Cam advance/retard

[andrewlandon67]

Y'all really are terrible influences, I've just moved to a college town and I'm spending a Friday night trying to work out what advancing the FK-8 in my '67 would look like. The main problem is that it's sitting under a cover in my mom's garage 60 miles away! In all seriousness, I am actually curious about what it would look like, and how it would affect my engine's performance if I were to advance the cam by a degree or so. The two main reasons to do this, as far as I can figure, are to increase dynamic compression on the intake stroke, and decrease reversion interrupting the intake charge coming down from the carburetor. You can see I've tried to model it on lined notebook paper in an attempt to kick my brain into thinking like an engineer, but unfortunately my drawing, handwriting, and technical illiteracy aren't doing me any favors. So if anyone's bored on a Friday and doesn't have their VW to work on, I'd love to hear some thoughts on the subject!

[Jim Ratto]

One degree most likely will not make a discernible difference.

Most of the time, installing an Engle cam with either a non-adjustable gear, or with "0" washers in an adjustable actually means it's advanced.

Advance and retard in a VW is a complex topic, ironically because all valve actions are locked to one stick (unlike a twin cam engine, but then you have to worry about valves meeting at times and places they really shouldn't).

This is my way of thinking, and I have no way to prove I am right, so I am open to friendly debate:

Straight up timing: Both intake and exhaust lobe midpoints (full lift) occur at same crank degree "distance" from TDC/BDC (in a perfect world, one would order a Web Cam, bolt it to gear, drop it in a motor and check timing, both midpoints would be "108"...)
An example (just pulling numbers from thin air here) @ 0.050" Intake; 24:60 / Exhaust 60:24 (here the duration @ 0.050" is 264 crank degrees [24+60+180 = 264])

Assuming symmetrical cam lobes, you can easily find midpoint of intake cam lobe:
264/2 = 132, now subtract your opening number (24) from 132, and 132-24 = 108 after TOP dead center
And now for exhaust, same thing, 264/2 = 132, subtract your closing number and that = 108 after BOTTOM dead center

Advanced timing: The cam is adjusted, when installed so it's intake midpoint isn't @ 108 ATDC, it will occur sooner in the piston's travel to BDC from TDC during the intake stroke.
so the same cam, but advanced 2 degrees will look like this:
Intake lobe; 26:58 / Exhaust lobe; 62:22
We do the math 26+58+180 = 264 (still)
And 62+22+180 = 264 (still)
But now look what happens....  264/2 = 132, 132-26 =  106 (this is intake, so instead of midpoint [i.e. "full lift"] @ 108 TDC, it's now happening @ 106 deg ATDC)
And on exhaust side....  264/2 = 132, 132-22 = 110 (so instead of exhaust full lift happening @ 108 degrees after BDC, it's now 110)
But what is happening in the intake tract, cylinder, and exhaust port and pipes compared to "straight up"?
I think we have to generalize, unless one were to take a known engine, and ONLY change cam timing, but NOTHING else, and monitor the changes in power and torque curves.

It is widely accepted that advancing timing will bring power on "earlier", but why?

It gets difficult to think about this in my head, because all the events are related, meaning it isn't really like the old drawings of "How an otto cycle engine works" that they teach you in high school shop class. And the more radical the cam, then the more the events are all mixed together. The crankshaft can't turn more or less degrees, it is mechanically limited to work in the cycles it does. If you use a cam that holds valves open longer, to get motor to rev and have good VE, then you've "used up" a greater portion of each cycle that the crank (and rod and piston- but that's going to depend on rod ratio and so on) is involved in.
Anyway, because of the interrelationships of valves opening and closing and sometimes both being open and sometimes both being closed and where is the piston and what is the load and what is the throttle angle and pressure difference in intake/cylinder vs atmospheric... where do we start?

