Let's talk dynamic compression

[morepowerjoe]

Hey guys. So while I'm waiting on my heads from TSP I'm trying to figure out the maximum dynamic compression I should run. On my last set up I had 9.39:1 dynamic compression and 10.95:1 static. Everything I read says you should stay under 9.0:1 dynamic compression for 93 octane but at 9.39:1 I didn't have any knock retard on the scanner with 28* of timing. So am I ok to run such a high dynamic compression or should I drop it down to 9.0:1. It all depends whether or not I mill the heads .030 or what thickness head gasket I use. I'm guessing that it's all in the tune but not completely sure.

[1JEWLDSSRS]

That would be a good one for Andrew, since you are running their cam.. Maybe?

[AlisoBob]

5:1 works well.......

[morepowerjoe]

Originally Posted by 1JEWLDSSRS

That would be a good one for Andrew, since you are running their cam.. Maybe?

Yeah I was kinda hoping he would chime in. Maybe Monday morning.

[morepowerjoe]

Originally Posted by AlisoBob

5:1 works well.......

Yeah with that low of dynamic compression I'm sure you could run 2 year old watered down 87 octane. lol

[morepowerjoe]

Here is the calculator that I have been using if anyone is interested in figuring out their dynamic compression. The hardest thing to figure out is how much over or under your piston is from the deck.

https://www.gofastmath.com/compressi...tio-calculator

[acutron42]

I reached out to Andrew Cammer. I ran this calculator along with 2 others & came up with 3 different results.

[morepowerjoe]

Originally Posted by acutron42

I reached out to Andrew Cammer. I ran this calculator along with 2 others & came up with 3 different results.

Were they far apart?

[acutron42]

I came up with 9.22, 9.66 & 9.89 dynamic. This is with all the same specs.

[morepowerjoe]

Damn that's crazy.

[acutron42]

I am sure Andrew will chime in. as far as deck clearance, divide your stroke by 2, add your rod length & compression height then subtract this from the deck height.

[morepowerjoe]

Where do you get the compression hieght from?

[acutron42]

Your compression height should be listed in your piston specs. Mine are 1.115. If I divide my stroke (4) by 2 & add 6.125 for the rod plus 1.115 for the compression height it is 9.240. A stock LS3 deck is 9.240. This comes out to zero deck clearance.

[acammer]

Hey fellas! My kinda thread.

Joe - I think you're calculating incorrectly. I know you have the SS3 VVT, and I know even without touching a calculator that it doesn't make anywhere near 9:1 dynamic with 11:1 static. My guess is that you're calculating with .050" valve lift valve events, which will make that IVC close event seem much earlier than it actually is at .006" of lift, which is what we consider "seat to seat" duration timing, and what should be used for dynamic compression calcs.

So, lets talk through how you extrapolate that value from your cam specs. Lets assume you don't have the valve events, so first lets calculate those. There are a lot of calculators, I like the simple one on Summit Racing here: https://www.summitracing.com/newsand...ing-calculator

The SS3 VVT is a 228/246 on a 112+6. Now, the other thing to keep in mind is the advance is variable on a VVT cam. We are most concerned with max dynamic compression values, so we'll use that full advance (+6) value for our calc - but it would be +2 in the fully retarded position at high rpm. This gives us a IVC of 40* ABDC, and that's at .050" of lift. Now, if you had extremely detailed cam information about the lobe, you could do this more exactly, but for our purposes, we're going to assume that there is an additional 55* of duration when looking at .006" seat to seat vs. 050". We take half of that (because the other half applies to the opening side of the lobe) and we add 27.5* of duration to the .050" IVC value. Now we're at 67.5* IVC.

So, now that we have a true IVC value, we can calculate the dynamic compression. I use the Wallace Racing calculators here: http://www.wallaceracing.com/dynamic-cr.php

67.5* IVC with a stock bore/stroke engine with a 6.125" aftermarket rod (I believe that's what's in yours) results in 8.56:1 dynamic compression. Plenty safe with even moderate 91 octane fuel.

Acutron42's got a sweetheart of a motor we did here at GPI. It's a 415, but it runs the same SS3 VVT cam. It's 12:1 static, and so that's a true 9.3:1 dynamic, the very upper end for pump 93 octane. It's an absolutely monster, it makes torque like an electric motor, and still has a top-end that runs up to 7000rpm quicker than the tach can keep track of. I don't recall the exact numbers, but if I recall correctly it did mid-upper 600s on the engine dyno. Should be ~550whp, all the torque, and all that with incredible street manners. We'd normally keep a pump gas street motor closer to 9:1 dynamic to leave some room in the tune-up for inconsistencies in fuel and tuning, but knowing Acutron42 like I do, and knowing we'd be doing the tuning, we felt comfortable in putting it a bit closer to the edge of the envelop. The results speak for themselves.