I pulled the trigger on some Brodix Race Rite 294s today. Now it's time to think about a camshaft that will make those heads do what I want them to do. I'm not a camshaft wizard, but I like to learn and share ideas. So here's where I'm at...

The short block is a 468 ci big block Chevy. It will be a true 10.6:1 compression ratio with the 115cc Brodix Chambers. I want to make peak power around 6,000 - 6,200 rpm and shift at 6,500. The car has a lot of gear with a T56 6-speed and 4.11 rear. It is also fairly light. I'm fine with a fairly rowdy idle.

So here is my logic so far...

This will be a hydraulic roller/pump gas combo that will see 6,500 rpm shift points. I think I need around 288-292 degrees intake seat duration to make peak hp at 6,000 rpm with the Brodix heads. Bullet Cams has a 291/236 lobe with .365 lobe lift (.621 at the valve). This is their 'CRA' series lobe which is a bit less aggressive and should rpm nicely. If I install this intake lobe on a 103 centerline (107 LSA), it will give me an 8.25:1 DCR, which is where I want it for pump gas.

On the exhaust side, my understanding is that the Brodix heads like about a 10-12 degree split due to their 65% I/E ratio. So I'm looking at a 306/248 lobe with .355 lobe lift (.603 at the valve). This is also a 'CRA' lobe which is an RPM friendly profile. These lobes are asymmetrical where they open the valve quickly and then slow down the closing to help with valve control.

So my first stab at this would be:

Advertised Duration: 291/306
Duration @.050: 236/248
Duration @.200: 157/167
Lift: .621/.603
LSA: 107
ICL: 103

I'm not married to any particular cam grinder as long as I can interpret their lobe catalog. I realize there are people that do this for a living. But it's fun to get nerdy with engines and learn and share ideas. All thoughts and discussion is welcome.

 

Is that a solid roller, hyd roller or flat tappet?


JIM

 

That's an important bit of info I missed! Lol. This will be a hydraulic roller. I already have a set of Morel 4603's for it. Valvesprings will be set up in the 170 seat and 480 over-the-nose range. I want this thing to rpm a bit for a juice cam.

 

Close to what I built,, 496, 10.5cr, Brodix RR 294's with 115cc. I used a Comp SR with a cast iron dist gear. .671 lift 254/260 on a 110 center. Haven't got enough miles on it to let it off the chain, but it feels like is should get interesting.

 

Yes, I chose the 294's over the 270's in case I don't like what I see in the bottom end when I tear it down. If that's the case I'll stroke it to 496.

 

As I understand it more cubes mellows a cam out and reduces RPM of said cam. IE my 400 reduces the same cam in a 350 by 500 rpm so, your 468 would do the same thing to a cam in my 406. I have a 234/244 @.050 cam it doesnt have the lift yours does but, dont you need to go higher duration to maintain the RPM band you want?

 

Kacy, I think the intake lobe duration is the hardest part about selecting the right cam. There are so many variables involved (bore size, stroke, head flow, runner size, etc.). I agree with you, that a intake lobe in the mid-230's seems small but these newer lobes are deceiving. Compare the following:

Comp XER lobe:

Advertised Duration: 294
Duration @.050: 242
Duration @.200: 155

The Bullet lobe I'm looking at above:

Advertised Duration: 291
Duration @.050: 236
Duration @.200: 157

Which is bigger with more area under the curve? The Comp lobe is quicker off the seat, but the Bullet lobe is bigger by .200" lift.

 

What made you go with a 107 LSA? Are you grounding any advance into the cam?

Instead of playing with my own ideas, I went to Chris Straub. I am building a similar engine, but going with the AFR 265's. He speced out a 229/229 .600 lift on a 109 LSA. I am curious as to what his lobes look like, but would have to get a cam doctor run to find out. The cam also has 4 degrees of advance ground in.

 

Call Chris Straub with all the details on your combo and get a custom cam.

With a big dollar engine and particularly with a lesser common combo custom cam all the way.


