Cams and supercharger
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Originally Posted by SStrokerAce
andereck , give up man engineermike is just that a ENGINEER so he knows everything. You are beating a dead horse. Your not going to convice him that scavenging works or that you can do it on a blower motor, but oh well.
Did I ever say that scavenging doesn't work on a supercharged engine? Actually, the whole point of my post is that it is easy to overscavenge a supercharged engine.
Mike
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Originally Posted by Vizard, posted by T/A KID
. . .the opprotunity exists to cam the engine to better low-end output and let the usually high capability of a turbine-type supercharger boost take care of the top end. ...it means selecting a cam with a TIGHTER LSA than would normally be used with a positve-discplacment-type supercharger. ...The earlier opening and closing will allow the engine to breathe more effectively at low rpm, trapping more charge weight to enhance low-end output.
Mike
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Originally Posted by andereck
With no overlap, or absolutely minimal, the clearance volume or unswept volume will contain residual gasses from combustion. Let's say that volume is 4.5 cubic inches with a boosted 346. That's 36 cubic inches across 8 cylinders. In a centrifugally supercharged application this volume can be filled with an air/fuel mixture under boost to contribute to the output of the engine. This can only be the case if the residuals are purged from the cylinder. This requires some overlap. In effect increasing overlap increases the fillable displacement of the engine.
Originally Posted by andereck
Regardless of the change if you consider the lift of the valves at that point there is no way you're going to dump 10% of the delivered airflow from your supercharger out the exhaust valve. How much time do you think is available at 6000 rpm for that to happen? With 94 degrees of overlap at .005" tappet lift its about .0026 of a second. At 3000 rpm its about .005 of a second. I would consider 94 degrees of seat to seat overlap extreme.
A test was done some time back to compare superchargers and turbochargers. An engine made 8XX hp on a supercharger, then made 950 hp on a turbocharger, then made 1000 hp on pre-compressed air, all at the same boost level. So, let's just assume that this engine was running 20 psi boost. That means that the turbocharged version had 20 - 40 psi backpressure, but it reduced power by only 5%. I know if you add 20 - 40 psi backpressure to any naturally aspirated engine, power will drop by way more than 5%. A while back, I reduce backpressure from 9 psi down to 6 on a NA engine and gained 13% power. My point is that perhaps the added backpressure from the turbocharger prevented overscavenging (good), but increased pumping losses (bad) for a net loss of only 5%.
I really wish someone would run a once-and-for-all camshaft overlap test on both turbo and supercharged engines. It would be really great to see every thing else stay the same, but just altar overlap and see the results. The cams could be something like:
test 1: 240/250-106
test 2: 232/242-110
test 3: 224/234-114
Mike
Originally Posted by engineermike
Did I ever say that scavenging doesn't work on a supercharged engine? Actually, the whole point of my post is that it is easy to overscavenge a supercharged engine.
Mike
Mike
Also if you can give me ANY idea of the difference in pressures of a cylinder, exhaust port and intake port during overlap of a supercharged vehicle you might undestand this. If you have seen anything work better back to back with high overlap vs. low overlap on a supercharged vehicle you might understand this, problem is you haven't and so you will continue to listen and live by logic that doesn't work. The only way you OVERscavenge a motor is when you have too much overlap for it, problem is you haven't SEEN too much before on a centrifigal blower motor and have PROVEN that is the exact problem. If you do run into this then you have another issue in the system that's holding it back, not the camshaft overlap.
Hell even you understand boost to backpressure ratios in turbos, all you have to do is extrapolate on that some more.
Well enough out of me, I'd rather do other things than share what I know about this if I have to argue these points.
Bret
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Originally Posted by SStrokerAce
If you have seen anything work better back to back with high overlap vs. low overlap on a supercharged vehicle you might understand this, problem is you haven't and so you will continue to listen and live by logic that doesn't work. The only way you OVERscavenge a motor is when you have too much overlap for it, problem is you haven't SEEN too much before on a centrifigal blower motor and have PROVEN that is the exact problem.
Originally Posted by SStrokerAce
Well enough out of me, I'd rather do other things than share what I know about this if I have to argue these points.
uhh... yeeea, so anyway.... back to the guy who started it
peak lift is one of least critical elements of a camshaft... within reason, lift is almost irrelevant in this situation.
what is much more important is timing... thats what everything is debated about above.
one of the basic concepts of a performance camshafts involves opening the exhaust while the intake valve is still closing.... this period while both intake and exhaust valves are open is the overlap...
the concept they're arguing about above, is how much (if any) overlap is ok for a supercharged motor... some argue that with both valves open, the incoming pressurized air will go right out the exhaust.
others argue that it works the same as a NA motor, and it doesnt matter, nothing weird happens..
and most of the smart ones are in between those two extremes... although there are alot of people who think any overlap is a "bad" thing on a boosted motor.
its one of those things where 100 people can do their own testing 100 times, and every one of them will draw their own conclusion..
but to sum up your original question.... the LIFT itself is not effected by the supercharger... but the overlap (and therefore the duration and LSA if you're going by basic cam specs) can be... so you most likely dont want a "huge cam"
Originally Posted by Pete 1
Hey guys, I `ve just been told you shouldn`t run highlift cams with a supercharger, is this correct, and for what reason ?
