Hitting the exact Compression Ratio Options

[malcolm2]

I am to the point on my rebuild that I need to make a decision. (IMG:style_emoticons/default/confused24.gif)

I am sure that this type of discussion can become very technical and fill volumes of books. I am not asking for that.

I am the kind of person that wants to know "why" a certain cam needs a certain CR. In order to make the best decision, I need answers. Or if it is a huge secret, that is fine too. I can go blindly along with the crowd.

Maybe it's really what. Has anyone experienced What an engine will do, or what will I see or feel if my cam mfg recommends 8.0:1 CR and my normal state (no shims) is 8.35:1 CR? To get to just about 8, I have calculated that I need about 2mm of deck height. I am currently sitting at 1.62mm average DH with a variance between cylinders of 0.15mm. So I need two 0.4mm shims and two 0.5mm shims and I will be dead nuts on 8:1. (IMG:style_emoticons/default/stirthepot.gif)

So the mfgr's value can be reached (if I can round up the shims), but I am curious to know how differently the engine will function if I go shim-less or even only get half way, maybe 8.2:1. (IMG:style_emoticons/default/WTF.gif)

[McMark]

Don't mix different height shims on the same side of the engine. The head must sit flat to the case, and if the cylinders are different heights, you'll have combustion leaks.

[mark21742]

Most cam manufacturers give you minimum compression ratio recomendations....if you go higher your engine will be a tad peppier....for every full point of compression (going from 10:1 to 11:1) you can expect a 3-4% gain in torque and HP acrossed the rpm range.
With longer duration cans you need higher static compression to get the same dynamic compression you would get from a shorter duration cam because, or the longer duration cam the intake valve stays opened longer after bottom dead center, and you don't start to build compression till the intake valve closes...

For instance my 408 in my 04 GTO had a static compression of 12.49:1, but since the intake valve doesn't close till 85 degrees after bottom dead center it comes out to only 8.22:1 dynamic compression.

Hope this helped a little

[r_towle]

Compression ratio and camshaft selection go hand in hand, although it is an extremely complex topic and one that can't be thoroughly explained in an internet forum.

The first thing to consider is that the static compression ratio is directly related to where in the rpm band peak torque will be made. The higher the compression ratio, the higher in the rpm peak torque will be made in a properly designed engine. If you have an engine with a relatively low redline, say 6000 rpm, a high static compression ratio will do you no good. You will make better power by keeping the static compression ratio low. Conversely, if you have a relatively high redline, say 9000, you need a correspondingly high static compression ratio. Determining how fast you can spin your engine is another topic entirely (search 'piston speed').

Keep in mind that most of these normal engines with normal valve trains have a peak RPM of 5500...really nothing left after that.

Proper valve geometry setup will result in a higher RPM range..and a more matched setup for the motor...cylinder versus cylinder.


'static compression ratio'. The formula for this is:

cylinder displacement + combustion chamber volume / combustion chamber volume

This is the 'normal' compression ratio you will hear guys talk about (i.e. 9:1, 10:1, 13.5:1, etc.).

'dynamic compression ratio' which is the effective compression ratio multiplied by the volumetric efficiency. This is the actual 'running' compression ratio the engine sees while it's doing its thing, hence the term 'dynamic'.


So what ends up happening is, the higher your static compression ratio, the later you want those intake valves to close to keep the effective compression ratio around 8:1. This means by definition your cam will have more duration. So the higher the static compression ratio, the more duration your cam will need. The lower your static compression ratio, the less duration it will need.

There is alot more involved...effective compression ratio is the Cr when the intake valve closes...to when the exhaust valve opens...and as the name implies...its what you really get for a CR...but its very tied to the camshaft selection.


When you buy a cam for your build you need to talk to the cam manufacturer and ask them what is best for your engine. Picking it yourself based on how slick the marketing terminology is, is a recipe for disaster and unfortunately that is how most cams are chosen.

You have other options if you do not want to shim your cylinders that much.
redo the chambers on the heads...make sure they match and CC them very accurately.

[malcolm2]

Compression ratio and camshaft selection go hand in hand, although it is an extremely complex topic and one that can't be thoroughly explained in an internet forum.

The first thing to consider is that the static compression ratio is directly related to where in the rpm band peak torque will be made. The higher the compression ratio, the higher in the rpm peak torque will be made in a properly designed engine. If you have an engine with a relatively low redline, say 6000 rpm, a high static compression ratio will do you no good. You will make better power by keeping the static compression ratio low. Conversely, if you have a relatively high redline, say 9000, you need a correspondingly high static compression ratio. Determining how fast you can spin your engine is another topic entirely (search 'piston speed').

'dynamic compression ratio' which is the effective compression ratio multiplied by the volumetric efficiency. This is the actual 'running' compression ratio the engine sees while it's doing its thing, hence the term 'dynamic'.

So what ends up happening is, the higher your static compression ratio, the later you want those intake valves to close to keep the effective compression ratio around 8:1. This means by definition your cam will have more duration. So the higher the static compression ratio, the more duration your cam will need. The lower your static compression ratio, the less duration it will need.

There is alot more involved...effective compression ratio is the Cr when the intake valve closes...to when the exhaust valve opens...and as the name implies...its what you really get for a CR...but its very tied to the camshaft selection.

