tiny exhaust

[gearhead]

I was just wondering the reasons why alot of the old (70's-80's) japanese vehicles ran such tiny exhaust systems?The rear tailpipe section on my truck looks like something a riding lawnmower would use,not a 2.0L four cylinder.My 81 subaru when it left the factory with a 1.6L engine with heads that flow so poorly they are an example of what NOT to do, came with 2" pipe.So was there some reason for the tiny exhausts-especially after the muffler?Just kinda something that was bothering me.The subaru was an example of a decent pipe.My 82 mazda 2.2L DIESEL came with 1 7/8".Replacing all that with some 2.5" tubing really woke the truck up.Surely the difference between 1 7/8 and smaller exhaust tubing and 2,2 1/4,and 2 1/2 tubing could have been that much right?

[bonvo]

noise and choking it back for emissions seems to be the only reasons but you need to keep in mind that all factory exhaust systems (with a few exceptions) are to restrictive 

 

subari ea71s didnt come with 2 inch exhaust i had to make a chunk of 2 inch a little smaller to patch the system on my wagon it looks bigger then it actually is due to the heat shields on it 

[JoeCool]

My truck has a MIDAS muffler on it now with what looks like 2" or 2 1/4" pipe going in, with a tiny 1 1/2" pipe coming out. The tailpipe and muffler hangers were broken and the pipe dragged the ground over bumps, so I tied up the pipe just before the muffler with a metal coat hanger, and cut off the tail pipe right behind the muffler. Got a little louder, and a little more power. For now that is my exhaust. I want to do something custom 2.5" with a rear exit eventually.

[Z-train]

Exhaust 101:

1)Do not compare diesel to gas-two entirely different animals.

2)Factory engineers were not idiots.Having said that,they had specs to meet that we don't.Once the choke point(smallest restiction is met(end of manifold)going bigger doesn't help much.

Tail pipes.All they do is change sound.They have ZERO effect on power or MPG's.

[datzenmike]

To an extent the exhaust cools as it goes down the pipe the gasses contract and a smaller pipe is large enough and cheaper. Cars don't need a large pipe diameter to do their jobs. Bigger isn't always better. For one thing gas speed should be kept as high as possible without restricting. High gas speeds help empty the cylinder. Trouble is this works best for a fairly narrow range and car makers don't care much about driving their product at 6K. It only has to work at usual speeds.

 

You can gain a small amount by increasing the pipe size to increase flow if expecting high RPMs or a modified better breathing motor. Too big a pipe and the gas speed drops and there is a loss or at best no change. Increasing the pipe diameter double, say from a 1.5" to a 3" will increase the cross sectional area by 4X!!!! not double. If you have a 1.3/4" pipe and you increase it to 2" you will increase the pipe flow by 30%. What motor needs a 30% increase like that??? Usually a 1/4" increase in pipe size is plenty.

[racerx]

Bigger exhuats does not mean better..a lot is just show and sound, which means more money in your end.  You have 2 markets going against your better judgment. The manufacturer who will do what it can to build as cheap as possible and meet standards, then you have the aftermarket parts people who will try to market their products to and say it will improve mileage, performance and of course the sound. Like you will see a lot of Honda's with their 3" tips or larger mufflers, sounds okay but look at how much they cost. 

[datsunaholic]

Smaller pipes can be beneficial.  In some instances a smaller pipe (not too small) helps low-end torque by enhancing extraction of burned combustion gasses,  Too large a pipe can cause exhaust gasses to linger due to reduced velocity, which can burn out the valves.  It's less of an issue with forced induction.

 

That being said, exhaust size was made mostly to get the best performance out the the emissions system.  Especially on EGR equipped vehicles and ones with catalytic converters.

[gearhead]

I just was under the impression that a 2.0L used a 2" pipe,and 2.2-2.4L used 2.25" to get the maximum flow at most useable RPM's for a street car.You can go bigger if all your driving is at high RPM..

 

I know diesels and gas engines are two completely different things.Gas exhaust is light and fast moving,Diesel exhaust is heavy and slow moving so diesel really likes bigger pipes to help flow that mass out.Even if you go slightly too big on a NA diesel, you will only loose a small amount at low RPM,but gain alot of mid-high rpm power which is where diesels lack at.

 

go a touch to big with gas and the engine will be a slug until your high enough up in the R's.

[racerx]

Good article about size of exhaust, specially what size is recommended..

 

http://www.sfxperformance.com/articles/446

[Z-train]

Diasagree-It's one of the worst i've read.He can't even get the definition of "fart can" right.He makes assumptions and blanket statements.Really NFG.

 

Exhaust gases leave the head around 1400 degrees.By the time they reach the muffler,they are down to 1000 degrees.Bigger pipes as you head towards the rear of the vehicle are not needed.

[datzenmike]

Also throws the words 'back pressure' around too much.

