valve size vs port diameter

[Fulvia-hf]

Dear Guy,
Can you shed some light on valve sizes vs throat size? In several articles including your book I read big valves increase torque. I can understand that big intake valves might do that by giving a better swirl into the combustion chamber but I cannot follow this theory for the exhaust valves. (my) Common sense says that the smaller the valve the less effort the exhaustgasses have to make to enter the exhaust port. So the exhaust valve should be as close to the throat size as possible. Correct?
(My Fulvia 1200cc has 39mm inletvalves with 35mm tapering to 33mm inlettracks and 33mm outlet valves with 29mm tapering out to 32mm exhaust tracks.
C&B cams with 10mm lift 302 deg duration.)
I quess there is a trade off somehow.

[Guy Croft]

William, ih

sorry not to have replied to this sooner, I actually forgot all about it.

Can you shed some light on valve sizes vs throat size? In several articles including your book I read big valves increase torque. I can understand that big intake valves might do that by giving a better swirl into the combustion chamber but I cannot follow this theory for the exhaust valves.

Provided that the inlet valve is not shrouded to any great extent - if it's 'bigger' it will tend to flow more air under lift. But it can only flow what the port and associated manifold etc supplies and the principal determining feature that sets that flow is the smallest cross sectional area in the inlet tract. It's nothing to do with swirl, which occurs as the piston comes up the bore and can be generated by port shape and offset. Yes, swirl can give faster burn but it's a tricky thing to experiment with and inducing movement to generate port-induced swirl lowers the volumetric efficiency. That is a trade off.. Less goes in but maybe it burns better.. See what I mean?

A bigger-than-original inlet valve will certainly tend to give more torque throughout the range even for same cam lift, but for a good flowing std size valve/seat/port combo is can be more cost effective to simply increase the valve lift and capture the full potential of the existing valve rather than go to the expense of fitting bigger ones. I say 'tend' because it has to be nicely executed. I have seen numerous rather poorly done 'big valve' conversions that flow less than standard valves thru the whole lift regime, 8V and 16V.

(my) Common sense says that the smaller the valve the less effort the exhaust gasses have to make to enter the exhaust port. So the exhaust valve should be as close to the throat size as possible. Correct?

I'll do my best with this. The key thing is what the port flows - too small and there will be a high pumping loss. That is when the cylinder pressure drops to atmospheric during the upward phase of the exhaust stroke and the piston is having to physically pump the gas out. Ways to minimise the loss are big ex port, early ex valve lift - say 110 deg btdc rather than 102deg to get a big pressure pulse early in the exhaust cycle to evacuate the cylinder. There's a lot more to cam timing than that of course but it will suffice for now.
There is a good rule of thumb used by many like me with a flowbench and that is that the ex port should flow an absolute minimum of 65% of inlet flow and preferably a lot more. I like to have 72-75% normally aspirated and 80% on turbo/supercharged units. You will see readily that is not something you can really work out on paper. So having determined an optimum port size - often enlarging in some areas to within 3-4mm of casting wall thickness - but it varies form head to head, you size the valve so that the valve throat is the same cross-sectional area as the port. It's not always possible to match the two, especially if it's a 4v combustion chamber with coolant passages thru the ex port splitter.

More often than not of course the valve size is set for you (unless you're doing a big valve conversion..) so the thing to watch is not go too thin on the contact face and leave some metal in hand there for subsequent overhaul by not cutting out the valve seat insert parallel thru, but giving it say 70 or 75 deg taper below the contact face. So in reality the size of the throat is set by that practical concern and rightly so.

My Fulvia 1200cc has 39mm inlet valves with 35mm tapering to 33mm inlet tracks and 33mm ex valves with 29mm tapering out to 32mm exhaust tracks. C&B cams with 10mm lift 302 deg duration.

A quick overview on those dimensions. We can ignore throat diameters for this. If we simply square the valve diameters, we see that your 39/33 valve combo has an E/I ratio of 1089/1521 which is 72%; so we could reasonably expect the ports to be about the same. if you measure the flow on the flowbench and the E/I ratio based on bare port flow is higher it means the inlet port is underflowing rather than the ex port is super-effective. If it's the other way round and the E/I ratio drops below 72% it means the ex port and valve is underperforming. You have to be rather careful with porting because more often than not it's easy to increase the inlet flow, only to find that you cannot get the x port flow up in by the right amount, and pumping loss becomes a real worry. Some engines suffer badly from this, Suzuki Swift, Sierra Cosworth 16v to name but two.

Your controlling section in the head is probably the 33mm region, cannot be sure because I don't know the valve throat diameter, and a 33mm port will flow 85cfm or more at 10" which is quite a lot. I'm no expert on Fulvias either although there are one or two on this site, maybe you're one too! I cannot tell you how much air they will flow with that 39mm valve fitted, but my guess is that a 39mm inlet with that port will be good for anything over 85bhp per liter, or 102bhp on your 1200cc unit.

I hope this is helpful to a degree, and as I said earlier, sorry for not picking it up sooner as it had some interesting questions.

GC

[Fulvia-hf]

Thanks a lot for your helpfull information.
It makes good sense but also tels me that without having the head on a flowbench porting is like grinding in the dark! Same change of ruining the head as to improve it.
The stock 1200 head has 36mm inlet valves and portsize of 35mm at the manifold (31 at the manifold/head split) to a throat of 30.
Exhaustport is is 31 at the manifold flange and roughly 29 at the throat (measured about 1,2" from the seat) Stock valve size is 32mm
My factory ported HF 1300 head has the manifold tapering from 35( carb size) to 33 at the head. the port in the head is tapering to 32 at the throat so about parallel. Valve upped to 39mm
Exhaust is 32,5 at the manifold and a throat of 31. Valve is 33mm
On the factory head is the valveguide hump removed both inlet and exhaust. Guides are tapered on the inlet and left thick in the exhaust. (stem 7mm)
These heads have about the same size inlet as exhaust ports and no way to make the inlet larger. (wall thickness is 1mm in places (where headbolts pass)) Making things worse is the fact that because of the basic design the difference in portlenght varies about 10cm between cilinder 1-2 and 3-4. So two ports are both narrow, long and almost parallel.
My head needs worn valves, guides and seats replaced. So since costs are being made anyway would you change valve sizes?
Oh, Standard hf1200 cams are about 9mm lift and 274deg in duration.
New C&B's 10/302 yet to be fitted.