So I was riding with a buddy today, he's got a 92 1200 sportster, and we were on the freeway. I have 1 tooth lower in the front so at about 65-70 mph I'm turning about 8k rpm, 9k rpm about 85mph. Of course I was going ***** to the wall, almost pushing it to the rev limiter. I notice when I'm up there screaming at 8k rpm (70mph) and I lay on the last 1/4 throttle the bike pulls HARD. Next thing I know I'm going 90mph and begin to get some bad speed wobble.

Why is this if the power supposedly drops off after 8k rpm like the dynos say? Is it possible...maybe....just maybe...with some bottom and top end upgrading, you can get this thing to spin to 12k rpm? This is a high rpm thumper for sure, and the 351 piston doesn't add THAT much reciprocating mass.

Any opinions?

 

Because dyno readings are taken with the throttle wide open, regardless of RPM. When you open that last 1/4 throttle, the bike is making more power than it did at 3/4 throttle. However, if you were already at WOT rather than cruising at part throttle, you'd feel the power drop off as you pass peak power.

Opinion: there's absolutely no point in trying to spin this engine beyond it's stock redline. The head and cams are far too restrictive to make power there anyway, besides the questions about reliability of other components at such high RPM. For the time and money that would need to be invested in the KLX, there are better options that would be better bikes in the end (KTM).

 

Hm...that makes too much sense hahaha :P

 

I've been thinking about this and playing with it. Given almost 50% more displacement than stock I wonder if we are close to what the carb can flow? In my experience, normally aspirated engines aren't close to limited by what the throttle body can flow, so once you open that butterfly much past 3/4 it's not really going to matter all that much - the engine will take the air it can given it's displacement and the other restrictions in the intake tract. Manifold vacuum will drop to zero or close to it.

But since we are trying to put way more air through the carb than originally intended, it may be more important that we really crank that thing wide open when wanting the power.

So I guess what I'm saying is maybe the issue isn't that the big bore has mad power at high RPM... maybe the issue is that the big bore really needs wide f'n open throttle at higher RPM to really shine. I've been ruminating on this and I'm thinking about a quick turn throttle for this reason.

Edit - So yeah, what Lutz said. lol

I'd really love to see dyno curves of the same bike with stock carb and with an oversized carb on the same engine build. Dyno should take the pumper aspect out of it and show direct difference in carb bore size, assuming jetting was correct on both, etc.

I guess. Or something like that. Comments? Somebody talk me out of buying a bigger carb.

 

Some good points, B. I think you may be on to something concerning the 34mm CV carb with the big bore kits...especially the 351. Concerning the pumper carb...and to keep it simple let's stay with the Mikuni...there's another performance element here that's especially applicable at higher rpm and WOT. The Mikuni has a totally open and smooth bore at WOT. It isn't discussed much, but I think this is one of the big benefits of these smoothbore carbs, the accelerator pump aside. I think this is totally why Bill Blue went to the Mikuni...not just for the accelerator pump. Bill's choice is a 34mm unit which is equal to the OEM CV carb, but the pump and the smoothbore are both big pluses. Look down the throat of the OEM carb. A notable obstruction in the form of a shaft and throttle plate are always in the flow...not so in the pumper smoothbore. Now, automotive carbs of all kinds have had throttle plates, and many of them performed quite well. Still, the best of those throttle plate carbs probably didn't have vacuum slides and most of had the benefit of an accelerator pump.

Now, if you go the route of the 36mm Mikuni pumper, you get the accelerator pump and 2mm additional throttle bore. Yes, it would take some very careful and consistent dyno runs to qualify absolute power gains. Still, anyone who has used the 34 or 36 pumper rarely goes back. I think Rob who posts here occasionally said he got rid of his 34mm pumper because it had too much response. That's his prerogative, but I doubt many of us would be dissatisfied with a good chunk of extra response and overall power.

On the smoothbore carb design, I'm old enough to remember and have owned early performance inline-4 motorcycles like the Honda CB750 and Z1-900/1000. I worked at a shop back then and saw the results first hand how these bikes responded with just the application of 1mm or 2mm larger smoothbore carb sets...no accelerator pump. The smoothbore design works, and I think the only reason we have OEM CV carbs is for emissions and cost.

 

Yup, that all makes great sense to me.

But.. (and obviously I'm a total newb to motorcycles) I suspect that the actual differences in true power between the uber-carbs and stock are probably very minimal. I can't help but think that if bolting on a carb would give 2HP that there would be dyno curves galore on here demonstrating such?

I'm thinking that the uber-carbs FEEL better because you don't have to crack them open as far to get that power... that the stock carb can do the same power but you really have to wrench it wide open to get it.

I've got a (subaru) WRX that I fool with the ECU tuning on, including the throttle by wire. It's amazing how fast you can make a car feel just by making that throttle curve more aggressive, even though you've added no power.

I can't speak towards the response of pumper vs. stock on these bikes since I have no experience, but I'd be very interested to know if there is truly more power with the bigger carbs, or if it's just more punch for a given throttle opening that makes it seem like there is?

Don't get me wrong, if it can definitively be shown that bigger carb = more power, I'll whip the checkbook out. But from all I've read here I'm not convinced that the difference isn't qualitative rather than quantitative.

