Why are 944 engines interference-type?
05-06-2002, 09:25 PM
#1
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Why are 944 engines interference-type?
For whatever reason, Porsche decided on using a timing belt instead of a chain. They also decided on making the valves hit the pistons if they are out of sync (like when the belt breaks).
Since you can't very well convert a timing-belt equipped engine to use a chain, I'm wondering if it's possible to remove the possibility of the valves touching the pistons. Especially on the turbos, which only have 8.0:1 compression ratios. Some pistons have valve reliefs cut into them, right? Would that trick work? How much would have to be cut?
I don't think you'd want to go to a cam with less lift, since it would reduce air flow so much. Making the compression even lower than 8:1 would also be a bad move. Is there anything else that would work?
(I don't have a 944 now, and when I get one I'm not planning on spending $1000s on the bottom end to *actually* do anything about this problem. I'm just curious what is possible, and what others think)
Since you can't very well convert a timing-belt equipped engine to use a chain, I'm wondering if it's possible to remove the possibility of the valves touching the pistons. Especially on the turbos, which only have 8.0:1 compression ratios. Some pistons have valve reliefs cut into them, right? Would that trick work? How much would have to be cut?
I don't think you'd want to go to a cam with less lift, since it would reduce air flow so much. Making the compression even lower than 8:1 would also be a bad move. Is there anything else that would work?
(I don't have a 944 now, and when I get one I'm not planning on spending $1000s on the bottom end to *actually* do anything about this problem. I'm just curious what is possible, and what others think)
05-06-2002, 09:39 PM
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i'm no expert, but i'd guess that to make the compression as high as it is, they had to make the engine interference to accomplish this. as far as chain goes... it would be a large chain! ever see a timing chain for american cars, pretty small... less stretch, you'd still have to tension and replace the chain, since there would be more links and therefore more possibility of stretching. the belt turns the water pump, it would be interesting having a chain driven water pump. well i'm interested to see the other responses, but i'm beleiving that they had to in order to acheive the compression that they did.
05-06-2002, 09:55 PM
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[quote]Originally posted by ERAU944:
<strong>i'm no expert, but i'd guess that to make the compression as high as it is, they had to make the engine interference to accomplish this.
</strong><hr></blockquote>
That seems to make sense in general, but I get confused when I look at compression ratios for the 924, 931, and 951.
<a href="http://www.connact.com/~kgross/FAQ/944faq02.html" target="_blank">http://www.connact.com/~kgross/FAQ/944faq02.html</a> :
83-86 944: 9.5:1
86+ 944, 8V: 10.2:1
87+ 944, 16V: 10.9:1
951: 8.0:1
924: 8, 8.5, 9, or 9.3:1
931: 7.5, 8, or 8.5:1
Now, from what I've heard, all 944/951/931/924S are interference type, and all non-931, non-S 924 are non-interference type.
So how can a 924 have 9.3:1 and be non-interference, while a 951 has 8.0:1 and is interference?
It must be because of the lift of the cam.
<strong>i'm no expert, but i'd guess that to make the compression as high as it is, they had to make the engine interference to accomplish this.
</strong><hr></blockquote>
That seems to make sense in general, but I get confused when I look at compression ratios for the 924, 931, and 951.
<a href="http://www.connact.com/~kgross/FAQ/944faq02.html" target="_blank">http://www.connact.com/~kgross/FAQ/944faq02.html</a> :
83-86 944: 9.5:1
86+ 944, 8V: 10.2:1
87+ 944, 16V: 10.9:1
951: 8.0:1
924: 8, 8.5, 9, or 9.3:1
931: 7.5, 8, or 8.5:1
Now, from what I've heard, all 944/951/931/924S are interference type, and all non-931, non-S 924 are non-interference type.
So how can a 924 have 9.3:1 and be non-interference, while a 951 has 8.0:1 and is interference?
It must be because of the lift of the cam.
