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Camshaft selection
- Thread starter John Brown
- Start date
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Anonymous
Guest
COLD-EH'":21xc67xx said:
Honestly...I've ran gear drives for years. Maily because the quality of chains were substandard in my opinion. As for harmonics (vibration), I honestly think ther is some truth to it to some degree, but I think most of this "data" was contrived from people not understanding that the harmonics STARTED further up the valvetrain (Pushrods and spring choice mainly) A gear drive once set will STAY set...A chain will have to be "guestimated" for stretch, and where the cam actually will be phased after break-in. The negatives I see with gear drives....Erratic timing when using the distributor (compounded when running a wet sump oil system) for ignition signal. A gear drive combined with a crank trigger (Especially with an external oil pump) is very accurate, and a good choice in my opinion. But if you're running nothing more than a distributor...I'd go with a chain. In a perfect world....I prefer a belt drive, dry sump, and a crank trigger..Probably more than u wanted to know, but what 411 I have to offer.
Felber
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Anonymous
Guest
COLD-EH'":3241w9ok said:
Do NOT, and I repeat DO NOT buy a camshaft from any mail order CATALOG. Ultimately, you should get a cam recomendation from your (or a reputable) engine builder. At the very least use one of them as a go-between to set you up with the right people to talk to. And if that's not an option..Call COMP cams (or your company of choice) direct..Ask for a tech, and have a custom cam ground. You can BUY it through Jegs, Summit or your local race shop, but put the time in to get a grind# from someone who knows cams. Turnaround through COMP is only a couple days. I'm not preching COMP, but they're all I'll use anymore.
Waterthunder
Well-known member
It’s all a balance the trick is to get the most cylinder pressure without going into detonation or restricting flow. I should say at least in most airboat applications. So each motor is different there are a hundred variables that effect how a motor will react to a cam change. The trick is to get the right cam for your application and generally with a stock motor the best cam for performance in an airboat will not cause your cylinder pressures to go to high. Generally if your cylinder pressures are to high that means you need more camshafts so under camming a motor will cause a detonation problem more than over camming.
Since I am the engine builder in this situation and I have never built an engine that I intend not to turn past 3200 rpm. I was wondering if one of you guys would guide me with an appropriate Competition cams part number. It's going to be direct drive, 496 BBC, peanut heads till it needs to turn more rpm. Edelbrock dual plane or DP air gap or coppy! any ideas!
I checked Competition cams web site instead of Summits. Lot's more choices but still did not see anything much in the 109 degree and under lobe seperation that would be useable to me. I did see the recomendation form so I'll have to fill one out and send it off to them.
:roll: However whats your opinion on what I picked from what they had? Solid mechanical, 1000/5600 RPM, PN 11-674-4, Grind # XS256, Duration IN/EH, Ad. 256/262, @.050 218/224, Lift .527/.532, 110 Degree lobe seperation. 8)
:roll: However whats your opinion on what I picked from what they had? Solid mechanical, 1000/5600 RPM, PN 11-674-4, Grind # XS256, Duration IN/EH, Ad. 256/262, @.050 218/224, Lift .527/.532, 110 Degree lobe seperation. 8)
found an interesting site with lot's of good information! Answered lots of questions. 8) http://www.kb-silvolite.com
After reading the Keith Black site I think I fugured it out. I will build the 496 at 10.5/1 and to kill some of the low rpm cyl pressure I will put in a CC 305H, 305 degrees duration, 253 degrees @ .050, 0.575 lift to breathe well at low rpm and 110 degree lobe seperation! 8) What do you think for a DD? :twisted:
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Anonymous
Guest
Big cubes like a bigger cam, but I'd talk it over with Comp.
With that cam, and 10.5 static compression, you'd end up with 7.2 dynamic compression. You really want to shoot for 8:1. Here's a calculator for dynamic compression
http://www.kb-silvolite.com/calc.php?action=comp
And you'll need to know how many degrees ABDC your intake closes, so here's the spec card for the 305H cam
http://www.compcams.com/Technical/Searc ... r=11-214-4
Also, with your 113cc chamber heads you're going to need a 20cc domed piston to get 10.5 static compression unless you run 0 deck clearance. Then you would get the 10.7 that the Eagle rotating assembly states with its 18cc domes
With that cam, and 10.5 static compression, you'd end up with 7.2 dynamic compression. You really want to shoot for 8:1. Here's a calculator for dynamic compression
http://www.kb-silvolite.com/calc.php?action=comp
And you'll need to know how many degrees ABDC your intake closes, so here's the spec card for the 305H cam
http://www.compcams.com/Technical/Searc ... r=11-214-4
Also, with your 113cc chamber heads you're going to need a 20cc domed piston to get 10.5 static compression unless you run 0 deck clearance. Then you would get the 10.7 that the Eagle rotating assembly states with its 18cc domes
Thanks for checking and taking the time to respond Red! 8) Just threw that out there to see if I could get a responce! :lol: The KB pistons are actually 17 cc and the cam IS more than I was planning to do. I found the Keith Black site before we went skiing and learn't a lot. I will see what deck height is when I trial assy. it the firs t time, thanks
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Anonymous
Guest
I went to school for awhile today over on the Comp Cams website.
