QuietTahoe

An intake stroke only takes up one half of one revolution. Therefore there is one intake stroke for every 2 revolutions. That gives you 3400 intake strokes per cylinder for 6800RPM.

QuietTahoe

As a reply to "How do you know if your car is over cammed"? One good way is that your MPH in the quarter is pretty good, but your ET is way too slow.

Another thought about overcamming: when you are slightly undercammed you only lose a little performance (and I mean a little); when you are slightly overcammed you lose a ton of performance! You can't compensate for being overcammed with other engine goodies. The only way to compensate is with steeper gears and less weight.

highgear

CFM needed for RPM?
For an NA motor the correct formula is: cubic inches X rpm = div by 3456 OR
346 x 6500rpm= 2249000 div by 3456 = 650.75231 CFM at 100% volumnmetric efficiency. (at one atmosphere of pressure) This is the max amount the entire motor can pump at this rpm. This about maxes out the stock MAF, or close to it. I have read where it is rated from 650cfm to 750cfm. Don't know which is correct. The entire intake system would have to support this flow.

Bink

Duh! My mistake...Thanks for the help! There are 4 cylinders on intake per revolution.... so I only had 1/2 accounted for. At 6800 it would be 680 cfm. At 6500 it would be 650 cfm - which coincides with highgear's formula. Now I seeit...Thanks again!
joel

Bink

Screened 85 mm MAF -max calibrated flow - 428.46 gm/sec = 704.51 CFM

Descreened 85 mm MAF -max calibrated flow - 443.60 gm/sec = 729.40 CFM

Earlier LS1 MAFs maxed at 428.52 gm/sec.

J-Rod

Since this thread is rolling right along, I think I will just jump in here.

Let me give you a real world example that many folks might understand better. When folks talk about overcamming and engine. The maxim is bigger is always better. But in many cases it isn't...

When you have an engine of W displacement with a target RPM of X, head flow of Y, then Z is your duration, period.

Now lets give a non-motor example. Take a glass and stick it under the faucet. turn it on. The glass begins to fill. At some point the glass is full and can take no more fluid. Thats your cylinder. Now, dump the water out and hold it under the faucet half as long as you did before it filled. Its now half full. If you open the faucet up more, it fills faster. So, you can keep the glass under there an even shorter period of time before it is full. So, lets ask ourself the magic question. If you hold that glass under the faucet until its past full, is it going to get fuller than it did when it was imply full. We all know the answer is no. It won't get any more full than it is when it is completely full. The glass example is something we can all relate to. At low RPM, think of that glass with water running over the sides as your engine. The water running over the sides is overlap. If you know that for X ammount of water leaving the faucet at this particular time that sticking the glass under the faucet for a certain period of time you will minimize the ammount of water spilled and maximize the filling of that glass. If you just stick it under there any old time you please, you may or may not get the ammount you want, and a lot may be spilled (read that as wasted). Sure if you just leave it under there longer, you may fill it up, or you may end up with a bunch rnning over the sides. The idea is to do it smarter....

This holds true for an engine. An engine at atmospheric pressure can only fill an engine so far (unless you get into Pro Stock, and things like wave dynamics and inertial supercharging. But thats outside the scope of this discussion right now). Duration is all about having less time to fill that cylinder once you have identified how long to prop that valve open, unless you plan on reving it higher. It does you no good to leave a valve propped opn some inordinate ammount of time over an engine with proper valve events and matched components which is open a shorter period of time. Here is an example from the SBC world from David Vizard. He looked at an engine with a stock cam 260/270 gross duration on a 112LSA an 41 degrees of overlap. He then swapped it for a "box cam" 278/290 gross 114LSA and 56 degrees of overlap. Under 300 the box cam lost power, but 3500 and up it made more power. 45 more HP at peak. So, now a "smarter" cam was designed for the car using all the information avaliable. This cam was a 272/278 108LSA with 56 degrees of overlap. This cam made more power across the board, all the low end plus 15HP over the box cam. The stock cam would idle at 600RPM and draw 18" of vac. and both aftermarket cams idled at 650RPM and both drew 15-16" of Vac.

So, how did a cam with a 108 beat a 114 across the board, idle just as good, and make more power.... Must be magic. Nope, its just proper selection of valve events and lobes. Ok, now I'm going to throw some material from David Vizard and Buddy Reher at you, along with a few of my own comments.