05-06-2002, 10:44 PM
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I did a little research about the differences between timing chains and timing belts and came across this info:
This is becoming the dominant method. Over half of the cars on the road use timing belts. There are several reasons for this:
a) It's easier to connect the camshaft and crankshaft together with a belt when the camshaft is far away from the crankshaft (like with overhead cam engines).
b) They claim the belt is quieter. Belts are lighter than chains, and every ounce counts to the manufacturers.
c) It's much cheaper and simpler to make a belt drive engine than any other way.
d) When the belt breaks it can destroy the motor, so it increases new car sales or repairs.
As for cars that almost always bend valves when the belt breaks: If you own one of these cars, replace the belt every 50,000 mile or... "YOU'LL BE SORRY!!!"
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]" />
This is becoming the dominant method. Over half of the cars on the road use timing belts. There are several reasons for this:
a) It's easier to connect the camshaft and crankshaft together with a belt when the camshaft is far away from the crankshaft (like with overhead cam engines).
b) They claim the belt is quieter. Belts are lighter than chains, and every ounce counts to the manufacturers.
c) It's much cheaper and simpler to make a belt drive engine than any other way.
d) When the belt breaks it can destroy the motor, so it increases new car sales or repairs.
As for cars that almost always bend valves when the belt breaks: If you own one of these cars, replace the belt every 50,000 mile or... "YOU'LL BE SORRY!!!"
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]" />
05-06-2002, 10:54 PM
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Sadly, the design of the 944 engine is pi$$ poor. Someone once said "Lousy design, brilliantly executed". So, we put up with the knowledge that a couple of grand hinge on the strength or weakness of a fabric belt.
That being said, there is really no reason that the original design had to be interference fit. It is flow that dictates breathing, and valve diameter can be increased, (especially when there are only two valves per cylinder), pistons can be notched, any number of ways clearance can be obtained. Interestingly, Volkswagen's VR6 engine uses a two stage chain. I like chains. I also like external water pumps that run off an accessory belt. I also like engines that do not require 40+ pounds of cast iron contra rotating to prevent the engine from tearing loose from its all to fragile motor mounts. This is the first vehicle I have ever worked on that requires a $400 tool to adjust a timing belt. And sometimes I wonder how much of this is Porsche Hype. Query...Who makes and sells the P9201 tool? Couldn't be Porsche, now, could it?
Interesting car. Hardly a month goes by that I do not threaten at least once to take it to a crusher and turn it into a coffee table. But, like the rest of you, I soldier on, fixing this, replacing that, and wondering which mileage marker has my timing belt's name on it.
Cheers!
Bob S.
That being said, there is really no reason that the original design had to be interference fit. It is flow that dictates breathing, and valve diameter can be increased, (especially when there are only two valves per cylinder), pistons can be notched, any number of ways clearance can be obtained. Interestingly, Volkswagen's VR6 engine uses a two stage chain. I like chains. I also like external water pumps that run off an accessory belt. I also like engines that do not require 40+ pounds of cast iron contra rotating to prevent the engine from tearing loose from its all to fragile motor mounts. This is the first vehicle I have ever worked on that requires a $400 tool to adjust a timing belt. And sometimes I wonder how much of this is Porsche Hype. Query...Who makes and sells the P9201 tool? Couldn't be Porsche, now, could it?
Interesting car. Hardly a month goes by that I do not threaten at least once to take it to a crusher and turn it into a coffee table. But, like the rest of you, I soldier on, fixing this, replacing that, and wondering which mileage marker has my timing belt's name on it.
Cheers!
Bob S.
05-06-2002, 11:47 PM
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[quote]
Now, from what I've heard, all 944/951/931/924S are interference type, and all non-931, non-S 924 are non-interference type.
So how can a 924 have 9.3:1 and be non-interference, while a 951 has 8.0:1 and is interference?
It must be because of the lift of the cam.
<hr></blockquote>
Well, to answer your question, the 924(non s versions) all had a 2 liter audi engine. This is not the same engine as the 944 series and 924 S series. The audi engine is a non-interference design and therefore its relative compression ratio is irrelevent when talking about interference.
