Does the 54mm piston raise the compression?

[turbowrenchhead]

Spec in the book says stock compression is 7:5 to 1. Dr atv's big bore piston says 9:1. I emailed dratv and asked. He stated that compression stays the same. Anyone know more details?

[jjsuthe]

Bore only changes the CC's To raise compression you would need some sort of domed piston or shave the head. At least that is how I understand it.

[turbowrenchhead]

From the pic on DRatv's site the piston has a dome.

[JohnR.]

Any time you bore an engine you raise the compression ratio. Even if the new piston is identical to the old one except for its O.D., the compression will go up because you have increased the size of the cylinder without increasing the size of the combustion chamber.

[AutoXer]

9:1 is higher than 7:1 the slight bore shouldn't make much difference

[JohnR.]

Since he didn't say what machine or even what the original bore size was but he did say it was a big bore piston I figured he was going to a substantially larger piston. Even with a much larger piston it probably wouldn't make that much of a difference. I saw that jjsuthe's post had some incorrect info so I wanted to correct it so misinformation doesn't get spread around. :)

[jensenracing77]

Any time you bore an engine you raise the compression ratio. Even if the new piston is identical to the old one except for its O.D., the compression will go up because you have increased the size of the cylinder without increasing the size of the combustion chamber.

i second this info.

[turbowrenchhead]

I have a ATC 90. The standard bore is 50mm. The big bore DrATV sells is 54mm and he advertises it at 9:1 compression.

My neighbor and I recently purchases this trike to pull an ice shanty. It will pull it but we would like to get more power out of it. I orginally though I would have the head shave to raise compression to make more HP. I started asking questions and ppl pointed me in the direction of changing the piston. Powroll.com sells a 10.5:1 piston. I thought that would be the way to go because I can get 110 octane locally and I can mix it with 93 to get a little higher octane. I'd rather have the power and deal with higher octane fuel. How ever the kit is $235 plus shipping. DrATV's is around $50 bucks. If I can I'd rather go with the cheaper one seeing that I paid $145 for the trike. I'm trying to keep this low budget.

[turbowrenchhead]

If the pistons are the same, lets say a flat head design and you put a bigger piston in. It won't change the compression ratio of any significance. If anything I would expect the compression ratio to drop a bit due to more surface area added.

[NINJA]

Okay, think of the engine as an air pump. If you increase the diameter of the piston in the pump, it can compress more air with each stroke. That is why the compression ratio goes up. Also, on a four-stroke engine, you do not shave the head to increase compression. The reason is so that your valves will not come into contact with your piston at TDC. Also it would cause excess slack in your cam chain, that your tensioner may not be able to compensate for. On a four-stroke, you put in a higher domed piston to raise compression. On a two-stroke you shave the head to raise compression.

[turbowrenchhead]

What about adding material at the top of the combusion chamber to raise compression? 7:5:1 is weak.

[NINJA]

Bad idea, your valves will contact it, your flame kernel will be disrupted, and your flow characteristics will be altered negatively.

[turbowrenchhead]

So my only choice is the powroll piston if I want to raise the compression?

[greenhuman]

The 54 big bore kit you need on the Dr ATV site is in the CT90 section. This kit will increase the cc's AND increase compression and increase HP.

[turbowrenchhead]

It looks like the same piston as the ATC 90.

[AutoXer]

have you considered a 140cc lifan motor? or a 125 for that matter, or maybe a nice atc200?

[turbowrenchhead]

Depends on what it costs to get one. Do they bolt in without fabrication?

[jjsuthe]

I do see this logic. Thanks for ironing that out!


Any time you bore an engine you raise the compression ratio. Even if the new piston is identical to the old one except for its O.D., the compression will go up because you have increased the size of the cylinder without increasing the size of the combustion chamber.

[turbowrenchhead]

Their is static compression and mechanical compression. If you have a turbo on your engine you can raise the static compression. Only mechanical changes to an engine can change mechanical compression. If you put in a larger piston that is the same style you you do not change mechanical compression.

[JohnR.]

That is absolutely false. Take a minute and do the math to figure out compression of any engine then increase the piston diameter while leaving everything else in the formula the same and you will see an increase in compression. Here's a link to the calculator if you don't believe me you can try it yourself:

Keith Black Compression Calculator (http://www.kb-silvolite.com/calc.php?action=comp)

Enter all the numbers for a real engine (I used a 350 Chevy with a 75cc head and a .030" gasket with a 4" bore) then add .030" or .060" inches to the bore and the compression will go up. Now try adding .160" (abot 4mm) to the figure and watch the compression jump up. Of course camshaft selection, cylinder head design, valve size, carb size and more will have an affect on the actual psi reading you will see on a gauge but that's a totally different topic.

John

[turbowrenchhead]

I should of said that you don't change it to any significance if that is the only thing you change. I can't see the compression being raised from 7:5:1 to 9:1 if the piston is the same style/ same dome.
Per what DrAtv advertises.

[Dirtcrasher]

If you want some more GUSTO, do the air box/exhaust/carb/mild porting stuff and that will add plenty of power over stock.

Or, buy a 350X and or 250R if you want some more JUICE! - EVEN in stock form' There's plenty of power there....

Tons of mods can be done to the simplest of motors.............

G-luck
DC

[JohnR.]

If it were a .5mm overbore I'd agree but 4mm is quite a bit and will add a point to the compression I'd bet. I don't have time to figure it out right now but maybe I will tomorrow.

[turbowrenchhead]

That would be cool, thanks. Also after looking at pics of the stock piston and pics of DrATV's piston. I think there is a little more dome there. I have decided that I'm going to purchase the piston when he gets his cams in. No sense in paying for shipping twice.

If I could find a bike for the little amount of money that we have got into the atc 90 I would get a bigger one. Believe me I looked around for a while before I found the deal that I got on this one. My neighbor and I liked this one because it fits in his suburban with the shanty on the roof. I doubt a 250 or a 350 would.

[JohnR.]

I would guess that the biggest you could fit in the Suburban would be a 125M. I was able to squeeze my 350X in my old 'burban years ago but the trike was lowered way down for the ice and even then it just fit with the bars laid down.

John