The Grunt Exhaust Manifold is a better shaped manifold that quickly replaces your stock, lower power producing manifold.
The changes are pretty clear, with the most obvious being the diameter of the outlet where it exits into the pipe. This dimension and the taper between the engine and pipe are critical for optimum power.
The stock one is just to big on the inner diameter, and you lose throttle response and acceleration because of it. And if you think the smaller diameter will hurt it, just realise that this is a smaller for a length of only 1" or so. Something like an FMF megabomb header, has this same smaller diameter for a 4" long length. So it's not a restriction.
A little too big or too small, or the wrong taper… and you lose something somewhere. To be optimal just takes lots of time on our end… in time on the lathe, on the dyno, and in the dirt.
Obviously we’ve already spent all that time, so you can just pop it on and get the benefits.
What you get as far as a power improvement goes is more “get up and go” at all throttle openings, but it’s mostly felt from idle to 1/2 throttle.
From sea level to 13,000 feet and with any header, it’s a brilliant, non-biased power increase and probably the best $100 you could spend. You will feel an increase in power from bottom to top and a big increase in throttle response.
You can see the difference in the shapes pretty easily. The stock manifold, on the right, goes from a smaller size coming from the exhaust port… to a larger size, where it exits into the headpipe.
The problem with the stock shape, is that the velocity of the exhaust gas slows down, at a time when it should still be speeding up. This slows throttle response and bottom end power.
Our Grunt, on the left, gets smaller, but not too small, as it exits into the header. This, is what makes a nice power gain for you.
It works with a stock header or aftermarket header, and you see a nice gain either way. Many people have noted, by the way, that the Grunt with the stock header works as well as an expensive aftermarket header with the stock exhaust manifold. And its around $400 less.
Big Grunt & Little Grunt
The usual question we get asked is “What's with the O-Rings?”
The bigger bikes have more power flowing through them and require a stronger seal, so that’s why the 450 / 500 size bikes have two O-rings and the smaller bikes only have one.
They are high temperature O-Rings used to make a perfect seal between the Grunt and the head pipe. This ensures that no air is sucked into the pipe, which helps eliminate back fires and ensures top performance.
It's made of steel, and the flange is .050" thicker than stock, so you know that you won't have failures.
The Grunt is Very Easy to Install
To remove the stock manifold and install the Grunt, the first thing to do, is remove the muffler. So you’ll want to pull the pipe springs from the pipe (if you don't have a pipe spring puller you can try needle nose vice grips to grab the springs and pull them).
The next thing to do is to remove the bolt that holds the pipe to the frame so you can slide the pipe forward, off of the manifold. It will move forward enough so that you can remove the manifold without having to remove the pipe completely.
Now you can get to the two 8-mm bolts that hold the flange to the cylinder, through the front. Remove them slowly at the end and keep you fingers on them to pull them out, so that they don't fall into the open pipe. If that happens, you'll say a bad word and then have to remove the shock to get the pipe off and get the bolt out.
You'll find that smearing grease on the flange to hold the gasket on and then on the gasket, to stick the whole assembly to the head, will help you a lot, as it will free up you fingers to get the bolts started into the head.
And then a little grease on the O rings, will help it all slide back together easily.
Again, the Grunt works with stock headers or aftermarket headers, and you see a nice gain either way for $400 less than buying a new header. You can pick the Grunt up below for only $99.95.