Now as for the purpose of an interference engine, there is one that sticks out in my mind. Think of the cycles of an engine, cylindrically speaking. To achieve a certain displacement with the smallest and consequently lightest block, interference makes perfect sense. The valves can "follow" the piston very closely on the down stroke, utilizing the available space to a maximum. Otherwise the block would require extra material to accomodate the mutually exclusive space of the pistons and valves in a non-interference engine. The same weight savings principle was used in selecting a timing belt instead of a chain. Its lighter, quieter, cheaper and easier to incorporate into a design. This should all make sense from a design standpoint as extra weight means more support for the weight, bigger brakes, beefier transmission, etc.
Now here's the thing that boggles me everytime I look into the engine compartment. Why in the world would they combine all these achilles heels together and couple them with a questionable water pump in the same belt system?!?!? Alone they are good ideas, however the design leaves absolutely no room for adjustment and maintenance error in the system.
Now, from what I've heard, all 944/951/931/924S are interference type, and all non-931, non-S 924 are non-interference type.
So how can a 924 have 9.3:1 and be non-interference, while a 951 has 8.0:1 and is interference?
It must be because of the lift of the cam.
<hr></blockquote>
Well, to answer your question, the 924(non s versions) all had a 2 liter audi engine. This is not the same engine as the 944 series and 924 S series. The audi engine is a non-interference design and therefore its relative compression ratio is irrelevent when talking about interference.
Now as for the purpose of an interference engine, there is one that sticks out in my mind. Think of the cycles of an engine, cylindrically speaking. To achieve a certain displacement with the smallest and consequently lightest block, interference makes perfect sense. The valves can "follow" the piston very closely on the down stroke, utilizing the available space to a maximum. Otherwise the block would require extra material to accomodate the mutually exclusive space of the pistons and valves in a non-interference engine. The same weight savings principle was used in selecting a timing belt instead of a chain. Its lighter, quieter, cheaper and easier to incorporate into a design. This should all make sense from a design standpoint as extra weight means more support for the weight, bigger brakes, beefier transmission, etc.
Now here's the thing that boggles me everytime I look into the engine compartment. Why in the world would they combine all these achilles heels together and couple them with a questionable water pump in the same belt system?!?!? Alone they are good ideas, however the design leaves absolutely no room for adjustment and maintenance error in the system.
05-07-2002, 12:10 AM
#7
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[quote]Originally posted by TomH:
<strong>This is becoming the dominant method. Over half of the cars on the road use timing belts. There are several reasons for this:
a) It's easier to connect the camshaft and crankshaft together with a belt when the camshaft is far away from the crankshaft (like with overhead cam engines).
b) They claim the belt is quieter. Belts are lighter than chains, and every ounce counts to the manufacturers.
c) It's much cheaper and simpler to make a belt drive engine than any other way.</strong><hr></blockquote>
It's not all that difficult to use a chain to drive the camshaft - especially a single camshaft. The belt is certainly lighter and to use a chain requires oil be supplied to the chain as well as a chain tensioner. Still, chains rule IMHO. The SR20 engine in both of my Nissan's uses a chain and I've only heard of one breaking and it was under warranty (clearly a fluke). I've examined many SR20 chains with very high miles and even with poor maintenance and none have shown any signs of wear.
It sucks that the 944 engine uses a chain, but it is what it is and we just deal with it. Can't do anything about it.
<strong>This is becoming the dominant method. Over half of the cars on the road use timing belts. There are several reasons for this:
a) It's easier to connect the camshaft and crankshaft together with a belt when the camshaft is far away from the crankshaft (like with overhead cam engines).
b) They claim the belt is quieter. Belts are lighter than chains, and every ounce counts to the manufacturers.
c) It's much cheaper and simpler to make a belt drive engine than any other way.</strong><hr></blockquote>
It's not all that difficult to use a chain to drive the camshaft - especially a single camshaft. The belt is certainly lighter and to use a chain requires oil be supplied to the chain as well as a chain tensioner. Still, chains rule IMHO. The SR20 engine in both of my Nissan's uses a chain and I've only heard of one breaking and it was under warranty (clearly a fluke). I've examined many SR20 chains with very high miles and even with poor maintenance and none have shown any signs of wear.
