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Is the exhaust header tube diameter too big for my engine?

General engine tech -- Drag Racing to Circle Track

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ELS
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Is the exhaust header tube diameter too big for my engine?

Post by ELS »

So I have a 1.6L inline 4 NA engine that I'm making a header for, because all local exhaust shops only sell pipes that were bent on a regular bender, not those mandrel benders that don't shrink the diameter of the pipe where it's bent.
so I thought of just using the pipes from the stock 2-1 section of the header, but they're slightly too big I think, if I gasket match the heads they'll be 32mm, but the pipe is 33.5mm, is this gonna cause insufficient scavenging pulses?
the exhaust is 55mm, I plan to make a 4-2-1 header, haven't decided on the 2-1 pipe section diameter, probably 40 or 45mm,
then it would be:
33.5mm - 45mm - 55mm

and also I calculated a required 4-2 runner length of 81cm for 6500rpm, and then additional 50cm for 4500... is this right? seems a bit too long.
the exhaust pipes I have are only 50cm in length, if I get desperate I can cut up a rusted but intact header for additional pieces
the cam (measured at 0.2mm lift) (camshaft degrees)
opens the intake at 4 degrees, closes at 140; lift is 9.5mm
opens the exhaust at 263 degrees, closes at 34; lift is 9,6mm
so that would be a 264-272 duration cam with 52 degrees of overlap.
exhaust port length is 95mm, intake port length is 105mm, valve spacing is 40mm, intake runner length is 110mm for the short runners, 150mm for the outside longer ones
give of take a few cm added in funky flow and what not :D
and also the carburetor depth, which is around 100mm

here's the cam profile if you're interested: (1.5x ratio rockers)
cam profile.PNG
the engine will be mostly operated from around 4500-6500rpm.

I reckon I could get 80cm of header pipes in the space I have, but then that additional 50cm for lower rpms if gonna be tricky if I dont want a big 2-1 section sticking out and catching rocks.
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mag2555
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Re: Is the exhaust header tube diameter too big for my engine?

Post by mag2555 »

Diameter is more important if a length, especially if the run of exh tube off of a port will have more then one 45 degree bend in it, or doubly so if there are two bends or more of 90 degrees.

Do you know what your heads flow, if so what is the exh to intake ratio of you have?

Do you have a cam in hand for the motor, and if so then how is that effecting the overall ratio?

I ask because when you bolt a tube up to a exh port if the tube is .080” ( 2.04 mm ) bigger in ID then the exh port you get pressure recovery to take place to the tune of picking up 3 to 5% more exh flow above 6.48 mm of valve lift, and this will effect the ratio.

If you need this added exh flow then as posted the header tube ID off the exh port needs to be bigger and it needs to a straight run for atleast 51.1 mm with the size exh valve I am guessing you have.

Whenever you have a bend of 45 degrees then the ID of that tube should go up by 25 to 30 percent and then go back down.

Any 90 degrees bends should go up in ID by 60 percent and then neck back down.

Look out for possible big added restrictions if you build with used tubing that has rust scale on the inside!!
You can cut a man's tongue from his mouth, but that does not mean he’s a liar, it just shows that you fear the truth he might speak about you!
ELS
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Re: Is the exhaust header tube diameter too big for my engine?

Post by ELS »

mag2555 wrote: Mon May 23, 2022 7:52 pm Diameter is more important if a length, especially if the run of exh tube off of a port will have more then one 45 degree bend in it, or doubly so if there are two bends or more of 90 degrees.

Do you know what your heads flow, if so what is the exh to intake ratio of you have?

Do you have a cam in hand for the motor, and if so then how is that effecting the overall ratio?

I ask because when you bolt a tube up to a exh port if the tube is .080” ( 2.04 mm ) bigger in ID then the exh port you get pressure recovery to take place to the tune of picking up 3 to 5% more exh flow above 6.48 mm of valve lift, and this will effect the ratio.

If you need this added exh flow then as posted the header tube ID off the exh port needs to be bigger and it needs to a straight run for atleast 51.1 mm with the size exh valve I am guessing you have.

