Another one for the pros, Head Questions
01-12-2006, 04:25 PM
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Hello, all
I don't think Porsche ever offered a turbo version of the "S" head. I would guess there is some basic design issue- a cooling problem or casting thickness variance- that make it unsuitable for a production car. Did they race a turbo 4-valve?
There are lots of articles/websites on combustion chamber theory. Quench or squish is a hot topic and opinions vary. In general people familiar with quench-type engines think you need it and people working on "open chamber" engines think it is a band aid. In your pictures, the 2-valve head is a classic "quench" design. The middle one I'd classify as open chamber. The bottom picture I would call a "squish" design. If the motor is designed around quench/squish, then you need to keep these areas narrow. If you use the valve-relieved piston with the bottom 4-valve, you would probably increase detonation sensitivity.
Some basics:
Your motor does three things. It is a pump, it is a combustion chamber, and it manages heat. Of the three, improving pump function will most increase performance. The two most important factors for pump function is intake charge mass and compression ratio. Charge mass is effected by pressure, temperature, and flow dynamics. Compression varies with "static" design and intake timing.
If your goal was to maximize combustion effectiveness, detonation would be a good thing. However, it exceeds most motor's ability to transfer heat/absorb mechanical shock so you get burnt pistons/hammered rod bearings. Cars in detonation often emit gray-sh-white exhaust smoke- it is aluminum oxide from burning piston crowns.
The most basic and most visible engine feature associated with detonation sensitivity is spark plug location. In 2-valve engines, the plug is off to one side so, for a given chamber shape, the maximum flame front travel distance is longer than if the engine had a more central plug. Your pictures illustrate this quite nicely. The longer a flame front travels, the more likely it will detonate. If an engine has squish/quench areas, it needs them. Unless you shorten the max flame travel distance some other way- adding a second plug is most common- or the squish is a "legacy" feature, removing it will lower the detonation limit.
Squish and quench are often interchanged but I think they are separate. Quench describes a combustion chamber designed to cool a traveling flame front by increasing the relative surface area. Squish an area of minimal piston/deck clearance design to exclude that area from the combustion process. The 2-valve has a defined chamber that tapers away from the plug. The taper increases surface area per cubic centimeter of combusting gas, more heat is removed, and detonation is avoided. The 4-valve head has flat areas around a defined oval chamber. These areas won't sustain combustion untill well after the primary combustion wave is spent. Personally, I don't think quench works but squish does. A closer look at the 951 head shows it has a lot of "squish" around the "quench." Simply put, areas under the squish participate in the pump function but not in combustion.
Some other generalizations: Intake heat is good for combustion, bad for pumping, heat management so cold intake increases power. Turbos reclaim exhaust energy but the common statements that they work by increaseing combustion pressure or effective compression ratio are misleading. They work much like a two-stage air compressor- by increasing the intake charge mass for the "second stage." What is called effective CR is really an estimation of the combustion detonation limit. Combustion chamber contours help manage where the fuel goes. The heart shaped chamber was patented by Harry Weslake in the early 50's for BMC (MG). In general, shapes help at lower RPM but can hurt at high speed. GM has had noise issues related to squish- carbon builds up on the piston and starts hitting the head. MG used to recommend "decoking" at regular intervals. The considered rod bearing routine maintenance at 60k ;-)
A couple examples:
http://www.chevyhiperformance.com/techarticles/94138/
Small block quench (or, in my opinion quench-squish) in it's classic form.
http://www.kb-silvolite.com/article....n=read&A_id=35
Kieth Black is a legend in drag racing and builds supercharded Chrysler Hemis. Without squish the ring lands would melt. His opinion is valid but shouldn't be generalized as this is a very specialized application. Classic hot rod vocabulary adds entertainment value. (article not by Kieth himself)
http://powerlab.mech.okayama-u.ac.jp...8/Html/003.htm
If you have the background and caffeine, the answer you seek is on this page. Or at least I think it is. My head hurt after reading a few titles.
http://www.erc.wisc.edu/modeling/mul...EM-UM_2005.pdf
Newer work by one of today's top researchers. Specific to detonation.
http://customclassictrucks.com/techa.../0405cct_head/
http://www.gmhightechperformance.com...s/0405htp_ls2/
Chevy heads- notice the Weslake widow's peak on the newer designs. Look closely at the "original" head. At some point GM moved the plug away from the valves- probably to deal with heat issues.
http://www.musclemustangfastfords.co...MMFF_MixMatch/
Ford Modular 4.6 info. Notice they dropped the Weslake peak on the newer motors, strong evidence that there is no "right answer."
http://www.custompistols.com/cars/articles/cracks_3.htm
Finally, an original Weslake MGA head showing it's age but the shape looks familiar.
