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Old 03-19-2008, 07:56 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, 08:27 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, 01:45 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, 01:03 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, 01:44 PM
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The real cam timing number you need to calculate accurate dynamic comrpession ratio is the real seat to seat "running duration" and intake valve closing point ""running closing point"

The actual real running valve timing duration is not that easy to determine as there are many factors that effect the valve motion in a running engine.
Calculating the dynamic running compression ratio based on the cams .050" timing and or the cams listed advertized duration is at best, an estimate
of the actual in motor running valve event duration.
Comp cams measures its advertized duration at .006" lifter rise. Crane measures advertized duration at .004" or .0045" for the most part.
These different checking points give different advertized duration specs for the same camshaft.
Not all cams with the same .050" spec, have the same advertized duration even when the checking height is specified and equal.
Different cam lobe designs have unique opening ramps which have a big effect on the actual in motor real valve seat to seat timing events.

So unless you know which cam design the dynamic calc was designed for specificy, it is a estimate of the real dynamic compresssion at best.
Sometimes a very rough estimate.
We haven't even touched on hyd lifter valving and internal clearance effects ( "lifter bleed rate") + oil pressure and viscosity and temp on actual running valve event timing on a hyd lifter valvetrain.
The combined effects can result in a sizable error in predicting running cylinder pressure with cam A compared to cam B even within the same brand name.
It is best to take the info from a dynamic cr calc using published cam specs with a grain of salt.
In my opinion both those cams would perform very well but would come up a bit short of 450 bHP in the motor you describe.

I would sneek the static cr up a bit more. Snug it up very close to 10:1 even, but not over. (measured) I would have one of the cam companies design a custom grind Hyd Roller cam with high(er) valve lift .560"++ ish using high rocker arm ratios to take advantage of the AFR heads flow superior air flow at higher valve lifts.
to build torque and power in the "street rev" range.
Something with 219 to 236@.050" with fast off seat valve action and high lift. The faster the off seat valve action, the better the valve train needs to be to control itself at high rpm. Cheap junk won't cut it, if you want fast valve action to get power making valve lift and lift per degree duration intensity.
The way a hyd roller cam lobes opening/closing ramps are designed, the "advertized duration" can be deceiving as to the real off seat valve action. it is not the same as a flat tappet cam of the same duration.

If you phone a Cam Co. and ask "ok here is what I have, blah blah blah, Whats the best cam to stuff in it?" you will most likely get a recomendation right out of their catalog.
Anyone can do that. If you want a better, more specific cam, you need to ask for a "Custom Grind" and get much more detailed on the build info. You'll probabily get refered to a different, high(er) level of tech assistance. You can even request so. But be prepared to get technical.
The more specific you get, the more specific they can dial in the cam grind.
I've found that when you work with them at that level, they can nail down the cam grind very well.

Street "driveability" is very subjective. A soon as you say or indicate that "oh "i don't want something too wild etc etc blah blah blah" the tech guy is going to go "mild" on his recomendation.
Most of the driveability issues on a street motor are really, lack of fine tuning. Wether that be raw knowledge in how to dial the whole thing in or lack of desire to bother to fine tune your combo to drive well or both.

AFR has done a good bit of reasearch on its heads on street type motors. they probabily can help you quite a bit on specific cams that have worked well in the past on 383's with their heads.
I'd go with more carb airflow than "750" especially on a dual plane intake manifold. You can kick that up quite a bit, Near 900 actual carb cfm is not too much on this motor. But you want to achieve that using a 1.4" venturii size.

Download comp cams' cam lobe library PDF and look at the different fast action hyd roller lobes in the 218-230 ish duration numbers and high valve lift .560" zone. .350" lobe lift.

There are racing camshaft designers/engineers like Harold Birkshire (spelling) "UD Harold"
{Ultradyne Cams, Lunati VooDoo cam series etc}
and others that design all this stuff for the major cam companies that provide custom cam design service for fee. Some, even produce the custom cam in house or have a major brand company do it for them (custom camshaft service) and ship it to you.
if you are willing to work with them allowing them to pick the push rod, rocker arm and valve spring/retainer combo to control valvetrain dynamics, they can stretch the custom cam design quite a bit.

In my opinion you can reach your goals but the catalog cams you have listed and the carb cfm is a little short.

Last edited by F-BIRD'88; 03-21-2008 at 02:58 PM.
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Old 03-21-2008, 03:05 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 03-21-2008, 03:55 PM
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9.8:1 is what I'm thinking also. Stick with the AFR's.
A custom ground hyd roller does not cost much (if any) more than a off the shelf hyd roller cam. The custom grind roller cams we got from Comp before were not real expensive and well worth it. They got stuff that is not in the catalog, trust me. I even custom ordered and had them make a catalog Hyd flat tappet cam for me for my 406 cause there was no available stock any where. All I had to do was pay ground shipping and they shipped it right out to my favourite retailer store where I get a lot of my stuff.
You need to get past the "Tier 1" tech assistance level and get down to the nitty gritty. (fast high lift valve action, beehive springs etc)
Nail down the camshaft grind before you start buying valvetrain parts.
You need to think "custom cam grind"
Again I'm thinking .560" valve lift, fast, short snotty duration, maybe single pattern on 108 LSA
820+cfm carb airflow. Thats a 750 that has been tweeked a bit.
I'd get back on the phone with Comp Cams and get a "custom cam" recomendation. Give them your email address so they have the time to let a higher level tech adviser (who will be busy) work on your project.
Do not give him wishy washy design limitations to work with. Let him do his job and design you a good combination That rocks. You have lots of room to work within.

383's and 406's built as you want for the street in the power level you are after, do not need to rev excessivley high to make the power so reliability should be good.

Doesn't cost you anything to get their "custom cam recomendation" and work from there. They are very knowledgeable once you get beyond the tier 1 tech advice level.

You can also go to the Speed Talk forum and pick a few of the cam designers minds there (UD harold) (Mike Jones) etc for a high(er) level recomendation.

Last edited by F-BIRD'88; 03-21-2008 at 04:07 PM.
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Old 03-22-2008, 02:16 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, 05:04 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, 08:14 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, 11:03 AM
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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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