Adam

 

Once the intake duration is known, your target Dynamic Compression Ratio (DCR) will determine where the Intake Closing Event (IVC) needs to occur. I need to close the intake valve around 68 degrees ABDC in order to reach my target DCR of 8.2:1. The Intake Closing Event (IVC) determines what Lobe Separation Angle (LSA) is needed. For a lobe with 291 degrees duration, I need it ground on a 103 intake centerline in order to close the valve at 68 degrees ABDC. Assuming 4 degrees advance, that puts my LSA at 107. For example, if I were to open up the LSA to 110 degrees (106 centerline), it would move my IVC to 71 degrees and lower the DCR below my target.

 

I have been a customer of Chris in the past and will be again. For me, this is a hobby where I try to learn things and understand why they are what they are. So, for this particular project, I want to figure out the answer (with help from others) as opposed to just being told what the right answer is. And if I'm wrong, I'm wrong. It's still gonna haul ***.

 

My recommendation.

Jones Cam Designs.

EHR 73360/76365
284/295@.006
238/250@.050
159/170@.200
.612/.620 w/1.7 rocker
108 LSA
104 ICL

These are Asymetrical and Inverse Radius (IR) lobes. I just about run them exclusively in my hyd roller engines. They are very stable and make lots of power.

http://jonescams.com/hydraulic-roller-tappet/

Will

 

There is no need to live with that poor flowing exhaust port. It is very easy to get that exhaust port up to 75 to 80 percent. There is nothing wrong with the port, it's all in the valve and seat.

Have a radius shape cut on the exhaust seat and move the 45 seat out to the outer edge of the valve. You want the 45 no wider than .080". I've run .060" on many engines for many years with no problems. Then narrow the 45 degree to the same width as the seat on the exhaust valve by cutting a 35 degree back cut. Do a little blending in the bowl and smooth the short side radius.

Your engine will run better with the better exhaust flow and less exhaust duration.

Mike

 

That looks like an excellent choice. Those IR lobes are much more aggressive off the seat than the Bullet CRA lobes I'm looking at (but with similar timing from .050" on up). I was being conservative on the lobe design because it's a full street car and I want to spin it fairly hard for a hydraulic roller. I got confirmation today that my heads are being set up with a +- 165/450 spring package.

Your cam choice would put my DCR at 8.42:1 and reduce total overlap from 84.5 degrees to 73.5 degrees. So it would have more snap on the bottom end and have better idle/driveabilty. Looks like a great choice if those lobes don't beat up the valvetrain on the street.

 

I ordered the heads complete from Brodix fully assembled. I intend to put them on a local flow bench and go from there. If I can make some cost effective gains without completely re-doing the heads, that is certainly an option. I don't plan to order the cam before I get local flow data on my specific heads. This really isn't a max effort build, it's more of a fun way spend some time and think about the combo. I don't mind crutching the cam timing to make the heads work, but if a simple tweak to the exhaust seat will make big gains I'm all for it.

 

This is 90% a question and 10% a comment: I thought that that opening and closing rates can have different acceleration / deceleration rates and that high speed valve closing events were a bigger deal in terms of longevity and valve train wear than high-speed valve opening events; no?

(My understanding I'm looking to confirm / refute: Valve bounce at close is more important to control with a deceleration on the close ramp vs. valve float via an aggressive opening ramp, which can be controlled by more spring pressure pretty effectively.)

I feel like I remember reading about the Comp Cams Extreme Energy lobes and the "sewing machine" sound and valve train and cam damage including someone who listed the exact deceleration rate at close of the comp XE closing lobe and then compared it to the Lunati Voodoo closing ramp that started decelerating upon close; again this is all from memory from a while back but I thought I remembered both lines having similar acceleration rates when two cams were chosen for comparison side-by-side but the Voodoo's had much better longevity simply because of slower decel rates prior to valve close. (You can fairly safely get some of the performance benefits of a faster opening rate (with good springs) with the longevity benefits of a slower decel rate upon close; aka when looking at accel / decel rates, the open and close rates should be viewed separately as they are sometimes different.)

