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Ok so I'm lost. I'm trying to put an LQ9 together as some of you know from reading my build thread. Anyway I was plugging numbers into a compression ratio calculator ( here ) trying to figure out how well it will run on pump gas. So from the cam card, which ABDC degree do you enter? The ABDC at .050 tappet lift or .006? If I use .050 I'm getting almost 10:1. I hope that's not right.
Use 43 , i typed all the info into the calculator I used and guessed on 8cc dished piston and got 7.72 : 1
Ok so this is my setup
4" bore
Stroke 3.622
Gasket thickness 0.051
Gasket Bore 4.020
Deck clearance -0.007
Head Chamber Volume 65cc
0cc Flat tops pistons, I used +1.7cc for ring land
Rod length 6.098
If I use 67 deg it comes out to 8.45. That sounds a lot better but I just want to make sue that's right.
Last edited by Phobos84; May 24, 2021 at 07:17 PM.
Depends on the calculator, I put in the specs for my flat-topped 5.3 on the calculator you linked, and that came in at 9.153, which is what I would expect, that uses advertised lift @ 0.006, so 67 is right for you.
Ok so I would enter the 67 deg. That makes me feel way better. So with 10.9:1 static compression I should have "about" 8.45:1 dynamic compression. So being it's aluminum heads I should be able to run this on 93 octane without too many problems. Thanks guys!
You should be set to run even 91 octane, 8.45 is well within the bounds, and with how efficient the LS heads are I wouldn't worry about it. Just don't copy the high octane table into the low octane table like some goofs do and you'll be fine with plenty of headroom.
The way you enter to get the lower number is correct... wouldn't make sense to check it with the valve more open was how I reasoned it on mine. Checking that against the number I have written down its about the same when I plug in my info. That being said it probably isn't really that close still. The one I used gives an estimate of cranking compression too... and it's about 30 psi low. I get low 190s cranking on a cold engine last time I checked where as most the calculations show it should be 160... there is a lot more at play than just that number imo. That being said it should run just fine on pump. I think you're overthinking.
There are two reasons for this. First I have never built a motor with this high of compression before. So I just wanted to make sure it was going to be ok. And the first numbers I got from the one calculator were crazy. And second is my job literally just paid me to sit in a swivel chair with a laptop in front of me for 8 hours and do nothing. I don't mean that there were other things that I should have done and just didn't. No I mean there was literally nothing to do but think deep thoughts about finding another job. Or calculate dynamic compression ratio.
Dynamic compression can only be calculated using 0.000-0.006" IVC numbers.
Calculators that ask you for 0.050" numbers just guess on 0.006" measurement by adding an arbitrary number like ~50 to the total duration (dividing it by 2 for the intake-only side) before giving you a dynamic compression reading.
Well with the .006 IVC degree of 67 that means that with a 65cc head I would have a dynamic compression ratio of 8.467 and a static ratio of 10.831.
So knowing that the compression ratio should be ok for pump gas now I want to check piston to valve clearance. Just to be sure I'm going to put a factory un-ported 243 head (64.45cc) on the motor with an old GM head gasket. Put 4 or so bolts in it lightly and check clearance using Play-dough. Then I'll send the PTV measurement to Advanced Induction and see if he thinks that milling the head from 69cc (final size after port work) down to 65cc would cause issue. I should be fine, but I got nothing but time so why not check.
Help me figure out Dynamic Compression
