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Exhaust question!


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#1 Johnboyee

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Posted 09 September 2015 - 07:16 PM

82 z22 King can, got weber running nice thanks guys for input, now onto next issue, previous owner ran an 1 1/2 ID straight pipe from the factory 2 to 1 collector and exited it out the vehicle infront of rear wheel, sounds like total crap and loud, truck does have air ride suspension so fram lays on the ground, I read the factory header was sufficient so sticking with that, was gonna run 2 1/4 ID pipe from factory collector, but problem I'm having is the pipe will hang lower than frame by way to much until after transmission. Since the piping at the collector is two 1 1/2 ID tubes, can I run two 1 1/2 tubes from the collector to past the transmission crossmember then turn it into one 2 1/4 pipe then resonator? I know it doesn't sound like much but every 1/2 inch of clearance helps and I believe this would give me the most clearance available as long as it doesn't inhibit performance. I am open to any other ideas with exhaust to gain maximum ground clearance!

#2 datzenmike

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Posted 09 September 2015 - 07:23 PM

I don't see why this won't work but I would use a 2" pipe after the two 1 1/2" join together. The exhaust gasses cool and shrink and this is why the pipes get smaller towards the rear. The Z series are not big revvers and there isn't a lot of exhaust to get rid of in the first case. Even 2" is probably too big but....


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#3 Johnboyee

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Posted 09 September 2015 - 07:51 PM

I was also toying with the idea of just running dual 1 1/2 all the way

#4 datzenmike

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Posted 09 September 2015 - 07:53 PM

Try it. I've heard they sound like shit but what does that mean exactly.


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#5 regularJJ

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Posted 17 September 2015 - 04:47 PM

Yah im sure it sounds and runs like shit cause stock this truck ranges 1 5/8" to 1 3/4" with a lot of bends in the pipe. Z22 does not run well when you fuck with the backpressure, as Ive recently found out. Im about to find out how it sounds at 1 3/4" with a thrush glasspack only, I suspect it will sound awful and will not create enough backpressure to run right.

 

I suggest piping it at 1 3/4", datzenmike is probably right 2" could work but is likely too much. If you want it to have a 2 1/2"-3" pipe hanging out the back just stick a fat tailpipe on there.



#6 datzenmike

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Posted 17 September 2015 - 07:17 PM

It isn't 'back pressure'. Back pressure is resistance to gas flow.  An engine will run better with less resistance to flow as the engine does not have to waste power pushing the exhaust out the pipe. Now in some cases a too small pipe will be the cause of this back pressure, but generally the factory pipe is correct enough. If you were to add a turbo charger or a larger cam and bigger or multi carbs and increase the amount of exhaust gasses this may need a slight increase in pipe size to compensate. What you want is a pipe small enough to have high gas speed but not to the point of bring restrictive. A tall order.

 

Generally a quiet muffler has more 'back pressure' and a cherry bomb or turbo style will have less. You will gain much more from having a less restrictive exhaust, than from increasing the pipe diameter.

 

Here's an example of pipe size and increasing the pipe size...

 

A 1 3/4" inner diameter pipe has a cross section of 2.404 square inches.

A 2" inner diameter pipe has a cross section of 3.14 square inches or an increase of 30.6%!!!!! That's one third larger!!!!. That's the same as an L16 that suddenly flows the exhaust volume of an L20B!!!

 

This is only a 1/4" increase. A 1/8" increase is still almost 15% increase in area.

 

Don't go crazy on increasing your pipe diameter, you don't need much to flow a LOT more.


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#7 regularJJ

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Posted 18 September 2015 - 05:11 PM

Hes allowing too much flow for what the engine needs, therefore he would need to restrict that flow with some 'backpressure', like the factory 1 3/4" creates.

 

If it ran better with more flow they would have built it that way. For example Im running 3" when its just the manifold and it runs like absolute shit. I didnt see him say he had a weber or anything if he did yes he would need more flow and less back pressure.



#8 Lockleaf

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Posted 18 September 2015 - 08:31 PM

Backpressure aiding an engine is a myth. That is not the advantage added by smaller pipe. All backpressure is negative to performance. The advantage added by smaller pipe is increased exhaust velocity. This increased exhaust velocity helps to create what is called scavenging, where the exhaust going out actually vacuums out the burned air from the cylinder which in turn helps vacuum fresh air in while the valves overlap. The result is a better filling of the cylinder with fresh air and fuel, thus creating more power. Good exhaust increases volumetric efficiency of the engine. Too big actually creates worse performance then factory overall.

