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Cam design basics ?

General engine tech -- Drag Racing to Circle Track

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CamKing
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Post by CamKing »

torquer wrote: I havnt come across a cam manufacturer that understands what happens when you put a 300 cfm restrictor plate under the carb.
Call me, that's what I get paid for.

First off, when working with a restricted inlet, don't look at flow in CFM. Look at it in mass(lbs per hr). This is the only way to get a true understanding of what's going on.
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Post by OldSStroker »

CamKing wrote:
torquer wrote: I havnt come across a cam manufacturer that understands what happens when you put a 300 cfm restrictor plate under the carb.

First off, when working with a restricted inlet, don't look at flow in CFM. Look at it in mass(lbs per hr). This is the only way to get a true understanding of what's going on.
Isn't that always how we should be looking at flow into an engine? AFAIK, OEMs use mass flow, but probably in gm/sec nowadays.

Jon
[i]"There are some people who, if they don't already know, you can't tell 'em."[b]....Yogi Berra[/b][/i]
[i]"Being able to "think outside the box" presupposes you were able to think in it." [b]--Bob Lutz[/b][/i]
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Post by CamKing »

OldSStroker wrote: Isn't that always how we should be looking at flow into an engine?
Yes, but there are plenty of people who don't.

I've been designing cams for restricted inlet engines since the early 80's, and I'm still suprised at how many good engine builders don't have a handle on what's really going on in the engine.
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Re: Cam Timing Events

Post by David Redszus »

I consider the intake opening and the exhaust opening points to be so close in equal importance, I do not know which is MORE important.

I would respectfully disagree with Harold.
The most important valve event is intake close. It will determine compression pressure and reversionary flow. The exact close angle will depend on induction tract pressure and cylinder pressure due to piston positon.

The second most important is exhaust valve open. If the exhaust valve opens too late, the efficiency of the blowdown period is compromised. If it opens too early, the blowdown is not improved but there could be some loss of combustion pressure on the piston.

Next in importance would be the intake valve open since it is the beginning of the overlap period. If the intake opens too early, we again have reversionary flow with exhaust gases backing into the inlet and a resulting dilution of charge. If it is too late, we will not obtain sufficient intake valve lift at the higher piston velocities.

The least important is exhaust valve close. It should be selected to keep the overlap period as short as possible but still evacuate burned gases.

The overall duration is a function of engine size and speed. Imagine the air as a string of pearls, attached to the piston crown. But the string is a rubber band. As the piston moves down, the rubber band stretches. After a while the rubber band contracts and tries to catch up with the piston, bringing the pearls into the cylinder. Even after the piston has reversed direction, the tension in the rubber band will continue to move pearls into the cylinder. Up to a point.

Larger engines have a longer string of pearls to drag into the cylinder. High reving engines have less time for the rubber band to stretch and contract. Remember the toy called a "Slinky". It was a tightly coiled wire that would move from a high position to a low position. By moving your hands up and down, you could make the wires move back and forth.

A major problem is that air is both stretchable and compressible. When valve events are adjusted for one engine speed, it may compromise another. You can't have your cake and eat it too. Unless...

...you use adjustable cam timing, adjustable inlet trumpets, adjustable valve lift, variable boost, etc.

A major consideration in the selection of valve events are pressure ratios; Pi/Pe (inlet pressure divided by exhaust pressure). Any gas will flow from a higher pressure to a lower pressure. That is what resonant tuning is all about.

When the exhaust valve opens, the blowdown gases exit at high pressure and travel down the exhaust pipe. But during overlap (180degs later) we would like a negative exhaust pressure to assist the scavenging process.

At inlet valve open, we would like a high positive pressure at the valve to prevent overlap reversion. At inlet close (270deg later) we would also like a high positive pressure to prevent reversionary flow.

The exhaust and intake tracts operate on different durations, different temperatures and different pressures.

The very best way to select valve events is to use inlet and exhaust pressure sensing vs crankangle on a running engine. Clint can elaborate on this topic in great depth, I'm sure.

