Milling the head for more compression?

[Big Jay]

Cutting valve pockets deeper in stock pistons is risky as they don't have much room in the top to do that. Also keep in mind that milling the head a bunch not only brings the valves closer to the piston, but also moves them out from centerline.

[Steve]

I might be wrong.

Yep, you're wrong.

It's not the first time, certainly won't be the last.

.

[wymple]

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.

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.

[RichDesmond]

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.

I'm not quite old enough to have worked on Model A's, so I'll take your word for it.

[wymple]

The 1st Ford Flathead was 5.5:1

[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.
.

[Longitudinal]

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.

[Longitudinal]

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.

[metalfab]

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.