Exhaust headpipe flange

[tekenaar]

Many here enlarge their exhaust pipes for better flow when they rebuild or "hop-up" their engines. There are many threads covering various approaches and recommendations for doing this . . . that is, from the headpipe junction all the way back to the rear of the car because that is relatively easy to do.

It's been a while since I've had to actually separate a head pipe from either of the stock cast-iron manifolds and hadn't given it any real thought until now. As you know from a couple of other on-going threads, I'm removing my ~40-year old 1.9 engine from my Kadett in order to investigate a #1 rod knock and to change the intake/exhaust over to the twin DFO "sprint" intake and exhaust.

I've already related how easy it was to separate the headpipe from the stock exhaust manifold by my "PB Blaster Prep" method elsewhere. Engine is still in the car and I noticed how much excess weld protrudes into the stock headpipe exhaust flange holes. Begs the question, when enlarging exhaust pipes and mufflers for better flow, does anyone bother to check this area of restriction?!

First attached pic shows outer pipe cleaned up and inner pipe as it came from the factory 40 years ago . . . quite a bit of flow restriction at the stock pipe! Second picture shows both flange pipe welds cleaned up . . .

[neuropel]

Otto,

I recently picked up a local stash of NOS opel exhaust pipes and as I was loading them I noted (and wondered) the same thing. Looks to me like it can produce, at least from appearance, a fair bit of restriction. I believe the 2.4 downpipes are manufactured a little better.

Todd

[wrench459]

Originally Posted by tekenaar I've already related how easy it was to separate the headpipe from the stock exhaust manifold by my "PB Blaster Prep" method elsewhere. Engine is still in the car and I noticed how much excess weld protrudes into the stock headpipe exhaust flange holes. Begs the question, when enlarging exhaust pipes and mufflers for better flow, does anyone bother to check this area of restriction?!

What goes in must go out.. so YES, we, as a collectively whole, look very closely at these things.
IMOA that is.

[tomking]

I noticed them when installing mine; therefore I ground them off and smoothed out the transition.

[tekenaar]

Originally Posted by neuropel Otto, I recently picked up a local stash of NOS opel exhaust pipes and as I was loading them I noted (and wondered) the same thing. Looks to me like it can produce, at least from appearance, a fair bit of restriction. I believe the 2.4 downpipes are manufactured a little better. Todd

From the looks of mine - the way they were originally, that is - and from some measurements I took, it looks like each downpipe was areally (area/flow) restricted by ~23%! . . . 1.5" ID (1.767 in²) - 2x .125" weld lip (radius) = 1.25" effective ID (1.431 in²), NOT GOOD!

[tekenaar]

Originally Posted by wrench459 What goes in must go out.. so YES we as a collectively whole looks very closely at these things. IMOA that is.

I like that you're a "half glass full" type, but, having read/heard about all the horror stories associated with first-time removal of 35-year-old headpipes from exhaust manifolds (bolt access, broken bolts, etc.), I doubt that many "newbie/novice" members here would actually include that when considering enlarging their exhaust from the "headpipe junction" back! . . . my take on it anyway.

Most will likely just cut the headpipes at the junction and be done with it . . .