Check My Math - Compression Ratio

[gbranham]

I was fiddling around with calculating my compression ratio this AM on my 427 Boss stroker with Twisted Wedge 11R heads. I came up with 9.38:1.

Bore = 4.125"
Stroke = 4.00"
Piston Dish = 17cc
Piston to Deck Clearance = .005"
Head Gasket Compressed thickness = .042"
Combustion Chamber Volume = 66cc

Is 9.38:1 correct? I've run the calculation a few times, and it seems right.

Greg

[Jeff Kleiner]

Assuming it's correct it seems that you're on the low end for static compression on an aluminum head engine. Of course your cam profile will dictate what your dynamic compression will be (somewhat less) but for pump gas you can generally safely run around 11:1 static and 9:1 dynamic.

Jeff

[gbranham]

Interesting. I used several online calculators, and I keep coming up with 10.38:1 CR. Not sure where my math came up with exactly 1.00:1 less than the online calculators.

CR.jpg

[Jeff Kleiner]

Interesting. I used several online calculators, and I keep coming up with 10.38:1 CR. Not sure where my math came up with exactly 1.00:1 less than the online calculators.

CR.jpg

That sounds better. Unless your cam selection results in a high dynamic compression it should be happy on pump gas.

Jeff

[gbranham]

That sounds better. Unless your cam selection results in a high dynamic compression it should be happy on pump gas.

Jeff

It's a mild cam...Trick Flow Specialties TFS-51403001 Trick Flow® Track Max® Hydraulic Roller Camshafts for Ford 5.0L | Summit Racing

This is driving me nuts, though, on my manual calculation vs online calculators...being off by 1.00:1. I'm a physics and math major, so this isn't hard stuff. Or maybe it is...hah!

Have a good weekend, Jeff!

Greg

[Jeff Kleiner]

It's a mild cam...Trick Flow Specialties TFS-51403001 Trick Flow® Track Max® Hydraulic Roller Camshafts for Ford 5.0L | Summit Racing

This is driving me nuts, though, on my manual calculation vs online calculators...being off by 1.00:1. I'm a physics and math major, so this isn't hard stuff. Or maybe it is...hah!

Have a good weekend, Jeff!

Greg

That's a Trick Flow Stage 1 and will be VERY mild. I used it along with Twisted Wedge heads when I built my old 302 and it was great but in a big inch engine it will be much less aggressive. It's .499/.510 and 221/225 degrees. I don't know what your end goal is but most 427 strokers that are shooting for upper 400s are running cams with ~.600+ and around 240-250 degrees or more duration. Just food for thought.

Jeff

[gbranham]

Thanks, Jeff. It'll be a weekend cruiser at best...mostly the occasional short jaunt around my winding country roads. It might see a few thousand miles a year, amd will be lucky to see 5000 RPM much.

[Dgc333]

Interesting. I used several online calculators, and I keep coming up with 10.38:1 CR. Not sure where my math came up with exactly 1.00:1 less than the online calculators.

CR.jpg

I get 10.38:1, years ago I validated the on line calculators and the ones I looked at all agreed with a hand calculation. Are you sure you didn't mess up the units. The piston dish and head volume are in cubic centimeters and everything else is in inches.

[gbranham]

I get 10.38:1, years ago I validated the on line calculators and the ones I looked at all agreed with a hand calculation. Are you sure you didn't mess up the units. The piston dish and head volume are in cubic centimeters and everything else is in inches.

I converted everything to cc, then went at it. I calculated the swept volume as 4.125" bore by 4.00" stroke, and then the compressed volume to be the combustion chamber volume+head gasket compressed volume+piston-to-deck clearance volume+17cc piston dish volume. Oh well. I'll just rely on the online calculators from now on. I hate that, though. Maybe I'll take another shot at it. Here's my current math. What am I doing wrong?

CR.jpeg

Thanks,

Greg

[gbranham]

I think I figured it out...I need to add compressed volume (93.29cc) to my swept volume, then divide by compressed volume. Voila! 10.38:1. I'll sleep better tonight!

Greg

[Dgc333]

I think I figured it out...I need to add compressed volume (93.29cc) to my swept volume, then divide by compressed volume. Voila! 10.38:1. I'll sleep better tonight!

Greg

I was looking at your calculation sheet and noticed that you hadn't added the dish, gasket and deck clearance volumes to your total volume. Just as I was to respond I noticed you had figured it out. 😁👍

[gbranham]

I was looking at your calculation sheet and noticed that you hadn't added the dish, gasket and deck clearance volumes to your total volume. Just as I was to respond I noticed you had figured it out. 

Yes, sir! Thanks for double-checking my math! I appreciate it.

Greg