Myths and Semi Myths
We are living in a world where off the shelf cams are specced out based on whether or not they produce a good improvement in a typical engine, not whether they produce an optimal improvement within the desired rpm range in your often unique engine. For the most part a 'typical' engine is a 350 small block Chevy, not an LSI or LS6! Without doubt the cam companies hold within their grasp the means to deliver power increases that rival the power-per-dollar ratio of a typical nitrous kit. But getting optimal valve opening events, which could change with even the smallest change of engine spec,would mean dyno testing a number of cams. For the masses without the resources to dyno test a batch of cams, this is bad news if optimal performance is the goal. The good news is things are changing, especially for those of us who are not modifying mainstream engines like the small-block Chevy.
A really well specced cam can be worth a lot in terms of output. Some years ago a well-known cam company boss said, "There was no such thing as a 100-horse cam for a small block Chevy." Using nothing other than this company's flat tappet hydraulic profiles DAvid Vizard (the guy who wrote a bunch of this) speced out a cam that, with no other changes in the 355-inch test motor, produced a 108-horse increase over a stock factory cam! This alone should demonstrate what a great deal a near optimally specced cam can be.

A cam choice based on duration figures of say 270 (street) 285 (hot street or semi race) and 300 (race) degrees is not, repeat not, the way to achieve optimal results, and here is my argument why. The dream of many hot rodders is to be able to run a really big cam on the street. It is totally practical to spec out a 300-degree advertised (or about 255 at .050) duration cam that will still allow you to drive around a traffic island at 1000 rpm in high gear without a problem. In addition to this it will produce a lope-free 650-rpm idle. Now that may sound like news to get excited about, but before you do let me put a giant damper on things. This cam would fall way short of the torque and power of a shorter cam correctly specced to meet the same idle and low-speed driveability requirements. This demonstrates that duration by itself does not come close to being the principal idle quality and low-speed driveability control factor. Since low-speed performance is not directly related to duration, it clearly cannot be used as the main selection parameter when speccing cams.


So what should be used as the principal cam speccing parameter—the LSA maybe? This proves to be a major control parameter so its use is a big step in the right direction. However, there is a big issue to be addressed here. Many cam companies talk about the LSA as if it is an adjustable feature. They will usually spec a wide LSA for street use and a tighter one for race use. The justification is that the wider LSA gives a better idle. The real truth here is that the LSA for optimum results, within a specified power band, for a given engine, is not adjustable. Only one LSA will produce optimal torque and power for a given engine spec and powerband. Any time it is suggested that the LSA be spread to improve idle quality it is a sure sign that the duration chosen is too long for the job! Sure a wider LSA will spread the powerband and improve the idle, but the price paid for spreading the LSA to achieve a certain idle quality is a substantial reduction in torque and power.

It is little known outside the cam industry that cam grinders often deliberately spread the LSA to protect overzealous hot rodders from the negative effects of an excessively long duration cam they insist on wanting to install.

Although the LSA is an important entity, its determination alone is still not enough to allow an optimally specced cam to be produced. In actuality it is more part of the answer rather than part of the question. (Remeber what we've been preaching about valve events)


One parameter on which to base a cam is overlap. Determining precisely how much overlap an application requires is complex and dependent on the intake/exhaust flow characteristics during the overlap triangle in relation to the cubic inch displacement of the cylinder. That's a mouthful and only the start of what we need to look at to properly select cams. With some new, more complex rules for cams selection looming here some of you may feel you would like to stick to the old method of cam selection by duration. You may be thinking that it's worked for you in the past so why change. I did not say selection by duration was a total failure. What I said was it is far from optimum. If you want to blissfully go along with the old traditional duration method in the belief that it will produce anywhere near optimal results (except by luck) be aware that the flat earth society is desperate and looking for new members!


Characteristic Relationships
The amount of overlap an engine requires to meet the application's needs depends primarily on the rpm involved plus the displacement of the engine in relation to the flow characteristics of the valves at low lift. Remember, low lift is where the valves are during the overlap period. Obviously the more rpm the engine is expected to turn, the more overlap required—but only up to a point. If the cubic inches get bigger more overlap is required for a given rpm band. On the other side of the coin, the greater the low lift flow of the valves, or the faster the valve opening acceleration, especially the inlet, the less the overlap required for optimum results. This is a valid reason for contradicting those head porters who tell you low lift flow does not matter. On the contrary—it does, and when you get your heads flow checked make sure you get flow figures at 25, 50 and 100 thousandths lift if they are to be used to aid cam selection.

From just the foregoing factors we can see that a complex interrelationship is forming. Once the amount of overlap has been decided (either by experience but preferably by computation—more about that later),

J-Rod

the next step is to determine what the optimum LSA will be. For a given rpm range and application there is, as I have already stated, only one that will be optimal. For a given cylinder head, the bigger the motor, the tighter the LSA required and visa versa. If the high lift airflow of the heads is increased it has almost zero effect on the LSA. If the low lift flow is increased the LSA gets wider and visa versa.

