Compression vs. Cylinder Bore?

[BM@Ronayers]

Can anyone tell me which is going create more power? I know it’s a crazy and maybe stupid question but I am stumped. I know more compression is good, right? Ok so here is my dilemma. I am thinking about putting a 330 Namura Big Bore Kit on my 300ex. Now the kit that I am looking at comes with the a piston that has the stock compression ratio 11:1 I believe, at least that is what I was told by the seller. Now, would I be better off getting the Kit or just get maybe a 14:1 compression piston? For the last few years I have been running a Wiseco 11:1 75.50mm piston. Or should I get the big bore and just get a higher compression piston to go with it? Thanks for any help.

[rooster300ex]

There is a sayin there is no replacement for displacement. Stock 300ex compression isn't 11:1 its more like 8:9:1 or 9:8:1. I've always been told that u need head work before makin it a 330.

[BM@Ronayers]

Originally posted by rooster300ex
There is a sayin there is no replacement for displacement. Stock 300ex compression isn't 11:1 its more like 8:9:1 or 9:8:1. I've always been told that u need head work before makin it a 330.
Yeah that sounds about right as far as the compression is concerned. I already have head work so I am just trying to decide if it would be worth going to 80mm with a lower compression piston or 76mm with a higher compression.

[usp4u]

as said, head work would be the key with either option.
something to think here is, what kind of riding will you be doing ? what kind of maintenice schedule will you be keeping ? Is it something you want to live with ?

Allota peoplejump right into a hi comp piston the first time they do a top end job. While it may offer an increase in performance, there is a trade off. Higher comp. will result in a couple different things. Jumping from a stock 300 comp. ratio of probably 8.5-9.0:1 all the way up to something like 13.0-14.0:1 will give a whole different feel to the engine, not all of it good: If you ride trails or tight woods, the machine will be much easier to stall/harder to keep running, and require more clutch work. This is because the piston has more pressure to overcome to be driven back down or get past topdeadcenter.
With higher compression comes more heat, not a good byproduct on an aircooled machine.
Over 12.0:1 you can figure on running race fuel($7-$12/gal.)
More throttle/rpm is generally needed for take-off to avoid stall.
If it's a woods machine, personally, I'd go to a big-bore with a more reasonable comp ratio.

On the MX side of things, higher comp. makes a little more sense. Most of the time you'll be moving along fast enough to keep the engine cool. However, in tighter more techinical sections like whoops or rythm sections you can more likely to stall given the increased engine braking outlined above.
Engine braking is increased. Depending upon the rider this can be a big deal or a non-issue. Let off the throttle in corners can feel like hitting a wall as an increase in comp while increase the effect of the engine decelerating. Rear tires can slide, suspension can "pack-up". This reason alone is why you will see people spend upwards of $2k on a slipper clutch as it "ratchets through" the engine braking and allows a more free-wheeling feel, like a two-stroke.

If it were me and it was a trail bike...big bore with reasonable comp. ratio.
MX bike: higher comp. maybe a bigbore as well.

p.s.- with increased comp. comes increased bottom end wear. there are now higher forces acting on the cranck/rod/bearings. just keep it in mind.

[BM@Ronayers]

Thanks for the info usp4u! I usually spend most of my time in the woods on this quad. I may jump on a mx track every now and then but not enough to set it up for that. I think I have decided what I am going to do. I have a brand new head that just got a stage 2 port/polish from PPEngines. With that I will be running a Webcam (236 grind) with the heavy valve springs and hard rockers. I want to keep my low end power but I want to have steady power throughout. I spoke to someone at Webcam and the recommended the 236 grind for my riding style. I believe I am just going to bore to 76mm, which is the most I can do with without resleeving, and run a 11:1 or 11.5:1. I want to keep the reliability of the quad and do not want to run into heat issues. As far as intake and exhaust are concerned I will be running my 400ex carb with a moose foam filter and a curtis sparks exhaust. I know the thing will not be a beast but hey it is still a 300 :D .

[usp4u]

sounds like a good plan. in my opinion there is always more power to be found in headwork/airflow than anywhere else. besides, it comes without affecting relaibility.
Although I'm sure someone will be along shortly to say that I'm completly wrong.;)

[thomasgangross]

i am new to the 300ex myself, picked one up last year, all my friends have the bigger quads and all, everyones right about not throwing in thousands of dollars into any motor unless your trying to acomplish a goal

theres many different ways to look at this, the 300 is a good machine, obviously its out of date with the bigger quads these days

the person usp4u seems really educated on the 300, i like his comments

i have done nothing to mine except a exhaust system, i race local in NY, harescrambles, and never once had a DNF, but also finished 15/25

if the high compression wont help in this type of riding, i think usp4u is right about the head work, everyone i talk to says the 300 is really underpowered in that department

to me the best thing to do is find any information from people that used to race these 300ex's because it wasnt to long ago these were the four stroke machines people were racing

keep posting and keep the 300ex's alive



:devil: :devil: :devil:

[usp4u]

Originally posted by thomasgangross

the person usp4u seems really educated on the 300, i like his comments

I have never owned or wrenched on a 300. but the priciples are the same for any of them.


Dont tell anyone, but most of my time is spending tuning on small block Mopars.;)

[Punkmaster Flex]

Originally posted by BM@Ronayers
Yeah that sounds about right as far as the compression is concerned. I already have head work so I am just trying to decide if it would be worth going to 80mm with a lower compression piston or 76mm with a higher compression.

80mm with a lower compression will not have as much power but will probably have good torque. You can still run on pump gaz too.

With a 14:1 setup youll have to run C12 or other high octane gaz. Which is a lot more expensive than 91 octane! ;)

I know what Im talking about, riding a 17:1 CR125 at the moment. Trust me you dont wanna see my gaz bills. :)

[Punkmaster Flex]

On my 300EX I had a 76mm 11:1 CP piston with a +10mm crank. Basically it turned the 282cc engine to 343cc. I never really had problems with it, but I think its pushing the enveloppe a bit if you cant do it by yourself. Stroked cranks are $$$.

[DEVINF450R]

Try running VP U4.2 or MR-11 $20 and $35 a gallon. Should see MY gas bill :rolleyes:

but usp4u is right on the money with what my engine builder says and does to our MX bikes. Headwork is an artform with a purpose and while a flowbench helps, its just a guideline. So chances are you can find someone who can get more power outta your head.

the difference between my stock head and my "National" Titan motorworks head felt like the difference of hopping off a 400ex and getting on a 450r. The headwork was the only change, I already had a 13.75:1 CP full race piston and cams

[usp4u]

it may sound like a cliche, but...all you doing is trying to build the most efficient air pump possible. in with the good air, out with the bad air.

[DEVINF450R]

Exactly... which means not lnly does the amount of air in have to be good, but it has to be at the most optimum angle flowing in, to flow out, AND burn the most fuel at the same time. Like I said, an art :macho