Milling the head for more compression?

[Don Pearsall]

What is the head milling limit for a stock GS1000L? I do need to do a valve job on mine and thought I could increase power a bit with a light milling for more compression ratio. I tried the search function but did not come up with much.


Thanks!
Don

[Nessism]

You can mill a fair bit but I don't think it will add much extra power.

BTW, if you use "advanced search" and toggle titles only, searching "milling" you will find several pertinent threads.

[GregT]

It's not really cost effective. Bigger bore with higher compression pistons is the best bang for the buck.

To go from 8 point something to 10.5 : 1 by milling raises questions of valve to piston clearance - and requires dialling in the cams.

[wymple]

You will have to explain to me about the cost effective comment. He will have the head off anyway, and boring + new pistons, rings has to be far more money. Depending on how much he cuts off, new pistons & such for clearance are optional? Smokey Yunick claimed power is directly related to compression, and a 10% increase generally gives a 10% increase in power. Does this not apply? He would need less than 10:1.

[Steve]

It was also mentioned that milling the head reduces piston-to-valve clearances, as well as changing cam timing.

I have not run the numbers to see how much needs to be milled off the head to sufficiently raise the compression, but I have a feeling that piston clearance and timing are definitely going to be affected.

Also, by the time you mill the head enough to raise compression, you will have a bit more slack to take up in the timing chain, requiring more changes there.

It might cost a few bucks more to bore the cylinders and install larger pistons, but I think there will be fewer details to look after in the long run. THAT is what might make it more "cost efficient".
.

[Buffalo Bill]

Cost will include expert labor: calculating correct amount to shave, measuring valve clearance before & after, degreeing the cams, measuring spring wear to determine replacing all or some springs.
You should do your own cost analysis, just be sure to cover all the details.
GregT is a long time race engine builder, useful advice there.

[salty_monk]

I guess the relevance of costs depends on how much of it you can do yourself, how much you value your time & how much you need to farm out...

In the situation where someone else is doing the work for you I'm 100% certain new pistons & rings would be far cheaper.

[GregT]

You will have to explain to me about the cost effective comment. He will have the head off anyway, and boring + new pistons, rings has to be far more money. Depending on how much he cuts off, new pistons & such for clearance are optional? Smokey Yunick claimed power is directly related to compression, and a 10% increase generally gives a 10% increase in power. Does this not apply? He would need less than 10:1.

Do you know someone who can deepen valve cutaways on pistons ? I do - but I'm using them regularly. To recut a set of four pistons - 2V - is dearer than a bore job.
It's more cost effective to start with fresh high com pistons - with new rings - than modify what you've got. An uncut head means no cam movement or camchain changes are needed.

I'm an admirer of Yunick - but what is easy on an American V8 ain't necessarily so on an OHC bike engine.
The head milling is a very small part of the collateral changes required.

[wymple]

Good enough for me. My only experience milling heads is automotive, no cam chain issues and more room for play, I guess.

" Do you know someone who can deepen valve cutaways on pistons"

Actually, I do. My sons are machinists and play with this stuff on their pulling tractors. I don't pay much attention to the detail they deal with. I know they get around 14:1 on their alcohol

[GregT]

Good enough for me. My only experience milling heads is automotive, no cam chain issues and more room for play, I guess.

" Do you know someone who can deepen valve cutaways on pistons"

Actually, I do. My sons are machinists and play with this stuff on their pulling tractors. I don't pay much attention to the detail they deal with. I know they get around 14:1 on their alcohol

When you're talking to them, tell them my machinist has programmes for his CNC mill for 4 valve and American auto parallel 2 valve heads. But for 2 valve Hemi like the GS it's use his older manual mill as he doesn't see enough of that layout to make it worthwhile to set up a programme. So it's major setup time and watch every step. Do they buy forged blanks and finish them ? That's what I used to do back when Arias and others would sell unfinished pistons overseas.

[Guest]

Smokey Yunick claimed power is directly related to compression, and a 10% increase generally gives a 10% increase in power. Does this not apply?

Power is directly related to a lot of things--where PCP happens, cam design, intake and exhaust port flow, header design, ignition advance, fueling, etc. All of those things have direct effects on power, but I would not expect a 1:1 linear relationship between CR and HP, all other things being equal.

As mentioned before, milling the head enough to have a noticeable effect on CR is going to retard the timing of both cams, which will adversely affect MPG and will push the power higher in the rev range. With only CR being changed, I would expect retarding the cams to be a subtraction from peak power rather than an addition. If I am correct, the loss from retarding the cams will take some part away from the gain made by higher compression. To get your cam timing back to where it should be, you will need adjustable cam chainwheels and the equipment and expertise to degree the cam based on valve lift. By the time the dust settles on all of those costs, you probably could have paid for overbore pistons with the CR you desire.

[wymple]

I did a quick google search & it seems most performance sites claim that compression bumps and nothing else increase HP by 2-3% per point of increase.

[Guest]

I did a quick google search & it seems most performance sites claim that compression bumps and nothing else increase HP by 2-3% per point of increase.

That seems about right, with a point being one whole number (from 9:1 to 10:1.) So, using my bike's numbers, just because I know them, a 10% increase in compression (from 8.8:1 to 9.68:1) by itself will give me about a 2.8HP increase, taking me from 93 to 95.8HP. I'll skip that and just eat a light lunch.

[Steve]

I understood "a point" to be 0.1.

To go from 9:1 to 10:1 is ten points, meaning about 20-30% more.

I might be wrong.

.

[RichDesmond]

I understood "a point" to be 0.1.

To go from 9:1 to 10:1 is ten points, meaning about 20-30% more.

I might be wrong.

.

Yep, you're wrong.

Given the ratios, I'm pretty sure "a point" is a full number. ~10% increase in compression -> 2-3% increase in power. Sounds about right.