Why are 944 engines interference-type?
05-07-2002, 07:23 PM
#16
Burning Brakes
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[quote]Originally posted by TomH:
<strong>
Personally, I like d) the best, ah, what was that term I learned in Marketing 301, "Planned Obsolesence" Design it so it brakes, so the consumer has to buy a whole new one <img src="graemlins/nono.gif" border="0" alt="[nono]" /> </strong><hr></blockquote>
I agree and this just makes me want to go hang a few car executives. Bastards!
<strong>
Personally, I like d) the best, ah, what was that term I learned in Marketing 301, "Planned Obsolesence" Design it so it brakes, so the consumer has to buy a whole new one <img src="graemlins/nono.gif" border="0" alt="[nono]" /> </strong><hr></blockquote>
I agree and this just makes me want to go hang a few car executives. Bastards!
05-07-2002, 07:28 PM
#17
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A new question all together...
Why are the belts so prone to failure on the 944 adn why do they need all this damn tensioning? Newer economy cars use belts for sure and can go 80-100k without a belt change or any worries, and also don't need tensioning. WTF?
ALso what does the new M96 engine use? Belt or Chain? Change intervals?
Also there are alot of serious track and race cars out there that 951 based. 400-500 hp has been done. How do they survive with this ****ty timing belt?
Why are the belts so prone to failure on the 944 adn why do they need all this damn tensioning? Newer economy cars use belts for sure and can go 80-100k without a belt change or any worries, and also don't need tensioning. WTF?
ALso what does the new M96 engine use? Belt or Chain? Change intervals?
Also there are alot of serious track and race cars out there that 951 based. 400-500 hp has been done. How do they survive with this ****ty timing belt?
05-07-2002, 10:44 PM
#18
Race Director
[quote]Originally posted by Danno:
<strong>Uh, compression ratio has everything to do with inteference. </strong><hr></blockquote>
I hate to disagree with you Danno, but I have to. You have oversold your argument. You are mostly right, but not 100% right. It doesn't have everything to do with interference. Valve lift has a significant impact on interference as well. I should think valve placement and angle in the head are also factors.
Otherwise I agree.
<strong>Uh, compression ratio has everything to do with inteference. </strong><hr></blockquote>
I hate to disagree with you Danno, but I have to. You have oversold your argument. You are mostly right, but not 100% right. It doesn't have everything to do with interference. Valve lift has a significant impact on interference as well. I should think valve placement and angle in the head are also factors.
Otherwise I agree.
05-07-2002, 10:50 PM
#19
Race Director
[quote]Originally posted by Danno:
<strong>Yes, people have re-shaped the piston tops to maintain the identical combustion-chamber volume... </strong><hr></blockquote>
OK, again, picking at straws, but.....
Piston shape has nothing to do with combustion chamber volume. The volume of the combustion chamber stops at the mounting surface of the head. To calculate CR, one must know the volume within the gasket, the volume from the deck of the block to the crown of the piston (wrong words, but you get the idea) and the positive volume of the dish or negative volume of the dome of the piston.
I know what you meant was total volume at TDC, but I just wanted to clarify for others.
<strong>Yes, people have re-shaped the piston tops to maintain the identical combustion-chamber volume... </strong><hr></blockquote>
OK, again, picking at straws, but.....
Piston shape has nothing to do with combustion chamber volume. The volume of the combustion chamber stops at the mounting surface of the head. To calculate CR, one must know the volume within the gasket, the volume from the deck of the block to the crown of the piston (wrong words, but you get the idea) and the positive volume of the dish or negative volume of the dome of the piston.
I know what you meant was total volume at TDC, but I just wanted to clarify for others.
05-07-2002, 10:52 PM
#20
Race Director
[quote]Originally posted by brh986:
<strong></strong><hr></blockquote>
Is that your car?
Did you attend Spenard-David?
I attended the school 5/29-5/31 in 89.
<strong>
</strong><hr></blockquote>Is that your car?
Did you attend Spenard-David?
I attended the school 5/29-5/31 in 89.
05-07-2002, 11:17 PM
#21
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There's lots of very reliable belt-driven interference engines out there: the Volvo engine in the 240 and 740 series is well-nigh indestructible (the B230, which displaces ~2.3 liters and has (I believe) higher than 8:1 compression). You replace the belt every 60K or so.
The difference is the material of the engine, I believe. With an aluminum block and heads, and the wide temperature variance range these 944 engines operate in, there is a lot of expansion and contraction going on during a duty cycle. Lots more, I suspect, than an engine with cast iron block and heads, which is what the Volvo employs.
This wide range of expansion and contraction on the aluminum engine means you need to use a timing belt; I don't think a simple chain would do well in that application. Someone mentioned a 2 stage chain, which might work, but that's more parts, more weight, more complexity.