It sucks that the 944 engine uses a chain, but it is what it is and we just deal with it. Can't do anything about it.
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05-07-2002, 12:14 AM
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This is an interesting question. The answer goes back to the 928 engine development program. According to Porsche AG Board Member for Development, Helmuth Bott...
"The new engine was to be derived from the concept of the 928 power plant for purposes of development, production, and customer service ease but, above all, to ensure its quality. Specifically, cylinder head design, valve drive and crankcase construction as well as material would be based on the 928 range." He continues by talking about containing development costs and capacity. "Combustion chamber development lies at the core of our efforts, and will continue to do so in years to come. Regardless of whether the concern is higher compression, stratified charge effects, new spark plug systems or valve arrangement, pre-development of a new layout can be pursued via the single-cyclinder experimental engine so long as the required relationship among these engines is maintained, and the most favorable complete engine of an engine family can be developed further, right up to production maturity. The results are then applicable to all other engines with modest outlay."
Perhaps we need to toss this question over the wall to the 928 list to get to the beginning of the story?
Keep the shiny side up,
"The new engine was to be derived from the concept of the 928 power plant for purposes of development, production, and customer service ease but, above all, to ensure its quality. Specifically, cylinder head design, valve drive and crankcase construction as well as material would be based on the 928 range." He continues by talking about containing development costs and capacity. "Combustion chamber development lies at the core of our efforts, and will continue to do so in years to come. Regardless of whether the concern is higher compression, stratified charge effects, new spark plug systems or valve arrangement, pre-development of a new layout can be pursued via the single-cyclinder experimental engine so long as the required relationship among these engines is maintained, and the most favorable complete engine of an engine family can be developed further, right up to production maturity. The results are then applicable to all other engines with modest outlay."
Perhaps we need to toss this question over the wall to the 928 list to get to the beginning of the story?
Keep the shiny side up,
05-07-2002, 01:00 AM
#9
Race Director
"The audi engine is a non-interference design and therefore its relative compression ratio is irrelevent when talking about interference."
Uh, compression ratio has everything to do with inteference. Check out what this 'ratio' is. It's the ratio of combustion-chamber volume to stroke-volume. The higher the compression, the smaller the combustion-chamber volume for the same displacement. This is simple physics that takes precedence regardless of who builds the engine.
"Now as for the purpose of an interference engine, there is one that sticks out in my mind. Think of the cycles of an engine, cylindrically speaking. To achieve a certain displacement with the smallest and consequently lightest block, interference makes perfect sense. The valves can "follow" the piston very closely on the down stroke, utilizing the available space to a maximum."
Check out what 'displacement' means. It's a measurement of cubic volume. With any given 'displacement' you choose, there's a certain 'volume' that goes with it. Interference or not, it doesn't change the bore & stroke of the engine.
"Otherwise the block would require extra material to accomodate the mutually exclusive space of the pistons and valves in a non-interference engine. "
You've just described a low-compression engine; one that has more volume in the combustion chamber than a high-compression engine; however, bore and stroke are identical.
The difference in size of the combustion chambers between a low 8.0:1 engine vs. a high 11.0:1 engine is just a few cubic-centimeter in size so the external engine size is irrelevant.
Uh, compression ratio has everything to do with inteference. Check out what this 'ratio' is. It's the ratio of combustion-chamber volume to stroke-volume. The higher the compression, the smaller the combustion-chamber volume for the same displacement. This is simple physics that takes precedence regardless of who builds the engine.
"Now as for the purpose of an interference engine, there is one that sticks out in my mind. Think of the cycles of an engine, cylindrically speaking. To achieve a certain displacement with the smallest and consequently lightest block, interference makes perfect sense. The valves can "follow" the piston very closely on the down stroke, utilizing the available space to a maximum."
