exhaust question

[twotoedbandit]

Hey guys,  I'm about to do a stainless exhaust for my 78 b210.  From what I have read on the forums I have decided to go with a 2in all the way back, and will likely be using an hks hi power muffler.  My question is, I am still running the stock exhaust manifold, so can I simply run 2in. from the exhaist manifold or should I taper the exhaust from the stock size to a 2 in pipe?  Will I run into exhaust flow issues if I go with 2in directly from the manifold?  Any advice is greatly appreciated guys!  Thanks!

[datzenmike]

2" is on the high side of large for an A15/14 engine. 1 3/4" would be plenty large enough. If easier to fit and cheaper I would go with the 1 3/4" pipe. Truth is that an unmodified motor doesn't need a larger pipe and you'll get more from a low restriction muffler. You want the exhaust speed to remain high so that is scavenges the cylinder and helps draw in new air. Going to too large a diameter pipe lowers the gas speed and you will loose some bottom power.

[twotoedbandit]

That makes sense, the motor is stock and running a downdraft weber. I'm thinking of going efi at some point but it definately needs a new exhaust for now. So, if I'm thinking correctly I could run 1 3/4 pipe from the manifold back and flare to the 2 in inlet on the hks muffler near the end. Possibly a glass pack in there too if it is too loud. Sound reasonable?

[datzenmike]

Yes. If running big cam and multiple carbs or turbo a stock pipe might then be restrictive and a larger (very slightly larger) pipe would help.

 

Pipe has to be small enough to keep the gas speed up but not so small that it restricts breathing at high RPMs.

 

Doubling the pipe diameter does not double the flow. It actually increases the flow by 4 times! A 1.5" pipe has a cross sectional area of 1.766 sq. in. If you increase the pipe diameter 1/4" to 1.75" the cross section is now 2.404sq.in. That's an increase of 36%!!!! Doesn't look like much but it is. A 2" pipe would be over 77%!

 

A 2.125" (2 1/8") pipe will flow double what the 1.5" pipe will. Enough for two A series engines.

 

I don't know off hand what the exhaust pipe diameter on the B 210 is just guessing it's 1.5"

 

 

I like a larger muffler. 1.75" pipe to a 2" flair into a 2" muffler. Tail pipes are usually smaller output. So 1.7/8" or whatever. Always try to run a tailpipe. They are quieter in the car and sound better outside. (I think) By law the tail pipe must exit to the outside of the vehicle and if under must be at least to the rear axle.

[twotoedbandit]

Awesome! Thanks for the advice. Without crawling under there to confirm, I believe stock exhaust is a 1.5. I'll start gathering the materials this week! Should I flare the connection from the manifold from 1.5 to 1.75 or just go 1.75 straight off the manifold? Not sure if it will make a difference just thought it might be smoother flow with the flare. Can't wait to get started!

[datzenmike]

Not enough to may enough difference to notice.

[datsunfreak]

Should I flare the connection from the manifold from 1.5 to 1.75 or just go 1.75 straight off the manifold? 

 

I would take the downpipe off, cut it off a few inches out from the collector, then flare it to fit tight inside the 1.75" pipe, then go from there.  :thumbup:

 

Side note, it may be a snug fit already. Shouldn't have to flare it very much.