Would anybody care to help me figure out my dynamic compression and cranking psi? Ive figured it with different calculators and get very different answers.

297 at .015

@.050

Int open 24 btdc close 56 abdc
Ex open 63 bbdc close 23 atdc

I havent degreed the cam, but have notes the previous owner wrote down. Maybe they will help..

Intake lobe

.133 @ tdc
.394 @ 107 atdc
0 @ tdc

24 btdc @.050
15 atdc @ .200
44 atdc @ .300
168 atdc@ .300
196 atdc@ .200
238 atdc @.050

Static is 11.4. 4.375 bore, 3.915 stroke, 6.385 rod.


Thanks

Look at this site ... just plug in your numbers and
it'll give you the cranking pressure
http://www.wallaceracing.com/Calculators.htm

Mr. P,

I used that calc and came up with 9.46. If I use the KB calc I get 8.68??

@.050

Int open 24 btdc close 56 abdc
Ex open 63 bbdc close 23 atdc

duration @50 int 260 degrees ex 266 degrees
LSA 108 degrees
ICL 106 degrees

If the advertised intake duration is 297, the exhaust duration must be 303 degrees

advertised
Int open 42.5 btdc close 74.5 abdc
Ex open 81.5 bbdc close 41.5 atdc

Using the wallace calculator and IVC 74.5 degrees atdc gives a dynamic compression ratio of 8.36:1.

[quoteUsing the wallace calculator and IVC 74.5 degrees atdc gives a dynamic compression ratio of 8.36:1.

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8.36 should be safe with pump 91, correct? Quench is .040, alum heads.

Thanks!!!!!

I use this one because it allows additions for boost and altitude. For me, altitude is a big factor. With your specs and 56 abdc, I get 9.66 at sea level. Change that to 74.5 atdc and I get 8.36.

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

Different calculators will give way different answers. This is why I struggle with what the "right" number to shoot for when you can get 4 different results.

There is a site that has the EXACT same calculator as the Wallace one, and tells you to specifically use the 0.050" number. Some say add xx° to your 0.050" ABDC number. There are others as well.

FWIW, the Wallace calc is off to my actual results. It is very very close if you use the 0.050". It makes no sense to me.

Also, mine is on the edge of detonation and I have a lower static than you, and a later intake closing. I'm at 1200 ft. My Wallace number is 7.4.

i made myself a little crazy worrying about dynamic c.r., since the most important parameter (duration) is, for all practical purposes, a judgement call. i don't worry anymore. i just select cam and compression based on common sense and experience anymore.

I like this one plug in your specs and @.050 (56 ABDC??) numbers 0 for boost
http://www.rbracing-rsr.com/comprAdvHD.htm

9.5 ish Dynamic cranking 195 ish on the edge of 91 93 should be fine assuming aluminum heads!

regardless of what the calcs say....IMO, 11.4cr, that cam, and 91 octane is going to be "iffY'....at best.
especially in Oklahoma summer heat.

IMO it's natural to use the seat timing, since that's when you actually start building pressure.

But, you have to carefully check what input data the calculator is expecting, wrong data in and the output will be incorrect.

I have made my own calculator, and cannot comment on different web-based calculators.

What should the DCR and cranking compression be for the different grades of today's gasoline (87, 89, 91 octane)?