So I am looking through the BBK X and H pipes with cats and they are considered off road only. How can this be? Wouldn't the addition of the cats be considered legal? Pypes just puts in the cats and calls it legal. Seems like there is some complication I am not seeing.

On a side note, is it really that bad to run an engine without an H or X pipe? Seems like the Superformance cars get along without them.

 

Absence of the stock catalytic converters. Many brands have hi-flow cats but will not pass some emission tests.

 

Yeah, I didn't think about some of the high flows not actually passing. Now that I look, it says 49 state legal. Works for me, as I am in the other 49 and don't intend to live in the other. Sorry California.

 

Werd. Just because somebody sticks in some cats in an aftermarket exhaust system doesn't necessarily mean those hi-flow cats will meet actual emissions standard. Thus, the safe thing is to market them as "Off Road Only" to keep the legal department from retiring to their day beds with a case of the vapors.

As to "why" either X or H pipes: cross plane V8s tend to send exhaust pulses down the pipes in rather unequal lumps. While that's great for that distinctive lumpy V8 sound, in terms of exhaust efficiency, it's like trying two fat guys through a narrow turnstile at the same time, resulting in increased back pressure and loss of power. Linking the two exhaust banks allows one of these fat guys to step into the other, empty aisle between exhaust pulses and thus send the exhaust lumps single file down the respective exhaust banks.

Other ways to address this inherent 90-degree V8 issue is to:

Unequal length headers. Basically, different pipe lengths for different cylinders can lessen the exhaust pulse traffic jam a bit but can result in tuning discrepancies between cylinders, where some essentially have shorty (hi rpm) headers while the others have long tube (low rpm) headers.

"Bundle of Snakes" exhaust systems such as on the GT40, however, its difficult to impossible to package that much contorted plumbing in a front engine car. These do audibly smooth out the sound of the exhaust too (listen to some old GT40 vids).

180-degree "Flat Plane" crank. A flat plane design, as often seen on Ferraris and other uber V8s does result in even pulses (intake and exhaust) along with a much lighter crankshaft and motor. However, they tend not to be as smooth and do indeed sounds rather "flat," at least until they get into the upper registers of the rev band where they sound fantastic.

 

I certainly see what you are getting at. It totally makes sense. I'm just thinking about opening up the exhaust on both banks collected down into 3" pipe but no x or h pipe but 3" out on both sides. Neglecting back pressure this should alleviate a good bit of the trapped flow right?

Thanks for the help I just wanted to learn some new things and look for an excuse to try out a separated dual exhaust some day.

 

While I liked your explanation, doesn't the uneven pulses have more to do with the firing order rather than a 90d crank?

Here's a tidbit I saved about Bullitt's (restricted) H-pipe. They came up with the packed tips to reduce the more noticeable pulses that occur due to the restriction in the H-pipe:

 

My explanation is a bit general and simple, and I would imagine differing firing orders would have some effect on exhaust tuning (but to some other detriment?). But I think the "lumpy" nature of 90 degree V8s are generally inherent and why all the effort is expended on more elaborate exhaust systems to ameliorate their negative effect, be that cross pipes, bundles of snakes or other.

The Bullitt exhaust tuning blurb I think illustrates the big exhaust pulses/lumps booming down each bank -- necking down the cross pipe to accentuate those pulses for a "lumpier" sound. Probably didn't help power any, but boy, those Bullitts sure did sound great.

I've always wondered why more serious racers (GT 40 program) didn't/haven't converted cross plane American V8s to flat plane crank configurations. While not too simple -- new crank, cam(s) and ignition systems at least -- significant (rotating and otherwise) weight can be saved not to mention the air/exhaust flow advantages. I imagine that at least with the GT40 program, the ability to use relatively off-the-shelf motors and parts contributed to reliability and reduced development efforts, and that it was fairly easy to slap on a bundle of snakes exhaust system on a mid-engined configuration.

Of course you'd loose a bit of reciprocating smoothness and that great cross plane exhaust rumble.

 

That would be my guess Rhumb, to date the only person named that I can remeber messing with a domestic 180 degree V8 is Smokey Yunick, I'm sure there were and are others since his time.

Its to bad Ford didn't carry the mod idea across the whole line, had the heads interchanged with an I4 the cam part would have been solved and making a Ford based 180 degree V8 would have only required a crankshaft (since a 180 degree V8 (even more than a 90 degree V8) is essentially 2 I4's sharing a common crankshaft)