As the compression ratio goes up the LSA needs to get wider, and the reverse happens if the CR is reduced. Although rod/stroke ratios, rocker ratios, thermostat temperatures, etc. affect LSAs their influence is less significant. Once the overlap and LSA have been determined, only one intake/exhaust duration figure will fit the numbers concerned. For instance, if for a single pattern cam where the overlap called for is 60 degrees and the LSA is 110, the duration can only be 280 degrees. To arrive at the duration when the overlap and LSA are known we take the overlap (60), divide by 2, add it to the LSA (110+30), then double it (140 x 2 = 280).

Exhaust Duration
From what we have discussed so far you can see how the duration is dependent on the overlap and LCA. Of course, we have not looked at the whole picture as of yet. Among other things there is the intake to exhaust duration that is needed. This is comparing the intake and exhaust flow and determining how much exhaust timing is needed to adequately get rid of the cylinder's contents at a specified rpm (usually projected peak power). This is influenced primarily by the intake to exhaust flow ratio of the valves, but it is far from the fixed value often supposed by cylinder head porters.

Assuming all the available space is used to accommodate the largest valves possible, the optimum exhaust to intake ratio is strongly influenced by the CR and the rpm.

Just for the record, when developing a head the higher the intended CR is the less exhaust flow needed in relation to the intake. A 10 to 1 CR may produce best results with the commonly used 75-80% exhaust to intake flow, whereas a 17 to 1 CR can produce better results with a slightly bigger intake and a slightly smaller exhaust producing about 65-70% exhaust to intake ratio.

Buddy Reher of Reher-Morrison backs this up in a discussion he had on flow benches.
Textbooks would lead you to believe that an exhaust to intake flow ratio of 80 percent is ideal - yet a typical Pro Stock head has exhaust ports that flow less than 60 percent of the intake runners. You can improve the exhaust flow tremendously with about 40 minutes of work with a hand grinder - but the supposed improvements will just about kill the engine's on-track performance. I know because I've been there.

We have also learned that low-lift flow (meaning anything below .400-inch valve lift in a Pro Stock engine with a .900-inch lift camshaft) is relatively unimportant. Think about the valve events in a racing engine: From the point when the valve first moves off its seat until it reaches mid-lift, the piston is either going the wrong way (that is, it is rising in the cylinder) or it's parked near TDC. The piston doesn't begin to move away from the combustion chamber with enough velocity to lower the pressure in the cylinder until the valve is nearly halfway open. Consequently it is high-lift flow that really matters in a drag racing engine. (We're not talking a street motor, but I wanted you to see what he big boys have to say about this).

The shape of the combustion chamber also has a significant impact on performance. A conventional chamber with deep reliefs around the valve seats and a relatively flat valve seat angle can produce terrific flow at .200 to .300-inch valve lift. Today a state-of-the-art chamber typically has 55-degree valve seats and steep walls that guide the air/fuel mixture into the cylinder to enhance high-lift flow. (ok, now back to David's stuff)...

When a cam is selected for a motor it has to match the flow characteristics of the cylinder heads used, be they optimum or not. In other words, it makes no sense to install a cam ground to suit a pair of heads you don't have. If the exhaust flow is too low the cam needs to compensate in part by having a slightly longer duration and, as you would expect, visa versa. As the operating rpm moves up the scale so does the exhaust duration required. If a very high compression and a lot of rpm are involved then the exhaust duration required can be 10-15 degrees more than the intake.


Valve Lift
Unlike a Hemi or a typical 4-valve-per-cylinder-type head, a parallel (or nearly so) valve, 2-valve cylinder head typically continues to flow more air up to lift values equal to as much as 35-40% of the valve diameter. The reason for this is that there is a flow pattern transition period that takes place at a lift value of about 25% of the valve's diameter. When this point is passed, if the port has been modified to support flow in this lift region, the port's flow efficiency actually starts to increase. This is the reason why 2-valve engines respond to high lift.

If you want to build a street motor with the most power, without any sacrifice of idle and low-speed qualities, then lift can become even more important than duration. The best street cams are those that seek to maximize lift while only adding a minimal amount of duration.
If you are building a stroker motor, be aware that the lift required increases virtually in proportion to the increase in displacement. Given some super-high-flow heads, a 4.8-liter (293-inch), LS-style motor may not need more than about 550 thousandths lift, but that same pair of heads on a 427-inch unit may well call for as much as 800 thousandths to deliver the best output.

