Dynamic vs Static Compression

[DeWynter]

Can someone explain this fully to me. I have a static compression of 11.25:1(aprox) and a dynamic of 9.7:1 Will this be fine on premium or will I be retarding the timing big time, or having to buy 110 octane gas? With the head change looming so close I want to make sure Im ok.
Full engine specs:
1970 350 4-bolt main
400 crank
Flat top two valve relief pistons 0.016 in the hole
57cc aluminum heads 1987 vette ported and re-valved to 2.0/1.6
Super Ram intake 1000CFM TB
LT4 Roller Rockers 1.6
Comp Cams 260/260 Retro roller cam 206/206 @ .05 and .5 lift intake and exhaust
1, 5/8 hedman headers 3" collector

[fastTa]

Can someone explain this fully to me. I have a static compression of 11.25:1(aprox) and a dynamic of 9.7:1 Will this be fine on premium or will I be retarding the timing big time, or having to buy 110 octane gas? With the head change looming so close I want to make sure Im ok.
Full engine specs:
1970 350 4-bolt main
400 crank
Flat top two valve relief pistons 0.016 in the hole
57cc aluminum heads 1987 vette ported and re-valved to 2.0/1.6
Super Ram intake 1000CFM TB
LT4 Roller Rockers 1.6
Comp Cams 260/260 Retro roller cam 206/206 @ .05 and .5 lift intake and exhaust
1, 5/8 hedman headers 3" collector

Your dynamic compression ratio combines both the values used to obtain static compression ratio and then also factors in the timing of your intake valve in relation to to crankshaft degrees (specifically the number of degrees ABDC). Or the IVC (intake valve closing point). This is important because the a/f in the cylinder does not begin to compress until the intake valve is closed. Since 99.999% of all internal combustion motors will close the intake valve after BDC (ABDC) and thus effectively shortening the stroke in relation to the beginning of actual compression, the DCR will always be lower than the SCR. Most performance motors will not close the intake valve until approximately 45-80 degrees ABDC so this basically takes alot of the stroke that is used to calculate the SCR.

Motors that have a high duration/lift cam operating at low RPM will suffer a loss of low end torque torque due to the fact that the effective compression ratio is reduced by the late IVC. Ideally the intake valve will close just before the incoming air stops and reverses direction. This guarantees that the maximum amount of air/fuel mixture has been drawn into the cylinder prior to ignition. This is when people talk about the cam "coming on". In order to ensure that the a/f is still compressed sufficiently over the reduced effective compression stroke it is necessary to increase the static compression ratio in order to make up for it. This is why high performance engines with aggressive camshafts usually tend to need higher static compression ratios than normal.

Given this, it is much more beneficial to base the ability of your motor to effectively run on pump gas based on DCR rather than SCR.

You will be fine on pump gas. My 383 LT4 motor was right around 12.2 SCR and it ran fine with no problems on 93 octane pump gas.

[Jay 02 TA ws6]

Your dynamic compression ratio combines both the values used to obtain static compression ratio and then also factors in the timing of your intake valve in relation to to crankshaft degrees (specifically the number of degrees ABDC). Or the IVC (intake valve closing point). This is important because the a/f in the cylinder does not begin to compress until the intake valve is closed. Since 99.999% of all internal combustion motors will close the intake valve after BDC (ABDC) and thus effectively shortening the stroke in relation to the beginning of actual compression, the DCR will always be lower than the SCR. Most performance motors will not close the intake valve until approximately 45-80 degrees ABDC so this basically takes alot of the stroke that is used to calculate the SCR.

Motors that have a high duration/lift cam operating at low RPM will suffer a loss of low end torque torque due to the fact that the effective compression ratio is reduced by the late IVC. Ideally the intake valve will close just before the incoming air stops and reverses direction. This guarantees that the maximum amount of air/fuel mixture has been drawn into the cylinder prior to ignition. This is when people talk about the cam "coming on". In order to ensure that the a/f is still compressed sufficiently over the reduced effective compression stroke it is necessary to increase the static compression ratio in order to make up for it. This is why high performance engines with aggressive camshafts usually tend to need higher static compression ratios than normal.

Given this, it is much more beneficial to base the ability of your motor to effectively run on pump gas based on DCR rather than SCR.

You will be fine on pump gas. My 383 LT4 motor was right around 12.2 SCR and it ran fine with no problems on 93 octane pump gas.

I understood that!!!!

[fastTa]

I understood that!!!!

I knew you would come out of the closet!! LOL

[Jeff_in_Atl]

I knew you would come out of the closet!! LOL

Jay's coming out of the closet?

[DeWynter]

Your dynamic compression ratio combines both the values used to obtain static compression ratio and then also factors in the timing of your intake valve in relation to to crankshaft degrees (specifically the number of degrees ABDC). Or the IVC (intake valve closing point). This is important because the a/f in the cylinder does not begin to compress until the intake valve is closed. Since 99.999% of all internal combustion motors will close the intake valve after BDC (ABDC) and thus effectively shortening the stroke in relation to the beginning of actual compression, the DCR will always be lower than the SCR. Most performance motors will not close the intake valve until approximately 45-80 degrees ABDC so this basically takes alot of the stroke that is used to calculate the SCR.

Motors that have a high duration/lift cam operating at low RPM will suffer a loss of low end torque torque due to the fact that the effective compression ratio is reduced by the late IVC. Ideally the intake valve will close just before the incoming air stops and reverses direction. This guarantees that the maximum amount of air/fuel mixture has been drawn into the cylinder prior to ignition. This is when people talk about the cam "coming on". In order to ensure that the a/f is still compressed sufficiently over the reduced effective compression stroke it is necessary to increase the static compression ratio in order to make up for it. This is why high performance engines with aggressive camshafts usually tend to need higher static compression ratios than normal.

Given this, it is much more beneficial to base the ability of your motor to effectively run on pump gas based on DCR rather than SCR.

You will be fine on pump gas. My 383 LT4 motor was right around 12.2 SCR and it ran fine with no problems on 93 octane pump gas.

I thank you for your info. I have been trying to get this info from other sources but it seems like everyone around here wants to keep it a secret. Or else they just arent really taking DCR into acount when building motors. I figured this was how it worked but just wanted to be sure.

[fastTa]

Jay's coming out of the closet?

He has a membership to the 4 bangers club!

[Jay 02 TA ws6]

He has a membership to the 4 bangers club!

Clean out the PM box deadbeat!

[fastTa]

Clean out the PM box deadbeat!

Done. Re-send it.