Do I need bigger primaries???
Thread Starter
Launching!
Joined: Dec 2005
Posts: 222
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Do I need bigger primaries??? I am currently having a 457 lsx built for my 03 silverado ss. I currently have pacesetter LT with 1 3/4" primaries and 3 inch collectors. At this time 1 3/4" primaries are the biggest available for my truck. I am wondering if custom headers with maybe a 1 7/8" primary or even a 2" primary would be benifitial to me. I am thinking they would, but I am also thinking that maybe the 1 3/4" primaries will give me more back pressure creating more torque to get my 5,300lb beast moving. I just want to make my truck as fast as possible. Any help would be appreciated. Here are the specs on the motor:
LSX block
-LS7 heads (stock intake side, but the exhaust has been reworked by WCCH)
-Ported LS7 manifold (jeremy Formato)
-4.200" bore
-4.125" stroke
-6.125" rods
-Custom CP pistons with a reverse dome of the ls7 combustion chamber
-SCR of roughly 11.2:1
LSX block
-LS7 heads (stock intake side, but the exhaust has been reworked by WCCH)
-Ported LS7 manifold (jeremy Formato)
-4.200" bore
-4.125" stroke
-6.125" rods
-Custom CP pistons with a reverse dome of the ls7 combustion chamber
-SCR of roughly 11.2:1
Launching!
Joined: Dec 2003
Posts: 251
Likes: 2
From: Portland OR
Optimum header pipe diameter depends on a bunch of different factors. The basic tradeoff with header primary diameter is that smaller diameters produce stronger scavenging pulses, which increases volumetric efficiency, torque, and horsepower - but if you get too small, backpressure on the exhaust stroke begins to take away more power than is gained by better scavenging, particularly at high RPM.
Something to consider: the exhaust valve on an LS7 is 1.60 diameter. The actual port diameter at the valve bowl is less than that, below 1.5 inch. The exhaust port is about 1 5/8 inch wide at its widest point. Putting a 2 inch header on an exhaust port like this is just a waste - the exhaust port will never flow enough to use the flow capacity of a 2 inch primary, but the intensity of the scavenging pulse will be reduced and you'll probably lose both torque and top end power.
Just my personal opinion, given that this is a truck, is that your existing 1 3/4 primaries are probably a pretty good tradeoff between mid range torque and top end power.
Something to consider: the exhaust valve on an LS7 is 1.60 diameter. The actual port diameter at the valve bowl is less than that, below 1.5 inch. The exhaust port is about 1 5/8 inch wide at its widest point. Putting a 2 inch header on an exhaust port like this is just a waste - the exhaust port will never flow enough to use the flow capacity of a 2 inch primary, but the intensity of the scavenging pulse will be reduced and you'll probably lose both torque and top end power.
Just my personal opinion, given that this is a truck, is that your existing 1 3/4 primaries are probably a pretty good tradeoff between mid range torque and top end power.
Thread Starter
Launching!
Joined: Dec 2005
Posts: 222
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Hmm....that makes sense. Would you still have that opinion if I told you my ls7's flow 250 cfm @.600 lift on the exhaust side after porting?
Originally Posted by 71CamaroLS1
Optimum header pipe diameter depends on a bunch of different factors. The basic tradeoff with header primary diameter is that smaller diameters produce stronger scavenging pulses, which increases volumetric efficiency, torque, and horsepower - but if you get too small, backpressure on the exhaust stroke begins to take away more power than is gained by better scavenging, particularly at high RPM.
Something to consider: the exhaust valve on an LS7 is 1.60 diameter. The actual port diameter at the valve bowl is less than that, below 1.5 inch. The exhaust port is about 1 5/8 inch wide at its widest point. Putting a 2 inch header on an exhaust port like this is just a waste - the exhaust port will never flow enough to use the flow capacity of a 2 inch primary, but the intensity of the scavenging pulse will be reduced and you'll probably lose both torque and top end power.
Just my personal opinion, given that this is a truck, is that your existing 1 3/4 primaries are probably a pretty good tradeoff between mid range torque and top end power.
Something to consider: the exhaust valve on an LS7 is 1.60 diameter. The actual port diameter at the valve bowl is less than that, below 1.5 inch. The exhaust port is about 1 5/8 inch wide at its widest point. Putting a 2 inch header on an exhaust port like this is just a waste - the exhaust port will never flow enough to use the flow capacity of a 2 inch primary, but the intensity of the scavenging pulse will be reduced and you'll probably lose both torque and top end power.
