Started from scratch with the Edelbrock, advice needed...

Page 2 of 4  


Thousands of pages could be written and have been written on proper cam selection. You can find probably 100 books on cam selection and maybe as many computer programs. Everybody can make a suggestion but cam selection depends on many factors, among them are static compression, valve size, rocker ratio, TQ stall speed, gear ratios and where you want your engine to make power. Any of your major cam manufacturers and large suppliers can take this information and suggest a cam. As far as RV cam's that is just a broad term used for cams designed for RV use that provide low end TQ. There are probably hundreds of grinds that could be considered "RV" cams.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Thanks David...
I'm probably going to go the route of speaking with someone at Comp Cams, Crane, or Edelbrock to determine which cam is right for me. Although, the stock cam for the 260hp motor is looking pretty good.
After viewing the specs of my 290hp motor and the 260hp, I see that the 260hp version does indeed produce more peak torque. Not surprising.
The two engines have the same heads, compression ratio, valve size, etc... But while my cam specs out like this:
Lift/Duration - .450"/.460" / 222/222
The 260hp cam specs out like this:
Lift/Duration - 0.390" / 0.410" / 195 / 202
Seems like a better choice.
I understand the lift, but the duration has me stumped. Is that the duration of the time that the cam is at rest (valves closed) or something else?
~jp
David Johnson wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Jon duration is the valve opening in degrees, based on one full engine cycle. You can run shorter duration and overlap with more lift and gain some power and torque that way.
Thanks David...
I'm probably going to go the route of speaking with someone at Comp Cams, Crane, or Edelbrock to determine which cam is right for me. Although, the stock cam for the 260hp motor is looking pretty good.
After viewing the specs of my 290hp motor and the 260hp, I see that the 260hp version does indeed produce more peak torque. Not surprising.
The two engines have the same heads, compression ratio, valve size, etc... But while my cam specs out like this:
Lift/Duration - .450"/.460" / 222/222
The 260hp cam specs out like this:
Lift/Duration - 0.390" / 0.410" / 195 / 202
Seems like a better choice.
I understand the lift, but the duration has me stumped. Is that the duration of the time that the cam is at rest (valves closed) or something else?
~jp
David Johnson wrote:

-
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Yeah, I just did some Googling and reading... If I understand correctly, 222 of duration would really mean, 22.2, correct??
The 260hp cam has less lift and less duration. It would probably still be a better choice, but if I were to maintain the same lift I have now, but with less duration, the motor would definitely gain some low-end torque at the expense of (unneeded) top end power, correct?
Not saying that's the way I'll go, but I want to make sure I understand it before I buy a new cam.
I do realize that the overlap at lower RPMs is causing the low-vacuum condition. I understand the how and why of that part.
What would be the negatives of running a duration of about .450"/.460" of lift, but with only 195/202 of duration?
I'm roughly guessing that I need my power to exist between 1000rpm and 3000rpm, probably peaking around 2000rpm... (rough speculation only here).
~jp
Shep wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Yeah, I just did some Googling and reading... If I understand correctly, 222 of duration would really mean, 22.2, correct??
The 260hp cam has less lift and less duration. It would probably still be a better choice, but if I were to maintain the same lift I have now, but with less duration, the motor would definitely gain some low-end torque at the expense of (unneeded) top end power, correct?
Not saying that's the way I'll go, but I want to make sure I understand it before I buy a new cam.
I do realize that the overlap at lower RPMs is causing the low-vacuum condition. I understand the how and why of that part.
What would be the negatives of running a duration of about .450"/.460" of lift, but with only 195/202 of duration?
I'm roughly guessing that I need my power to exist between 1000rpm and 3000rpm, probably peaking around 2000rpm... (rough speculation only here).
~jp
Shep wrote:

The problem you run into with short duration and high lift is the valves slam down. The lobes have to be so sharp that your just kinda drop off the edge. Cams ground like this also slap up just as bad. You can find a high lift short duration cam but you are going to sacrifice valve train life. Hell for a couple hundred bucks these cam companies will grind anything you want. This is where I fell in love with roller cams. Not only do you have nearly 0 wear but you can get away with more radical designs without beating your lifters to death. You can achieve higher gross lift with rocker's of a higher ratio. But then you get into valve spring issues. Also remember when you get into this job, lifters MUST be changed with the cam and if you pull one apart you intend on putting back together the lifters MUST go back into the same hole they came out of. If not you will have very rapid cam and lifter wear.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Well, more than likely I will be using something GM would consider a "stock" cam.
I'd read that about the "sharp" lobes on a cam being a problem. At this point, I'm thinking of paying someone to do the work, unless it's ridiculously expensive.
I'm tired of wrenching--at least for the next month or so ;-)
~jp
David Johnson wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Way back when I used to be fond of Competition Cams in 4x4's and RV's. They were great cams and I would never use a stock cam if I went through the trouble to replace it. ----------------- The SnoMan www.thesnoman.com
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Got any recommendations offhand for a good streetable cam profile?
I'm not looking to build a stump-puller, rather I just want my truck to start and run, and go down the road the way it's supposed to. It may be pulling a utility trailer or pop-up camper in the future.
Thanks,
~jp
SnoMan wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:
I built up a GMC 4x4 with a with a Competition Cam H260 in a 350 4bbl with a 4 speed granny gear tranny back in the late 70's. It had a nice power curve and a good idle (it pulled well from about 1500 RPM to 5000 RPM) and it would make a nice RV cam with a automatic. It always felt ready a will to go when you wanted it to and had nice sharp throttle responce. If it was a stick I think I would try a H252 next time for a little more torque below 1500 RPM.

