Hi Guys,

Putting a new exhaust on my A15 in a few weeks. I'm planning on going with a 2.5" system, a few people have reccomended this already, and I have seen some people with worked A12's even getting good results.

Some very basic specs on the motor:
A15 +1mm o/s approx 10.2:1 comp
Twin 40mm sidedraft dellortos 32mm chokes H89 Ported oval port head & JTS GX manifold
Wade 140 grind cam (40/70 ish)
JTS 4:2:1 extractors with long secondarys
Currently running a 2" system with one large baffled non- straight through muffler.

The problem I have is it starts to lose power around 6700 when the cam should keep going to around 8000.

Thinking of going to the 2.5" system with a nice straight through muffler and a resonator.

Does anyone have any real- world experience in going to a 2.5" system on an a series?

I'm looking for comments on the loss of bottom end and gain in top end that the 2.5" system would give me?

Cheers

Matt

I like the jts headers for the a12 but my 1608cc a15 quad mikuni setup loved
the 4-1 headers with 2 inch system, a hotdog and a supertrapp all the way to 7500,
and still wanted to keep going like a motorbike. This is where 4 cyl 1-1.4litre
bikes have it right with their 4-1 headers. Also remember the 82mm stroke makes
more torque than the a12 and a14 in the lower ranges so 4-2-1 helps torque but
not really the best for higher rpm.
For my needs I prefer the 4-2-1 for the street in a mild setup as Im not interested
in going beyond 7000rpm these days.
Also have you looked at your ignition? as your setup looks like it should be happy
way over 7K rpm.

id go for a 2 or 2.25 inch max system and get rid of the restricting muffler. a semi offset muffler should let it breath well and
keep the noise down.

i wouldnt go 2.5 inch unless its race only

oh wait... you said you wanted expert opinions..
disregard my advice

I'd go 2.25" with a V-tech pipe aswell.... I run 4 into 1 also
But triY would be a better street compromise
Use decent mufflers... not crappy $25 hot dogs

Is this exhaust for a street engine or race motor? Street engine.... I would be building a full mandrel bent 2 inch ( 51mm ) system with a single muffler! Race motor with 8000rpm rev limit I would up-size the pipe to 2 1/4 ( 57mm ) again build the system with mandrel bends. Stay away from "Push Bent" exhaust systems, They crush the pipe through the bends and rob the system of flow!! Exhaust systems are only as good as its smallest point!! I dont see the need to run a 2 1/2 inch ( 63mm ) exhaust unless you are running the motor to a 9000-10000rpm rev limit all day long eg full race spec engine. If you have a exhaust system that's to big you get what I like to call "Gas stall" that's where there isn't enough velocity in the system to move the gas quick enough and the gas cools down and becomes heavy, This slows the flow down, Exhaust gas moves alot faster when its hot, The quicker you can get the gas out the end of the tailpipe the beter!!


Heres a link to a company who make some good quality mufflers

http://www.flowmastermufflers.com/index.php?page=9

Nothing wrong with a cheap hot dog and no mustard :)
The supertrapp did the work and the 2inch was for my 7500rpm redline
yet easily went to 8 and had very good midrange.
Whats up with your ignition?

Thanks for the info guys, keep it coming!

I should clarify it does rev past 7k, its just that the power starts to die off from about 6800 onward.

Looks like a nicely built 2 1/4 might be the go, its a fairly agro motor except for the lowish comp at 10.2:1, and it is a street application.

Cheers

Matt

I'll probably go 2" ,2 1/4" max

2 1/4 press bent equal to 2 inch mandrel, and cheaper if your paying for it no building it.

Yeah press bends are not nice.