With advanced timing, the intake valve opens soon than it would have if cam was straight up. So the piston is caught earlier in it's trip from BDC to TDC during what we learned was the "exhaust stroke" as the intake valve opens. I would think the efficiency of the exhaust pipes need to play a role here. If the exhaust is a good one and helps to scavenge (or has scavenged) the cylinder of waste gases, then the early opening is a good thing, but that would help @ rpm goes up, not at lower rpm- so why is it helping to advance to increase lower-middle rpm output? Let the piston keep traveling.... now it's spent it's dwell time (hopefully not too long- my old boss used to say to fans of long rods "a piston standing still ain't doing any goddamn work") and is now headed back from TDC to BDC during the "intake stroke" and now we know the pressure drop in cylinder and intake tract is more severe, as a tidal wave of air/fuel fog rushes into cylinder through open intake valve. Full lift occurs for a degree of crank rotation @ 106 after TOP dead center. Charts that I have from Philip H Smith's "Scientific Design of Exhaust and Intake Systems" book show from his tests that in a high rpm engine, maximum pressure drop occurs somewhere around 75-80 degrees after TDC. How this relates to moving full lift point to happen earlier, I'm still foggy on. I think the real prevailing reason the advanced timing typically brings in power earlier lies mostly in early (earlier) intake valve closing. At this moment, there is no overlap. The exhaust valve has been closed for some time now. Cylinder pressure (given a well designed intake tract, with proper length and port design) started increasing while piston was still on it's way to BDC (again I refer to Smith's published tests) and by 40-30 degrees BBDC it has risen above atmospheric. THEN BDC, and then the piston begins to travel back to TDC, and if the intake valve closes earlier, your compression cycle starts earlier (again, exhaust valve is waiting, closed).
I've had instances and have read of similar cases where advanced cam timing motors are a little sensitive to ignition advance and curves. It's pretty easy to see why.... considering the above.
This took a bit to get out of my head and typed out, hopefully later on we can talk about exhaust with advanced, and then talking about retarded cam timing.

Here's something interesting:
Both Porsche (with various 4 cam Carrera 4 cyl ) and Lamborghini (with 3.9L Miura "S" V12) both found significant hp/torque gains with advanced cam timing. Porsche ran intake midpoints all over the place, from 110, 104, 97 and 92 ATDC
Wouldn't it be cool to sit down with the engineers behind those decisions and ask them "Why?"

see ya

Jim

[andrewlandon67]

So quite a bit of those first few paragraphs were roughly my same thought process, if a bit more scientific and less all in my head. The rest of it though... holy hell Jim. I do have some theories, all to be taken with several grains of salt, but I'm just trying to picture it out in my head with nothing but a fairly basic knowledge of the pushrod VW engine and the Otto cycle engine in general so bear with me. I believe part of the reason that moving the intake cam lobe centerline closer to your maximum pressure drop can increase power is simply because the engine, as a whole, has closer to the larger amount of space with the most amount of pressure difference (cylinder to atmospheric) possible, so essentially it has the largest straw to pull through closer to when it is pulling its hardest. As far as the exhaust valve/system goes, I'm quite a bit foggier on that but I think it makes some amount of sense that not only does the exhaust help to pull the intake charge in, but that the (in my mind) denser intake charge would help to displace the hot and less-dense exhaust gasses. Some of the power increases gained with an earlier-closing intake valve I think could be up to having less of the cylinder's stroke from BDC to TDC (or vice versa) taking place while the intake valve is open, which means less of the air/fuel mixture is escaping back into the intake port, but again, that's just my thought process.

Like I said earlier though, this is almost entirely all coming out of my head as a result of spending too much time digging around on this forum and to be totally honest, I'd enjoy quite a lot to have the time, money, and resources to build a test motor to check one variable at a time ie; cam timing, lobe shape, and so on.

Thanks to Jim Ratto for the slightly blown mind on a Sunday night!

[Jim Ratto]

I took a camshaft class in 2000, from Dema Elgin. He professed best place (in piston travel from TDC to BDC) to have valve fully open is when rod is 90 degrees to crank throw.

If you dig long and deep enough in Bosch Automotive Handbook, you'll find a graph that shows maximum pressure differential in cylinder happens @ 72 degrees ATDC.

best books I can suggest on these topics are:
Four Stroke Performance Tuning by AG Bell
the above mentioned book by Philip H Smith
and Gene Berg's technical articles (all of them).

[andrewlandon67]

Jim, between your big post on this thread, Mr. Elgin's website, and the Phillip H. Smith book that's currently on its way from Amazon, I have some serious reading material until I can get myself and my bug settled up here! Thank you again for providing a young-ish, bored college student with this much inspiration and knowledge! Even if I never end up messing with cam timing, it's awesome to work my brain a bit on something outside of school and a non car-related job! I do wonder what you mean by saying that an Engle cam installed with 0 washers or a non-adjustable gear is still advanced though. Are they just ground slightly off or something?

[Jim Ratto]

I do wonder what you mean by saying that an Engle cam installed with 0 washers or a non-adjustable gear is still advanced though. Are they just ground slightly off or something?

Check out timing card, this is how Engle expects the cam to be installed in your engine,
Intake opens 24, closes 54, so I'll do the math, 24+54+180 = 258; 258/2 = 129; 129-24 = 105
Exhaust opens 60, closes 18, 60+18+180 = 258; 258/2 = 129; 129-18 = 111
So Engle's card has the cam going in 3 degrees advanced.

