trying to understand cam logic more

JRW31526

Member
okay, on my other threads i found out that the 360 will take a long time to get riled up, but tell me this, people say huge cams take a while to get going, but the 360 with champion rockers would make less lift at the valve than the 315 with 1.2s but the 315 dosent take a long time to get going, and on the other hand the 315 with 1.3s takes a long time to get going too, what im saying is the 315 and the 360 have close to the same lift at the valve and they dont act the same, can someone explain this? i may be miss understanding something about the fundamentals of cam works, maybe someone could clear this up for me?
 

flattop1

Dawg 89
A lot like algebra don't always understand it but it works.
A couple degrees here an there make a difference, same with lift. and as stated ramp design.
best to trust the experts.
study design and theory/cam profile
 

alvin l nunley

Premium User
Lift rates, timing, duration, total lift and overlap (the time both vales are open at the same time.) are all part of it. And even then, it’s complicated. It’s why so few make cams.
All cams are going to have an RPM where they reach peak torque. If you set the clutch to slip at that RPM on the starts, there will be no “long time to get riled up” that is if they make more torque than the other cams at peak torque. It could be that a cam has a low peak torque, but the torque curve is flatter. The flatter the torque curve, the more peak HP it will make at the higher RPMs.
I’ve been an Isky fan since the 60’s, and I trust they know what they’re doing.
I forget what they call it, (getting old you know) but there is a small “bump” at the start of the lifting of the valve, and this “bump” is there to take up any compression that could accrue when the valve starts opening. They had a name for it, but for the life of me I can’t remember it.
I know Isky has it, I learned about it from their ads in the 60’s, but I don’t know if it is in their single cylinder cams. I suspect it is. Do you have any ideas on whether other makes have it?
 

rebsfan4

New member
Lift rates, timing, duration, total lift and overlap (the time both vales are open at the same time.) are all part of it. And even then, it’s complicated. It’s why so few make cams.
All cams are going to have an RPM where they reach peak torque. If you set the clutch to slip at that RPM on the starts, there will be no “long time to get riled up” that is if they make more torque than the other cams at peak torque. It could be that a cam has a low peak torque, but the torque curve is flatter. The flatter the torque curve, the more peak HP it will make at the higher RPMs.
I’ve been an Isky fan since the 60’s, and I trust they know what they’re doing.
I forget what they call it, (getting old you know) but there is a small “bump” at the start of the lifting of the valve, and this “bump” is there to take up any compression that could accrue when the valve starts opening. They had a name for it, but for the life of me I can’t remember it.
I know Isky has it, I learned about it from their ads in the 60’s, but I don’t know if it is in their single cylinder cams. I suspect it is. Do you have any ideas on whether other makes have it?
Al,

Are you speaking of the compression release on the exhaust lobe of the cam? It opens the exhaust valve ever so slightly to aid in letting of just enough compression and reducing kickback when starting the engine.
 

astrowrench

Member
Lift, Duration, & LSA
Designing a camshaft is all about juggling the duration, lift, and lobe-separation angle to optimize the shape of the powerband for a given application. If you have a camshaft with the same lift and duration, you can change where the power and torque come on in the same rpm band. For example, in a Pro Stock or Pro Mod motor, we set most of them on a 116- to 118-degree LSA, whereas we used to run them at 108-112 degrees. The tighter LSA created too much torque down low, which resulted in tire shake. Using a wider LSA moved the power higher up in the rpm band, eliminating tire shake and making the power more useable. Likewise, in certain oval track classes we run into rules that limit maximum lift. In situations where we're limited to 0.410-inch lift, we run upwards of 248-250 degrees duration at 0.050 and are still able to make power at 7,000 rpm. As you can see, lift and duration go hand in hand, in terms of making power while the lobe-separation angle affects where in that powerband you'll make peak power and torque. Think about it @9000rpm the valves whack the seats about 75 times a second see how long it takes to snap your fingers


Thanks "Big ED"
 

level600a

New member
I think Russ at NR Racing posted this in the old forum I believe it is one of the simplest and best way to understand the cams.
Here is his post:

There are four important numbers on a cam. ICL (intake center line), LS (lobe separation), duration, and lift.

