Calculating Dynamic Compression Ratio...

[Jim V.]

Using the Keith Black software (http://kb-silvolite.com/calc.php?action=comp) to compute static and dynamic compression ratios and am confused by a rquired entry field.

"Intake Closing Point (degrees)ABDC @ 0.050 lift plus 15 degrees".

I am using a 929 cam with the following specs... Duration 195/202 degrees, LCL = 112 degrees, and .390/.410 lift. Where would I get the intake closing point (.050 lift) ABDC for this or any cam?

The old gray matter can't seem to get a grip on this one..

Thanks
JimV

[Scott Marzahl]

Re: Calculating Dynamic Compression Ratio...

I'm sure Duke has the opening and closing points....Duke????

[Scott Marzahl]

[Duke W.]

Re: Calculating Dynamic Compression Ratio...

The information you have is insufficient to make the calculation. I don't know why these cam vendors won't provide at least a full set of high level specs. Knowing duration and LSA doesn't provide enough data to determine cam INDEXING to the crankshaft. In order to determine this you need at least one point of maximum lift and LSA or both points of maximum lift.

The points of maximum lift for the 929 cam are 108 ATDC for the inlet and 116 BTDC for the exhaust. The LSA in cam degrees is simply the arithemtic average of these two numbers, or 112.

So if the inlet is at peak lift at 108 ATDC and the .050" lifter rise duration is 194 (my number based on the engineering lobe data that I have) the lifter will be back at .050" rise on the closing flank at 108 + 97 = 205 ATDC, which is past BDC, so subtract 180 from 205 and you get the inlet .050" lifter rise closing point at 25 ABDC. It's ACTUALLY 26 because the lobe is asymmetrical, so use 26 as the actual .050" closing point in the calculation.

The .050" lifter rise timing points for this cam are (12)-26/37-(15). Timing points are usually specified as inlet open degrees BTDC - inlet close degrees ABDC/ exhaust open degrees BBDC - exhaust close degrees ATDC. In the above specs, parentheses are used to indicate a NEGATIVE number. For example, inlet .050" lifter rise opeing is achieved at 12 degrees ATDC, which is the same as NEGATIVE 12 degrees BTDC.

Duke

[Jim V.]

KB Compression Software Results...

The Keith Black compression calcultor gave the following results for my 300hp, OE cam, OE carb/intake/exhaust/heads, and using the Hypereutectic KB156 and other nominal data (composite head gasket). I used the 26 degree ABDC intake valve closing spec plus the 15 degree as directed by the software for the dynamic compression computation. YES DUKE...I will get dead nuts deck height prior to disassembly! At this point I am trying out the software with a few potential piston solutions.

Looks like the static is a tad low at 9.3 (target 9.5)...but the static a tad high at 8.5(target

[Jim V.]

oops..previous msg clipped short...

Entered data for OE everyhing except..Felpro comp.head gasket & KB156 pistons:

Head volume 62cc
Piston Head Vol 7cc
Gasket thickness .038
Gasket bore 4.166
Cyl bore 4.030
Deck Clearance .022 (KR 156= 1.678 minus OE 1.671)
Stroke 3.25
Rod Length 5.7
Intake Closing Point ABDC @ .050 lift plus 15 41 (26 + 15)

Concerned with a potentially doggy perf with the 9.3 SCR and preignition issues with the 8.5 DCR....what cha think

Details...details...details...that what this hobby is about right?

Thanks
JimV

[Scott Marzahl]

Re: oops..previous msg clipped short...

Looks like someone is having fun! You can see what a few thousand's difference makes. You have several options for head gaskets to play around with too. You can punch in a graphite/Steel Corteco .028" or SS shim gasket at .018" thick and see what the compression difference makes. Your actual exisiting deck height is going to vary from your calculated too.

Also when you cc your heads for an example, they may end up being 64 CCs area and if you then select say a KB157 piston for calculating purposes, with a MLS head gasket of .041" thickness and a deck height that in reality may end up being closer to .030", your SC ratio will be in the 9.6 ballpark. Isn't math fun!!

[Duke W.]

Re: oops..previous msg clipped short...

I'm assuming the piston volume is actually "negative" as it adds to chamber volume because the crown is flat with valve notches. Most CR calulators assume you are inputing net piston dome volume, which subracts from chamber volume, so a flat top notched piston volume is input as a negative number. Be sure you understand the sign convention of the calculator you are using.

DCR can vary because there is no "standard" point to peg inlet valve closing.

A static ratio of 9.3 is fine, but you don't really know what it's going to be unless you measured the deck clearance prior to teardown and calculate the new clearance based on new piston compression height variation, or you can wait until the shortblock is assembled, measure deck clearance, and then select a head gasket to target a range of about 9.5. It won't be doggy at that CR.

Remember that the "advertised CRs" were typically a good half point or more higher than as actually built. On a '63 SHP/FI engines if everything is nominal the CR actually does achieve 11.25 with a shim gasket, but many were built with double gaskets which reduced CR about 0.5 and high decks typically took at least another quarter point.

I ran the nominal numbers for an early 300 HP engine (61 cc chamber, 029" deck clearance, -5.4 cc piston volume) with a .018" shim gasket, the true CR is only about 9.8, but 10.5 was advertised. I doubt if many cylinders left Flint at more than a true 9.5.

http://www.csgnetwork.com/compcalc.html

The advertised CR numbers from that era were about as realistic as the power ratings!

Duke

[Jim V.]

KB156 spec. & Dynamic Comp. Ratio

The KB156 spec shows 7cc, despite being an appearent flat top with notches. I will have to call to verify the plus 7.

The debate on the veracity of the KB dynamic calculator aside, is a dynamic comp. ratio of 8.5 problematic for pre-ignition for a OE 300hp?

Yep...my middle school mathematics teaching experience is really paying off now..

Thanks again guys!

JimV

[Duke W.]

Re: KB156 spec. & Dynamic Comp. Ratio

Any debate about the specifics of DCR is fruitless unless the context of the calculation is understood. Try the DCR calculator I referred to and compare.

A specific DCR calculation is not the be all end all.

Duke