Dynamic Compression Ratio Calculator

[folkenheath]

I wrote this Excel spreadsheet a while back to help calculate various things during an engine build to assist in certain part selection, and help everything work together. Many people will be able to fill the inputs in by simply reading some of the notes listed below the data on the front page. If not, the instructions are included on a separate sheet in the spreadsheet. The general layout has two rows of information (A & B) side by side for comparison purposes. The inputs are at the top (shaded gray), which is the information you will need to fill in for the "Outputs", or answers to be correct. The inputs to the right are optional, depending on your needs.​

Click on the link, then click on the download arrow(top right).​

https://drive.google.com/file/d/1QV-BjnSUmEEI-N3chrNw32duaoKQzdeP/view?usp=sharing

EDIT: See post #33, I added turbo and fuel pump/injector sizing to it...

 

[yellowK20]

Hey that didn't work for me?

 

[yellowK20]

Never mind it works now lol Thanks!!

 

[CGT80]

Number of Cylinders 8 Bore (in) 4.155 Stroke (in) 3.750 Connecting Rod Length (in) 5.700 Anticipated Redline (RPM) 5500 Gasket Volume (cc)1 9.0 Piston Volume (cc)2 -5 Comb Chamber Volume (cc) 70 Deck Height (in)3 9.000 Extra Quench Height (in)4 -0.005 Intake Closing(deg)5 61 Boost Pressure (lbs/in2(psig))6 0 Volumetric Efficiency (%)7 100



Outputs A Cylinder Volume (in3) 50.85 Swept Volume (in3) 5.06 Engine C.I.D. 407 Engine Liters 6.67 Bore/Stroke Ratio 1.11 Connecting Rod Ratio 1.52 Compression Height (in) 1.425 CFM Flow 647 Compression Ratio 11.05 Dynamic Compression Ratio 9.11

 

[CGT80]

That's pretty cool. Wish I had this a year ago when I had my 406 built. I don't know if I got all of the specs right. My builder was shooting for 10:1 CR. Head guy wanted 10.5:1, but we didn't want detonation issues. Calculator says 11.05:1

 

[folkenheath]

That's pretty cool. Wish I had this a year ago when I had my 406 built. I don't know if I got all of the specs right. My builder was shooting for 10:1 CR. Head guy wanted 10.5:1, but we didn't want detonation issues. Calculator says 11.05:1

Thank you, glad you like it.

You might want to look at your quench, I label it "extra quench", meaning, any extra space above it at TDC. So a negative number would mean your piston is above the deck at TDC. I would guess that should be .005, not -.005? That will be a lower compression number. Did you input your correct cam specs? If so the dynamic compression is a good number to look at to. Don't forget, there is detailed instructions on the other sheet in the file in addition to the helper notes at the bottom.

 

[CGT80]

I didn't even know there was another sheet. I'm sure the piston is out of the hole by 0.005". The head guy I used was set on getting the quench tight. He said he has run as little as 0.025" or so. I think my felpro gasket was supposed to be 35-40 thousandths compressed. The intake number is from the cam specs-270h magnum from comp cams. I also know that the head was 70 cc to start, but the heads were milled and the combustion chambers may have had material removed, so I don't know the exact new volume. I am also running TRW flat tops with valve reliefs.

 

[folkenheath]

Well it sounds like you have it under control, but that's some pretty high compression and a really tight quench, I wouldn't be surprised if you had slight contact at higher RPMs, but it may not hurt anything.

Aluminum heads or iron? Carb or EFI?

I think you may be over the limits of pump gas. With timing, gearing, chassis etc you may be able to get it to run without detonating but you will probably be sacrificing power vs lower compression and more timing, or simply using higher octane fuel like race gas or E85

 

[CGT80]

Edelbrock performer aluminum heads, 600 cfm performer carb (this is a smog engine), thermal barrier coatings on valves, intake, and possibly intake ports of heads, exhaust cross over plugged on one side and restricted on the other.