Hope this helps,
Bill
I don't think Porsche ever offered a turbo version of the "S" head. I would guess there is some basic design issue- a cooling problem or casting thickness variance- that make it unsuitable for a production car. Did they race a turbo 4-valve?
There are lots of articles/websites on combustion chamber theory. Quench or squish is a hot topic and opinions vary. In general people familiar with quench-type engines think you need it and people working on "open chamber" engines think it is a band aid. In your pictures, the 2-valve head is a classic "quench" design. The middle one I'd classify as open chamber. The bottom picture I would call a "squish" design. If the motor is designed around quench/squish, then you need to keep these areas narrow. If you use the valve-relieved piston with the bottom 4-valve, you would probably increase detonation sensitivity.
Some basics:
Your motor does three things. It is a pump, it is a combustion chamber, and it manages heat. Of the three, improving pump function will most increase performance. The two most important factors for pump function is intake charge mass and compression ratio. Charge mass is effected by pressure, temperature, and flow dynamics. Compression varies with "static" design and intake timing.
If your goal was to maximize combustion effectiveness, detonation would be a good thing. However, it exceeds most motor's ability to transfer heat/absorb mechanical shock so you get burnt pistons/hammered rod bearings. Cars in detonation often emit gray-sh-white exhaust smoke- it is aluminum oxide from burning piston crowns.
The most basic and most visible engine feature associated with detonation sensitivity is spark plug location. In 2-valve engines, the plug is off to one side so, for a given chamber shape, the maximum flame front travel distance is longer than if the engine had a more central plug. Your pictures illustrate this quite nicely. The longer a flame front travels, the more likely it will detonate. If an engine has squish/quench areas, it needs them. Unless you shorten the max flame travel distance some other way- adding a second plug is most common- or the squish is a "legacy" feature, removing it will lower the detonation limit.
Squish and quench are often interchanged but I think they are separate. Quench describes a combustion chamber designed to cool a traveling flame front by increasing the relative surface area. Squish an area of minimal piston/deck clearance design to exclude that area from the combustion process. The 2-valve has a defined chamber that tapers away from the plug. The taper increases surface area per cubic centimeter of combusting gas, more heat is removed, and detonation is avoided. The 4-valve head has flat areas around a defined oval chamber. These areas won't sustain combustion untill well after the primary combustion wave is spent. Personally, I don't think quench works but squish does. A closer look at the 951 head shows it has a lot of "squish" around the "quench." Simply put, areas under the squish participate in the pump function but not in combustion.
Some other generalizations: Intake heat is good for combustion, bad for pumping, heat management so cold intake increases power. Turbos reclaim exhaust energy but the common statements that they work by increaseing combustion pressure or effective compression ratio are misleading. They work much like a two-stage air compressor- by increasing the intake charge mass for the "second stage." What is called effective CR is really an estimation of the combustion detonation limit. Combustion chamber contours help manage where the fuel goes. The heart shaped chamber was patented by Harry Weslake in the early 50's for BMC (MG). In general, shapes help at lower RPM but can hurt at high speed. GM has had noise issues related to squish- carbon builds up on the piston and starts hitting the head. MG used to recommend "decoking" at regular intervals. The considered rod bearing routine maintenance at 60k ;-)
A couple examples:
http://www.chevyhiperformance.com/techarticles/94138/
Small block quench (or, in my opinion quench-squish) in it's classic form.
http://www.kb-silvolite.com/article....n=read&A_id=35
Kieth Black is a legend in drag racing and builds supercharded Chrysler Hemis. Without squish the ring lands would melt. His opinion is valid but shouldn't be generalized as this is a very specialized application. Classic hot rod vocabulary adds entertainment value. (article not by Kieth himself)
http://powerlab.mech.okayama-u.ac.jp...8/Html/003.htm
If you have the background and caffeine, the answer you seek is on this page. Or at least I think it is. My head hurt after reading a few titles.
http://www.erc.wisc.edu/modeling/mul...EM-UM_2005.pdf
Newer work by one of today's top researchers. Specific to detonation.
http://customclassictrucks.com/techa.../0405cct_head/
http://www.gmhightechperformance.com...s/0405htp_ls2/
Chevy heads- notice the Weslake widow's peak on the newer designs. Look closely at the "original" head. At some point GM moved the plug away from the valves- probably to deal with heat issues.
http://www.musclemustangfastfords.co...MMFF_MixMatch/
Ford Modular 4.6 info. Notice they dropped the Weslake peak on the newer motors, strong evidence that there is no "right answer."
http://www.custompistols.com/cars/articles/cracks_3.htm
Finally, an original Weslake MGA head showing it's age but the shape looks familiar.
