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Just so I am not confused, what specific kind of engine you're building that you're referring to here? It's important that we all talk about a specific engine combo because everything has to match. So here's my understand of what engine you're talking about: It's a stroker with stock intake and exhaust manifolds and that can thus pass the California visual emissions tests? And for which you port the heads in a certain way that increases the low-lift flow. Compared to stock heads on an otherwise similar short block, compression, and cams those ported high low-lift flow heads give the same low-end torque but higher top-end power? You have higher compression ratio and longer-duration camshafts which also have a higher overlap, but that combination is set up in the way that it passes the California tail-pipe test for both nitrous oxides as well as for hydrocarbons without catalytic converters. And the highway mileage is up over 3 mpg compared to bone stock engine? If I've got everything right here so far, how much peak hp per liter and peak torque per liter does that engine make?
Originally Posted by GregBBRD
I've spent a tremendous amount of time working on the combustion chambers of the 928 head....experimenting with shapes and angles to unshroud the valves. Making changes, testing, making further changes, and more testing. I've worked for hundreds of my own hours with port shapes to increase airflow in the lower lift ranges of the valves....significantly improving airflow below .250". (See my post above, explaining the reasons.)
These efforts have continued to improve my engines. Horsepower continues to increase, without any loss of torque in the mid range.
My street engines are now to the point where they will pass a California emissions test...without a catalytic converter....even with more aggressive camshafts. My HC and CO numbers are virtually zero, with very low NOX....not just to pass an emissions test (as with VW), but in normal operation.
I add converters to my engines....to make them visually legal.....not to clean up the exhaust.
Additionally, all of my clients report significant fuel mileage increases in the highway cruise mode (3-4 more mpg over stock.)
For me, this points towards significantly increased efficiency....brought about by greatly increasing cylinder filling capability and greatly increased combustion efficiency.
I'm curious as to why nobody has converted a 928 to a Miller cycle engine ( same type of combustion cycle as in the Mazda Millenia posted above).
All that is needed is a source of low rpm positive intake pressure.
Plus a set of custom cams with extreme amounts of intake duration to the point where the piston is in the compression stroke and the valve is still open (for around the first 20% of the stroke).
Sounds like a possible option for more power as you can get a lot more air into the engine.
Because in practice it is more about efficiency than power production.
Miller cycle in the Millenia engine effectively reduces the compression stroke vs the expansion stroke - they use the supercharger (twin-screw in Mazda's case) to "push back" to keep denser air in the (effectively smaller) cylinder.
The trade off is that the reduction in pumping loss (required to compress the inlet charge) is greater than the loss required to drive the supercharger.
Improved power is sort of secondary to the design, but fuel efficiency is on par with the non-supercharged version.
However, I think you might be onto something here.
Mazda KL came in lots of varieties but the "basic one" was a 2.5L making about 160hp.
Compare that to the 928 5.0 at 320hp.
The miller cycle engine was a supercharged 2.3L making 210hp/tq.
So a 4.6L 928 with 420hp and good gas mileage?
Here are some photos of the EMC winning modular Ford engine. A couple of things to note.
First, the valves are sunk and may look to someone like they are "intentionally shrouded". Every effort is made to reduce the low-lift flow between intake and exhaust ports during the overlap. The squish pads extend pretty far to the sides of the intake valves in the center, for example. Not that this is not done just to increase compression, as the piston had to be milled for two small bowls to lower compression.
Second, the intake valve pockets in the piston have been "ported". I suspect, but don't know, that part of the motivation was to increase low-lift flow on the squish-pad side of the valve. That's the good kind of low lift-flow flow, as it's not going straight out the exhaust.
The cams were then specified to compensate for these changes. That engine had 230 degree intake and exhaust cams at 0.05" and 98 degree LSA installed straight up. It made a stupid amount of torque throughout the competition rev range. 11.4:1 compression and headers, though.
Here are some photos of the EMC winning modular Ford engine. A couple of things to note.
First, the valves are sunk and may look to someone like they are "intentionally shrouded". Every effort is made to reduce the low-lift flow between intake and exhaust ports during the overlap. The squish pads extend pretty far to the sides of the intake valves in the center, for example. Not that this is not done just to increase compression, as the piston had to be milled for two small bowls to lower compression.
