Dynamic Compression Ratio - help me compute

[ykf7b0]

There are numerous static and dynamic compression calculators on the web and I have used many but one number that needs entering for dynamic compression ratio is eluding me and I cannot locate it. I have a MP Purple shaft part number P4120235AE and it is the old 282/484 cam. The number I'm trying to locate is for the intake valve closing point in degrees ABDC @ .050 lift plus 15 degrees. The intake valve closes at 70 degress ABDC but I'm not sure how to compute this number at @.050 lift. Any experienced help is appeciated!

 

[moper]

Another reason I'm not a fan of the MP camshafts... If you can't find a record of someone that did profile one, I'd suggest that you profile the lobe and work it out that way if it's that critical.

 

[ykf7b0]

I think I have got it worked out by searching and computing. Thanks

Figuring Intake Valve Closing Point for Cam selection and DCR calculation.
To figure this you take the advertised intake duration and divide by 2.
Then you add the LSA to this number.
Then you subtract out the ground in advance. I did advance my cam 2 degrees upon installation.
Then you subtract 180
This gives you an actual intake valve closing point to use in the DCR calculator.

It is believed that anything over 8.7 to 1 dynamic compression is the danger zone for pump gas. This is not to be confused with static compression.

 

[mentor70]

i imagine that is with Iron cylinder heads.

 

[ykf7b0]

I know static compression can be (should be) increased with aluminum heads but based on what I've read so far the dynamic compression should also limited to less than 9 to 1 compression with aluminum heads and pump gas.

 

[moper]

Just my opinion here...
While it's important to keep dynamic in mind, you're better served by minding the static and getting that right. While the dynamic ratio mechanically stays consistent, the cylinder pressures that ratio should provide change as the engine reaches the camshaft's power band and cylinder filling becomes more of a function of the physics of a runing engine. As a result I tend to use lower figures. Static is much more important IMO. For an open chamber iron head I'd keep dynamic under 8.25:1 if it has to run on pump fuel and has a moderate cam. Also, the higher performing the engine as a whole, the more room you have to leave for that pressure increase. That's why a larger cam might indicate a dynamic of 7.5:1 or less. Because with the right static ratio, heads, intake, carb, headers, and exhaust that engine will really fill the cylinders at the cam's sweet spot.

 

[ykf7b0]

Good point moper. I have a 10.15 to 1 static engine and I can increase this to 10.59 to 1 by going with steel shim head gaskets. I have 915 closed chamber iron heads with a 284/484 purple shaft and I needed to know I wasn't getting into detonation with my dynamic ratio. This is the only reason I was concerned with the dynamic. I don't want to mix racing fuel with my mainly street driven 440. (It is a 10 second car with 175 hp shot of nawws). I know more than one shivvy owner/builders that didn't pay attention to the dynamic compression ratio and wound up with a semi slow car and needed a mix of 104 and 93 octane to drive it on the street.

 

[moper]

Given the nitrous, I'd keep it lower. The 284/484 isn't my favorite - IMO you could do much better with something more modern - but it will work.

 

[ykf7b0]

That old purple shaft is working and sounding too good for me to change it now.