An excellent treatise on the topic was written by T.W. Asmus, Chrysler Corp, and is available from SAE as #820749, Valve events and engine operation.
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Re: Cam Timing Events

Post by CamKing »

David Redszus wrote: The least important is exhaust valve close. It should be selected to keep the overlap period as short as possible but still evacuate burned gases.
I would completely disagree with that.
The engine needs the exhaust to pull on the intake at overlap for a given amount of time. Reducing that time(duration) can kill the power.

If your theory was correct, restrictor plate engines would have 30 degree shorter exhaust lobes, since there is very little to evacuate.
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Re: Cam Timing Events

Post by David Redszus »

CamKing wrote:
David Redszus wrote: The least important is exhaust valve close. It should be selected to keep the overlap period as short as possible but still evacuate burned gases.
I would completely disagree with that.
What is it that you disagree with,.... but still evacuate burned gases? Perhaps you would like to retain the burned gases.
The engine needs the exhaust to pull on the intake at overlap for a given amount of time. Reducing that time(duration) can kill the power.
The exhaust does not pull on the intake any more than the intake pushes on the exhaust. It is the inlet/exhaust pressure ratio that matters, not the exhaust valve open time. The later the exhaust valve is held open, the more difficult it is to maintain a negative pressure in the exhaust and prevent a backflow of burned gases.

Please note that we did not say to reduce overlap duration; only that excessive overlap causes backflow problems.
If your theory was correct, restrictor plate engines would have 30 degree shorter exhaust lobes, since there is very little to evacuate.
Well, in fact they do. A non-restrictor engine might typically have an inlet duration of 268 and exhaust of 274 (measured at that dopey .050" again). A restrictor plate engine would have cam timing reduced by about 30 deg to an inlet of 238 deg and an exhaust of 243 deg. A restrictor plate engine would also have about 1.0mm less valve lift as well. And this restrictor plate, cam choked, engine would also make about 45 hp more (at a lower rpm) than with unrestrictor cams.

Thank you for validating my theory. Actually it is not theory, it is science.
But its not mine.

The camshaft optimization of restrictor plate Cup motors was done by Novak & Kach at Ford Motor Co, and published in symposium notes for the Motorsports Engineering Conference. SAE #962514.

The wisdom and credit belongs to them.
dacaman12

Re: Cam Timing Events

Post by dacaman12 »

David Redszus wrote:I would respectfully disagree with Harold.
The most important valve event is intake close. It will determine compression pressure and reversionary flow. The exact close angle will depend on induction tract pressure and cylinder pressure due to piston positon.
Wouldn't it depend on your definition of "most important"?

Harold defines "most important" as "has the greatest effect on power output". According to this definition, intake close can, in no way, be considered the most important event.

If intake closing was "numero uno", changing it would have the greatest effect on power output. Closing the intake valve, say, 8° late would have a substantial effect. The loss of compression and the addition of reverse flow would murder low end torque.

These thing DO NOT happen. Our torquiest cam is one of Harold's that closes the intake valve 8° later than the the very best Comp grind. It outpowers that same Comp grind throughout the entire power curve, especially on the bottom.

This gain proves that the opening events are MORE IMPORTANT than either closing event. If nothing else, it shows how much effect the opening events have on intake closing.

Intake closing can't possibly be the "most important" if it is affected by the opening events so much, can it?

You CANNOT outpower Harold's truck cam by dicking with the intake closing event. We've tried!!!

This is why intake closing is trumped by BOTH opening events events.

However, the closing ramps do have the greatest effect on dymanic stability, though. The Comp cam would sack a set of springs in one race.

P.S. The Asmus parameters NEVER make the most power, in our engines, at least.

Disclaimer:

These are only the results. I'll leave it to the engineers to explain(or explain away) my observations.

If the theory doesn't match the results, change your theory.
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Re: Cam Timing Events

Post by CamKing »

David Redszus wrote: It is the inlet/exhaust pressure ratio that matters, not the exhaust valve open time. The later the exhaust valve is held open, the more difficult it is to maintain a negative pressure in the exhaust and prevent a backflow of burned gases.
No matter what the inlet to exhaust pressure ratio, if the exhaust isn't open long enough to pull on the intake, it'll hurt power.
There are many things besides duration that effect the pressure in the exhaust, and if you design the exhaust lobe and exhaust port correctly, you can increase the overlap duration without losing the negitive pressure in the exhaust. The exhaust closing point is not the least important point, and it hasn't been for the 25 years I've been designing cams.
Please note that we did not say to reduce overlap duration; only that excessive overlap causes backflow problems.
Sorry, but when you said that the exhaust closing point is the least important, you're contadicting your above statment.

