Need help determining exhaust backpressure/flow
Thread Starter
Race Director
Joined: Nov 2003
Posts: 12,221
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From: Overland Park Kansas
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
Need help determining exhaust backpressure/flow
I'm trying to deduce my sidepipes flow rate (since the manufacturer can't find records of that info). More importantly, I'm trying to verify/deny their claim the system is good to 450hp.
According to Lawrence-Keech (makers of my discontinued SideEffects sidepipes), the stock L98 catback induces a 44" of backpressure. When the stock catback is replaced with SideEffects, it drops to 31" of backpressure. Measurements were made at @ 600cfm.
Also, in LK sales literature, it says the sidepipes flow 35% better than the stock catback. The same brochure shows the L98 stock muffler as flowing 275 cfm. Since the muffler would be the primary restriction in the factory catbac, I'm wondering if 1.35 x 275cfm = sidepipe flow per side?
(The total system flow would be about 750cfm I'm thinking).
OTOH, since there's more than 28" of backpressure at 600cfm, should I assume the sidepipes actually flow a bit less than 600cfm?
My ultimate goal is to determine how much power the sidepipes rob from a 450hp motor (measured at the crank). If not, I'm hoping someone can at least make sense of the numbers provided and determine if the two measurements are consistent -- pointing to the correct cfm for the system. Using that, I could figure it out.
According to Lawrence-Keech (makers of my discontinued SideEffects sidepipes), the stock L98 catback induces a 44" of backpressure. When the stock catback is replaced with SideEffects, it drops to 31" of backpressure. Measurements were made at @ 600cfm.
Also, in LK sales literature, it says the sidepipes flow 35% better than the stock catback. The same brochure shows the L98 stock muffler as flowing 275 cfm. Since the muffler would be the primary restriction in the factory catbac, I'm wondering if 1.35 x 275cfm = sidepipe flow per side?
(The total system flow would be about 750cfm I'm thinking).
OTOH, since there's more than 28" of backpressure at 600cfm, should I assume the sidepipes actually flow a bit less than 600cfm?
My ultimate goal is to determine how much power the sidepipes rob from a 450hp motor (measured at the crank). If not, I'm hoping someone can at least make sense of the numbers provided and determine if the two measurements are consistent -- pointing to the correct cfm for the system. Using that, I could figure it out.
Last edited by GREGGPENN; Dec 29, 2008 at 11:35 PM. Reason: Reworded post for clarity
Melting Slicks
Joined: Aug 2003
Posts: 3,394
Likes: 393
From: Lakeville MI
The values listed appear to be in units of cm of mercury not inches.
44 cmHg = 8.5 psi
31 cmHg = 5.994 psi
These appears to be reasonable values for back pressure.
You can add a pressure tap somewhere in the exhaust system and measure it to confirm.
Also for 450 crank HP, you would want the entire system to flow approx. 990 cfm (450 x 2.2) or 495 cfm per muffler. It would appear that your mufflers fall short based upon the posted information.
44 cmHg = 8.5 psi
31 cmHg = 5.994 psi
These appears to be reasonable values for back pressure.
You can add a pressure tap somewhere in the exhaust system and measure it to confirm.
Also for 450 crank HP, you would want the entire system to flow approx. 990 cfm (450 x 2.2) or 495 cfm per muffler. It would appear that your mufflers fall short based upon the posted information.
Last edited by tequilaboy; Dec 30, 2008 at 11:48 AM.
Thread Starter
Race Director
Joined: Nov 2003
Posts: 12,221
Likes: 446
From: Overland Park Kansas
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
One email I got says 30hp for a short-runner intake (shifted around 6k rpms), but only 5hp for a hogged out TPI that gets shifted around 5k rpms. (Of course the TPI doesn't develop 450 crank HP either -- with everything else being held constant in a simulation).