Hope this helps,
Bill
01-12-2006, 07:29 PM
#19
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Geo, I pulled that from Forced Induction Performance Tuning by A Grahm Bell. Hella good book, the best I have found by far in reguards to boost. So far it seems to be right on every time, so I'm trusting it when it comes to the head design stuff as well. It just doesn't go into great detail so basicly it's a start ans a teaser.
Bill, they did race a turboed 4 valver, it did quite well for it's first race, completed the LeMans 24 hr race and finished 7th overall IIRC. I think it was a threat to the precious 911 and it was not pursued any further. I'll have to get to those links and see what is to be learned. Hopefully it's not above my head at this point. Thanks for the info man, I'll have to read it a few more times to really absorb it. Thanks!
Patrat, thanks for the pic,it appears to have a squish area, can't be sure just from the pic. Also appears to be a smaller volume chamber too. Homework and comparison will tell. Thanks.
I'm not going to run stock pistons from any of these cars, the pics are merely for comparisons of design.
Thanks again guys!! Andy
Bill, they did race a turboed 4 valver, it did quite well for it's first race, completed the LeMans 24 hr race and finished 7th overall IIRC. I think it was a threat to the precious 911 and it was not pursued any further. I'll have to get to those links and see what is to be learned. Hopefully it's not above my head at this point. Thanks for the info man, I'll have to read it a few more times to really absorb it. Thanks!
Patrat, thanks for the pic,it appears to have a squish area, can't be sure just from the pic. Also appears to be a smaller volume chamber too. Homework and comparison will tell. Thanks.
I'm not going to run stock pistons from any of these cars, the pics are merely for comparisons of design.
Thanks again guys!! Andy
01-13-2006, 12:18 AM
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Now, I haven't taken the time to read this entire post, so if I'm wasting space, my apologies.
Quench is a good thing. As the piston is travelling up, and the space between the piston and the quench pad becomes prapidly smaller, the air/fuel mixture will be squished out. This action promotes a tumbling action in the air/fuel mixture. This results in a more efficient burn.
It also keeps the mixture toward the centre of the cylinder, which keeps the number of hotspots within the combustion chamber to a minimum.
Just think of a boosted engine as an engine with very high compression.
Hope that helps...
Oh, I also plan to use 928 heads as an alternative to the 944 16v heads. I think there will be more performance and reliability there...
Quench is a good thing. As the piston is travelling up, and the space between the piston and the quench pad becomes prapidly smaller, the air/fuel mixture will be squished out. This action promotes a tumbling action in the air/fuel mixture. This results in a more efficient burn.
It also keeps the mixture toward the centre of the cylinder, which keeps the number of hotspots within the combustion chamber to a minimum.
Just think of a boosted engine as an engine with very high compression.
Hope that helps...
Oh, I also plan to use 928 heads as an alternative to the 944 16v heads. I think there will be more performance and reliability there...
01-13-2006, 12:42 AM
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That is what I am finding out also. I'm recovering from my lack of knoweledge, say it with me, squish and quench are gooood. Now I need to design a piston that is crazy enough to work....
Now that I have an explanation of why it works, it makes sence that it would help and not bring on detonation. I'm actually hoping that the 944S head has some squish area now, lol. Please ignore my moodswings.
I beleive that effective CR can be calculated but only with the measurements of the air in the cylinder. Boost, density, temp at a specific engine speed and calculated with the static CR. I guess it's something for a guide but I don't know how accurate it would be. I'm not going to put alot of time into that one, it's waaay over my head for me to even get it close without a full testing facility and a lab(even then i wouldn't know what to do with the equipment anyway, lol). All kinds of stuff happens in that little hole called a cylinder. In short, you are right Geo, not in the real world and not just a calculation to work on paper.
I guess it's about time to start looking at piston design and the crazy little stuff that looks like crop formations on the top.
Now that I have an explanation of why it works, it makes sence that it would help and not bring on detonation. I'm actually hoping that the 944S head has some squish area now, lol. Please ignore my moodswings.
I beleive that effective CR can be calculated but only with the measurements of the air in the cylinder. Boost, density, temp at a specific engine speed and calculated with the static CR. I guess it's something for a guide but I don't know how accurate it would be. I'm not going to put alot of time into that one, it's waaay over my head for me to even get it close without a full testing facility and a lab(even then i wouldn't know what to do with the equipment anyway, lol). All kinds of stuff happens in that little hole called a cylinder. In short, you are right Geo, not in the real world and not just a calculation to work on paper.
