Best Cam Upgrade for stock 360???

jpnjim wrote:

Nice analysis Grey!

Thanks for taking the time to post it!

I have a couple questions if you don't mind:

"3) INTAKE FLOW CAPACITY"

^

Is there a formula, or computer program you use to get this number?

I've never seen it put like that.

EVO:

"Open it too late though and exhaust can't escape, resulting in pumping losses and intake charge dilution, both bad."

^do you think there's a real risk of opening the exhaust too late in an off the shelf cam?

or was that part mostly just to simplify the explanation?

I understand the risk of too much overlap from a late exhaust event, and the reversion you could get,

but I hadn't considered there's a point where the exhaust would be too "lazy" (not enough 'pop') from opening too late.

I guess a wider exhaust lobe would help here,

it would open soon enough to benefit from the higher cylinder pressure earlier in the cycle,

while still be being wide enough to have decent overlap as it as closing.

My belief has been that using a wider exhaust than intake always comes at the expense of bottom end torque.

Thanks again,

I enjoy reading your cam explanations. Smile

I worked up a spreadsheet with all of the common aftermarket off-the-shelf AMC V8 cams a while back that does all the calculations. It started out as a super detailed DCR calculator, but then I took it a bit deeper and added a ton of crazy features. I'll hunt down that thread and post the latest version of the spreadsheet on it.

For the intake capacity I'm taking the cylinder head flow test data for a typical head, and using that with the camshaft lobe profile to plot the cylinder head flow curve corresponding to the cam lobe lift curve. The area under the lift curve of the cam doesn't dictate the capability of the cam if the head runs out of capacity before the cam reaches full lift. Plotting the flow capacity curve gives a real representation of how they work together, and will show you how a better head will get more out of the same cam than a stock head would. Calculating the area under the capacity curve gives us a nice single number we can use for comparison. The spreadsheet lets you pick between generic head types, but it's good enough for cam comparison purposes.

The exhaust opening event location is something I don't know enough about in order to say exactly where it should be for a given application. But based on experience and common sense you can see how opening it really early will give you a louder exhaust because there will be higher pressure in the cylinder when you open it. Also, since that pressure is released so it can't get converted to work.

The overlap period is coming from the exhaust closing event, and how it overlaps the intake opening.

1968 AMX 390 w/T5

Yes indeed - the spreadsheet compares the attributes of 103 different off the shelf cams (well, sometimes they are the same cam just with a different name). Both of the Summit cams are in there.

On this engine combination the Summit K8601 cam would give a DCR of 6.09, which is pretty low, when you consider that most engines will run 87 octane with a DCR up to 7.00. And if I was building the engine and wanting the best power from pump gas I'd aim for something closer to 7.75 and run it on 93 octane. So you can see that 6.09 is really not optimal.

That being said though it's hard to get and 8.3:1 engine up to a DCR of mid to high 7's with the off the shelf cam offerings that are out there. The best are coming in just shy of 7.0. One thing you can do to increase the DCR is to advance the cam a few degrees, and on these lower compression builds that might be a good solution. When doing that though keep in mind that the EVO point moves as well, so for example the nasty Crane H-272 EVO of 51 degrees gets even worse at 53 degrees. But cams with a earlier EVO like the Isky or Lunati would actually end up with a pretty traditional EVO point after you advance them 2 degrees.

1968 AMX 390 w/T5

Ok, I just loaded the latest version so the thread should be toward the top of the list now and easy to find. It seemed to be working ok, but when I add new stuff there's always a chance it causes some sort of problem and may need a little tweak. Let me know if you have any questions.

1968 AMX 390 w/T5

Sounds to me like it is close to the Comp268H and Isky 270HL

Comp 268H
Advertised duration: 268 intake/268 exhaust
Duration at .050 in. cam lift: 218 intake/218 exhaust
Gross valve lift: .456 in. intake/.456 in. exhaust
Lobe separation: 110 degrees
Basic rpm range: 1,500 to 5,500
Good low end torque. Broad powerband. Excellent for towing or performance. Slightly rough idle.
Recommended compression ratio: 8:1 to 9.5:1

ISKY - CAM ONLY Good All-Around performance. Fair idle. stock Converter. 370-411 Axle Ratio. Up to 650 CFM Carb. 9.51 Compr. RPM-Range (2000-6000) Valve Lift (.475 .475) Valve Lash hot (.000 .000) ADV. Duration (270 270) .050 Duration (216 216) LC 109

I have a Sig Erson cam in my 70 AMX 360 (9.2 comp, 4 speed, 3.54 gear) that has the same specs at .050 as that Howard

cam. I am sure that it may not be the latest and greatest, but it is easy on the valvetrain, idles well, and pulls strong to 5,500 rpm.