I am working on getting some numbers crunched and here is where I am at.

Someone suggested I use this on-line calculator http://www.wallaceracing.com/cr_test2.php
to put some numbers into which I have done and read as such

Number Of Cylinders: 6

Bore:3.40

Stroke:4.12

Head Chamber (cc's):From what I have read slants are in the 54 to 62 CC range. I purchased new oversized valves, I may use them in this head not sure yet. That is going to affect the CCs so I am going to enter a figure of 54 CCs here so that I have a starting point to figure out how much I would have to mill off the head just to get to 9.1 ratio I am after.
I will do the actual measuring tomm hopefully to see where I am truly at.

Valve Relief / Dome (cc's):I have been told to enter 0 here cause my pistons are flat top standard

Gasket Thickness: Not sure on this, I have been told a fel-pro compressed reads .037 thick. Can someone verify this. I plan to use a Fel-Pro when assembling this engine as it is only a starter/tuner engine

Gasket Bore Diameter:I will be using a stock standard Fel-Pro head gasket so I am entering 3.40 here

Deck Height: My deck height at this point is .155

Enter this info in and I get

Your displacement is 224.44 CID
The Compression Ratio is: 8.42

Can someone help me with the few questions above and show me the formula that I would need ( and how to use it ) to see how much I would have to remove from the head and how that figure will change into CCs to get to my 9.1 ratio

If I use a head chamber CC of 48 I end up with this....

Your displacement is 224.44 CID
The Compression Ratio is: 9.01

which is about where I am thinking I would like to be but I do not know how to figure how that calculates into how much has to be removed from the head to get there.

Hope this makes sense.

You will need to enter 3.500" for the bore of the head gasket and not the cylinder bore size. But with these numbers, your compression is 7.74:1. And this is calculating 62cc for the head volume. It's been posted on many sites that an average of .006" of material removed equals 1cc reduction. But this is only to be considered a baseline, and your results may vary slightly.

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There's no such thing as too much cam....only not enough engine!

Thanks, so 3.500 is the cut-out size for a standard head gasket?

Can you ( or someone ) confirm my train of though then on this.

Lets say I have a 54 CC head which with all the other numbers plugged in as above ( with the exception of the change in 3.500 head gasket ) would give me a 8.39 compression ratio.

Figuring in 6 thous. approx per CC would mean that I would need to remove 36 thous. from the head to yield a 48 CC head chamber in which will put me at 8.97 compression ratio. ( close enough to the 9.1 Id like to be )

Does this sound like I am figuring it right?

Theoretically. But remember that the .006" per cc rule is just an approximation, and the machine shop should confirm this as they proceed with surfacing.

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There's no such thing as too much cam....only not enough engine!

The .0066 per cc rule is spot on as long as the head being surfaced is above the 44-46cc chamber measurement...once it dips in that range the cut starts into the shelf and the measurement is no longer close...
With the head casting variances, it's typically safe on builds under 10:1 SCR to be able to "round" your cut up to the nearest whole as it will not change things much (i.e if you are wanting a 10cc reduction on an untouched head you'd take .066...most machine shops will charge you by the depth, so .070 will not cost any more than the .066 and still get you what you want...and cover a lack of target compression if the head gasket doesn't truly compress to .039...).

Either way, the machine shop should check to make sure they got you "on target".

At 8.9 or less you're not talking big gains, once over 9 with the appropriate prep and cam the engine completely changes is demeanor (and better when you use adders like bigger exhaust and carb).

So just to clarify are you stating the machine shop should be cutting some then doing the CC process themselves and re-evaluate the situation?

Ok thanks, Ill need to absorb some of this but whats your feeling on maximum compression without having to use premium fuel?

It's all in the build, and your cam choice and phasing...and what you want out of it.

I have daily driven a 10:1 engine on 87...I actually did the engine break in and first 8 drag race runs on my last race engine at 11.7:1 on 92 premium before I bumped the timing and ran 110 (probably only needed 100)...

Short duration stock/RV cams will not take a large increase in compression as well and you will be limited to anything below 9.5:1...start thinking of using a 270 duration cam and 10-11:1 becomes no problem....12:1 is race gas range, but if the cam is large enough the bleed down at overlap can put the DCR back into the pump gas range.

