Thread: 355 build HP and Torque?

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Originally Posted by rollie715

Thanks for the reply. Where do I find the 29 deg ABDC number?
The CompCam link says 56 deg. Am I reading it wrong?
Here's the link:
08-410-8 - Magnum

If it was 56 deg, would the 18cc dishes still be ok? I hope to be verifying the cc's on the heads in the next couple days and can calculate the actual SCR and DCR before I order the pistons and have the block bored.

I purchased and am studying the following two books by David Vizard. Thanks for the suggestion.
How to Rebuild Your Small-Block Chevy
How to Build Max Perf Chevy Small-Blocks on a Budget

Glad to see that you have taken the initiative on buying and reading books relative to your project. Good job!

There are several different ways to measure the opening and closing points of the intake and exhaust valves. The 56 degree figure is measured at a tappet lift off the base circle of the lobe of 0.006", (six thousandths of an inch). That's pretty much standard for a hydraulic tappet camshaft and is used by most of the cam grinders, shown as "advertised" duration. For some reason, Comp does not publish the tappet lift at 0.050" in their specs like some of the other grinders do, so you have to figure the 0.050" lift for yourself. You need the 0.050" lift figure to use the Dynamic Compression Ratio calculator on the Keith Black Piston site. It asks for you to add 15 degrees to the 0.050" figure in order to calculate the DCR.

Take a piece of paper and draw a circle like the circle seen on the 08-410-8 timing diagram, the one where the 106 degree intake valve centerline is shown. Make a pencil mark at about 7:30 O'Clock as is shown in the diagram. That is the centerline of the intake valve, in other words, that's the max valve lift position. On either side of that 106 mark, the valve lift is decreasing. Somewhere inside the circle, write the 0.050" duration, shown in the diagram as 206 degrees. Draw a line under that and write 103 below the line. This is just to help you keep track of the numbers you need to use. 103 is half the 0.050" duration of the intake valve.

Now, if we assume that the action is going counter-clockwise on the circle, we can begin at the 106 degree intake valve centerline and go clockwise toward top dead center to find the point where the intake valve is beginning to open. If we move 103 degrees (half the 0.050" duration) from the centerline of 106 degrees, we find that the intake valve opens at -3 (minus 3) degrees BTDC. The intake valve opening is always expressed as opening before top dead center, so on a short cam like this one, we have to show the open as minus 3 BTDC, because it actually occurs at ATDC going counter clockwise. But since we must always show the intake open as some value BEFORE top dead center, we will use the minus sign to show the opening point properly. I may be spending too much time on this, but I want to make sure you understand what you're doing.

Now, we have the intake opening at -3 BTDC. If we go counter-clockwise from that point down to bottom dead center, we have travelled 177 degrees (180, or half a circle, less 3). Now we have to figure how many degrees past BDC the intake valve closes. If we know the 0.050" duration of this grind is 206, then we can subtract 177 from 206 and find that the intake valve closes at 29 degrees ABDC @0.050" tappet lift. How cool is that?

You can do the same thing with the exhaust valve if you want to, just do it in reverse, where the exhaust centerline would be at about 4:30 PM and you would go counter-clockwise back to TDC. Of course you will need the exhaust centerline number. You can figure that from the lobe separation angle. We know that the LSA is a mathematical function, whereby the intake centerline and the exhaust centerline are added together and divided by two to find the LSA. On the cam card, the intake centerline is shown as 106 and the LSA is shown as 110 degrees. If we subtract the IC from the LSA, we find a difference of 4 degrees. If we double that to 8 degrees and add it to the IC, we find an exhaust centerline of 112 degrees BTDC. Both EC and IC are expressed at some value BEFORE TOP DEAD CENTER.

If anything in this tutorial is unclear, please ask questions. I want everyone to understand how to do this procedure.

Other grinders, such as Lunati, will give you all the 0.050" info right up front. Here's an example....
Cam Spec Card :: Lunati Power

One other thing while I'm on camshaft timing.....the difference between advertised duration and 0.050" tappet lift duration will be an indication of the agressiveness of the lobe lift. Some of the younguns on here and other forums seem to think that the quicker you get the valves open, the better for performance. While that is basically true, there has to be some consideration for how mechanical things work together. You can YANK the valves off their seat and SLAM them back onto the seat only so many times before things begin to fail. Using a softer valve open and close can extend the life of all the valvetrain components and this seems like a good thing on a street motor. If you want more flow past the valve, use more duration and more SCR to match it.

If you subtract the 0.050" duration from the advertised duration, you will begin to get an idea of how radical the grind is, from an opening and closing standpoint. In other words, let's say we have a cam with 280 degrees advertised duration and 220 degrees 0.050" duration. The difference is 60 degrees, or 30 degrees on each flank of the lobe from 0.006" valve lift to 0.050" valve lift. Now, let's look at a cam with 230/280. The difference between advertised and 0.050" tappet lift is 50 degrees, or 25 degrees on each flank of the lobe from 0.006" lift to 0.050" lift. Can you see that there is more effort on the tappet face/cam lobe? You're doing the same amount of work in 25 degrees, that the easier cam is doing in 30 degrees, and you're extending the force over a longer period of time, which makes the cam live longer, theoretically. Again, if you want more 0.050" duration, then use a hotter cam with the same difference between the events, such as a 230/290 cam. Roller tappet cams will eliminate most of the problems associated with flat tappet cams in my opinion.

You can get away with snappy valve action on a roller cam because there is no way for the lifter to scuff the lobe, but on a flat tappet cam, you'd better do your homework. Crane Cams used a 62 degree difference (for instance, 230/292) on their flat tappet cams for years and years. I haven't looked at them lately, so I'm not sure what they use.

To throw a monkey wrench into the works, most grinders show mechanical (as opposed to hydraulic) lobe duration at advertised valve lift and 0.020" valve lift.

A couple more thoughts.....
When the motor is all together and after you have done the initial cam break-in and have the valves adjusted properly, place a piece of thin glasses lens cleaning paper (or doobie paper) between the rocker tip and the valve tip, then roll the motor over by hand through 2 complete revolutions. Remove the paper and inspect. It should be compressed, but not torn. If it's torn, then the rocker geometry is off and there's more work to do. This assumes you have adjusted the sweep of the rocker tip across the valve tip and minimized travel from closed to open and vice versa with different length pushrods or whatever.