I'm planning a top end rebuild for my '95 XL1200 this winter, and am considering some performance upgrades while I have the engine apart. The bike already has a Dynatek 2000p ignition, but the engine is stock to the best of my knowledge. The goal is to keep the rebuild under $1k total, so that probably rules out any head work for the time being.

I'm going for a "mild street" build (i.e. can reliably run 91 octane in 90+ degree weather with no pinging), and am trying to decide between Andrews N4 or N6 cams. My understanding is the N4s are designed to be run with stock compression, while the N6s need a 10.5:1 compression ratio or higher to run correctly. If this is accurate, I'm looking at two options: run N4s and keep the stock pistons, or run N6s and install high-comp pistons. Wiseco sells two part numbers that I am looking at: K1685, which are forged 10.5:1 domed pistons; and K0213PS, which are cast pistons from the Tracker line that have more of a flat-top appearance. The K0213Ps are marketed for 883 to 1200 conversions and are quite a bit cheaper than the forged pistons ($220 vs $345).

My question is, would both piston sets be compatible with the N6 cams? I've seen the K1685s mentioned in other threads, but never the Trackers, and I would prefer the cheaper option for this 30 year old bike if at all possible. Additionally, I haven't found any dyno sheets to compare the N4 and N6, so I do not know if the horsepower-per-dollar trade off would favor the N6 option.
Any advice is appreciated, as this is my first time tearing into a HD motor.

 

Some friendly advice. Reconsider what you're planning on doing. N4's are a great cam but not much bang for the buck with a stock motor. I don't think N6 is a bolt in cam for a rigid mount Sportster. Some good info on cams here. If you want a solid upgrade for around $1k buy a 1250 or 1275 kit from Hammer and a CV performance kit for the carb and call it a day. Then do the cams later when you have the cash for headwork. All this assumes you've already done the intake and exhaust as that is usually the first stage.

 

The operative cam spec is the intake valve close (IVC) timing. It's what determines how much of the compression stroke is actually used for compression. Watch this video to understand more about this relationship and how it's caclulated:
But the bottom line is that the earlier (lower) the IVC, the more compression you extract from a given compression ratio, and conversely, the later the IVC, the less compression you extract from a given compression ratio.

A quick comparison of IVC's:

• Your stock "D" cams: 41 degrees
• Andrews N4's: 46 degrees
• Andrews N6's: 50 degrees

IVC not only drives how much compression you get from a given compression ratio, it also drives the rpm range of the cams more than any other spec. The later the IVC, the higher up the rpm range where you'll make max power.

Using the program described in the video (you can download that program here), let's just run some numbers. First, the stock motor pencils out like this:



There are some assumptions built into this, for example how far down the bore the piston sits when at TDC varies, but it's in the ballpark. Your bike was basically designed for about 167psi of cranking compression.

Now if you just swapped in the N4's, with no changes, it comes out like this:





Literally all I did was change the IVC from 41 (D cams) to 46 (N4's). As you can see, it drops about 5psi out of the cranking compression.

So the N4's with the stock compression ratio is a pretty soft combination. Really those cams work best at something in the range of 10:1 to 10.5:1.




Above is the N4's at a 10:1 compression ratio (8cc dome). As you can see, it works out to about 189psi of cranking compression.




Above is the N4's at a 10.5:1 compression ratio (11.5cc dome). As you can see, it works out to about 201psi of cranking compression.

Finally, let's run the N6's at 10.5:1, since you mentioned that combination:



So N6's at 10.5:1 nets you about 196psi of cranking compression.

The sixty-four dollar question however is "how much compression can you handle without creating a detonation problem?"

To answer that, its' helpful to understand how resistant the design is to detonation. Here's a picture of your stock combustion chamber:




The thing to notice about this chamber is that it's just a big bowl, i.e. a hemispherical shape, more commonly referred to as a "hemi" head. Hemi heads are touted for their unimpeded flow. There's nothing in the way of the air as it enters and exits the chamber.