----------------- The SnoMan www.thesnoman.com
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Jon R. Pickens wrote:

Hi Jon,
Cam duration is a measurement of how long,in rotational degrees, the valve is more than .050" off the seat. In the example you mention it is actually 222 degrees of crankshaft rotation. Overlap is how long, again in degrees of crankshaft rotation, both intake and exhaust valves are open (at the same time). A diesel engine probably won't have any overlap, or very little. A gas engine operates at much higher RPM and can benefit from the "flow through" of gasses that happens during overlap.
Cam specs can be checked by using a degree wheel on the nose of the crankshaft and a dial indicator on one of the lifters. Set TDC on the degree wheel, then rotate the crankshaft in the direction of normal engine rotation, note the crankshaft position when the lifter has moved up .050" in its bore and again when it has returned to that point. The amount of degrees that the crankshaft has rotated is the duration of that particular cam lobe. I say this because some cams do not have the same profile on intake and exhaust, though most do. And also, some cam grinders require that their cams be measured at .0 degrees of cam lobe lift, and then others at some other point, but most will check at .050. The checking point of .050" is often used because at that lift actual flow of gasses is beginning to take place.
One poster mentioned one cycle, make note that one cycle of a 4 stroke engine is 720 degrees.
There is much to learn about four stroke engines and cam technology. You are off to a good start, just never be afraid to ask, the only dumb/stupid question is the one you don't ask.
George
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
George your cam spec checking instructions are right on except you must find absolute tdc to get meaningful open and closing numbers relative to tdc or bdc, you can get the actual degrees of opening without doing this, but it doesn' t relate to how agressive the cam timing may be and what the intake centerline is. Anyway minor issue and based on your post I am sure you know this.

-
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Shep wrote:

Shep,
You are right of course. I should have been more specific in my statement of "Set TDC on the degree wheel" in the second paragraph. You might want to reread my response. It is there, I just didn't go into great detail.
I didn't mean this small discourse to be a class in degreeing a cam. I figure Jon is learning a great deal here and just pointing him in the right direction is all a news group can do. Aren't newsgroups neat for exchange of ideas and assistance?
George
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Jon R. Pickens wrote:

Jon,
As Shep pointed out, correctly I must add, setting TDC correctly is vital to checking a cam profile. The following procedure will get you very close.
Turn the engine till the piston for the cam lobe you are preparing to check is at or very near to TDC. Then move the piston to nearly BDC. Next set a stop that will contact the piston about a half inch or more prior to the piston reaching TDC. Rotate the engine by hand till it contacts the stop, make note of the location (degree marks) of the degree wheel markings. Then rotate the engine the other direction till you contact the stop again. Again note the location of the degree wheel (degree marks) and adjust till you get the same number of degrees either side of TDC when moving the crank toward the piston stop. Tighten the degree wheel and then remove the stop. You have now "set TDC" on your degree wheel. That is what I didn't elaborate on but should have.
This little procedure will prove invaluable as in later years you may want to do some experimental work on changing centerline offset to alter the performance characteristics of a particular camshaft. Advancing the camshaft relative to crankshaft position (relative to "heads up" or "degreed location") will lower the RPM at which peak torque occurs, slightly reduce total horsepower and increase low RPM throttle response, which is very good for street use. Doing just the opposite (retarding the cam) will have the effect of increasing the RPM point where peak horsepower will occur, will kill low RPM response and might lower or might increase overall horsepower depending on the camshaft grind.
The above is a broad generality and may not be true for all cams, the more radical the cam the less this will be true. Tuner beware.
Do not try this until you have also mastered the procedure of checking piston to valve clearance. That is vital. Eyebrows in pistons are meant to be machined or cast there, not impact forged while running in the engine.
Once you have the tools and the know how, you have gained freedom from just being one of the sheep. The world of performance is yet to be conquered. Go for it.
Good luck.
George
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
George, again great description, exactly how it should be done for exact tdc, removes all the variables.

-
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Shep,
Thanks, I thought about it after reading your last post and decided to write the "rest of the story" so that if Jon truly wants to know how it is done he will have that to read.
George
Shep wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

If you stay with 87, you are really hamstringing yourself here and loosing MPG and power. Given today fuel price, better gas is only a few percent more.

This is a lttle differnt because it depends on whether you have manifold vacum or throttle port vacum.

