"Higher" Compression Question

[DSGCOBRA03]

When your at the stock 8.5:1 base compression, generally the highest boost level you want to go to on pump gas (91 in my area) is around 17-18lbs. If I were to put 9.7:1 pistons in, what would be the highest boost level safe on 91 pump? (This would be on a twin screwed engine.)

 

[Midnight_Cobra]

+1

I'm in the same boat as you.

 

[cozmo2806]

Probably 12-14 PSI

 

[Quick Strike]

You will bring the dynamic compression ratio (DCR) above 8.6:1 if you use a static compression of 9.7:1 with the stock 03/04 Cobra cams degreed in their stock position. That DCR is too high for a pump gas engine and will detonate easily on pump gas. Pump gas engines run best between 7.5 and 8.0:1 DCR. You will need to consider cam profiles that take advantage of the higher static compression and produce a lower DCR if you want to use that much static compression (9.7:1). The advertised intake closing point is a major consideration in determining the DCR with respect to static compression.

 

[SlowSVT]

You will bring the dynamic compression ratio (DCR) above 8.6:1 if you use a static compression of 9.7:1 with the stock 03/04 Cobra cams degreed in their stock position. That DCR is too high for a pump gas engine and will detonate easily on pump gas. Pump gas engines run best between 7.5 and 8.0:1 DCR. You will need to consider cam profiles that take advantage of the higher static compression and produce a lower DCR if you want to use that much static compression (9.7:1). The advertised intake closing point is a major consideration in determining the DCR with respect to static compression.

I'm a little confused with this post. The static compression of the Terminator is 8.5:1. (unit of combustion chamber volume divided by piston sweep volume). Dynamic compression takes into account the valve timing which will always be lower since the valves opens before TDC and closes after BDC. Dynamic compression numbers will always be a bit muddled because they don't take into account the charge velocity and volume entering in and out of the cylinder head which is governed by a variety of factors. I suppose you could just do the math regarding the valve timing but the number you will arrive at will be somewhat meaningless since there are so many other factors at play.

To the OP

While raising the (static) compression in theory will increase the power output in a supercharged engine but what that does is limit the volume of charge that can be packed into the cylinder head before detonation occurs. The amount of heat energy in the charge is fixed (gasoline is about 18,000 BTU's per pound). The maximum power that can be obtained will be achieved just before detonation occurs. In a supercharged engine you can actually increase the amount of power produced by lowering the static compression allowing you to pack a higher volume of charge into the engine compared to a high compression FI engine. Some people get a bit confused by this as I have learned so I made a diagram to help simplify the explanation.

Keep in mind I exaggerated the figures in the diagram to make it easier to comprehend and they don't represent what would be typically used in our engine

I know this is not the question you were asking. Running 17-18 lbs of boost on an 8.5:1 4.6 engine with 91 octane rated fuel is over the limit the fuel will tolerate and it's almost certain you will blow your engine. Raising the compression ratio to 9.7:1 will limit the boost level the fuel will tolerate even further (I think they limit the boost on a 10:1 4.6 engine to around 8 lbs). 15-16 is as far as I would go with 8.5:1 running 91 octane but your ignition timing will also be a factor. If I were you I would keep your engine at 8.5:1 and allowing the engine to ingest a higher volume of charge for reason I explained above.

 

[Quick Strike]

I'm a little confused with this post. Dynamic compression numbers will always be a bit muddled because they don't take into account the charge velocity and volume entering in and out of the cylinder head which is governed by a variety of factors.

This should help clear it up for you a little (from a very good DRC article - read it this time): "Dynamic compression ratio should not to be confused with cylinder pressure. Cylinder pressures change almost continuously due to many factors including RPM, intake manifold design, head port volume and efficiency, overlap, exhaust design, valve timing, throttle position, and a number of other factors. DCR is derived from measured or calculated values that are the actual dimensions of the engine. Therefore, unless variable cam timing is used, just like the static compression ratio, the Dynamic Compression Ratio, is fixed when the engine is built and never changes during the operation of the engine."

 

[SlowSVT]

This should help clear it up for you a little (from a very good DRC article - read it this time): "Dynamic compression ratio should not to be confused with cylinder pressure. Cylinder pressures change almost continuously due to many factors including RPM, intake manifold design, head port volume and efficiency, overlap, exhaust design, valve timing, throttle position, and a number of other factors. DCR is derived from measured or calculated values that are the actual dimensions of the engine. Therefore, unless variable cam timing is used, just like the static compression ratio, the Dynamic Compression Ratio, is fixed when the engine is built and never changes during the operation of the engine."

I never stated anything about cylinder pressure when inquiring about your post (that was addressed to the OP). As a matter of fact I'm even more confused by this response. No need to clear anything up for me. I'll take your word for it.

Russ :beer: