...that write-up on striking the balancing act between creating torque and horsepower in any motor captures the essence of what engine building's all about. I should memorize that paragraph, chanting it through the halls like a mantra.

I don't aspire to become an engine builder but the more I keep asking this kind of stuff the more I get into it, ya know? I wish I had a friend or some friends who work on bikes and do these kind of mods so I could watch them and learn it myself.

That HQ 500 sounds like the cam for me when the time comes.

I have yet another question that ties in with cam configuration and engine management: how does more exhaust back pressure help the engine make more torque? We've all seen kids with asian tuner cars and they put the coffee can exhaust on the back for freer flow but this robs torque. Why?

Anyway you guys kick major donkey *** (pun intended) and thanks for all your help.

John

 

Based on the charts I've seen and my personal experience the 255's don't shift the TQ curve to the left, but rather move it straight up. The stock cams give a very flat and to some of us pleasing TQ curve and moving it up just makes more of what you already have.

Stock cams will be quieter in the valve-train and especially in the exhaust tone.

IMO if you plan on doing mods I would not go the Nightrider route (IED and variants), but would instead buy a real tuner.

How much is enough for you is impossible for us to even guess at. You can do a cam swap for little money, as low as $200 if you have the right tools and DIY.

 

That's a big can of worms, and it's too complicated to capture in a couple of sentences.

There's a fair amount of misunderstanding about exhaust back pressure and power production. Back pressure is the resistance to airflow due to downstream restrictions and it doesn't help power production one bit. Delta pressure, on the other hand, is the drop in pressure that occurs when a gas (or fluid) travels across a piping component. The larger the Delta P value, the higher the pressure drop is, and the faster the exhaust gas will travel through the system.

Make your exhaust tubing too small, and the air flow becomes restricted, slowing the gasses, and increasing reversion at the cylinder, which hurts power production. That's backpressure. However, if the exhaust tubing is too large, the gasses will not accelerate to the maximum possible, which reduces the Delta P value. What we want is the tubing to be just the right size to effect maximum velocity through the exhaust, in order to increase the pressure drop behind the exiting gasses.

Our objective is to maximize the flow into and out of the cylinder. The incoming charge is a seperate event, but directly related to the exhaust out of the motor, especially during the valve overlap period. The drop in pressure behind the exiting gasses can help draw in the fresh charge, which helps maximize cylinder fill (scavenging). So we want the highest flow rate possible throughout the power band, and this is where we start running into a conflict.

For a given amount of exhaust pressure at the exhaust valve, a smaller diameter pipe will provide higher flow velocity than a larger diameter pipe. However, at a higher rpm, that same pipe won't flow enough volume to make maximum power. If we put in a large diameter exhaust, the flow velocity slows at low rpms, and torque production suffers. It's similar to the balancing act that occurs with cam design. Your motor acts differently at low rpms than it does at high rpms, but we expect it's components to handle both.

Just like in cams, bigger isn't always better, and some times, it makes things worse.

There's a lot that goes into an exhaust design. Generally, you want equal length pipes, and longer pipes tend to help torque production at lower rpms, and shorter pipes tend to increase hp at higher rpms. Likewise, smaller diameter pipes tend to help low rpm torque production, and larger pipes increase hp at higher rpms. A 2-1 system, where the primary tubes come together in a collector helps maximize scavenging.

So, having said all that, why do we sometimes intentionally try to slow down exhaust gasses? Probably because the exhaust wasn't designed properly, and we're getting wave reversion at the cylinder. Energy waves occur with our exhaust pulses due to the rapidly expanding gasses. They travel down the pipe at a very high rate of speed, and when they reach the end part of it makes a 180 (due to the low pressure of the ambient air) and travels back up the exhaust pipe. The reversion wave passes through the exiting exhaust gasses (which are traveling about 5 times slower), and tries to re-enter the combustion chamber. If the exhaust valve is open, the reversion wave pushes against the incoming fuel/air charge, harming power production. If the exhaust valve is closed, the reversion wave turns around again and heads back out. That is what we want to happen, and eventually the wave will dissipate all it's energy going back and forth.

If our exhaust system isn't properly matched to our motor, the reversion wave can make the bike very sluggish at low rpms. If we install torque cones, sometimes we can slow down the exhaust gasses enough to match cam timing. However, a better idea is to have a properly designed exhaust without restrictions, and a properly tuned bike.

 

I'm not a cam expert but as mentioned you can't look at the lift numbers, overlap, or anything else independently. IMO the stock cam was the MOCO's compromise on emissions, with little thought on how to maximize performance.

With the 96, I think the key is to get the motor to open up and breathe. A stage one is the first step, but changing cams unlocks the real power. After that the the main considerations are where YOU want the torque and power curves based on what kind of riding you do and what type of comfort you want in the long run.

The nice thing is with all the dyno charts floating around, all the homework has been done for you. All you need to do is match the dyno curves with what you believe best fits your style.

Again, I'm no expert, its just an opinon about an imperfect science.

 

I'm no cam expert but I do know this HQ500 cam works, I have a buddy that put the same cam in his bike last winter and got the same results.
I use a Big Sucker 2 air cleaner, a FatCat exhuast with quite baffle, Hq500 cam and a Harley race Tuner with a full Dyno tune. This combo flat works. I believe the Racetuner and the dyno tune are the keys to making these new engines run so well, a lot of people don't do it because of the expense and IMHO that is a mistake.
I can tell you that the over size exhaust slows down the exhaust velocity and that's what kills the torque on those small engines, I found that first hand when I had a local muffler shop put a better looking and sounding muffler on my Scion XB, they installed this garbage can sized muffler and it totally destroyed the cars performance until I built a slip in baffle that brought the size down to 1 3/4 and that resorted the performance and kept the nice sound.

 

That's very true, but it goes farther than that. You can't look at any cam independantly of the rest of the motor, especially heads, compression, fuel delivery and exhaust design. They work together in a dynamic state, and pushing any one component out of balance can throw the whole system out of whack.

You are spot on with your observation that the stock cams were designed to meet emissions and not to maximize performance. Fortunately, the aftermarket can design cams for "race only" applications that don't have to take into account emissions requirements. At least not yet. I'm sure the EPA would love to close that loophole, and are probably looking at ways to do just that.

 

I'm also very surprised this hasn't been done already.

 

i would love to see numbers on a 96 with 10.1 compression,street ported heads with a good cam.i would love to get my numbers up to at least 96hp and 105 or so on tq. iv seen big numbers on 95s over 100/100 and i dont see why we cant get close with a 96 since its stroked .

 

Check out Fuelmoto's dyno sheet on a 96in with TW555 cams. I believe it has the HP around 96-97 and the torque over 100 with just the cams