From: Bored&Stroked (Original Message) Sent: 1/3/2004 12:34 PM
Cam Terms and Clarifications:

I believe some cam clarification on terms might help -- here are 4 basic terms you need to be aware of if you're comparing cams, profiles, etc.. This is especially true if you're talking to cam grinders about custom cam grinds - or want to compare between cam grinders. You'll need to know HOW they measure and spec their cams -- or you can't really compare.

a) Lift: on flathead cams was always measured as the gross lift the cam profile delivered -- from the base circle/heel of the cam to the max lift of the lobe. On OHV engines, the term lift takes into account a known rocker arm ratio -- like 1.5 to 1. Since flatheads don't have rockers -- it is the NET lift of the profile itself.

b) Duration: (number of crankshaft degress the valve is open) can be measured a couple of different ways. In the old days, most grinders measured it at .010 or .020 lift off of the base circle (heel of the cam) -- in later years most cam grinders changed to duration at .050 lift instead. This was done to make the figure more accurate in that at .050 the opening ramp is further along and it tends to compare cam grinder to cam grinder patterns more closely. The figures that you're seeing in these old books and magazines are probably at .010 (though I don't think they specified it). When you measure it at .050, the overall duration figure is alot less.

c) Valve Overlap: This is the number of degrees where the exhaust valve is still open - on it's closing ramp . . . and the intake valve is starting to open on it's opening ramp. In early gasoline combustion engines, no overlap was used . . . therefore they had really short valve durations. At some point in the 20's or 30's (I believe), somebody figured out that increasing the duration and overlap really helped performance. Having the exhaust valve still open when the intake starts to open uses the exhaust "pull" out the exhaust port to help start the intake charge entering the chamber -- before the piston has started down and has generated it's own vacuum. Also, leaving the valves open longer (duration) increased the density of the intake charge and allowed the headers to better scavenge the exhaust gasses out. Overlap/duration amounts steadily increased from the 30's where little/none was used all the way to today, where the guys running flatheads at Bonneville are using lots of it. (Just look at a cam timing tag -- see the degrees the intake starts to open BTDC and the exhaust is still open ATDC).

When you're running lots of overlap, the performance of the engine at low speed will drop (less horsepower than stock in many cases), it will idle really rough (the sound we all like), but will run well at mid to full-throttle speed when getting a full intake charge and good exhaust scavenging is very important and the overall latency of moving intake and exhaust gases is more of an issue.

d) Lobe/Profile Separation: This is the number of degrees between the intake and exhaust profile centers. You'll typically see figures around 106 - 110 degrees. I think most of the Isky grinds are about 108 degrees. With racing flatheads, people are now increasing the lobe separation to pick up horsepower -- I believe due to the poor exhaust port designs and inefficiencies of the flathead block. On some blown motors, we want to decrease the valve overlap (due to the intake charge being "pushed" out the exhaust ports) --- yet still maintain the overall duration of the valve timing. You'll now see some values between 112 - 116 degrees in the blower cams that people are grinding.

Enough of my lengthy babble . . . I've even bored myself once again . . .

From: 36fordguy Sent: 1/3/2004 1:20 PM
Inaddition to the above cam comments, an important facts are being overlooked namely how fast the pickup ramp is & how long the cam holds the valve open at max lift. Remember its the area under the curve that lets the gases flow , not only the duration & overlap. Ed came out with 404 which was a good race cam but this cam had to ground on a steel cam and even in the "old" days these were hard to come buy. Later Ed came out with the 400 jr which he ground on a cast iron cam. These cams didnot have a radical "profile" but did have close cam timing as the 404 or potvin 425. The casr iron cam is not as strong as the the steel cam and would "flex" more at high speed and fail more often. Most of the time you could not identify it as cam failure because it scattered the engine

From: Dave McLain Sent: 1/8/2004 9:39 PM
A few more things to think about while on the subject of camshafts. Flathead cams must be quite radical dynamicly for several reasons. First of all, there is no rocker arm to give any kind of lift multiplication, therefore, all valve lift must come from the camshaft lobe and nothing else. It's a good thing that the flathead engine has a very large tappet diameter, it needs it. Also, since the valvetrain is very simple and light it can handle a very aggresssive profile pretty easily without alot of noise or parts breakage.

One of these days I'm going to get a cam checking fixture for our shop, an Audie Cam Pro or something similar. Then it would be very cool to get together a few examples to test. We would then have alot more good info on those old cams for these engines, how much more aggressive is the 404 vs the Isky 400Jr really??

From: Bored&Stroked Sent: 1/10/2004 3:30 PM
I've degreed quite a few flathead cams and graphed the profiles/ramps on graph paper to visually compare them -- especially their opening/closing ramps. Even had a set of Harley KR Flathead roller cams to compare with. I'm just now getting all my parts/engines/cams to Columbus, but hope to have things setup in a manner that I can work once again sometime this summer. It is really interesting to see how the profiles work out -- especially when you compare cams like the 404-A, 425 Eliminator, 450 Erson "Awful-Awful", etc. I'm fortunate to have these cams -- along with a few rollers. Once I get all my data put together and graphed into an Excel spreadsheet, I'll share it with the group. Where are you located Dave? Would be nice if you were somewhere in my neck of the woods! I'm in Columbus, OH.

From: Dave McLain Sent: 1/10/2004 10:22 PM
I'm in central Missouri about 80 miles southwest of StLouis, so I'm pretty far away from Ohio. I've got a program from Performance Trends on my computer that's very good and helpful for graphing cams. It's called Cam Analyzer and it makes it easy to enter your data every two crankshaft degrees. With a little care you can be pretty accurate, but it takes quite a while to graph both lobes of a camshaft.

I'll bet some of those old cams have really crummy dynamics. It's also true that some of that might not matter all that much in a Flathead engine because of the simple, lightweight valvetrain, but modern designs would certainly be better. I just wonder how many profiles are really out there for a lifter as big as the Flathead lifter dameter.

From: Flat32 Sent: 1/11/2004 4:47 AM
I just Google searched for the Audie website and downloaded their Cam Pro demo. Dave, it is slick. Their flow testing stuff looks interesting as well.

From: Dave McLain Sent: 1/11/2004 8:54 AM
I've got a friend, Scott Johnston from Re-In-Car Nation Automotive who has a home made bench with an Audie Flow Qwik. He specializes in porting Big Block Ford castings and it's really worked excellent for him. He ported the SCJ heads we used on the Engine Masters' Challenge engine and he used it for all of his testing. The numbers it generates are virtually identical to the ones from my Superflow bench, we made sure to verify that while we had the chance by testing the same port on the same head and comparing notes.

I got to check out the Cam Pro at the PRI show, it's a really cool device. I also talked with Kevin at Perf Trends, he's getting his cam checking fixture together too. Andrews also makes one but their stuff is way out of my league. That guy is VERY knowlegeable about camshaft mechanics and they do lobe design for many of the top camshaft companies.

If you are into entering your data by hand I would recommend the Cam Analyzer from Performance Trends. It takes time, but if you're careful you can produce very professional results when checking that "mystery" camshaft.


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