But all that stretching the belt undergoes as the engine temperature cycles is hard on it, too, and I suspect it shortens its life. In light of all this, the engine -should- have been made a non-interference type and the belt tensioning -should- have been made simpler to perform. It's a definite shortcoming.
Thaddeus
The difference is the material of the engine, I believe. With an aluminum block and heads, and the wide temperature variance range these 944 engines operate in, there is a lot of expansion and contraction going on during a duty cycle. Lots more, I suspect, than an engine with cast iron block and heads, which is what the Volvo employs.
This wide range of expansion and contraction on the aluminum engine means you need to use a timing belt; I don't think a simple chain would do well in that application. Someone mentioned a 2 stage chain, which might work, but that's more parts, more weight, more complexity.
But all that stretching the belt undergoes as the engine temperature cycles is hard on it, too, and I suspect it shortens its life. In light of all this, the engine -should- have been made a non-interference type and the belt tensioning -should- have been made simpler to perform. It's a definite shortcoming.
Thaddeus
05-07-2002, 11:53 PM
#22
In one of the last 2 Excellence issues there was an article on the Holy 944 Grail, the 924 Carrera prototype. There was a picture of it's timing chain and it is easy to see why they went to a belt as it is very long and complicated. It also drives both cams. I think there are lots of issues here. The 944/928 engine owes much to TOP (Thermally Optimized Porsche) research. This undoubtably dictated many of the factors which led to an interference design in the quest to get maximum efficiency and power within the range of available fuels.
The choice of belt over chain was probably because the budget and the selling price were both limited. Also noise probably was a factor especially as noise and vibration were major factors that Porsche wished to address vs the Audi motor.
We must remember that it was well into the 944's lifetime that Porsche came up with a definitive tensioner for the 911, so chains are not the perfect answer either. As was mentioned before the thermal cycle stretch is significant and probably the main reason for belt replacement rather than breakage or wear. (The "broken" timing belt is more likely a collapsed idler or stretched belt). As to more recent utilitarian cars having less maintenance, they benefit from more recent technology. If someone offered a 100000mi belt would you really trust it? Which begs the question that if the idlers stc become the real problem, the belt itself isn't that expensive to replace when you're in there anyhow.
My final grumble is about "Planned Obsolensce". It seems an insult to the designers to suggest it. These cars are frequently in wonderful shape when decently maintained in spite of 6 digit milages and being 10-20 years old. I have seen somre things be more complicated than they need be (it is after all a German car!) but not so much because of shoddy manufacture.
My simple opinions.
The choice of belt over chain was probably because the budget and the selling price were both limited. Also noise probably was a factor especially as noise and vibration were major factors that Porsche wished to address vs the Audi motor.
We must remember that it was well into the 944's lifetime that Porsche came up with a definitive tensioner for the 911, so chains are not the perfect answer either. As was mentioned before the thermal cycle stretch is significant and probably the main reason for belt replacement rather than breakage or wear. (The "broken" timing belt is more likely a collapsed idler or stretched belt). As to more recent utilitarian cars having less maintenance, they benefit from more recent technology. If someone offered a 100000mi belt would you really trust it? Which begs the question that if the idlers stc become the real problem, the belt itself isn't that expensive to replace when you're in there anyhow.
My final grumble is about "Planned Obsolensce". It seems an insult to the designers to suggest it. These cars are frequently in wonderful shape when decently maintained in spite of 6 digit milages and being 10-20 years old. I have seen somre things be more complicated than they need be (it is after all a German car!) but not so much because of shoddy manufacture.
My simple opinions.
05-08-2002, 01:39 AM
#23
Race Director
[quote]Originally posted by Thaddeus:
<strong>This wide range of expansion and contraction on the aluminum engine means you need to use a timing belt; I don't think a simple chain would do well in that application. </strong><hr></blockquote>
On the contrary.
The Nissan SR20 family of engines is all aluminum and uses a single chain. Pretty damned indestructable too. I've torn down a few of them with high miles and looked closely at many others and those chains just don't show any wear are are far more trouble free than the belt on the 944.
<strong>This wide range of expansion and contraction on the aluminum engine means you need to use a timing belt; I don't think a simple chain would do well in that application. </strong><hr></blockquote>
On the contrary.
The Nissan SR20 family of engines is all aluminum and uses a single chain. Pretty damned indestructable too. I've torn down a few of them with high miles and looked closely at many others and those chains just don't show any wear are are far more trouble free than the belt on the 944.
