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What is the best exhaust pipe size to use for a NA KLZE? I currently have 2 inch.

It varies based on what your goals are.

Be prepared for many posts of "2.5>2.25" or "2.25>2.5".

There are posts/web sites that go into detail on how this is calculated. For a 2.5 non aspirated engine with the horsepower the ZE has 2.25" is touted as being optimum. Some will say 2.5" will allow the engine to reach peak power better at higher RPMs. Bottom line is, unless .2 - .4 of a second in the quarter mile is important to you 2.25 is probably fine, IMO.

http://www.exhaustvideos.com/faq/how-to ... eter/#easy

http://www.magnaflow.com/07techtips/faq/question10.asp

One final comment. The ZE runs rich with either a KL31-36 or the Probinator chipped ECU and our curved neck intake. I have a 2.5" exhaust and you'll find that with the increase in exhaust flow the issue of a rich running engine is exasperated.

the ze always runs rich?!?!? Almost everyone with a ZE has those combinations...

How could we get it to proper fuel delivery. A FPR? or Megasquirt? or is there a simpler fix?

kulluminati777 wrote:the ze always runs rich?!?!? Almost everyone with a ZE has those combinations...

How could we get it to proper fuel delivery. A FPR? or Megasquirt? or is there a simpler fix?

Not sure what your saying but if this is the first you've heard of ZE's running rich then you've been living under a rock,,,I meant that in a nice way tho I have to say that my 96 OBD2 with a KL02 vaf and 2.25 exhaust ran almost spot on as they say.

Yeah 2 1/4 is probably optimum, but I have 2 inch.

Ya size matters but only to a point. The parts you buy are important too. Quality high flow cat and a well fitting system with the exception of that $158.00 SS headers from Ebay. They're cheap and they rock.

2" makes for a torque monster, but you will feel it drop off above 5-5,500 ish RPMs.

For a relatively stock KL, I would personally stick with 2.25" pipe, but it really depends on your goals. Street driven only, I would say 2.25".

I noticed differences when I went from a straight through exhaust to the MS back box and a real resonator... It dropped off in the high end.

The truth is it would not matter if we had a properly tuned full length headers available. but seeing as all there is is a universal ebay headers available, you need to use the rest of the exhaust to attain the goals you are looking for. I am running 2.5" all the way back to a none baffled corkscrew style resonator and then to the MS back box which is 2" ID. I have been contemplating running a 3" midpipe and cutting open the MS back box and making it a proper 2.5" but I am too lazy at this point. perhaps when I convert the piping to mandrel SS.


kulluminati777 - You can solve the richness with a piggyback system.

2.5'' I find works wonders that's what im running with and a highflow cat and reactive muffler sounds amazing

I'm running 2.25 and it seems quite good.

As for the richness. I would have agreed, but I'm going to dyno again after all my errors are fixed. I ran 11.5 AFR and then it dipped to 10 or so in the higher rev range. I found I had a faulty knock sensor and O2 sensor code also. These would have to affect the AFR by at least a point or two.

If my engine ran at 12-13:1 AFR all the way, I'd say that's not too rich.

Many dyno graphs on probetalk show people running from 12-14:1 AFR with chipped ECU's on their KLZEs. Perhaps there are a number of us running with error codes and richness can be fixed without using megasquirt and other ECU management.

Ok, running rich is not a problem of the kl engine. ALL engines run rich.
That's because of at least 2 reasons (there may be more, but I'm not an engineer, so I don't know them):

1. Fuel is heavier than air, therefore, it has a higher inertia. To prevent a lean condition on sudden acceleration, ECU's are calibrated to run a slightly rich mixture.

2. A stricly stoichiometric mixture burns very hot. A little unburned fuel acts as coolant, especially for the exhaust valves, that have pretty much no other way to cool down.

Then, on the issue of the exhaust pipe diameter, there's an eternal compromise between exhaust gas velocity and restriction.
An engine produces more exhaust gases the higher RPM it runs at (yeah, duh!). That means that restriction will increase with RPM.
But to achieve a good exhaust gas velocity (and, with it, a good cylinder scavenging effect), you need to impart energy to the gases, which means restriction.
So, if you have a smaller diameter pipe, you will get a higher gas velocity at lower RPM. That will translate into better scavenging, better cylinder filling, and, ultimately, better torque, at those lower RPM. Perfect for street use, or for racing on very twisty tracks, where low end torque is way more important than final speed.
On the other hand, if you have a bigger pipe, your high RPM horsepower and torque will increase because now that "sweet spot" when perfect gas velocity means perfect cylinder filling moves up the RPM range (because you need to produce more gases, so they can move fast enough down the exhaust pipe), and because the pipe is less restrictive at those higher RPM, but your low end torque will suffer, because, in the low RPM range, there's just not enough gases being produced, to achieve a suitable gas velocity, and, with it, a good scavenging of the combustion chamber. That'd be a good setup for a NASCAR engine, for example, but not for a street car.

So, like MM92 said, there's no "perfect pipe diameter" for a given engine. The best pipe diameter is dictated by your application, and, even then, it's never "perfect", but just the best compromise.