Right. I'm confused. check this out.

I have a 350 punched .040 over.. I wanna go 10:1 static compression.. (55cc chambers, reverse dome pistons).

I currently have a cam with the following specs.. 282/307 advertised duration, 230/245 at .050".. 114 lsa, 112 ica.

If you calculate it out, at .050" it is 8.9:1 dynamic compression, and 9.5 degrees of overlap.

If you calculate it out using the seat-to-seat method, it's 7.47:1 dynamic compression, and 64.5 degrees of overlap.

This in itself is confusing to me.

Now my machinest tells me "Joe, that cam was great when you were gonna run the s-trim, but for n/a use you cant use it. it will detonate.."..

So he recommends a comp 306 cam, that a lot of lt1 guys use.

Comp 306 is 290 / 307 advertised, 230/244 at .050".. 112 lsa, 108 ica.

At .050" its 13 degrees overlap, and seat to seat is 74.5 degrees of overlap.

Now the odd thing is, on either calculation, .050" or seat to seat, the dynamic compression is the same as my other cam.. Yet the overlap is way large..

Now personally I think 74.5 degrees of overlap is gonna tell the o2 sensor to go screw, but he seems to think I "need" to change cams.

Any opinions?


btw, desktop dyno thinks my cam makes a few more HP than the 306.. My machinest thinks the 306 cam should make about 70hp more..


-- Joe

 

I can understand your confustion. I'm not familiar with the calc of DCR but I would offer this based on the seat and .050" specs. I can't see any way that your current cam would have a problem with knock due to too much dynamic compression. Most cams marketed as "computer friendly" have less than 50 degs overlap measured from the seat figures. Since the overlap of both of the cams you mentioned is quite a bit more, knock shouldn't be an issue at all but effort will be required to get either to play nice with the ECM. I don't see the CC306 being 70 HP stronger.

 

It is the intake valve closing point/degree on the cam that determines the DCR. Nothing else regarding the cam. If both cams have the same degree at which the intake valve closes than the DCR will be the same.

 

Dynamic compression is nothing more than the old static compression ratio calculation but starting calculations from the point the intake valve closes (instead of from piston BDC). All that matters is when the intake closes- no other cam specs are taken into account in a DCR calculation. End of the day it comes down to duration and ICA affecting the DCR calculation- nothing else makes any difference in it.

BTW- you're off on your DCR calculations somewhere. Your current cam has a lower DCR than the new one based on .050 valve events (the intake on the current cam closes later than the new cam). Using advertised duration it's about a wash between the two but "advertised" duration numbers are difficult to compare becuase they are not measured at any commonly accepted amount of lift (can be anywhere from .001 to .006 typically for hydraulics, considerably more for solid cams). Always difficult to compare different cams based on advertised duration numbers.

 

Ok got it...


With the first cam you listed i came up with 10.5:1 static and 7.8:1 dynamic. Thats just plugging in numbers out of my head for the piston specs...


What are you using to calculate this?


Yes both cams have the same dynamic CR because the exh valve still closes at the exact same time. when the exhaust valve closes is when the compression starts to build. If it closes sooner you have a higher DCR if later its less DCR...

Both cams give you a dynamic stroke of 2.49701117177702

If you want, PM me your email and i can send you this wicked dynamic CR calculator i have.

 

I downloaded this one by Pat Kelley which I've been using. Which one do you have?

specs:

Je reverse dome pistons. -16cc dish, felpro 1010 (4.166 x .039), piston is .005" in the hole, 4.040" bore. chamber on heads is 55cc after angle milling them. I figure 10:1 static..

-- Joe

 

Im using pat kelleys program too, it kicks ****!

 

So stick with the .050" numbers?

So the moral of the story is, for a N/A setup, think i'm better off with my current cam, the 306 cam, or something like the CCA-08-432-8. (only 65 degrees overlap, but more low end torque). DCR seems about 9.1 using this calculator

United Engine and Machine Co.

That too high eh? On the seat-to-seat method calculator, it comes up as
7.87:1 dcr.

-- Joe
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I don't see the CC306 being any stronger either. I think maybe my machinest feels that the more overlap will let me run a bit more spark advance..

I ran this cam and tuned idle using '749 when I had the blower, and it sucked *** in closed loop. 02 just was useless, and the wideband wasn't much better.

I wonder if I'd be better off with a cam like a 08-432-8. Less overlap than both, but seems to be a strong torque builder.

-- Joe

 

When using the United Engine calculator use the actual intake valve closing degree point. Don't use the .050 as it gives a false reading. The best is the Pat Kelly DCR calculator which was mentioned above.