 

If an L20B uses a 2" pipe then a Z22 will need a pipe that is 10% larger in cross section. A Z24 needs a 20% larger cross section. Assuming the motors are exactly the same except the displacement....  which they sure ain't. This works out to...

 

L20B.... 2" diameter

Z22...... 2.1"

Z24...... 2.175"

 

Not worth the bother is it? I think Nissan runs a 2" down pipe on the 720 Z24. Going up 1/4" on any exhaust is likely more than enough for the average motor to 'open it up' without slowing gas speeds down too much and suffering a loss.

 

A 2.24" pipe is 26.5% larger than a 2" pipe. Does your L20B make so much power that it really needs 26% more exhaust pipe????? The exception is if running a turbo or multi carbs and racing at 7-8K.

[Z-train]

I'm running a 2.25 head pipe on my 2.3 stroker.Plenty of pipe-any bigger and it would hurt it.

[gearhead]

like I said 2" on 2L and 2 1/4 on 2.2-2.4L but I am pretty sure the factory exhaust on my 74 620 is not 2",its down right tiny.

[Laecaon]

like I said 2" on 2L and 2 1/4 on 2.2-2.4L but I am pretty sure the factory exhaust on my 74 620 is not 2",its down right tiny.

 

 

So what about on my sisters minivan with a 3.8L V6?

 

Should she have a 3.8" exhuast?

[Dat Lurka]

What about my caddy with the 8.2?  :rofl: 

 

 

The 620 is 1.75 and opening it up will make a difference. Just chop it off under the cab and you'll have plenty of flow. 

[datzenmike]

like I said 2" on 2L and 2 1/4 on 2.2-2.4L but I am pretty sure the factory exhaust on my 74 620 is not 2",its down right tiny.

 

 

 

The '74 should have a smaller L18 so 1 3/4"- 1 7/8" maybe.

[HRH]

2 1/4 pipe is absolutely plenty for and Datsun 4 cylinder that has been played with.  For a stock L20, 2" is fine.

 

Think of a hose vs. a drain pipe.  A one cubic inch volume of water will move very nicely down a one cubic inch volume of hose without restriction and will keep flowing well, as in, the stream coming out of the hose will push far past the end of the hose before it falls to earth.

 

A 3" drain pipe let's say, with the same cubic inch volume of water will dribble down the drain pipe and dribble at the end.  Bad flow=bad performance.  It won't scavenge out of the cylinder head very well and you'll be rewarded with less than ideal combustion gases not being evacuated.  If you were to increase the flow, think of how the water rushes in a storm drain and splooshes about, hitting every non-straight section.  Same way with exhaust gas.

 

Matching pipe to cylinder head flow is quite important.  If you're running 3" pipe on your 2.0 Datsun motor, it better have some sort of forced induction.

[gearhead]

my 74 has a l20b,Just I don't think the exhaust was changed with the engine.Might be why 110 hp in a 2300 lb truck didn't feel that fast to me..because the engine wasnt actually making its 110 hp.

[DanielC]

Back to the original question.

"I was just wondering the reasons why alot of the old (70's-80's) japanese vehicles ran such tiny exhaust systems?"

 

I have four 521 Datsun pickups.  The earliest was made in 09/69, the latest was made in 03/71.  Going by the serial numbers, in that 19 month period, Datsun made 347,037 521 trucks. 

In every car company that mass produces vehicles, there is an accountant, that goes over every part of every vehicle, asking how can we reduce the cost of this vehicle, and the account will focus on every single part.

 

In my example above, if Datsun had a choice of spending only 1 yen on a bigger pipe for the exhaust system, or not, by going to the smaller, but cheaper pipe, Datsun just saved about 350,000 yen.

 

A smaller pipe, is easier to bend.  A smaller pipe, easier to fit between the engine, torsion bars, and transmission.

 

 

Now, to exhaust theory.  Something that is being ignored in this discussion is the fact that exhaust flow out of the engine is not constant.  it starts and stops.  Another factor affecting exhaust flow is pressure waves in the exhaust system, or sound waves.

When the exhaust valve opens, a high pressure pulse starts moving down the manifold, faster than the actual gas flow.  When this high pressure pulse gets to a place that opens up bigger, it is reflected back up the exhaust pipe, as a negative pulse, and likewise if it meets a restriction, it is reflected back as a positive pulse.

This principle is used in a two stroke motorcycle engine.  The expansion chamber type of exhaust pipe is a pipe for a short distance, a increasing in size megaphone, a short piece of big pipe, and a reverse megaphone, and finally the pipe that dumps in to the atmosphere.

