Dynamic Compression Ratio Help

[Shabadoo]

I've been attempting to calculate my dynamic compression ratio and am having some trouble. I don't know if the trouble is in the accuracy of my numbers, how I'm using the numbers, or the formulas I'm using. Possibly someone will see this and be able to help. Here are all the numbers. Most of them I found online on various websites so once again, accuracy may be off.

This for a 2000 2V 5.4 with PI heads and non-Pi pistons. I'll begin with calculating static compression ratio:

Assumed:

Stroke: 4.165 in.
Bore: 3.552 in.
Connecting Rod: 6.658 in.
Deck Height: 10.079 in.
Chamber Volume: 42 cc
Gasket Thickness (Compressed): 0.036 in.
Piston Dish: 11 cc (18 cc for PI pistons)
Compression Height: 1.22 in.

Calculated:

Assembled Deck Height: 0.1185 in.
= Deck Height - [(Stroke/2) + Connecting Rod + Compression Height]

Cylinder Volume (CV): 41.27155399 ci
= 0.7853982*Bore^2*Stroke

Deck Clearance Volume (DCV): 1.174232688 ci
= 0.7853982*Assembled Deck Height*Bore^2

Head Gasket Volume (HGV): 0.356728918 ci
= 0.7853982*0.036*3.552^2

Piston Dish Volume (PDV): 0.6712607 ci
= 11*0.0610237

Cylinder Head Chamber Volume (CHCV): 2.5629954 ci
= 42*0.0610237

Compression Ratio: 9.661:1
= (CV + DCV + HGV + PDV + CHCV)/(DCV + HGV + PDV + CHCV)

I know this isn't the most accurate but it seems to be where I need it to be. If you replace the 11 cc non-Pi pistons with the stock 18 cc PI pistons you get 8.94:1 which is close to the advertised 9:1.

Now to dynamic compression ratio:

Assumed:

Advertised Duration:
- Intake: 233
- Exhaust: 239

Duration @ 0.050:
- Intake: 184
- Exhaust: 191

Intake Open: -18
Intake Close: 22
Exhaust Open: 33.5
Exhaust Closes: -22.5

Lobe Separation Angle: 114

Numbers for the camshaft obtained from the following websites:
- http://www.crower.com/media/pdf/2008b/67-97.pdf
- https://www.crower.com/valve-timing-chart/#114

I honestly could not find formulas to calculate dynamic compression ratio. I used a few online calculators which gave me numbers between 9.4:1 to 10:1 which seems extremely high. Now I did raise my compression ratio but even doing the same calculations with the stock compression I saw 9:1 which is still very high from what I've seen.

I'm not sure where to go at this point so I'm hoping someone will see this and be able to steer me in the correct direction. If anything, this will maybe help someone out whose trying to do the same.

 

[reece146]

There is a DCR calculator on the Jeep Strokers web site. It may be generic enough to use as is or maybe you can "steal" the formulas out of the web script.

I remember seeing a web site with full work up and discussion on calculating DCR. I'll see if I have it bookmarked next time I'm on a computer.

 

[Shabadoo]

I found that Jeep Strokers website you talked about and got the following results:



Everything seems to check out and the 8.65:1 dynamic compression ratio sounds about where one would expect it. Thank you for pointing me in the right direction. I did find the code for the calculations which the website uses and I'm currently working on getting all the formulas from that. If you are still able to find that website you mentioned which had a full work up, I would appreciate it. No worries though if you can't.

 

[reece146]

I don't know if these links will give you any new data or maybe you've already seen them. I didn't vet them myself recently but they are in my bookmarks - other things are in the foreground (docker swarms - oh my!) and I don't need to descend into this rat hole of brain candy.

http://cochise.uia.net/pkelley2/DynamicCR.html

http://www.crankshaftcoalition.com/wiki/Dynamic_compression_ratio

http://www.crankshaftcoalition.com/wiki/How_to_choose_a_camshaft

http://www.crankshaftcoalition.com/...atio_compatibility#Intake_valve_closing_point

HTH

 

[Shabadoo]

Thank you for all the links reece. I managed to pull all the calculations from the Jeep Strokers source code and for the most part they match those in your first link. I haven't had the time to thoroughly read through the other links but after a cursory glance it looks like there is some great info. It will help a lot.

For all viewing this thread, when I get more time I will post my DCR calculations for review. For now I have attached the excel spreadsheet I made which I used to calculate static and dynamic compression ratio. The values in it are for my application specifically but it can be used for any engine. Part of why I created this thread was to not only get some help myself but to provide a resource for others who may be trying to do the same thing I did. I did not physically measure anything to get these numbers therefore, outputs should be taken more as estimates.

 

[reece146]

Cool. I'll take a look at the spreadsheet tomorrow (hopefully).