Compression Ratio

Are you still talking about that same combo you posted about earlier? If you are do you plan on spraying it or adding any forced induction?

With N/A you could shoot for 9 and to 10:5 and still be streetable. When you get over that mark streetability is usually out the door, but there are exceptions.

If you are or plan to use a supercharge or turbo down the road stick below 9 down to 8.

The intake is not going to effect you compression ratio. That is determined by cylinder head, bore, stroke and gasket thickness. Anything in the combustion cylinder part of the engine.

I am not a intake expert so I will let others chim in that.

I would suggest going on the web and looking at compression calculators. They were a huge help to me when building my motor as the what effects what.

Thats the simple explanation anyway.

AnthonyR23 said:

Is there a compression ratio I should be targeting with components during a build.. eg. 10 to 1

Click to expand...

You need to look at what the cam recommends. Cams with a good bit of overlap like more compression. If you think you would consider a turbo or supercharger you want to keep your
compression as it is.

enyawix said:

You need to look at what the cam recommends.

Click to expand...

I 100% agree

backfocus said:

The intake is not going to effect you compression ratio. That is determined by cylinder head, bore, stroke and gasket thickness. Anything in the combustion cylinder part of the engine.

I would suggest going on the web and looking at compression calculators. They were a huge help to me when building my motor as the what effects what.

Thats the simple explanation anyway.

Click to expand...

You are talking about "static" compression, and has only a little to do with dynamic crompression... which is what the engine is really seeing. The camshaft has a big roll in this area.

You are talking about "static" compression, and has only a little to do with dynamic compression... which is what the engine is really seeing. The camshaft has a big roll in this area.

Click to expand...

Yes I was, as his question was vary vague and didn't give any info on the setup he wanted, or what his plans were for it. It honestly sounded as if he didn't know anything about a motor, no offense Anthony, it was just the way it was worded.

I know the cam plays in dynamic compression. Especially since it plays in the opening and closing of the valves in the combustion camber. Ed helped me with this as well when he made the cam, which is going in my motor.

I was just giving a crude explanation of what I thought he was asking.

347HO said:

You are talking about "static" compression, and has only a little to do with dynamic crompression... which is what the engine is really seeing. The camshaft has a big roll in this area.

Click to expand...

Yes, when he said "compression ratio" that IS static compression.

backfocus said:

Yes I was, as his question was vary vague and didn't give any info on the setup he wanted, or what his plans were for it. It honestly sounded as if he didn't know anything about a motor, no offense Anthony, it was just the way it was worded.

I know the cam plays in dynamic compression. Especially since it plays in the opening and closing of the valves in the combustion camber. Ed helped me with this as well when he made the cam, which is going in my motor.

I was just giving a crude explanation of what I thought he was asking.

Click to expand...

Ah... I see your point.

Fast63 said:

Yes, when he said "compression ratio" that IS static compression.

Click to expand...

I know where AnthonyR23 is going with this question.
So, since you know also... please explain to him what static compression will do for him on choosing parts and prior to assembling the engine?

Then, explain static compression while the engine is running and how it does or does not affect emissions? Depending on your answer, then explain why.

Follow that up with your explanation of what static compression he needs to start out with for his "emissions" engine.

Anthony I it is great that you are trying to learn about this, and I didn't mean you didn't know anything. The original question was just too. vague. I myself am not going to claim to know everything either, but I just relearned most of this stuff myself in building my motor, so it is still a little fresh in my head. Although we will see if it pays off when the motor is started up.

I and many people would ask though that you please you paragraphs in your posts. When it is altogether it is difficult to read.

Now on to the question. I know that many of the Trickflow heads do have emission provisions on them, which ones I am not sure at the moment. But I had this article saved that might help with the explanation of compression. Also you might look an find virtual engine dyno 2000, or something like that. It has compression calculators in it that are interesting to use to see what effects what.

Also have you talked with ED. He will probably steer you toward an AFR head, but will still work with you no matter what. As long as you contact him an assure you are buying from him, he is one of the best resources for you as far as setup and emissions.

Here is the article, it is for chevy, but the idea is what counts.

From Hot Rod Mag:

Dynamic Compression
How to calculate dynamic compression on a small-block chevy

Dynamic Compression
-June 2007-

Question: I am building a 355 Chevy for the street using aluminum flat-top pistons with valve reliefs that at TDC sit 0.024 inch below the deck. The heads are 64cc iron Vortec 062s with screw-in studs and Crane PN 10309-1 springs and retainers for added lift. The cam is a Crane 282-H06. I am able to calculate the head gasket thickness necessary to achieve a static 10:1 compression ratio, assuming the valves are closed for the entire compression stroke. However, with this cam, the intake valve is open until 34 degrees ABDC. Should this be taken into consideration? If so, how do I calculate the actual working compression ratio in an effort to achieve a 10:1 ratio?
Peter Illemann
Goderich, Ontario, Canada

