Thread: Which Heads?
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Old 8th August 2012
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aswracing aswracing is offline
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Bravo, not sure if you're aware of this, but NRHS only just recently started porting heads themselves. Before that, they farmed all their head work out to Norlin Racing and Head Service. Well, Dan Norlin created HAMMER PERFORMANCE specifically to provide a second retail outlet for his legendary head work, and now HAMMER is the only place to get this work.

Read about his head porting process here. It's a CNC based process that's been under continual development and refinement for almost a decade now. Thousands of XL heads have been run through it over the years and it's put more XL's over 100hp that anyone else. Dyno sheets and pictures of this work are all over the XL Forum, the HAMMER site, and even NRHS's site. If you want the work that generated all the dyno sheets on the NRHS site, and is pictured there, it's only available from HAMMER PERFORMANCE. I encourage you to do your homework; I'm confident you'll discover that HAMMER PERFORMANCE has the technology and experience to do the best job possible on your heads, and the proven track record to back it up.

Anyway, to answer your question, assuming your '03 is not a 1200S, you have what we call the "hemi" head, so named because it has a big round hemispherical chamber:



This chamber has no squish band, and therefore it generates very little chamber turbulence. It's just not an efficient design. You can read more about the various HD chamber configurations here.

With respect to the ports, as delivered your heads have a very low floor (which hurts flow) and also a very squared off bowl. For example, take a look at these three pictures, in particular look down into the bowl near the area where the numbers are stamped.


Your heads. See the sharp transition in the bowls, next to the stamped numbers?


A Thunderstorm head. See how they smoothed that area? The roof through the bowl area has a longer radius, making for a nicer turn.


A rubber mount 1200 head is improved even more. Notice how the sharp line is virtually gone.

The floors are also improved between the three generations of heads. Thunderstorm and XB heads also have much bigger valves, 1.810"/1.575" vs. 1.715"/1.480".

Anyway, what I'm trying to point out here is that there have been some pretty significant improvements in the heads since your '03 heads. And although HAMMER Dan can fit the bigger valves and make the ports much better than they are, the late model heads are a far, far better platform to start with and ultimately can make a lot more power. You really have to decide whether it's worth pouring money into the hemi head casting. Even the 883 casting, which is pretty cheap to come by used, makes a better performance platform than the hemi head, as bitpusher pointed out. HAMMER Dan's SMASH porting service on 883 heads has been proven to support nearly 100hp, in fact. Read this thread for some good technical info on SMASH 883 heads. Also read this article describing all the various heads for Evolution XL's.

One big advantage to going with the rubber mount 1200 heads, or even the Thunderstorm or older Lightning heads for that matter, is that you could use our trick new 30 Degree Reverse Dome Pistons in your 1250 kit.



Virtually all really high horsepower XL motors use 30 degree pistons, but historically, a 30 degree dome with decent squish area has required a high compression ratio. These new ultra light forgings give the advantages of a tall 30 degree dome - lots of squish area, improved breathing, and a more efficient burn - but do it with a pump gas friendly compression ratio. These pistons are also based on CP's "X-Forging" ... google it for lots of good reading ... but the bottom line is that it's ultra strong and very light. Lighter pistons reduce vibration - very important in a pre-rubber mount - as well as reduce the wear and tear on the lower end and add power. This is the most advanced 1250 piston you can buy.
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Last edited by aswracing; 8th August 2012 at 15:06..
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