What kind of LSA would a NA motor like given the same RPM range?
Probably around 106 or so. LSA done properly on a turbo should be for delaying when the intake valve opens. The theory is to let the piston go down abit, let the pressure of the exhaust that was trapped in the cylinder go down, then open the intake valve.
Heres is an issue or two I see with this: (If some one is reading and doesn't know, Lobe separation angle or LSA is the number of degrees between the center line of the intake and exhaust lobes and then devided by two, so 110* LSA means the center of the intake and exhaust lobes are 220* apart on the cam, which would be 110* crankshaft degrees of crankshaft rotation)
On a normal cam when you run a higher LSA you basically split it between exhaust and intake. IE.. the intake opens later and the exhaust closes sooner. Since the intake opens later it will also close later. The closing point of the intake valve is probably the main factor for what rpm the motor makes its power at. The later closing of the intake valve is what bleeds off cylinder pressure at lower rpms and hurts low/mid range power. They do this because the late closing can allow extra air to pack into the cylinder at higher rpm. Basically once the air gets moving fast enough its own momentum does a better job of filling the cylinder. It can over come the effect of the piston traveling back up, and the air didnt have enough time to try to bleed back out anyway. Now if you put the extra LSA degrees all into delaying the opening of the intake valve this would be basically the same thing as retarding the cam. This is what a turbo cam would do to get that trapped exhaust pressure down. Since you put all the delay into opening the intake valve then you also wind up delaying the closing of the intake valve by the full amount. This would make the motor take the full hit in low end and midrange power.
Here is the problem I see in my little brain (it may not really be a problem, I just see it as one)
A wide LSA for a turbo specific cam would mean that the intake valve would close really late for its size. Basically a 6000 rpm N/A lobe would have the closing point of a normal 7000 rpm lobe. So you would have all the draw back of the 7000 rpm lobe but none of the advantages of it. You wouldn't have the extra fill time of the longer duration or the better filling you could get from the higher lift that could be put on the larger lobe.
The solution to this problem is to run a shorter duration intake lobe. For a given closing point of the intake valve you would have to shrink the intake lobe 4 degrees for every 1 degree you want to add to the LSA. So if you wanted to go from 108 to 114 LSA and retain the same intake valve closing point you would have to make the lobe 24* duration smaller! Thats a lot of duration to give up, but if your running alot more back pressure than boost then it wasn't doing nothing but hurting you anyway.
Lets get this so complicated that we all want to go kick the neighbors dog!
Compression Ratio:
Turbo motors run usually around 8 to 1 comp. That breaks down into 7 parts cylinder displacement plus 1 part for the combustion chamber, head gasket piston dish ect. You would then take the total of those two and devide it by the 1 that gives you the "to 1" part of the 8 to 1. After the exhaust stoke if the "to 1" part is sitting there at twice the pressure thats in the intake manifold you would have to let the piston go down into the cyl until one of its 7 parts was given up. You would then have doubled the volume and cut the pressure in half so now its equal to the intake manifold pressure and dropping. Now the pressure in the intake manifold can push some air past the intake valve and into the cly. That extra 1 part we gave up just to get the pressure down just turned our 231" motor into a 198" motor. The worse the back pressure ratio gets the more shrinkage there is and shrinkage is bad!
Lets kick it up one more notch!
The more exhaust that was trapped in the motor then more heat that was trapped in it as well. We're going to have 180 cc's worth of very hot exhaust gasses that are going to be mixing with and trying to ignite the 540cc's of our fresh charge. I've never heard this mentioned before but I can not see how this couldn't have an effect on ignition timing and octane requirements.
Can you see how the boost vs back pressure can have a HUGE effect on a hipo cam selection? If boost is higher than BP you can have the valve opening and filling the cylinder where a normal turbo car could not. You could run a bigger lobe with more duration and lift w/o sacrificing any low end or mid range power, infact you could really increase the low,mid, and high end power all at the same time. Cams are not supposed to be able to do that, but a turbocharged motor puts a cam into a really parculiar predicament.
I'm going to need some freekin prosac by the time I get this figured out! I need to go buy a horse and buggy.
Any more feed back? and did anybody actually read this post?
Jason
