10 Ways To Make Your 4-stroke Faster

[Doug.]

TEN WAYS TO MAKE YOUR FOUR-STROKE FASTER

Most showroom stock four-strokes are delivered in such a high state of tune that it isnt easy to get more power out of them. It takes the expertise of a skilled engine tuner to even try--and they dont always succeed. But, as we careen towards a four-stroke future, the four-stroke hop-up business will solidify. The MXA wrecking crew sat down with a host of four-stroke tuners to analyze the ten most basic ways to get more power out of a thumper.

NUMBER ONE: HOPPED-UP GAS

There isn't an easier performance mod than pouring VP Racing's Ultimate 4 race gas in the tank. It only takes a few seconds and offers instant rewards. The increase in octane, power yielding oxygenents and hydrocarbons (with a greater energy potential) results in a fuel that can produce 6 percent more power than standard pump gas. That's a three horsepower increase on a 450 and two horses more on a 250.

NUMBER TWO: EXHAUST SYSTEMS

With prices running as high as $800 for a titanium system, performance four-stroke pipes have become status symbols. As with a two-strokeÕs expansion chamber, a four-stroke exhaust system is tuned to produce the optimum powerband. The major elements of pipe design are: (1) head pipe length, (2) head pipe diameter, (3) overall tuned length, (3) taper of S-bends, (4) diameter of the muffler core and (5) length of the muffler. Unlike Formula 1 or road racing exhaust pipes, a four-stroke pipe has to deliver a broad and usable powerband--instead of maximum horsepower.

In the future, expect to see tapered head pipes, steps, stamped shapes and exotic metals (like Inconel) being used to deliver more low-end, a broader mid and extra top. Not only can aftermarket exhaust pipes add a horsepower or two, but they can save as much as five pounds over stock systems.

NUMBER THREE: CUBIC CENTIMETERS

Since the day that Nicolaus August Otto fired up the first four-stroke engine in 1876, four-stroke designers have been boring them out. An increase in cubic centimeters is the time honored method of making a four-stroke faster. There is one caveat, however. Most four-stroke engines, save for XR50s and TT-R125s, are already at the legal displacement limit. Thus, an increase in bore will make your 250 or 450 four-stroke illegal for AMA racing. The rule, however, doesn't apply to the Vet classes or play bikes. While few racers would consider a big-bore 450, a large displacement 250 could be your ultimate Vet weapon. Expect to gain two to three horsepower. Rick Peterson Motorsports makes 302cc versions of all the popular 250 four-strokes.

NUMBER FOUR: STROKE IT

Stroking a four-stroke engine also increases the displacement (making the bike illegal for AMA Pro racing). To stroke an engine, a tuner relocates the big-end bearing further outboard on the crank halves. In most cases, the connecting rod is shortened the same distance that the big end bearing is moved. This is done because the larger sweep of the big-end bearing would otherwise send the piston into the valves. When the rod is shortened, the piston tops out at the standard top-dead-center measurement.

At bottom-dead-center, the shorter rod and repositioned big-end bearing bring the piston a few millimeters further down the bore. A piston with a skirt cutaway is usually required to provide enough crank clearance. A big-bore gives more sheer horsepower, while stroking a thumper broadens the torque curve.

NUMBER FIVE: HIGH-PERFORMANCE CAMSHAFTS

Performance cams are available with lobes that are timed to open the valves earlier and to keep them open longer (known as dwell). As a rule of thumb, the best high-performance cams do their best work in the range where the power is already concentrated. The most noticeable camshaft change is in the way the power is delivered. Automobile hop-up shops have done a bumper business in cams for decades. Motocross racers have not embraced different cam shapes, but this will change.

NUMBER SIX: HIGH-FLOW PORTING

Unlike on a two-stroke, four-stroke porting is done in the head. There are no ports in the cylinder wall of a four-stroke. To properly port the head on a four-stroke requires a flow-bench. A flow-bench measures how much vacuum it takes to draw air out of the intake port and exhaust flange. Porting and polishing the cavities of a four-strokes cylinder head can increase the flow rate for more performance. In some cases, larger valves can be fitted to improve the flow rate, but this mod normally only works on four-stroke engines of a lesser tune.

NUMBER SEVEN: INCREASED COMPRESSION

One of the easiest ways to get a harder hit and better low-to-mid power is to increase the compression ratio. On a two-stroke engine, compression is increased by milling the head (which decreases combustion chamber volume). On a four-stroke, the compression ratio is increased by using a piston with a higher dome. A high-compression piston typically delivers more punch at the cost of some top-end. Stock compression is typically in the 12:1 range. It can be upped as high as 13:1. Be careful, increases in compression might require higher octane gasoline.

NUMBER EIGHT: BORED-OUT CARBS

As with a two-stroke engine, carburetors offer wonderful tuning opportunities. On most small displacement play bikes, like TT-Rs, KLXs and XRs, it is de rigeur to replace the stock carb with a larger one. The larger the bore of the carb, which is measured in millimeters, the more fuel it can flow. As a rule, big carbs increase mid-and-up power (at the cost of low-end). Why do big carbs hurt low-end power? In order to flow fuel through a larger opening, the engine has to create more vacuum. The larger the carb throat the greater the vacuum required to draw the fuel into the engine. At low rpm settings, many four-stroke engines donÕt produce enough air velocity to get the fuel flowing--thus, low-end power is lost until the air flow is sufficient (at higher rpm). Conversely, smaller carbs flow fuel more easily, start better and have better throttle response.

