Exhaust bore size
- Thread starter Ianrhall
- Start date
51 MERC-CT
Well-known member
Bore size would be appox. 1.650( I.D. of tube )
O.D. of tube is 1.750
Don't know about Brockmans but all the Smithy's I've seen take 2" O.D tubing
tubman
Well-known member
Just a question here. Do you chose pipe size for sound or performance? I thought for optimum performance, the bigger the better, When I built my '36 with a loaded 276" Merc 20 years ago, I used Fenton headers and 2" pipes all the way back (A real pain to get to fit, BTW). I thought it sounded fine and I had way too much power for the "traditional" drivetrain.
The in and out on the Smithys maybe 2" but have you cast your eye down inside? They are quite a chunk smaller in there. I'd imagine the flow is awfull. In his huge amount of testing, Flat Dog (may he rest in the sound of flatmotors) stated that smithys were no good for performance. Having looked inside, I can see why.
As for pipe size, the accepted size is 1 3/4", on a stocker that's fine. Start going up in breathing ability, your gonna need a bit bigger. 2" would do most engines, anything bigger than about 2 1/2" is to big mostly. There is an optimum pipe size, that relates to breathing ability, for optimum exhaust gas speed, for good scavenging. To big, and the gas speed is to slow, to small, and the gas speed is to fast.
Just edited this in, blown engines are different, the exhaust can be infinitely big, with no loss in power.
Of course most of this is nonsense, when hung off the back of a Flathead with any of the stock or off the shelf exhaust manifolds or headers. They are all way, way to short for 90% of the flatheads out there. And the stock ports are stupidly angled on the ends, they don't point anywhere near the same direction as the headers. Far to many "reflected" pressure and gas and sound waves, for the exhaust slug to deal with before you can even think about a "tuned" header/exhaust system.
If you haven't read John's book (JWL) you'd probably be amazed how much power (on an honest dynomometer) is released when you pull of the stock manifolds and replaced it with off the shelf headers and twin pipes.
Moral of all this waffle,
Stick on 1 3/4" or 2", you won't make a damn difference on a stock to fairly warm flathead, just sound slightly different twix the two.
Martin.
As for pipe size, the accepted size is 1 3/4", on a stocker that's fine. Start going up in breathing ability, your gonna need a bit bigger. 2" would do most engines, anything bigger than about 2 1/2" is to big mostly. There is an optimum pipe size, that relates to breathing ability, for optimum exhaust gas speed, for good scavenging. To big, and the gas speed is to slow, to small, and the gas speed is to fast.
Just edited this in, blown engines are different, the exhaust can be infinitely big, with no loss in power.
Of course most of this is nonsense, when hung off the back of a Flathead with any of the stock or off the shelf exhaust manifolds or headers. They are all way, way to short for 90% of the flatheads out there. And the stock ports are stupidly angled on the ends, they don't point anywhere near the same direction as the headers. Far to many "reflected" pressure and gas and sound waves, for the exhaust slug to deal with before you can even think about a "tuned" header/exhaust system.
If you haven't read John's book (JWL) you'd probably be amazed how much power (on an honest dynomometer) is released when you pull of the stock manifolds and replaced it with off the shelf headers and twin pipes.
Moral of all this waffle,
Stick on 1 3/4" or 2", you won't make a damn difference on a stock to fairly warm flathead, just sound slightly different twix the two.
Martin.
Last edited:
Talkwrench
Well-known member
I maybe wrong here but on a flathead [unless a race motor] I doubt you could tell the difference between 1 3/4" and 2" it would be more in the sound difference. Again see Johns book.
In the 1960's my father ran twin 1 inch pipes on his Pilot with no mufflers and as a kid I loved the cackle. When I experimented in the 70's with my F1 I ran 2 inch pipes with '*****' mufflers, but the best sound I got was running 5 feet of 3 1/2 inch tail pipes behind these mufflers that exited under the running boards in front of the rear wheels. As a bigger kid now I'm back to running the same sort of mufflers with 2 inch pipes to the tailgate, attached to Speedway centre-dump headers. No noticeable difference in performace compared with the stock manifolds and single pipe I had on previously.
