USAC Midget Exhaust Header specs

[hoffman900]

Mostly interested in lengths. It seems engine builders are still favoring very long 4-1 primaries, and just curious if they're really as long as they look, or are just exaggerated because of how small the cars are. I know the Honda K24's are near 380bhp @ 9800rpm and 260lb-ft of torque at around 7800rpm. They certainly seem long for that rpm range, but I know they're searching for driveability as well, so I don’t think they’re around the second harmonic, but look pretty close.

[fastvette]

Seems like most all dirt cars use long tube headers. I think it's like you say for drivability. Smooths out the power band. At least that's my thoughts.
On the dyno shorter tubes will make more peak power.

[hoffman900]

Seems like most all dirt cars use long tube headers. I think it's like you say for drivability. Smooths out the power band. At least that's my thoughts.
On the dyno shorter tubes will make more peak power.

Second harmonic should work over a narrower band.

The WoO builders have been experimenting with 4-2-1’s on and off over the last ten years or so. Some run them, some don’t, but I would have expected them to catch on more in the Midget ranks, and always wondered why you still see the long primary 4-1’s still.

[gruntguru]

Mostly interested in lengths. It seems engine builders are still favoring very long 4-1 primaries, and just curious if they're really as long as they look, or are just exaggerated because of how small the cars are. I know the Honda K24's are near 380bhp @ 9800rpm and 260lb-ft of torque at around 7800rpm. They certainly seem long for that rpm range, but I know they're searching for driveability as well, so I don’t think they’re around the second harmonic, but look pretty close.

https://frontstreet.media/2021/04/02/th ... -learning/
The linked article says the headers are 1 7/8" off the head stepping to 2" for the rest. Scaling from the photos, I get about 18" total primary plus the port - say 3.5" - Total 21.5". That is probably about right for 7,800 rpm with that small ID.
Before the step I get about 4" + port 3.5" = 7.5 long"

[hoffman900]

Mostly interested in lengths. It seems engine builders are still favoring very long 4-1 primaries, and just curious if they're really as long as they look, or are just exaggerated because of how small the cars are. I know the Honda K24's are near 380bhp @ 9800rpm and 260lb-ft of torque at around 7800rpm. They certainly seem long for that rpm range, but I know they're searching for driveability as well, so I don’t think they’re around the second harmonic, but look pretty close.

https://frontstreet.media/2021/04/02/th ... -learning/
The linked article says the headers are 1 7/8" off the head stepping to 2" for the rest. Scaling from the photos, I get about 18" total primary plus the port - say 3.5" - Total 21.5". That is probably about right for 7,800 rpm with that small ID.
Before the step I get about 4" + port 3.5" = 7.5 long"

Thanks. I was too busy to put it in CAD and scale it to match. I suspect it's some as what I said, they're small cars, so appearances are deceiving.

[digger]

If that’s a 2” pipe my painted on eyes say that looks more than 9x diameter

[juuhanaa]

Mostly interested in lengths. It seems engine builders are still favoring very long 4-1 primaries, and just curious if they're really as long as they look, or are just exaggerated because of how small the cars are. I know the Honda K24's are near 380bhp @ 9800rpm and 260lb-ft of torque at around 7800rpm. They certainly seem long for that rpm range, but I know they're searching for driveability as well, so I don’t think they’re around the second harmonic, but look pretty close.

https://frontstreet.media/2021/04/02/th ... -learning/
The linked article says the headers are 1 7/8" off the head stepping to 2" for the rest. Scaling from the photos, I get about 18" total primary plus the port - say 3.5" - Total 21.5". That is probably about right for 7,800 rpm with that small ID.
Before the step I get about 4" + port 3.5" = 7.5 long"

You crazy Man say 1 7/8" and 2" ID or OD doesnt matter small?

I have been told my 2 liter Renault with 29,6mm exhaust valves and 1450mm2 exhaust port at manifold flange is hogged out



-juhana

[hoffman900]

Mostly interested in lengths. It seems engine builders are still favoring very long 4-1 primaries, and just curious if they're really as long as they look, or are just exaggerated because of how small the cars are. I know the Honda K24's are near 380bhp @ 9800rpm and 260lb-ft of torque at around 7800rpm. They certainly seem long for that rpm range, but I know they're searching for driveability as well, so I don’t think they’re around the second harmonic, but look pretty close.

https://frontstreet.media/2021/04/02/th ... -learning/
The linked article says the headers are 1 7/8" off the head stepping to 2" for the rest. Scaling from the photos, I get about 18" total primary plus the port - say 3.5" - Total 21.5". That is probably about right for 7,800 rpm with that small ID.
Before the step I get about 4" + port 3.5" = 7.5 long"

You crazy Man say 1 7/8" and 2" ID or OD doesnt matter small?

I have been told my 2 liter Renault with 29,6mm exhaust valves and 1450mm2 exhaust port at manifold flange is hogged out



-juhana

Yeah, I wouldn’t call it 1 7/8” od small with that tubing, at that power level. NASCAR ran that off the head when pushing 870bhp and WoO 410ci’s run 1 7/8” od tubing off the head and they can make 1000bhp (but useable power, they’re in the mid 900s). That’s about 115-120bhp/cylinder.

Always cracks me up seeing street cars / bracket cars with 2.25” od primaries on 650-750hp engines.

[juuhanaa]

https://frontstreet.media/2021/04/02/th ... -learning/
The linked article says the headers are 1 7/8" off the head stepping to 2" for the rest. Scaling from the photos, I get about 18" total primary plus the port - say 3.5" - Total 21.5". That is probably about right for 7,800 rpm with that small ID.
Before the step I get about 4" + port 3.5" = 7.5 long"

You crazy Man say 1 7/8" and 2" ID or OD doesnt matter small?

I have been told my 2 liter Renault with 29,6mm exhaust valves and 1450mm2 exhaust port at manifold flange is hogged out



-juhana

Yeah, I wouldn’t call it 1 7/8” od small with that tubing, at that power level. NASCAR ran that off the head when pushing 870bhp and WoO 410ci’s run 1 7/8” od tubing off the head and they can make 1000bhp (but useable power, they’re in the mid 900s). That’s about 115-120bhp/cylinder.

Always cracks me up seeing street cars / bracket cars with 2.25” od primaries on 650-750hp engines.

Yeah, but aren't those high hp engines stuck in terms of torque, while Honda makes torque over a wide rev range?

My 2l pump gas Renault junk produces an unofficial 1.5ft lbs/cid at 4000rpm using a total of about 27in long 4-1 headers stepped 9cm from the cylinder head and 38mm to 41.5mm. Those 4-1 headers also do not have collector choke shown in the Honda picture and i have yet to experience an oversized ID.



-juhana