If you are an LO206 racer, you have probably seen social media posts and forums over cam twisting the LO206 without removing the seals from the engine.
Cam twisting is brought to light primarily through Krybaby Performance. The controversy concerns the lack of a camshaft centerline spec in the rulebook. In the Briggs and Stratton Lo206 Rule Book, the camshaft is allowed an 8-degrees tolerance at any specified point. The argument is that wide tolerance enables an engine builder room to adjust the centerlines for an advantage.
It has caused concerns but also a lot of debate.
Suppose you have followed any of the forums or posts about cam twisting. In that case, Krybaby is almost always discredited or dismissed in some way which only plays into the conspiracy of the situation. However, regardless of your feelings, their point is valid, and I want to explain that.
Here is what I'm aiming to answer:
- Briggs Changing the Rules.
- How does cam twisting influence power?
- How does this translate to track performance, and how will it affect my racing?
- Are there drawbacks to cam twisting?
Briggs Changing the Rules.
First, Krybaby is not wrong to expect Briggs to address and make a resolution with the rules. Briggs has posted about the 3,600 data points of their camshaft. And everyone, including Krybaby, would agree that there isn't much variation out of the box, which only worsens the issue when the rules allow such a wide tolerance.
Briggs can easily tighten the rules. Krybaby suggests 1/2 degree, which may be tight but shows their confidence in Briggs's ability to produce the engine with that close specs. Even if it were a degree, it would reduce the impact twisting the cam could make. I can only assume reasons for the tolerance to be so broad, from manufacturing and assembly tolerance to giving room for errors during tech. Protesting and having engines sent to Briggs to validate rulings can be costly to the racer. When you have a sealed bottom end, you do not expect intentional tampering as much as human error.
Cup Karts North America has amended the rule to address the issue. However, they only put the rule on the intake side. Krybaby also pointed out that the intake isn't the problem as much as the exhaust. To understand how we have to discuss camshaft theory.
How does cam twisting influence power?
Cam twisting is not new in any class that requires a stock camshaft, whether it's the clone, flathead, or automotive racing. Camshafts with the same lift and duration can still dial in the power band by changing the centerlines and lobe separation of the cam.
While the low lift, low duration, and no overlap can limit gains even when changing centerlines, it can't prevent them. Only the rules can.
There is a lot of depth and mystery to camshaft theory, and I won't be able to cover everything, but I'll try to make it simple within the context of the Lo206.
Changing the centerlines of the camshaft influence the valve events of the camshaft to cause them to happen sooner or later.
One of the first valve events that are looked at is the intake valve closing which can influence the dynamic compression of the engine. The sooner the valve closes, the higher dynamic compression can be made. However, even within the rules, this would vary very little. Air is lazy and takes time to get moving. During the later part of the intake cycle, in reality, during the beginning of the compression cycle, the air has more inertia that helps fill the cylinder. If the intake closes too soon, you could lose the opportunity to fill the cylinder better. Again this is a low lift/duration camshaft with no overlap that limits what we can gain. There is more to it, but I'm making generalizations for time's sake. For the LO206, the intake centerline can be a wash.
The next event is the exhaust opening event. Most forget the exhaust valve opens during the end of the power cycle, known as the "blow down" phase. Our goal is to best time the exhaust opening for the combustion to use its energy most effectively without causing resistance during the exhaust cycle. If we open the exhaust valve late, the exhaust gases are left in the cylinder, or worse, the combustion is still expanding when the piston begins going back up. Your engine is wasting power to overcome this resistance. If we open the exhaust valve early, then we reduce cylinder pressure which would push the piston downward. There can be many parts of an engine's design to consider when the exhaust should open, but the more rpms you turn you want to open the exhaust sooner. For that reason, you usually see more duration on the exhaust to broaden the powerband. But it appears the Briggs cam may be a single pattern(same lobe on the intake and exhaust)camshaft.
Now that you have the idea let's contextualize it with the Briggs Lo206. The engine is rev limited to 6100rpms; peak hp is around 5250rpms(give or take), and clutch engagement which is around peak torque, is 3400-3600rpms. Peak torque is the max efficiency of the engine, so from that point on, the engine is making less torque and power(horsepower is only a calculation of torque and rpm).
We don't race below 3400rpms, so we don't care what happens there. We aim to move the exhaust to open sooner to broaden the powerband until we reach our max rpm or start losing power around 3400-3600 rpm. We have yet to test this with the Lo206, but with a Honda clone of similar specs opening the exhaust around 42-43 degrees at .050 lift (which doesn't need the additional 2 degrees the Briggs rules allow) can move peak power 500-600rpms while maintaining peak torque around 3600rpms. Not only does this improve peak power, but it increases average torque from 3600-6100rpms. While these are different engines, it shows performance potential.
By setting a centerline rule, you also control the opening and closing events since the profile on the cam can't change and, as Briggs has stated, has thousands of data points on a cam that uses straight-cut gears, which has more accurate repeatability than helical cut gears, so it shouldn't need as wide of tolerance if the centerline rule is implemented.
Hopefully, you better understand the camshaft theory and how this validates Krybaby's concern over the current written rules.
How does this translate to track performance, and how will it affect my racing?
