Milling head for higher compression?

[Suzukfan78]

Im just curious about this and am definitley not going to tear into my engine untill it loses compression, or has other internal problems hopfully far in the future. I see a lot of bikes around here that have gone with over bore kits that will also raise your compression ratio due to piston geometry. Would it be much cheaper to gain some significant HP by simply milling down your cylinder head to acheive a higher compression ratio and changing your timing curve/running premium gas only to deter any detonation issues? Also how much HP do you think is safe for these bikes untill you would strongly reccomend the crank to be welded?

[rosco15]

The other benefit to aftermarket pistons is more displacement as well as the higher compression. With the different modifications I've done the bigger pistons have given me the most bang for the buck.

[RacingJake]

Milling the head can raise the compression but you may run into valve to pistons clearance caused by the valve's are now closer to begin with. This means you'll have to retard the intake cam to gain more clearance resulting in any gain by milling the head has just been lost to get the V-P clearance back.

I think it would be better to leave the head alone if it's in good shape and work on exhaust, carbs and gearing. If it's a street bike I wouldn't worry about the crank too much.

[Steve]

Actually, the process of milling the head will retard both cams. :shock:

Since the cams will be a few thousandths closer to the crank, and the cam chain did not shrink, both cams will be slightly retarded. I don't have any idea how many degrees per thousandth taken off the head, but it could add up. I would think that slotted sprockets and some very careful measuring would be necessary to restore the cams to top dead center, then check for possible valve/piston interference issues.

It is good that you are not looking to do this just because you can ("am definitley not going to tear into my engine untill it loses compression"). If you are looking into seriously increasing the horsepower, get a bigger bike. If you already have the biggest bike, then a big-bore kit will be necessary. I realize this takes all the fun out of things like a 7/11 kit, but I am trying to keep it practical here. 8-[


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[RacingJake]

You got to watch out when retarding the exhaust cam cause thats a good way to smack the piston with the exhaust valves. The exhaust valve may still open to much when the piston cross's TDC on the exhaust stroke. To much advance on the intake and to much retard on the exhaust can cause a few problems.

[Suzukfan78]

Just curious...

I was just curious becuase you can do this to small block chevys with no clearence issues and gain HP. Im not going to change anything internal to the engine untill somthing breaks. When my engine does finally wear out or is pushed to hard I will go with new pistons, welded crank...the works. I know I could just buy a busa for cheaper, but its a lot of fun to tinker with these old bikes and make them faster than people expect. I owned a busa for a year; and I miss it a lot. I WILL get the new one in a couple years (even if kids come a long in the meantime :-D). Unfortunatley Im waiting for my driving record too cool down.:? Even when I had that amazing machine, the old GS would still make me grin ear to ear. I read that a lot "dont worry about making your GS faster...just buy a newer bike"; Yes, getting these bikes to perform is expensive and No, they will never be like a GSXR1000, but hell; its worth every penny to change and upgrade your bike into what you want. More precisley; its worth every penny if you plan on keeping it. If I were to sell now I would lose a couple grand for what I put into it. For me approx. 25% of the joy of owning a motorcycle is changing it too suit your personal taste. The GS is a great, timeless bike and I will never sell it.

[first timer]

can anyone explain squish band (i think that is what it is called) the clearance between the head and the piston top. Over on my SV forum they seem to talk alot about the importance of that, and over here it is never mentioned. If you milled the head wouldn't you affect this squish band thus effecting things good or bad?

[txironhead]

If you are looking into seriously increasing the horsepower, get a bigger bike. If you already have the biggest bike, then a big-bore kit will be necessary.

You could always upgrade to a Boss Hoss......\\/......or build your own Y2K bike......

[Steve]

you can do this to small block chevys with no clearence issues and gain HP.

You can do this with small block Chevys because the relationship of the crank and cam do not change when you mill the head. Yes, the head moves a bit closer, but only the valves are affected. Most of these engines use hydraulic adjusters, so even that is not a problem. In extreme cases where a LOT was milled off, you might have to use shorter rocker arms.

We do not have that option with our overhead-cam engines. 8-[

Most of your stock Chevy engines are also relatively low-compression engines with lots of room between the valves and pistons. You can raise the compresion a bit by milling, and still not get the engine into the 'radical' category. The two manuals I have here at the house show the compression ratios for an 850 as 8.8:1 and a 1000 as 9.2:1. The 850 could probably stand some increase, but the 1000 is already approaching the point where it might need a higher grade of fuel. I couldn't tell you what the clearance is between the valves and pistons, but I can guarantee it's not much. :shock:


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[drhach]

I think the other reason that it works so easily on a small block Chevy is that they take the same amount from the whole surface of the head. In other words, they square it up in the fixture and evenly mill it. In our case, I believe that the heads are typically angle milled. That's where the cam timing issues start. As far as I know, the tolerances on most Japanese fours are already so tight that you can't just lop of a consistent amount of metal across the whole head like you can on a Chevy. Our bikes already have way higher CRs and there isn't much left to play with. So they angle mill the heads.

[Suzukfan78]

Thanks...

Great explanations, Thanks guys.:-D

[Big Jay]

I think the other reason that it works so easily on a small block Chevy is that they take the same amount from the whole surface of the head. In other words, they square it up in the fixture and evenly mill it. In our case, I believe that the heads are typically angle milled. That's where the cam timing issues start. As far as I know, the tolerances on most Japanese fours are already so tight that you can't just lop of a consistent amount of metal across the whole head like you can on a Chevy. Our bikes already have way higher CRs and there isn't much left to play with. So they angle mill the heads.

When we normally surface these heads, it is done evenly across the entire head. This lowers the head ( and cams ) in relation to the crankshaft and thus retards the cam timing. You can take quite a lot off of these older heads. The limit factor is the intake seat.

Now when doing a full max compression cut, you angle mill it. As stated above, the limiting factor on how far you can go is the intake seat. It is larger than the exhaust seat, so it sticks up higher.

When angle milling, the cutter is cocked over so it cuts more from the exhaust side than the intake. This way you can cut right down to both seats at the same time. This makes the combustion chamber as small as it is possible to do.

Jay

[RacingJake]

can anyone explain squish band (i think that is what it is called) the clearance between the head and the piston top. Over on my SV forum they seem to talk alot about the importance of that, and over here it is never mentioned. If you milled the head wouldn't you affect this squish band thus effecting things good or bad?

It could pose a problem if your squish was say around .020 near the edge of the pistons and you milled the head .020 and were using raised pistons. That's why it's always good to clay the pistons and degree the cams checking the clearance's when you go past stock. You should of seen me working on my 1166cc 13.5-1 piston's when I stuff them into a 750 22cc head.

Squish helps with increased turbulence to increase power and also helps reduce detonation issue's when set right. Normaly it's the outer band area around the piston and the chamber that forces the air/fuel into a smaller area of the combustion chamber when the piston reaches TDC.

Here's my next set of heads

[first timer]

so basically when getting your head done, you need to make sure the guy doing it knows how squish band works and you will need to bring him the whole motor so he can clay it up to check clearances. I would love to do all this but i need to watch someone do it once first.

[RacingJake]

If your running flat tops and mill the head you should'nt run into any problems if you don't go hog wild. But if you change to high compression pistons or head swap I'd clay the pistons. You also have to watch out for the car machine guys cause they can remove alot of material.