However, there are plenty of times where a small step up can show improved performance. Often manufacturers leave performance on the table in a tradeoff with noise/fitment/cost of larger exhaust. Sometimes a maker will purposefully choke an engine to reduce air flow, making the engine weaker, but increasing mpg.

Half Pint ('71 510 Goon) http://community.rat...agon/p?=1269361

 

Audrey ('69 Roadster 1600) http://community.rat...rey-the-ratster

 

Tiny Havok ('85 720 DIY built Crewcab 4x4) http://community.rat...ache/?p=1395455

 

720 Shenanigans http://community.rat...20-shenanigans/


#9 regularJJ

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Posted 19 September 2015 - 07:58 AM

Ah, apologies I misunderstood how backpressure works. DIdnt realize a bigger pipe actually hurts the exhaust velocity which is what causes the loss in low end torque.

 

Original point stands tho and I still think all these 720's look fucking stupid as lowriders with 3" pipes  :poop:



#10 Lockleaf

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Posted 19 September 2015 - 08:08 AM

^^^^ Yes to this. :)

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#11 datzenmike

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Posted 19 September 2015 - 09:20 AM

Backpressure aiding an engine is a myth. That is not the advantage added by smaller pipe. All backpressure is negative to performance. The advantage added by smaller pipe is increased exhaust velocity. This increased exhaust velocity helps to create what is called scavenging, where the exhaust going out actually vacuums out the burned air from the cylinder which in turn helps vacuum fresh air in while the valves overlap. The result is a better filling of the cylinder with fresh air and fuel, thus creating more power. Good exhaust increases volumetric efficiency of the engine. Too big actually creates worse performance then factory overall.

However, there are plenty of times where a small step up can show improved performance. Often manufacturers leave performance on the table in a tradeoff with noise/fitment/cost of larger exhaust. Sometimes a maker will purposefully choke an engine to reduce air flow, making the engine weaker, but increasing mpg.

 

I'm starting to like you....

 

'Backpressure' (if there even is such a thing) is what I call resistance to flow. Gas that is overcrowded in the pipe and more is crammed in behind it. In some cases, maybe a small amount, in all cases only at very high RPMs, the pipe size and number and sharpness of bends will offer some resistance to flow. At low and moderate speeds it's an empty pipe and gasses have an easy way out through it. Now anything else, like a resonator, a catalytic converter and specially a muffler, DOES resist flow by blocking the path of the gasses trying to get out. The pipe size and bends are nothing compared to the resistance to flow provided by a muffler.

 

A muffler resists sudden loud pressure pulses in the exhaust and delays their exit over time. Think firecracker sharp POP, vs, burning the same amount of gun powder. Time delay dilutes the volume. Generally the quietest muffler is most restrictive. Reducing this restriction AND being quiet (or acceptably quiet) is difficult.

 

As lockleaf stated, gas flow must be kept high rather than low for scavenging. This is free power wasted by a larger pipe and slower gas speeds. Any exhaust not pulled from the cylinder has to be pushed out by the upwards movement of the piston on the exhaust stroke.

 

So, again don't go crazy on exhaust pipe diameter. Bigger is not always better. The factory pipe is probably close, so make a modest increase in diameter if making a cam choice or larger carb. Proportionally larger if adding a turbo as the exhaust volume will be half to double more than stock. This does not mean to double the pipe size. Doubling the diameter increases the cross section 4 times!!! Very roughly to double the cross section multiply your pipe size by 1.4 to increase by about 30% add 1/4" to your pipe size.

 

You will gain more from replacing a restrictive muffler than you will from a larger pipe. Yes a small pipe diameter increase 'might' get you some gains but it's equally possible the factory pipe is good enough and now you have dropped the gas speed and lost something. In order to gain anything there as to be losses you can recoup. Your system might be good enough.

 

Keep this in perspective. Exhaust restriction is most pronounced at full throttle and high RPMs. The rest of the time there is lots of room in that stock pipe for the exhaust. So where do you drive most? Racing? with ultra high RPMs and full throttle? Or mostly normal part throttle cruise?


