[PontiacMatt72]

When tryin to select a cam, obviously there are multiple factors that affect the final decision. What I'm curious about is: what is the difference between selecting a cam for a manual transmission and selecting one for an automatic transmission?

For example: 461, 6X-8 heads (240-250 cfm), (approx.) 9:1 compression, 3.42 gears. in a 72 Lemans, high performance street car, (approx.) 3800 lbs.
- two transmissions to consider: M-20 (2.56 1st) or TH400. If you picked the "ideal" cam for each transmission, what would be the difference between the two.

I'm planning on running a solid roller, custom grind, somewhere around the tune of...
- Comp Cams XE custom grind Solid Roller
- 240/246 duration @.050"
- .570/.576 Lift (1.5 ratio rockers)
- 110 LSA
- 106 ICL

This was spec'd for an automatic. what variations should be made for a manual tranny, if any?

[Skip Fix]

"Generally" with a stick and more gears and a clutch to slip at low rpm you can go with a cam that has a narrower band, usually a closer lobe sep.

[Steve C.]

Single-plane intake or dual-plane intake ? Can influance the decision.

A wider LSA figure, i.e. 112-116 degrees, moves the lobe centerlines further apart and will smooth your idle due to the decrease in overlap that it creates, and idle vaccumm should increase. Some suggest good for use with a automatic transmission in some instances. The lower the figure, i.e. 106-110 degrees, moves the centerlines closer and will increase bottom end power, but your idle will suffer along with it. Some suggest not as much of a factor with a stick car.

[PontiacMatt72]

Single-plane intake or dual-plane intake ? Can influance the decision.

Haven't decided yet. either Performer RPM (dual plane) or Torker II (single plane).

As I said, cam was spec'd for auto trans., but have decided to go with the 4 speed instead.

["QUICK-SILVER"]

Originally Posted by PontiacMatt72

As I said, cam was spec'd for auto trans.

With what kind of converter?
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To me, the whole point is keeping/using the cam/engines basic RPM range.

Don't tell the cam and it wont know if it's running auto or stick.

It never bothered any of my cars anyhow. The 4 speed always felt better but didn't show it at the track.

[DiamondJim]

That is a pretty healthy cam for 9 to 1 compression. It will be lazy down low in an auto unless you use a higher stall converter, around 3500 or more. Sticks are more fun to drive but Pontiac power will break parts. I would not recommend the M20 and sticky tires. Goodluck -Jim

[Steve C.]

The use of the word "power" in regards to the bottom end could be a bit misleading in my post above. Not power as in horsepower. According to Comp Cams the narrow lobe separation moves torque to lower rpm and increases maximum torque.

http://www.compcams.com/Technical/FAQ/LSAproperties.asp

And this from Engine Builders magazine:

"Transmission – Manual or Automatic? Automatics typically need more low end torque, especially in a heavier vehicle. But this can be affected by the stall speed of the converter. A higher stall speed allows the engine to rev higher before it grabs, but hurts fuel economy in a street driven vehicle. If the engine will go in front of a manual transmission, how many gears does the transmission have, and is the gear spacing wide ratio or close ratio?"

http://www.enginebuildermag.com/Arti..._camshaft.aspx

[PontiacMatt72]

With what kind of converter?

Hadn't chosen a converter yet, but was thinking somewhere around 3000 to 3500 stall. but as I said, no longer going with automatic...

I would not recommend the M20 and sticky tires.

I don't plan on ever running "sticky" tires. It's just a high performance street car.
Chose the M-20 because of rear gear ratio (3.42) found a pretty good deal on local M-20 that supposedly doesn't need to be rebuilt. Just so happened to be the wider ratio M-20 (2.56 1st as opposed to 2.52). I'm gonna stick it in as it is, and if it breaks i'll rebuild it with wide ratio M-22 gears (if possible). Have not bought a clutch yet, but will definately not slack off and will get a good quality one.

[stickboy]

Unless you want to rev it up all the time, I would consider it MORE critical to have smooth idel in a stick car because it is always directly connected. So maybe a bit smaller on the cam and on a 112...230, 236. You're 3K stall didn't care about any response below that, you're 8.75:1 1st gear ratio is gonna buck and you'll have to slip the clutch a lot with that much cam. 3800# is a lot of mass to get moving. Plus if you plan to cruise in 4th around town at 2000, better make sure it'll be happy to drive down there...I'd go one step smaller and wider on the cam in a "street" stick car.

Race is a different story, then you can rev it to the moon and dump the clutch, who cares what it does at slight throttle at 1800.

Plus carb and timing tuning will be key to make it drive nice on the street.

I run the same exact cam you're thinking of except w/ 1.65's......but my 1st gear ratio is 10.37 compared to your 8.75.

But I would still recommend one step smaller and wider...fwiw.

[i82much]

Keep in mind a "wide ratio" muncie actually has a pretty tall first gear compared to most modern manual transmissions. If I recall correctly I could go about 50 mph in 1st gear with my m-20 and 3.55 gears in my 65 gto ... doubt you could hit 30 if it was a T-5. And 3.42 is not a lot of gear for a stick car.

[lust4speed]

I hate to contradict the Comp Cams sheet that you quoted, but I don't agree with their definition of decreasing LSA. I would say that a smaller LSA brings both ends of the torque curve closer together while raising total torque. So instead of decent torque being available at 1,800-5,200 with a maximum of 482, a tighter LSA might produce a usable curve of 2,600-4,400 with a maximum of 520.

I have run the same engine with an auto and an M20, and in my opinion the auto with a higher stall was more comfortable on the street than the 4-speed with a large cam. I'd go smaller than their recommendation and spread the LSA out to 112-114 for the 4-speed. Again, this is not worrying about dropping the hammer and letting all hell break loose because any cam is going to shred tires -- this is about being in traffic on a cruise route or with a cop on your tail and you want the car to act civilized taking off.