Exhaust size question

[scoti]

X or H pipe causing a loss in power or torque is a new one to me. They help equalize between the sides & can help tame the sound depending on the set-up.

The header primary size is typically based on displacement, air flow capability, & cam size. For an old school 350 w/mild compression, 1 5/8 headers work well & 2.25 duals would be fine for most street driven rides. I doubt you'd notice much difference w/2.5" duals. 383-400ci motors w/some additional compression would prob like the larger 1 3/4" primary tubes & definitely would be better suited for 2.5" minimum pipe for duals.

 

[SS_Malibu]

If I could design an exhaust just the way I want it would include and x pipe. I would prefer the x over the h for the purposes of installing a single wideband o2 sensor downstream of the x. I believe a single sensor downstream of the x will still get a decent mix of both engine banks, since the x will cross and mix the exhaust of both engine banks.

I use a single wideband o2 downstream of an x on my 92 Foxbody. It's a nice way to monitor the actual real time fuel conditions.

I suppose the only way to do it better is two wideband sensors, one on each pipe.

El Correct O. While I don't know much about O2 sensors an "X" helps to pull scavenged exhaust from each bank and balances the exhaust behind the "X" pipe.

 

[mikester]

I got bored and here is a page from an old spec sheet from 2014. I doubt the updated it since, but, it's what they used to recommend.

View attachment 186785

Thanks for posting that ! I'd have to dig through the receipts to find that page. LOL
I wound up using 1 5/8 headers on this motor. Maybe I goofed but I doubt it will make a big difference. I used 1 3/4 Shoenfelds on the 406. The shorter set makes it so much easier to get at the filter.

 

[565bbchevy]

I used 1 3/4 Shoenfelds on the 406. The shorter set makes it so much easier to get at the filter.

That is why I have remote filters on both my Gbodys, easy access away from the headers.

 

[JD1964]

Here's and example of the wideband o2 and the AFR being displayed on the gauge. It's a single sensor downstream of the x pipe. This is a 3rd gear pull from about 45mph to redline. You can see higher 14's at the start, then I floor it and it fattens up to mid 10's, then at the top end leans slightly above 11:1 before I let off. Pushing about 10psi here. Vortec V3-SCi

 

[ck80]

Thanks for posting that ! I'd have to dig through the receipts to find that page. LOL
I wound up using 1 5/8 headers on this motor. Maybe I goofed but I doubt it will make a big difference. I used 1 3/4 Shoenfelds on the 406. The shorter set makes it so much easier to get at the filter.

Whelp.... depends on a big ol balancing act of everything. On the face of it, you think, 1/8" not that much different right?

But a 1.625" diameter circle rounds down to a 2.073 Sq inch area. Going up to a 1.750" diameter bumps up to a 2.405 Sq inch area.

The 1 5/8" passage area is only 86.195% the size of the 1 3/4" passage. Or if you'd like the other way around, the 1 3/4 has 116% the volume capacity of the 1 5/8.

I actually got bored (pain-related insomnia) and can credit motorheadmike making an exhaust out of his wife's new cookie sheets as inspiration to read up about exhausts some a few months back. I am far, far from an expert, and instead consider myself knowing just enough to ask questions of others. If what they say makes sense and fits then I go with their advice. If it doesn't seem to jibe, I compare to other opinions.

Having said that, brainstorm? You may be able to counteract/offset any losses from the smaller header size somewhat by running an extension behind the collector and ahead of the cats? I've read tests, and I've read anecdotal posts by guys who ran cars a variety of ways on the strip - open headers, vs headers with full exhaust, vs headers extensions then exhaust. What they reported in their slips was the open headers and the headers with extensions plus exhausts saw similar times. But, speeds were higher with the collector extensions. When you account for the extra weight of the exhaust, plus higher speeds, supports some older (mid 2000s) car craft articles about dyno tests with extensions between 8-18" increasing hp and torque midrange.

So maybe a question to ask the exhaust guy is his thoughts on that matter.

Also, not knowing his thoughts on muffler as you identified, hard to say if any flow rate would benefit much from the bigger pipe elsewhere based on what you have in your headers, cats, and mufflers for which one creates the biggest bottleneck.

Same mindset of ss once and don't do it again for rusting is the slightly bigger pipe in case you ever slightly upgrade the motor.. maybe won't need to do it again.

 

[JD1964]

Seriously though, regardless of the final decision on the exhaust design, consider including o2 bung or bungs welded in for widebands should you ever want or need them. They can be capped with the screw in plugs until needed.

 

[CopperNick]

Strictly speaking, being able to weld is an asset but you can assemble an exhaust system using mechanical connections and arrive at a decent system.

What I have used in both my vehicles is the Vibrant Stepped pipe clamp. This is a band clamp that is shouldered so that the fat end fits over the bell or hub in the pipe and the smaller end matches the pipe being slipped into place. Once you have slipped one section into another, you tighten up the bolts and you have a tight connection that, if you need to sometime down the road, you can disassemble without needing a torch or air saw. I throw a little Never-Sleeze at the bolt threads, just as a precaution.



Nick

 

[ck80]

Looks like the only true dual kit w/o any cross over pipe is the Jegs stainless kits but won't ship until the end of Dec.

JEGS 30525: JEGS Exhaust Kit - Header-Back Dual G Body Exhaust Kit 197801988 GM - JEGS

Note: Exhaust kit is designed for 1978-88 G-Body that was originally equipped with a SB-Chevy. Does not fit ANY Cutlass Supreme model. Designed for us

www.jegs.com

Searching & it looks like the only kits now out there for our cars would be this one, the Dynomax aluminized 2.25" kit. Then there is the Hooker "LS" kits w/ X in 2.50" & 3", the Pypes w/ X in 3" w/o cats or 2.5" w/ cats ($$$), and one from American Racing in 3" ($$$$). Looks like we're now loosing options on kits got our cars.

If you look around further, other companies are discontinuing stuff left and right.

Peek at holley's clearance section for how many universal cats and x-pipes are marked down....

I'm not sure how much is related to "our cars" and how much is related to things like the noise/exhaust modification laws going around similar to the NY one