Oversized valves

[apostate]

great we are getting somewhere so you agree that if you want to use 40.5mm (wherever that number came from) you would use atleast 43mm which is a larger valve than 42mm. hence oversize valves are good and you answered your own question. that is your opinion, but you haven't shown us where you or someone else has used this and proven it is better than something else

40.5 mm is an extreme porting procedure to an OEM 885 head. There's a risk of ruining the head if one goes beyond this. I do not take into consideration aftermarket heads since I am not familiar with such.
Such an extreme porting is generally needed for pure race applications. And having in mind that valve & seat material in race applications is of a much higher quality, those throats can still be closed with 42 mm valves. For durability and reliability sake most people use 43 mm valves.
40.5 throat does not make any sense for a street engine with a moderate camshaft. 39.5 throats that can help a 3.1-3.2L M20 with a Schrick 288 (for example) do not need a 43 mm valve at all. Lower displacement and/or camshaft combos definitely do not need oversized valves, just the opposite - performance will suffer if such are used.

there are various definitions of curtain area, none of them are actually correct when you do some CFD and see how the air moves and see the trajectory and how it isnt the same all the way around the valve and how it changes at every lift. its next to impossible to define a universal relationship to cover every combination of seat angle, width, chamber type as it all comes into to it. Simplifying it it makes life easier, some use the middle of the seat, some use the ID some use OD, whatever they are simply empirical relationships rules of thumb to make things simple. if you read the work of blair you would know this as this is what he says. the valve and seat sizes have a relationship between them so it doesnt matter how you define the curtain but it shows that at lower lifts the curtain (however you define it) governs flow obviously the top cut(s) angle and width, seat angle and width and bottom cut(s) angles and their with all come into it

I agree in general. But valve outer diameter is still a factor that influences the suction process together with many other parameters of the valve. I wont elaborate on this, it will get too long.

the modern trend for race designs is 50 or 55 degree seats if 45* isnt used, 30* was used years ago, few use it these days. even david vizard rarely seems to use it despite discussing it alot in his books

I guess I am old school.

Thanks for the discussion, Digger. It made this thread at least worth something.

I am out.

[digger]

40.5 mm is an extreme porting procedure to an OEM 885 head. There's a risk of ruining the head if one goes beyond this. I do not take into consideration aftermarket heads since I am not familiar with such.
Such an extreme porting is generally needed for pure race applications. And having in mind that valve & seat material in race applications is of a much higher quality, those throats can still be closed with 42 mm valves. For durability and reliability sake most people use 43 mm valves.
40.5 throat does not make any sense for a street engine with a moderate camshaft. 39.5 throats that can help a 3.1-3.2L M20 with a Schrick 288 (for example) do not need a 43 mm valve at all. Lower displacement and/or camshaft combos definitely do not need oversized valves, just the opposite - performance will suffer if such are used.

yeah so like it been said to you before it depends on the application and what you are trying to do.

to end this, as for valve size to throat size i advise people to google "valve throat size" and see what guidelines some of the professionals out there recommend and make up their own mind. i've not seen any respected professional mention anything north of 92% like the OP says, many dont even like as high as 92%

here is a couple such hits, both Joe mondello is one of the most respected cylinder head guys who ever lived and Larry just ports heads, builds exhausts and dynos them all day long and makes is own software that is the best BFYB software you will ever own (PIPEMAX) so what would he know ;)

http://www.hotrod.com/articles/ccrp-1011-measure-the-diameter-of-the-exhaust-throat/

http://hotrodenginetech.com/pipemax-creator-larry-meaux-on-race-engine-head-porting/

there are many good books around

or even buy a DIY flow bench kit and test for yourself and hit the dyno if you doubt them.

[bmwm42]

Smells like 5toes

[apostate]

https://www.youtube.com/watch?v=V4NZRNU18ZQ

[2002tiiguy]

lol shes good but holy random batman

[ForcedFirebird]

Didn't read every comment, but what I have found is simply plopping a larger valve in an 885 head does nothing. Actually it hurts flow unless the bowl/throat is worked (easily done on a Serdi).