Don’t worry too much about lift. The Honda head (i.e. port design) does not respond much once the lift at the valve exceeds about .350. A .275 lift cam with 1.3 rockers will achieve this. We have spent a lot of time on the flow bench, and unless the head is radically changed you will not see much gain past .350 lift.

Duration is another story. This is how long the valve is kept open. Hondas love lots of duration. We have run as much as 300 at .050, but 260 seems to be a good compromise better power and tractability. As duration goes up, so does the rpm at which max power is made. At the same time idle quality goes down along with low-end torque. An engine with a lot of duration will scream and make lots of power but can be a dog off the corners and/or trying to get started, not to mention idles at 3000 rpm.

Typically 210 to 220 makes for high torque motors that start and stop a lot, use low rpm stall/clutches, and see a wide range in operating rpm. These motors will peak somewhere between 4500 and 5500, and will pull to 6500.

250 to 260 are for the 7000+ motors that don’t see RPM much below 5000.

Needless to say, 230 to 240 cams fall in the middle.

One interesting thing about duration, the smaller the carb, the more duration can help a motor. With small carbs, usually the cam/ports will flow more than the carb can deliver. The only way to improve power is the hold the valve open longer. In other words, where going from a 240 to a 260 cam on a motor running a 28 mm Mikuni will only show a 10% improvement in peak HP, doing the same on a motor with a stock carb can show a 20% improvement.

Almost as critical as duration, is the ICL. This is the intake opening and will range from 98 to 116 degrees. It is also referred to retard or advance in the cam. This determines the where the peak torque will occur. Most cams fall in the 102 to 110 range. FYI: Honda 160s are around 105 and GX200s around 110 from the factory (retarded for emissions).
A cam with a 102 will have power range/peak around 1000 to 2000 rpm lower than a cam with 110 ICL. It will also have considerable more low-end torque and will pull a lot harder off the corners. For very, very small tracks this can translate into significantly lower lap times despite having lower HP. For Boats, winch boarding, rock climbing ATVs, these cams can make huge differences. Depending on the application, it is possible to stall a motor that has a high ICL, whereas advancing the cam produces great results.

At the other extreme, a cam with 114 ICL might peak in the 9000-rpm range. On a large track (1/2 mile) where the rpms never get below 7500, it would be unbeatable. However, you would need a clutch that engages at 5000+ just to get moving. Not always fun for a street toy.
The other important number is the LS (lob separation), which determines the amount of overlap. These will range from 100 to 115, but most cams will be in the 106 to 110 range. As a general rule the lower the number, the more HP, but the with a smaller power band. However, the wider (less overlap) the LS, the more dynamic compression a motor can build. For high compression engines, overlap is needed to bleed off compression at lower RPMs preventing detonation. Anything over 11 to 1 should be using a 106 to 107 LS. For lower compressions, it depends on how wide a power band is desired. If you want a power band of only a few 1000 rpm (typical for oval tracks), then go with a narrow LS, however if your rpm ranges from 2000 to 7000 (road courses), then you may want a wide LS.

The duration, ICL, and LS all work together and changing any or all of these numbers can have a significant impact on how a motor performs.

One final note, what works on the dyno does not always work on the track and vice versa. What works great in a Mini Bike may not be the best for a Kart, a dirt track cam is not going to be the same as a paved cam, etc, etc.

www.nr-racing.com

The part number of our cams tells the ICL and LS. The first number is the lift, the second set is the CL, and the last set is the LS. For example

280 0207 .280 lift 102 ICL 107 LS
252 0607 .252 lift 106 ICL 107 LS
252 0207 .252 lift 102 ICL 107 LS
252 0211 .252 lift 102 ICL 111 LS
274 0607 .274 lift 106 ICL 107 LS
 

Yakattack

Member
And some more info.... from NR.Racing....
Food for thought on heads, more cam info near bottom...
THANX for sharing RUSS!!!