One of the comments frequently said to dismiss the possible advantage of cam twisting is related to poor driving and handling. To paraphrase, "you only care about cheating because your driver parks it in the corners."
That comment is valid because the power advantage likely will not improve your spot on the track if you are a mid-pack racer. If you don't have the skills or the setup, cam twisting isn't what puts you in the winner's circle.
However, what if you are a top 5 driver battling for wins?
We must consider that good drivers that can nail the setup gain more from cam twisting than the average driver. That is the argument.
Let's consider race strategy. In the Lo206 class, you'll often see groups form during the race. The experienced front runners understand the race isn't won on the first lap, so they work together by not racing for position early on and use bump drafting to use their momentum to push the kart in front forward, then run them back down with the draft. The purpose is to separate themselves and reduce the number of karts they will have to battle with during the race's closing laps. If some of the front runners have cam-twisted engines, they can pull away sooner, they may be less dependent on other karts to break away, and may further reduce the number of karts battling it out at the end.
Here are some ways that additional powerband can help:
1. If a competitor has the same gear ratio, your additional power will help you accelerate to your top speed sooner. Since the LO206 is limited to 6100rpms, max speed is also limited, but getting there quicker can open up better opportunities for passing and make it more forgiving on your driving.
2. The common understanding of gear ratio changes is for better bottom-end or top-end. But it may come at a compromise, one for the other. In other words, you have to decide which is better at reducing lap times, a better bottom end for the tight sections of the course or a better top end speed for the straightaways. Whichever you choose, the broader powerband will either compromise less or do the job-specific job better.
3. Environmental conditions such as headwinds(ex. 20mph winds down the straightaway) or tracks where aero or drafting are essential. Suppose you are in a situation where you have to compromise your top end to overcome the wind resistance. In that case, the additional power may require less compromise to maintain a better average speed.
4. Changes in track conditions: If the track tightens up over the day or rain washes the rubber off the track, and you have to make sudden tuning adjustments, more power can make it more forgiving if you don't get it 100%.
Ultimately we're talking about a wider tuning window where we are less gearing-sensitive and more forgiving on our driving.
I believe most people miss the point by dismissing the gains in performance without considering the improvements made in chassis/gear tuning.
Now, will this affect your racing? There is a low chance that it will change the outcome. As I said, a mid-pack racer isn't going to turn into a winner until he gets closer with their driving and setup. Competitive racers will not waste good engines on races that don't offer a reward for winning, and since Briggs isn't offering prizes for their weekly racer series, you're not winning much more than bragging rights for local races. You are most likely to see these engines if you run a prestigious event or if there is money on the line. And at the end of the day, we're not racing dynos. You can still lose to the better driver.
Are there drawbacks to cam twisting?
It would seem so far that cam twisting is the way to go if you expect to find an edge in the competition. However, it's more challenging than it sounds.
Since the cam is being twisted inside the engine through the lifter bores, you risk damaging different engine components. Talking with racers and engine builders, aware of cam twisting and conscious of the failures that come with it. Most often, the cam bosses that hold the cam in the block break. We also suspect damaging the tappet surface or stem could occur, or damaging the lobe surface or twisting the cam in a bind. For the amount of effort it takes to twist the cam, you may find a way of removing and reinstalling the bottom-end seal and not risking the engine, which is another reason for the rule change.
While there may be tools and techniques for cam twisting and improving your chance of success, the human factor is out of anyone's control. We would not encourage someone with little to no engine building or mechanical experience to try twisting the cam in your engine. If you are an engine builder, you should also take precautions that you could also damage a customer's engine. Since Briggs Lo206 is hard to come by, it may not be worth the risk if you can't replace the engine.
There is a paradox within the spirit and intent of how rulebooks are written and how they are applied; on the one hand, wordage such as "if it doesn't say you can, then you can't" while the scrutineers and techs are usually trained to view the rules as long as the specs are not outside the rule you are given the benefit of the doubt that you didn't manipulate the engine. Some want to call cam twisting illegal or cheating, but how do you prove the engine didn't come with beneficial cam timing if Briggs allows a wide tolerance?
It's similar to when engine builders were manipulating spark plugs, there was no rule against it, but Briggs chose to change the spec plug. It was no different when seals were being removed and reinstalled; Briggs decided to change the seals and obsolete engines. It doesn't make sense not to make a simple rule change that doesn't require the racer to change a part or would make their engine obsolete except for those manipulating their cam timing.
Now that it is more public knowledge, there is usually more interest in experimenting for the average racer. While it may do very little to change the outcome of a few races, more than likely, there will be a pile of damaged Lo206 engines. If the rules are amended too late, you have a bunch of useless, illegal engines. Briggs is already backlogged on supplying engines this year, and if the demand for replacement engines, heads, and short blocks increases, it will only put more strain on the program. As someone who races the LO206 class, I would be less worried about getting the most performance from your engine and start worrying more if the LO206 will survive the next couple of years.
I believe it's in the best interest of the Lo206 program for all parties that Briggs reevaluate the cam rules. Adding a centerline rule will help tighten the cam spec rules, and whatever that cam rule may be, the exhaust opening at .050 lift should not be more than 39-40 degrees before bottom dead center.
We could continue our argument, but we want to know your thoughts.