05-08-2002, 04:59 AM
#24
Race Director
"Can the engine be reengineered by using pistons with valve reliefs and possibly shorter crank throws to move the piston away from the valve, yet still fill the combustion chamber to get a decent compression ratio?"
Sure... Here's some photos of the stock 951 pistons:
And a cross-section of the combustion chamber:
In the second photo, I cut out some deeper valve-reliefs and piled up the material on the other side of the piston to keep combustion-chamber volume the same.
Here's another way to do it with aftermarket JE pistons:
"You are mostly right, but not 100% right. It doesn't have everything to do with interference. Valve lift has a significant impact on interference as well. I should think valve placement and angle in the head are also factors."
Yes, I admit that I committed one of my own pet-peeves; blanket all-encompassing black&white yes/no statements. Ok, let's go into the nitty-gritty details here. George is right on about the valve train as well (more later).
Now when I say 'combustion chamber' I mean the space defined by the opening in the head, the headgasket and the volume added or subtracted by the dome on the piston. This net 'combustion chamber volume' can then be used with the 'bore & stroke volume' to determine the compression-ratio.
Additionally, the 'combustion chamber shape' is a very, very important factor in piston-interference and efficient burning of fuel as well (how to extra the most power from any given piece of fuel). With 2-valve heads, large valves with lots of lift is required (0.434/0.472" lift on the 951) to fill any particular combustion chamber volume. However, 4-valve heads typically use a lot less lift like 0.220-0.250" for the same displacement (allows higher-revs too). This means 4-valve heads can have higher-compression with the same amount of piston-interference.
In addition to the interference issue, 4-valve heads has some performance-enhancing benefits, namely fast combustion due to quick propagation of the flame-front. A high-compression 2-valve head tends to have a very large convex dome projecting into the pocket in the head:
This stretches out the combustion-chamber shape into a thin crescent. This means that when the spark is ignited, even at the very center, it still has to travel quite a ways to get to the edge of the chamber. This gives slow and lazy combustion, not the best for power (that's why dual-plugs are an upgrade for 911s). Contrast that with the more centralized triangular shape of the 951 head above. However, on the 951, the spark still has to travel from one side of the chamber all the way across to the other. Time for a 4-valve head with a centralized spark-plug.
A 4-valve head on the other hand, has a much narrower included-angle between the valves (15-20 degrees). This flatter combustion chamber shape, with the valves pointing downwards towards the crank more, doesn't require as much of a piston dome to achieve the same compression-ratio. With a centralized spark-plug and a flat pancake-shaped combustion chamber, the flame-front propagation and burn is quick for more power.
"Why are the belts so prone to failure on the 944 adn why do they need all this damn tensioning? "
Partly has to do with the specifications of the belt. On my Supra, the timing belt was about twice as wide at 30mm and it was thicker too. I only had to replace that belt ONCE during the 10-years and 250k-miles that I had the car. I think the addition some Kevlar fibre would really help out the smaller 944/951 belts.
And the arguments against a chain I think are just rationalizations (size, weight, complexity). The cam chain on my Honda motorcycle is barely larger than a bicycle chain and it spins 4 cams off the center of the crank at up to 13,000rpm! It uses a slipper-type tensioner like the 944S2/968 so those have to be replaced every once in a while. An automatic hydraulic roller tensioner like a lot of Japanese cars would make chain-tension worry-free.
Sure... Here's some photos of the stock 951 pistons:
And a cross-section of the combustion chamber:
In the second photo, I cut out some deeper valve-reliefs and piled up the material on the other side of the piston to keep combustion-chamber volume the same.
Here's another way to do it with aftermarket JE pistons:
"You are mostly right, but not 100% right. It doesn't have everything to do with interference. Valve lift has a significant impact on interference as well. I should think valve placement and angle in the head are also factors."
Yes, I admit that I committed one of my own pet-peeves; blanket all-encompassing black&white yes/no statements. Ok, let's go into the nitty-gritty details here. George is right on about the valve train as well (more later).
Now when I say 'combustion chamber' I mean the space defined by the opening in the head, the headgasket and the volume added or subtracted by the dome on the piston. This net 'combustion chamber volume' can then be used with the 'bore & stroke volume' to determine the compression-ratio.
Additionally, the 'combustion chamber shape' is a very, very important factor in piston-interference and efficient burning of fuel as well (how to extra the most power from any given piece of fuel). With 2-valve heads, large valves with lots of lift is required (0.434/0.472" lift on the 951) to fill any particular combustion chamber volume. However, 4-valve heads typically use a lot less lift like 0.220-0.250" for the same displacement (allows higher-revs too). This means 4-valve heads can have higher-compression with the same amount of piston-interference.
