I think the CA on the chart means Canada
If you open it up you'll find 145 main. Don't know anyone who's found a 125.
So don't worry about it just dig into it as told

Don't you have California- I'm from the State and here to help you- types looking over your shoulder? :'(

WarriorPoet287 wrote on 03/29/19 at 13:26:36:



I got a MAC system when a rep from the company came to this site and offered one in a contest which I won.  My engine is not stock, nor is my exhaust system.  I think it runs pretty good.  
I found the system did not work well for me.  The muffler had a good sound and is capable of being loud but at idle to cruise settings it had a deep rumble and was not objectionable at all.  Get on it hard and it volume definitely increases but in a good way.  I liked the muffler.
The header has a 1.75”ID.  My header has a 1.6”ID.  A stock header has a 1.3”ID.  1.3” is adequate for stock engine with something like a Dyna muffler.  For a modified engine you need more, at least 1.5” ID if not 1.6” which works great on mine.  1.75”ID is too large.

Some numbers:

Tube ID               Tube area.              

(A). 1.3”ID.          1.280” sq.
(B). 1.5”ID.          1.830” sq.
(C). 1.6”ID.          2.142” sq.
(D). 1.75”ID.        2.479” sq.



The area inside the tube increases as follows...A-B 43%; A-C 60%; A-D 76%.
B-C 17%; B-D 35%; C-D 16%.

The percentage increase in the area of the tubes increase dramatically with just seemingly very small increase of tube ID.  How much slower must the exhaust travel when the space is enlarged so much ?  A lot. The result is poorer performance unless you improve breathing throughout.  The more efficiently the air can enter and exhaust exit the engine the more power you can make.
Get too big and results just drop.

Recommended header size for an engine making _____ HP
25-35hp/cyl       1.3”ID
35-50hp/cyl       1.5”ID
45-60hp/cyl       1.6”ID
with some cross over between them.

The header size (up to 35 hp) isn't the problem  it's the shape of the exhaust channel between the exhaust valve and the exhaust header(that's the " bottleneck.") Grinding out the exhaust port (carefully ) should show a slight improvement even on a stock motor.
    elevation has to due with jetting ,higher elevation requires smaller jets (less fuel) due to less oxygen , near sea level the opposite.
    With the exhaust we are concerned with velocity (flow) rather than pressure. The object is to remove as much of the exhaust gases as quickly as you  can from the cylinder ,to allow more room for the incoming fuel/air mix.(more hp).

The flex pipe is inexpensive and can be purchased at any auto parts store.  It has a 1.5" ID and the OD is just the right size to allow it to be slipped into your 1.79" ID header.  You have to exercise it a bit to loosen it up prior to insertion.  As you are feeding it in it will start to bind a bit.  Each time it starts to hang up you simply pull out on it a little and then continue feeding it in.  Lube it up with some anti-seize prior to insertion.

Well, I did the inspection...….and......this is tough to report......my flexible pipe ran away.  That sucker migrated a few inches down the header.  I am sorry to report that I got flummoxed when I saw that flex pipe down inside the header.  I was majorly concerned about retrieving it and lost sight of why I had the pipe off in the first place.  I started digging around trying to snag the flex pipe before I got any good photos of the extreme heat filler.  I can tell you that a lot of the filler was gone, so I am not thrilled with the inspection.  How's this flex pipe look?

Here's a shot of the little bit of filler that remained after I finished my ballistic removal procedure.  It's a dirty job but somebody's gotta do it.