2 cents...

Ok, so where does a guy start, what are the stages.

I think you prob. remember what I am going for and its use but just in case.

1986 225 slant six, 4 spped truck trans with a granny gear, 3.55 gear ratios, six thousand + pound 4 wheel drive truck, big tires ( 33 and maybe eventually 35s)

Id like the best fuel economy and would like to build it for optimum efficiency. Not going for a sound and certainly do not have any dreams of spinning the tires. Speed is not important, Id just like to know that the little engine is doing all it can to do all it can if you get my meaning.

So far I have a lower end re-built using new .040 over pistons, stock rings. This I do not want to change, I do nt want to get back into the bottom end.

Head is off, I have purchased the engine builder valves,, the 340 springs, a rollermaster double timing chain, Super six set-up. Electronic ignition, I can use the Super six dist or any variation you feel would work. Everything else is open.

No chosen cam, can do about anything I wish with the head.

What or where do you suggest as a starting point, if I were your customer and you were building the engine to suit my desires what would you do?

At this point all I am doing is collecting info and trying to make a plan for the best build but of course I lack the experience.

My research (read that reading what Josh Skinner, DusterIdiot, SlantSixDan, CharlieS, and other have posted) suggests that the max DYNAMIC compression ratio that can be used with regular (87 octane) pump gas is aout 8.2:1.

Now, achieving that dynamic compression ratio is where you must balance cam profile, cam phasing, and combustion chamber volume. The same cam in the same combustion chamber advanced or retarded will create a different dynamic compresison ratio. The same cam at the same phasing but in a larger or smaller combustion chamber will create different dynamic compression ratios. The same combusiton chamber and cam phasing done with a differnt cam will also chage the dynamic compression ratio.

This is why you must do all the measuring, math, and cam selection prior to having the head or block milled to achieve a specific dyanamic compression ratio.

Too be quite honest I am doing the best I can to learn what all of these things mean, I could easily spend a long long time trying to articulate every facet of the internal combustion engine so that I make the absolute best choices on my build.

I appreciate every bit of help I get from everyone on these forums, I am getting closer every day but I am still a long ways away.

Ill just keep asking questions and keep poking along.

I will work now then on selecting a cam if you guys can agree that this is where I need to start.

I contacted Orgeon cams, I attempted to have him help me to choose the absolute best grind for my engine and what I wanted to do with it.

He was very nice to talk with but he had very little interest in helping me to choose what was best for what I was after, he said he did not do the internet after I asked him to e-mail me some suggested cam specs ( for you guys to look over ) instead he made suggestions on cam blanks he felt might work good for me and gave me the sigh when I asked him to re-peat himself on the numbers he was quoting. ( so I could write them down )

I wrote them down, came here and asked what you guys thought, no replies so I figured no-one knew what to make of the numbers.

Again very pleasant otherwise and maybe he was just having a tiring day.

Might was the key word.

I will contact some other cam manufacturers and see if I can find a tech guy interested in the slant engine enough to spend some time with me.

Stupid question but is there any sort of on-line computer deal that I can enter a bunch of info into and have it tell me whats best.

I have seen some on-line cam helpers, they ask the same questions such as comression ratio, valvetrain geometry, combustion chamber CCs ect ect.

I feel like I am chasing my tail

I had to do a search but on March 23 of this year I contacted a guy at Erson named Russ. This was his reply..........We custom grind slant six cams regularly. We can make just what you need. Because I'm not a salesman, you'll need to speak with Eric Bolander. He's a great technical consultant, so custom cams are his specialty. Give him a call at 800 641 7920 ext 8003. He's on West Coast time, so his phone is off until 11 AM Eastern time. If you have any trouble at all contacting him, let me know. I'll get you in touch.
Thanks
Russ


I never did follow thru with this but I will do so starting tomm. Thanks for any future help with this.

I also contacted Comp Cams, have some e-mails, filled out the required info they asked for the best I could, they asked vehicle weight, intended purpose, gear ratios and all the minute specifics of the engine.

I did fill in the best I could with what I knew and could find on-line. I was contacted by one of their sales reps....Josh sumpthin I think. He had ZERO interest in helping me to find the BEST cam for what I was after. He only tried to sell me what was avail off the shelf.