When the motorcycle engine is running at the RPM the pipe was made for, the exhaust port opens, the high pressure pulse travels to the opening megaphone part of the pipe, and is reflected back to the cylinder as a negative pulse, and this negative pulse actually helps suck more exhaust out of the cylinder, and in fact it sucks a good portion of fresh air fuel mixture out.  As the original pulse travels farther down the expansion chamber, the positive pulse meets the reverse megaphone, and is then reflected back as a positive pulse, and again, if the engine is running at the right RPM, this positive pulse arrives back at the cylinder just before the exhaust port closes, and pushes the fresh air fuel mixture back into the cylinder, just before the exhaust port closes again.   This is what causes the "surge" of power when a two stroke motor cycle engine comes "on the pipe".  This works very well on a two stroke engine, because both the intake or transfer ports are open when the exhaust port is open, and the exhaust port opens first, and last in the two stroke cycle.

 

The same sonic principles occur in a multi cylinder four stroke engine, but they are way more complicated.  We all have heard shorter headers are better for high RPM power, and the same is also true of intake manifold runners.  On a multicylinder engine, it is also possible to join different cylinders together, to allow one cylinder to help scavenge the other cylinder.   On a four cylinder Datsun L-16manifold, there is a reason that cylinders 1 and 4 are joined, and cylinders 2 and 3 are joined together, and finally the joined pipes are  joined again to make one pipe.

[Z-train]

Back to the original question.

 

 

1)A smaller pipe, is easier to bend.  A smaller pipe, easier to fit between the engine, torsion bars, and transmission.

 

 

Now, to exhaust theory.  Something that is being ignored in this discussion is the fact that exhaust flow out of the engine is not constant.  it starts and stops.  Another factor affecting exhaust flow is pressure waves in the exhaust system, or sound waves.

When the exhaust valve opens, a high pressure pulse starts moving down the manifold, faster than the actual gas flow.  When this high pressure pulse gets to a place that opens up bigger, it is reflected back up the exhaust pipe, as a negative pulse, and likewise if it meets a restriction, it is reflected back as a positive pulse.

This principle is used in a two stroke motorcycle engine.  The expansion chamber type of exhaust pipe is a pipe for a short distance, a increasing in size megaphone, a short piece of big pipe, and a reverse megaphone, and finally the pipe that dumps in to the atmosphere.

When the motorcycle engine is running at the RPM the pipe was made for, the exhaust port opens, the high pressure pulse travels to the opening megaphone part of the pipe, and is reflected back to the cylinder as a negative pulse, and this negative pulse actually helps suck more exhaust out of the cylinder, and in fact it sucks a good portion of fresh air fuel mixture out.  As the original pulse travels farther down the expansion chamber, the positive pulse meets the reverse megaphone, and is then reflected back as a positive pulse, and again, if the engine is running at the right RPM, this positive pulse arrives back at the cylinder just before the exhaust port closes, and pushes the fresh air fuel mixture back into the cylinder, just before the exhaust port closes again.   This is what causes the "surge" of power when a two stroke motor cycle engine comes "on the pipe".  This works very well on a two stroke engine, because both the intake or transfer ports are open when the exhaust port is open, and the exhaust port opens first, and last in the two stroke cycle.

 

The same sonic principles occur in a multi cylinder four stroke engine, but they are way more complicated.  We all have heard shorter headers are better for high RPM power, and the same is also true of intake manifold runners.  On a multicylinder engine, it is also possible to join different cylinders together, to allow one cylinder to help scavenge the other cylinder.   On a four cylinder Datsun L-16manifold, there is a reason that cylinders 1 and 4 are joined, and cylinders 2 and 3 are joined together, and finally the joined pipes are  joined again to make one pipe.

1) Faulty logic.The engineers designed the motor to meet certain standards and what the motor needed to reach those goals.

 

2) Yes,exhaust flow is a "constant",not like water from a hose,but in relative terms it is a constant.Comparing two strokes is just as bad as comparing diesels.

[crackerjack69]

I thought exhaust flows in pulses.... The important part for flow is the pulses being released from the downpipe in a proper pattern so they don't interfere with each other. Which the Datsun engineers accomplished quite well. As long as the pulses are arranged properly by the manifold runner lengths, the only thing that matters past the downpipe is the ability of the pipe diameter to transfer exhaust to the back of the vehicle where it can exit with less offense to the occupants of the cab. So like Mike said, going a whole lot bigger will at best create no change in performance. And at worst, hinder the cylinders from being properly evacuated. This is essentially a "If it aint broke, don't fix it" situation. Unless you are using your Datto for high performance applications, the stock exhaust has worked for 40+ years, and will certainly continue to.

[DanielC]

The last time I looked at my engine, it seems the exhaust valve is closed from slightly past TDC on the intake stroke, all the wat throught the compression stroke, and for a good part of the power stroke.

[ssnapz]

My stock pipe on my 85 z24 was 1 3/4, I run 2 1/2.now and it loves it. I do run my motor to 65 to 7 though. And power don't start till 3.

[Z-train]

Your borderline too big.And that Z-24 isn't make much power above 5500.

[ssnapz]

I have a mild cam, Holley carb, and headers. With the right outside temp it will pull to 7k. Hot out only 6500. Power don't start till 3k