Well, in fact they do. A non-restrictor engine might typically have an inlet duration of 268 and exhaust of 274 (measured at that dopey .050" again). A restrictor plate engine would have cam timing reduced by about 30 deg to an inlet of 238 deg and an exhaust of 243 deg. A restrictor plate engine would also have about 1.0mm less valve lift as well. And this restrictor plate, cam choked, engine would also make about 45 hp more (at a lower rpm) than with unrestrictor cams.
A restrictor plate cup engine that turns 8,000 rpm runs a 304/312 seat duration cam. A non-restricted cup engine turning 8,000 would require a 304/312 seat duration cam. The non-restricted engine would be pulling in a lot more air at 8,000, yet it would require the same amount of overlap duration. That flies in the face of your theory.
Last edited by CamKing on Tue Jan 29, 2008 12:11 pm, edited 1 time in total.
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Post by Rick360 »

David Redszus wrote:Well, in fact they do. A non-restrictor engine might typically have an inlet duration of 268 and exhaust of 274 (measured at that dopey .050" again). A restrictor plate engine would have cam timing reduced by about 30 deg to an inlet of 238 deg and an exhaust of 243 deg. A restrictor plate engine would also have about 1.0mm less valve lift as well. And this restrictor plate, cam choked, engine would also make about 45 hp more (at a lower rpm) than with unrestrictor cams.
Cam King wrote:Wrong.
A restrictor plate cup engine that turns 8,000 rpm runs a 304/312 seat duration cam. A non-restricted cup engine turning 8,000 would require a 304/312 seat duration cam. The non-restricted engine would be pulling in a lot more air at 8,000, yet it would require the same amount of overlap duration. That flies in the face of your theory.
Two theories. Tell us which Cup team is running each of your cams and we'll see who's theory is right in a few weeks at the Daytona 500. 8)

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Intake Valve Closing

Post by UDHarold »

Let us examine 3 cams:

25 BTDC-75 ABDC 280*, 115 LSA/ICL
35 BTDC-75 ABDC 290*, 110 LSA/ICL
45 BTDC-75 ABDC 300*, 105 LSA/ICL

All three close at the identical intake closing point.
Does your common sense tell you they will make the same power, or different power curves, each desireable for different situations?
I have seen unsymmetrical cams, with delayed closing points, make better power than similar duration symmetrical cams, even from my early days at Competition Cams, cira 1977.
My current-designed NASCAR Truck cam shuts the intake valve 8*, right, EIGHT degrees, later than a similar duration cam from a major cam manufacturer.
According to the dyno, my design makes better torque and horsepower, and revs a little higher.
I must being doing something wrong.

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Re: Intake Valve Closing

Post by digger »

UDHarold wrote:Let us examine 3 cams:

25 BTDC-75 ABDC 280*, 115 LSA/ICL
35 BTDC-75 ABDC 290*, 110 LSA/ICL
45 BTDC-75 ABDC 300*, 105 LSA/ICL

All three close at the identical intake closing point.
Does your common sense tell you they will make the same power, or different power curves, each desireable for different situations?
I have seen unsymmetrical cams, with delayed closing points, make better power than similar duration symmetrical cams, even from my early days at Competition Cams, cira 1977.
My current-designed NASCAR Truck cam shuts the intake valve 8*, right, EIGHT degrees, later than a similar duration cam from a major cam manufacturer.
According to the dyno, my design makes better torque and horseower, and revs a little higher.
I must being doing something wrong.