If you want the engine to do its best, you want 12 to 14 BTDC intial with ported vacum of about 10 to 12 degress (it will not be a issue at WOT) and you want a total mechanicl advance of 38 to 40 degrees by about 3800 RPM or so (12 to 14 initial and 26 degrees or mech advance) Also do not concern yourself with a different torque converter at this time because they engine is not set up right yet and higher stall converter make a lot of extra heat and with tall effective gear ratios they can crerate serious heating problems that can lead to early tranny failure.
You should be able to get about 15 or 16 inches of vacum with that cam at a idle. Try 12 BTDC and run it a bit. If it pings, slow down advance curve or icrease octan of both but do not retard intail timing. (I used to race SB's in the 70's and 80's and know them well)

----------------- The SnoMan www.thesnoman.com
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Replies inline...

I only use 93... I only quoted that from the Jegs site as a point of reference...

Ported, stock replacement advance module.

Yeah, I don't plan on changing out the converter. It gets me moving right off idle, and if I just let off the brakes from a standstill on flat pavement, I'll start moving immediately.

I did achieve that much vacuum in the past, but it was running quite a bit richer. I'll advance the timing back to 15 BTDC and start from there. That's about where it was when I had the vacuum that high. I'll play with balancing the idle mixture screws and idle speed to get it leaned out as best I can. My only real way of measuring it is to stand downwind of the tailpipe and see if my eyes and nose burn, LOL...not the most healthy way to diagnose the problem! But at least I know when it's better.
Hopefully the vacuum increase will help some, but I know I'm never going to see the full potential of this motor without swapping the cam for something else. I'm probably just gonna stick with the 260hp version because it's a proven, stock cam for that same motor. Again, the profile for the 260hp cam is 0.390" / 0.410" lift, 195 / 202 duration.
~jp
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I would not say that at all. Intail timing needs to be at least 12 to 14 degrees with that overlap and you may need to rejet the carb some to but we will get to that later. First find the sweet spot fof the idle. If you had about 9.5 or 10 to one compression it would really talk when your foot was in it. Higher CR's are very important when you start to run higher overlaps is you want decent lower RPM responce and good idle. . ----------------- The SnoMan www.thesnoman.com
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Ok, Time-out on the cam talk... let's talk carbs for a sec... I have a big question about that.
First of all, it's already been rejetted. It was too rich with the stock .098" jets, so I installed .092's. Then it was too lean because I didn't swap the metering rods accordingly.
I now have some .070"x.047" metering rods (stock has a .075" cruise mode) in there.
Edelbrock has yet to answer what to me is the most important question right now... At what vacuum level do the rods go to CRUISE mode. Yes, I know with my 4" springs that the carb will hit Power Mode at 4" Hg of vacuum--I can see it with the vacuum gauge in the truck while going down the road, as well as feel it. It hits Power at EXACTLY 4" of Mercury.
But at what point do the rods come up out of the jets to engage Cruise mode? I know it's a gradual change. The 4" springs means that it fully hits Power mode at that vacuum level, but what are the rods doing at say, 6 or 7" Hg?? That's my question... My lean spots starts around 7-8" vacuum, and quits around 5". Power mode kicks in at 4", everytime.
I put in the 5" springs, but it was immediately so rich that I couldn't hardly start the truck. I tried 3 times, and then when it finally started it literally smelled like a gasoline refinery in my carport. Ran like Hell too.
I really want to get out there and install the .095" jets and try it with the .071"x.047" rods.
Something tells me that my lower-than-normal vacuum condition is causing the power staging in the carb to be way out of whack.
I'm just having a hard time finding the balance.
Thanks,
~jp
SnoMan wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:
Okay, you are really taking me way back now. Your carb is kinda patternd after the old Quadrajet that GM used for many years and that I came to like and respect off road because of its tunabilty and central fuel bowl that worked well off axis. I used to have a whole mess of jets and rods (primary and secondary) for those things that I have not seen for years now. When you make changes to the main jets and rods, you want to make gradual changes and only one thing at a time (jets or rods) and no more than 2 jet (or rod) sizes at a time. If 98's were too rich I would try 96's with same rods. Run them for a while and see how it runs and watch plig color and tailpipe soot. You can also "tweak" the mixture a bit by adjusting the float level in 1/8 inch steps if need be. THe spring on the metering rods is another matter and I will tell you how I did it back then. I would strech or compress the springs to tweak the setting some because again you want gradual changes. Measure spring lentch accurately and increase or decrease its length (by strecthing or compressing) in 1/4 inch steps or less. Small steps here too. ALso remember that sometime too rich a mixutre can be confused with too lean on off idle responce so sometimes you need to adjust the acceleratior pump too here because if you get the part throttle just a hair lean before enrichment and add a little extra gas with pump, it can make for very crisp and jumpy throttle responce (it is a balancing act) You might start out with decreasing the accelerator pump flow now and see if responce changes because if it does not you are allready too rich at transfe. If it starts stumbling they you are close to proper transfer ratio and you may just need a little bigger shot. Am I confussing you or are you following my drift here? (I used to love playing with carbs back then as it can be a real science with nice rewards for those with the patience to tweak them properly)

----------------- The SnoMan www.thesnoman.com
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

    Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.