[andrewlandon67]

That's really interesting, because the tag that came with mine looks like this. I wonder why they switched it up so drastically over such a period of time. It also makes me wonder what mine would run like if I installed it to that tag instead of the one it came with.

[Jim Ratto]

My personal experience with how street engines with 2x 2-throat carbs run regarding advanced or straight up

Number one, the BEST all around street cam I've used since 1989 has been the Pauter R6E8, with 1.25 rockers. I qualify the word "best" with the overall forgiving nature of this cam, and each time I've run it on the street it has been set up with intake lobe center @ 104 ATDC. Idle quality, ease of starting, ease of carburetor adjustment and calibration, willingness to accelerate from low-to-reasonable rpm, and it's ability to run hard when it is asked to, has left it my favorite, and someday, I will use this grind again, set to same timing.

Engines with aggressive cams and modest compression tend to run better with ILC @ 104-105 ATDC, especially off idle. Add compression and they run even better.
I've experimented with a few popular cams set @ 108/108, and I'm always disappointed with the result. The engine is tougher to jet, they seem to want a ton of timing, the idle tends to be erratic, they're more "cold blooded" when first fired up, and the top end, in my experience is no better than if the same cam was set 3-4 degrees advanced.

Full-on race engines are a different story, due to compression and less concern with idle quality and engine's forgiveness @ low-mid rpm.

[Bad bug]

One degree most likely will not make a discernible difference.

Most of the time, installing an Engle cam with either a non-adjustable gear, or with "0" washers in an adjustable actually means it's advanced.

Advance and retard in a VW is a complex topic, ironically because all valve actions are locked to one stick (unlike a twin cam engine, but then you have to worry about valves meeting at times and places they really shouldn't).

This is my way of thinking, and I have no way to prove I am right, so I am open to friendly debate:

Straight up timing: Both intake and exhaust lobe midpoints (full lift) occur at same crank degree "distance" from TDC/BDC (in a perfect world, one would order a Web Cam, bolt it to gear, drop it in a motor and check timing, both midpoints would be "108"...)
An example (just pulling numbers from thin air here) @ 0.050" Intake; 24:60 / Exhaust 60:24 (here the duration @ 0.050" is 264 crank degrees [24+60+180 = 264])

Assuming symmetrical cam lobes, you can easily find midpoint of intake cam lobe:
264/2 = 132, now subtract your opening number (24) from 132, and 132-24 = 108 after TOP dead center
And now for exhaust, same thing, 264/2 = 132, subtract your closing number and that = 108 after BOTTOM dead center

Advanced timing: The cam is adjusted, when installed so it's intake midpoint isn't @ 108 ATDC, it will occur sooner in the piston's travel to BDC from TDC during the intake stroke.
so the same cam, but advanced 2 degrees will look like this:
Intake lobe; 26:58 / Exhaust lobe; 62:22
We do the math 26+58+180 = 264 (still)
And 62+22+180 = 264 (still)
But now look what happens....  264/2 = 132, 132-26 =  106 (this is intake, so instead of midpoint [i.e. "full lift"] @ 108 TDC, it's now happening @ 106 deg ATDC)
And on exhaust side....  264/2 = 132, 132-22 = 110 (so instead of exhaust full lift happening @ 108 degrees after BDC, it's now 110)
But what is happening in the intake tract, cylinder, and exhaust port and pipes compared to "straight up"?
I think we have to generalize, unless one were to take a known engine, and ONLY change cam timing, but NOTHING else, and monitor the changes in power and torque curves.

It is widely accepted that advancing timing will bring power on "earlier", but why?

It gets difficult to think about this in my head, because all the events are related, meaning it isn't really like the old drawings of "How an otto cycle engine works" that they teach you in high school shop class. And the more radical the cam, then the more the events are all mixed together. The crankshaft can't turn more or less degrees, it is mechanically limited to work in the cycles it does. If you use a cam that holds valves open longer, to get motor to rev and have good VE, then you've "used up" a greater portion of each cycle that the crank (and rod and piston- but that's going to depend on rod ratio and so on) is involved in.
Anyway, because of the interrelationships of valves opening and closing and sometimes both being open and sometimes both being closed and where is the piston and what is the load and what is the throttle angle and pressure difference in intake/cylinder vs atmospheric... where do we start?