Thanks
Thanks
what is much more important is timing... thats what everything is debated about above.
one of the basic concepts of a performance camshafts involves opening the exhaust while the intake valve is still closing.... this period while both intake and exhaust valves are open is the overlap...
the concept they're arguing about above, is how much (if any) overlap is ok for a supercharged motor... some argue that with both valves open, the incoming pressurized air will go right out the exhaust.
others argue that it works the same as a NA motor, and it doesnt matter, nothing weird happens..
and most of the smart ones are in between those two extremes... although there are alot of people who think any overlap is a "bad" thing on a boosted motor.
its one of those things where 100 people can do their own testing 100 times, and every one of them will draw their own conclusion..
but to sum up your original question.... the LIFT itself is not effected by the supercharger... but the overlap (and therefore the duration and LSA if you're going by basic cam specs) can be... so you most likely dont want a "huge cam"
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Well I really wanted this to be civil and thought provoking, but its neither which is unfortunate. I will continue to experiment with my own stuff in this area and hopefully learn from it while still enjoying myself. There is nothing presented here that counters my philosophy besides just what the status quo says. I certainly don't believe that anybody has put together a test that included 1300+ cfm of compressed air and was able to maintain that flowrate. I don't see that happening.
Peace all.
Peace all.
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Originally Posted by MrDude_1
one of the basic concepts of a performance camshafts involves opening the exhaust while the intake valve is still closing.... this period while both intake and exhaust valves are open is the overlap...
First, who do you think you are being reasonable on this board?
Second, you have the above backwards and probably know that but I didn't want anyone to get confused.
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From: Upstate NY
Originally Posted by MrDude_1
you know, someday id like to meet you just so i can hear how sarcastic it actually sounds when you say something.... 
(and i mean that in a good way)
(and i mean that in a good way)
Originally Posted by SStrokerAce
Ben, Vizard doens't know anything. Tons of guys will tell you that!
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From: Albuquerque NM - The Land of 8000ft DA
Originally Posted by SStrokerAce
andereck , give up man engineermike is just that a ENGINEER so he knows everything.
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From: K-W, Ontario
I'd like to toss in a few points that I picked up from some of you that have
not been mentioned in this discussion:
- LSA does not determine overlap alone
- Overlap at the lobe, is not equal overlap at the valve (pre-load, rocker ratio, floating...)
- Compression can't begin until both valves are closed. Cylinder pressure
is determined by IVC, and VE (< all that encompasses cylinder filling).
- Overlap is tuned to work within a small range of RPM. You can't grind a
a camshaft to equal efficiency over the entire operating range...especially
in a non-variable environment.
In a race motor, does it really matter if some of the charge is wasted outside
of the tuned area?
not been mentioned in this discussion:
- LSA does not determine overlap alone
- Overlap at the lobe, is not equal overlap at the valve (pre-load, rocker ratio, floating...)
- Compression can't begin until both valves are closed. Cylinder pressure
is determined by IVC, and VE (< all that encompasses cylinder filling).
- Overlap is tuned to work within a small range of RPM. You can't grind a
a camshaft to equal efficiency over the entire operating range...especially
in a non-variable environment.
In a race motor, does it really matter if some of the charge is wasted outside
of the tuned area?
Last edited by Adrenaline_Z; Oct 18, 2006 at 03:51 PM.
Originally Posted by andereck
First, who do you think you are being reasonable on this board?
Second, you have the above backwards and probably know that but I didn't want anyone to get confused.
Second, you have the above backwards and probably know that but I didn't want anyone to get confused.
oopsie... you're right.
it should say:
...opening the intake while the exhaust valve is still closing..
not the other way around.. good catch.
Originally Posted by sscam68
NOt all of us are like that. I'm an engineer and if engineering has taught me anything, it's taught me how little I truly know!
Mike, i'm not giving out the data for free... sorry.
Bret
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Originally Posted by SStrokerAce
Mike, i'm not giving out the data for free... sorry.
Can you at least share what you think is not enough, just right, or too much overlap? At this point, no one has even defined the amount of overlap we're talking about. Who knows, maybe we all agree in the end. . .
Mike
Last edited by engineermike; Oct 18, 2006 at 05:02 PM.
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From: Albuquerque NM - The Land of 8000ft DA
Originally Posted by engineermike
How convenient for you to not have to back up anything with data. . .
Who in their right mind would give out that kind of information for free? Especially the kind that cost THEM time, effort and $$$$ to gather?
Seriously, search through their posts and pick up on the crumbs and start doing research. You'll learn A LOT, you won't become an expert but at least you'll gain an appreciaton for this stuff.
Last edited by sscam68; Oct 18, 2006 at 09:10 PM.
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Originally Posted by sscam68
Who in their right mind would give out that kind of information for free? Especially the kind that cost THEM time, effort and $$$$ to gather?
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From: Albuquerque NM - The Land of 8000ft DA
Originally Posted by engineermike
INTMD8 has tested 4 cams in his turbo setup and post full results from each. Also, I've been privy to just about all the cam testing that Thunder Racing did to develop cams like the TRex and CheaTR. Last time I checked, Thunder and LS1Speed were still in business and doing well.
if you've been so "privy" to TR's testing why don't you go ask them, have them give you the answer? Have them foot the bill. Last edited by sscam68; Oct 18, 2006 at 11:28 PM.
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Originally Posted by sscam68
if you've been so "privy" to TR's testing why don't you go ask them, have them give you the answer? Have them foot the bill.
Mike
BOSS APPROVED!