[Testiphony]

I found two different mods that would be awesome if they were installed on the Harley-Davidson Street. Feel free to add to this list or comment on the mods that I describe below.

The first awesome mod that I want to install on the Street is this Two Brothers Racing Comp-S Exhaust. The Comp-S Exhaust is made especially for Harley-Davidson bikes. This exhaust set-up does great things for performance. It shaves weight off the bike while adding power and torque. The exhaust also offers enhanced ergonomics without sacrificing aesthetics. Of course all these performance upgrades come second to the awesome exhaust note that the TBR Comp-S Exhaust.

The Comp-S Exhaust comes in two different finishes. One is brushed and the other is ceramic. The exhaust set-up is finished off with carbon fiber end caps, badges, and the high-temperature wool and packing material ensures reliable, stable performance.

The second mod I am interested is an engine bore kit that could double the power of your Harley. I am not sure that this particular engine bore kit is compatible with the Harley Davidson Street, but wouldn't it be cool if it was?

S&S Cycle is now offering a 1250 Big Bore Kit for Harley Sportsters. It can be installed on the 883 and the 1200 versions.

According the the dynojet screenshot, the 883 Sportster in its stock shape delivered a maximum of 45.4 horsepower, whereas the 1250 Big Bore Kit provided 99.7 hp. Rear wheel torque went up from 47.3 lb-ft to 81.6 lb-ft. That is pretty incredible!

The kits are available in two versions, with flat-topped or pop-up pistons, with the latter increasing compression. S&S says that the flat-topped pistons offer 10:3:1, while the pop-up increase compression to 11:2:1 with both stock and CNC ported heads.

Imagine doubling the power of the Street for just $989.95.

http://www.autoevolution.com/news/how-to-double-the-power-of-your-harley-davidson-sportster-77736.html

 

[Desert Tortoise]

There is probably nothing wrong power wise with the stock exhaust. Sportsters all come with staggered dual exhausts, which are terrible for all around power. But a 2 into 1 like the Street has, with enough internal volume in the muffler (the rule of thumb is that the air box and muffler volumes should be ten times cylinder displacement) you probably won't be able to improve engine output with another exhaust. At best you might loose some weight off the bike, but don't expect any power gains.

 

[harleyhoonigan]

TBR makes great products with this one looking amazing.
Aren't their stuff sort of pricey?

 

[fkystreet]

geez that is one big bore kit

wonder how big you can go on the 750 and 500 model

 

[Yusef]

Even if its not an actual power increase, if you can shave enough weight off then you'd definitely feel the difference on the HD Street. Installing a big bore kit on the HD Street would amp up the power by quite a bit. I just wonder if it is possible to install that bore kit on the HD Street. If that one doesn't fit though than I'm sure there would be another.

 

[Desert Tortoise]

geez that is one big bore kit

wonder how big you can go on the 750 and 500 model

The Buell XBRR race bike that preceded the liquid cooled 1125 Helicon engine were based on Sportster architecture, but the stroke was reduced and bore greatly increased, giving them a displacement of 1339 cc.

A 1200 cc Sportster has a bore and stroke of 88.9 mm X 96.77 mm respectively. It's an undersquare, low revving turd of an engine. The Buell XBRR has a bore and stroke of 103.6 mm X 79.4 mm respectively. A 50 cc overbore on a 1200 cc Sportster is nothing. What Buell did to that engine to make the XBRR was very radical.

 

[Desert Tortoise]

Even if its not an actual power increase, if you can shave enough weight off then you'd definitely feel the difference on the HD Street. Installing a big bore kit on the HD Street would amp up the power by quite a bit. I just wonder if it is possible to install that bore kit on the HD Street. If that one doesn't fit though than I'm sure there would be another.

It is comparatively simple to put a big bore kit on an air cooled twin or single. New barrels, new pistons and rings, maybe a little machining on the head to clear the larger diameter pistons and you are there. You can do the work in a day or two max if the overbore is not too great. On a liquid cooled bike, however, you cannot get to carried away with an over bore without worrying about coolant passages and such.