I know my mechanical advance is off from where I need it-I am still working on this part. This is in a 79 c20 pickup that weighs 6500 lbs, 4.10 rear and 33" tires, towing torque converter in a th400 that has all the good stuff in it, gearvendors overdrive.

I have some detonation when it is warm. I set the msd mechanical advance, but didn't get the same curves that they did in their instructions. My engine guy agreed to build it to 10:1 CR so I am sure he did that. I think some of my data for the spread sheet is probably off. My cam is also advanced 4 degrees to help low end torque. The guy that did the head work and specified the lower end specs. has proven that he knows his stuff, but we still held back the compression ratio since the truck is so heavy.

I should have gotten more specs when I had the engine built, but I let the top end guy and the engine builder work together to ensure they would be on the same page.

 

[folkenheath]

I have changed the original link because I don't have the comcast account any more it was saved on previously.

 

[80' 427]

For heavy trucks you guys sure push the edge of what I would use. I have 9 to 1 383 and it rattles if I lug it. Next time it will be 8.5 or less. Even in trucks impco recommends 9.5 as the highest compression to prevent detonation on 100 octane. Of couse quench and head design will make a big difference but a 3000lb camaro and a 6000lb truck are like comparing Oxy/act welding with SMAW. Sure the concept is the same but the typical usage is way off.

 

[folkenheath]

For heavy trucks you guys sure push the edge of what I would use. I have 9 to 1 383 and it rattles if I lug it. Next time it will be 8.5 or less. Even in trucks impco recommends 9.5 as the highest compression to prevent detonation on 100 octane. Of couse quench and head design will make a big difference but a 3000lb camaro and a 6000lb truck are like comparing Oxy/act welding with SMAW. Sure the concept is the same but the typical usage is way off.

I agree with you completely. I also don't lug it. My 489 is 10.2:1 and I haven't had it ping so far. But I have a 10" converter in it too, and with a big block and a 5500 lb truck that's a lot more stall than the same type converter in my 3200 lb regal with a small block. So I don't think I can get it in a position to detonate with that converter. The moment I put the ski boat behind it the stall is slightly more and it still doesn't detonate, but I don't tow it very far with that, or I use the trailblazer.

If I had a manual I could see how it would be easy to ping if I held at it lower RPM, but with a manual you can probably get a stock engine to ping.

I also think I don't use my truck for the typical useage trucks are meant for either.

Thats also why I like the dynamic number, it takes into effect the small cams in trucks, and you can definitely see you need lower static compression to have the same dynamic compression with a small cam.

 

[80' 427]

Not you specifically but most others on here are a bit hp crazy. My camaro makes about 850 on the bottle with the last set up but the new one is getting a bit lower compression. My k10 is the one that pings and it has a 700. It tends to lug it and want to shift into OD at too low a speed when crusing the country. I had a s10 converter (2200ish) in my 90 k1500 and it was great till you were backing up to a trailer on an incline, made for some 2 foot driving. Honestly my K30 has 7.8-8 to 1 compression and sure it would run better with more but I like how easy it turns over if I should happen to kill it. I have had to make a few ign curve mods to keep a strong low end (it has 10 degrees of mech advance). It is set at 20 and advances to 30 which was where we found peak hp on the chassis dyno with the lpg.

 

[folkenheath]

Not you specifically but most others on here are a bit hp crazy. My camaro makes about 850 on the bottle with the last set up

I'm sorry but those two sentences are funny, others are hp crazy, but you have 850 on the bottle. I also feel like people here are always saying they don't need hp to wheel, and I am always wanting more. ha ha

And be honest, you are dropping the compression on the camaro so you can use more juice? or are you hoping you can use more timing for more power with the same juice?

My k10 is the one that pings and it has a 700. It tends to lug it and want to shift into OD at too low a speed when crusing the country. I had a s10 converter (2200ish) in my 90 k1500 and it was great till you were backing up to a trailer on an incline, made for some 2 foot driving. Honestly my K30 has 7.8-8 to 1 compression and sure it would run better with more but I like how easy it turns over if I should happen to kill it. I have had to make a few ign curve mods to keep a strong low end (it has 10 degrees of mech advance). It is set at 20 and advances to 30 which was where we found peak hp on the chassis dyno with the lpg.

