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Discussion Starter · #1 · (Edited)
I don’t know for you but I want the most Performance for my money and considering today’s Fuel Prices and Low Octane, Dynamic Compression is becoming a BIG issue for me.

I choose a 9.6.1 Static Compression along with a Timing Control Unit thinking that I could extract the maximum amount of power from an ounce of 87 Octane Pump Fuel by selecting a compatible Hi-Perf yet Efficient Camshaft – if such a thing exist!

I’m referencing a 468cu BBC but I do not believe Cubic Inches is a factor when it comes to Fuel Octane Vs Dynamic Compression for Engine Performance.

It is my understanding for any engine, that Dynamic Compression is Inversely Proportional to the Cam Duration; such that, Dynamic Compression Decreases as the Cams Duration Increases. It is understandable that the intake valve stays open longer and therefore closes later in the compression stroke for Hi-Rev Profiles in order to charge the cylinder with more fuel and oxygen because the event decreases as the revs increase. To relieve Dynamic Compression Loss you can choose a Cam with a Shorter Duration, faster lobe ramps and Higher Lifts. Of course one performance enhancement is not complete unless accompanied by other options such as; larger valves, bigger intake runners, bigger carburetors, bigger exhaust ... etc: but this is not what I want to discuss.

I’m referring to how to choose a Cam Profile that will give you the Most Power and Efficiency given your Static Compression Ratio without Detonating or Loosing Power and Wasting Fuel by spewing it out the tail pipe because the Dynamic Compression is either Too Hi or Too Low for to days Pump Fuel.

I’m into Touring 3.08 gears behind a BBC built strong enough to handle 7 Grand but for those into Hi-Ratio Gears and Week End Drags, a Fuel Octane Vs Dynamic Compression Balance means Performance and in either case Camshaft selection is critical.

Here are some Questions that have plagued me when building an engine. I’m very interested in your experiences, solutions and opinions to what is relevant or irrelevant.

1/ How are Compression Pressure readings relative to Dynamic Compression?

2/ What is the optimum Dynamic Compression for to days Pump Gas?

3/ How much Dynamic Compression is required to turn 7000 rpm?

4/ Is there a Formula for Calculating Dynamic Compression?
 

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And hopefully you aren't locked into 9.6:1 static. You'll find that you can easily go higher.
 

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Discussion Starter · #6 · (Edited)
Well I had a go at Pat Kelly’s DCR Calculator.

Recommended Dynamic Compression for the Street is between 7.5 and 8.2

I’m running a 454 +.030”, L2465F30 + 25.70 Dome Pistons and 781 castings119cc Semi Open Chamber Large Oval Port with 2.18/1.88 Manley Server Duty Pro Flow Valves. Donovan Gear Drive(makes changing cams & degreeing easy) Jomar Girdle, Pro Mag Rockers, Ti-Trtainers and 10 degree Locks with appropriate Dual Valve springs for the Cam. A Modified 4150Hp Holley 830 cfm 80511.1 with annular Boosters on an Edelbrock RPM Air Gap and Mallory VI Ignition with Mechanical Advance Magnetic Breakerless Distributer. Morrosso 6 qt Side Kick Oil Pan and Milidon Full length Diamond Stripper. Full Tube 2" header through a 2 1/2" full length Exhaust and Flow Masters. 15"x8" steel rims on the front and 15"x10" steel rims on the back . Tires are Comp T/A's

I’m into Touring with lots of night driving so I like my 3.08 Gears.

Bore 4.280
Fel Pro Head Gaskets .039” and 4.300” Dia
Deck to Piston Distance at TDC .020”
Rod Length 6.135”
Cam installed ILC 104 Degrees

Piston to Valve Clearance Intake/ Exhaust : .100”/.150”
Possible room for more cam advance to increase Dynamic Compression and Torque.

Static Compression of Engine 9.8:1

I Calclulated 3 CAMS I have installed ILC (Intake Lobe centre) @ 104 Degrees

1/ Crane Energizer 266 H10: AD 266/266 Lift .499/.499 LSA 110

Dynamic Compression using DCR Calculator: 8.3
Some what High for to days Octane and limits advance but man what Torque
from idle to 5400rpm. Great cam for an 8:1 Static Compression Engine.

2/ Comp Cams XM278H-12: AD 278/292 Lift .564/ .566 LSA 112

Dynamic Compression using DCR Calculator: 7.97
This is one MEAN CAM. Excellent Torque and revs easily over 6000 rpm
Has an exhaust note to die for. Might run on 87 Octane if you retard timing
but likes 91 Octane the Timing advance is somewhat limited.

3/ Comp Cams 288AR-10: AD 288/288 Lift .623/.623 LSA 110

Mechanical roller with Crower Hippo 66291H-16 Rollers.
Dynamic Compression using DCR Calculator: 7.7.
I haven’t enough road time with this cam but looking forward to it. So far it seems to idle nicely at 800 rpm and pulls away without chugging in any gear. I believe this cam should handle 87 Octane given the 7.7 DCR yet handle Higher Octane with plenty of room to twist the Distributor or advance Timing.

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I guess if I wanted to up the Static Compression from 9.8:1, I could shave the heads .020" and run a thiner Head Gasket but I'm not into Drag. Long Distant Night Driving Yes. Water injection huh your right. I could always increase the advance on a Cool evening with high humidity.

I traded my 4 Series Carrier for a 3 Series so I could run 3.08's. But guess I could up the Static Compression to around 10.25. Put a 4 Series back in and setup 4.10 gears, install Square port runners and run a Cam with longer Duration and Higher Lifts and put on Street Slicks but I would have to say good by to Touring. Street Racing is not my scene but consider me on the Open Road were my Starting Line speed is between 40 and 80 mph :eek:))

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I did another DCR run on the 288AR-10 Cam. Just by installing an .018 OEM Steel Shim Head Gasket, I can up the Static Compression to 10.6:10 this will give me an 8.3 Dynamic Compression for the 288AR-10 Mechanical roller - Worth considering but I'm not that faithful in this particular gasket and I really don't want to shave .020" off the heads.

Another matter - I should have CC'd the Volume of the 781 Combustion Chamber after installing the Larger Valves. I would not be surprised if the Volume dropped from 119 to approx 114cc's. I checked with Mortec and the 781 casting spec at 118cc's for the 781 - siish!. Now I postulate that my Combustion Chamber is a min of 116cc's and with a .039 gasket my static compression would be 10.1:1; in which case, the 288AR-10 would give a DCR of 7.92 and the XM278H-12 an 8.25 DCR - This seems to be a PERFECT range but I'll never know until I CC the combustion chambers - lol

I will just have to wait until I get a chance to do a good work out with the 288AR-10 to decide whether the XM278H-12 is the cam for me.
 
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