 

[StoneFree]

There is probably nothing wrong power wise with the stock exhaust. Sportsters all come with staggered dual exhausts, which are terrible for all around power.

Hate to burst a bubble but I've worked in the aftermarket automotive and motorcycle header development and manufacturing industry and individual equal length exhausts provided the highest performance in both HP and torque. That's why racing engines use them. Two-stokes are different but I won't go into that.

Two-into-one exhausts are best performance-wise if the exhaust tubes are equal "air-resistance" which is a combination of bends and length. In looking at the general configuration of the Street 500/750 exhaust system it looks pretty good but a flow test would tell more.

One thing that people are already getting "wrong" is the belief that "bigger is better" when it comes to motorcycles. I own a 1200cc Sportster and you know how many times I've needed that much power? Never. I do road trips of a thousand miles and more and never have I ever need to crank on full power and I've obviously never come close to riding that motorcycle as fast as it will go (it's geared for about 110 mph).

The 500 and 750 engines stock will provide more than ample power for riding on the streets and freeways.

Let me provide a little historical context. In 1969 Honda introduced the new CB750 and it was very fast considered to be a "big" motorcycle engine. The H-D 500 engine alone is more engine than most people will ever need.

That isn't to imply it isn't fun to tweak it just a little for a little more power you don't really need but it is fine right out of the box. Harleys are made for "tweaking" but it's really for fun more than anything else. Nothing wrong with having fun.


StoneFree

 

[Desert Tortoise]

Staggered duals make power over a very narrow rpm range, usually way up near red line. Over the rest of the rpm range they make significantly less power than a good 2 into 1 exhaust. They are fine for flat track racing or drag racing where the engine is wide open most of the time, and part throttle performance is not a consideration. Made to meet noise regulations, however, they are so restrictive they are probably the worst choice for an exhaust on a street motorcycle.

Remember the rule of thumb for a proper muffler is that muffler volume should be ten times cylinder displacement. If you look at a stock V-Rod exhaust, those two big end cans are what ten times 1130 cc looks like. You cannot put such large mufflers on a staggered dual system and have something that looks halfway decent (but they would hang down so low and be so wide you would not need a kick stand, just lean the bike to the right and set on the front muffler!)

A 2 into 1 exhaust can be built to use the exhaust pulse from one cylinder to scavange the other cylinder over a wide rpm range and also have a sufficiently large muffler to allow proper flow. You cannot do this with the sort of staggered dual exhaust Harley sells. Like anything in engineering it is a compromise, but a well designed 2 into 1 exhaust will make more power over a wider rpm range than any staggered dual system.

 

[StoneFree]

A 2 into 1 exhaust can be built to use the exhaust pulse from one cylinder to scavange the other cylinder over a wide rpm range and also have a sufficiently large muffler to allow proper flow. You cannot do this with the sort of staggered dual exhaust Harley sells. Like anything in engineering it is a compromise, but a well designed 2 into 1 exhaust will make more power over a wider rpm range than any staggered dual system.

This is actually important for two-strokes but not really for four-strokes that have an exhaust cycle where the piston forces the exhaust gases from the cylinder under presssure. Two-strokes don't have that and only the expansion of gas causes the exhaust with the venturi effect of a second cylinder providing additional suction to pull the exhaust from the cycliner. A single cylinder two-stroke relies on the velocity of the escaping gas to pull the remaining burnt gases out of the cylinder.

Let me share a story. I was the crew chief on a local NASCAR racing team for several years. We tried four different exhaust configurations including a 180 system that fed the exhaust from both sides into two different collectors. Guess what, no change in the horsepower on the dyno between any of them. We settled on the 180's because they sounded the best and not because they improved performance. It was very much like carberation (we weren't allowed to use fuel injection) because there was no difference between a 750 cfm and an 850cfm because a small block Chevy (358 cid) engine will only pump about 725 cfm anyway.