The key to making power with a Honda head is mild porting, smoothing out the short side radius, blending the bowl, and shortening the valve guides by .100". Together these will yield about 60% better flow than stock. Trying to remove any material (i.e. making the port larger) tends to hurt performance and there is really not enough aluminum to do so. In fact raising the floor of the port improves flow quite a bit. Honda ports (esp. the exhaust) are intentionally oversized to promote cooling. The ports should also be left rough since this creates a boundary layer that improves flow. Best is too sand blast the ports after porting/smoothing. This type of surface will flow much better than a polished surface, plus it helps fuel atomization. The exhaust is not so critical since carbon buildup creates a rough surface on its own.

A good three-angle valve job will allow up to 15% improvement in flow on a GX160/200 head. Unless running huge Mikunis or Tilly’s and turning 7000+ rpm, the stock valve sizes work very well and are capable of making up to 16+ hp. Stainless valves do flow a bit better than stock stuff. For high RPM operation (with big carbs & cams) the 27.5 mm intake valve will help and will allow 20 hp. We have tested 28.5 & larger valves but have not seen any improvement. In fact, we have seen power decreases on the dyno using these. The exhaust valve size should be left alone as it is too big to begin with. A few years ago we installed a smaller seat along a 120 intake valve in a GX160 exhaust port. The motor showed improvement, esp. on the low end.

It is important to know that actual flow through the head is also a function of valve lift. Needless to say, the more lift, the more flow. (Note: higher lifts can decrease velocity, which in turn decreases torque). For the most part, GX200s show improvement as lift is increased. However, on the flow bench a GX160 race head (big valve, ported, milled) does not show any additional flow after about .350 lift. Dyno testing combos above .350 lift (i.e. .400 lift) has shown only minimal high RPM improvement usually at great losses at the low end. Also, the Honda valve train does not do well with a lot of lift. There are a lot of geometry issues that one must address. In fact 1.3 rockers should be avoided as they really play havoc on the geometry. We have seen many failures from using 1.3 rockers with big cams. You can get them to work, but custom valves, spring pocket, and other head work is needed. Stock with stock or 1.2 ratios if possible.

The best combinations for max power tend to be big duration cams (~260 at .050) at .330 to .350 lift. Cam with .280 to .290 lift using 1.2 rockers or .308 cams with 1.1 rockers tend to work the best. However for most applications (i.e. all around performance), a .270 to .280 lift 220 to 230 duration cam combined with 1.1 or 1.2 rockers will be the best set-up. Cam timing is also critical…..
 

astrowrench

Member
Might as well Finish the article by Russ (NR)....... 18cc vs 14cc Heads...The chamber (on both styles) are bowl shaped. As you mill the head it removes part of the chamber and actually unshrouds the valves. A 18cc head milled .080" has much better flow potential than an unmilled head. I say potential because edge treatment can effect the flow. (i.e. round off the edges). We have a digital SuperFlow flow bench and have flowed 100's of heads in all kinds of different configurations with every type of porting one can imagine. We have backed up a lot of this with dyno testing (which does not always agree with the flow bench). In the end, unless major work is done to the chamber, the 18cc heads will always outflow the 14cc.

The only advantage to a 14cc head is that compression can be increased to around 10.3 to 1 by just bolting on ahead. You need to mill a 18cc head .050" to achieve that. However, a .050" 18cc head can make more power than an un-milled 14 cc head....Thanks Russ.......I'm willing to bet (1) Perfectly used stock exhaust valve, THAT all the Builders and Cam makers phones will start Ringing off the hook soon...I have 3 messages already....as a lot of cams will be sold/changed today and this Thread is going to be along one. Wait until SCHOOL gets out!!! ....LMFAO This is going to be most Interesting
This is why I use mostly 18cc Heads for my completed heads
 
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