From what I'm gathering, Piston dome, HG, and deck height, would determine c/r. Anything else that Iv'e over looked anywhere??? I've been reading up on Z6 all motor builds and most gas here is 89 and you can find a few that have 91octane , so I'm curious while researching if I have missed something so far? This is also assuming the crank and rods are stock btw. Thankz

I'm in "ALL MOTOR" THREADS FOR A REASON (don't need the "just turbo it answer" thnx)

 

Cams.. you bleed off compression and actually how high you spin the motor does have some effect on it..

lol and anyone who's in all motor and tells you to just turbo it , is an idiot...
It doesn't take alot , just a good combo of parts and tuning to be quick all motor.
I'm building a motor for a guy right now.. 12.1CR z6 with 100shot..should roll out fairly well on the motor and be pretty quick on the dope.

 

when you are talking about allmotor builds displacement is the key to make good horsepower so is not all about compression you can always sleeve to achive maximum bore and stroke combine all these with some good high lift cams a set of 550cc injectors and a 11:/12: compression pistons and you can tune on 91 octane all day you can also tune on e85 if it is with in your reach it all depends on a good dyno tune

 

Thankz ALL MOTOR, If you can shoot me a pm of those specs please. I'm more into a mean DD kind of build. I'm looking to create that perfect recipe of power, reliability, and a D series

Nice info midnighteg-6! I don't think e85 will be a big thing locally anytime soon sadly, but worth a look for a long term back porch build #2. I'm on a tight budget and have plenty of time to do it right the first time I hope. Also I don't plan to break any records or anything over the top, just a simple and some what modest Z6 build. I think 140ish whp with room to grow is the goal I'm looking at. The hardest part is gonna be finding a tuner local, I might have to load her up on a U-Haul lol

 

Combustion chamber volume "affects" compression. So do rod length, crank size, wristpin location, piston dome volume, head and deck hight, headgasket thickness, etc. Even things like valves and spark plugs can affect compression.

 

Well, there are actually two types of compression:

Static, which is simply the volume in the cylinder when the piston is at Bottom Dead Center (BDC) divided by the volume in the cylinder at Top Dead Center (TDC). Chamber volume, headgasket bore and thickness, piston dome/dish, deck clearance, and bore and stroke all play a part in your final Static Compression Ratio. If you want to get real **** with it, you can even calculate the volume around the piston above the top ring.

Dynamic attempts to go a step further and takes into account valve events to determine when the compression actually begines. Dynamic Compression Ratio is typically calculated by the volume in the cylinder when the valve closes (which is typically well after BDC) divided by the volume in the cylinder at TDC. Since you need to know the piston's position at a specific crank angle, rod length and crank stroke come into play as well.

 

Thankz bambam, I can see how the dish of the valves would play part, would thier size also? and how? How do spark plugs effect compression?

EG1834, it is the Static numbers that most refer to the I'm assumeing?
Is calculating the only way to come up with these numbers are is there a more physical aproach like gauges and testers?

If you made too much compression how does one detune it to run 91? my only thoughts is timeing or getting inside and changing things?

 

Yes, the SCR is usually what someone means when they are talking about compression ratios. Some people think DCR is important, some don't. I am one of those that don't.

The SCR is calculated based off of measurements taken with instruments such as micrometers, dial bores gauges, and burettes. You can measure the volume of the combustion chambers or piston dome/dish with a burette, and measure certain parameters of the shortblock such as bore and stroke, deck clearance, and headgasket bore to find out your volumes at TDC and BDC. Here are the formulae when the measurements are in inches.

• Swept: bore^2 x stroke x .7854 x 16.39 = cc’s
• Head Gasket: bore^2 x crushed thickness x .7854 x 16.39 = cc’s
• Deck Clearance: bore^2 x distance in(+) or out(-) of the hole x .7854 x 16.39 = cc’s
• Piston: Measured Dish(+) or Dome(-) in cc's
• Combustion Chamber: Measured in cc's

If you add all five of the totals up you will get your total volume. Add the last four and you will get your compressed volume. Divide the total volume by the compressed volume and you have your Static Compression Ratio.

If you end up with too high of a SCR, usually the best thing to do is to use a dish piston or cut the dome down so that you not only reduce the SCR, but you also take weight off of the piston. Hondas are nice in that you don't have to rebalance the crank if you cut the dome down. It's probably not a good idea to do it on a bossted setup since it will likely reduce the piston's integrity under high cylinder pressures. If you were to try tuning around too high of a SCR, you'll end up making less power and less efficiency than if you had just gotten it right the first time.

 

Well seeing as how valves and spark plugs are both protruding into the combustion chamber, they're total exposed surface area can affect combustion chamber volume. Think of bosch platinum+4 v s. A normal style spark plug.

 

correct, both of these things affect combustion chamber volume slightly.

