stock catback info
04-01-2009, 01:25 AM
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stock catback info
Everyone is always saying to cut the 2.25 restriction out at the rear axle and the stock exhaust will be a true 2.5 all the way back RIGHT?
Well, if you measure the inlet at the stock resonator, it is a hair over 2 inches....... 2 and 1/16th to be exact. It's right from the flange and runs an inch or so back into the resonator before it opens back up.
So to the people that went to the trouble to do an axle back .....you might want to check this cause it's one hell of a restriction.
Well, if you measure the inlet at the stock resonator, it is a hair over 2 inches....... 2 and 1/16th to be exact. It's right from the flange and runs an inch or so back into the resonator before it opens back up.
So to the people that went to the trouble to do an axle back .....you might want to check this cause it's one hell of a restriction.
04-01-2009, 09:07 PM
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Well i'm surprised no one has anything to say about this since most give people with GMPP exhaust **** for going with too small of an exhaust . Then tell you you should do an axle back to have a true 2.5 inch exhaust.
04-02-2009, 11:04 AM
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Crap, my buddy was going to bend a s/s axle back for me. guess I have to spend more money and wait longer. Thanks for the info, you saved me from wasting time and money.
04-02-2009, 11:08 AM
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interesting, I still think it would be benificial to replace this and open it up, blow through a straw 1 inch long and blow through a straw 5 inches long, notice a difference?
04-02-2009, 11:10 AM
#5
rukkee .... does the GMPP exhaust have this restriction ??? Or is it a true 2.25in exhaust all the way through ?? If it is a consistent 2.25in thats prob. where everyone gets the gain from the smaller than stock exhaust. Not to mention the GMPP has a less erratic bend right before the rear axle. Let me know what you find, because I keep going back and forth deciding if I want to mess with the exhaust.
04-02-2009, 11:18 AM
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This comes from redlineforums. Very useful information.
Hi ! Here are my flow calculation for some LSJ engine parts :
ADDED COMMENT : These flow numbers are calculated at ambiant air temperature AND pressure.
Stock Throttle Body (67 mm) : 625 CFM.
EDITED : Stock Head : One intake port : 250 CFM @ stock 0,395" valve lift. One exhaust port : 145 CFM @ stock 0,395" valve lift. Stock head is said by GM to be efficient up to 275 WHP; it is clearly possible to make more WHP than that, but in a unefficient way (several HP losses caused by heat and restriction).
Stock Exhaust manifold ("Header") : 425 CFM.
Stock Downipe (DP) with cat : Plus or minus 370 CFM or less.
Stock Cat-back Exhaust (2,5" inlet to 2,25" outlet with stock muffler) : 300 CFM.
Now here are some replacement parts :
GMPP Exhaust manifold ("Header") : (30 more CFM than stock) : 455 CFM.
Shorty or long tube Header with 1,75" primaries and 2,5" outlet : at least 500 CFM.
Shorty or long tube Header with 1,75" primaries and 3" outlet : at least 725 CFM.
Shorty header with 1,5" primaries and 2" outlet : Waay not enough, who cares LOL.
Stock DP w/o cat : 425 CFM.
2,5" DP with VERY HIGH FLOW cat : plus or minus 450 CFM.
2,5" DP w/o cat : 500 CFM.
2,75" DP with VERY HIGH FLOW cat : plus or minus 475 CFM.
2,75" DP w/o cat : 600 CFM.
3" DP with VERY HIGH FLOW cat : plus or minus 500 CFM.
3" DP w/o cat : 725 CFM.
Stock catback with only the rear muffler replaced (the last portion is still in 2,25" tubing) : 350 CFM.
Stock catback with last portion replaced for 2,5" tubing and straight through muffler : 450 CFM.
2,5" catback : From 450 to 500 CFM (must be a straight through muffler).
2,75" catback : From 550 to 600 CFM (must be a straight through muffler).
3" catback : From 650 to 725 CFM (must be a straight through muffler).
Hope this can help ...
Regards )
ADDED COMMENT : These flow numbers are calculated at ambiant air temperature AND pressure.
Stock Throttle Body (67 mm) : 625 CFM.
EDITED : Stock Head : One intake port : 250 CFM @ stock 0,395" valve lift. One exhaust port : 145 CFM @ stock 0,395" valve lift. Stock head is said by GM to be efficient up to 275 WHP; it is clearly possible to make more WHP than that, but in a unefficient way (several HP losses caused by heat and restriction).
Stock Exhaust manifold ("Header") : 425 CFM.
Stock Downipe (DP) with cat : Plus or minus 370 CFM or less.
Stock Cat-back Exhaust (2,5" inlet to 2,25" outlet with stock muffler) : 300 CFM.
Now here are some replacement parts :
GMPP Exhaust manifold ("Header") : (30 more CFM than stock) : 455 CFM.
Shorty or long tube Header with 1,75" primaries and 2,5" outlet : at least 500 CFM.
Shorty or long tube Header with 1,75" primaries and 3" outlet : at least 725 CFM.
Shorty header with 1,5" primaries and 2" outlet : Waay not enough, who cares LOL.
Stock DP w/o cat : 425 CFM.
2,5" DP with VERY HIGH FLOW cat : plus or minus 450 CFM.
2,5" DP w/o cat : 500 CFM.
2,75" DP with VERY HIGH FLOW cat : plus or minus 475 CFM.
2,75" DP w/o cat : 600 CFM.
3" DP with VERY HIGH FLOW cat : plus or minus 500 CFM.
3" DP w/o cat : 725 CFM.
Stock catback with only the rear muffler replaced (the last portion is still in 2,25" tubing) : 350 CFM.
Stock catback with last portion replaced for 2,5" tubing and straight through muffler : 450 CFM.
2,5" catback : From 450 to 500 CFM (must be a straight through muffler).
2,75" catback : From 550 to 600 CFM (must be a straight through muffler).
3" catback : From 650 to 725 CFM (must be a straight through muffler).
Hope this can help ...
Regards )
04-02-2009, 07:52 PM
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I'm going to say that the GMPP exhaust it 2.25 all the way threw , but i will verify it this week end when i work on it. I'm going to try and use a dremel tool on the stock resonator and open it up to atleast 2 and 1/4th . I'll let you guys know if there is enough meat inside the flange to do it . I really don't want to take the GMPP off cause it sounds so good....... but it's just to loud now with the downpipe .
04-03-2009, 02:26 AM
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I'm going to say that the GMPP exhaust it 2.25 all the way threw , but i will verify it this week end when i work on it. I'm going to try and use a dremel tool on the stock resonator and open it up to atleast 2 and 1/4th . I'll let you guys know if there is enough meat inside the flange to do it . I really don't want to take the GMPP off cause it sounds so good....... but it's just to loud now with the downpipe .
I don't really understand fully what your saying about the 2 1/16th part.
i would really like to salvage the res and still have a full 2.5
thanks
04-03-2009, 02:32 AM
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i think i still want to do axle back and use stock resonator is the 2 1/16th part fully fixable by a muffler shop maby like they can hack off and reweld a new part to stay 2.5?
I don't really understand fully what your saying about the 2 1/16th part.
i would really like to salvage the res and still have a full 2.5
thanks
I don't really understand fully what your saying about the 2 1/16th part.
i would really like to salvage the res and still have a full 2.5
thanks
When you look at the front of the RES you will see what i mean.
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