I guess it's about time to start looking at piston design and the crazy little stuff that looks like crop formations on the top.
01-13-2006, 12:57 AM
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Static compression is a good place to start. Pretty straight forward -> the volume of the cylinder and combustion chamber minus the displacement of the piston dome. Figure out the volume at BDC and at TDC and you have your static compression.
The compression of the engine will change as revs increase. Cam timing, parts heating up, rod stretch, etc...
The 944S head is known to crack. Just an FYI. I'm actually looking at a 928S head with some port work and slightly reworked combustion chambers....
The compression of the engine will change as revs increase. Cam timing, parts heating up, rod stretch, etc...
The 944S head is known to crack. Just an FYI. I'm actually looking at a 928S head with some port work and slightly reworked combustion chambers....
01-13-2006, 01:26 AM
#24
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OK, repeat after me.....
There is no such thing as dynamic or effective compression ratio. CR is a purely static number.
Cylinder pressures are another issue and there are other ways to measure this and they are also heavily influenced by VE as well as CR.
There is no such thing as dynamic or effective compression ratio. CR is a purely static number.
Cylinder pressures are another issue and there are other ways to measure this and they are also heavily influenced by VE as well as CR.
01-13-2006, 01:13 PM
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This is the lift/downforce arguement all over again. Taylor was a genius and we used his text. I separated the two illustrate how the term quench is misleading. The end of combustion occurs very near peak cylinder pressure. This is about 10-20 degrees after top dead center. The term quench gives me a mental picture of cooling walls first slowing then extinguishing and advancing flame front. I here squish and I see an area of the chamber too narrow to support rapid combustion. Growing up, a neighbor's wife was very large. I was enjoying a refreshment with the man's brother and he said, "I it were me, I'd make all the doors a foot wide while she was out. She could come back when she fits." In my mind, that is how squish works.
Detonation vs preignition is another example. Detonation has little effect on peak pressure timing- it would still be around 10-20 atdc. However, it increases temperature and pressure at the point. In detonation, the ignition event doesn't occur too soon, it occurs too violently.
Taylor's purpose in rigidly defining these terms is to eliminate confusion. How you picture squish/quench or even how they work is less important than clearly defining how a narrow "flame killing/preventing" region can help manage detonation.
His argument against effective compression ratio is different. Effectve CR is interesting but it is a derivative of other variables. For engine design, it adds no value. It is the engine equivelant of the meal lunch. If I call someone I've never met and say "You need to make lunch," the odds that I get what I want when and where I want are pretty slim. Still, most people think lunch is a real event and it paints a mental picture.
In science, it is less important that you agree than it is that you understand each other. Is squish the same as quench? Ask a thirsty cockroach.
Have fun,
Bill
Detonation vs preignition is another example. Detonation has little effect on peak pressure timing- it would still be around 10-20 atdc. However, it increases temperature and pressure at the point. In detonation, the ignition event doesn't occur too soon, it occurs too violently.
Taylor's purpose in rigidly defining these terms is to eliminate confusion. How you picture squish/quench or even how they work is less important than clearly defining how a narrow "flame killing/preventing" region can help manage detonation.
His argument against effective compression ratio is different. Effectve CR is interesting but it is a derivative of other variables. For engine design, it adds no value. It is the engine equivelant of the meal lunch. If I call someone I've never met and say "You need to make lunch," the odds that I get what I want when and where I want are pretty slim. Still, most people think lunch is a real event and it paints a mental picture.
In science, it is less important that you agree than it is that you understand each other. Is squish the same as quench? Ask a thirsty cockroach.
Have fun,
Bill
01-13-2006, 04:35 PM
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Originally Posted by Bill.e1
For engine design, it adds no value.
01-15-2006, 03:37 PM
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Originally Posted by Skunk Workz
Strangely,by careful consideration of this "no-value" parameter we have managed to run 30 psi boost in a pumpgas engine with 9:1 static compression...
A quick example that is relevant to this thread. With a twin cam head, you can change cam timing individually. This gives you another tool for tuning. By retarding the intake, you lower the "effective CR" and reduce valve overlap. This allows you to increase the boost without detonation or overscavening (blowing intake charge out the exhaust).
Notice how everyone runs higher CR than in the 70's/80's? Valve overlap is bad for emissions so production cars have to close the intake later to maintain airflow. This reduces the "effective CR" so mechanical CR's are bumped to compensate. Obviously other factors also apply. The supercharger's popularity surge is also, at least partially, emissions driven. They allow older designs to make power running emissions-spec cams. They keep exhaust gases hot so converters light earlier. Last but not least, it is faster/cheaper to supercharge an older engine than it is to develop a new one.
Regards,
Bill