Which cam is hotter, 20/21 or a GE40? And how do I read this chart?

[Shuie]

Spin off question from the thread about my P&C's.

An S cam is too 'hot' for Max Moritz 98mm P&Cs. No problem, it makes for a screamer of an engine when using the 98mm RSR pistons and carbs.

I've read that a 20/21 is the hottest cam that can be run with the flat dome Max Moritx pistons. To the best of my knowledge, my 3.2 short stroke with the MM 98mm pistons has run a GE40 cam and 40mm Webers for many years.

The following chart gives me the impression that the the GE40 is pretty close to the S, which we know has clearance issues with the MM pistons.







Is it pure luck that my engine ever worked or are the GE40 and 20/21 cams similar? Is one a better choice because of the induction (GE40 with high overlap for carbs and the 20/21 for CIS?). Also, how do I use this chart to determine how 'hot' a cam is or what P&C's it can be used with?

TIA

[KobaltBlau]

The problem is that cams aren't one dimensional, so you can't just say one is hotter than the other and be done with it. You have duration, lift, and overlap coming into play.

I used the word "hotter" in the other thread, but do what I say, not what I do

I'm assuming the data in that chart is correct.

Originally posted by KobaltBlau
The GE40 is really more like a slightly hotter solex cam except with 5 deg wider lobe centers (from the data you found) The lobe centers are a significantly closer than 964s, certainly than the 20/21s. Usually this is the key to valve clearance with CIS dome pistons (wide lobe centers)

the 20/21 has a bit more lift than the GE40s, but the intake and exhaust durations are almost the same. the big difference is the lobe centers, the GE40s are only 5 deg wider than an '911S' cam, and the 20/21 are 20 deg wider (!) than the GE40s.

Yes, the GE40s would be good for carbs/mfi but would not work with CIS (too much overlap). The GE40s should make more power with carbs/mfi than the 20/21, The 20/21 of course works with CIS.

[camgrinder]

The durations for the 20/21 and the GE40 are not measured at the same valve lift on that chart.
I believe the 122 lobe center number is incorrect as well.
I think it should be 112.

Comparing them with numbers at .050":
20/21 238/226@.050" .485"/.452" valve lift
(Taken from the webcams webpage)

GE-40 256/242 @.050" .470" .440" valve lift.

The GE 40 is longer duration cam than the 20/21. It also has closer lobe centers. Both of these parameters put the valve closer to the pistons at TDC.

[jluetjen]

That chart is mine (and a little out of date now). I created it based on reported numbers to try to answer some of the questions that you're asking since many of the cam manufactures do not seem to be interested in presenting data that can be compared company to company. Camgrinder was not posting on this BBS when I started gathering that information so I went and compiled the data myself. I agree that there are no "better" or "best" cams, only different. I'd say that I agree with John D's conclusions, but that's sort of redundant since he as access to more hands-on measurement information then I have since he's in the cam grinding business and I'm not.

BTW, the peak torque and HP engine speeds are as reported by the factory for the different grinds. Using a regression analysis, I estimated the red numbers. They are by no means exact since there are other factors such as CR and port dimensions which affect the engine performance, but they did give a general sense as to the "radicalness" of the different cams. You'll notice that I was able to estimate peak torque engine speed for most of the cams, but not peak HP. This is because there is a pretty strong coorelation between duration and the peak torque engine speed. The peak HP engine speed depends on other factors such as overlap which is not published for a lot of cams.

Oh yeah, I'll be updating my spreadsheet based on Camgrinder's new information.

[rdane]

John,
FWIW my 3.0 with 20/21 cams had peak HP @ 6400 and torque @ 5500
my 3.4 with little changes from the 3.0 internally and the same 20/21 cam has peak hp @ 6100 and torque @ 5200