Estimating CR from Compression test

[jack46]

I read in a couple posts about determining CR from compression test. I thought if a car had 165 psi compression test then it would be 10 : 1

 

But have been told this is not true.

 

Is there a way to determine CR

 

I thought

 

Compression = (CR +1)*14.7 ie CR ~= (Compression - 15)/15

 

at 165 CR = 10

 

Thanks

[JMortensen]

Cam duration will drastically effect the compression psi. If you have STOCK components, you can check vs the suggested compression in a FSM, otherwise you can't really tell based on a compression test.

 

If you know what you have, ie what head, shaved head, what block, what pistons, bored cylinders, you can figure it out.

 

Anything else is peeing up a rope IMO.

 

Seems like the stock turbo guys are seeing about 145 psi, and thats with something like 7.2:1. I ***THINK*** the FSM says 185 for stock NA, that's with somewhere in the 8.5:1 neighborhood in most years.

 

Jon

[jack46]

Ok thanks I once thought it worked, then was told it didn't. Then I saw it posted here, where someone suggested it did work but never got a reply...So I will let it die thanks

[Kevin Shasteen]

You cant automatically assume a 10.0:1 SCR simply because you obtained a 165 PSI during a Compression Test: this assumption does not take your Cam's Duation, Displacement, nor Atmospheric Pressure into consideration.

 

Your cylinder pressure is indicative of your Dynamic Compression Ratio (DCR) as it is factored in with Atmospheric Pressure (AP)...not your Static Compression Ratio (SCR). This dance that takes place between your DCR, SCR, and AP is meaningless if your Intake System leading up to your Cylinder is mismatched to your power needs as Airflow Velocity will have been compromised. For a hot street engine you need your Cross Sectional Area of the Intake Ports to be at least .80 to .85 of your Cylinder Head's Intake Port and .90 for a dedicated race engine.

 

There is no such thing as "Vacuum" in your intake system-what you are reading is the Pressure Differential between the Absolute Pressure in your Intake Ports as that pressure comes into contact w/the Ambient Atmospheric Pressures. To know the Absolute pressure in your Intake Manifold's ports you will use the Barometric reading and not the AP readings. So if your Intake Port reads 16inches Hg at idle-then you would subtract that from the Barometric reading of your Altitude, like so:

 

29.9 - 16 = 13.9" Hg

 

DCR is indicative of how much Volume remains above the piston after the Intake Valve Closing (IVC) occurs. Remember, the IVC takes place after the piston has passed BDC.

 

One of the formulas on the Grapeape website states this formula for calculating the PSI rating:

 

PSI = DCR^1.2 x AP

 

PSI = Pounds Per Square Inch

DCR= Dynamic Compression Ratio

1.2 = Exponent that represents adiabatic pressures (how pressure responds to thermodynamics)

AP = Atmospheric Pressure

 

Obviously you wont know your DCR unless you have a Crank Angle Chart and your cam specs, specifically your "Seat to Seat" Duration spec. Because we dont have the Crank Angle Chart specific to your engine nor your Cam's Seat-to-Seat Duration spec; we wont know your DCR-so we will have to work our math equation backwards.

 

TRANSPOSING THE EQUATION

If you already know your PSI and it is 165 then the formula from the Grapeape website looks like this:

 

165 = DCR^1.2 x AP

 

The first step would be divide your AP by itself and then also divide your AP into the 165. Let's say you are at Sea Level-so your AP would be 14.7. So the next step would look like this:

 

165 / 14.7 = DCR^1.2 x (14.7 / 14.7)

 

Upon working this out your equation now looks like this:

 

11.22 = DCR^1.2

 

The next step is a little more complicated at first glance since we are dealing w/an Exponent; but once you understand how to transpose an exponent it really isnt that difficult. What you do is divide the Exponent into [1] and your answer will then be applied to the 11.22 as an Exponent. The answer to this last step will be your DCR; voila-you now have your theoretical answer to the question, "What is my DCR?"

 

So, divide 1.2 into 1, like so:

 

1 / 1.2 = .8333

 

Now apply this sum to the 11.22 as an exponent, like so:

 

11.22^.8333 = 7.498, or 7.5:1

 

This is your DCR....as in THEORETICAL DCR. If you wish to know your SCR you will have to "Guestimate" your Total Chamber Volume and apply that guestimation to the Compression Ratio Equation. The Compression Ratio Equation is:

 

(Cylinder Volume + Total Chamber Volume) / Total Chamber Volmue = CR

 

Since we are dealing w/DCR and not SCR our CR is actually known as the Effective CR, or as the politically correct math nuts call it: CRe. By either name it is still the amount of volume remaining above the piston at the moment that corresponds w/the IVC. So you ask, "What does this have to do with the price of chickens in China?" Not much-just making sure you unerstand the difference between DCR & SCR as far as the PSI readings are concerned.