Discouraging

"BEST" is a pretty subjective adjective. "Best" will mean different things to different people and will depend entirely on the intended use.

If I remember correctly, you are building this engine for a heavy truck in street daily driving. You will want a camshaft (and engine build package) that will maximize torque in the under-3500 RPM range.

A few years back there was some heavy discussion on the board between the cam profile gurus and what developed out of it was some custom designed cam profiles for slant sixes. For low RPM torque builds, the profiles that were recognized as the "best" were the Erson RV-10 and RV-15 profiles. Do a board search for RV10 and RV15. Alternatively, search for "Erson Group Buy" to find the thread.

As far as the terminology I used, I will try and explain it here.

"Compression ratio" Usually when someone talking about engines refers to the compression ratio of an engine, they are referring to what is more accurately called the "static compression ratio." This number is a ratio between the maximum cylinder volume when the piston is at BDC vs. the compressed volume of the combustion chamber with the piston at TDC. In other words, assume you had a 100cc maximum cylinder volume that was squished by the piston into a 10 CC compression chamber. That would be 100:10, or 10:1 after fraction reduction.

However, this number is only moderately useful because no internal combustion engine has the intake valve fully closed at BDC. All engines have the intake valve close slightly after BDC so some of the gases that would otherwise be compressed escape back into the intake tract. This leads to:

"Dynamic Compression Ratio" he dynamic compression ratio is the compression ratio the engine will actually "se" while running. Dynamic compression ratio takes into account the fact that the intake valve hangs open a while once the piston begins to move back up the cylinder from BDC. Dynamic compression ratio is controlled by camshaft profile and camshaft phasing since those two factors will determine when in relation to the piston movement the intake valve closes.

So what are "camshaft profile" and "camshaft phasing" you may ask?

"Camshaft Profile" This is the actual shape of the lobe on the camshaft. In a slant six the lifters ride directly on the camshaft and transfer the bump on the camshaft up through the pushrod, into the rocker arm, and then down into the valve stem where the motion is translated into valve lift. The profile of the cam tels you how tall the bump on the cam is (and therefore how much the valve will open once the rocker arm ratio is factored in), how long the intake valve will remain open after a certain amount of valve lift, and when the valve opens and closes (again, usually measured from a certain point of valve travel such as .050).

"Camshaft Phasing" This is simply the position of the camshaft in relation to the crankshaft. The cam is tied to the crank through the timing chain. The "phasing" is the relation between the crankshaft position (and, therefore, the position of the piston in the cylinder bore) and the orientation of the camshaft.

For example, lets discuss TDC on the compression stroke since it is a position most of us are familiar with. When the engine is said to be at TDC on the compression stroke, several things are going on. First, the piston is at the very top of its stroke at the very top of its travel up the cylinder. Compression will be at its highest (I'm ignoring compression due to pre-TDC fuel ignition for purposes of this explanation) and the valves will be fully shut. When the piston is at the top of its stroke the camshaft will rotated to a position where both lobes for cylinder #1 are completely out from under the lifters and the valves will therefore be completely closed. As the piston moves back down the bore on the power stroke and then back up the bore on the exhaust stroke, the camshaft will continue rotating and the exhaust valve will begin to open. At the top of the exhaust stroke the exhaust valve will be closing and the intake valve will be opening. As the piston goes down the bore for the intake stroke and moves into the compression stroke, the intake valve will close. However, the intake valve will not close immediately at BDC on the intake stroke.

This video does an excellent job explaining the basics of the internal combustion engine. The whole video is worth watching, but the discussion about the four stroke cycle and valve events in relation to cylinder position takes place at 7 minutes in.
http://www.youtube.com/watch?v=gfr3_AwuO9Y
I encourage anyone interested in engines to watch this video.

So, putting this all together, the "dynamic compression" the engine will see depends largely on when the intake valve closes. Depending on the camshaft profile (which is lift and duration) and the camshaft phasing (which is the relation between the cylinder position and the peaks of the camshaft lobes), the intake valve will close at a different point in the travel of the piston therefore making the dynamic compression increase or decrease.

Hopefully that helps a bit.

It took me about a year to wrap my head around all of this stuff.