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.