I don't really think that we are viewing camshaft dynamics too differently. Anyone who says that "you need this cam for a turbo engine no matter what the circumstance" is misinformed. I figure most of the guys building really bad street engines or race engines pretty much have a handle on what they need to run. The big LS thus lower duration camshaft was really meant for a more usable power curve.
Here are a few experiences I have had over the years...not necessarily form VW aircooled engines...but street car engine never the less. About three years ago, two guys came in for a dyno run. Bother were really your plane Jane pumped up small block Chevy. One was in a 69 camaro the other in a 70 Nova. When they showed up.....both guys agreed that the Nova was going to have quite a shot more power. That was based on the fact that the Nova would pull away from the camaro when they would screw around next to each other in short stop light bursts. Neither one of them had ever been to a drag strip.
They were both very shocked to find that the hp number on the camaro was substantially higher than that of the Nova. But...the Nova built power much sooner and made a real flat torque curve. The camaro was kind of low power then at 3,800 rpm it rocket ships up into big numbers and held them there until it hit the rev limiter.
Now....if the camaro had lower gears, a higher stall converter (or four speed) he would have pulled away from his buddy in those short races...but in doing that would have made his car less friendly to drive (for most). So I guess what I am saying...is that I like to make a street driven engine more friendly for my customers. Myself...I personally don't mind a nasty narrow, sharp power band car on the street...but I feel that the masses would like to roll on the throttle from a very slow speed, have boost jump up right away and lay them back in the seat. No clutch slipping or high rpm launches...just from a near engine lug to power.
Just out of curiosity... what are the IO and EC degrees are on the cam you are running in your car right now?
Camshaft grinds for turbo
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scott the viking
- Posts: 3864
- Joined: Fri Apr 23, 2004 8:35 am
Ya know....I was just sitting in my office thinking about the last answer I gave here...And I really think it's kind of vague and droll. I have personally done years of dyno testing on literally thousands of different cam grinds and for me come flop out a general answer the way I did was kinda of B.S.
So....I thought I would elaborate more on what I have found....I will try and keep this from getting too "techy" and wordy. I don't want to bore everyone to death.
Okay...here it is...
When I say that a turbo camshaft should have less overlap than a NA car...that is somewhat true...but really is focused on guys who have bought too small of turbo for their car. By the way...this is VERY common. Most stock turbo vehicles run too small of turbo. See...proper turbo selection is more important that many may think. Here is what happens in most stock turbo cars. The turbo is too small, the exhaust is to restrictive, therefore, the pressure in the exhaust manifold or header is much greater than the pressure in your intake manifold from the boost. The "band-aid" for this is using a camshaft with less overlap. This shuts the exhaust valve sooner, so all that pressure built from too small of a turbo and restrictive exhaust manifold does not get much time to rush into the open intake valve. After all...the pressure in your intake IS lower than your exhaust pressure in this engine.
Now on a turbo that is sized correctly for your engine...your exhaust pressure before the turbo will be lower than the amount of boost you are making. In other words...there is more pressure in your intake manifold than in your exhaust...This is a good thing. However...even on a correctly sized turbine the exhaust pressure and the intake pressure will get closer to the same pressure as the engine rpm increases and then the exhaust pressure will pass the intake pressure. If everything is working like it should...this won't happen until you are near the top of your power curve. So to state the obvious here...the goal is to have as little backpressure as possible with the most amount of boost.
I have even experimented with cams that have negative overlap. I.E....The exhaust valve closed with the piston at top dead center and the intake valve would not open until this piston was 17 degrees (or more) down in the bore. This was to see how it would work with a MASSIVE amounts of boost.
To get to the point here...what I have found is that a street engine that has the correctly sized turbo and it running 15 pounds of boost or under...they like 114 and sometimes two degrees tighter overlap. When I went tighter than that...even on a properly matched turbo...my problem was not the exhaust trying to force it's way back into the intake...but rather the fresh air and fuel mix trying to be forced out the exhaust. This did make for a pretty cool fire show, because a lot of unburned fuel was getting lit off of the turbine housing.
So...I have found too much overlap to hurt me even with the turbo being the right size. If it's too small it forced the burnt fuel into the intake and if it was just right it forced the fresh air and fuel out the exhaust without burning all of it in the cylinder. I have ran into this same situation with supercharged engines as well...What is happening is they are really working too good. With a NA engine, the overlap was needed to help pull in the fresh air/fuel charge from the intake, it has now just become an open door for the exhaust, the fresh air/fuel and whatever else you sucked in, to escape. With everything working so well and under so much pressure it won't bother to stop in the cylinder and get burnt...It will just freight trains right on by and out the exhaust. So it need to be stopped by shutting the exhaust valve before it has time to escape.
