[Formulabruce]

Originally Posted by Formulajones

I didn't go with the allowable 1.5 compression bump

Block is zero decked.

Heads are bone stock 1970 ram air III castings (no porting allowed) They were milled .010 to ensure flatness and a good seal. Afterwards they CC'd at 74. Compression came in at 10:1.
Camshaft is installed advanced to increase cylinder pressure and give me a slight boost in low end torque (you'll see why in a minute)

Breaking this down a bit....having done this same thing before.... Don't take offense, just trying to justify your results..
1. Seems the CR did get bumped.. which IS logical in this build.
2. Decking the block , Read below a bit from Jim hand on how Both decking and lowering timing because of it, can and does change the CR.
3. Heads. Stock heads are 72 cc, how does one slice off 10K and get 74 cc?
4. Advancing cam to "increase cylinder pressure" >> Fact>> Does Increase the CR....
Im NOT being critical, just stating that what you did do had a huge effect on performance which deviated alot from stock. ( I do like it, I did it, and I didnt like the CR at "dynamic" of 10.5) but I wasnt drag racing it much.
Read some of Jim Hands comments on block decking and its effects...
Quote >>Jim Hand "Deck Height or Deck Clearance:

This most important step consists of measuring the distance from the assembled piston tops to the surface of the block deck (deck clearance), and milling as necessary. The general feeling is that the total quench or squish distance should be about .040". The quench distance is the compressed thickness of the head gasket plus the deck clearance. As most of our Pontiac head gaskets compress to about .042", that means we want about 0 deck clearance. The quench area is the flat part of the piston that would contact a similar part of the head if you had .000 assembled quench height. In a running engine, the .040 quench height decreases to a close collision between the piston and the cylinder head. The shock wave from the near collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, averages the chamber temperature, reduces detonation and increases power. The shock wave also provides better fuel/air mixing, and this allows the fuel to ignite better and burn faster. A faster burning fuel charge means less timing is required for optimum power output. An example of this--after running my 462 for years with a factory deck height of about .020, we set the deck to 0. There were no other significant changes to the engine (new rings and bearings, but same cam, heads, intake and exhaust systems). The optimum timing setting prior to the change was 34 degrees - that provided the fastest MPH and quickest ET. After the change to 0 deck, the optimum timing using the same Amoco gas changed to only 30 degrees total mechanical. Not only did the lowering the deck RAISE THE CR by several tenths of a point, but by retarding the timing 4 degrees, we were later able to INCREASE the CR even higher due to the optimum lower timing setting. " END Quote<<

[Formulajones]

Originally Posted by Formulabruce

Breaking this down a bit....having done this same thing before.... Don't take offense, just trying to justify your results..
1. Seems the CR did get bumped.. which IS logical in this build.
2. Decking the block , Read below a bit from Jim hand on how Both decking and lowering timing because of it, can and does change the CR.
3. Heads. Stock heads are 72 cc, how does one slice off 10K and get 74 cc?
4. Advancing cam to "increase cylinder pressure" >> Fact>> Does Increase the CR....
Im NOT being critical, just stating that what you did do had a huge effect on performance which deviated alot from stock. ( I do like it, I did it, and I didnt like the CR at "dynamic" of 10.5) but I wasnt drag racing it much.
Read some of Jim Hands comments on block decking and its effects...
Quote >>Jim Hand "Deck Height or Deck Clearance:

This most important step consists of measuring the distance from the assembled piston tops to the surface of the block deck (deck clearance), and milling as necessary. The general feeling is that the total quench or squish distance should be about .040". The quench distance is the compressed thickness of the head gasket plus the deck clearance. As most of our Pontiac head gaskets compress to about .042", that means we want about 0 deck clearance. The quench area is the flat part of the piston that would contact a similar part of the head if you had .000 assembled quench height. In a running engine, the .040 quench height decreases to a close collision between the piston and the cylinder head. The shock wave from the near collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, averages the chamber temperature, reduces detonation and increases power. The shock wave also provides better fuel/air mixing, and this allows the fuel to ignite better and burn faster. A faster burning fuel charge means less timing is required for optimum power output. An example of this--after running my 462 for years with a factory deck height of about .020, we set the deck to 0. There were no other significant changes to the engine (new rings and bearings, but same cam, heads, intake and exhaust systems). The optimum timing setting prior to the change was 34 degrees - that provided the fastest MPH and quickest ET. After the change to 0 deck, the optimum timing using the same Amoco gas changed to only 30 degrees total mechanical. Not only did the lowering the deck RAISE THE CR by several tenths of a point, but by retarding the timing 4 degrees, we were later able to INCREASE the CR even higher due to the optimum lower timing setting. " END Quote<<

Actually, my compression ratio is lower than "advertised", which is key when looking at the rules. Since these engines never actually had their "advertised" compression ratios in reality, which I explained above. That's what I was trying to get accross. I could have built a 12.25:1 engine if I wanted to, but I chose to keep it pump gas friendly. I could have done all the tricks we are talking about starting with a 12.25:1 engine and been within the rules. I gave up about 8-10% of potential HP with this engine, but,,,I'm not running $8 a gallon gas either.


Zero decking the block, which I do on all my builds, does raise compression, however my main reason is for tighter quench, which as you read Jim Hand touched on. That tight quench promotes swirl and mixture in the chamber which fights detonation. Again detonation is my main concern since this was a pump gas motor.


Not all stock heads will CC at 72. I've been over this before in past posts. They vary give or take a few CC's. 72 is just a number that GM tossed out but I've found with many castings that this number is not set in stone. Mine actually CC'd at 76 after new seats and valve job done, with an extended tip spark plug. Valves were not sunk (that hurts flow ) Then they were milled .010 and CC'd again, came in right at 74CC. This actually worked out in my favor however to keep my compression ratio down a tad, because the overbore and deck bumped it slightly.


When talking compression we have to talk about dynamic and static. Advancing my camshaft didn't increase the static (or advertised) compression ratio. What advancing the cam does is change the dynamic's of the engine, or increasing dynamic compresson (creating more cylinder pressure) by closing the intake valve sooner. It's not exactly the same thing but they do affect how the engine runs. I'm more or less changing the power band of the engine to put it in simple terms. It's just a tuning tool.