Of note on mufflers there are zero restriction mufflers that, while quieting the sound, to not impair engine performance one iota.

All normally aspirated engines are basically nothing more than an air pump. If we remember that then all we're concerned with really is allowing the engine to breath both on the intake and exhaust side. Restricting either the intake or exhaust hurts the engine but as long as there is no real restriction it will peform perfectly just like it's supposed to.

 

[StoneFree]

BTW if a person really wants to improve performance then have a more aggressive cam made for it. Getting the valve open faster and longer is a key to performance because that is a restriction on the engine.

 

[Desert Tortoise]

You apparently don't know much. Exhaust wave scavanging very much applies to four stroke engines, even if you could not figure it out. Read and learn.

https://www.rbracing-rsr.com/exhausttech.htm

 

[Desert Tortoise]

BTW if a person really wants to improve performance then have a more aggressive cam made for it. Getting the valve open faster and longer is a key to performance because that is a restriction on the engine.

Depends on what kind of power band you want. For a street bike, loading the power in the last 1000-2000 rpm might not be a very good choice. Increasing airflow with stock cam profiles can often yield a more useful power increase in the rpms you use the engine. Sometimes you get better results from detailed porting and valve/seat cuts that are informed by a flow bench, raised compression and attention to things like velocity stack and throttle body diameters than you get from lumpier cams. Longer duration and/or greater lift can increase peak horsepower, but at the expense of your midrange horsepower and torque. Lumpier cams also put stresses on the valve train, even in overhead cam engines, that increase wear and decrease durability. Again, it depends on what you want out of an engine.

If all you need is a single max rpm screamer, then cam it to the moon and use straight pipes. You won't have midrange, it will be useless on the street, but it will make max power in competition. If you need to make power over a wider range of rpms, then less lift and duration, sometimes even smaller valve sizes make more sense. This is why 883 heads with their smaller valves are often a better choice when building an 883 Sportster to a 1200, and the reason BMW used smaller valve and carb diameters on the R100G/S than they used on R100RSs and R100Ss. And, a two into one pipe is essential. Want to kill the midrange on an old BMW flat twin? Put an aftermarket pipe on it that does not have the two crossover pipes found in the stock system. Those crossover pipes are essential for exhaust scavenging and exhaust scavanging is essential on those engines for mid range power. Even Harley has to hide a cross over pipe between the front and rear pipes of their OEM staggered dual systems to give them some midrange power. Without it they fall flat on their face. I am surprised you do not know these things.

 

[Yusef]

Once you spend money on a bore kit though, would it be cheaper to just buy a more powerful Harley from the get-go? How much do these bore kits usually cost? If its upwards of $1,000, then that is going to be a fine line.

 

[fkystreet]

BTW if a person really wants to improve performance then have a more aggressive cam made for it. Getting the valve open faster and longer is a key to performance because that is a restriction on the engine.

or just bore it out.

 

[Desert Tortoise]

Once you spend money on a bore kit though, would it be cheaper to just buy a more powerful Harley from the get-go? How much do these bore kits usually cost? If its upwards of $1,000, then that is going to be a fine line.

Buy the 500 and then add an 800 cc or 900 cc big bore kit. Even if the kit costs $1500 it is a good value. I have a hunch you will see not only big bore kits but double overhead cam heads and different intake systems for these bikes. The intake and fuel injection strike me as a place where a lot of horsepower could be found. Double overhead cams, allowing use of bucket and shim lifters rather than rocker arms ought to be good for another 1000 - 1500 rpm on the redline, and more power. I see lots of ways to make power with these bikes. Picture an 800 cc, double overhead cam, dual intake CVO Street with a male slider fork, dual Brembo front discs and 17 inch wheels at both ends. Bandanaheads will experience vertigo at the thought!