For those of you who's turbo systems are not working at this level of efficiency....you also need to tone down the overlap...but for the exact opposite reason. Your fresh fuel and air won't be rushing out your exhaust valve...but rather it will be hindered from coming in to the cylinder at all. This would be due to the fact someone left the door open too long in a room full of pressure. That pressure has now come back into your cylinder and is looking for a place to go. This place just so happens to be in your intake valve...because that door was left open too long and that room is full of much lower pressure that the one it just came from. Just like water...if you plug off one hole it will look for another hole without as much pressure.
So what I have personally found is that too much overlap is not turbo friendly...no matter how well your turbo is sized. As far as off the shelf turbo grinds cams are concerned...Most of them are designed with the idea that you are not running the right sized turbo. So they run a wide separation. If you do have the proper sized turbo and want to buy an off the shelf camshaft....I would suggest to look the type of grind that is specifically made for nitrous or a supercharger.
So....I thought I would elaborate more on what I have found....I will try and keep this from getting too "techy" and wordy. I don't want to bore everyone to death.
Okay...here it is...
When I say that a turbo camshaft should have less overlap than a NA car...that is somewhat true...but really is focused on guys who have bought too small of turbo for their car. By the way...this is VERY common. Most stock turbo vehicles run too small of turbo. See...proper turbo selection is more important that many may think. Here is what happens in most stock turbo cars. The turbo is too small, the exhaust is to restrictive, therefore, the pressure in the exhaust manifold or header is much greater than the pressure in your intake manifold from the boost. The "band-aid" for this is using a camshaft with less overlap. This shuts the exhaust valve sooner, so all that pressure built from too small of a turbo and restrictive exhaust manifold does not get much time to rush into the open intake valve. After all...the pressure in your intake IS lower than your exhaust pressure in this engine.
Now on a turbo that is sized correctly for your engine...your exhaust pressure before the turbo will be lower than the amount of boost you are making. In other words...there is more pressure in your intake manifold than in your exhaust...This is a good thing. However...even on a correctly sized turbine the exhaust pressure and the intake pressure will get closer to the same pressure as the engine rpm increases and then the exhaust pressure will pass the intake pressure. If everything is working like it should...this won't happen until you are near the top of your power curve. So to state the obvious here...the goal is to have as little backpressure as possible with the most amount of boost.
I have even experimented with cams that have negative overlap. I.E....The exhaust valve closed with the piston at top dead center and the intake valve would not open until this piston was 17 degrees (or more) down in the bore. This was to see how it would work with a MASSIVE amounts of boost.
To get to the point here...what I have found is that a street engine that has the correctly sized turbo and it running 15 pounds of boost or under...they like 114 and sometimes two degrees tighter overlap. When I went tighter than that...even on a properly matched turbo...my problem was not the exhaust trying to force it's way back into the intake...but rather the fresh air and fuel mix trying to be forced out the exhaust. This did make for a pretty cool fire show, because a lot of unburned fuel was getting lit off of the turbine housing.
So...I have found too much overlap to hurt me even with the turbo being the right size. If it's too small it forced the burnt fuel into the intake and if it was just right it forced the fresh air and fuel out the exhaust without burning all of it in the cylinder. I have ran into this same situation with supercharged engines as well...What is happening is they are really working too good. With a NA engine, the overlap was needed to help pull in the fresh air/fuel charge from the intake, it has now just become an open door for the exhaust, the fresh air/fuel and whatever else you sucked in, to escape. With everything working so well and under so much pressure it won't bother to stop in the cylinder and get burnt...It will just freight trains right on by and out the exhaust. So it need to be stopped by shutting the exhaust valve before it has time to escape.
For those of you who's turbo systems are not working at this level of efficiency....you also need to tone down the overlap...but for the exact opposite reason. Your fresh fuel and air won't be rushing out your exhaust valve...but rather it will be hindered from coming in to the cylinder at all. This would be due to the fact someone left the door open too long in a room full of pressure. That pressure has now come back into your cylinder and is looking for a place to go. This place just so happens to be in your intake valve...because that door was left open too long and that room is full of much lower pressure that the one it just came from. Just like water...if you plug off one hole it will look for another hole without as much pressure.
