720 Exhaust Options

[datzenmike]

I'll say 2" on the down pipe.

[Z-train]

Second,thought...agree with Mike.

[that-son]

i remember reading somewhere that back pressure of any kind is bad. straight pipe!

 

anyway, big mistake.  i was being cheap again and instead of going with an  $80 magnaflow, i went with a $30 raptor.  cruising around town gets really exhausting (no pun intended).  sounds like a really nasty subwoofer.

[datzenmike]

Call it restriction. You want the gas speed to be as high as possible down the smallest diameter pipe that does not itself cause a restriction. A tall order as what works at 6K is too big around town driving. If only you could change the pipe diameter as you need it like variable valve timing.

 

Bill don't you have an L16 intake on that LZ dyno engine? (something?) Popular wisdom would say run a larger L20B intake that is less restrictive for better flow but the L16 has higher air speed and the torque numbers are very impressive. I know it the intake but it does demonstrate how bigger isn't always better.

[Z-train]

Mike-correct.I used the L-16 to get the higher intake speed to promote cylinder filling as the cam is all done by 5000 rpm.THe motor was built for low end grunt.

 

 

 

 

 

With Mike Klotz' adapter welded on.

[nismo24096]

Motorcycle exhaust is always an option. I am running straight pipes right off the front exhaust manifold and its "Y"ed off right before the rear axle to run the two mufflers I have.  It does not pop or anything like that when you let off of it like most straight pipes does and with the baffles in the mufflers its even quieter than the video. ps listen with good speakers or headphones so you can hear the low rumble..

[mmitchell57]

Nismo, that is a whole other ball park I never thought about. pretty neato. I already got my exhaust installed. I stuck with the stock diameter, hi-flow cat, and glass pack. Sounds good at low speed, but start to sound like total ass at about 3k +.  Last glasspack I had took a while to break in. Once it was broke in, it sounded good. I'm hoping this one is the same way.  Looks stock, sounds ok. i'll focus on other things that need attention but eventually look at the other options. However, the stock motor doesn't really pump enough junk to worry about it.

[Onderma]

I cracked my muffler, and it kinda got ripped off, so now I'm running stock exhaust, with a cat, except from the muffler and back is ripped off, anyone know if this is bad to run? I understand a muffler gives back pressure, so without my muffler am I doing harm to my engine?

I too am running the stock cat with my rusted muffler gone. Early on I realized it was a bit too loud and id lost some of the giddy up, so I tore the neck out of the old muffler, bent it and bolted it up to the end. I wasn't sure if my Idaho Official redneck exhaust tip would do the trick but it really did. It quieted it down a bit (enough) and gave me a little more back pressure so I even noticed the low end torque go back up to normal.

[harlow426@msn.com]

How noisy is a 720 with the ex cut off @ cat? Super obnoxious? May not get pipes done b-4 Canby & was wonderin' if I could wing it with no muffler.

[datzenmike]

Only the po po might care. No one at Canby will. Fill yer boots.

[Gradyfest]

I like all this exhaust talk, I'm in the research stage. I am taking care of my exhaust leaks from the front first. I am about to seal up my main leak at the manifold and will be working back. I drive a 1980 720 with a L20B and am just now getting serious about exhaust leaks and replacment options. After my manifold leak, this spot is next, (I'll just be patching good metal into it for now.)

 

 

 

[Gradyfest]

After that is this little guy... It's kind of exciting to shop for a muffler. I'm looking for a muffler that is close to OEM and is quite.

 

 

 

I can't wait to not here exhaust leaks anymore. Not gaging or dry heaving every morning when I go to kick the choke down will be pretty nice as well.

[DanielC]

See this thread:

http://community.ratsun.net/topic/61167-how-i-replaced-a-521-exhaust-system/?hl=exhaust

[Stoffregen Motorsports]

After that is this little guy... It's kind of exciting to shop for a muffler. I'm looking for a muffler that is close to OEM and is quite.

 

 

 

I can't wait to not here exhaust leaks anymore. Not gaging or dry heaving every morning when I go to kick the choke down will be pretty nice as well.

Dude, are you running Yokohama rallye tires?

[Gradyfest]

Dude, are you running Yokohama rallye tires?

 

They are Yokohama 742S 7.00R15s. That are the absolute closest thing to the old rating G78-15s that came on the 720 4x4s skinny 5.5" rim. They are 31" tall and snow RADIALS, ride soft! Also being only 7" wide they make a huge difference in turning ease with no power steering. I love them.

 

[Onderma]

I've tuned my carb to the back pressure a little better and gotten used to living with out the muffler. A few friends told me it sounds very retro military when I roll up. It's quieter than many Hondas. Under acceleration the higher rpm throw out a guttural throb, sounds like nothing else. I don't want to say it sounds good to all people, for me it completes the vibe nicely though.