Answer: The compression ratio listed for any engine combo is always the static compression ratio, which is fixed via component selection and machining practice when the engine is built. As you've surmised, it assumes the cylinder is full of air and fuel at bottom dead center (BDC) and that all the mixture is compressed into the top dead center (TDC) combustion cavity. But an engine really doesn't begin making serious compression until the intake valve closes and seals the air/fuel mix in the cylinder. That's why engines with identical static compression ratios can have significantly different cranking pressures, as seen with a common compression gauge. Although part of this is due to displacement differences (a larger-cubic-inch engine is a bigger pump), the main influence is camshaft design. By changing certain cam parameters, it's possible to bleed off cylinder pressure on the bottom end, decreasing fuel octane sensitivity, even though its static compression ratio remains unchanged. The actual cylinder pressure an engine sees is often referred to as dynamic compression, because (unlike the static built-in compression ratio) it changes dynamically according to camshaft variations. The most important of these variations is the intake closing point, because it extends beyond BDC into the compression stroke. Closing the intake later aids top-end power at the expense of low-end torque. Down low, where the engine is most likely to detonate, the late intake closure bleeds off cylinder pressure, effectively dropping the dynamic compression ratio.

Rod length and crankshaft stroke also play into the dynamic compression equation because they alter the piston's position within the cylinder at the intake-valve closure point. How all this comes together is explained by KB Pistons' John Erb: "Consider a 10:1 Chevy with a 3.48-inch stroke, 6-inch center-to-center rods, and a very hot race cam that closes the intake at 90 degrees after bottom dead center (ABDC). This engine will think it is running with 6.17:1 compression and will be happy with 80-octane fuel. As a general rule, the best available pump gas will work with an 8.0:1 dynamic compression ratio. To get 8.0:1 with the preceding rod, stroke, and cam intake closing event, you would need about a 13.2:1 static ratio."

Erb's 8.0:1 dynamic guideline is for classic, old-school, all-iron engines. An advanced, high-tech engine can go a point or so higher-but even with the latest high-tech electronically managed engines, actually shooting for a 10:1 dynamic ratio is way too high. Calculating the relationship between piston position in the bore and intake closing point and then determining the effective dynamic compression ratio requires some pretty serious math, but KB Pistons has a Web-based calculator that makes it simple-just plug in the requested numbers.



Hope that helps a little! I have never had to pass emissions, so I am not sure, but I would stay on the conservative side if I were worried about that, Under 10:1 for sure. I have more to add, but I want to see what other have to say as well.

347HO said:

I know where AnthonyR23 is going with this question.
So, since you know also... please explain to him what static compression will do for him on choosing parts and prior to assembling the engine?

Then, explain static compression while the engine is running and how it does or does not affect emissions? Depending on your answer, then explain why.

Follow that up with your explanation of what static compression he needs to start out with for his "emissions" engine.

Click to expand...

Are you just trying to be some kind of prick or something? Either way, I'll bite.

I really can't give you any advice on emissions for you because you live in Ontario and I have no idea how they conduct emissions tests over there. I can tell you that if you lived in California like I do, you would fail emissions immediately with a manifold other than stock at most testing places and would have to search around for someone to do it. As far as what comes out your tail pipe, I don't think you will increase your emissions much or at all by having a better flowing intake. If the intake eliminates the EGR valve function, your NOX results will go up quite a bit, and that's not good for passing. Better heads also increase NOX, but again you should use heads that are 50 state legal in order to make sure all your smog equipment is doing its job, like the twisted wedge heads. In order to guarentee a passing of the smog test, you really should use the stock cam, maybe throw on some 1.7 roller rockers, or when you get a custom cam ground, specifically tell the grinder that you need to pass smog and you don't want to inch by. Tell him your whole combo and what your requirements are for passing.

When choosing parts, you should always look at all the parts that will effect your compression before you buy anything. First thing to do is select what compression ratio you want. If you want 10:1 compression, use any combination of parts you want to get you there. The easiest way is to select a 347 kit that says you will get 10:1 compression with a 58cc head and then buy a 58cc head.

Static compression is your compression ratio. It is not how much PSI is in the cylinder when you're cranking it over and measuring it, or when it's idling. As soon as you want to know the PSI in there, you are talking about dynamic compression which is measured when the parts are moving. The cam has an effect on this, as well as your static compression (the ratio), and I think better flowing heads and intake can effect it by allowing more air into the cylinders to be compressed. So, I am not sure what "Then, explain what static compression while the engine is running" means because static compression is only measured by the ratio. That is why it is called static, because the engine is not moving.

The static compression will have an effect on emissions, but not a lot, unless you are going with a crazy amount of compression. I have read material saying that a higher compression ratio will effect your CO, NOX, and hydrocarbon count, but others say it will only affect your NOX count, and from my experience it only affects the NOX, and if it does affect CO and hydrocarbons, it is very little. I run 10.13:1 in my 85 stang and I measured very little hydrocarbons and CO, but my NOX was decently high. Definitely well below the maximum allowed on all accounts. So I would say not to worry about the compression ratio in regards to emissions, just don't go nuts, stay 10:1 or under, because you need to run pump gas anyway, and make sure your other parts are emissions compliant. There are many engines today from the factory with around 10:1 compression so if it was a huge problem, they wouldn't use that much static compression.

Give us a little more detail about the emissions standards in your area. Do they do a visual inspection? If so, how detailed is it? Do you know what the limits are of the emissions that you can put out?