BBC compression mapping

[Chevy454]

This is a continuation of another post on this board, but I ran across some info that I felt prompted starting another thread. For those of you who are looking for a little more from you LS-6 or L-72 build up, read on...

In the earlier thread, Jim Lerum posted the compression mapping for a 454, which looked like this:

Intake valve closer/ Compression
IVC 68 degrees = 11.54cr
IVC 69 degrees = 11.64cr
IVC 70 degrees = 11.74cr
IVC 71 degrees = 11.86cr
IVC 72 degrees = 11.96cr
IVC 73 degrees = 12.06cr

As Jim says, "this formula involves % cylinder volume vs crankshaft angles". Well, at the time, he didn't have the numbers for a 3.76" stroke, but I have since ran across them. Here is what I found:

Intake valve closer/ Compression
IVC 68 degrees = 11.60cr
IVC 69 degrees = 11.71cr
IVC 70 degrees = 11.81cr
IVC 71 degrees = 11.91cr
IVC 72 degrees = 12.03cr
IVC 73 degrees = 12.14cr

What these numbers should be telling us is what the target compression ratio should be given a certain IVC (intake valve closing). Well, Jim guessed the IVC of the #3863143 cam to be 70 degrees, and I used the same range of numbers to map the 427. BUT, one problem I'm running into is finding the correct "seat to seat" timing of the cam. I've found them measured at .006", .015", and .050", but they don't seem to match up with what I need. So, anyone have a 3863143, a degree wheel and a cam doctor? [img]/ubbthreads/images/icons/grin.gif[/img]

[JLerum]

Rob,

Glad you enjoy the book. For anyone who really gets into the dynamics of engine building it encompasses a LOT of what I think makes an engine so much better than most. You have heard it before that "it is all in the details".

As stated Rob has now compression mapped the L-72 with its GM cam. You may be wondering so what does this mean? It simply means that dynamic compression is extremely important in physical process of making an engine the best airpump possible. In a nut shell,... make all the peices work together as best as you can to get the last ounce of performance possible.

Rob.........Don't forget the IVC is not just measured anywhere. It should be measured at the .028 lash setting with the degree wheel on the crank <font color="red"> </font color> the cylinder head and valve train in place. As the intake valve rocker just releasing contact with the valve stem you are then at IVC.

The cam doctor did not work well with the lobe measuring at less than .020! It ran some really huge numbers that obviously would be a mistake in calculations. In my mind the short block needs to be built to the specs and then the decking of the head and gasket thickness with a advancing or retarding of the cam to get the end result.

Jim

[JLerum]

Remember their are margins of air in the manufactoring of parts.....Cam lobes may be set for the number one cylinder but the rest may be off do to error in cam grind etc....

The valve stem of the number one valve needs to be the same height as the rest as it is part of the measurement the way I've discussed above.

Also remember that their is strech in the timing chain of about 1 degree after intial breakin.

Carbon will also eventually take its place on the pistons and combustion chambers slightly raising the compression.

Make sure you can sleep at night as everything will not be perfect but it sure is fun trying!!!!

Jim
PS...........It is a sick world.

[COPO PETE]

You guys are too high tech for me. Sounds interesting though!
Peter

[Chevy454]

<blockquote><font class="small">In reply to:</font><hr>
Don't forget the IVC is not just measured anywhere. It should be measured at the .028 lash setting with the degree wheel on the crank the cylinder head and valve train in place.

<hr></blockquote>

So, when on the cam doctor, should you measure seat to seat timing @ .024" for the IVC?

[JLerum]

I should of said .024 for the intake lash but I did a miss print. Yes, the cam doctor should of been set up for for the .024 intake and the .028 for the exhaust.

Remember, the solid lifter cams duration should be about 8-10 degrees less compared to an equil hydrolic cam. I believe that is what you would see if placed on the cam doctor with the right lash.

Jim

[Casey Marks]

Is this old or "new" math ????

Just curious ......... [img]/ubbthreads/images/icons/wink.gif[/img]

Rob, are trying to cheat ???

[Seattle Sam]

Sounds to me like Rob is paying attention to the details, that's where you win. It's not cheating to set your engine up to optimal specs using the allowable factory parts.... [img]/ubbthreads/images/icons/cool.gif[/img]

-Sam

[Chevy454]

Well, Casey, it's "new" to me, and it involves "math", so I guess "new math" it is! I've just been "reading" some new Christmas presents! Still learning...

[JLerum]

Casey,

All in the details! I know you know that. I hope you can get Marlin to the track. More the merrier epecially if it is a chevrolet. Hows the Impala coming along?

JIM