Whenever you have a bend of 45 degrees then the ID of that tube should go up by 25 to 30 percent and then go back down.

Any 90 degrees bends should go up in ID by 60 percent and then neck back down.

Look out for possible big added restrictions if you build with used tubing that has rust scale on the inside!!
unfortunately I can't flow the heads, but the valve sizes are 37mm intake valve, 31.5mm exhaust, the valve angle is 19.3 degrees, non-crossflow wedge head.
bore is 79mm, R-S ratio is 1.5 with a stroke of 80mm.
the exhaust ports turn 90 degrees pretty sharply right after the valve, and then I'll need a more straight 90 degree turn where the port exits.
I think the slightly bigger diameter of the pipes will be good then, since I'll have slight bends all along the pipes.

I have some diagrams of the head if that helps:
image0d02.gif
(exhaust on the left, intake on the right)

but I think I'm going at this backwards, I first need to tune the intake manifold, and I plan to use DCOE's in the future, I should probably make the exhaust manifold tuned for an ideal intake runner lenght for a set of DCOE's rather than the current 2barrel manifold.
tuning the intake to 6000rpm, an ideal runner and port length should be ... 1.13 meters?
I've heard that people usually tune for the 2nd and 3rd harmonics so that would be half and a third the size right? so if I have a 57 cm intake runner+port+carb+velocity stack until the airbox I'd only need a
45,5mm long 4-2 section.
and then still the 51cm 2-1 section... but if I lift my requirements from 4500rpm down low to 5500, that's only 20 extra cm.
(excuse me if I made some huge error, I'm new to this)
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ELS
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Re: Is the exhaust header tube diameter too big for my engine?

Post by ELS »

actually going over my calculations, I would want the vacuum formed when the exhaust valve of the 1st cylinder closes to travel at the speed of sound to where the 4-2 section merges, then have that pulse travel back the same distance (when header pipes are equal length) to cylinder 4, trough the cylinder, trough the intake and to the plenum, yes?
then if the duration between exhaust valve closing of the 1st cylinder and 4th cylinder overlap period is around 380 crank degrees,
then 380 * seconds per degree (at 6500rpm) * 340m/s = 3.313 meters, split that in half and I get 1.66 meters
then subtract the port, valve spacing, intake port, runner, carburetor, and velocity stack length (which I calculated to be ideal at 57cm)
166cm - (9.5cm + 4cm + 10cm + 57cm) = 85.5cm would be the ideal length of the 4-2 section for my exhaust header, tuned to 6500rpm with my setup.
then for the 2-1 section I'd need an extra 73 cm for 4500rpm.
that 73 cm will be a pain for sure to get... but I'll try to get it as long as I can then.

now I really get why it sucks to have long gearing :lol:

and then with a header tuned to 2 rpm's so far apart, could it be possible of having a dished power curve? although probably would make descent power overall, that would have most power right where you shift, and have a tendency to break traction.
or not likely?

Oh and I forgot to address that exhaust flow recovery you mentioned, well there's no way I'll be able to get a straight run of 51cm from the exhaust port, but I can try making it straight for as long as I can, probably 20cm would be no problem.
but I thought a constant curve would be more beneficial, like trying to keep the curve of the exhaust port into the exhaust header. but I guess exhaust gases don't have a ton of mass to make that a problem :?
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Re: Is the exhaust header tube diameter too big for my engine?

Post by digger »

Diameter off head is fairly heavily driven by hp per cylinder with some small tweaks for CR and cam specs but those are fairly secondary effects.

32” primary for 6500 sounds about right. I wouldn’t use longer primary as you can use the secondary/ collector to “tune in” the midrange and bottom end as total system length is a tuning variable

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Re: Is the exhaust header tube diameter too big for my engine?

Post by ELS »

well I'm hoping for around 150hp, that would be 37.5 per cylinder
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Re: Is the exhaust header tube diameter too big for my engine?

Post by Walter R. Malik »

Just a personal thought, I think the 2nd section is a bit to big, (maybe 42mm).
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