Really stupid question about dual exhaust pipe sizes

[df2x4]

lol. the idiot that says "THAT is sweet!" is a perfect example of the quality of information to be found these days with a youtube search. proves beyond a doubt one needs no intelligence or integrity to make money.

I mean I agree for the most part but come on, it is pretty sweet.

 

[Erik the Awful]

Or you weakened the vacuum signal on the carbs booster.

The only difference was the headers. Intake, cam, everything was the same except the headers. Saying the vacuum signal was off would mean I was jetting richer for no reason, but the plugs showed I was running lean. I was able to jet richer because I was more effectively evacuating the cylinder, i.e. less backpressure. It could be true that I was losing some of that to overlap.

Back pressure is not going to prevent an engine from dropping a valve due to weak valve springs.

Valvesprings pull in the other direction. I've always heard warnings not to run a motor without headers or a manifold bolted in place, and I've seen a valve that had been "sucked". If that was all BS, I won't argue, but I'm going to need some evidence.

 

[Mark Gilbert]

The only reason I ever worried about too short of exhaust was that I worried that it would allow the valves to cool too quickly when the motor was shut down. Now that may not really be a concern, but it was the old wives tale that I had heard and made at least some sense. It also may have only applied to engines running different fuels that may run the exhaust valve hotter.

 

[Redneckgeriatric]

The only difference was the headers. Intake, cam, everything was the same except the headers. Saying the vacuum signal was off would mean I was jetting richer for no reason, but the plugs showed I was running lean. I was able to jet richer because I was more effectively evacuating the cylinder, i.e. less backpressure. It could be true that I was losing some of that to overlap.

Valvesprings pull in the other direction. I've always heard warnings not to run a motor without headers or a manifold bolted in place, and I've seen a valve that had been "sucked". If that was all BS, I won't argue, but I'm going to need some evidence.

i want evidence too. how about the blow torch theory. reversion pushing air on redhot valves. reversion is your enemy. reversion is your enemy, etc:. happens alot on exhaust tubing that spent more dollars advertising than designing.

 

[evilunclegrimace]

no one wants backpressure. it is exactly what is says.....pressure. what you dont want is reversion which is allowed when using too large/short an exhaust.

Reversion is caused by backpressure.

https://help.summitracing.com/app/answers/detail/a_id/4805/~/exhaust-reversion

 

[Erik the Awful]

If just past your exhaust ports you open the exhaust big and wide, your exhaust velocity stalls. You are then more susceptible to reversion. That's why I stepped my headers - my exhaust velocity might slow, but it keeps moving, and since the next step is bigger, there's still room for the exhaust to expand toward the exit. That's also why megaphone headers are so effective. It's most effective if you can keep the exhaust flowing straight for at least two inches before you have any bends in the primaries. I didn't have that option on the Cadillac headers.

You must be registered for see images attach

They look like hell, but they flow like the mighty Mississippi. No, I do not have a tubing bender. Mandrel bends are fairly overrated, and this was a Lemons car, so I wasn't going to spend big dollars on the exhaust.

Years ago there was an engineer named Paul Yaw who built rotary engines in his shop on the west coast. He did a lot of bench testing with an airflow meter he built, and discovered a lot of this and wrote really good technical articles. Mazda contracted him to do some work on the Renesis motors, but when the rotaries faded out he started working on LS motors. I wish I still had those articles saved somewhere.

Doh! I just remembered his name, googled it, and found his old articles!
http://web.archive.org/web/20051222093839/http://www.yawpower.com/techindx.html

He now runs Injector Dynamics.
http://injectordynamics.com

 

[Redneckgeriatric]

Reversion is caused by backpressure.

https://help.summitracing.com/app/answers/detail/a_id/4805/~/exhaust-reversion

they have reversion and backpressure confused. backpressure will build a wall reversion cannot penetrate. backpressure is .....pressure building in an area from valve face to exhaust tip.backpressure is a flow restriction.
reversion is atmosphere allowed to travel into the exhaust system because there is not enough exhaust velocity to keep it out. heat in exhaust allows higher velocity and helps keep reversion at bay. imagine cool air running up your pipe and hitting warm air halfway up pipe. it causes a little thunderstorm sorta. this is a restriction! if pipe stays cool and allows reversion to travel far enough, it causes AFRs to go crazy and give wrong data to the ecm which causes rich or lean spots in the tune. then you try and tune that out, but it is based on lies givin to the ecm. lambda goes nuts.

 

[Redneckgeriatric]

If just past your exhaust ports you open the exhaust big and wide, your exhaust velocity stalls. You are then more susceptible to reversion. That's why I stepped my headers - my exhaust velocity might slow, but it keeps moving, and since the next step is bigger, there's still room for the exhaust to expand toward the exit. That's also why megaphone headers are so effective. It's most effective if you can keep the exhaust flowing straight for at least two inches before you have any bends in the primaries. I didn't have that option on the Cadillac headers.

You must be registered for see images attach

They look like hell, but they flow like the mighty Mississippi. No, I do not have a tubing bender. Mandrel bends are fairly overrated, and this was a Lemons car, so I wasn't going to spend big dollars on the exhaust.

Years ago there was an engineer named Paul Yaw who built rotary engines in his shop on the west coast. He did a lot of bench testing with an airflow meter he built, and discovered a lot of this and wrote really good technical articles. Mazda contracted him to do some work on the Renesis motors, but when the rotaries faded out he started working on LS motors. I wish I still had those articles saved somewhere.

Doh! I just remembered his name, googled it, and found his old articles!
http://web.archive.org/web/20051222093839/http://www.yawpower.com/techindx.html

He now runs Injector Dynamics.
http://injectordynamics.com

damn thats ugly . lol J/K, that was a lot of work and will make great midrange tq!
how many noticed your tri=y design?

 

[evilunclegrimace]

they have reversion and backpressure confused. backpressure will build a wall reversion cannot penetrate. backpressure is .....pressure building in an area from valve face to exhaust tip.backpressure is a flow restriction.
reversion is atmosphere allowed to travel into the exhaust system because there is not enough exhaust velocity to keep it out. heat in exhaust allows higher velocity and helps keep reversion at bay. imagine cool air running up your pipe and hitting warm air halfway up pipe. it causes a little thunderstorm sorta. this is a restriction! if pipe stays cool and allows reversion to travel far enough, it causes AFRs to go crazy and give wrong data to the ecm which causes rich or lean spots in the tune. then you try and tune that out, but it is based on lies givin to the ecm. lambda goes nuts.

That is no where near correct But you go on believing what ever you like

 

[Erik the Awful]

A little more diplomatic is to explain that reversion is gases in the exhaust moving in the wrong direction, which is usually caused by backpressure. Also remember that hotter air will have more pressure than cooler air.

Your exhaust gases are a collection of individual pulses from each cylinder. If each pulse is moving with high velocity, you are going to have more time between the pulses, and more reversion between the pulses. This is also why H and X pipes are more effective, it uses the venture effect to put a vacuum on the opposite bank of cylinders, preventing reversion there, which is reciprocated when that bank fires a pulse past the merge.

All that said, I learned all this reading other people's lab work. My own experience is limited to building Cadillac headers and a couple exhaust systems.