92TripleBlack

I got the ATI procharger brochure for LT-1 motors last night. They have a graph that shows how much boost you can add to a motor in relation to its compression. I know that a high compression motor can have less boost than a lower one. I also know that its not linear. You can add much more boost than the amount lost in compression so you can add 8 lbs. boost to a 10.4:1 compression but more like 15 lbs. to a 9:1 compression. This means more air is going throught the motor and its making more power. My question is why?

The second question is they have a self contained lubricating and a cheaper non-self contained lubricating version. Is the Self Contained one as sturdy, reliable, and capapble or should I go with the oil line version? pros and cons of each please.

The last quesiton is that if I get a procharger for a stock LT1, and at a later date I decide to build the motor to work with it more by going to a lower compression stroker motor, do they have the parts and ability to go to the higher boosts cheaply and easily.
Thanks for your feedback

mn_vette

I believe I can answer your first question. The thing to think about it the combustion chamber size. If you increase the combustion chamber size you can actually fit more air particles in there(before detonation occurs), creating a bigger explosion. Increasing a combustion chamber size will in effect lower the compression ratio. This is done by dishing the pistons or working the heads both will work.

Since detonation occurs when the gasses are compressed, I believe with less compression there is a smaller chance of detonation, then you are able to increase the boost more than proportionally





[Modified by mn_vette, 2:45 PM 10/1/2002]

Bruce

I got D1-SC self contain is simply for easy install without running T for oil line etc. Self contain is better because it aslo run cooler. To change the SC oil simply buy the vaccum pump to pump the old oil out that is. It also have a drain plug but it impossible to put a socket ot wrench underneat to loosen it.


Bruce
93LT1 Conv w/ZF6
Blown D1-SC/N2O
ARE built 383ci/Stage II heads

Chefskeez6

I can definetly help with your understanding of boost. Compression ratio is defined by the volume of the combustion chamber + (thickness of the head gasket X PIE X radius^2) + the volume of the cylider when the piston is at the bottom of its stroke, this is ur swept volume. Then you need the volume of your combustion chamber and head gasket when the cylinder is at the top of its stroke, the combustion chamber volume won't change unless you have domed pistons which your stock LT1 won't have. You divide the total vulome when the cylinder is at the bottom of its stroke by the total volume when the cylinder is at the top of its stroke.

Now that we understand how we calculate compression ratio understand why the cylinder fills up with air. Even without getting into the complications of valve opening sequences its simply air pressure. At sea level i think it is some where around 14.7 PSI. A super charger is adding a boost to the air pressure. This is why people living at higher elevations can get away with running higher boost. The more air you can cram into the combustion chamber the more power you can make.

The art of jockeying compression ratios and boost level is difficult. IT involves a lot of math, trial and error, and complex fuel air mapping. Some engine builders wil even lower compression to something like 7 to 1. Lowering compression allows for high levels of boost but makes the engine a dog on the low end when its not under boost.

A formula to help calculate boost levels

final compression ratio = ((max boost + 14.7)/14.7) X the built compression ratio of the motor. PLease don't use this formula to make any real decisions its from memory, i'll see if i can find it in a book later and post again.

safe levels of final compression ratio for centrifugal style blowers on 92 octane gas is about 13.5 to 1. 13.5 to 1 may seem to high but it is considered safe because you achieve maximum boost at higher rpms when the engine is under less load.

Realistically on a stock LT1 motor i would run absolutely no more than 6 lbs of boost. More can be achieved with a stronger short block.

I hope this answered some of your question.
Oh yeah and to change boost levels i think you just need to change the pulley size on the blower.

Chefskeez6

the more i look at that formula the less correct it appreas i will find the correct formula tonight and post it.

Light84vette

Isnt the amount of power/14.7=X. Then 14.7+Boost pressure times X= power or hp or torque which ever one your looking for. For exapmle a 300hp engine with 10psi would come out to about 500hp. But the question I want to know is how much boost can you run with 10.5 C/R and 8.5 C/R. ;)

92TripleBlack

According to Procharger, a 10.4:1 LT1 motor can hold 6 LBS of boost, 8 LBS if you intercool it. A 9:1 can hold more like 14 LBS of boost.

gcrouse

When calculating effective CR of a supercharged engine, the combustion chamber at TDC is always a fixed volume. At BDC, the cylinder will hold a certain volume of air at standard atmospheric pressure (14.7psi). By adding say 7.35psi of boost, we are putting 50% more air in that cylinder, so when you divide by the combustion chamber volume, the effective compression ratio is also 50% higher. With 14.7psi boost, you are double the amount of air and doubling the effective compression ratio. Intercooled systems can usually run an ECR of 16:1 if tuned properly. This of course is a perfect scenario ignoring volumetric efficiency, quench effects, charge intercooling, etc, etc, etc.

Second question- definitely get a self-contained model. I would say they are waay more reliable; they just added an internal pump to spray oil on the bearings/gears instead of relying on engine oil pressure. No more lubricating with hot engine oil with acids and crap in it. And you're not taking oil outside the motor which is always good.

As far as compression, I don't like to go too low because it'll run like crap when not under boost. I think 9:1 CR is a nice compromise because it will still run below 3000rpm and you can put 12-14psi on it, depending on intercooling, tuning, etc. With your current 10.4:1, I'd be thinking a P1sc with big pulley to make about 5-6psi. When you blow the motor and rebuild it with lower compression, upping the boost is simply a matter of swapping a smaller pulley on, about $60.

P.S. who let you out of the audio forum? If you make too much power the exhaust note might drown out that nice stereo system...

Bill Reid

George nailed it...

If you need a quote on a system let us know... we are an authorized Procharger dealer would be happy to provide one :seeya

:cheers: