Factory side exhaust vs factory 2.5" under the car exhaust

[Whitehall]

I had my 70 LT-1 on a chassis dyno this Summer. Currently, just because I like the look, I have '69 style side exhausts on the car. The horsepower curve went 'flat' at approximately 4500 rpm producing 230 HP at the wheels. A contributor to the results, I'm certain, was that the valve lash was loose, but I'm more curious to know if there is data indicating a reduction in HP when using the factory side pipes. Given the incredible wealth of information in the 'exhaust' department on this forum, I'm hoping for an answer! Thank you.

 

[Maniacmechanic1]

The Baffles inside of the Corvette sidepipes are known to knock down 100 -150 Hp.
The More HP present the worse it gets for Power loss.
Remove the Baffles alltogether & power will be back if the Carb jetting & ignition timing curve is right to match.

 

[chromebumpers]

Welcome to Grumpy’s Mr. Whitehall,

I have Hooker side pipes with the supper quite inserts on my 71 base 350. Solid lifters suffer greater hp and even more torque loss on any brand baffled pipes. At least for Hooker, the inserts they make now have a better flow through design and there are a couple aftermarket suppliers making low restriction thin wall inserts for those that like “some” tamed down sound.
In 2003 a few guys in our local Corvette Club managed a “Dyno what you brought” Day at Bryner Chevrolet. At that time I had a ‘70 LT-1 with the ‘69 sidepipes just like you. I remember like it was yesterday, my stock LT was running better than most at 342 hp and 405 tq at the rear wheels and after I skinned my knuckles and half a day of time, my $2,500 NOS ‘69 exhaust was a thumbs up by everyone. I managed to get in last of the day Saturday. With the weather very close in temp and humidity to the first test I was pissed to loose 38 hp and just a hair over 50 ft lbs. The side pipes stayed until I sold the car in the spring ‘04 at which time I returned the LT back to OE.

I have read about similar numbers with other guys and these pipes. There is Hot Rod Magazine story (Sorry no date or the story title remembered) that mentioned this swap. I’ll try to find it.

 

[Grumpy]

side pipes without headers are mostly just for looks, in most configs its going to kill or restrict power,

adding the full side pipe header and side pipe config without the inserts can provide very good power, and they were used on the l88 road racing corvettes as they provided the best power curve option available once correctly configured once the muffler inserts are in place, in the side exhaust, its a castrated exhaust, in the case of most inserts, but you can fabricate baffles that both reduce the sound and don,t significantly reduce power


one reason side exhaust on corvettes were and are used is the design allowed decent ground clearance AND long efficient header primary tube header designs.

http://garage.grumpysperformance.com/index.php?threads/c3-side-pipe-exhausts.793/#post-30576

http://garage.grumpysperformance.co...ackpressure-hurting-your-combo.495/#post-8333

http://garage.grumpysperformance.co...impressed-with-dougs-headers.7831/#post-39146

http://garage.grumpysperformance.co...horty-and-full-length-headers.1303/#post-8221

http://garage.grumpysperformance.co...acing-entire-exhaust-on-a-c3.9296/#post-35280

http://garage.grumpysperformance.com/index.php?threads/spiral-mufflers-anyone.10365/#post-42721

http://garage.grumpysperformance.co...ource-for-fabricating-sidepipe-headers.13159/

http://garage.grumpysperformance.com/index.php?threads/calculating-required-exhaust-pipe-size.11552/

http://garage.grumpysperformance.co...-guys-that-just-slap-on-factory-headers.3155/

 

[Whitehall]

So, some seemingly varying results, but I think my first step should be to measure the back pressure with the current set up. I'm hoping measuring from one of the AIR ports on the exhaust manifold would work? There will be no news on this front until the Spring.

Can't risk burned legs, so the 'better' option isn't one.

Thank you! Ed

 

[Maniacmechanic1]

So, some seemingly varying results, but I think my first step should be to measure the back pressure with the current set up. I'm hoping measuring from one of the AIR ports on the exhaust manifold would work? There will be no news on this front until the Spring.

Can't risk burned legs, so the 'better' option isn't one.

Thank you! Ed

You can try it.
Checking exhaust back pressure is usually done ex downstream like with an oxygen sensor removed on late model EFI vehicles.

 

[Maniacmechanic1]

I have found many plugged up catalytic converters checking exhaust pressure so the test is Valid.

 

[Whitehall]

Thank you all, and, at some future date, I'll update you on findings, such as they may be. And, it's nice to have learned, via you, that there are other options which will fit under the factory covers.