Check out what 'displacement' means. It's a measurement of cubic volume. With any given 'displacement' you choose, there's a certain 'volume' that goes with it. Interference or not, it doesn't change the bore & stroke of the engine.
"Otherwise the block would require extra material to accomodate the mutually exclusive space of the pistons and valves in a non-interference engine. "
You've just described a low-compression engine; one that has more volume in the combustion chamber than a high-compression engine; however, bore and stroke are identical.
The difference in size of the combustion chambers between a low 8.0:1 engine vs. a high 11.0:1 engine is just a few cubic-centimeter in size so the external engine size is irrelevant.
05-07-2002, 01:05 AM
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Ok, This was mentioned on another board. It never went anywhere, but I'm curious.
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?
Has anyone tried to do this?
<img src="confused.gif" border="0">
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?
Has anyone tried to do this?
<img src="confused.gif" border="0">
05-07-2002, 01:50 AM
#11
Race Director
Yes, people have re-shaped the piston tops to maintain the identical combustion-chamber volume (therefore same compression-ratio). Yet with deeper valve reliefs to reduce the degrees of interference.
Note that changing the crank-throws will reduce your displacement. It doesn't have anything to do with interference because if you use longer rods to place the piston tops at the same location at TDC, you'll still have the exact same interference as before.
In fact to maintain identical compression as before, you'll have to use even longer rods than the reduction in stroke to get an even smaller combustion chamber than before. Thus making interference even worse.
Note that changing the crank-throws will reduce your displacement. It doesn't have anything to do with interference because if you use longer rods to place the piston tops at the same location at TDC, you'll still have the exact same interference as before.
In fact to maintain identical compression as before, you'll have to use even longer rods than the reduction in stroke to get an even smaller combustion chamber than before. Thus making interference even worse.
05-07-2002, 02:25 AM
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Ok,
Let's leave messing with the crank throws out of this for a minute. You say that people have changed piston shapes to reduce the interference. Has anyone eliminated the interference? It seems like there would be a market for a low compression piston (especially for the turbo or supercharged cars) that wouldn't destroy itself if the belt came apart. Guys are spending 400+ for pistons, certainly someone can come up with one that work for this application.
Here's a sample for the guys with 4 valves. Notice the raised section in the middle to fill the combustion chamber. (Note: this is not a Porsche piston, just trying to make a point)
Let's leave messing with the crank throws out of this for a minute. You say that people have changed piston shapes to reduce the interference. Has anyone eliminated the interference? It seems like there would be a market for a low compression piston (especially for the turbo or supercharged cars) that wouldn't destroy itself if the belt came apart. Guys are spending 400+ for pistons, certainly someone can come up with one that work for this application.
Here's a sample for the guys with 4 valves. Notice the raised section in the middle to fill the combustion chamber. (Note: this is not a Porsche piston, just trying to make a point)
05-07-2002, 06:57 PM
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Danno I think this will clarify much of my previous post. It wasn't very well thought out now that I read it all.
[quote]
"The audi engine is a non-interference design and therefore its relative compression ratio is irrelevent when talking about interference."
Uh, compression ratio has everything to do with inteference. Check out what this 'ratio' is. It's the ratio of combustion-chamber volume to stroke-volume. The higher the compression, the smaller the combustion-chamber volume for the same displacement. This is simple physics that takes precedence regardless of who builds the engine.
[QUOTE]
If you read what I wrote again, I specifically included the word RELATIVE. I shoud have been more specific in stating relative to the porsche block. Relative meaning that the relative compression ratio between two totally different blocks are absolutely not comparable when speaking about interference. Point being that because a certain interference engine has "such and such" compression ratio doesn't automatically mean that another engine out of a different block with higher compression ratio also has interference.
[QUOTE]
"Now as for the purpose of an interference engine, there is one that sticks out in my mind. Think of the cycles of an engine, cylindrically speaking. To achieve a certain displacement with the smallest and consequently lightest block, interference makes perfect sense. The valves can "follow" the piston very closely on the down stroke, utilizing the available space to a maximum."