So what I have personally found is that too much overlap is not turbo friendly...no matter how well your turbo is sized. As far as off the shelf turbo grinds cams are concerned...Most of them are designed with the idea that you are not running the right sized turbo. So they run a wide separation. If you do have the proper sized turbo and want to buy an off the shelf camshaft....I would suggest to look the type of grind that is specifically made for nitrous or a supercharger.
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bugsterkafer
- Posts: 784
- Joined: Mon Dec 01, 2003 5:30 am
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Billyisgr8
My turbo cam had a 106 lobe center and 284 duration in my 1584cc singleport with 35 x 32 valves using a T3-50 trim 42 intake 48 exhaust and a 1 1/2 exhaust header. Boost started at 2500 and was 15 pounds at 3800. Using the 1/8 mile mph it would have been a 110 mph run in the 1/4.
So how much faster would a 114 lobe center cam make my car? I'm thinking not faster at all. If anything my cam will give me better torque when off boost, then once the turbo kicks in hang on.
Kevin
So how much faster would a 114 lobe center cam make my car? I'm thinking not faster at all. If anything my cam will give me better torque when off boost, then once the turbo kicks in hang on.
Kevin
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scott the viking
- Posts: 3864
- Joined: Fri Apr 23, 2004 8:35 am
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Buggsy
- Posts: 1457
- Joined: Tue Jan 01, 2002 12:01 am
I always thought a guy should probably measure his exhaust pressure verses his boost pressure, and then adjust his exhaust housing size and possibly his camshaft overlap so they stayed pretty close to even, but I've never seen anyone post any exhaust pressures, and I've been to lazy to do it myself. This sound logical?
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turbo_bob
- Posts: 1312
- Joined: Mon Feb 10, 2003 12:01 am
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Marty
- Posts: 5802
- Joined: Fri Nov 16, 2001 12:01 am
I am thinking you are wrong.Billyisgr8 wrote:My turbo cam had a 106 lobe center and 284 duration in my 1584cc singleport with 35 x 32 valves using a T3-50 trim 42 intake 48 exhaust and a 1 1/2 exhaust header. Boost started at 2500 and was 15 pounds at 3800. Using the 1/8 mile mph it would have been a 110 mph run in the 1/4.
So how much faster would a 114 lobe center cam make my car? I'm thinking not faster at all. If anything my cam will give me better torque when off boost, then once the turbo kicks in hang on.
Kevin
Sure your car is smoother and drives around real nicely now, but a 112 LC cam would give you MORE overall power.
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Marty
- Posts: 5802
- Joined: Fri Nov 16, 2001 12:01 am
Yes that IS the correct way to "dial in" your setup.Buggsy wrote:I always thought a guy should probably measure his exhaust pressure verses his boost pressure, and then adjust his exhaust housing size and possibly his camshaft overlap so they stayed pretty close to even, but I've never seen anyone post any exhaust pressures, and I've been to lazy to do it myself. This sound logical?
You would need to datalog to get it right though.
Peak versus Peak wont tell you too much.
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fuey22
- Posts: 187
- Joined: Sat Feb 11, 2006 11:50 am
now were talking
this has to be one off the most informative posts ive red on the forums for a long time.now people are being educated instead of b.sed. i always said i would much rather do buisness with people who teach and explain why they suggest that product or setup. when i built my turbo setup i ask questions on here and was pretty much discuraged by the answers i got.
the cam and turbo question are like the big no no.....so i talked to pat at cb and he answerd my questions without any bs so guess who i did buisness with. and have been happy with the whole setup. im glag this is getting back to what it is intended for its a forum not a buisnes.thanks scott super post..........
the cam and turbo question are like the big no no.....so i talked to pat at cb and he answerd my questions without any bs so guess who i did buisness with. and have been happy with the whole setup. im glag this is getting back to what it is intended for its a forum not a buisnes.thanks scott super post..........
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Billyisgr8
It has been proven that tighter lobe centers improve bottem end torque, and give peak hp #'s at a lower rpm than a larger lobe center. So hypotheticly it should help getting the car moving till on boost. In the real world probably not much difference in acceleration in my car.scott the viking wrote:Explain please.Billyisgr8 wrote: If anything my cam will give me better torque when off boost, then once the turbo kicks in hang on.
Kevin
My car is a drag car that gets driven on the street to get ice cream or for testing. Where I live there is no traffic so for being smooth I don't have hours behind the wheel to compare to my other car. As for the cam I have it is what I had on the shelf at the time I built the engine so I used it. Is it correct probably not, but it sure worked in my 1584cc singleport, can I get more power out of a singleport, yes , this is where trial and error come into effect now.