UDHarold
obviously without experince on my type of engine what would your gut recommend for better lowend without lowering topend?

current cam is IO-IC-EO-EC

32*BTDC - 84*ABDC - 80*BBDC - 28*ATDC

cam has 0.038" lift at TDC

stroke 89.6mm
bore 86mm
~3.1L
I-6 configuration
10.5CR
12V head 20% flow improved

basically it hass very late closing intake (which i pressume is killing my lowend), early exhaust open and not much overlap (or lift during overlap for that matter) hurting midrange and topend but making it ok as a daily driver.
Compared to a 2.8L of same type with simlar CR std head with 32-72-66-26 (0.069"lift at TDC) mine is down 20-25% on torque down low and only has more torque from just below 5k upwards with power peaking at 6k and 10% higher.

cheers
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Post by caddycarlo »

Ok I just read two pages of what you guys took time to write and argue over and what did I learn ?

Not Much

The ordinal question was basically asking for how do I choose a cam ………….. This question comes up every day in shops everywhere …………… It seems like a simple enough question …………. Though the inability to answer such a question would suggest otherwise ……….


All My life I have worked on engines …….. sometimes as a hobby and sometimes for money ………. I stood behind a counter selling speed parts for years and this question came up every day ………………. Now all of you are CAM GODS as far as us regular guys are concerned …………. And Over the years I have talked to your kind on the phone in attempts to make better decisions …………

And after reading this and going by my past I think I can safely say you guys are just making guesses ……… now these guesses are based on years of watching what happens on a dyno when you change some part of the cam profile and seeing what worked and what did not. So depending on your memory and experience with the engine type you are dealing with they may be very good guesses or they may not …….. but they are still guesses

After all if you were not guessing you could say if you give me such and such info I can tell you where the intake closing point should be or opening or whatever
But the problem with the whole cam thing is that there is a whole lot of variables from air speed and total flow to carb size to ramp speed to rpm band and so on and so on …..

And assume I know the basics on how a cam works and that if I close the intake too late the air will revert and that if I have too much overlap the low rpm air will be diluted …………. And Tell me how I am to choose a cam ? how do I know that the cam in the engine is closing the intake to late ? What will I see? How am I to know when to open the intake valve? And how do I know I have too much overlap ?

Or am I to just choose one of you to call and ask ? Will you have time for me and the hundreds a day like me ? Or am I to decide by a line of info and a rpm band in a catalog ? I do not even know if the engine that the rpm band was taken from is like my engine after all it is not like they flag cams as street with iron heads and no flow ……… So how do I choose ?

If it is just based on what you know then how then I to choose which one of you to call ? and how will I know that it is you I will talk to and not just some guy you got to answer the volume of calls ?

If you hire guy to answer the phones like comp or cane does what guidelines are you going to give them to answer the phones ? can’t you just give me these guidelines?

If I do Call then what info will you need to make a very good guess ?

All's this guy wanted to know was how could he choose better than he did last time

Can you not ask for the info you need from him to help ? Or is the guessing way worse then I fearfully imagine it is ……………………………………
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Post by Procision-Auto »

I don't think these guys are here to give away cam specs so the average
member can run off and grind their own cam, or buy from someone else.

I'm sure if the same questions were asked in their shop with money in
hand, the end user would get much more information and a camshaft
they required.

When I read posts from the cam design members, I take away something
new each time, however I don't expect them to hold my hand and give
away secrets, or do all the work for me without a dime to their name.

Try this experiment:

Walk into a speed shop empty handed and start asking questions about
a product.

Observe response.

Do the same thing with a few $100.00 bills in hand.

Observe response.

Having said that, when I read through this post I can make better judgement
on LSA and some timing events for the application.

Unless we're willing to spend hours on a dyno/track to test these
tips, we have nobody to blame but ourselves for accepting the info.

Hopefully nobody here is expecting an internet cam recommendation
to be 100% optimized for their motor?
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Post by CamKing »

caddycarlo wrote:I think I can safely say you guys are just making guesses ………
Actually, I use a very involved proprietary equation.
Maybe you can tell me how I should best explain how to pick out a cam?

I could give out the equation, but I don't think that would be a good business move on my part.
I could ask for all the info on the engine, and make a recommendation, but that wouldn't explain how I came up with it.



Waiting your advice
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Post by caddycarlo »

so as far as you are concerned someone should call and talk to to so you can help them out ..............


what info do they need to have on hand for your formula to work ?
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