matching cam to axle ratio

[G_Body_Enthusiast]

I'm looking at getting a Comp Cams Extreme Energy 4x4 cam, http://www.summitracing.com/parts/cca-cl12-231-2 , but I want to know what gear ratios would work well with it.

Here are the specs:
Cam Style: Hydraulic flat tappet
Basic Operating RPM Range: 600-4,600
Intake Duration at 050 inch Lift: 206
Exhaust Duration at 050 inch Lift: 214
Duration at 050 inch Lift: 206 int./214 exh.
Advertised Intake Duration: 250
Advertised Exhaust Duration: 258
Advertised Duration: 250 int./258 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.432 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.453 in.
Valve Lift with Factory Rocker Arm Ratio: 0.432 int./0.453 exh.
Lobe Separation (degrees): 111

The operating range is 600-4600 RPM so I think it'd be fine to use with a tall gear like 2.41 or 2.73.

 

[online170]

Stockish 350 right?

First off, the Xtreme Energy line is excellent! I have high praise for those cams.

But you need to look at the big picture. Gear ratio is only part of the puzzle. The big question is, how will you use the car? How do you want it to behave? To be honest, that cam seems a bit small. Its suited for towing or rock crawling.



If you can tell us how you will use the car, it would be extremely helpful.

 

[G_Body_Enthusiast]

Basically it'd be for a stock 350, 8.5:1 compression, 72 cc heads, some kind of dual plane intake like a 2101 performer, 1 1/2" tube full length headers, dual exhaust. For a daily/weekly driver, looking for economy and torque, don't need a high RPM range.

 

[Yav8]

Cam looks good the the motor you have. I would do a upgrade on the heads to set of Vortecs 96-2001 castings or a set of the earlier truck heads from 85-95 with the center bolt valve covers. Your getting a new intake anyway, the heads alone would be a big improvement.

 

[DRIVEN]

I run an XE256 in a 400 that's similar to yours -- low compression, Performer intake, Q-jet, small tube headers, dual exhaust. It makes ridiculous torque! Tows like a diesel but nothing much past about 4000rpm. My gearing is 3.08 and seems like a decent match and the extra stroke of the 400 even lets me get away with an overdrive on the freeway. One thing to know, if you haven't already read about it yet, is that the XE line has a reputation for being a bit noisy under the hood. Mine is but not a big deal to me. FWIW it's in a '66 El Camino that gets used primarily for hauling and towing.

 

[G_Body_Enthusiast]

I'm not concerned about valve train noise as long as it doesn't mean premature wear on the parts. Using that cam will require springs, so using vortec heads means getting them machined (according to Com). Comp recommends 981-16 or 26981-16. I'd be inclined to using the 26981-16 springs, http://www.summitracing.com/parts/cca-26981-16. Stock springs for the vortec heads are 1.25" OD, the Comp springs are 1.24". Why would the heads need machining? Different installed height?

I want to keep the compression 9:1 or less, I don't need high compression for this engine. With a 4.030 bore, 3.48 stroke, 64 cc head volume, .041 compressed head gasket, +12.3 cc pistons and a .010 deck clearance, I get about 9.3:1. That's a little more than I would like, what octane is needed for that?

 

[Yav8]

The EX256 does not need a upgrade in the valve springs. Its only .448 in and .456 ex so the stock vortec spring will work just fine. The vortec heads can handle more compression then the old smog heads because of the new fast burn design. If you want you could go to a .051 head gasket and drop is a little more.

 

[G_Body_Enthusiast]

A .051 head gasket would yield a compression of just over 9.1:1. Acceptable but quench would suck. I could probably accept a 9.3:1 compression. Summit's version of the vortec head with a 67cc volume would bring it down to 9.08:1 with a .041 head gasket. Looking at the pics, the combustion chamber isn't the same so I doubt they would run like vortec heads but the price is about the same either way. I'd rather go with the vortecs. The heads I was looking at are summit's 72 cc heads.

Summit's 72 CC heads, 165cc intake runner:
http://www.summitracing.com/parts/sum-1 ... /chevrolet
Summit's vortec head, 67cc volume, 175 cc intake runner:
http://www.summitracing.com/parts/sum-1 ... /chevrolet
and the actual vortec head:
http://www.summitracing.com/parts/nal-1 ... /chevrolet

I was only looking at the 72 cc heads to keep compression down.

 

[online170]

All looks good. I highly recommend a 2.73 gear or even a 3.08.

I had a car with a 2.56 once, and it felt lazy. Passing on the highway was no fun, and i almost didnt even need 3rd gear with 28" tire. 2nd gear on highway was about 3200rpm....


2.73 was the perfect gear for a 99% street driven car, and still felt like it had low end grunt. 3.08 was more grunt still, but i didnt like the rpm at 75mph.

 

[G_Body_Enthusiast]

My tires are 25.4", I like rotating my tires since it's just a street car. However, with a 3.08 gear and 25.4" tires, I get 3054 RPM with a 3 speed automatic. Which is not too bad at all. 40 MPH is 1630 RPM, 2.41 is 1275 at 40 MPH and 2391 RPM at 75 MPH. A 2.73 gear is right between the two, 2709 RPM at 75 MPH and 1445 RPM at 40 MPH. 2.41 X 2.52 is 6.07 for first gear, 2.73 x 2.52 is 6.87 and 3.08 x 2.52 is 7.76. Even the 3.08 isn't that radical for a non OD transmission.

The Kia I drive has higher RPM at 75 MPH than a 3.08 gear would. The 2.73 gear would put be me back where I was with the 200c (2.74 x 2.41) but it would also raise the cruising RPM but only by a couple hundred RPM. The 3.08 gear would put me over what the original first gear/axle ratio figure was by 1.1, which is a nice step up in acceleration. But it gives me an extra 600+ RPM at 75 MPH and 355 RPM at 40 MPH. I won't see 75 MPH very often, most of the time it will be 50 MPH or less. I know 2.41 will not work, I think 2.73 would work ok but I think 3.08 would be the best option for a 3 speed automatic.