So you think my car would be faster with a 114 lobe center Marty? I was allways under the impresion that peak HP would not change with different lobe centers just how torque and HP look under the curve. You have the experience with this so you know better than I do. That is good news to hear, since any extra hp out of a singleport is in the right direction for a 11 second run.
Kevin
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Matt Davis
- Posts: 1503
- Joined: Thu Mar 15, 2001 12:01 am
Not the 2 engines I help out with. MUCH larger than that. I have the ones cam card in front of me. He asked me not to display it, but it isn't close to 236 @ .050turbo_bob wrote:Kenny Duttweiler is a small duration guy, his turbo cams are in the 236 @ .050 range.Matt Davis wrote:I've had this same convo with Kenny Duttweiler when I was at his shop and some other turbo "gurus" including Linginfelter
I don't agree with the small cam guys, they don't agree with us.
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mike thompson
- Posts: 1047
- Joined: Tue Apr 29, 2003 8:39 am
Buggsy's comments about observing pressure in both sides of the system is something I did a while ago.
You just hook up the boost line to a legnth of copper tubing (to absorb and radiate heat) on a 1/8 NPT coupling welded in to the collector. Observe where the pressure flattens out- this is where the wastegate controls boost. On mine, the gauge read lower by a couple of pounds or more ( don't remember the exact #). Removing the muffler made a slight reduction but not worth the xtra noise.
Bob Tomlinson wrote in Turbomannia (to the effect) that more boost pressure than exhaust is something to shoot for and usually, only race engines attain.
Kind of made me feel good.
Mike
You just hook up the boost line to a legnth of copper tubing (to absorb and radiate heat) on a 1/8 NPT coupling welded in to the collector. Observe where the pressure flattens out- this is where the wastegate controls boost. On mine, the gauge read lower by a couple of pounds or more ( don't remember the exact #). Removing the muffler made a slight reduction but not worth the xtra noise.
Bob Tomlinson wrote in Turbomannia (to the effect) that more boost pressure than exhaust is something to shoot for and usually, only race engines attain.
Kind of made me feel good.
Mike
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Billyisgr8
Is a narrow lobe center ideal for a turbo, depends on how much overlap there is, small duration means small overlap...... it seems to work ok for me, is it the most effecient, probably not , data logging is the only way to find out. I am in this for fun so I just run what I have since I am my own sponsor.
Maybe we just need to have a set # for what is acceptable for turbo camshaft overlap. Like 28 degrees. I just read yesterday about using a 126 degree lobe seperation on a camshaft. That is alot but it worked.
Here is a quote from Crane directly from there web page
Lobe separation is the distance (in camshaft degrees) that the intake and exhaust lobe centerlines (for a given cylinder) are spread apart. Lobe separation is a physical characteristic of the camshaft and cannot be changed without regrinding the lobes.
This separation determines where peak torque will occur within the engine's power range. Tight lobe separations (such as 106?) cause the peak torque to build early in basic RPM range of the cam. The torque will be concentrated, build quickly and peak out. Broader lobe separations (such as 112?) allow the torque to be spread over a broader portion of the basic RPM range and shows better power through the upper RPM.
And another quote
the narrower (smaller numerically, also called 'tighter') the lobe separation (the average of the two lobe center figures) the more powerful the cam will be in its power range, but the lower the mileage, and the higher the emissions. It also will likely have a rougher idle, and somewhat poorer off-idle throttle response, even if it's not a long duration cam. These effects become pronounced when the value falls below 108 °.
* conversely, the wider the lobe separation, the power curve will be wider, but not as high peaking. Typically, values over 111° will have, logically, benefits of the opposite of those negative effects mentioned in regard to smaller lobe separations. Most all original equipment cams use a wider lobe separation to give a smooth idle, smooth off-idle throttle reponse, lower emissions, and better fuel economy, at the the expense of mid & upper-mid range torque.
Turbo cams also require wider lobe separations, as do some very long duration race cams. In both these cases it is to avoid excess overlap
Kevin
Maybe we just need to have a set # for what is acceptable for turbo camshaft overlap. Like 28 degrees. I just read yesterday about using a 126 degree lobe seperation on a camshaft. That is alot but it worked.
Here is a quote from Crane directly from there web page
Lobe separation is the distance (in camshaft degrees) that the intake and exhaust lobe centerlines (for a given cylinder) are spread apart. Lobe separation is a physical characteristic of the camshaft and cannot be changed without regrinding the lobes.