In addition to the interference issue, 4-valve heads has some performance-enhancing benefits, namely fast combustion due to quick propagation of the flame-front. A high-compression 2-valve head tends to have a very large convex dome projecting into the pocket in the head:
This stretches out the combustion-chamber shape into a thin crescent. This means that when the spark is ignited, even at the very center, it still has to travel quite a ways to get to the edge of the chamber. This gives slow and lazy combustion, not the best for power (that's why dual-plugs are an upgrade for 911s). Contrast that with the more centralized triangular shape of the 951 head above. However, on the 951, the spark still has to travel from one side of the chamber all the way across to the other. Time for a 4-valve head with a centralized spark-plug.
A 4-valve head on the other hand, has a much narrower included-angle between the valves (15-20 degrees). This flatter combustion chamber shape, with the valves pointing downwards towards the crank more, doesn't require as much of a piston dome to achieve the same compression-ratio. With a centralized spark-plug and a flat pancake-shaped combustion chamber, the flame-front propagation and burn is quick for more power.
"Why are the belts so prone to failure on the 944 adn why do they need all this damn tensioning? "
Partly has to do with the specifications of the belt. On my Supra, the timing belt was about twice as wide at 30mm and it was thicker too. I only had to replace that belt ONCE during the 10-years and 250k-miles that I had the car. I think the addition some Kevlar fibre would really help out the smaller 944/951 belts.
And the arguments against a chain I think are just rationalizations (size, weight, complexity). The cam chain on my Honda motorcycle is barely larger than a bicycle chain and it spins 4 cams off the center of the crank at up to 13,000rpm! It uses a slipper-type tensioner like the 944S2/968 so those have to be replaced every once in a while. An automatic hydraulic roller tensioner like a lot of Japanese cars would make chain-tension worry-free.
05-08-2002, 08:57 AM
#25
Race Director
[quote]Originally posted by Danno:
<strong> In addition to the interference issue, 4-valve heads has some performance-enhancing benefits, namely fast combustion due to quick propagation of the flame-front. </strong><hr></blockquote>
They also tend to have better quench areas that help resist detonation.
Quench forces the bulk of the mixture towards the spark plug, leaving little mixture at the outer edges of the piston/combustion chamber.
<strong> In addition to the interference issue, 4-valve heads has some performance-enhancing benefits, namely fast combustion due to quick propagation of the flame-front. </strong><hr></blockquote>
They also tend to have better quench areas that help resist detonation.
Quench forces the bulk of the mixture towards the spark plug, leaving little mixture at the outer edges of the piston/combustion chamber.
05-08-2002, 02:32 PM
#26
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My understanding from a very experienced Porsche mechanic was that the 944 Turbo engines are NON interference type. At least in stock condition. The effect of more clearance because of lower compression ratio.
05-08-2002, 04:54 PM
#28
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[quote]Originally posted by Danno:
<strong>Here's another way to do it with aftermarket JE pistons:
</strong><hr></blockquote>
Do you know a part number, or the cost? I'm curious why these aren't used all the time to do a rebuild on a 951.
Thanks,
Miles
<strong>Here's another way to do it with aftermarket JE pistons:
</strong><hr></blockquote>
Do you know a part number, or the cost? I'm curious why these aren't used all the time to do a rebuild on a 951.
Thanks,
Miles
05-09-2002, 04:24 AM
#29
Race Director
"My understanding from a very experienced Porsche mechanic was that the 944 Turbo engines are NON interference type. At least in stock condition. The effect of more clearance because of lower compression ratio. "
Heh, heh... that mechanic probably hasn't tried turning the engine over by hand with one of the valves fully open eh? (with timing-belt disconnected)
The piston pictured above is for a custom-built bored-out engine. With steel sleeves, it comes out to a 104mm bore I think. You can't run bare aluminum pistons inside the aluminum (Alusil) bores of our engines. They need to be iron-coated, hence the lack of plentiful aftermarket pistons. I think U.S. Custom is one of the few that can (or will) iron coat your pistons.
I was wondering if the Telflon coatings offered by some outfits like Swain may be durable enough to last. I'll have to check with some people who've been using that coating and the ceramic tops...
Heh, heh... that mechanic probably hasn't tried turning the engine over by hand with one of the valves fully open eh? (with timing-belt disconnected)
The piston pictured above is for a custom-built bored-out engine. With steel sleeves, it comes out to a 104mm bore I think. You can't run bare aluminum pistons inside the aluminum (Alusil) bores of our engines. They need to be iron-coated, hence the lack of plentiful aftermarket pistons. I think U.S. Custom is one of the few that can (or will) iron coat your pistons.
I was wondering if the Telflon coatings offered by some outfits like Swain may be durable enough to last. I'll have to check with some people who've been using that coating and the ceramic tops...
