call it fuel for simplicity) is carried in the airstream so good airflow is the essential means for getting it in and certainly in every case so far I have been able to demonstrate bhp gain where there has been flowbench gain, though they are not linearly related of course.

I hope that is of some help.

[sumplug]

Thank you Guy.
Some time ago, Rimflow valves became popular in fitment to Pintos for example, so are your proposed valves going to follow a similar design?
What about angles of valves in the head in their running regime? Valves running straight with 90 degrees to port will flow poor compared to angled valves where the port is much straighter. But there must be a limit at what angle you can run the valve?
Also, does or will a 5 valve head flow better then a 4 valve head? I know F1 tried 5 valves [Honda for example], but was shelved for 4 valves.

Andy.

[Guy Croft]

Re valve angle have a read of 'all the power is in the head' in GC V/W.

No, I have no test results on rimflow items and I have not yet seen a 5 valve head.

GC