Lets say I wanna run a 12lb ATI Procharger...I have heard that LS1 compression ratios are not blower friendly.....so out of curiousity where would I want the compression ratio to be and how would you get it there? Thanks.

 

Stock motors will take 6-7lbs of intercooled boost ok. Any more than that, you'll need different heads and/or re-worked forged internals. 9:1 would be my guess as a place to start. SLP will be (or already is) selling crate motors with that compression ratio for forced induction applications.

 

I've read that stock bottom end will take 8psi but I'd probably run 7 to be safe. As far as the compression you'd want to run high boost it should be between 8-9:1

 

Not more than 9:5 to 1. Also as a side note I do not think "aluminium" blocks should be supercharged, maybe I have old school thinking, however a cast iron block is perferred.


SteveC

 

Tell that to NHRA & the Top Fuel/Funny Car boys.

 

Don't get hung up on boost numbers. Just as in N/A, it is all about airflow. You can make a ton of boost by shoving a potato up your exhaust pipe, but you won't make any power. Decide how much power you want to make, then make adjustmenst to your setup to get there. Forced induction power is made by flowing more air than the engine is capable of flowing naturally aspirated. Boost is resistance to forced airflow. As stated earlier, you can safely run 7-8 psi on an all stock motor. This is based on a given pulley setup designed to spin a s/c to a certain rpm which will result in an output of x cfm. You can add headers and a free-flowing exhaust and make more power, but less boost even though the s/c is still spinning the same rpms and flowing the same cfm. Why? You have reduced an airflow restriction, improved the volumetric efficiency, and increased cylinder pressure, which makes more power. Now you add a cam and free-flowing heads. Again, you make more power, but boost has now dropped to 4 psi even though the s/c is still spinning the same rpm. Again, improved efficiency. So, you change pulleys to spin the s/c faster. You make more power, but boost is back up to 8 psi because you have the same restrictions you had at 4 psi but more air being forced into the intake. You also have more cylinder pressure. So watch out. 8 psi was safe on the all stock motor because you were only creating x amount of cylinder pressure. Now you are still only making 8 psi, but cylinder pressure has increased substantially and the bottom end might not hold. Remember this is a hypothetical situation and is for illustration purposes only. I am not saying if you add headers, cam, and heads, and spin the s/c until you make 8 psi you will blow your motor, I am just saying you can't judge anything by boost numbers. They are a poor benchmark of power.
Check out my sig. I am making 550+ rwhp at only 6psi. This is because my engine flows very well. I promise you I am making significantly more cylinder pressure than a stock motor at 8 psi. Hence the forged internals. Of course the bigger cubes makes a big difference, especially in the torque numbers. I plan to pulley up in the near future to spin the s/c faster. This will result in more cfm, more power, and probably higher intake air temps and boost numbers. I then have to find where my restriction is to continue to flow the same amount of air, but at a lower boost level. High boost numbers are not impressive, they only mean your engine is not flowing good enough to handle the amount if airflow the s/c is creating. High boost is hard on everything; intake, plenum, intercoolers, tubing, and the s/c itself. A good way to reduce high boost numbers is to reduce static compression ratio. This can be done by installing lower compression pistons or larger combustion chambers. If you are seeing high boost numbers and decide you need to reduce static compression ratio and your bottom end is already stout, one of the best ways is to get some heads with larger chambers that have been optimized for forced induction. For example, my heads from TEA have very good flow numbers and my cam is optimized to take advantage of those numbers. If you are interested in this dynamic, just ask, but this post is already getting too long. The larger chambers and free-flowing exhaust ports are a big reason why my power is up and my boost is low. My static compression ratio is about 9.88:1, only slightly less than stock. When all is said and done, I expect to make 650 rwhp at 10psi on pump gas. I know a guy who made 822 rwhp on pump gas, but he was spinning his s/c fast enough to make 18 psi on an already free-flowing engine. Needless to say, it didn't last, but it could have lasted longer with a little better tune and if the compression ratio had been dropped enough to improve the volumetric efficiency. Don't worry about the strength of these blocks, they will take all the power you want to throw at them on the low side of 1000 rwhp. Sure some guys have blown them up but they had other problems like detonation, excessive rpms, excessive cylinder pressure, and heat build up. Again, you can safely run 8 psi on an all stock motor, even more with a conservative tune, and make upwards of 500-525 rwhp, but you better start thinking about a forged bottom at that point, not to mention drivetrain upgrades.

Getting back to your original question, a stock compression ratio of 10.1:1 is FI friendly, but only to the point where cylinder pressure and heat becomes excessive and detonation occurs. Detonation can be staved off by increasing octane, reducing compression, cooling the intake charge, running a richer air/fuel ratio, and decreasing timing, or combinations of these, all to varying degrees. A lower compression ratio will allow you to flow more air before it reaches the same cylinder pressure and detonation point, but a lower compression ratio is less efficient out of boost than a higher compression ratio. So it still boils down to how much power you want to make and what you want to use your car for. We know what all stock can handle. All forged stock cubes/compression can easily take 12 psi, more with high octane, alky/h20 injection, but again, it is way better to run less psi and flow more air. Personally, for a street driven car, I think it is better to reduce the compression a bit with a set of FI optimized free-flowing heads with larger chambers and get a good blower cam that is optimized to perform to the head flow numbers. Then, if you later decide FI is way too expensive, you can sell off the s/c kit, cam, and heads, pop on some nice milled 5.3s, get cam optimized for the new heads, and go nitrous for a while. It all depends on what you want out of your car.

Sorry so long, but I tend to get long winded when I am tired.

 

Less than 10:1 on a street car.

Mike

 

Depending on engine size, boost levels, and type of power adder it really varies.

Anywhere from 9:1-9.5:1 on a 346ci Street motor that will be supercharged or turbocharged.

Other applications like the Magnacharger like a higher compression like 10:1-10.5:1, overall it really depends on how the pressure is being applied.