Wow, the lpg likes low timing I guess? Never used that stuff, I hate the smell in the exhaust, in the fork trucks for example. Do you ever get that smell from your truck exhaust?

I tried limiting my mechanical to 12 degrees in the 489, in ran a little better but with 36 at RPM is was well over 20 degrees cranking and a couple times it dieseled when I shut it off so I put it back to around 21 base timing, with the vacuum can it's closer to 38 or so at idle anyway.

I will admit with the headers, compression, timing, and stock type starter with the 4.25" crank it does turn over slow if it gets heat sinked real good, especially when I shut it off after a big romping in the sand.

 

[80' 427]

Well I dropped compression for a few reason. 1) I was going to drag week again this year and 2 of my old SRP pistons were collapsed and I wanted to push it harder. The new ICONs weren't available in as big of a dome. I run it on pump gas off the bottle and inject 114 with the N20. I believe a hugh dome can steal some hp. The plan was to try for 1000 flywheel this year. The roller I had was for N/A so I got a new cam, better rods, new pistons and a Big shot (300hp+). Now I decided to go to school so I need to finish my brodix heads and assemble it, then let it sit for 3 or so years.

Honestly if you have the a/f right it doesn't stink too much. I can smell it once in a while but my pipes end just past the cab. I have it to the lean side of the curve since that is what LPG like the best. Most strong smelling LPG rigs have someone that doesn't understand them tuning them. The think it is like a gas powered engine and it isn't. Rich is bad and will melt exhaust valves and detonate.
We will see if my new idea works or not. Since my carb is just 460 cfm, I just bought a second vaporizer and carb and will be installing it on a tunnel ram. My thought is since they are variable venturi carbs the low speed fuel drop out that happens with a gas carb on a tunnel ram won't happen and it will pick up torque similar to a TPI intake.

Honestly mostly my truck smells like what an engine smells like after you decarbon buy trickling water into the carb. Not really like lpg. Still 1000 times better than an eye burning gas carb. Getting old those make me sicker than I used to be. I used to be able to spend hours on the chassis dyno with the door shut but now a few mins and my chest hurts and my head will be splitting. I would rather be cold.

 

[CGT80]

I am in a different line of work now, so I took all of my tools out of my truck. It probably weights less than 5500 lbs with the empty aluminum boxes and aluminum ladder rack. It weighed 6500 lbs or so with tools and supplies.

MSD finally sent me a new shaft with weights and springs. It is now spot on with what their charts show. I am at 12 degrees initial and 21 centrifugal for 33 total at just under 3k rpm and then my vac advance adds 16 degrees to that.

This is on a 10:1 406 sbc with worked over aluminum heads and thermal barrier coatings on the internals.

The new ignition map made a huge difference. It doesn't run quite as warm now, but the big difference is that the engine makes power even at 210 degrees F. coolant temp. at the head. Before, if was really down on power when it was warm. The hottest it has gotten has been 220 with the AC going, stuck in traffic on a 100+ degree day.

I get slight pinging when it is very hot. I think my next step will be to adjust my vac. advance stop to less than 16 degrees. When I give it a little gas, the pinging goes away. I don't know if it is due to loosing vacuum or if it is from the carb stepping up to a fatter spot on the needles and dumping in more fuel.

I haven't towed my trailer since the new mods. It weighs 9k lbs and really brings out any issues with the engine.

The power is much better with the extra weight removed and the new tune. It will now chirp the 33" BFG all terains from 1st to 2nd gear (TH-400 and 14bff, with gearvendors overdrive- not a light weight drive train).

The guy that built my heads wanted to run 10.5:1 but the engine builder was only comfortable with 10:1 for this application. I wanted as much torque as possible and knew I was pushing the limit. So far I am very impressed with the amount of power this engine has.

 

[folkenheath]

Well I dropped compression for a few reason. 1) I was going to drag week again this year and 2 of my old SRP pistons were collapsed and I wanted to push it harder. The new ICONs weren't available in as big of a dome. I run it on pump gas off the bottle and inject 114 with the N20. I believe a hugh dome can steal some hp. The plan was to try for 1000 flywheel this year. The roller I had was for N/A so I got a new cam, better rods, new pistons and a Big shot (300hp+). Now I decided to go to school so I need to finish my brodix heads and assemble it, then let it sit for 3 or so years.

Honestly if you have the a/f right it doesn't stink too much. I can smell it once in a while but my pipes end just past the cab. I have it to the lean side of the curve since that is what LPG like the best. Most strong smelling LPG rigs have someone that doesn't understand them tuning them. The think it is like a gas powered engine and it isn't. Rich is bad and will melt exhaust valves and detonate.
We will see if my new idea works or not. Since my carb is just 460 cfm, I just bought a second vaporizer and carb and will be installing it on a tunnel ram. My thought is since they are variable venturi carbs the low speed fuel drop out that happens with a gas carb on a tunnel ram won't happen and it will pick up torque similar to a TPI intake.

Honestly mostly my truck smells like what an engine smells like after you decarbon buy trickling water into the carb. Not really like lpg. Still 1000 times better than an eye burning gas carb. Getting old those make me sicker than I used to be. I used to be able to spend hours on the chassis dyno with the door shut but now a few mins and my chest hurts and my head will be splitting. I would rather be cold.

So you did drop the compression for more power!

I agree on that as well, if you can have the same compression ratio with a dish or a dome the dish will make more power all other things equal. However, I don't think that overrules the effect of more compression with a higher dome.

That make sense with the LP and tuning. I just know when I am in the warehouse or the shop with the LP running fork trucks I hate the smell, but that is indoors. Never really noticed it outside.

I am in a different line of work now, so I took all of my tools out of my truck. It probably weights less than 5500 lbs with the empty aluminum boxes and aluminum ladder rack. It weighed 6500 lbs or so with tools and supplies.

MSD finally sent me a new shaft with weights and springs. It is now spot on with what their charts show. I am at 12 degrees initial and 21 centrifugal for 33 total at just under 3k rpm and then my vac advance adds 16 degrees to that.

This is on a 10:1 406 sbc with worked over aluminum heads and thermal barrier coatings on the internals.

The new ignition map made a huge difference. It doesn't run quite as warm now, but the big difference is that the engine makes power even at 210 degrees F. coolant temp. at the head. Before, if was really down on power when it was warm. The hottest it has gotten has been 220 with the AC going, stuck in traffic on a 100+ degree day.

I get slight pinging when it is very hot. I think my next step will be to adjust my vac. advance stop to less than 16 degrees. When I give it a little gas, the pinging goes away. I don't know if it is due to loosing vacuum or if it is from the carb stepping up to a fatter spot on the needles and dumping in more fuel.

I haven't towed my trailer since the new mods. It weighs 9k lbs and really brings out any issues with the engine.

The power is much better with the extra weight removed and the new tune. It will now chirp the 33" BFG all terains from 1st to 2nd gear (TH-400 and 14bff, with gearvendors overdrive- not a light weight drive train).

The guy that built my heads wanted to run 10.5:1 but the engine builder was only comfortable with 10:1 for this application. I wanted as much torque as possible and knew I was pushing the limit. So far I am very impressed with the amount of power this engine has.

Glad you got it running better. So did you ever put your numbers in and see what the dynamic compression was?