Check out what 'displacement' means. It's a measurement of cubic volume. With any given 'displacement' you choose, there's a certain 'volume' that goes with it. Interference or not, it doesn't change the bore & stroke of the engine.
<hr></blockquote>
Now as for this I did make a mistake. I should have said to achieve a better compression ratio most efficiently. I was kind of thinking an extra step ahead in my mind replacing that potential lost compression into extra displacement to make up for it. My mistake. This actually pertains directly to what you wrote about my misunderstood first comment and I totally agree with you : compression ratio has everything to do with interference.
[quote] "Otherwise the block would require extra material to accomodate the mutually exclusive space of the pistons and valves in a non-interference engine. "
You've just described a low-compression engine; one that has more volume in the combustion chamber than a high-compression engine; however, bore and stroke are identical.
The difference in size of the combustion chambers between a low 8.0:1 engine vs. a high 11.0:1 engine is just a few cubic-centimeter in size so the external engine size is irrelevant. <hr></blockquote>
You're absolutely right. I just described a low compression engine, just as I meant to. The object being that there is a very slight (manufacturers care about everything) advantage to be gained through interference. Mostly the compression and a tiny bit with material and weight savings. However, I agree that the amount is still very small.
[quote]
"The audi engine is a non-interference design and therefore its relative compression ratio is irrelevent when talking about interference."
Uh, compression ratio has everything to do with inteference. Check out what this 'ratio' is. It's the ratio of combustion-chamber volume to stroke-volume. The higher the compression, the smaller the combustion-chamber volume for the same displacement. This is simple physics that takes precedence regardless of who builds the engine.
[QUOTE]
If you read what I wrote again, I specifically included the word RELATIVE. I shoud have been more specific in stating relative to the porsche block. Relative meaning that the relative compression ratio between two totally different blocks are absolutely not comparable when speaking about interference. Point being that because a certain interference engine has "such and such" compression ratio doesn't automatically mean that another engine out of a different block with higher compression ratio also has interference.
[QUOTE]
"Now as for the purpose of an interference engine, there is one that sticks out in my mind. Think of the cycles of an engine, cylindrically speaking. To achieve a certain displacement with the smallest and consequently lightest block, interference makes perfect sense. The valves can "follow" the piston very closely on the down stroke, utilizing the available space to a maximum."
Check out what 'displacement' means. It's a measurement of cubic volume. With any given 'displacement' you choose, there's a certain 'volume' that goes with it. Interference or not, it doesn't change the bore & stroke of the engine.
<hr></blockquote>
Now as for this I did make a mistake. I should have said to achieve a better compression ratio most efficiently. I was kind of thinking an extra step ahead in my mind replacing that potential lost compression into extra displacement to make up for it. My mistake. This actually pertains directly to what you wrote about my misunderstood first comment and I totally agree with you : compression ratio has everything to do with interference.
[quote] "Otherwise the block would require extra material to accomodate the mutually exclusive space of the pistons and valves in a non-interference engine. "
You've just described a low-compression engine; one that has more volume in the combustion chamber than a high-compression engine; however, bore and stroke are identical.
The difference in size of the combustion chambers between a low 8.0:1 engine vs. a high 11.0:1 engine is just a few cubic-centimeter in size so the external engine size is irrelevant. <hr></blockquote>
You're absolutely right. I just described a low compression engine, just as I meant to. The object being that there is a very slight (manufacturers care about everything) advantage to be gained through interference. Mostly the compression and a tiny bit with material and weight savings. However, I agree that the amount is still very small.
05-07-2002, 07:21 PM
#15
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[quote]Originally posted by ERAU944:
<strong>i'm no expert, but i'd guess that to make the compression as high as it is, they had to make </strong><hr></blockquote>
8:1 is actually pretty LOW compression
<strong>i'm no expert, but i'd guess that to make the compression as high as it is, they had to make </strong><hr></blockquote>
8:1 is actually pretty LOW compression