Oversized valves

[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

[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.

[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.

[2002tiiguy]

lol shes good but holy random batman

[apostate]

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

[bmwm42]

Smells like 5toes

[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.

[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]

That's why I earlier stated that 40.5 mm throat is a risky venture with a 42 mm valve. 39.5 mm throat is not. Refer to page 1.


Same comment as above.


I disagree. On a 2V 885 head you can go way up to 93-94% with no reliability issues if you keep the seat main angle at 45 degrees. Race designs can go down to 30 degrees but this is not considered reliable for street applications. OEM % is too conservative for obvious reasons - valve & seat alloys of inferior quality used for mass production.


Curtain area is calculated based on the internal diameter of the seat. Valve diameter is referred to as a function of that ID of the seat (a generally accepted coefficient). When you put an oversized valve over a certain ID of the valve seat, all those considerations change considerably for worse.


Wrong. The curtain area is a function of the valve seat internal diameter and the valve lift. And no, it is not a linear relationship.


What about stroke?

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. go out and test your theory an show us the result

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. the throat size affects what you can do with the angles, how wide they can be. too large throat in comparison to the valve limits how well you can manipulate the air through the hole so you need to balance it carefully

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

[digger]

Good explanation.

A couple of questions -

What is the relationship between kinetic energy and inertia?

Why do longer runners help increase torque? And is that at all rpms or a certain band depending on length?

inertia is akin to mass

kinetic enegy is 1/2mV^2

So there is a component of inertia in the energy. But if you use an inertia explanation it doesn't allow you to get the area velocity effect. If it was inertia a big port would do the same job since the bigger port has more inertia the bigger it gets

organ pipe theory, the lower harmonics (longer lengths) tend to produce slightly stronger pulses (less losses i guess).

[apostate]

the issue if you use a seat throat of 40.5mm and 42mm valve you can not transition from the parallel throat to the 45degree seat very well as there isnt any metal to put a bottom cut (s) of 60 or 65 or 70 or whatever by the time you cut a seat wide enough to be durable and transfer the heat. so it wont be a very efficient design

That's why I earlier stated that 40.5 mm throat is a risky venture with a 42 mm valve. 39.5 mm throat is not. Refer to page 1.

there is a range of typical throat vs valve size often between 0.85 and 0.92 is "normal" range. your 40.5/42 = 96.5% i doubt you will find anyone using or advocating this with an oldschool OE port configuration with 45* seat.

Same comment as above.

Some more modern stuff use 92%, but some people use 55 or 55* seats to so horses for courses.....but often people like 88-90% its not any sort of gospel but going too far one way or another doesn't seem to be successful as im told sometimes the engine runs better despite the flowbench numbers being slightly worse.

the OE 885 head is 36.7/42= 87%. On a modern 4V engine you probably could use a higher % than an old 2V as the port is much more straight and downdraft IDK.

I disagree. On a 2V 885 head you can go way up to 93-94% with no reliability issues if you keep the seat main angle at 45 degrees. Race designs can go down to 30 degrees but this is not considered reliable for street applications. OEM % is too conservative for obvious reasons - valve & seat alloys of inferior quality used for mass production.

the other thing you are not considering is the curtain area. The curtain area dictates flow upto lift of 0.25D in the ideal theoretical sense even though it is not so clear and distinct. So even despite having a minimum cross section upstream of the valve the bigger valve still helps at the low to medium lift ranges. it isnt until higher lift that the port, inner seat throat and minimum CSA start to dictate flow

Curtain area is calculated based on the internal diameter of the seat. Valve diameter is referred to as a function of that ID of the seat (a generally accepted coefficient). When you put an oversized valve over a certain ID of the valve seat, all those considerations change considerably for worse.

the bigger valve has a bigger curtain area, its only a linear relationship but it is there. A 4V head has way more curtain area than a 2V engine and it this is part of the reason a 4V engine needs less duration than 2V engine for the same hp. Less duration is always better if you can meet your hp goal.

Wrong. The curtain area is a function of the valve seat internal diameter and the valve lift. And no, it is not a linear relationship.

often people use bore size as a means to size the valve, some rules say for a hemi a max of 53% of bore is good for maximum effort.

What about stroke?

[digger]

Ha! I'm trina wrap my head around how to grind a 45mm valve to fit a 40mm seat.

I did learn some things in the article posted above.

"In fact, the area approximately ½-inch above and below the intake valve is the most influential factor of the entire inlet flow path. Maintaining good airspeed past the rapidly opening and closing valves is the engine builder’s top priority."

"Steady-state airflow past an open valve (on a flow bench) is one crucial measure, but starting and stopping that column of air (and fuel droplets) many times per second is not conducive to the smooth transfer of the fuel/air medium from the induction system."

ForcedFirebird talks about this, that a flow bench only gets you so far.

Also, I didn't realized the "tulip" shape was different on intake and exhaust valve stems for flow. Trina catch up.