 

People often overlook spark plugs and valves as items that can affect compression. Bosch+ 2s or+ 4s are the only spark plugs that come to mind that have a greater surface area exposed in the combustion chamber. However their ability to accomplish what theyre designed/marketed to do will more than likely offset any potential gains from the increase in compression. Flat faced or larger valves will also slightly affect compression.
Going into more depth on cylinder head volume, combustion chamber shape will also affect compression i.e. P72 (gsr) design vs. PR3 (B16/ITR/CTR) design. Combustion chamber shape/design also affects other things.........hmmmmm.

 

I've often visioned getting some of the top 15-20 OG people on here together to design and build a motor. I can only imagine the ideas we could come up with. 350whp bseries? The combined knowledge/experience/resources would be unbelievable.

 

Thankz All,
I really appreciate the info, it'll give me more to look into. You guys made that pretty to understand . I don't think I'll be a master tomorrow but I'm a step in the right dirrection

 

if you could get the top 15-20 og ppl back in here posting i would be happy

 

This man knows his ****. Listen to him.
However I feel that DCR is important, since thats the "compression" during the actual operation of the engine. Too high and you get knock regardless of what the static compression is. Right?
If your 11:1 engine is getting some knock, this means your dynamic compression is too high at the wrong time. Changing camshafts would change your DCR but not your SCR. Now your knock has disappeared and you never changed your static compression. This is why I think dynamic is more important. Of course the biggest factor in DCR is STR...Right?

 

Hey thanks for the kind words!

My opinion towards DCR is that it doesn't seem to be as useful as SCR. Without busting out my Excel Spreadsheet, I think to get into a problem of too high of a DCR, one would have to run either a ridiculously small cam (ie early intake closing) or a SCR already higher than what may be generally considered "pump gas friendly". While I think DCR does have its merit, using sound judgement for selecting a SCR to run on a given octane and selecting a camshaft profile for the intended RPM range would result in a better engine combination than selecting the SCR and camshaft to chase down a predetermined DCR.

Of course, thats just my opinion. It may change tomorrow, lol.

 

Agreed. Isnt it also very hard/near impossible to actually measure your DCR once you get into some higher rpms? Not necessarily due to the VTEC system, but from the actual air being ingested at different rpms and load?
So this explains why on some engine setups, a smaller cam makes more power than a larger cam due to its higher DCR,right? And generally speaking the higher the DCR(without knock), the more power will be made...
Please correct me if im wrong here...

Reading on HT for years, you start to see the people who know their ****, and the ones who "fake it till they make it" LOL. Stick around long and youll be able to pick them out too. All of my automotive training and education throughout the years coincides with just about everything you say. Thank you for sharing your knowledge!

 

That intake valve closing point that determines the DCR also determines when the intake valve stops filling the cylinder. At less RPM and slower airspeeds in the port, there isn't as much interia cramming air into the cylinders so an earlier intake closing and higher DCRs may make more power at that range. However, at higher RPM and higher airspeeds, you can hang that valve open longer as inertia continues to fill the cylinder even as the piston is coming back up, so in that case lower DCRs may be better. I bet you will see the same trend if you compare a VTEC lobe to the non-VTEC lobe.

So as far as determining DCR based on cam events, I wouldn't say the higher the DCR (w/o knock) the better unless DCR took into account the actual mass of air being injested and not just the volume. I think it would be better to say the highest average cylinder pressure w/o knock is best, but that kind of measuring capabilites are out of reach for average guys like us. Total bummer.

And again, you're too kind, lol. I know what you mean about the some of the "fakers" on here. They get pretty hostile if you disagree with them, that's all I'm gonna say about that...lol. I'm definetly sticking around though, I am actually learning a lot on here reading old build threads and such.

 

This is heading towards the topic of VE.....

 

Now you guys are over my head, but I like it. I didn't even concider Vtec as a factor of c/r, but more of a way to optimize it at certain rpms within the power band? You all have great info, thankz for sharing

I got one other question. let me take a step back to noob land for a moment. When you test compression on a motor does that have any correlation to c/r? And if so how? I get the fact a engine has to have good compression to opperate correctly and to achieve good combustion but are those numbers connected to the c/r? I don't mind asking stupid questions as long as I get smart answers lol

 

Thats a good question. You cannot directly co-relate compression test results vs. static compression ratio. Even the smallest things like cam timing and valve clearance can affect compression results. So no you cant accurately say an 11:1 cr engine will be exactly 250psi or 9:1 is exactly 200psi...Know what I mean?
A compression test is mainly just to take a quick look comparing cylinder to cylinder. The numbers themselves arent that important.

 

Cool, I gotcha! I really didn't think they did but hey, obviously I'm a noob so I didn't want to over look something so little. Now that I know they arn't related I can move forward lol, Thankz

 

It is slightly related to dynamic compression though.

 

http://www.empirenet.com/pkelley2/DynamicCR.html


Good read.

 

Well yeah. the higher the dynamic compression, the higher compression test results should be (id think).
Now what if we did a compression test with vtec enabled..Lower compression test readings but would it be a lower or higher dynamic compression ratio. Going by the intake valve closing event, it should be lower right?