optimum exhaust o.d.?

[PinoyDat210]

hey, i disassembled my exhaust and took out the cat and rusted and f'd up muffler that was just hanging their. I now have a straight pipe unto about midway of the car into a cherry bomb glasspack(definately too short). Now, im thinking about running some 2 inch pipe the rest of the way and bend it to stick out infront of the rear wheel on the driver side. I was just wondering which diameter would have the best flow and performance for the engine.? would 2 inches be strecthing it?

 

Note: i have a 1980 datsun 210 fu series engine with the 310 chassis.

[datzenmike]

You have an otherwise stock motor? Then around 1.5 to 1.75" is plenty large enough. If you go too large you can actually harm the performance. You want to keep the exhaust speed up. Stick a 2" pipe length on the end sticking out from under the car and cut it diagonally so it's oval and looks larger.

 

You say the glass pack is 'too short' ... so do you mean LOUD? If so, running it out to the driver's side will move it closer to your ears. What about another longer glass pack or turbo muffler or resonator to tone it down? Deepen the sound, get rid of the pop.

[ggzilla]

The optimum? The FU engine is carefully tuned -- even has a more expensive carburetor -- designed to work with 1.5" exhaust and return 43 mpg from the A14.

 

After the pre-muffler (or cat), use any larger size you want -- it will not affect performance. The pipe from the exhaust manifold to the pre-muffler is the most important part. Guys that remove it lose some MPG.

[PinoyDat210]

Sweet, cause i just kept the 1.75 pipe that came directly off the exhaust manifold then ran the straight into the cat and cut it just infront then added the cherry bomb glass, Then i am going to add a 6x6 inch length, 2 inch o.d., with a 90 degree bend and a 4inch radius to turn it more to the drivers side and then run it through a 2 inch o.d. jones resonator, and then out the side with a jones 2inch to 2.25 flare rolled tip. I think its going to sound pretty sweet. Because the last time a had it running it was massive loud and beasty but kept popping you could definately here it was too short.

 

Also, i was wondering if enlarging the exhaust pipe by a somewhat, could help increase low-end torque and power? I have heard things like this before but would just like to clarify.

[ggzilla]

Enlarging the exhaust pipe reduces low-RPM power.

 

It might (or might not) increase high-RPM power.

With a stock A14:

* Oval port head -- 1.75" for max power

* Round port head -- 1.5" for max power

[josh_t]

perfect. i was just looking for this. i have no cat and its not my fault, it was like that when i bought it. and the exhaust is 1.5 until right below the drivers seat, where it steps up to 2.25 for the rest of the way to the bumper. i would like to go up to a 2 or 2.25 the whole way for sound purposes eventually and because i'm ocd and like the uniformity. the first inch or so would be stock diameter obviously so would it really kill the low end that much? i know my 4 wheeler did better with a muffler than straight piped and i have two mufflers on my car so would that help some? its a reversed glasspack and a (not as ricey as most) fart can.

[ggzilla]

Yes, your mileage is probably off right now.

 

When you go to new exhaust you probably won't notice any loss because it will be louder. Mileage or timing (take it to the drags) is your best way to tell. Or you could pay for a dyno test.

[josh_t]

i'm not super concerned about the mpg. i expect it to be noticeably less than the epa stated even if it was new considering how i drive and its old. and its actually a pretty quiet car with the setup i have now so it shouldnt be overbearing when its all hooked up right with no leaks. when i get to college i can use the dyno for free.

[josh_t]

i want to go with a weber eventually so i can hopefully compensate for any losses with some jetting.

[datzenmike]

As I see it, and I'm no expert here, this is only opinion...

 

Smaller pipe at low speeds keeps the gas speed up. Exhaust has to travel faster in a smaller pipe. The exhaust gains momentum and inertia and this helps drag any residual exhaust in the cylinder out with it. This means there is less waste burnt air and gass left to dilute the incoming fuel and air. In some cases at certain RPMs while the exhaust valve is still open and the intake starting to open it will help drag some of the intake air in to the cylinder as well. Remember that at low speeds the intake air is experiencing a stop and go motion as the valve opens an closes. The engine actually has to work to draw air in so anything that helps that increases low speed power. A large diameter exhaust pipe will lower exhaust speed and more exhaust gasses will be left in the cylinder to dilute the incoming gas and air from the intake making it leaner. Aditionally the motor will work harder to draw the intake air in and there is a power loss. Yes the carb can be adjusted to enrich the mixture and gain some of this back.

 

Small pipe at high RPMs sees the closed valve interval so short the intake air traveling so fast that it's own inertia causes it to bunch up against the closed valve and when opened it rushes in by itself. The exhaust pipe now becomes a restriction because it is too small and there is a power loss. At high RPMs a larger exhaust pipe will benefit power making.

 

The ideal exhaust pipe would increase it's diameter as the engine speed goes up.

[josh_t]

i understand the scavenging effect pretty well. and i noticed that most 4 cylinder cars spend more time in the mid range than anywhere else so i guess good low end torque wouldnt be much of an option if i want power anywhere else as it is a tiny engine and thats a pretty big limitation.

[ggzilla]

Small pipes help the mid-range.

 

For maximum power in the high-RPM band they should be a little bigger, but not too big.

 

Note that your FU engine has the small port head, so large exhaust pipe won't help increase the power.

[josh_t]

I have the H72 head not the FU. Mine is regular oval port. Its a 76. The FU head is a 728 and is a round port for 77-79.

[ggzilla]

Why didn't you say so - in that case go with 1.75" exhaust. The 1980 FU uses a round port head too. 1981-1982 FU uses small oval ports.

[josh_t]

haha sorry. would i go 1.75 until the glasspack and then go 2.25 after that like it is now? at the moment its 1.5 until right after where the cat would be and then it is 2.25 the rest of the way. i really dont want to re-do everything including the mufflers. i'm pinchin pennies pretty hard here.

[ggzilla]

Yes you can do that.

[josh_t]

i'll draw somethin up tomorrow and post it to give you an idea where i'm comin from.

[datzenmike]

Keep in mind that a doubling of pipe size increases the cross sectional area by 4 TIMES!!!! For example if you are running a 1.75" stock pipe, to increase the size by 50% you only have to increase the diameter to 2.15" an increase of only 0.4 ", under 1/2 inch.

 

A small increase makes a huge difference so go easy.

[josh_t]

well, i'm not changing much from where it is. all the size changes are alreasy done other than the first section.

[ggzilla]

If you are not concerned about optimum power or optimum fuel economy, then it really doesn't matter which size you use.

[josh_t]

well at the moment i'm really not worried about anything but keeping my feet from dragging the ground and my exhaust falling off, so just getting the whole setup welded on there would be a big plus. i have been getting as much rust as possible from off the floors and putting a coat of black primer on the inside and out to keep it where its at for now. found out my front floor pans are holy because my cowl vents were full of leaves and not draining. it also explained the sounds of leaves entering my blower motor and making some pretty nice noises. it was a rolling leaf vacuum for a minute there.