I was right, this did get very technical! My head just exploded! Actually, I have the cam installed. I did not see any slick marketing. I explained my proposed use of the vehicle and the chosen fuel induction and took the recomendation. I did not really get a range of CR, just a recommendation.

My heads are back from their trip to Athens, so I am going to have to make adjustments with shims. Better safe than sorry, I will start looking for the shims. Any suggestions about who sells the best variety of thicknesses?

[r_towle]

Jake Raby
or
European Motorworks

rich

[malcolm2]

Don't mix different height shims on the same side of the engine. The head must sit flat to the case, and if the cylinders are different heights, you'll have combustion leaks.

That is how my Deck Height Variance worked out...cyl 1/2 are slightly larger (needs the 0.4mm shim) than cyl 3/4 (needs the 0.5mm shim). I assume that side to side shim difference is OK?

[MartyYeoman]

You have other options if you do not want to shim your cylinders that much.
redo the chambers on the heads...make sure they match and CC them very accurately.

You can also flycut the tops of the pistons to achieve a uniform quench zone.
Marty

[malcolm2]

You have other options if you do not want to shim your cylinders that much.
redo the chambers on the heads...make sure they match and CC them very accurately.

You can also flycut the tops of the pistons to achieve a uniform quench zone.
Marty

Now I am lost. I am shooting for more deck height, won't cutting the cylinders lower the deck height?

NOT A LIE: I had a guy that worked for me once, he told me, "I have cut that board twice and it is still too short."

[r_towle]

more deck height is either more shims or deeper chambers in the heads.

I suppose you could also shave the pistons.

The key to shimming is you need the head to sit square.
That is why both cylinders on a specific side need to be shimmed the same.

Interesting that you are getting different readings on opposite sides of the motor...did you have the case milled flat?

Rich

[malcolm2]

more deck height is either more shims or deeper chambers in the heads.

I suppose you could also shave the pistons.

The key to shimming is you need the head to sit square.
That is why both cylinders on a specific side need to be shimmed the same.

Interesting that you are getting different readings on opposite sides of the motor...did you have the case milled flat?

Rich

No milling. I did have it checked over. I checked the deck height with the same P/C on all locations. I figured that might reduce the variation, but now that I think about it, I need a smidge of variation. I will try each P/C in each loc and see what I get.

[r_towle]

if you had the case "checked over" they should have tested and measured each cylinder register in the case to ensure all four are the same distance from the centerline of the crankshaft...its kinda important and its honestly not to late if your measurements have a variation....better now than 100-400 miles after you finish.

Are you bolting down each cylinder and using a plate on top to get an accurate reading?

What is your measuring technique?

This motor requires a partial assembly...measure for shims (one ring, or all rings to keep the piston centered) and then take it all apart to put in the proper parts...

torque down the cylinders each time.
Check before and after you put in the shims to make sure you are right....
Then do final assembly.

So, really its two assembly times, but sometimes three if you are cautious.


RIch

[Jake Raby]

It is common to achieve different true deck heights on opposite sides of the engine. It is generally a .005-.007 differential from the 1-2 bank to the 3-4 bank with 3-4 always being tighter.

The cam that he is running was developed and tested with a maximum and minimum compression ratio. It doesn't mean that he is being forced to run this CR. It simply means that the stories about how awesome this cam has worked for others that he may have read might not be applicable to him.

(people wonder why I stopped recommending anything to anyone under any circumstance)

[malcolm2]

It is common to achieve different true deck heights on opposite sides of the engine. It is generally a .005-.007 differential from the 1-2 bank to the 3-4 bank with 3-4 always being tighter.

The cam that he is running was developed and tested with a maximum and minimum compression ratio. It doesn't mean that he is being forced to run this CR. It simply means that the stories about how awesome this cam has worked for others that he may have read might not be applicable to him.

(people wonder why I stopped recommending anything to anyone under any circumstance)

Not running yet, still building. Truthfully, I have heard no stories, you and I had a phone conversation, and I went with it. There is that expectation of greatness. Shouldn't there be? I am simply inquiring, curious and respectful of the opinions of folks that have the experience.

Thanks for the tidbit of knowledge above. Validates my findings.

[Jake Raby]

There is that expectation of greatness.

I am trying really hard to lessen that expectation with everything other than our turn key engines these days.. Because too much is outside our care and control otherwise.

You are doing your homework. Good luck with the engine- Luck plays a major role in successfully building an engine.

[malcolm2]

There is that expectation of greatness.

I am trying really hard to lessen that expectation with everything other than our turn key engines these days.. Because too much is outside our care and control otherwise.

You are doing your homework. Good luck with the engine- Luck plays a major role in successfully building an engine.

I am a fan of statistics, so I like to shift the odds of good luck in my favor. You have done alot of homework for folks like me. Keep the info coming. BTW: I did find the cam specs/details with the CR range. I had only remembered you telling me the max #. And I took that as the target. A talk radio guy here in Nashville calls that "van Gogh listening". He only had 1 ear you know.

I found some shims and my CR will average 7.986:1 (IMG:style_emoticons/default/driving.gif)