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#12 Lockleaf

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Posted 19 September 2015 - 12:12 PM

Thanks Mike. I take that as a compliment :)

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#13 Scomber

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Posted 20 September 2015 - 09:48 AM

New guy here.  

I'm looking to get my second Datsun:  a 720.  My first was a 521, 18-15 years ago.  What I'm driving now is an '86 Toyota Pickup, 2.4L, 1 3/4" pipe.  In the last year, I've replaced the muffler and the cat, but not at the same time.  If I had it to do again, I'd do both at once and use 2" pipe, for the simple reason that 1 3/4" pipe and exhaust parts are getting to be an odd size out there.  I couldn't find a slip-fit universal cat, and had to use a 2" and make my own half-assed adapters.  

I'm not a speed and power freak.  I'm not a fanatic about everything being original.  I want a truck that gets me from here to there and hauls stuff efficiently.  And when it breaks, I want to be able to fix it with the least fuss.  That's why I'd go 2" all the way.



#14 datzenmike

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Posted 20 September 2015 - 10:12 AM

I think the 720 (Z24 at least) has a 2" down pipe and it may continue past the cat to the muffler. The tail pipe is smaller.


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#15 regularJJ

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Posted 20 September 2015 - 03:05 PM

Scomber I had NP finding 1 3/4" exhaust parts at autozone, the problem is you cannot find the original pipe for 1980-1982 anymore. 83 and on is fine they still have the parts diagrams on system as well. Check my post about 'need advice on 82 4x4 exhaust system', I was surprised how easy it was to rebuild a trashed system. The old mufflers are crap never seen a good one, toss it and throw on something else also plenty of 1 3/4" stuff out there. And from what I seen the pipe seems to always rust through just past the front pipe or between the cab and bed as water leaks straight through both places. A 1 7/8" sleeve fits over the existing parts you can get those off the sales floor at any parts store, I welded mine on but I suppose you could use a u clamp? For the cat, you may be able to find a 1 7/8 to 2 1/8 adapter, but not at autozone. Magnaflow sells a cat with 1 3/4" in/out diameter part #94003, havent tried it got lucky mine only has 125k or less on it and is in good shape.

 

And thanks for the extremely detailed info mike, thats why I like you heh. Im lazy tho man Im still gonna call it backpressure :sleep:



#16 DIY 1985

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Posted 11 December 2017 - 12:55 PM

I'm looking into measuring back-pressure at the 02 sensor. Sixth owner, and I don't trust the old catalytic converter. Has anyone done this? My exhaust fabricator only takes cash payment; should I ask him to toss the cat for straight pipe? I want to install a new EGR system with a Back Pressure Transducer and don't want to incorrectly tune the Weber jets with EGR if the exhaust is partly plugged?

#17 datzenmike

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Posted 11 December 2017 - 05:50 PM

If you don't have emissions testing you can straight pipe it.

 

Just so you know, there is no EGR at idle, when the engine is cold or when at full throttle so if jetting for performance the EGR does not matter.


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#18 DIY 1985

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Posted 12 December 2017 - 07:23 AM

I'm tuning for altitude. If EGR makes the air/fuel mix leaner it would still influence my choice of a primary idle jet on a progressive 32-36 DGV carb because it is still in play at part throttle, just not exclusively? And there's some wiggle room within the idle mix screw. I plan to install a new O2 sensor and a readable air-fuel guage. Where I typically drive, the elevation varies over 5,000 ft.

#19 datzenmike

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Posted 12 December 2017 - 07:39 AM

EGR does not make the air fuel ratio leaner. What it does is displace some air fuel and replace it with inert (no, or very little free oxygen) burned exhaust gasses that do nothing for the combustion process. This will lower peak combustion chamber and exhaust gas temperatures and less oxides of nitrogen are produced. Again... full throttle is unaffected for climbing hills or passing. Mileage is unaffected as this is pretty much the same as lifting off the gas slightly. At moderate speeds you would just step down on the gas for more power. You won't feel or know anything is happening.


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#20 DIY 1985

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Posted 13 December 2017 - 05:17 AM

I was assuming that the pressure from the exhaust manifold, via EGR valve, would offset intake manifold vacuum, and with less being pulled through the carb venturis less gas would make it to the motor.

I thought that EGR was a means of getting less fresh-air/fuel to the motor when the need is to simply fill combustion chambers rather than supply power needed?