Second, the intake valve pockets in the piston have been "ported". I suspect, but don't know, that part of the motivation was to increase low-lift flow on the squish-pad side of the valve. That's the good kind of low lift-flow flow, as it's not going straight out the exhaust.
The cams were then specified to compensate for these changes. That engine had 230 degree intake and exhaust cams at 0.05" and 98 degree LSA installed straight up. It made a stupid amount of torque throughout the competition rev range. 11.4:1 compression and headers, though.
Also note that the cylinder head has been unshrouded all the way to the very edge of the cylinder, to allow the air to better enter/exit .....It makes my head hurt trying to figure out why they welded up that set of heads you showed, which increased the shrouding.
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
Any thoughts porting the intake valve pockets for more low-lift flow in an actually running engine?
Keep in mind that the valves spend a very tiny amount of time sitting this close to the piston. Also keep in mind that the piston is chasing the exhaust valve and has slowed significantly when it catches up with the valve.....peak flow is long gone. The other thing that is happening (when the valves are this close to the piston) is that with properly designed intake system, that opening intake valve has an inertial pulse behind it (or in your case, positive pressure)....which is pushing out the remainder of the burnt combustion products (hopefully at the important rpm ranges.)
Also note that the cylinder head has been unshrouded all the way to the very edge of the cylinder, to allow the air to better enter/exit .....It makes my head hurt trying to figure out why they welded up that set of heads you showed to increase the shrouding.
Whay may look like shrouding in the photos works like a diffuser in the real life. I have one set of flow numbers for those heads, and they flow better than any of the other heads I've ever held in my hands. The flow is high enough at mid lifts that there's no need to go to larger-diameter lifter buckets for any conceivable 928 engine that I could end up with.
I have every dimension of the head measured also. The numbers are in a spreadsheet, almost enough to put them in a three-dimensional computer model if I go ahead and buy the software that can do that.
Once I can find someone with a flow bench that has a tumble measurement honeycomb wheel, they'll go on the flow bench again. When there's so much flow, really more than enough, I want to know whether these heads also produce tumble as efficiently that I think they do based on the valve and port angles.
Then there's the question whether the head burns efficiently. By simple logic, those heads will burn fast. The distance from the spark plug is short because the combustion chamber is compact.
For 928, I'm leaning towards the conclusion that it's really the efficiency in the flow vs tumble tradeoff that determines how good the heads are. That's just a hunch at this point, it's not like I've tried a bunch of different heads on actual engines.
Whay may look like shrouding in the photos works like a diffuser in the real life. I have one set of flow numbers for those heads, and they flow better than any of the other heads I've ever held in my hands. The flow is high enough at mid lifts that there's no need to go to larger-diameter lifter buckets for any conceivable 928 engine that I could end up with.
I have every dimension of the head measured also. The numbers are in a spreadsheet, almost enough to put them in a three-dimensional computer model if I go ahead and buy the software that can do that.
Once I can find someone with a flow bench that has a tumble measurement honeycomb wheel, they'll go on the flow bench again. When there's so much flow, really more than enough, I want to know whether these heads also produce tumble as efficiently that I think they do based on the valve and port angles.
Then there's the question whether the head burns efficiently. By simple logic, those heads will burn fast. The distance from the spark plug is short because the combustion chamber is compact.
For 928, I'm leaning towards the conclusion that it's really the efficiency in the flow vs tumble tradeoff that determines how good the heads are. That's just a hunch at this point, it's not like I've tried a bunch of different heads on actual engines.
Makes sense. It will be an interesting experiment, if you get to it.
The reality of these 928 engines is that there is so little money, effort, and people working on development that almost everything is a "hunch".....all one can do is "go for it" and see how their idea works. I'm absolutely convinced (along with my wife) that none of the development hours I've spent making pieces and developing products will ever pay me back for the time spent. Most all of my pieces will take years and years to even repay just what it costs to make them, because of the economics of scale required to produce them.
If I was interested in making money, all that time would have been better spent doing service work on late model Porsches.
However, that is boring and requires no creative thinking.
Keep in mind that the valves spend a very tiny amount of time sitting this close to the piston. Also keep in mind that the piston is chasing the exhaust valve and has slowed significantly when it catches up with the valve.....peak flow is long gone. The other thing that is happening (when the valves are this close to the piston) is that with properly designed intake system, that opening intake valve has an inertial pulse behind it (or in your case, positive pressure)....which is pushing out the remainder of the burnt combustion products (hopefully at the important rpm ranges.)
