heads

[94sporty]

what is the difference between 883 and 1200 heads?

[aswracing]

317

[aswracing]

No seriously, it depends on which year heads you're comparing to which.

Evolution 883 heads have been remarkably unchanged over the years. All the chambers are about 49cc (although the earliest 4-speed versions were bathtubs the rest are hemi's), with 1.585 & 1.350 valve sizes.

So you have the big change in '91 with the 5-speeds that all the heads got (on center tappets causing the pushrod holes to move, and crankcase breathers integrated into the carb mount holes). And then you have the next big change in '04 with the rubber mounts (7mm valve stems, beehive springs, right front motor mount hole relocated) and then you have the efi changes in '07 (milled down manifold surfaces and the new flanges with the wider bolt spacing). But by and large, the basic head design has not changed.

1200's on the other hand have changed a lot. The original 1100 heads were bathtub 62cc chambers with 1.844 & 1.610 valves similar to the Big Twins of the day. Then in '88 with the introduction of the 1200 they went to the hemi head with it's 67cc chamber and 1.715 & 1.480 valves. Probably the worst Evolution head ever made. Then in '91 the 5-speed version of the hemi head came out for the on-center tappets and crankcase breathing through the heads. Then in '96 they came out with the "Lightning" head which was essentially a hemi head with some material added to the chambers to get the volume back down to 62cc and bump the compression (unfortunately shrouding the valves badly in the process). The Lightning head was sold over the counter and also came on Buells of the day, then was introduced on the 1200S in '98 and became the SE head. Then the Thunderstorm came out in '98 on Buells with it's 67cc chambers with a squish band and matching domed pistons, 1.810 & 1.575 valves, and much improved ports. Then the XB heads came out in 2003 on Buells with it's 62cc bathtub chambers and 1.810/1.575 valves in 7mm with beehives and better ports yet. Then that basic head design was put on all 1200 Sportsters starting in 2004 with the rubber mounts. Then the EFI version came out in '07 with the milled manifold surfaces, wider flange pattern, and relocated motor mount hole. And I haven't mentioned the 2 rubber mount SE heads they've offered.

So when you ask what's the difference between 883 and 1200 heads, it depends on what you're comparing to what. Mostly which 1200 head you're talking about.

We have all this documented with pics and what not showing the differences here: http://www.hammerperf.com/ttxlheads.shtml

[FSG]

317

that sounded like Max   

[aswracing]

317

that sounded like Max   

Ouch!

[chopper]

317

that sounded like Max   

My first thought, too.

[94sporty]

Say a 1994 883 and a 2000 1200

[aswracing]

Say a 1994 883 and a 2000 1200

Probably the most fundamental difference between the 1200 and 883 heads of any years are the chamber sizes and how the same basic geometry gets adapted to the two different displacements. 1200 heads of that vintage have a 3-1/2" diameter 67cc "hemi" chamber:



Valve diameters are 1.715" intake and 1.480" exhaust.

883 heads also have a "hemi" chamber, but it's only 49.5cc and 3" in diameter:



And the valve diameters are 1.580" intake and 1.350" exhaust.

This smaller chamber means that you can't just toss these heads on over a flat top 1200 conversion. Flat top 1200 pistons will put you at almost 12:1 compression under stock 883 heads. So either you have to do something to make the chambers bigger, or more commonly, you use what's called a "conversion" piston which is dished in the middle of it to bring the CR down to a more typical 10:1 or so:



One big, big advantage to using 883 heads in a conversion as opposed to the 1200 hemi head is that you'll gain a squish band. Because you've got a 3-1/2" diameter piston under a 3" chamber, the chamber overhangs the bore for 1/4" around the perimeter. Here, I have a drawing that shows this:



This squish band greatly increases chamber turbulence and reduces detonation. Notice how the conversion piston pictured above has a 1/4" shelf around the perimeter, this is the reason why. Beware of conversion pistons with a narrow shelf there, they cost you squish band surface area.

The smaller diameter of the 883 chamber is part of the reason it's so much smaller in volume, but the other big reason is less obvious at first glance. The 883 chamber is also quite a bit shallower than a 1200 chamber. Therefore 883 valves have longer stems to reach it. The valvetrain geometry is all the same between 1200 and 883 heads, i.e. same angles (27 & 31) and the valve stem tips are in the same place, it's just that the 883's valves are longer to reach the shallow chamber and smaller in diameter to fit within the smaller diameter chamber.

These longer valves have implications on using 883 heads with big cams. Because the valves are longer and closer together to reach the small diameter, shallow 883 chamber, you can very quickly run out of valve to piston clearance if you use cams with much overlap at all. Likewise, you try to go very big on the valve diameters and those high overlap cams are going to cause valve to valve clearance issues too.

So any serious work to an 883 head that makes the valves big and/or is done to accommodate high overlap cams requires the valves to be sunk in their seats pretty significantly to gain the clearance needed. When you sink a valve, you also have to unshroud around it heavily to restore low lift flow. Together, those operations make the chamber quite a bit bigger. In fact, they often make it enough bigger to use a flat top or even a domed piston. In fact, this is our piston of choice when doing performance work to 883 heads:



It implements a 30 degree squish band which is the ultimate design for these heads. But I digress. The point here is that the valve lengths and chamber sizes are different, and you have to deal with it when using 883 heads in a conversion, and carefully match the head work, piston configuration, and cams. There are essentially an infinite number of ways to make the various trade-offs needed in those three areas to get all the parts to work together, and exactly how you make those decisions will have everything to do with the final result.

The other big difference is in the ports. 1200 hemi heads have some really bad ports. In the pic of the 1200 head above, you can see one of the big problems. Look down in the bowls, in the area where you see the number. See what a sharp angle that is? The air comes in and essentially hits that wall on the long side and is forced into a sharp corner. Air hates that. The 883 head has that same situation, however, because the valve is so small, there's material available to smooth that transition a lot in the process of reworking the head for bigger valves. By the way, the long side was greatly improved with the "Thunderstorm" head which came out in '98, and improved even more with the "XB" head in 2003. Twin Cams got a similar improvement over their predecessors in 2006.

It's not shown in the pics, but it's kind of a similar situation on the short side. Both 883 & 1200 heads have a terrible short side, much too low and shaped poorly. But since the 883's chamber is so shallow, the short side is longer, and since the valve is so small, when you open up the valve size you have a lot of material to remove and you can, in the process of doing so, pretty radically reshape and improve the short side. You can't make it great, but you can make it pretty good. In the end, just like with the long side, you can make a much better short side if you start with an 883 head. By the way, as with the long side, the short side was very much improved in the Thunderstorm head, then improved some more in the XB head, and the 2006 Twin Cam head also has a much better short side than it's predecessors.

So anyway, bottom line, in stock form the 1200 hemi head will make more power than an 883 head, due to it's larger valves. However, if you're going to do performance work, the 883 head is a much superior piece because it's chamber gives you a squish band and it's ports give you more material to work with in a couple of critical areas. We have customers who've made as much as 110hp on reworked 883 heads, it can be made into a pretty decent performance head. The 1200 hemi head, not so much.

[PanHeadRed]

Some really nice posts aswracing, excellent.

[94sporty]

Looking to send my heads to hammer .....but I want to know what I would be spending on getting the best performance out of my 883 heads. Not good with all this stuff. Add them done once but I think they just cleaned them up. Do you need to k of what all I have on it?