 

[Livin On]

Buy the 500 and then add an 800 cc or 900 cc big bore kit. Even if the kit costs $1500 it is a good value. I have a hunch you will see not only big bore kits but double overhead cam heads and different intake systems for these bikes. The intake and fuel injection strike me as a place where a lot of horsepower could be found. Double overhead cams, allowing use of bucket and shim lifters rather than rocker arms ought to be good for another 1000 - 1500 rpm on the redline, and more power. I see lots of ways to make power with these bikes. Picture an 800 cc, double overhead cam, dual intake CVO Street with a male slider fork, dual Brembo front discs and 17 inch wheels at both ends. Bandanaheads will experience vertigo at the thought!

Jeez Desert, thats a ton of knowledge y'all just threw out in this thread. Quite looking forward to your eventual build thread. Learn something new everyday

Actually I do have a question for you, I've seen guys with cafes or scrambles that will jack the rear end ever so slightly with a barely taller shock. They say the rationale is that the taller rear loads the front end better for sharper handles. Is this a worth while consideration?

 

[Desert Tortoise]

Jeez Desert, thats a ton of knowledge y'all just threw out in this thread. Quite looking forward to your eventual build thread. Learn something new everyday

Actually I do have a question for you, I've seen guys with cafes or scrambles that will jack the rear end ever so slightly with a barely taller shock. They say the rationale is that the taller rear loads the front end better for sharper handles. Is this a worth while consideration?

Yes, that is exactly right. It also gets the pegs higher off the ground for greater cornering clearance. The only thing you have to be careful about is this, as you raise the rear, you reduce rake and trail. Reducing rake quickens the steering, but you need to have enough trail to keep the bike stable. If the bike has a lot of trail to begin with, say 108-110 mm, you can get away with raising the rear 25-35 mm and not run out of trail, keeping in mind that trail declines as the bike is leaned in a corner, and declines if you compress the fork under hard braking. This is why you see XR-1200 race bikes and before that old 883 Sportster race bikes with jacked up rear ends running steering dampers on the front fork. The riders raised the rear so high to gain cornering clearance and load the front end that they ran out of trail and the bike is unstable in corners. A steering damper is a Band-Aid for a poorly thought steering geometry or the sort of kludge you use to make a street bike something track worthy.

On my old K100RS, a tech friend installed a triple clamp from another model BMW that had less offset, the distance from the center of the steering head to the center of the fork tubes, than the original triple clamps. This took a lot of detailed machining so don't think you can reproduce this at home. Decreasing offset increases trail. Find the geometry on line and you will see this is true. Increasing trail allowed us to both raise the rear about 35mm but also to slide the fork tubes up in the triple clamps 25 mm, lowering the front end. Unlike a stock, slow but sloppy steering original K100RS that degenerated into a death weave if you push them hard in the corners with loaded saddlebags, this bike turns right effin now, but never even hints at instability. I have even run it with a blown out shock and no wobbles or weaves. The rear was pogoing up and down but nothing fed back into the steering. This bike is invincible loaded.

The Street Rod had so much trail stock I could get away with both a smaller diameter front wheel (also reduces trail)(actually smaller wheels at both ends, I used XR-1200 wheels) and raising the rear 13-14 mm.

I bought that blue bike in late 1984 and it about to turn 295,000 miles. See if you can figure out the other changes I have made over the years.

 

[sunny_sahdev]

you have complete changed its looks.. very impressive

Sent from my GT-N7100 using Tapatalk

 

[Desert Tortoise]

you have complete changed its looks.. very impressive

Sent from my GT-N7100 using Tapatalk

Appearance is secondary. It is all about how the bike works on the road. The blue K100RS has all it's original bodywork and is the stock color (after a couple of repaints) so it doesn't jump out as being obviously modified. But, I have completely changed the function of both bikes, especially that of the "Blue Bovine". Most other riders have no idea what they are looking at when they encounter the Street Rod (that's a Harley?) and are shocked at how fast it can be hustled down a twisty road.

Figured out what I did to the Blue Bovine yet?