Small Six Valve Question

[addo]

This is related to the idea that the exhaust valve has to have 80% of the flow capacity of the intake unit.

If you have a 1.649" intake valve, this suggests a 1.475" exhaust. But it's 1.388" Surely they didn't get it so wrong...

Yes, I realise this theoretical calc ignores the back profiles and stem size/existence.

Moving up to a custom shortened Manley Race-Flo valve with 1.785" inlet size, this infers 1.596" for the exhaust. 1.600" is a stock head size. Again, in a shortened, prepped version to optimise flow.

So, if one were to go to these larger sizes, shrouding becomes an issue. That exhaust is a "just fit" by the smallest of margins!

Which is worse? A large valve with shrouding issues, or a smaller valve with less theoretical flow ability? Is the effect different over RPM operating bandwidths? What would be the most likely problems of overshrouding? Scavenging or filling? Overspending?

Thanks, Adam.

 

[MustangSix]

Don't confuse flow rate with valve size.

 

[StrangeRanger]

Flow area of a valve is valve diameter multiplied by lift. If the exhaust valve has more lift it can be smaller.

Flow area of the bowl is the choke point. Thats bowl area minus stem area and needs to be the same as the valve flow area. Small valves have disproportionally smaller bowl areas because the valve stem uses up a bigger portion of the area.

 

[addo]

I didn't really think I was confusing valve size and flow rate. It's the potential of each possibility that puzzles me.

So, is it reasonable to surmise that Ford chose the smaller exhaust valve, and added proportionally higher lift, so that flow would develop in a way most complementary to the restrictive exhaust ports and manifold?

Plus, as I understand it, to notch up performance with the smaller valve size, the lift has to increase appreciably. The cam gets lumpier, the "sweet spot" more defined and overall tractability drops. Also, valve train stresses increase.

But it's the shrouding bit I don't get. Not denying it or anything. But it's kinda like someone saying "Don't go out in the dark". I'm asking "Why". Will the swirl be altered? Will the valve heat unevenly? Will I be famous? Will I be rich?

I don't believe there's anything on the Forum Tech currently with stock valve size flow rates in a ported head. Nor am I decrying the reccomendations of those who say 1.5" exhaust, 1.75" inlet valves work, but simply trying to put some meat behind the reasoning.

If all else fails, I will bench a stock head, then the P&P one, and post results. If I drop in the larger valves, makes it hard to go back, should they prove unsatisfactory.

Adam.

 

[StrangeRanger]

Addo,
I've got a spreadsheet set up to handle a lot of the valve size calculations (too much time on my hands once) You've given me the intake and exhaust sizes, if you give me the valve stem diameters I can plug those into the sheet and run the numbers backwards to see what minimum exhaust valve diameter and minimum intake and exhaust lifts should be for a given intake size.

The 300 turned out to be modeled on nearly all of Ricardo's rule of thumb proportions. It would be interesting to see how the 200/250 family does

 

[CZLN6]

Howdy All:

Happy New Year to All!

Wow! this has been a great series so far. Ironically, we're working on a flow bench series of runs to begin to quantify a starting point and the effects of changes. In the real world there are several other conditions that limit valve size, most significant is the 3.68" stock bore diameter. Another flow bench hurdle is to find a plexiglas cylinder to replicate a 3.68" stock cylinder. The closest we've found so far is a 3.78" 307 ci Chevy sleeve.

Another hurdle is stock exhaust manifold; then the stock exhaust port as cast. The first 3 series will be with both the stock log exhaust manifold and with a header. After that all tests will be with a header. The real world hurdle is the price of the header plus installing a port divider.

Our plan is to start with a late model, flat-topped log head, probably a D7BE-AB, which will have stock 1.75" intakes and 1.38" exhaust valves. The benchmark will be the head as pulled from the engine, dirt, crud and all. Lifts of .100", .200", .300", and .400" will be the standard. We will likely use a .450" lift for the later tests. Next, it will be cleaned up and be given a standard 3 angle valve job. Next will be a back cut on the intake valve. At this point more involved modifications will take place, such as pocket porting, polishing and reshaping/unshrouding the chamber, opening the carb hole in the log, milling the deck surface and, finally, installing a 144 ci intake valve @1.467" in the exhaust side.

The intent is to evaluate and quantify, in terms of $ and % of cfm increase, the relative value of each each change. The focus will be on getting the most out of a USA log head at a reasonably affordable price.

Addo- I realize that this is slightly different then your original querry, but I'd appreciate your thoughtful consideration. Is there anything else we should consider, given these perameters?

Jack- What do you think?

Flow bench time is pricey, so we hope to make each change useful.

Adios, David

 

[Hot 6t Falcon]

Adam,

Good Question.

My simple answer is a question. Which do you think is better - a partially shrouded 1.38 valve or a partially shrouded 1.47 valve?

Both David and I use 144ci 1.468 intake valves as exhaust valves. But we also unshroud them. The valve has the correct stem diameter and only requires recutting the seat to install.

