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Old 03-19-2008, 08:56 PM
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Cam Selection and Dynamic CR

Hey guys,

I guess you could consider this another "which cam?" question, but hopefully my request will be a bit more in-depth. Whenever somebody asks this question, a popular answer is "call the cam companies". Well, I did that, and I'm still not sure.

Pertinent Information:
* 1970 Nova, approx. 3200 lbs.
* 383 C.I small-block, factory roller cam
* AFR 195 heads, flat-top pistons
--> 9.55:1 static compression ratio, .045" quench
* 5.7" Rods
* Typical dual-plane intake, 750 cfm carb, full-length headers
* 3.73:1 rear gear, street tires
* 4-speed manual transmission
* Power brakes, manual steering

Usage:
* Daily driver
* 91 octane gas
* RPM range 1500-6000 max

My goal is 450ish horsepower and similar torque within my rev range, with a nice flat torque curve while being very driveable on the street and running power brakes. I called Crane and Comp, gave them the above information, and here are their recommendations:

Crane:
Cam PN. 109671, card: http://www.cranecams.com/index.php?s...71&lvl=2&prt=5
Adv. Duration: 278/286
Duration @ .050": 216/224
Lift: .509/.528
LSA: 112

Comp:
Cam PN. XR276HR, card: http://www.compcams.com/Technical/Se...umber=08-423-8
Adv. Duration: 276/282
Duration @ .050": 224/230
Lift: .502/.510
LSA: 110

I've been leaning towards the Crane because of the lift vs. duration numbers, and because the Crane tech was super cool compared to the Comp tech who was more focused on selling me stuff than helping me out.

According to my calculations, the DCRs of each are:

7.73 for the Comp
7.61 for the Crane

So they are very close and right where I want to be, but the Crane cam beats the Comp in the lift department pretty well.

This little exercise in math has shown me how misleading manufacturer cam specs can be. Upon first glance looking at the commonly referenced number of duration at .050", I thought the Crane cam was "shorter" and would result in a higher DCR, while the Comp cam looked "longer" and would result in a lower DCR, based on my SCR of 9.55:1. But this turned out not to be the case and the real information is in the advertised durations and valve event timings.

I don't know, I'm burnt out by all of these calculations and comparisons and I would appreciate it if you guys could lay my inhibitions to rest. Which numbers are more important to performance under my proposed usage? Should I put more focus on lift (which is what I've been taught) or on DCR? Which one will behave better as a daily driver? Which will provide more power?

Or are they very similar to each other and I probably wouldn't be able to tell the difference?

Thanks

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Old 03-20-2008, 09:27 AM
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the comp cam will run better. 110 lsa will have better throttle response and the duration at 0.050" is better for your setup (4 spd and 3.73 gear). The crane would be better it you were running a 3.42 to 3.55 gear and wanted a smoother idle.

however, you don't need to run more exhaust duration than intake duration with those heads.

I also think you cr is a little low. 10:1 would be better. And I would use a cam duration of 230/230 at 0.050" with 108 or 110 lsa when using a 3.73 and 4 spd
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Old 03-20-2008, 02:45 PM
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Yes, I expected them to recommend a single-pattern cam, so I was surprised.
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Old 03-21-2008, 02:03 PM
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How about idle quality? I'm not overly concerned about it as long as it doesn't die when I hit the brakes, but Comp says their cam has a "choppy idle".

Will the 112 LSA on the Crane provide a better idle?
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Old 03-21-2008, 04:05 PM
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F-Bird, THANK YOU! That is some excellent information.

You're right, the DCR calculations I did are a bit off, or at least the Crane number is. The calculator I use goes off of the .006" advertised numbers, and so does Comp for their advertised numbers, so for my purposes at least I think that number is pretty close. Crane's advertised numbers are given at .004", which I missed, so that number is a bit off. For all calculations, valve event timing was taken into consideration.

I'm not an engineer by any means and so I realize that my calculations are only going to get me close. But this is not a race engine and so I think (I hope?) that my calculations will be enough to get me where I want to go. My only goals are:

* Keep it tame on the street
* Extract as much power as possible while doing so
* Run it on 91 octane at most

I am willing to do what is necessary to raise the static compression if I need to, but it will have to be limited to the heads as the rest of the components are already established. Whether that means switching to a comparable head with a smaller chamber (like Trick-Flow 72cc) or milling the AFRs (which AFR can do before shipping), it is still open. Again, the only requirement is that it run on 91 octane with no timing compromises. Would 9.8:1 be more appropriate?

I would like to stay away from custom grinds at the moment because the budget is already stretching thin. I will be buying new rockers, so higher ratios are definitely a possibility. I just wasn't sure I would need them. I don't need to hit 450 HP, that is kind of a "wouldn't it be cool" goal, but 400+ would be nice and I don't think that is asking too much.

The lift vs. duration and issue is what initially drew me to the Crane; the ramp speeds look to be a bit more aggressive. However, I can't go too radical in the valve train to where I start sacrificing reliability for just a bit more power. Like you said, aggressive ramp speeds means a bulletproof valve train and while I'm willing to accept that and spend the money for it, to a certain point, but I don't want to take it too far, know what I mean? So, how about sticking with an off-the-shelf cam and using higher-ratio rockers to take advantage of the high-lift flow numbers?

I plan to order the heads assembled, but customized from the vendor with the appropriate springs. The rest (lifters, push rods, rocker arms) I will purchase separately as needed.

Anyway, thanks again for the excellent post, F-Bird. This is the kind of discussion I was hoping for.
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Old 03-22-2008, 03:16 AM
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Cool, thanks. I'll give them a call next week and see if I can get some information out of them. I always thought there was an extra charge for custom cams, but that would be awesome if I'm wrong.

I brought up the 9.8:1 number because that's what I would arrive at with a 72cc Trick-Flow head. I can get the same by having the AFRs milled, but AFR wants $260 for an angle mill with intake correction, or $104 for a flat mill (not sure which I would need, but can get that information from them when the time comes).

The one big issue that I am still a bit foggy on is the LSA. I know that generally speaking, a wider LSA will improve idle, but if I'm understanding this correctly, it's really just a band-aid that the cam grinders use in their off-the-shelf cams to compensate for buyers over-camming their engines. Still, when you suggest an LSA as tight as 108, I get a bit concerned. If I go the route of getting a custom-ground cam, do you think the designer will be able to work with the timing/durations and the overlap enough to provide a decent idle even on a 108 LSA?

I realize this question is one I can have answered when I actually call them, but it will be a few days before I can make that call and I'm curious. I think I'll stop by Speed Talk as well and introduce myself.

Thanks again.
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Old 03-22-2008, 06:04 PM
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Arcuden:

Running a 383 with a CC 274XE flat tappet hydraulic, '0' deck, 0.039 quench, static CR is 10.30, dynamic CR is about 8.2. This is in a 3200# C3 'vette with a 4-speed and a 3.36. 35* total timing, all in by 2600 RPM.

Good street manners and it runs on 89 octane.
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Old 03-22-2008, 09:14 PM
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Glen, that is very encouraging, thanks! May I ask what heads you are running?
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Old 03-23-2008, 12:03 PM
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Quote:
Originally Posted by Arcuden
Glen, that is very encouraging, thanks! May I ask what heads you are running?
TF 23*, 64cc chambers, 195cc runners.
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