Theory behind the need of PCIII for race exhausts

[snowmannn]

Hi, I know that if you put on race/aftermarket exhausts you eventually need a fuel remap/PCIII in order to get the appropriate fuel/air mixture in the engine. Do you know why this is so? Using a sporty air filter like K&N makes more sense because more air enters the combustion chambers but what does the race exhaust do to affect the burning process?

I can only think of lower pressure in the exhausts because they are "open" but what would that change inside the engine ?

Thanks!

 

[MRGM]

I'll try to not get too technical...

If you open the exhaust the effect is to lower the force required to evacuate the consumed fuel/air mixture from the cylinder. The lower pressure results in a more complete evacuation of gasses. With the air filter you increase the amount of air that can enter the cylinder. All of this means that now you're putting more oxygen in the cylinder than was intended from the factory, and thus are running even more lean than the bike was setup with.

Enter the PC3. We can correct this imbalance in the fueling to deal with the lean condition. More importantly, we can fuel to achieve more power and smoother delivery. Surprisingly, the calibrations for the engine are not created to deliver the best power. The are a compromise between emissions, fuel ecomomy, heat generation, etc. We only really care about the performance and without emissions tests, we can turn up the fuel even if it would send the engine over allowable levels of NOx/CO/particulates/etc.

 

[snowmannn]

MRGM, thanks a lot for the info! What about the case when I only change the exhaust with a more open one but I don't touch the original air filter and I don't put a PCIII? There should only be faster evacuation of gases but would that affect the a/f mixture?

 

[MRGM]

You WILL affect the A/F ratio, it may not be enough to necessitate a change. IMO even a bike with stock exhaust can benefit from a fuel map adjustment.

:thumbup:

 

[snowmannn]

I see . And what's the reason - is it again only the lower pressure and faster gas evacuation?

 

[Bruce McCrary]

We're dealing with specific pieces that were engineered to be a precise specifications but 'allowed' tolerances due to mass production. Your injectors were designed to flow at a rate of "X" but in the process of building them in huge numbers a tolerance of "+" or "-" "X%" is allowable. So, if your injectors are allowed say a 2% variance, and your fuel pump another 2%, theoretically you could be down 4% from 'stock' with a stock engine.

Then too, the ECU is only capable of adjustments within a relatively narrow window. The same bike that is delivered to the altitude of Denver is also delivered to Miami at sea level. Obviously that can't work, and make big swings in adjustment based on other atmospherics, not to mention changes in the 'system' like a free flowing air filter, air box mods and exhaust swaps.

Bruce

 

[bmccrary]

pieces that were engineered to be a precise specifications but 'allowed' tolerances due to mass production. Your injectors were designed to flow at a rate of "X" but in the process of building them in huge numbers a tolerance of "+" or "-" "X%" is allowable. So, if your injectors are allowed say a 2% variance, and your fuel pump another 2%, theoretically you could be down 4% from 'stock' with a stock engine.

I'm so proud of you!

He used engineered, tolerances and variables in the same paragraph. I am rubbing off on him!

-bryan

 

[Bruce McCrary]

Bite me.

Don't forget who taught you the basics and fundamentals of what you think you know...

Dear ol' Dad

 

[Got-boost?]

IMO even a bike with stock exhaust can benefit from a fuel map adjustment.

:thumbup:

Due to EPA standards the rich and lean conditions over the mapping on a stock FZ1 is amazing too rich in some areas then very lean in others. This is true if I am not mistaken on must FI bikes. I am keeping the stock exhaust and just buying the PC to adjust the mapping, the shop I go to has a awesome Tech there.

 

[snowmannn]

Hmm, it's good to know that 'stock' is not really that stock .

I'm actually interested in the physical/chemical/whatever processes that cause the combustion process to change when *ONLY* the exhaust is replaced/removed. So far I understood we get faster gas evacuation... is there anything else and does this *alone* (i.e. NO air filter mods) cause changes to the a/f mixture?

 

[Hellgate]

Hmm, it's good to know that 'stock' is not really that stock .

I'm actually interested in the physical/chemical/whatever processes that cause the combustion process to change when *ONLY* the exhaust is replaced/removed. So far I understood we get faster gas evacuation... is there anything else and does this *alone* (i.e. NO air filter mods) cause changes to the a/f mixture?

Read this book: Four-Stroke Performance Tuning, A. Graham Bell, Haynes Press.

In a short 327 pages you'll have a full understanding of the combustion process and the up-stream and down-stream processes that impact that process.

 

[MRGM]

A big part of the change in evacuation in gasses, is that the exhaust gas that remains in the cylinder is an inert gas which lowers the peak combustion temperature for the next cycle and in turn that lowers NOx. The amount of EGR is set by the factory using the valve timing and unless you change the cams you can't change it much other than going to a new exhaust and air cleaner.

 

[Speedygonzales]

You don't need an PC III, you can get your bike on a dyno and play with the CO adjustments.

A PC will allow a more precise adjustment per rpm, so you can really give the bike more power in the midrange. The CO adjustments will have affect on the whole rpm range.

A rapidbike wil give even an more precise tuning ( per 250rpm ).

I would recommend putting the bike on a dyno to get the right CO, even with just after market can's. I had really good results with it! As other people have said before me, a standard bike is set up for a large number of costumers. Who could live in area's where the air is a lot different from eachother. A bikebrand has to set up a bike so it runs on all those places from stock, it might not run ideal but it runs good. A dyno will make it run ideal for your conditions.

 

[snowmannn]

Now that's what I was looking for ! Thanks to all of you for the valuable info!