 

Once you have guestimated your Total Chamber Volume you can now plug that figure into the CR formula and work the math accordingly. Let us say you tallied your Total Chamber Volume to be 75 cc and our engine is a SBC 350. First calculate your individual Cylinder Volume and convert it to cc's also

 

350 / 8 = 43.75 cubic inches

 

now use the 16.387 constant to change your cubic inches to cubic centimeters

 

43.75 x 16.387 = 716.93

 

Now plug our Cyl.Vol and guestimated Total Chamber Vol into the CR formula and your equation now looks like this:

 

(716.93cc + 75cc) / 75cc = 10.559, or 10.6:1 SCR

 

To guestimate your Total Chamber Volume you would need to know:

 

1) Cylinder Head Combustion Chamber Volume

2) Piston -Dish or +Dome Volume

3) Piston Ring Land Volume: for a small block assume 1.3 cc

4) Piston Deck to Block Deck Clerance Volume

 

Kevin,

(Yea,Still an Inliner)

[RB26powered74zcar]

Kevin.... this may be common stuff to alot of you guys out there, but to me to see someone write & completely understand things like you just passed on to jack46, is a wonderfull gift ya got their bud!!

 

Sorry I have nothing to offer but a kudo

I only wish I had those talents...

 

joel

[jack46]

Thanks Kevin

 

I didn't like the way you did part of it but answer is the same I used logs to get rid of exponent but nice litttle trick doing it your way will need to look at that again, but the answer is the same so your way is also right.

 

Thanks again

[jack46]

I used the formula on a couple motors on engine analyzer and they don't agree. I have a feeling this formula is maybe a 10% swag which is fine. From the engine analyzer I have it seems to be about 8% off. Thanks again is interesting reading

 

Jack

[Guest Phil1934]

Nice explanation, but why did you calculate the intake vacuum in psi if you did not use it again?

[Kevin Shasteen]

Thanks Joel, but after looking at what you did to your car on your site I would say your restoration to your car was much harder than my learning the math involved to an engine...its just a choice-both involve extreme perseverance.....Very nice car you got there. If I had your car I would not be embarrassed of still being an inliner.

 

Jack,

 

About the 8-10% off thing; welcome to theory I believe this is where you have to understand that no matter where you get your data there will always be some level of tolerance built into that paper calculations. Even if you paper calc's are done on a nice engine sim as the one you have. Paper theory calc's are designed to get you in the ball park and from there the R&D takes over: the better your data-the less R&D required to get you that much closer to your objected goal. I was looking at the magazine rack last night and I think it was "Hot Rod" that lists 10 or 20 ways to calculate the power of your engine. They rated each approach on a scale from 1-5. They gave engine sims a 3, 4.5 depending on which program you are utilizing...so as you can see, even the very best engine sims are not perfect. Somewhere along the way the car will have to jump off the chalk board and take to the street for a little R&D. Just my opinion-FWIW.

 

Phil,

 

About the Intake Absolute Pressure statement-I guess I was reading too many other posts on HybridZ last night as I know I read on some thread someone talking about Intake Vacuum and at the time I put it in this thread I didnt realize it was not pertinate to this specific thread...sorry for the confusion.

 

BTW: I think it is also important to remember when dealing with Crank Angle Charts for the sole purpose of building your engine in such a way that your DCR & SCR compliment one another that David Vizard states a hot street engine can get away with a 7.5:1 to 8.0:1 DCR and for a dedicated race engine you would need an 8.8:1 to 9.0:1 DCR. Just something I forgot to mention in the previous post.

 

Kevin,

(Yea,Still an Inliner)

[Guest Phil1934]

I'd forgotten I bookmarked this site

http://www.rbracing-rsr.com/comprAdvHD.htm

It calculates DCR and cranking compression and allows for boost and ambient air pressure. I ran 3 cars through I'm building and was surprised. A 10.5:1 455 Olds in a '39 Olds had a DCR of 7.4 with a 79* IVC. A mild 9.7:1 350 I'm putting in my nephew's 260Z has a DCR of 8.1 with 60* IVC and a blown Mercruiser 470 in a Model Y for myself with 8.5:1, and IVC of 66* and 10 PSI has a DCR of 11.33!

[Kevin Shasteen]

Phil,

 

Pretty cool when the numbers start talking to you-dont you think? 8)

 

Jack,

 

Something else I forgot to mention about engine sims-at least it is something I have discovered: I dont know about the more expensive simulations but the cheaper simulations will usually ASSUME your Intake Manifold Absolute Pressure is "Optimum" when they make their calculations. So when we build our combo's in some software-just be aware if the intake ports are being assumed prior to plumping your money down for the actual hardware...again just my .02c's worth on what I have noticed on the 1st & 2nd generation Desktop Dyno's.

 

In the real world if you mismatch your DCR & SCR along w/mismatching your Intake Manifold Cross Sectional Ports then Airflow Velocity is severely compromised) and your engine will not run that great at low rpm's or not at all w/out an idle in the 1000-1500rpm range. For the track this is okay but for a street engine/daily driver this is a troublsome characteristic to put up with in stop/go rush hour.

 

Q: Does Engine Analyzer assume a perfect/near optimum intake port pressure when calculating enigne output?

 

From what I have discovered, providing intake manifold pressure is optimum, the relationship between your DCR & SCR will determine the rpm range where your peak tq/hp surface. The closer your DCR is to an 8.5:1 the sooner your tq/hp peak numbers will enter the picture while the more your DCR is minimized from an 8.5:1 DCR into the 7.5:1 DCR the further up the rpm range your tq/hp will be delayed prior to surfacing. The further your DCR is compromised from down from the 7.0:1 into the 6.5:1 DCR and lower the closer you are entering detination territory as well as ensuring a 1000-1500rpm idle: which would require a more "dedicated race engine" DCR between 8.8:1 to 9.0:1 to maximize your compressions (maximize your engine's output)....according to Vizard-that is.

 

Kevin,

(Yea,Still an Inliner)