 

[CGT80]

I put some numbers in to get dynamic compression, but I don't know all of the values for my engine. Below is what I came up with. I thought my head builder said something about having a high static compression ratio to make more power, but a lower dynamic to make the engine still perform and live in its intended application. My friend who also used the same guy for other work, and is familiar with his theories and builds, said the cranking pressure would likely be high on my engine and the VE could be above 100% due to the amount of head and intake work. I haven't done any tests on my engine and it has not been on a dyno. I am also only running a 600 cfm elecbrock carb and the engine still pulls 5 inches of vacuum at 5500rpm at full throttle and the exhaust only makes 1-2 psi of back pressure for a very short period of time=likely needs a bigger carb.

Engine builder shot for 10:1 comp-head wanted 10.5:1 but agreed to go a little lower, but that is not what this spreadsheet comes up with.

Inputs A Number of Cylinders 8 Bore (in) 4.155 Stroke (in) 3.750 Connecting Rod Length (in) 5.700 Anticipated Redline (RPM) 5500 Gasket Volume (cc)1 9.0 Piston Volume (cc)2 -5 Comb Chamber Volume (cc) 70 Deck Height (in)3 9.000 Extra Quench Height (in)4 -0.005 Intake Closing(deg)5 61 Boost Pressure (lbs/in2(psig))6 0 Volumetric Efficiency (%)7 100



Outputs A Cylinder Volume (in3) 50.85 Swept Volume (in3) 5.06 Engine C.I.D. 407 Engine Liters 6.67 Bore/Stroke Ratio 1.11 Connecting Rod Ratio 1.52 Compression Height (in) 1.425 CFM Flow 647 Compression Ratio 11.05 Dynamic Compression Ratio 9.11

 

[folkenheath]

Inputs A
Number of Cylinders 8 Bore (in) 4.155
Stroke (in) 3.750
Connecting Rod Length (in) 5.700
Anticipated Redline (RPM) 5500
Gasket Volume (cc)1 9.0
Piston Volume (cc)2 -5
Comb Chamber Volume (cc) 70
Deck Height (in)3 9.000
Extra Quench Height (in)4 -0.005
Intake Closing(deg)5 61
Boost Pressure (lbs/in2(psig))6 0
Volumetric Efficiency (%)7 100

Outputs A
Cylinder Volume (in3) 50.85
Swept Volume (in3) 5.06
Engine C.I.D. 407
Engine Liters 6.67
Bore/Stroke Ratio 1.11
Connecting Rod Ratio 1.52
Compression Height (in) 1.425
CFM Flow 647
Compression Ratio 11.05
Dynamic Compression Ratio 9.11

Based on your inputs, you have a .030" over 400 with a 350 connecting rod and a flat top piston with a 5 cc valve relief? If you have the stock 400 rod it would be 5.565". You can see it makes a very small difference in the dynamic ratio though, the most important thing is the intake valve closing point.

Also, your extra quench is negative. If your piston is in the hole it's a positive number. (adds to the combustion chamber size).

If I correct the quench to .005" I get 10.78 static and 8.89 dynamic. Which is pretty high for pump gas. You may have made more power with slightly less compression so you could use more timing. Limiting your timing to 33 to combat detonation is hurting your performance. Double check your piston, head, and cam specs to make sure your input numbers are correct, if they are, you may want to convert to E85 or race gas to get your timing and power you should have.

 

[CGT80]

I would have to look up the build sheet. My 383 had 350 rods, but with this block, the builder may have used 400 rods. He used Scat 9000 series rods and crank. Cam is Comp 270h magnum 224/224 @50 470 lift. Don't know the other specs of hand. TRW hyper pistons stick out of the cylinders by 0.005"
Quench should be 30-35 thousandths from combustion chamber to piston top. This head guy and my friend that has used his parts, said his engines require less advance than most due to the efficiency of his work. I don't have any proof one way or the other on that statement. You could be right about running less compression and more advance-I haven't heard much about one vs. the other.

I might still be able to run more centrifugal timing if I slow the rate of advance and/or limit the vac advance more. I think most people run less than 16 degrees on their vac advance for modified engines. I think I remember MSD stating that the dizzy I have is set for 10 degrees from the factory (I have a different can than what msd used-mine is adjustable too).