However, at the same time, they're noted for a lack of chamber turbulence. This lack of chamber turbulence means the air & fuel don't get mixed well, which makes them detonation prone. This is the reason your bike came from the factory with such low compression. Hemi chamber designs are very detonation prone, which is why almost nobody makes them anymore. Even the Chrysler Hemi doesn't use a hemi chamber design, it's just a marketing term dating back to the muscle car era when they actually last made a hemi chambered motor.

Now look at the chamber design found in 2004-2022 XL1200's:




Notice how it's a bathtub shape instead of a hemi. It has big flat surfaces on each side of the chamber. As the piston passes through top dead center on the compression stroke, fuel & air gets trapped between the top of the piston and these big flat areas. That's called a "quench" or "squish" band area. The fuel and air gets squeezed out of this area and shoots into the combustion chamber. Think of stepping on a tube of toothpaste.

The result is much better mixing, which in turn means much better resistance to detonation. This is a modern chamber design.

Just as an example, all 2004-2022 XL1200's, with this chamber, came from the factory with a 9.7:1 compression ratio and a set of cams with a 25 degree IVC. It calculates out to about 200psi of cranking compression. That's factory stock!

Now you might think they paid a price in flow, since the valves are shrouded by the bathtub walls. The air has to go around those shelves as it enters and exits the cylinders.

But they both massively increased the valve sizes and improved the port shape, as compared to your heads, and they actually flow a ton better than your heads.

So you see what I'm getting at here. Sure, you can raise the compression of your motor with your current heads, using domed pistons. But those heads are very detonation prone and that's going to constrain how hard you push it. But run the numbers and know your combination ahead of time. Personally I advise people to stay under about 180-185psi with those chambers.

With a set of the modern heads, you can push it quite a bit harder, gaining substantial torque and power in the process, and actually have less detonation, not more. So the ideal solution here is to update your heads. As a bonus, you can run a set of the late model heads over your stock pistons and get 10:1 when paired with thin gaskets. The thin gaskets will tighten up the squish clearance (ideally you want it in the .030-.040" range), making the squish band more effective, and actually reducing detonation despite the higher compression. If I had a 1200 of your generation, and I wanted more power, a set of the modern heads would be the first thing on my list. They would make more difference than anything else you can do.

 

Very good video indeed!

 

Lots of really great info above. I am still of the opinion that for the cost , time, and effort to replace pistons I would put a big bore kit on there. Buy that is just me. Also, if you move to '04+ heads you will need to change the jugs or it will look weird due to the larger fins on the newer bikes. Out of curiosity I am wondering how having the '03- heads reworked by a place like Hammer Perf compare just putting '04+ heads (or similiar design) on there? Also, it looks like N6's are bolt in for the '95. I stand corrected. Awsracing also mentioned rpm range brielfly, and I think that is also an important consideration for a street bike.

 

"How does having the '03- heads reworked by Hammer compare just throwing '04+ heads (or similiar design) on there?"

Can Hammer do much with the 91-03 1200 heads? Or, would it be better to start with 883 heads and have them reworked? Given you keep the fin profile as current.

It looks like you only gain ~5 HP (w/04+ heads) by going to a 1275 kit over a stock 04+ 1200, is that really worth it for $1000? Gaining 30+ HP by upgrading an 883 was well worth $1000 to me, but if I already had a 1200, not sure the small gain would be worth it.

And it sounds like it's still about the 91-03 head design.

John

 

My thought is simply if you're after hp you're going to go big bore one way or another so why spend that much on pistons for your 1200 just to run hotter cams when you could just roll that money into a big bore kit. And it's just easier. Pay Hammer the $50 or whatever to install the piston and boom it's plug and play. Otherwise you've got to get pistons/rings, have the cylinders honed, gap the rings. For someone who's never done this kind of work before they might pay a shop to do it and then likely you're near or at the cost of the big bore kit.

Regarding Hammer's head work on the 1200 heads. It appears they use a different approach on the 1200 vs the 883 heads. If you look here you can see the 1200 heads are cut out like the newer heads, whereas the 883 heads cuts outs are spherical.

 

Thank you for the very thorough and detailed reply! I did not know the chamber design changed so much in '04. Your writeup on the different head styles over on xlforums provided some good context too.

