Milling the head for more compression?

[GregT]

Some old gasoline engines from the past had compression ratios down around 6:1. A point rise made a lot more than a 2-3% increase on those.

The 1st Ford Flathead was 5.5:1

They were made for the fuel available. About 70 octane - and uneven quality.

Power was "adequate" for the roads and vehicles of the time.

 

[wymple]

I think HP was around 55-60. Pretty lame now.

 

[bitzz]

I'm going to be "that guy"
From what I have read here I am guessing no one here has ever REALLY built a performance motor.

Let's start with the old maxim: There's no replacement for displacement.
That went out the window 30 years ago.
The replacement is RPM and compression. We can NOW build higher revving motors with higher compression and have them hold together.
Adding compression is a valid hop up BUT it only part of the job. Higher compression with the same amount of fuel is going to get negligible gains.
More compression + more fuel + more RPM = more HP
So NO, you're not going to see a 10% increase in power with a 10% increase in compression, without doing the rest.

IF you want to "deck" the head, you need to know a couple of things.
What your ACTUAL displacement is
What your ACTUAL head volume is

(Displacement + head volume) / head volume = compression ratio

What your piston to valve clearance is
Now you know what your ACTUAL compression ratio is.
Now figure out what you WANT for a compression ratio and do the math. That will tell you how much material you need to remove from the head surface.
Now subtract that number from the piston to valve clearance, so you know you won't be "tinging" valves.

Here is where some experience comes in.
How much piston/valve clearance do you need?
Usually... typically... 0.1" is safe, BUT that number is dependent on a lot of stuff: how elastic are your rods? how is the valve train? Good springs? Spring rate? How good is the bottom end?
On a Norton Commando, with stock rods, the closest I'll go is 0.11, but put Carillo rods in the mix and I can drop that to 0.080".
On my Yamaha two strokes I run 0.030 piston to head clearance.

You're not removing enough material to worry about the valve train. The valve timing (cam to cam) will remain the same, you're not changing the relationship between the cams, you're changing the relationship between the "cams" and the crank, that will be taken up by the cam chain tensioner, like it was designed to do.
... but if I was doing this I would move the cam lobe centers out a bit to take advantage of my new compression.

Smoky said we can go 11.5/1 on pump gas.

 

[Grimly]

From what I have read here I am guessing no one here has ever REALLY built a performance motor.

I think that's probably news to some.

 

[cowboyup3371]

I'm going to be "that guy"
From what I have read here I am guessing no one here has ever REALLY built a performance motor.
.

Have you met Rapidray?

 

[Steve]

I'm going to be "that guy"
From what I have read here I am guessing no one here has ever REALLY built a performance motor.

Hi, "that guy".

Your profile says you joined about 5 years ago, but you only have twenty-some posts, so it's safe to say you don't visit here very much or very often. We have SEVERAL on the forum that not only have built a "performance motor", they actually have businesses and build them as their main source of income.

Rapidray is in California and has worked with several racing teams, including Vance & Hines.
Stetracer is in the Midwest. Not sure if he has a business, but at least builds his own race engines.
GregT is in New Zealand and does quite well.

There are others, but I won't bother looking them all up. Be careful with your broad statements.

On my Yamaha two strokes I run 0.030 piston to head clearance.

OK, that's piston-to-head, but what about the VALVES???
Yeah, I know that two-strokes don't have overhead valves, so why bring in 2-strokes?

You're not removing enough material to worry about the valve train. The valve timing (cam to cam) will remain the same, you're not changing the relationship between the cams, you're changing the relationship between the "cams" and the crank, that will be taken up by the cam chain tensioner, like it was designed to do.

I'll accept the first part, but not the last. Probably not removing enough from the head to make much of a difference, and the cam-to-cam timing won't change, but unless you put a second tensioner on the FRONT part of the change, the cams WILL be slightly retarded because the length of the chain did not get shorter, like the distance from the cams to the crank.

Smoky said we can go 11.5/1 on pump gas.

Which "Smoky"? Smokey Yunick? I have no idea if that 11.5:1 concept was his, but even if it was, that was an understatement.
My wife's ride has a 12.0:1 compression ratio and not only runs on pump gas, it runs quite well on 87 octane pump gas. :-\\\

Please be careful with your claims and accusations. We're not a bunch of kids, we didn't just fall off the turnip truck, we DO know what we are talking about.

.

 

[Guest]

The 1st Ford Flathead was 5.5:1

Yup. And some of that was because engineers hadn't figured out how far they could push compression. The rest of it is because of the poor fuel quality and consistency. Before the first cars hit the road, gasoline was nothing more than a useless byproduct of making kerosene.

 

[wymple]

No replacement for displacement still has it's value. All the power adding tricks you can come up with also apply to a bigger volume. A 500 CI engine with the same tricks applied to a 350 CI engine the 500 will make more power. Yes, you can turbo, supercharge, cam & compress, nitrous, whatever, but you can also do those things to the bigger engine.

 

[Guest]

From what I have read here I am guessing no one here has ever REALLY built a performance motor.

Careful, bro. Some of us have a great deal of experience with building performance engines.

Let's start with the old maxim: There's no replacement for displacement.

This is actually true and always will be. Compression, good ports, cam timing, and RPM can be considered substitutes for displacement, but not replacements. Saying that compression and RPM are a replacement for displacement is like saying that handling is a replacement for horsepower or vice versa.

So NO, you're not going to see a 10% increase in power with a 10% increase in compression, without doing the rest.