With advanced timing, the intake valve opens soon than it would have if cam was straight up. So the piston is caught earlier in it's trip from BDC to TDC during what we learned was the "exhaust stroke" as the intake valve opens. I would think the efficiency of the exhaust pipes need to play a role here. If the exhaust is a good one and helps to scavenge (or has scavenged) the cylinder of waste gases, then the early opening is a good thing, but that would help @ rpm goes up, not at lower rpm- so why is it helping to advance to increase lower-middle rpm output? Let the piston keep traveling.... now it's spent it's dwell time (hopefully not too long- my old boss used to say to fans of long rods "a piston standing still ain't doing any goddamn work") and is now headed back from TDC to BDC during the "intake stroke" and now we know the pressure drop in cylinder and intake tract is more severe, as a tidal wave of air/fuel fog rushes into cylinder through open intake valve. Full lift occurs for a degree of crank rotation @ 106 after TOP dead center. Charts that I have from Philip H Smith's "Scientific Design of Exhaust and Intake Systems" book show from his tests that in a high rpm engine, maximum pressure drop occurs somewhere around 75-80 degrees after TDC. How this relates to moving full lift point to happen earlier, I'm still foggy on. I think the real prevailing reason the advanced timing typically brings in power earlier lies mostly in early (earlier) intake valve closing. At this moment, there is no overlap. The exhaust valve has been closed for some time now. Cylinder pressure (given a well designed intake tract, with proper length and port design) started increasing while piston was still on it's way to BDC (again I refer to Smith's published tests) and by 40-30 degrees BBDC it has risen above atmospheric. THEN BDC, and then the piston begins to travel back to TDC, and if the intake valve closes earlier, your compression cycle starts earlier (again, exhaust valve is waiting, closed).
I've had instances and have read of similar cases where advanced cam timing motors are a little sensitive to ignition advance and curves. It's pretty easy to see why.... considering the above.
This took a bit to get out of my head and typed out, hopefully later on we can talk about exhaust with advanced, and then talking about retarded cam timing.

Here's something interesting:
Both Porsche (with various 4 cam Carrera 4 cyl ) and Lamborghini (with 3.9L Miura "S" V12) both found significant hp/torque gains with advanced cam timing. Porsche ran intake midpoints all over the place, from 110, 104, 97 and 92 ATDC
Wouldn't it be cool to sit down with the engineers behind those decisions and ask them "Why?"

see ya

Jim

 
Jim explain how the 180 come about in this calculation.

[andrewlandon67]

One degree most likely will not make a discernible difference.

Most of the time, installing an Engle cam with either a non-adjustable gear, or with "0" washers in an adjustable actually means it's advanced.

Advance and retard in a VW is a complex topic, ironically because all valve actions are locked to one stick (unlike a twin cam engine, but then you have to worry about valves meeting at times and places they really shouldn't).

This is my way of thinking, and I have no way to prove I am right, so I am open to friendly debate:

Straight up timing: Both intake and exhaust lobe midpoints (full lift) occur at same crank degree "distance" from TDC/BDC (in a perfect world, one would order a Web Cam, bolt it to gear, drop it in a motor and check timing, both midpoints would be "108"...)
An example (just pulling numbers from thin air here) @ 0.050" Intake; 24:60 / Exhaust 60:24 (here the duration @ 0.050" is 264 crank degrees [24+60+180 = 264])

Assuming symmetrical cam lobes, you can easily find midpoint of intake cam lobe:
264/2 = 132, now subtract your opening number (24) from 132, and 132-24 = 108 after TOP dead center
And now for exhaust, same thing, 264/2 = 132, subtract your closing number and that = 108 after BOTTOM dead center

Advanced timing: The cam is adjusted, when installed so it's intake midpoint isn't @ 108 ATDC, it will occur sooner in the piston's travel to BDC from TDC during the intake stroke.
so the same cam, but advanced 2 degrees will look like this:
Intake lobe; 26:58 / Exhaust lobe; 62:22
We do the math 26+58+180 = 264 (still)
And 62+22+180 = 264 (still)
But now look what happens....  264/2 = 132, 132-26 =  106 (this is intake, so instead of midpoint [i.e. "full lift"] @ 108 TDC, it's now happening @ 106 deg ATDC)
And on exhaust side....  264/2 = 132, 132-22 = 110 (so instead of exhaust full lift happening @ 108 degrees after BDC, it's now 110)
But what is happening in the intake tract, cylinder, and exhaust port and pipes compared to "straight up"?
I think we have to generalize, unless one were to take a known engine, and ONLY change cam timing, but NOTHING else, and monitor the changes in power and torque curves.

It is widely accepted that advancing timing will bring power on "earlier", but why?