This separation determines where peak torque will occur within the engine's power range. Tight lobe separations (such as 106?) cause the peak torque to build early in basic RPM range of the cam. The torque will be concentrated, build quickly and peak out. Broader lobe separations (such as 112?) allow the torque to be spread over a broader portion of the basic RPM range and shows better power through the upper RPM.
And another quote
the narrower (smaller numerically, also called 'tighter') the lobe separation (the average of the two lobe center figures) the more powerful the cam will be in its power range, but the lower the mileage, and the higher the emissions. It also will likely have a rougher idle, and somewhat poorer off-idle throttle response, even if it's not a long duration cam. These effects become pronounced when the value falls below 108 °.
* conversely, the wider the lobe separation, the power curve will be wider, but not as high peaking. Typically, values over 111° will have, logically, benefits of the opposite of those negative effects mentioned in regard to smaller lobe separations. Most all original equipment cams use a wider lobe separation to give a smooth idle, smooth off-idle throttle reponse, lower emissions, and better fuel economy, at the the expense of mid & upper-mid range torque.
Turbo cams also require wider lobe separations, as do some very long duration race cams. In both these cases it is to avoid excess overlap
Kevin
Last edited by Billyisgr8 on Wed Dec 20, 2006 8:31 am, edited 3 times in total.
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scott the viking
- Posts: 3864
- Joined: Fri Apr 23, 2004 8:35 am
Was thinking maybe a little something to help with what lobe separation is may help us get on the same page.
Q: What is lobe separation?
A: Lobe separation is the angle in camshaft degrees between the maximum lift points of the intake and exhaust valves. It is affected by the placement of the intake and exhaust lobes on the camshaft.
Q: What does lobe separation do?
A: Lobe separation affects valve overlap, which in turn affects an engine’s power curve, idle quality, idle vacuum, and rpm range. Duration, lobe separation, and valve overlap all affect each other. For example, if you want to increase valve overlap without changing duration, you can decrease the lobe separation angle. Increasing lobe separation angle without changing valve overlap requires increasing the duration.
Q: What is valve overlap?
A: Valve overlap is the angle, in degrees of crankshaft rotation, that the intake and exhaust valves are both open. This occurs at the end of the exhaust stroke and the beginning of the intake stroke. Increasing lift, duration, and/or decreasing lobe separation increases valve overlap.
Q: What does valve overlap do?
A: At high engine speeds, valve overlap allows the gasses exiting the cylinder through the exhaust valve to help pull a fresh air/fuel mixture into the cylinder through the intake valve. Increasing valve overlap helps high-rpm power, but reduces low-rpm power and idle quality.
As the spread between the lobes tightens, the lobe-separation number gets smaller and overlap increases since the center-lines of the two lobes are coming closer together. A larger lobe separation angle means less overlap because the lobe center-lines are moving farther apart. This gets tricky because if you increase duration, this automatically increases the overlap with the same lobe separation angle. This is why you will see big cams with wider lobe separation angles since the cam grinder is attempting to limit the amount of actual overlap between the two lobes.
Q: What is lobe separation?
A: Lobe separation is the angle in camshaft degrees between the maximum lift points of the intake and exhaust valves. It is affected by the placement of the intake and exhaust lobes on the camshaft.
Q: What does lobe separation do?
A: Lobe separation affects valve overlap, which in turn affects an engine’s power curve, idle quality, idle vacuum, and rpm range. Duration, lobe separation, and valve overlap all affect each other. For example, if you want to increase valve overlap without changing duration, you can decrease the lobe separation angle. Increasing lobe separation angle without changing valve overlap requires increasing the duration.
Q: What is valve overlap?
A: Valve overlap is the angle, in degrees of crankshaft rotation, that the intake and exhaust valves are both open. This occurs at the end of the exhaust stroke and the beginning of the intake stroke. Increasing lift, duration, and/or decreasing lobe separation increases valve overlap.
Q: What does valve overlap do?
A: At high engine speeds, valve overlap allows the gasses exiting the cylinder through the exhaust valve to help pull a fresh air/fuel mixture into the cylinder through the intake valve. Increasing valve overlap helps high-rpm power, but reduces low-rpm power and idle quality.
As the spread between the lobes tightens, the lobe-separation number gets smaller and overlap increases since the center-lines of the two lobes are coming closer together. A larger lobe separation angle means less overlap because the lobe center-lines are moving farther apart. This gets tricky because if you increase duration, this automatically increases the overlap with the same lobe separation angle. This is why you will see big cams with wider lobe separation angles since the cam grinder is attempting to limit the amount of actual overlap between the two lobes.