Fortunately, putting lobe lift onto an LS1/LS6 cam is helped considerably by the fact this valvetrain was designed from the outset to be a roller-style setup. The advantage the cam has is that it is designed on a much larger core diameter than usual.

This means a larger base circk can be used. This in turn means that for a given pressure angle between the cam profile flank and the roller lifter,, the acceleration can be made higher. In short this means more lift, better dynamics and less stress. Any cam profile used should take these advantages into account rather than use a generic SB Chevy roller profile.

I'll continue with this later. Right now its time for bed, I just spent the last sevral hour debugging a big issue at work, and I'm tired.

Colonel

CHRISPY

Excellent Writeup J-rod!!!

FASTONE

Good post J-rod,although I don't totally agree with everthing you stated,You did make me think about some things differenty such ex. port flow on race engines with optimum headers and collectors.Also how high compression engines can use less duration on the exhaust.Now on LDA things get interesting!!!!I just built a 408 SBC with9.95CR, 6in. rods,185CC fully ported Vortec heads with2.02 and 1.6 REV valves(way too small for 408,right??)I went with a Comp .230 intake and .236 exhaust,.544 lift and .555, now for LDA, I was thinking of 110 to 112 since it was a street car,well about 6 monthes ago CHEVY-HI-PERFORMANCE magizine built a similar 408 but it had 5.7 rods, cam was .230,.480 lift on 109 LDA,installed at 107ATDC, THE 109 LDA caught my attention!!I'm just using this motor as an example as it relates to the GEN-111'S as well. This motor had outstanding torque(475@2400RPM,525@3500,)I know part of this was the small heads, but cam showed a lot of promise.CHP later put some AFR heads and 1.6 roller rockers and it made 490 HP still with a great torque curve, this lead me to reason that the 109LDA had a lot to do with this mild engines output. POINT being made is this cam was a little out of the norm,as side note to back up the narrow LDA is that GMPP is selling a 383HT(crate engine) and its cam is 108LDA now this is sold as a truck engine for torque?????By the way my 408 which is a hydralic rollercam idles very well at 750 RPM and pulls my car very well even in third gear in city traffic, and to think I was worried about the idle and streetability.Also this cam is extremely guick reving and pulls very hard to over 6000RPM .This is with 3.42 gears and 28' tires!This is just my 2 cents, this is a great thread its getting interesting to say the least, I'm curius to see how DenzSS ends it,but enjoying everyones different theories on CAMSHAFTS AND V.E. of an engine,keep it coming!!!!!!!!!!!! By the way you LS-1 loyalist I'm building one right now,it will be different!!!!!

QuietTahoe

Great post J-Rod, I agree with most of what you say as long as optimum peak power is the goal. The problem is when you add emissions regulations to the mix. Having worked for a few major performance components manufacturers in my career, I realize that they have to put forth a product that meets government regulations or at least shows some token respect for government regulations. The performance aftermarket is under continual scrutiny by regulators and "tree-huggers". Most of the major cam manufacturers have cam designers that are aware of all of your points. Catalog grinds are there to provide a good "bang for your buck" to the average guy, but most all of the cam companies have literally tens of thousands of lobe profiles that can be put on a stick with any LSA that you want. All a guy has to do is call, give the details of his ride and, last and most importantly, be honest with himself and with the cam company about what he wants. I had my own engine rebuilding operation for 24 years and you can get great help and custom sticks without any aggravation at all from most of the cam companies. Readers just have to understand that everything is a compromise. There is no such thing as a perfect camshaft. There is only a best camshaft for a limited application under a limited set of circumstances. Change the circumstances, including weather and altitude, and the cam changes! Too many readers don't appreciate that.

You do a great job educating people, are you a engineer/teacher?

FASTONE

Denzss, I THINK I JUST GOT IT,280CFM is a flow per minute measurement,thats the flow thats needed at peak intake air velocity to fill the cylinder kind of like a MPH measurement, am I close or out in left feild?????I was looking at the question as a volume measurement not a speed measurement, I GOT IT RIGHT????FASTONE

DenzSS

Yup. Cubic feet per minute. That is what is required to provide the cylinder with all of the air/fuel that it needs at a certain RPM.

FASTONE

I can't beleive it , DenzSS you made me think alot about that formula and the 280 cfm , then it hit me out of the blue this morning.I kept trying to see it as a volume needed to fill the cylinder and not the speed at which it is filled, cfm is a speed measurement and a volume measurement.

Beavis5.3

Can't throw NASCAR into this equation. Those motors constantly run at high RPM's (And I mean constant 7000-9000 RPM). You can't apply that to street enignes that will only briefly see slightly above 6000 RPM, maybe. Plus, NASCAR motors run 15:1 compression ratios, don't see anyone with a street 348 LS-1 running that high a ratio.