They mention a special cam grind that's available for small blocks that's called a "4 - 7 Swap" ..... it changes the firing order, and they say that it improves the torque curve and reduces crankshaft distortion. No other changes are necessary other than swapping the plug wires around since #4 and #7 are both at the top at the same time.
Anybody tried it? Just on paper I can see how it might be smoother running.
BF
They mention a special cam grind that's available for small blocks that's called a "4 - 7 Swap" ..... it changes the firing order, and they say that it improves the torque curve and reduces crankshaft distortion. No other changes are necessary other than swapping the plug wires around since #4 and #7 are both at the top at the same time.
Anybody tried it? Just on paper I can see how it might be smoother running.
BF
A
Anonymous
Guest
Don't use the 305H. It is a very lazy cam designed in the late 80's. I know that sounds short and silly, but that is VERY far from being a good choice on damn near any engine that does more than cruise the boulevard sounding good. If you can stand some maintenence, I would go with a solid cam for higher ramp rates, or at the very least, use a Extreme Energy hydraulic/Modern ramp design, and also consider a 1.8 rocker. The generally used measurement for dynamic compression is based on degrees of valve opening to to degrees of crankshaft rotation. I.E. a different way to look at your static compression ratio. What I in my own weird way consider dynamic compression is actual cylinder pressure under running conditions. Meaning that every engine combination has a different VE (Volumetric Efficiency) curve. An engine with both lower static as well as the standardized measurement for dynamic compression can easily see more cylinder pressure than one that is higher on both counts if the package is correct. If the engine is designed to fill the cylinders via camshaft and cylinder head selection, static and dynamic numbers are pointless. The choice of the peanut port head makes sence somewhat, but a Edelbrock performer head would actually perform much better, not to mention weigh a TON less.
As for the 4/7 swap...I use them pretty regular. They tend to idle better, and in some application have better throttle response, as well as more power across the spectrum. But this is very minimal to nill in lower HP applications. Take it from someone who was skeptical of them, and tested quite a few...From my experience. The main plus...Easier on #4 main bearing, and generally a more "crisp" engine.
I know I probably just made another mess with a half-assed reply, but I also hope I put some brains to work for the guys on a budget who like to build their own stuff. And if all else fails....DO NOT use the internet for advice. Call the cam company of your choice. INSIST on someone with airboat experience, and have ALL your DETAILS ready...And then see what they have to say. Otherwise, spend the money and get an engine that performs in an airboat from someone who knows what they're talking about.
Appologies in advance for any typos or lack of 411...I just got home, read the post and two-finger typed off the top of my head. LOL
Forgot to mention....I've been drinking also. I tend to get creative under these circumstances, but also miss some details along the way. So, take that as my disclaimer
Felber
As for the 4/7 swap...I use them pretty regular. They tend to idle better, and in some application have better throttle response, as well as more power across the spectrum. But this is very minimal to nill in lower HP applications. Take it from someone who was skeptical of them, and tested quite a few...From my experience. The main plus...Easier on #4 main bearing, and generally a more "crisp" engine.
I know I probably just made another mess with a half-assed reply, but I also hope I put some brains to work for the guys on a budget who like to build their own stuff. And if all else fails....DO NOT use the internet for advice. Call the cam company of your choice. INSIST on someone with airboat experience, and have ALL your DETAILS ready...And then see what they have to say. Otherwise, spend the money and get an engine that performs in an airboat from someone who knows what they're talking about.
Appologies in advance for any typos or lack of 411...I just got home, read the post and two-finger typed off the top of my head. LOL
Forgot to mention....I've been drinking also. I tend to get creative under these circumstances, but also miss some details along the way. So, take that as my disclaimer
Felber
Well I just got back from skiing, my old buddy from the west coast just left, and I've been drinking. Like I said I was just having fun with that 305. I was actually thinking the XS 256/57 solid, I like solids! :twisted: Peanuts are just because they are fresh and they should work at 3200 rpm. Bigger and better later! :twisted: I just don't want to run into detonation with the 17cc hyperutectics and don't want to go as low a C/R as the -3 CC pistons. I also don't know if the rotating assembly seller will cut the tops down and then ballance the assembly for me, I'll have to call and ask! The shops up here told me they want $360.00 minimum to ballance where US supplier wants $225.00 and that comes with harmonic ballancer and flex plate. :? Comp Tech recomended a hydraulic that I would have considered, but don't know if he was an airboat person. A hydraulic would work fine in this application and the CC ones I have tried are some of the nicest around I think but I do like solids! 8)
Waterthunder
Well-known member
There is somebody on this site that I had Comp Cams grind a custom 4/7 swap cam for. The only reason he installed it is because he wanted his motor to smooth out at idle he had some serious problems. I will ask him if it solved any problems. I believe there can not be any HP gain from a firing order swap cam in an airboat application because of the RPM’s we run. With Iron heads on pump gas I like to run no more than 10.25 to one and with aluminumheads I try to stay around 11.25 to 11.50. However goofy cam grinds and different type pistons and deck heights change all of this so you need to have all of your information on a matching combination. If you want a good combo PURCHASE THE CAM COMP RECOMENDS spend the extra hundred bucks for a custom grind. If you buy a shelf cam it will not be the best cam for your application.
Waterthunder
Well-known member
P.S listen to Felber and dont run the 305H it's a step above the 3/4 race cam in technology. In a direct drive it would be the laziest thing on the water!