Just my personal opinion, given that this is a truck, is that your existing 1 3/4 primaries are probably a pretty good tradeoff between mid range torque and top end power.
Launching!
Joined: Dec 2003
Posts: 251
Likes: 2
From: Portland OR
My guess - and it is just an educated guess - is that you will get more power under the curve with 1 3/4 primaries, unless you're going way big on the cam. And going way big on the cam would probably be a mistake in a 5,300 lb truck with (I assume) an automatic tranny.
Just another point to consider - exhaust flow at .600 lift is actually not as important as exhaust flow below .200 lift. Most exhaust gas is leaves the cylinder during exhaust "blowdown", within 50 - 70 degrees of crank angle after the exhaust valve begins to open on the power stroke. The rest of the exhaust gas is "pumped" out during the exhaust stroke - higher lift exhaust flow does help here.
FWIW, the LG motorsports guys are building 600+ RWHP LS7s with ported heads in 3100 lb M6 'vettes, and they're still using 1 3/4 primaries on their headers. Given the economics of the Z06 customer base, if LG could show 15 HP more from a 1 7/8 primary and charge $1K extra for it, they'd do it ...
Just another point to consider - exhaust flow at .600 lift is actually not as important as exhaust flow below .200 lift. Most exhaust gas is leaves the cylinder during exhaust "blowdown", within 50 - 70 degrees of crank angle after the exhaust valve begins to open on the power stroke. The rest of the exhaust gas is "pumped" out during the exhaust stroke - higher lift exhaust flow does help here.
FWIW, the LG motorsports guys are building 600+ RWHP LS7s with ported heads in 3100 lb M6 'vettes, and they're still using 1 3/4 primaries on their headers. Given the economics of the Z06 customer base, if LG could show 15 HP more from a 1 7/8 primary and charge $1K extra for it, they'd do it ...
Thread Starter
Launching!
Joined: Dec 2005
Posts: 222
Likes: 0
Wow, I wasn't aware the lg is using 1 3/4" primaries. My cam will most likely be in the mid to high 230's on duration.
Originally Posted by 71CamaroLS1
My guess - and it is just an educated guess - is that you will get more power under the curve with 1 3/4 primaries, unless you're going way big on the cam. And going way big on the cam would probably be a mistake in a 5,300 lb truck with (I assume) an automatic tranny.
Just another point to consider - exhaust flow at .600 lift is actually not as important as exhaust flow below .200 lift. Most exhaust gas is leaves the cylinder during exhaust "blowdown", within 50 - 70 degrees of crank angle after the exhaust valve begins to open on the power stroke. The rest of the exhaust gas is "pumped" out during the exhaust stroke - higher lift exhaust flow does help here.
FWIW, the LG motorsports guys are building 600+ RWHP LS7s with ported heads in 3100 lb M6 'vettes, and they're still using 1 3/4 primaries on their headers. Given the economics of the Z06 customer base, if LG could show 15 HP more from a 1 7/8 primary and charge $1K extra for it, they'd do it ...
Just another point to consider - exhaust flow at .600 lift is actually not as important as exhaust flow below .200 lift. Most exhaust gas is leaves the cylinder during exhaust "blowdown", within 50 - 70 degrees of crank angle after the exhaust valve begins to open on the power stroke. The rest of the exhaust gas is "pumped" out during the exhaust stroke - higher lift exhaust flow does help here.
FWIW, the LG motorsports guys are building 600+ RWHP LS7s with ported heads in 3100 lb M6 'vettes, and they're still using 1 3/4 primaries on their headers. Given the economics of the Z06 customer base, if LG could show 15 HP more from a 1 7/8 primary and charge $1K extra for it, they'd do it ...
Originally Posted by adams2003
Any body else have any comments or opinions?
Thanks for any help.
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hoenstly with a motor that large torque isnt an issue. and with a motor that large i would want something that can flow well up top. most ppl with larger cubed motors are seeing good gains by going from 1 3/4 to 1 7/8s. hell i think even stock LS7s run
1 7/8 headers so why even think of running smaller
as for a 1 7/8 header i have them on my truck and im sure they would work on yorus as well.
IMHO your leaving to much on the table in your setup with 1 3/4s
1 7/8 headers so why even think of running smaller
as for a 1 7/8 header i have them on my truck and im sure they would work on yorus as well.
IMHO your leaving to much on the table in your setup with 1 3/4s