[720inOlyWa]

After  the fuckwads at the Lacey, Washington Midas store installed an unbaffled muffler- backwards- and then quoted $300 to replace the exhaust flange gasket, I made tracks to Eddie‘s Blue Flame Automotive and had Ed Blood do the job right. Ed installed a Honda Accord muffler and while it does not growl at all- a desired outcome in this circumstance- it does give the nicest little Japanese pickup truck exhaust note you could ever want to hear. Sweet and clean.   My 4x4 probably needs a muffler too, and it will get a similar job done. Just sayin’.

[Z-train]

I've tuned my carb to the back pressure a little better and gotten used to living with out the muffler.

The former,I've never heard of and the latter is a bad idea.

[Onderma]

I have a sizeable exhaust tip on now, seems to work

[Z-train]

Seems..operable word.

[datzenmike]

I don't like the word back pressure, but... Exhaust flows out with force by itself. At low speed there is plenty of time and pipe for this. Problems arise when the volume of exhaust begins to crowd the systems ability to flow it all. What's left in the cylinder is pushed out by the piston on the exhaust stroke and uses up engine power. The amount left in the cylinder depends on resistance (or lack of) to flow in the exhaust system eg. small muffler, small pipes, bends, plugged cat or muffler baffles. The more exhaust left in the cylinder the more work the engine does pushing it out. On top of this a properly sized pipe will allow the exhaust to travel at high speed and inertia will help empty the cylinder more efficiently. The only problem is that what works at one RPM is either too large or too small at another.

 

The exhaust left in the cylinder and combustion chamber at the beginning of the intake stroke dilutes the gas and air that enters. A properly sized pipe with few bends and low restriction muffler will allow good 'scavenging' of the exhaust gasses from the cylinder and can even help draw in intake air during the valve overlap period. Again the pipe size works best at a certain RPMs. A 3" pipe is not the answer. This would likely work best at 9,000 RPMs and is counter productive. Just remember that doubling an exhaust pipe diameter increases the flow capacity by 4 times!!!!! A 1/4" to 1/2" increase in size is likely all you need. Your engine spends 99.999% of it's time idling around at low speed so set your aim at mid range performance.     

[DanielC]

I know barely enough to know I do not know a lot about exhaust science.

 

I do know this.  One subject rarely mentioned in discussions about exhaust systems is the effects of sound pressure waves, or pulses.  The exhaust gasses do not flow evenly and smoothly.  

Consider a cylinder, after the plug has fired,, the piston is travelling down the cylinder.  Sometime BEFORE bottom dead center, the exhaust valve starts to open, just barely, at first.  There is still fairly high pressure gas in the cylinder, but the piston is moving pretty slowly, and it is not much use to keep the exhaust valve closed to recapture that energy.

As the exhaust valve starts to open, a high pressure pulse, or sonic wave goes away from the cylinder, at the speed of sound, in the hot gas.   That high pressure pulse moves more rapidly than the actual flow of the exhaust gas, initially.  As the exhaust valve opens more, the gas flow increases, and speeds up.  But the speed of the exhaust gas in limited to the speed of sound.  Near BDC, the piston is moving slowly.  But as the piston moves up, it picks up speed, until about 75 degrees BTDC.  At around that point, if the engine is revved up, piston speeds can be as high as 3,000 to 4,000 feet per second.   Sound travels around 1,100 feet per second in cool air, at sea level, I do not know off hand how fast the speed of sound is in a hot exhaust gas.  So at high engine speeds, the piston is actually compressing the exhaust gas again, trying to get it out of the cylinder, even though the exhaust valve is wide open.   Around this point, (75 degrees BTDC) you hope the exhaust flow is the greatest, and the piston starts slowing down.  Ideally, the speed of the exhaust flow will be at some point greater than the speed of the slowing piston, and then the exhaust flow will help suck, or draw the remaining gas out of the cylinder.

But remember the high pressure sonic pulse created when the exhaust valve first opened?  If the pipe expands, or is open, that pulse can be reflected back, AGAINST the flow of the gas, as a negative pulse.   Similarly, if the pipe contracts, or is partially closed off, the high pressure pulse is reflected as a high pressure pulse.

And not only is there a high pressure pulse made when the exhaust valve just opens, another sonic pulse is made if the engine is running fast enough for the piston to travel above the speed of sound, briefly, in the exhaust stroke.

 

 

 

[Lockleaf]

I strongly suggest David Vizard's book "How to build horsepower in anything" for anyone wanting to understand the actual science behind exhaust system design and the entire internal combustion process from start to finish. Well written, clear, and based on empirical evidence from thousands of engine builds and dyno tests.

[Onderma]

By tuning my carb I mean that I adjusted the idle mixture, it probably needed to be done before but wasn't completely clear to me until the lack of exhaust allowed the truck to stutter like a mofo. At this point in time it is running well. However, I ran some seafoam through the fuel and it cleaned out a nice (and clearly very old) crack in my header. I've reached the limit of half-assing the exhaust so I plan on installing pace setter headers and welding in a glass pack so I can move on to more entertaining fixes like the seats.

[Onderma]

'Back pressure' was clearly the incorrect phrase. I won't let it happen again, z-train.