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high compression vs. stroker?

By yz480doinwork,
in YZ400F/426F/450F/450FX

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yz480doinwork

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Hey i have a 2006 special edition yz 450f. i currently have a Atena 478 big bore kit (which comes with a low compression piston) and i have ported and polished my head along with some air box mods. I am trying to decide weather i should by a Hot Rods +3mm stroker crank or a venom 14.25:1 high compression piston. I also eventually entend to run a boondocker nitrous kit. Considering i already have a big bore kit what would be the best choice, stroker or pisten? what will give more power and last longer in the long run considering i will be running nos.

thanks for your input

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CRF250X Rider

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I don't really know to much about the yzf engine specifically but I'm pretty sure that strokers aren't the most reliable. Since the short piston skirt it tends to rock back and forth more and isn't the best thing for your engine. I would go high compression, Or if your going balls out, why not stroker, high compression piston, turbo, nitrous and a jet pack? On second though a couple packs of stickers and a new set of graphics would probably be be more bang for your buck. Rather reliable too.

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mkporn

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The stroker kit is not going to add unreliability. The high compression will be worse and more of a nightmare simply because of the added cost of fuel and tuning and added stress on a already highly stressed piece of the motor.

The stroker kit should not require any machining as it is a drop in if it is the one you are buying out of the cat.... Tru, you are adding 3mm to the stroke, but the piston s not coming out of the bore far enough to worry about tilting from being unsupported by the cylinder.

The whole idea is to get as much air fuel stuffed into the cylinder as possible and the stroker will help with that. You have already big bored it and done head work so the next logical place to go is stroke. Remember, there is no replacement for displacement....

Stroker motors have very little problem when done right and will be just as reliable.....to a point.

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grayracer513

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The stroker kit is not going to add unreliability.

Yes, it will. In fact, anything at all that increases the output of the engine puts more load on it.

The issue with increasing the stroke is that the piston speed is increased, which is its own reliability issue, and this increases the load on the rod and rod bearing at high RPM exponentially. Furthermore, if the rod is shortened to compensate for the added stroke, so that a cylinder spacer plate is not required, the rod angle is increased, which also loads the crank more.

Big bore kits add piston weight, again adding stress to the rod/bearing/crank.

High compression is perhaps one of the least intrinsically stressful modifications, adding only the stress of additional output as long as detonation is kept under control. Port work, and improvements to the intake and exhaust are completely harmless in and of themselves, adding no direct strain on anything other than to increase the amount of power transmitted through the rotating assemblies.

NOS systems and superchargers both fundamentally accomplish the same thing; a larger than normal volume of fuel and oxygen are burned by the engine, producing a larger than natural amount of stress.

The extent to which any of this actually makes the engine less durable depends on what the engine is designed to deal with. Any of it will increase wear if used to its full potential.

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mkporn

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Yes, it will. In fact, anything at all that increases the output of the engine puts more load on it.

The issue with increasing the stroke is that the piston speed is increased, which is its own reliability issue, and this increases the load on the rod and rod bearing at high RPM exponentially. Furthermore, if the rod is shortened to compensate for the added stroke, so that a cylinder spacer plate is not required, the rod angle is increased, which also loads the crank more.

Big bore kits add piston weight, again adding stress to the rod/bearing/crank.

High compression is perhaps one of the least intrinsically stressful modifications, adding only the stress of additional output as long as detonation is kept under control. Port work, and improvements to the intake and exhaust are completely harmless in and of themselves, adding no direct strain on anything other than to increase the amount of power transmitted through the rotating assemblies.

NOS systems and superchargers both fundamentally accomplish the same thing; a larger than normal volume of fuel and oxygen are burned by the engine, producing a larger than natural amount of stress.

The extent to which any of this actually makes the engine less durable depends on what the engine is designed to deal with. Any of it will increase wear if used to its full potential.

like i said, to a point. and in my "OPINION" a stroker kit will provide more reliability than a 14.5 comression ratio when in conjunction with NOS....The key is NOS.

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grayracer513

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That's a matter of detonation suppression, just as it is when using a supercharger. It's easier to understand with the supercharger example, but it's essentially the same issue. NOS and 'chargers both increase the base density of the fuel/air charge, raising the ultimate pre-ignition pressure levels. Superchargers obviously do this by compressing the intake air to 1 or 2 times atmospheric pressure before it enters the chamber.

NOS does not do this by the same method, though, so the problems are slightly different. NOS uses nitrous oxide to displace some of the intake air that would normal be aspirated. The idea is that air is 21% oxygen, while NO² is close to 50% oxygen by weight, so the oxygen available to burn fuel with is increased, even though the base charge density is no different. Additional fuel is added with the NOx to retain the stoichiometric balance, and the result is that more fuel is burned than normal. This creates higher preflame pressures during the combustion process, which, as you correctly point out, can result in loss of detonation control.

So, you are right, to some extent. To the degree that high compression ratios contribute to loss of combustion control, they should not be mixed with NOS systems without a great deal of caution.

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sandrappy

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If it was me i would do both. I have the hot rods +3mm stroker crank in my bike. If you go with it have it balanced with the piston you choose to run or it will vibrate your arms to sleep. once its balance it will run very smooth, but to answer your question. Higher compression piston will give you more power all threw out. If you go from 12:1 to 14:1 you will gain approximately 6% more power all threw out the power band i.e. you take the hp graph of your bike and add 6% to it everywhere. With a stroker crank you change the shape of the power band such that the power band is wider. The power will come on sooner and last longer.

Also remember when you go with that a stroker crank you will be adding comression. I actually chose to go with the 13:1 CP piston as it is a very nice piston with the stroker crank i am just shy of the 13.5:1 mark. I live a high elevation so i can get away with running this combo on pump gas.

If I were you i would go with a better piston as the Anthem pistons are usually not very good, and while your doing it throw in the stroker crank. Also a good set of cams will help. Web's +5's gain ya about 3 hp if i remember correctly.

Oh and if you want realiability keep it stock. if you dont like being the last to the top of the hill then mod it. There is nothing wrong with high compression or stroker cranks as they have proven to be reliable and last a long time, but they still won't last as long or wear as good as a stock motor will.

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mkporn

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I was answering his question minus the drival.....You forgot to mention the effect of N that effectively reduces the ambiant charge tempature which also increases the potential for power increases............nitrogen is an inert gas and provides no effect other than the reduction of charge temp.....but that is a whole other lesson in itself.

My point was that he asked a simple question and I gave him a simple answer to keep him out of trouble. Ideally you couuld forgo the piston and shave the head, change valve grinds, put nos on it, drop some dry ice in the tank, run alcohol or methane and change all kinds of other things and realy confuse the issue while making huge power and make the thing totally unreliable.

I appreciate you stepping in, it saves a whole load of typing on my part.....guess thats why you are the mod and the guy people come to for the for the facts.

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aggiemoto99

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man you guys are stuffed with knowledge. but let me get one thing straight. buying a hotrods +3mm stroker it comes with a shorter rod so no head machining or head shims are required? and the cams sandrappy was talking about are called "web's +5"?

That or a piston pin location higher in the piston than before, or a combination of shorter rod and higher pin position, I don't know I have not seen or built one.

I have heard good things about webcams.

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ThumperKid250F

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im building a ice bike out of my 06 yz 450f and i just got my crank back from falicon crankshafts. they did the supercrank mod, longrod stroker kit.. the supercrank mod is a blueprinted crank, its polished, fully welded and inspected.. they ask for your whole piston kit so they can balance it all..

give them a call they know there stuff.

http://www.faliconcranks.com/crank_svc.html

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