Also for what it's worth, I'm on a straight street SF, so super quick bursts to get the front up aren't really a concern for me... I can see how the pumper would be way better for that off-road even if dyno numbers didn't show higher.

 

Interesting thought on the bigger bore carb and concerning throttle opening. I figure if the ratio takes 2 full fists (as it does stock) the bike's throttle will obviously open slower and feel pretty sluggish even with the pumper carb. I have the zeta quick turn throttle(before big bore) and right away I noticed alittle more than 1/4 turn is closer to what my almost 1/2 throttle felt before the upgrade. On my CV carb I really only feel long vaccum hesitation during WFO clutch-less shifting, at 10mph just wack the throttle and she powers up quite quick.

Quicker throttle ratio + pumper carb might be the reason some folks don't need the jerky throttle response as Rob said, but I guess in theory if you have the glorious tm-36-68 that TNC teases me about in many threads, you could adjust the accelerator pump to activate at a later time in the throttle turn thus eliminating a TOO jerky response.

Concerning carb diameter, I would assume that the smoothness of the throat like TNC said has a greater effect than larger diameter opening, but putting both together is probably why the tm-36-68 runs so smooth, once again as TNC said in another thread, wide open. This could also equate to why the larger pumper gets better mpg at cruising speed, more free flowing air.

 

Most dyno readings are not taken at a set rpm. They are taken over the range of the run using full throttle. If not, the dyno would never create an actual power curve. Usually the run is made in an upper gear to avoid tire slip when the throttle is run up. My friend's V-Max was done in 4th gear.

Torque is the ability to perform work in a rotational direction of measure and is the real raw power number. Horsepower is derived from the torque value and is the ability to move 33,000 lb 1 foot in 1 minute (a comparison that could relate to the proverbial work horse quite literally) it is measured using rpm and torque. These two numbers are fed into a formula -- torque times RPM divided by 5,252 -- to arrive at horsepower. that is why the horsepower figure always is higher than the torque figure. Here's some stuff about it.
There was another article in either Cycle World or Motorcyclist, but I forget where and when. The whole thing was reviewed in excellent terms. Wish I had it.

Now the question, are you talking power in torque or horsepower. Torque runs out before redline, but the engine will still pull beyond that torque peak. How far depends on the tune of the engine and rev limiter.

In a side note, a big bore is a change of the tune and may or may not allow higher revs. The good part of the big bore is usually that it spreads the power gain over the entire range if compression is pretty much the same as the original. So with more power you can pull a taller gear, giving more top end. Reliability with all else the same is also the same. If compression is higher the gain is bigger overall, but may not be as strong down low, I think.

I like bolt ons that help like carbs and pipes, but when it comes to diving in the engine, there is no replacement for displacement. Done well, it is totally reliable, where cams may make the engine performance peaky and harder to ride. If the money and time is there, a big bore and taller gearing is the direction I would go every time over high compression and cams.

My own big bore experience is with my KLX650 and an OEM Vulcan piston pushing the engine out to 678cc and about the same compression. My bike pulled stronger over the stock bike everywhere. I had a pipe and Dial-A-Jet (fuel adder over brass jetting) which helped too. We had another stock KLX with a Vulcan piston built. They ran the two off. The stock big bore jumped out from the start around 2500-3000, but at about 3500 my bike with the pipe and DAJ started to pull and walked on past the other bike.

It used to take some time to wind up to 90-95, but with the increased displacement and pipe/DAJ it gets 90-95 quite easily. I've not pushed it over 100 at this point but top before was around 108. I think it's probably about the same since the redline seems about the same. The power spread over the entire range, but didn't push the rpm range up. That was fine by me, I am about reliability. The big bore opportunity just presented itself when we redid the top end after cam drive problems. The piston was laying around for free... what can I say... it had to be done.

 

Maybe I can better elaborate on what caused me to come to ask this question as I repeated the test today.

When I'm cruising at 8000 RPM and wack the throttle WFO it pulls. When I'm at 9 RPM and wack the throttle WFO it REALLY pulls, until it hits redline of course.

 

So I think I might understand if there actually is a question to start...

The way horsepower - the amount of work over time - runs higher than the torque curve is covered in the general information and that with your 351 it will pull up to red line or point where working capability tapers off, easier due to increased torque - ability to do work, which maxes out at a lower rpm - over the entire range. Pretty simple. The maximum ability to do work over time is stronger if not higher. Thus you pull stronger until the power is either limited or falls off and won't pull any longer. I actually called up a dyno chart and it shows the KLX has the ability to overrev well into the 5 figure range. The engine can spin it that high without breaking and is still pulling, but not as strong. THAT is what you are experiencing. Most red lines on bikes are above the actual peak HP limit. It is often referred to in tests as the ability to pull past power peak when needed, without endangering the engine. The KLX has the ability to rev five figures easily without breaking, in spite of the fact that the HP peak is at about 8000 rpm. The horsepower and torque curve drop off nice and gradual allowing significant over-reving the engine. A NICE BONUS!

The old Honda V45s from 1982 would pull like crazy past red line, but the valve train would not keep up and with no rev limiter in those models you can guess the results - all eight results! One friend did it with his Sabre on a missed shift. In 83 Honda put in a rev limiter to save the engine.