NUMBER NINE: CLUTCHES

The extra weight and torquier power delivery makes a four-stroke tough on clutches. If you pump up the horsepower with race gas, exhaust systems, big-bore kits, camshafts or flowed heads, the stock clutch may not be sufficient. Hinson makes baskets, pressure plates and hubs that guarantee longer life.

NUMBER TEN: WET SUMP

The Yamaha YZ250F and YZ450F can have their lubrication systems switched from dry sump to wet sump with aftermarket kits. These kits eliminate the YZ-Fs oil tank and, in the process, save two pounds of oil and oil lines. Wet sumping doesnÕt necessarily make a bike faster, but it does make it lighter. Wet sump YZ-Fs will need to have their oil changed more frequently.

In the future, a new breed of four-stroke oil will increase horsepower. These new oils will have the lubricating qualities of a 30-weight, but the viscosity of a 0-weight (comparable to water). These low viscosity oils will reduced drag on the mechanical parts and improve everything from throttle response to top-end.

Motocross Action Magazine

3-18-05

[velosapiens]

or you could just buy a bigger bike and not have to do anything to it.

i rode a husaberg fe650 a couple weeks ago. no need for engine mods of any kind on that bad boy.

[Woods_Rider]

Or buy a 2 stroke and get twice as much power for the same amount of displacement. ?

[tnt250]

I'm not sure that inconel in your exhaust system is going to do any favors as far as weight is concerned. Interesting article though.

[rubenz]

i don't think pouring race gas into my bike would do anything at all. just like it doesn't do anything in stock cars/trucks that run on 87-91 octanes. but that's just what i have heard most motor tuners say but they could have been lying.

[Doug.]

i don't think pouring race gas into my bike would do anything at all. just like it doesn't do anything in stock cars/trucks that run on 87-91 octanes. but that's just what i have heard most motor tuners say but they could have been lying.

Thats just what you've HEARD? Dont knock it 'till you've tried it. ?

I run VP U-4 in my YZ250F and there is a noticable increase in power.

[rubenz]

Thats just what you've HEARD? Dont knock it 'till you've tried it. ?

I run VP U-4 in my YZ250F and there is a noticable increase in power.

yeah and alot of folks say that they get more power using 91 octane on a car that is made to run 87. when i run 91 gas in my bike it does aboslutely nothing. when i run 91 ocatane in my truck it does nothing. now if my bike was supposed to run 110 ocatane and i usually run 91 and then switch to 110 then there will be a power difference becasue that is what it needs to run optimal. so basically what i'm saying is that just run the gas that the bike needs, not higher octane and not lower.

[velosapiens]

i don't think pouring race gas into my bike would do anything at all.

as i understand it ultimate-4 is oxygenated, so you can jet richer and the gas is actually carrying the extra oxygen with it. that's where the extra power comes from.

i believe you are correct wrt to traditional race gas formulas which you would only take advantage of if you had a highly modified engine.

i still think it's cheaper just to get a bigger bike in the first place than put $10/gal gas into my bike. 4-5k miles of riding each year makes my gas budget pretty high anyway.

[Doug.]

You only need higher octane if you raise the compression (in a thumper). VP U-4 is race gas that is oxygentated, not high octane.

[FakeRacing]

Hmm, I think they forgot one... put a faster rider in the saddle. Most modern bikes have more stock potential than a mere mortal will ever tap outa them

[go-smaller]

good article for sure ?

[frankstr]

There's no replacement, For displacement.... ?

[cms_austx]

Hmm, I think they forgot one... put a faster rider in the saddle. Most modern bikes have more stock potential than a mere mortal will ever tap outa them

But don't forget the source...w/ MXA, it's often difficult to tell the difference between the ads and the stories. They're all about selling stuff!

[rubenz]

You only need higher octane if you raise the compression (in a thumper). VP U-4 is race gas that is oxygentated, not high octane.

ahh. ok i get it. i thought it was just higher octane. i'm not sure why i thought that. it's just the first thing i think about when people say race gas. ?

[MX145]

ahhhh.... the smell of race fuel ! VP's Ultimate4 will boost the power in dyno runs, etc... however you may want to consider the after effects of running it. It can cause fast deteriation to many parts in and associated to your motor. Have you ever been around a garage, box van, trailer, etc... with it around?? You will definately know.... it has a very distinct odor that doesn't stay in the tank !! I would recommend running the appropriate rated fuel for your engines modifications which may include U4. The higher octane isn't always best as some may believe. VP's website has some good info. on different fuels they offer for the appropriate application.

[ovrrdrive]

Hmm, I think they forgot one... put a faster rider in the saddle. Most modern bikes have more stock potential than a mere mortal will ever tap outa them

I don't think I've ever wished at any time that my 450 had any more power than it does already. ?

Interesting article though.

[YZ250F_Rider]

ahh. ok i get it. i thought it was just higher octane. i'm not sure why i thought that. it's just the first thing i think about when people say race gas. ?

The reason you thought that is some cars can take advantage of higher octane ratings of premium gas by advancing their ignition timing.

This is one of the missing steps as well. You can advance the base timing of a thumper and gain more power everywhere, but you have to run a higher octane fuel when you do.

[Lightningale]

Or buy a 2 stroke and get twice as much power for the same amount of displacement. ?

...and burn three times the fuel and ten times the oil. ?

[JohnnyOfast]

...and burn three times the fuel and ten times the oil. ?

,,,,, and spin the tire, not find any traction, and sit still like a dragster smok'n the tires at the line. ?

[cms_austx]

,,,,, and spin the tire, not find any traction, and sit still like a dragster smok'n the tires at the line. ?

Hey, somebody needs to alert RC how much he's hurting his chances of winning by racing that 2 smoker!!! ?