Playing around with the Hammer calculator, here is what I am getting for my stock motor with 04+ heads (62mm chamber) and the 0.030 head gaskets you mentioned:




And here are the results with N4s and stock pistons:



Followed by 8mm domed pistons with N4s and N6s, respectively:




Based on these results, my original question about the Tracker flat top pistons is answered. With their recessed domes, they may lower compression when paired with any head design. It makes sense that they would be marketed for 883 conversions, since as I understand, the 883 heads are more prone to detonation and require lower compression ratios.

 

This is definitely an option I'd consider. If going the big bore route, I would probably install 04+ heads and cams this winter and save the cylinders for later, just to spread the cost out. Either way, I'm sold on new heads being the best first step. The numbers speak for themselves.

Let's say, hypothetically, I ran 04 heads over stock pistons with a N4 or N6, even if it was just for a season or two while planning to eventually install a big bore kit. Would either cam even be worth it without high comp pistons? Obviously the static compression would be lower, but there are benefits to the greater lift, duration, etc. Plus, the advertised rpm range of both cams is where I do most of my riding.

If it's between high-comps and a big bore as the end goal, the high-comp with cam option would be quite a bit cheaper, especially since I would be doing the work myself. In that case, the question would be which cam to run with a domed piston, which mainly comes down to how much compression the 04+ heads can safely handle.

 

Oh man, are you opening a can of worms

Look at those pictures of the chambers again side by side:




OK so the chamber difference we already discussed, a hemi vs.a bathtub. Note however that the hemi head is missing the material needed to create a squish band.

Now look down into the ports, into what we call the "bowl" are, near where you see those stamped numbers.Particularly the intake port (the bigger valve). Notice the difference in the roof of the port in that area (aka the "long side").

See how the hemi head has a sharp transition there? And notice how the late model head has that area totally smoothed out. Once again, just like the chamber, the late model head has additional material added there to smooth that whole transition. Again, the hemi head is missing material.

There's one more place, not pictured, where the late model head has material that the hemi head is missing, and that's the floor (aka the "short side"). The late model head has a considerably higher floor.

It's the difference in length between the short side and the long side that creates port turbulence, which reduces flow. Chamber turbulence is good for detonation resistance as already discussed, but port turbulence is a very bad thing that reduces flow.

By making the short side longer and the long side shorter in the newer heads, they achieved a less turbulent port, because the difference between the short side and long side is less. If you ever put these two heads onto the flow bench, you can really hear the difference. The hemi head just howls at you, the port is so bad. The later head is silent by comparison. And the flow numbers reflect it, too. Of course, the much larger valves in the later heads is a big part of that as well.

And yet the late model head's ports are actually smaller. Any head porter will tell you this: if you can achieve a given flow with a smaller port, it'll make more power.

But my main point here is that the hemi head's issues all come from missing material in key places. Which means the only way to make a really good head out of them is to weld them up and start over. Well, welding on a head to that degree opens up a whole pandora's box of issues. It's something you really don't want to be doing to a head if you can avoid it.

And I'm sure you can appreciate that by the time you weld it up and shape & size the ports properly and change all four seats so that you can run big valves, you've put a ton of time and work into it. And it's still not going to be as good of a head as a late model head because it'll still have small fins.

That's not to say the hemi head can't be improved for a reasonable price. It most certainly can, and if the customer has a fairly modest power goal, say something up to 90-95hp, you can certainly make a case for working with the hemi heads. A low cost porting job in combination with a 1275 kit and a set of N4's will get you there and won't break the bank.

But personally, I'd ditch the hemi heads if you have a power goal greater than that. Either get a set of late model 1200 heads or get a set of 883 heads and have them reworked. 883 heads give tons of material in all the right places and really great heads can be made out of them.

In terms of power supporting capability, the difference between a hemi head and a late 1200 head is much more than 5hp. The hemi head in stock form will support maybe 80hp tops. I've seen stock late model 1200 heads break 100hp numerous times.

Now sure, if you just swap the heads on an otherwise stock 2003 XL1200, the difference may only be 5ish hp. But that's because other things are holding the motor back. The power of the motor is defined by what holds it back. By swapping the heads, you've removed that constraint and run into a different one.