Question Lots Of Questions
Question Lots Of Questions Ok gurus help me out What size valves would you use on say a 422 with a rear mount turbo system with Gt76 with a .81 a/r stock or 2.08/1.60 on a ported set of 317 with 72cc's? what would be the ups and down? Also on FI I have read that 90/90 setups are not needed ? Last why small cams? I understand Lsa but why not big lift and duration? Last When does back pressure become a problem with turbos? 
Im not really a noob but I just cant grasp this 
Im not really a noob but I just cant grasp this 
TECH Resident
Joined: Dec 2004
Posts: 845
Likes: 0
From: K-W, Ontario
I'll leave the first 90% of your post alone, but I'll reply with the following:
High pressure in the exhaust ("backpressure") at any point in the cylce is
bad news for exhaust gasses leaving the cylinder.
If there is any resistance to flow, there will be losses in the amount of
clean charge that can enter the cylinder.
Contrary to some who tell you that "some backpressure is good", it's actually
the total opposite.
The best exhaust system to have is a clear unobstructed pipe which is cut
to the length best suited for the RPM window used.
Anything else is a compromise, or bad combination of valve timing, RPM and
intake/exhaust dimensions.
High pressure in the exhaust ("backpressure") at any point in the cylce is
bad news for exhaust gasses leaving the cylinder.
If there is any resistance to flow, there will be losses in the amount of
clean charge that can enter the cylinder.
Contrary to some who tell you that "some backpressure is good", it's actually
the total opposite.
The best exhaust system to have is a clear unobstructed pipe which is cut
to the length best suited for the RPM window used.
Anything else is a compromise, or bad combination of valve timing, RPM and
intake/exhaust dimensions.
9 Second Club
Joined: Feb 2002
Posts: 985
Likes: 7
From: Lake Orion, MI
Here is my simple version to some of your questions. Remember, an engine is an air pump. It makes power by getting as much air as you can into it with the right amount of fuel.
In an N\A world, you are limited to how much air you can suck into a cylinder at one atmosphere when the piston draws down. So big openings and big intake pipes allow the sucking of the air charge to happen as quickly as possible. There are naturally a proper balance between the use of big lift, big duration and big intakes for each engine combo.
Now bring in the turbo. The cylinder is still the same volume but we are able to get more air by compressing the air charge at levels higher than what we find a sea level normally. This compressed charge is able to fill the cylinder with more air and quicker than we need most of the time. So we no longer need the big a lift or big duration cam because we are pushing the air into the cylinder with as much air as it can handle at a given boost level. More boost means more air until we hit the maximum air that the turbo can move.
Again, the bigger valves are a nice to have but the turbo will do all the work whether they are there or not.
Rick
In an N\A world, you are limited to how much air you can suck into a cylinder at one atmosphere when the piston draws down. So big openings and big intake pipes allow the sucking of the air charge to happen as quickly as possible. There are naturally a proper balance between the use of big lift, big duration and big intakes for each engine combo.
Now bring in the turbo. The cylinder is still the same volume but we are able to get more air by compressing the air charge at levels higher than what we find a sea level normally. This compressed charge is able to fill the cylinder with more air and quicker than we need most of the time. So we no longer need the big a lift or big duration cam because we are pushing the air into the cylinder with as much air as it can handle at a given boost level. More boost means more air until we hit the maximum air that the turbo can move.
Again, the bigger valves are a nice to have but the turbo will do all the work whether they are there or not.
Rick
You might want to read the link in the FAQ. It does explain some cam theory....
http://forcedinductions.com/help.htm
http://forcedinductions.com/help.htm
Ok
Originally Posted by DrTurbo
You might want to read the link in the FAQ. It does explain some cam theory....
http://forcedinductions.com/help.htm
http://forcedinductions.com/help.htm
Also Backpressure, I understand this is not good when you have too much on a turbo app but what limits the amount of back pressure? I know that on a turbo app you want the pipes all the same size, Ok i got that but for rearmounts is it the same in aspect? For example a 3in header going on a 2.5in Y, Would this cause more back pressure or less?
