This is what i get from the calculator from your engine info:
4.250 bore, 4 stroke 6.135 rod with 6.2 boost ant 9.6:1 comp ratio at 1000 feet altitude.



Camshaft, Rod Length, Boost and Altitude Correction to Compression


Your engine summary is as follows: Bore 4.25 inches, stroke 4 inches, rod c-c length 6.135 inches, with a static compression ratio of 9.6 :1. Your camshaft specifications call for an inlet valve closing of 42 degrees ABDC (after bottom dead center).

Your chamber volume is 108.13 cc's. With this camshaft your dynamic, or effective stroke is 3.63 inches. Your dynamic compression ratio is 8.60 :1 corrected for cam timing, altitude, and rod length. Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 174.55 PSI. Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 6.2 PSI is 12.23 :1.

Knowledge is power.




you are at 12.23:1 dynamic and the stock merc 525sc is arond 9.18:1 at 1000 altitude (feet).

So i dont know at wich ratio we are to high with this calculation......

While all the calculations are cool, I would be more interested in what others have done, and done successfully. Your 525SC is pretty common engine, that lots of guys on here have hopped up in various ways.

For starters, going with Aluminum Heads is a big help in fighting detonation. Polishing the combustion chambers can also help prevent any hot spots.

Boost psi can be tricky. For example, like you said, your boost will drop based on better flowing heads, and cam profile. So lets say stock maybe you were driving the blower at a 2:1 Ratio and making 5psi. Now, with the mods, you may have to drive the blower at 2.3, or maybe even 2.45 to make your goal of 6psi. Now you are getting into not only heat, but you will also really be "beating up" the air in the blower. Lets say you put a decent duration cam, and prop for 5500. Your blower will be turning either 12650 rpm or 13500RPM at WOT. Where stock at 5200RPM engine speed and 2:1 Ratio, you were turning the blower at 10,400RPM.

If your gonna keep the 177, i'd keep the drive ratio around 2:1. Like Kvogt said, keep it at 4-5psi, and run it. Can you make more power by upping the boost, yes, but not with a 177 on a stout 454ci. With good heads, properly selected hyd roller cam, modest static compression, 600HP is about where you'll be at 5psi. If 650-700 is what your looking for, more boost will get you there, but your gonna need a bigger compressor to do it.

Very interesting and 12.23 sound much more realistic to me and how I think of compression ratios. 13.4 was way high. I'm not sure what chamber volume is though and where you got that? My heads are 116cc.

My real numbers entered I get 12.45 dynamic at 42*abdc. Loading the cam straight up would have increased the dynamic to 12.58 (40*). That is a great calculator, thanks for sharing. It would be nice to get some correlation though. I'm going to be redegreeing both cams in the next couple weeks so I may go back to the straight up 107ICL recommended.

Also, for reference. On my 460CI engines, with Dart Aluminum heads, aggressive hyd roller cam, compression around 8.4:1, with a 177 weiand driven at 2:15:1, i got about 4.5psi. Swapping to a 250 B&M setup, 1.6:1 ratio gave me 5psi. 1.8:1 gave me 6.5-7psi.

I went with a 40% larger blower, although to get equal boost psi, I only slowed the blower 25%. Why is that, I dont know. Is it because the air charge is cooler, hence lower boost psi in the intake, or was the weiand a more effecient design than the B&M, I dont know. I do know that the best I got out of the 177 setup at 5psi was about 78mph on a cool day. The 250 setup at 7psi, got me 84mph on a cool day, and that was with fat jets and conservative timing. Probably a couple more mph in it with tweaking.

For your chamber volume, this calculator does not take care on your actually deck height, head gasket thickness and your actually squish.

Probably calculating with standard spec for that.

I know that i have a limitation with this blower. I am also looking for a 250 or 256 blower if i can find one. But for now i will start with the 177 in prediction of a bigger one.

What is your cam??? just curious

Another interresting DCR link....

http://members.uia.net/pkelley2/DynamicCR.html

Realy interresting...

That's cool, I've never seen that calculator with the boost built in. This is the one I've always used for NA stuff.

http://kb-silvolite.com/calc.php?action=comp

I can't give you an exact number for the dynamic comp. ratio, but I can tell you this. I have found that I as long as I stay under 190 psi on the dynamic cranking compression I'm OK on 93 octane with a good tune. For 91 octane, I would assume that number would be around 175-180 psi. Good luck.
Eddie