Valve to throat ratio. Is a bigger valve always better?

[Bobby Shaftoe]

The initial message was deleted from this topic on 17 August 2017 at 01:35

[Pumaracing]

http://www.pistonheads.com/gassing/topic.asp?h=1&a...

[PeterBurgess]

The key word is 'blend', this means the valve throat will not be at 90 degrees to the 60 degree cut so not as wide as you calculate. It may be worth having someone cut the 60 down to size with a 75 degree 'bottom' bottom cut as it were, this will give you a better idea of the shape required. With the larger inlet valve go for a 1.5 mm 45 degree seat.

I hope this helps.

Peter

[DVandrews]

I have a spreadsheet knocking around the calculates throat size, ideal port size, seat width and Inner seat diameter given the valve size, it will also suggest an exhaust valve size. It ties up fairly closely to David Vizards formulae.

Dave

[PeterBurgess]

Hi Dave

The only problem is the sheet will be generic not specific which can only be based on empirical work, the spreadsheet will produce somewhat black and white sizes, when there are at least 50 shades of grey

Peter

[DVandrews]

It's a starting point.. No more no less..

Dave

[Pumaracing]

Some further interesting information on the last page of this thread and what it links to..

http://www.pistonheads.com/gassing/topic.asp?h=0&a...

Even on an F1 engine with almost perfectly downdraft ports the optimised seat and throat profiles are much as I describe in the thread I previously mentioned. 85% to 87% throat to valve diameter ratio and seat widths of 4% to 4.5% of the inlet valve diameter. There is no point opening up ports or throats larger than is required to flow the maximum amount of air the valve itself can pass. Even on very downdraft ports that's nowhere near a throat to valve diameter ratio of 0.92. If you've read somewhere that this is a target to aspire to on road modified engines then you can discount that advice without a second thought.

[PeterBurgess]

If you can get up to visit us I would show you. Throats are not round below the 60 and they are not 90 degree throats even on the short side of the port, tis experience. Heads also vary and you may well find you almost have a venturi throat with some variants of coring.

Peter

[PeterBurgess]

Is that not what NCK has done?

Peter

[PeterBurgess]

When you cut the seats on either a standard head (throats too small) or fit larger valves in either std or modded head you will end up with masses of bottom cut to blend in, this, if done by removing two sides of a triangle(as it were) will give a gentler entry angle into the immediate valve seat, this should flow more than a more vertical entry into the seat. Maybe it is a matter of where valve seat becomes throat or then port is confusing you? USA calls it valve bowl which kind of implies (to me) seat down to guide area

Peter

[DVandrews]

Bobby,

if you send me a mail with your direct email address I will send you a copy of the spreadsheet.

Dave

[PeterBurgess]

Hi Bobby following sequence to show cutting and blending seaat for 41.3mm inlet valve.

First pic, cut 30/45(1.5mm)60 seat, note how cutter has dug into insert at bottom so plenty to remove to blend.





second pic, hand cut 70 degree botton cut to reduce 60 to 1.5mm.






Third pic, fettle with carbide burr





Fourth pic, blend with sander band.





Fifth pic, fan grinder finish.






Last pic, 36 mm valve will not go to bottom of insert so throat less than 36 mm which is 87% of the inlet valve size. I will hold on to the head and you are welcome to come along and measure.




Peter

[PeterBurgess]

Shame when people do that to heads, can you squeeze 43 inlets onto the seats or maybe have larger inserts fitted for the inlets and let the ex valves take care of themselves with the slightly larger ones?

Peter

[Bobby Shaftoe]

I do actually have a set of 43/37mm valves spare, only problem being they're on a 5/16 stem (with a triple groove retainer) rather than 11/32.

I tried to source the guides, Paul at REC suggested they were Ford X-flow/Essex/Pinto but a call to burton power later it appears the OD of the ford guide is way too small. Not sure if the pinto triple groove collet fits the rover valve spring retainer either.

I think the cost of seats/guides/collets etc and the associated machining work would put the 43mm's way over my budget.

[PeterBurgess]

Don't chuck 'em away though, as heads get rarer and rarer they will begin to look better and better to work on

Peter

[Bobby Shaftoe]

Well finally got round to getting the heads off and recutting the seats. A quick and dirty test on my flowbench showed 167cfm with the recut seats and 1.63" versus 158cfm with standard valves. (My bench isn't calibrated to anything in particular but a stock unported rover head flows around 136 cfm peak, all at 28" h2o)

[Pumaracing]

Digging out my old flow bench data from 20 plus years ago and converting from 25" to 28" your stock unported, modified stock valve and modified 41.4mm valve flow figures are right on the money. You will have picked up about 15 bhp when you get it back together. 265 bhp if it was 250 before.

[PeterBurgess]

Pleased to be of assistance, I hope the photos helped you with your journey.

Peter

[226bhp]

Bobby Shaftoe said:

Well finally got round to getting the heads off and recutting the seats. A quick and dirty test on my flowbench showed 167cfm with the recut seats and 1.63" versus 158cfm with standard valves. (My bench isn't calibrated to anything in particular but a stock unported rover head flows around 136 cfm peak, all at 28" h2o)

With valves in or not and if so at what lift?