250 Six banger exhaust question

[Hair]

I'm running a stock 250 except for a set of headers and an HEI that I added.

I finally got it out the other day for the first test drive (open headers).

It ran fine, but felt like it had lost a lot of low end power compared to what it had in stock fashion.

I figured that it was due to the open headers. Am I correct in thinking this?

If so, here's my delima, I was planning on running dual exhaust from the headers out the back using 2.25" pipe and two turbo mufflers. Would that be too big? Originally I think it only had like a 1.75" single pipe. Should I maybe go with 2" pipes or maybe 1.75" or will the 2.25" work fine?

I was going to use 2.25" because the headers came with a 2.25" adapters.

I've heard both, some say that there's no such thing as not enough back pressure, but I've noticed from personal experience that on smaller engines especially (like motorcyle engines) no back pressure DOES cost power. I've never messed with a straight six before, until now, so I just don't know.

What about it, will dual 2.25" be okay?

[culp7]

its the backpressure thats causing the loss of power, i've seen it before

[Wrenchbender Ret]

If you lose power after a better flowing exhaust, your fuel mixture is too lean. You can probable stand some more ign. advance also. I would use 2 in pipes. That is plenty large & will sound better.

[mikajo39]

Just need to tune it, backpressure kills horsepower no matter what people say.

[slomotion]

You're going a little too large.
In a book titled "Chevrolet Inline Six-Cylinder Power Manual" by Leo Santucci, the exhaust systems are relatively small in diameter. In Santucci's book he explains that a radical, cammed, all out drag 292 only has 1-7/8"-to 2" header tubes. Based on this, a street 250 wouldn't need the volume you're thinking about. A rule of thumb is; "an engine should flow 75% of the intake capacity through the exhaust, and there's no advantage to flowing more."
In the book there's a whole chapter on headers/exhaust....worth the price over and over.

[292farmer]

from experience i know what u mean the open headers with no exhaust kill the low end torque, i ran them like this for 3-4 wks before making an exhaust im using 2"ID and the low end is now right up to before if not more. headers seem to cause motors to run lean try upping a jet size, or rebuild carb if havent done yet as that may be slowing fuel flow, check filters too. torque naturally needs back pressure thats why headers with longer collectors favor torque, as opposed to "shorty" headers. stick with 2" as that will favor the 250 best especially if its mainly stock. good luck!!

['72customdeluxe]

Originally Posted by slomotion

You're going a little too large.
In a book titled "Chevrolet Inline Six-Cylinder Power Manual" by Leo Santucci, the exhaust systems are relatively small in diameter. In Santucci's book he explains that a radical, cammed, all out drag 292 only has 1-7/8"-to 2" header tubes. Based on this, a street 250 wouldn't need the volume you're thinking about. A rule of thumb is; "an engine should flow 75% of the intake capacity through the exhaust, and there's no advantage to flowing more."
In the book there's a whole chapter on headers/exhaust....worth the price over and over.

that's the header primaries i'm almost sure of. He is saying two 2" exhaust pipes after the collectors

[slomotion]

Thanks 72Customdeluxe, after reading that back, I guess it was a little confusing. All I was trying to say was, in this case, more isn't really better.
Do try to find the book though, it's an excellent reference on sixes.

[Hair]

Thanks guys,

Slomotion: I got that book, looked yesterday for section on exhaust and didn't see one, looked again after your remarks and found it, pages 122-123. First rule on there says it's better to er in favor of being too small and bigger is USUALLY NOT better. Thanks

292farmer: I have a brand new never used NOS carb that I put on when I added the HEI and headers (got if off ebay for $10, love it) It seems to operate like a dream. If some pipes don't bring everything up to par then I'll look at the jetting. You said that you are running 2" pipes. Are you running duals all the way out, and also, what type of mufflers did you go with?... and is your motor basically stock like mine?