You're saying the same thing that most here are saying. If we believed that 10% increase in CR = 10% increase in power, half of us would have 18:1 compression right now. I know I would.

you're not changing the relationship between the cams

True.

you're changing the relationship between the "cams" and the crank, that will be taken up by the cam chain tensioner, like it was designed to do.

Not true. The exhaust and intake cams will be retarded. The tensioner is on the slack side of the chain, so the cams are going to rotate backward relative to engine rotation when the head becomes shorter. With a normal mill job, the mount that the cams are retarded is only academic. When milling for compression, e.g. taking off .080" or .100" to make the chambers appreciably smaller, those cams are going backward and the amount is going to be noteworthy. The opposite is true (i.e. the cams advance) on engines with the tensioner on the draw side of the belt/chain.

 

[Guest]

I would lightly shave the head, and degree the cams to create either more low end or more top end which ever suits your needs. Clean up the ports, and do a multi angle valve job. Then the icing on the cake would be to make sure your AFR (jetting) is dialed in. A group of small things that enhance each other can become greater than the sum of their parts. Not a lot of $$. Also do a leak down test and see if the rings are sealing good, if not replace them when you open it up.

 

[bobgroger]

Why has no one mentioned a thinner base gasket? That is the cheapest and easiest way to move the piston closer to the head and increase compression. Measure the deck height and valve clearance first, of course. Base gaskets are available as thin as .005 for the 1100e

 

[GregT]

Why has no one mentioned a thinner base gasket? That is the cheapest and easiest way to move the piston closer to the head and increase compression. Measure the deck height and valve clearance first, of course. Base gaskets are available as thin as .005 for the 1100e

Because the OP is working with an 8 valve GS1000. The different thickness base gaskets are used on the 16 valve motors to set the squish clearance. Not applicable to the 8 valve engines as they don't have a squish band to speak of.

 

[Guest]

Why has no one mentioned a thinner base gasket? That is the cheapest and easiest way to move the piston closer to the head and increase compression. Measure the deck height and valve clearance first, of course. Base gaskets are available as thin as .005 for the 1100e

when you remove the base gasket you move the piston closer to the head. Which can be very bad if you don?t have the proper piston to head clearance. When you mill the head your not moving the pistons any closer to the head your making the combustion chamber smaller.
You base gasket thickness is determined by the piston to head clearance nothing else when building a preformance motor.
compression should only be increased by milling the head or using aftermarket pistons.

 

[rphillips]

That just don't sound right. If you remove the base gasket, you don't raise the piston at all, you lower the head toward the piston by lowering the cylinder block that has the head mounted to it, piston is at exact same place. Only way to raise the piston is stroker crankshaft, longer connecting rod, or taller piston. Either way the closer the piston gets to the head, the higher the compression will be. If I'm thinking right !

 

[Steve]

"Raise the piston" (by removing the base gasket) or "lower the head" (by milling it), you are reducing the combustion volume.

As he mentioned, though, milling the head only shrinks the volume of the combustion chamber, it does not reduce the piston-to-head clearance.

.

 

[Guest]

"Raise the piston" (by removing the base gasket) or "lower the head" (by milling it), you are reducing the combustion volume.

As he mentioned, though, milling the head only shrinks the volume of the combustion chamber, it does not reduce the piston-to-head clearance.

.

Good luck with that theory.

 

[rphillips]

Now I see, base gasket reduces deck height. Milling the head does not. Milling the head still does reduce the piston to valve clearance.

 

[Guest]

That just don't sound right. If you remove the base gasket, you don't raise the piston at all, you lower the head toward the piston by lowering the cylinder block that has the head mounted to it, piston is at exact same place. Only way to raise the piston is stroker crankshaft, longer connecting rod, or taller piston. Either way the closer the piston gets to the head, the higher the compression will be. If I'm thinking right !

I am using .010" as a number not that they make an .010" gasket. Imagine if you have a .010" base gasket, and you put the cylinders on. The piston will be at a certain height in relation to the cylinder deck if you measured it. Now if you remove the .010" gasket and reinstall the cylinders, they will go on .010" further down because the gasket isn't there. If you measure the piston height in relation to the deck of the cylinders the piston would be .010" higher in the cylinder with an .010" gasket removed. You are not changing the stroke, just relocating the TDC height of the piston in relation to the cylinder deck height. You are effectively raising the piston in the bore by reducing the base gasket thickness.

"Raise the piston" (by removing the base gasket) or "lower the head" (by milling it), you are reducing the combustion volume.

As he mentioned, though, milling the head only shrinks the volume of the combustion chamber, it does not reduce the piston-to-head clearance.

.

Milling the head or using a thinner base or head gasket will reduce the combustion volume by; reducing the piston to head clearance by the amount of metal removed from the head, or by the thickness removed from the base or head gasket. Removing thickness of the base gasket places the piston slightly higher in the bore by the amount removed from the base gasket, thus reducing the piston to cylinder head clearance at TDC. Milling the head or using a thinner head gasket places the roof of the combustion chamber closer to the piston by the amount shaved from the head, or by any reduction of thickness removed from the head gasket thus reducing the piston to cylinder head clearance at TDC. Any of these mods alter cam timing but not a great deal.

 

[Guest]

Now I see, base gasket reduces deck height. Milling the head does not. Milling the head still does reduce the piston to valve clearance.

unless you recut the valve deeper in the seats also.
I have a 1150 head with .105” milled off and the valves cut .040” deeper in the seats. This is on a street bike.

 

[Guest]

Alway make sure you have the proper clearances no matter how you build it.