[Edit] I found a quote from UD Harold where he seems to indicate since 1977 he designed the opening and closing ramps separately:

"I really should charge for this, but........
When designing a cam, you must first decide whether to make it symmetrical, or unsymmetrical--opening and closing sides different. I have done unsymmetrical cam design only since 1977, although some of my symmetrical cam designs of the early 1970s are still being made today.
Each side needs at least 2 seperate equations, and I have used as many as 5 or 6, per side...... A design needs a ramp, and a flank, and all my work at Competition Cams was of this sort, what I call '4-Equation' cams, opening ramp, opening flank, closing flank, closing ramp.
Each equation has to have starting and ending conditions, at least 4 of each, and they are: Displacement, Velocity, Acceleration, and Jerk. Where 2 equations meet, they must meet in ALL 4 conditions, or you get infinite spikes in Jerk, at the least. This means the equation has to have 8 exponents, although some exponents may be 0, and some of the coefficients may be 0.
Exponents may vary from 0 to as high as 99, although I've never used any that high. Picking the right exponents is one of the major factors in good cam design. It is learned only through trial and a lot of error. As a point of interest, Displacement, ie---cam lift, always has the exponent 0.
Acceleration at the nose means there has to be an exponent of 2 present, to give a real 2nd derivative.
I have equations written to help design cams as far as the 4th derivative, Harvey's 'Snap'. Once you get into 5 or 6 0s before the 1st signifigant digit, ie---.000001", one-millionth of an inch, I wonder if there is any noticable benefit in further pursuit of derivatives. It's a very good cam grinder indeed that is more accurate than .0001".......
That's enough to get you thinking for the next year.....

UDHarold

BTW, I am self-taught in cam design, although I did get a B in Differential Equations in Jr. college...."



Adam
P.S. I really love the part where he was pretty much the undisputable world's best cam designer but felt it necessary to state that he only received a B in Differential Equations in Jr. College! lol!

 

Maybe it'll run better with more exhaust flow, maybe not.

Darin Morgan has made some pretty bold statements about air bench flow numbers on exhaust ports and just how little correlation there is between how an exhaust port flows at room temperature and 1 PSI when in an operating engine it's operating at 500+ PSI and 1,300F or more...

Direct quote:
Personally, I care very little about how much the exhaust port flows. It’s at the very bottom of my list of importance. I don’t try and make the exhaust port flow as much as possible, because I have been down that path hundreds of times and it’s a dead end street! The pressures the exhaust port has to deal with in the running engine are hundreds of times greater than the flow bench could possibly simulate. Ask yourself this: why would you design a port using 1 PSI test pressure at 70°F, when it’s actually flowing at 500PSI+ and 1300°F?

http://darinmorgan.net/frank-talk-about-flow-numbers/


I find this subject particularly interesting when comparing CFM #s for intake and exhaust ports between AFR and other top-tier SBC 23 degree heads. (Many of them have caught or slightly surpassed AFR on the intake ports, but not the exhaust ports on the flow bench, BUT if the exhaust CFM #s aren't actually what matters then what does the extra $300-$400 get you on a running engine.....?!?...)

Adam

 

I have the same understanding as you do about controlling the rate on the close ramp to prevent valve bounce. If you look at the Comp XER lobes, they are fast everywhere. If you look at the Advertised Duration vs. Duration @.050 they really snap the valve open too.

 

I have never really understood the dynamic compression ratio measurement. Wouldn't it vary with rpm. I have seen people pick a cam based on a certain dynamic compression ratio number at low rpms to keep it from detonating. But at high rpm, when those cylinders start filling, in some cases it would rattle. Maybe I am understanding it wrong. Love your car!

Bill

 

You could probably get a good custom grind from Straub and take some of the guessing out of it for you.

But for reference,

On my 496 with probably a tad less head flow then you'll have, I peak at 5750 rpms, shift at 6000-6100.... That's with an Isky hyd roller 238/248*@.050", .578/.608", 110 LSA. I do have 1.75 rockers on the intake side tho....so its probably close to .600"I.

I would think this cam in a 468" would peak a couple hundred later....It has mild lobes, easy on valve springs(kind of) and great driveability. (12" vacuum in mine).

I think a custom cam would get the most out of your combo, however I do like a proven product, mine is certainly proven...lol


Good luck on the new build Neil....