It gets difficult to think about this in my head, because all the events are related, meaning it isn't really like the old drawings of "How an otto cycle engine works" that they teach you in high school shop class. And the more radical the cam, then the more the events are all mixed together. The crankshaft can't turn more or less degrees, it is mechanically limited to work in the cycles it does. If you use a cam that holds valves open longer, to get motor to rev and have good VE, then you've "used up" a greater portion of each cycle that the crank (and rod and piston- but that's going to depend on rod ratio and so on) is involved in.
Anyway, because of the interrelationships of valves opening and closing and sometimes both being open and sometimes both being closed and where is the piston and what is the load and what is the throttle angle and pressure difference in intake/cylinder vs atmospheric... where do we start?

With advanced timing, the intake valve opens soon than it would have if cam was straight up. So the piston is caught earlier in it's trip from BDC to TDC during what we learned was the "exhaust stroke" as the intake valve opens. I would think the efficiency of the exhaust pipes need to play a role here. If the exhaust is a good one and helps to scavenge (or has scavenged) the cylinder of waste gases, then the early opening is a good thing, but that would help @ rpm goes up, not at lower rpm- so why is it helping to advance to increase lower-middle rpm output? Let the piston keep traveling.... now it's spent it's dwell time (hopefully not too long- my old boss used to say to fans of long rods "a piston standing still ain't doing any goddamn work") and is now headed back from TDC to BDC during the "intake stroke" and now we know the pressure drop in cylinder and intake tract is more severe, as a tidal wave of air/fuel fog rushes into cylinder through open intake valve. Full lift occurs for a degree of crank rotation @ 106 after TOP dead center. Charts that I have from Philip H Smith's "Scientific Design of Exhaust and Intake Systems" book show from his tests that in a high rpm engine, maximum pressure drop occurs somewhere around 75-80 degrees after TDC. How this relates to moving full lift point to happen earlier, I'm still foggy on. I think the real prevailing reason the advanced timing typically brings in power earlier lies mostly in early (earlier) intake valve closing. At this moment, there is no overlap. The exhaust valve has been closed for some time now. Cylinder pressure (given a well designed intake tract, with proper length and port design) started increasing while piston was still on it's way to BDC (again I refer to Smith's published tests) and by 40-30 degrees BBDC it has risen above atmospheric. THEN BDC, and then the piston begins to travel back to TDC, and if the intake valve closes earlier, your compression cycle starts earlier (again, exhaust valve is waiting, closed).
I've had instances and have read of similar cases where advanced cam timing motors are a little sensitive to ignition advance and curves. It's pretty easy to see why.... considering the above.
This took a bit to get out of my head and typed out, hopefully later on we can talk about exhaust with advanced, and then talking about retarded cam timing.

Here's something interesting:
Both Porsche (with various 4 cam Carrera 4 cyl ) and Lamborghini (with 3.9L Miura "S" V12) both found significant hp/torque gains with advanced cam timing. Porsche ran intake midpoints all over the place, from 110, 104, 97 and 92 ATDC
Wouldn't it be cool to sit down with the engineers behind those decisions and ask them "Why?"

see ya

Jim

 
Jim explain how the 180 come about in this calculation.

There are probably more qualified people than me to explain this, but since it's my thread, I figure I'll give it a shot. The reason that this calculation has the 180 added in is due to the fact that in a 4-stroke piston engine, the camshaft spins at half the speed of the crankshaft. What this means is that since the crank is spinning twice as fast as the cam, and since we read cam duration in terms of crank degrees, we have to add the amount of time the cam spends between the opening and closing points to our math, and the best way to do this is to add the 180 degrees that are between TDC and BDC, which is where the majority of the valve's time open is spent. If the cam were to spin at the same speed as the crank, and still be measured in crank degrees, we'd be adding 360 to our duration figures, but of course if this were the case then the engine wouldn't work very well as a 4-stroke.

[Bad bug]

Thanks for the explanation i was on track with my thoughts then. I will ask more questions as i read through again. It's strange how the cam card for both FK8 cams are different, wondering if the buyer ordered it this way.

[andrewlandon67]

Thanks for the explanation i was on track with my thoughts then. I will ask more questions as i read through again. It's strange how the cam card for both FK8 cams are different, wondering if the buyer ordered it this way.

That's actually an interesting theory... Jim's timing tag certainly looks older than mine, so it could just have been how they did it whenever he got that particular tag. I'm not entirely sure as to where my cam timing actually is (Forgive me Father Ratto, for I have sinned and been lazy) because we kinda just put the bottom end together without worrying too much about it, but I have been curious as to where it sits. Maybe I'll get bored this winter and yank my heads off and take a look, or better yet, just leave the damn thing alone until it actually needs to come apart.