Yes, the 885 can benefit from larger valves (mainly intake to offset the odd intake to exhaust ratio), but they heads are real finicky about changing the port shape - I wouldn't suggest doing it without flow testing, though. All the small block stuff you learned does not apply to the m20 heads.

This probably isn't the best outlet to go into great discussion about fluid dynamics, but Digger knows as well as I do that a 1mm over valve isn't going to kill your velocity, and again the m20 is slightly different, so all the "rules" about throat vs valve etc isn't typical.

[LateFan]

This probably isn't the best outlet to go into great discussion about fluid dynamics.....again the m20 is slightly different, so all the "rules" about throat vs valve etc isn't typical.

I'd like to see you and digger start a conversation or primer about this very thing elsewhere. Some of us sort of grew up on the typical hot rod talk of high compression, huge valves, huge lift, long duration, big ports...and there's so much more to it that that. Some of the math would probably go over my head, but I'd like to learn more about the concepts and understand it.

The Alpina hemi-type combustion chamber vs the current thinking of say, the Bimmerheads or MM-style head. Smooth finished vs the surface texture of some heads. Valve / seat size, and more importantly the shape of the path as it passes around the valve. OEM intake runner diameter and restrictions upstream. Max torque vs high rpm hp in a streetable engine.

Anyway, just a request - I love engine theory and the WHY of it all. Or any reading you might suggest as a start.

[digger]

The Alpina hemi-type combustion chamber vs the current thinking of say, the Bimmerheads or MM-style head. Smooth finished vs the surface texture of some heads. Valve / seat size, and more importantly the shape of the path as it passes around the valve. OEM intake runner diameter and restrictions upstream. Max torque vs high rpm hp in a streetable engine.

For combustion chambers take a look at modern chrylser hemi chambers and also what what Jon Kaase did with the Boss Nine head which has similar valve layout to the m20. They are not a true hemi and would be classified as a semi hemi more similar to an m20 head than a “conventional” hemi.

http://www.jonkaaseracingengines.com/html/kaase_boss_nine_-_semi-hemisperical_big_block_ford_cylinder_head.html

The combustion chamber (concave funnel shape) guides the flow from the port into the chamber for longer distance past the seat so it slows the flow more progressively improving pressure recovery. Look at a venturi, one of the big factors affecting the efficiency of a venturi shape is the divergent angle so if you use the combustion chamber as part of the divergent portion the better it is.

The smaller the combustion chamber the easier it is to get higher CR without excessive dome shape which can block the flame front path.

The more compact the combustion chamber is the more thermally efficient it is.

The more “amount” of squish you have the more turbulence you introduce in the combustion chamber which leads to a faster flame speed and resistance to detonation.

The less advance the engine needs without being prone to denotation the better. if you need 35* advance the spark plug firing 35* before TDC thus produces combustion pressure that works against the piston which is moving up, so called negative work which is wasted.

So for alpinas conversion of the 885 semi hemi to more of a true hemi appears to be the wrong direction IMO and opposite of what others do for both an OEM head and a high performance aftermarket head. Unless you are doing a top fuel engine or similar the hemi chamber is definitely not the best.

Bimmerheads appear to just use standard chamber exactly what BMW did, it is a safe logical option.

There is merit in different textures but the MM stuff seems to be on the wrong scale and likely to attract small pockets of mixture that won’t burn properly especially ontop of the pistons. The texture people have shown to provide improvements appear much finer (smaller scale) than the large ridges MM use.
The MM engine don’t appear to make any more power than a corresponding spec engine without the surface turbulence though to properly test the effect of the texture would be thousands of dollars on fully instrumented engine dyno.
You also don’t want sharp edges in the combustion chamber or piston tops neither external nor internal edges as it creates hot spots or dead zones respectively.

death by a thousand small cuts, the cumulative effect of many small effects can produce a much larger effect