Of course whether this matters at all depends on the cam. If the valve pocket is way too deep and if the cam barely lifts the valve at TDC, then porting those valve pockets isn't going to help (and might conceivably hurt, but probably not).
Regarding the tiny amount of time, the valve overlap is arguably the most critical part in the induction cycle. With well tuned exhaust, you have high vacuum in the combustion chamber. If one doesn't get the intake flowing during overlap, it's hard to recover from it later in the cycle.
The 928 four-valve head flows very well at low lifts, so I don't think it's getting more low-lift intake flow that is critical for the overlap in our engines. I think its a lot more important where the intake flows. If everything else including pressures are great, it's still the case that the fresh exhaust charge can go straight out the exhaust. (Or if the tune is off at some rpms or exhaust back pressure is too high, the exhaust goes straight to intake.) I am looking at this piston porting possibility not so much from the angle of getting more flow but thinking if its possible to get the intake flow in the right place during the overlap.
Piston manufacturers offer this sort of features for custom pistons. For two-valve heads, they seem to be called "radiused" valve pockets. For four-valve heads, they seem to be called clip or slant cuts, depending on how they are done. Clip cut style would be the one applicable to the S4 piston. So it's not like I'm the first person to invent the wheel here.
(A digression: Regarding the intake tuning, the intake-valve closed pipe resonance tuning that people normally think about when they think about intake tuning may or may not be needed. There's often a conflict between what intake runner and plenum would tune well for the IVC-IVO period and what would tune well for the IVO-IVC period. If one can set up the exhaust to create a strong pull in the combustion chamber at IVO, then one doesn't have to try to tune the intake for the IVC-IVO period and can tune the intake for the IVO-IVC period without compromises. It's my sense that the high hp/l normally aspirated engines have mostly been built this way.)
Of course whether this matters at all depends on the cam. If the valve pocket is way too deep and if the cam barely lifts the valve at TDC, then porting those valve pockets isn't going to help (and might conceivably hurt, but probably not).
Regarding the tiny amount of time, the valve overlap is arguably the most critical part in the induction cycle. With well tuned exhaust, you have high vacuum in the combustion chamber. If one doesn't get the intake flowing during overlap, it's hard to recover from it later in the cycle.
The 928 four-valve head flows very well at low lifts, so I don't think it's getting more low-lift intake flow that is critical for the overlap in our engines. I think its a lot more important where the intake flows. If everything else including pressures are great, it's still the case that the fresh exhaust charge can go straight out the exhaust. (Or if the tune is off at some rpms or exhaust back pressure is too high, the exhaust goes straight to intake.) I am looking at this piston porting possibility not so much from the angle of getting more flow but thinking if its possible to get the intake flow in the right place during the overlap.
Piston manufacturers offer this sort of features for custom pistons. For two-valve heads, they seem to be called "radiused" valve pockets. For four-valve heads, they seem to be called clip or slant cuts, depending on how they are done. Clip cut style would be the one applicable to the S4 piston. So it's not like I'm the first person to invent the wheel here.
(A digression: Regarding the intake tuning, the intake-valve closed pipe resonance tuning that people normally think about when they think about intake tuning may or may not be needed. There's often a conflict between what intake runner and plenum would tune well for the IVC-IVO period and what would tune well for the IVO-IVC period. If one can set up the exhaust to create a strong pull in the combustion chamber at IVO, then one doesn't have to try to tune the intake for the IVC-IVO period and can tune the intake for the IVO-IVC period without compromises. It's my sense that the high hp/l normally aspirated engines have mostly been built this way.)
It seems like it would also depend on what the combustion chamber looked like.
For example:
.....I'm trying to envision piston position in relationship to that flat area that got added into your combustion chambers. Before, during, and after overlap....seems like the welded area would be blocking anything that was done on the outside 1/4 of the valve pocket on the piston. Any relief out the side of the valve pocket would seem like it would be in a fairly useless area to affect airflow.
I been drawing cross sections (in my mind) and it seems like the piston is going to have to be pretty far in the hole, before that added welded wall isn't in the way of airflow to the outer part of the actual piston.....where most of any pocket porting is done.
03-17-2017, 09:18 AM