It doesn't seem like our 885 head has the same problem with deep shrouded valves as that American head shown...
[ATTACH]114664[/ATTACH]

(MM 885 head)
[ATTACH]114665[/ATTACH]

...although I understand now where if the valve is too close to the cylinder wall, you're not getting full flow all the way around the valve opening.

the isnt many problems with the m20 heads and shrouding they are semi hemi, the valve goes away from the bore walls as it lifts it is only 8mm offset laterally (minimum shrouding when = 0 true hemi are like this) . as an example there is a 2.8L m20 running with 45.5mm valve and 85mm bore. this head was flow tested with various size bore adapters on the flow bench to check for shrouding and flow stability. some of the best pushrod stuff like shown uses canted valves to reduce shrouding the m20 doesnt need too much work in this area compared to others

[digger]

Sticking around here just a bit more only due to Digger...
@ Digger - yes, your schooner analysis is almost on spot. Let's assume that porting (head and manifold) is done properly and the valve seat internal diameter is set to say 40.5 mm. Why would one want to close this throat with a 43 mm. valve vs a 42 mm. instead? How would this affect ramming effect and air trapping into the cylinder? Furthermore, imagine that you close the mentioned valve seat with even bigger valves like 44 or 45 mm.? Good idea?

Another overlooked issue with 885 head is the combustion chamber shape. It is such for a reason. Port (and valve) angles are such for a reason. Head port shape and angle in/out and length/height position (inlet&exhaust) are further variables when talking of a SYSTEM as some folks (not mentioning them) very much like to address it. Head port angle is crucial to velocity and ramming. Stock M20B25 manifold is done not as a piece of art (though it seems like one) but to address all these issues with the 885 head design. It wasn't easy or cheap to produce it back then in the 70's.

Air-pocket traps are crucial too (kids, do not port-match your stock manifolds to a stock head at home). Overlapping affects ramming too. Especially when you have a good designed free-flowing exhaust system (lol, again talking about a SYSTEM, sorry).

Ovesized valves over restricted internal diameter valve seats? I don't thing so.

Cheers.

the issue if you use a seat throat of 40.5mm and 42mm valve you can not transition from the parallel throat to the 45degree seat very well as there isnt any metal to put a bottom cut (s) of 60 or 65 or 70 or whatever by the time you cut a seat wide enough to be durable and transfer the heat. so it wont be a very efficient design

there is a range of typical throat vs valve size often between 0.85 and 0.92 is "normal" range. your 40.5/42 = 96.5% i doubt you will find anyone using or advocating this with an oldschool OE port configuration with 45* seat.

Some more modern stuff use 92%, but some people use 55 or 55* seats to so horses for courses.....but often people like 88-90% its not any sort of gospel but going too far one way or another doesn't seem to be successful as im told sometimes the engine runs better despite the flowbench numbers being slightly worse. this is where you need dyno data not just flow bench data

the OE 885 head is 36.7/42= 87%. On a modern 4V engine you probably could use a higher % than an old 2V as the port is much more straight and downdraft IDK.

the other thing you are not considering is the curtain area. The curtain area dictates flow upto lift of 0.25D in the ideal theoretical sense even though it is not so clear and distinct. So even despite having a minimum cross section upstream of the valve the bigger valve still helps at the low to medium lift ranges. it isnt until higher lift that the port, inner seat throat and minimum CSA start to dictate flow

the bigger valve has a bigger curtain area, its only a linear relationship but it is there. A 4V head has way more curtain area than a 2V engine and it this is part of the reason a 4V engine needs less duration than 2V engine for the same hp. Less duration is always better if you can meet your hp goal.

often people use bore size as a means to size the valve, some rules say for a hemi a max of 53% of bore is good for maximum effort.

[LateFan]

Ha! I'm trina wrap my head around how to grind a 45mm valve to fit a 40mm seat.

I did learn some things in the article posted above.

"In fact, the area approximately ½-inch above and below the intake valve is the most influential factor of the entire inlet flow path. Maintaining good airspeed past the rapidly opening and closing valves is the engine builder’s top priority."

"Steady-state airflow past an open valve (on a flow bench) is one crucial measure, but starting and stopping that column of air (and fuel droplets) many times per second is not conducive to the smooth transfer of the fuel/air medium from the induction system."

ForcedFirebird talks about this, that a flow bench only gets you so far.

Also, I didn't realized the "tulip" shape was different on intake and exhaust valve stems for flow. Trina catch up.

It doesn't seem like our 885 head has the same problem with deep shrouded valves as that American head shown...


(MM 885 head)


...although I understand now where if the valve is too close to the cylinder wall, you're not getting full flow all the way around the valve opening.

[jeffnhiscars]

Well, the seat diameter changes with the valve diameter, no? You don't increase valve size without machining out the chamber and pressing in a larger seat. If not, of course the larger valve is pointless.

Jeff - thank you. My son is studying physics and we got talking about this so I could wrap my slow brain around it.

Inertia is connected to Newton's first law. It takes force to get a mass moving, it takes force to stop a moving mass. In the case of our engine, the force is difference in pressure, correct?
Kinetic energy = 1/2 mass x velocity squared, measured in joules.
Momentum (P) = mass x velocity.


EDIT - digger should be in bed at this point - it's 3 in the morning.

I wrote that to help get my own mind around it :)