Beavis5.3

QuietTahoe's comment makes me think of another good point. Too many guys aren't realistic about what their car is gonna do. Like I mentioned in an earlier post, guys talk about cams that go up to 6000-7000 RPM, how often does a street motor really see this? Not often.

Yeah, every guy wants to be able to boast he's got the baddest, meanest motor on the street. But boasting is radically different from actually PROVING you have the baddest, meanest motor on the street.

As Dirty Harry said "A good man always knows his limitations."

Most guys don't know squat about cam design. No insult to anyone, just a fact. So why not consult an expert, someone at one of the cam companies who is. Odds are they've seen a dozen other guys setting up a motor the same way and know what will work, and what won't.

The wrong cam can cost you some serious bucks, thousands of dollars in some cases. The right cam can not only save you money, but also let you get the most form the other parts you have.

2002Z06yellow

so if i wanted a cam what should i do? i obviously cant design one, and the off the shelf products are not optimum... what can i do to gain the most power and and still maintain reliability?

Beast96Z

iTrader: (21)

Talk to a professional builder. I reccomend giving EDC a call. (EDC is his screen name)

DenzSS

Ed is very good.

J-Rod

Ok, so lets look at this from a different angle. As has been stated, for a given engine witha given airflow "budget" there is an ideal duration for given RPM. Now, my question is how do we determine that? What is the math to make that calculation?

Now my second point to that is this. If you look at all the various cam lobes out there, you will see that just looking at the gross duration, or the .050 doesn't really tell the whole story. So, the question then becomes this. If you can determne the required duration, can you influence that duration with the speed of the lobe? I'll give an example Suppose you are using a comp XE Marine lobe that specs out at 242 degrees of duration @ .050. You throw in a XE-R lobe that specs out at 232 @.050, and a XE-R lobe with 242 @.050




I used these cam lobes in a discussion in my cam thread to illustate that there is more to a cam than just duraion @ .050.

Lobe 3726:
.006 281
.050 232
.200 153

Lobe 3356:
.006 298
.050 242
.200 153

Lobe 3731
.006 291
.050 242
.200 163

I was pointing out if you did a cam with the 3356 lobe, you could consider switching to a 3726, or you could go the other way. To be easier on your valvetrain you could switch from the 232 to the 242. Now, one has less lift than the other, but I am just trying to show that there is more to a cam than .050 where everryone gets hung up on.

From SStrokerAce
The lobe area is the key here, and with the 3731 and 3356 there is a big difference that is much easier to see in a graph than in a bunch of numbers but the area number still shows how much more is actually there.

3731 = 30.26 inch deg
3356 = 27.90 inch deg

Which means that the 3731 lobe has 8.46% more area. That means the lobe is open 8.5% longer letting in more air = more power.

Now the 3726 vs. 3356 comparison is interesting too....

3356 = 27.90 inch deg
3726 = 28.25 inch deg

So now the same .200 duration, 10 degs less .050 duration and still more area. 1.25% more.

If you want to get back down to the same lobe area then you have to go down to the 3725 lobe with 27.88 inch deg of area. That's a 230 duration @ .050 with 151 @ .200, looks like the extra lift helps that one out.


Ok, now Chris' comment about lobes on the stret are as follows.

Agressive and mild lobes. Again they are used to work with the airflow. A killer set of heads on a NA car that will see mostly street use. . .mild lobes. If the head is flowing good air, then you can easily open the valve and give the engine a smooth broad power band. Take a good set of heads, mismatched runner volume...undersized for the CID of the engine, you need an agressive lobe to pack that cylinder. Positive manifold pressure cars need mild lobes.


Ok, so what about one that see some track duty? A max effort street car... I'm not talk about your 50K mile a year car, more like your weekend warrior. Would you still go the same way? Do you take the road of a more agressive opening and closing, and if you do and your valve events stay the same then overlap should decrease. Which is preferable?


Quiet Tahoe: I'm a computer nerd who love cars, and has been fortunate enough to be able to talk to folks a lot smarter than I am about cars, a little of which have been ale to pick up on... Again, I put in a lot of my own commentary my simplistic style of trying to make it palletable to folks who may not be as technically inclined comes from trying to explain technical things to non-technical managers (i.e. - Here is why this does what this does, and why it does it, and in my case why it costs so much...).

Thanks for the props. Again, I can't take credit for a lot of that, as my sources for it were some pretty smart folks (David Vizard , and Buddy Reher)...