Good Luck

 

[Anonymous]

Adam, I don't know why shrouding is a bad thing. With a small bore, I would think... put the largest dang valves you can fit. An example I can give you. Back in the '70s I had a 273 small block mopar V8. This engine had 3.625" bores with 1.78 intakes and 1.50 exhaust. The hot set-up on this thing was to use 340 heads with the 2.02 intakes and 1.60 exhaust. Certainly the bores had to be notched and obviously there was a lot of shrouding going on but the increase in performance was dramatic. However, low-speed throttle response did suffer. Looking back 360 heads with 1.88 valves might have been better. Anyway the ports must be able to flow the increased valve size, but we are looking for performance are'nt we?

 

[MustangSix]

One of the big constraints in a 200/250 head buildup is the availability of valves. The short length and 5/16" stem makes it unique and also limits the spring selection.

I'd like to see you ream the guides to 11/32" (cheaper than new guides) and install stock 1.78" small block Ford intake valves with the appropriate springs. On the exhaust side the 1.45" SBF valves. Both are taller, but they are cheap and accessible.

 

[rbohm]

8) on the intake side, if you contour the chamber properly you can use the shrouding to your advantage by inducing a swirl to aid combustion. on the exhaust side however you want as little shrouding as possible with in reason, but as long as said shrouding is not out of line a larger valve is still better.

 

[StrangeRanger]

Addo

I punched the valve dimensions into my spreadsheet and the results may be a bit surprising. Taking the stock 1.649" intake with a .312" stem and allowing .070" for seat width results in a flow area in the bowl of 1.71 sq. in. The equivalent intake port diameter would be 1.48." The required intake cam lift to maintain the cross-sectional area would be .361" In a 200 this will theoretically deliver optimum VE at 3460 RPM and acceptable VEs from 990-5930 RPM.

On the exhaust side I had set my sheet up for 75% of the flow area. I'm not sure of where that came from nor can I verify your 80%. I need to get back to the library and re-peruse Sir Harry.
Anyway at 75% flow area the above valve would require an exhaust bowl area of 1.23 sq. in. The 1.388 dia /.312 stem valve with a .070 seat width gives an area of only 1.11 sq. in. A 1.475" dia valve would be required to make the 75% flow area. The 1.475 valve would have a required port diameter of 1.28" and a minimum required cam lift of .310"

Increasing the intake valve to 1.785" gives an intake flow area of 2.05 in. sq. The required intake port dia. would be 1.62" and the required intake cam lift becomes .396" This combination would have a theoretical VE peak at 4140 RPM and be in the acceptable range from 1180-7100 RPM

If you set the exhaust up for 75% of that you need a valve diameter of 1.595" an exhaust port diameter of 1.40" and a minimum exhaust cam lift of .342"

Now all this assumes that Ricardo's empirical model of flow areas, intake velocity and VE applies to the Ford head which it very likely does and that the valves and ports are the limiting factor, which in the US head they clearly are not, but may well be in the Oz head. It will be very interesting to see David and Dennis's data.

 

[addo]

Interesting stuff, indeed.

I think my 80% came from Monsieur Daly... Honourable source? Or stems are slightly larger (.3415 or so), but the SBF are the same, IIRC.

One thing - are you punching in my nominated exhaust sizes to get the port size, or are you working backwards from the bowl size, to arrive at these figures? That VE RPM and bandwidth with the larger valves seems pretty tractable.

The inlet port size is the challenging part - I will have to measure and check here, to see. Our log castings are pretty tidy and consistent inside the runners. My exhausts are 32mm square at the flange, with a ¼" or so corner radius. I really worked hard to open it up right to the valve guide boss. I munched all the guide bosses pretty hard because I'll be fitting roller rockers - so minimal side loads.

With tidy, smooth portwork, will a venturi effect compensate for a comparatively minor restriction? Either on the intake or exhaust side... The undercut valves should help things along in the immediate area behind the valve face.

One thing seems clear - Ford engineers were hobbled by the exhaust manifold - why, one ponders?

Dennis and David - sorry if I've been non-contributive. 13 hour days on site tend to get rather numbing. Enjoying this discussion; it looks like yielding desirable results, and our aims are roughly parallel. ('Cept I'll probably never tow a horse trailer...)

Regards, Adam.

 

[StrangeRanger]

I worked backwards from bowl size to get the valve sizes. No suprise that it confirmed your numbers though.

 

[Anlushac11]

8)

Has anyone looked at using Ford 2.3L OHC valves?

My Haynes manual says that the diamater of the stems are very similar, The oversize stems sizes overlap, and as far as I can tell the length is very close too. I plan on pulling a valve out of my 88 2.3L turbo head and my 80 I6 head as soon as I get it off and doing a side by side comparison.

Stock valves on the 2.3L are 1.73" intake and 1.49" exhaust. Racer Walsh, a 2.3L pewrformance place sells stainless valves in that size for $12 each. If they can be made to fit with minimal work that might be a source for cheap valves. Without a running car I cant get over to toolplace and rent a Sawzall to cut exhaust pipe.