big bore/combustion chamber

[RifRafRacing]

How big of a bore can you run before you have to machine the head to match the larger bore size?It looks like even with the 78mm pistons Im using,you have quite a mismatch between the piston diameter and the smaller combustion area of the head.

[Big T]

I don't have an exact answer, but I remember my machinist chamfered the edges of my combustion chambers when I went to the 1085 kit in my GS1000.

He said something about hot spots and compression ratios.

[drhermanstein]

How big of a bore can you run before you have to machine the head to match the larger bore size?It looks like even with the 78mm pistons Im using,you have quite a mismatch between the piston diameter and the smaller combustion area of the head.

Good point. I'm going to check with my local crazy hp bike engine builder

[Todd Horton]

If the big bore pistons are made for that particular model they should fit. Even with the large dome pistons they are made so that the outer edge has a flat area for where the combustion chamber isn't big enough. If it were me, I wouldn't chamfer this area as this gives a squish effect to the combustion chamber ,actually helping the combustion process and also allows the edges of the piston to remain cooler. This in turn helps to deter detonation. TH

[rosco15]

What Todd said. I have friends that run 1428's with no mods to the head.

[lecroy]

Agree with Todd and Ryan. I am running 83mm with a stock head but my pistons are a dish shape. Friends of mine have several 1500+cc with much higher compression. Lots of head work, but they did not change the shape of the combustion chamber. Just some massive valves, hard welded rockers and some very big ports.

[drhermanstein]

How big of a bore can you run before you have to machine the head to match the larger bore size?It looks like even with the 78mm pistons Im using,you have quite a mismatch between the piston diameter and the smaller combustion area of the head.

Local Suzuki Drag freak says unless you're going stupid large, leave well enough alone. If you are going big then a little scuff to take away sharp edges.

Herman

[jrp]

This is something I have wondered about, since I am considering an increase in bore.

If I understand the design correctly, Suzuki's TSCC uses the squish areas that are machined into the head to accelerate the speed of the swirls which helps complete combustion more quickly.

When you increase bore, you are increasing the squish areas. It is not clear to me how this changes the dynamics related to the speed of the swirls. Does it speed it up? Slow it down? No change at all? I assume that this could only be answered by correct modeling.


Jeff

[RifRafRacing]

yeah,Its just one of those things that crossed my mind.how big can you go with the intake and exhaust valve size without major head sugery?Im ready to assemble the bottem end,so Im starting to think ahead about the topend.Ive got a mildly ported head with g4 cams and good springs.It needs freshening and I figure increase the valve size while Im at it....(its getting hard to hide the money shelling from her that goes with these builds).

[rosco15]

I've been working lots of side jobs lately and selling some parts I don't need. Time to take mine apart as well.

[Deesel]

I don't have an exact answer, but I remember my machinist chamfered the edges of my combustion chambers when I went to the 1085 kit in my GS1000.

He said something about hot spots and compression ratios.

Is always a good idea to make sure there are no sharp edges in your combustion chamber- which can get heated up quickly, cause hotspots which will cause preignition and detonation.

[Todd Horton]

As the squish area increases so does the turbulance in the combustion chamber. This accelerates the burn to help ignite the mixture. Works on any engine two stroke, or four. When the squish is very tight and or large it may cause you to actually retard the ignition timing to prevent detonation. Any time you have to retard timing you are headed in the right direction. :-D TH

[Big Jay]

Most of the off the shelf big piston kits are made to work with the stock combustion chamber. However, the custom pro stock type pistons have the sides of the domes clear out to the edge of the piston so that there is no squish band area. These require the combustion chambers to be machined to match.

Jay

[RifRafRacing]

Suzuki GS topend

thanks Jay,this has probably been addressed on here before,but I came up empty searching old threads.Im thinking of running a dry block with a topend oiler.Is this a good idea for a street/strip application?Do you simply plug the holes in the upper case?

[rosco15]

I'm running my bike like that now. I don't ride mine on the street. Tap the holes in the top of the cases and install plugs. Tap the four holes in the bottom of the head and install plugs there as well. With the top end oiler kit install the distribution block onto the top of your cases without the blockoff sleeve.