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How To Measure for Compression Ratio Calculations / Vitara Evaluation

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How To Measure for Compression Ratio Calculations / Vitara Evaluation

Old 01-22-2006, 11:54 PM
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Default How To Measure for Compression Ratio Calculations / Vitara Evaluation

Hey Guys,

Below is a rough draft of a write-up we are doing for compression ratio calculation. We took notes while doing these calculations for our Vitara/D16A6 build, and wanted to see what you guys thought. Jeff has already pointed out a couple of issues, regarding our concern about the deck heigth. (We were concerned, because our numbers seemed about half a point off.) Let me know what you think, and be nice dammit.

Craig

How to check and calculate compression ratios.

The article below describes the method we use to check compression ratios. This method is commonly referred to as "CC'ing" a Cylinder". It is called this because you are checking to see how many Cubic Centimeters (cc's) the cylinder contains.

This article was inspired by the ongoing discussions around using the Suzuki Vitara pistons, in a Honda D series engine. In this article we hope to show how to check the compression ratios of these pistons versus the stock pistons in a D16A6 engine. The engine used, has no modifications, and the only preparations for writing this article, were to clean, hone, and then deck the block just enough to straighten out the sealing surfaces. We have used our low-cost TunerToys H-Beam connecting rods to install these pistons, which required the notching of the block. The use of these rods does not affect the compression ratio of the engine/piston combination.

The following items are used in this write up...
  • 100Ml Burrett
  • Rubbing Alcohol 32oz
  • Green Food Coloring
  • Vaseline (or some other light colored grease)
  • Plexiglass (1/8" to 1/4" thickness, approx 4" square)
  • Dial Indicator (optional)
  • Small Funnel (optional)

We had planned out the article in advance, so we assembled the engine with three of the new TT/Vitara piston/connecting rod combinations, and one factory piston and rod. The factory piston/rod we had just held on to when we had removed them from the engine before sending it off to the machine shop. This allowed us to check out the cylinder and piston volumes on both types of pistons in the same block, removing some of the variables in the setup.

The process of calculating compression ratios is fairly simple. Compression Ratio is the result of dividing the total volume at bottom dead center (BDC), by the volume at top dead center (TDC). Imagine you have a volume of nine units when the piston is at BDC, and a volume of one when the piston is at TDC. As the piston moves from BDC to TDC, those nine units, get compressed down to one unit, given you a compression ratio of 9 to 1 or "9:1". Pretty simple isn't it.

So let's figure out how to determine those volumes for our engine. We must measure the volume of the cylinder at BDC and TDC. First we'll measure volume at TDC. The basic idea is to place the piston at TDC, cover the cylinder with the Plexiglass, and then fill the void between piston top and Plexiglass with alcohol, measuring how much alcohol we put into the void. The alcohol is dispensed with a burrett, which is basically a long tube, marked off in milliliters, with a valve at the bottom opening. As you dispense liquid from the burrett, you read how much the liquid level has decreased from the starting level. When you subtract the starting liquid level from the finishing liquid level, you end up with the amount dispensed. Below is a picture of the burrett that we used for this article. It is about three feet tall, and holds 100mL.



A closer picture of the bottom end of the burrett...



Now we need to position the piston at TDC...



To place the piston at TDC, we placed a dial indicator over the piston, and rotated the crank to bring the piston to the top of the cylinder. As the piston approaches TDC, the dial indicator reading increases, then peaks out as the piston reaches the top of it's stroke. It's at this point that the dial indicator reading will stop and reverse direction as the piston begins to travel back down the cylinder. We used the dial indicator to stop the piston right at the top of it's travel (TDC), but stopping the rotation right as the dial indicator stops moving. You can do about the same thing by feeling the piston as it reaches the top of the cylinder, and then moving the crank back and forth to feel out the top of the piston's travel (TDC). With the piston at TDC, you can remove the dial indicator and place it aside.

If you have not already done so, you'll need to prepare a piece of Plexiglass to cover the cylinder bore. The 4" square may be sufficient to accomplish this on a 3" cylinder diameter, but may need to be larger or smaller, depending on your application. You will need to place a hole in the Plexiglass, large enough to admit the end of the burrett. The burrett is used to dispense the alcohol into the cylinders, in order to measure their volume. We learned the hard way that you should place the hole in the Plexiglass towards the edge, and not in the center. An example of a correctly placed hole is pictured below...




The reason for placing the hole at the edge is because as you add liquid to the cylinder, the lowest part of the cylinder fills first. If the engine is on and engine stand, it is not level, and the cylinder will fill unequally because it is tilted. This means that the liquid will reach the level of the hole, before it reaches the highest point in the cylinder. Once it reaches the hole, you cannot add any further liquid, and the remaining air is trapped inside the cylinder. In order to be accurate, we have to get all the air out of the cylinder, replaced by liquid. On an engine stand, the hole should be at the forward most part of the cylinder, near the cylinder wall, and not in the middle. We had to level out the engine using bricks under the stand's wheels, in order to use our Plexiglass with the hole in the center. Here is the Plexiglass cover we used for the write up...



So even though we have our hole in the center, we decided to continue on with the procedure, in order to complete the write-up. So with one end of our engine stand up on bricks to level out the engine, we forge ahead...

To place the Plexiglass onto the cylinder to be checked, you must first apply some Vaseline around the top of the cylinder wall, to seal the Plexiglass in place...



With this done, place the Plexiglass over the top of the cylinder, pressing it down into the Vaseline to seal it into place. Press it down firmly to seat it against the top of the cylinder.



Now that the piston is at TDC, and the cover is in place of the piston, we need to put enough liquid in to the cylinder to replace all of the air. We know that stock A6 pistons have a dish of about 3.4cc according to online research. So we need to put at least that much liquid into the burrett. We used alcohol in the burrett because it has a low viscosity, and it evaporates (no rust). We added green food coloring to the alcohol to make it easier to see in the pictures. So using the funnel, we had some liquid to the burrett and took a reading of the liquid level.

Reading the burrett is as easy as holding it vertically, and reading the liquid level. Here is an example of taking a reading of 75.2ml. The higher numbers are at the top, so readings are taken reading down, instead of up...



Our initial reading after filling the burrett with green alcohol, was 75.2ml. We then held the tip of the burrett over the hole in the cover, and turned the valve to slowly dispense the liquid into the cylinder. Dispensing is a two hand job, so we have borrowed a picture to illustrate...



It didn't take long before the cylinder was filled to the bottom of the hole in the cover. You can see in the picture, that all of the air in the cylinder, has been replaced by green liquid...



So now the reading on the burrett is at 71.0ml. We subtract this reading from our beginning reading of 75.2ml (75.2 - 71.0) and we are left with 4.2ml. That's a little different from what we found online, so we did it again to be sure. Our second attempt resulted in the same reading of 4.2ml, so we continued on with our procedure.

The next step is to move the piston to the bottom of the cylinder, and perform the same procedure again.

It would be difficult to read a dial indicator at the bottom of the piston stroke, so we moved it to the next cylinder. This allowed us to read the neighboring cylinder at TDC, while our target cylinder was it BDC, using the same "peaking" procedure. Otherwise, the procedure is the same as checking the top of the cylinder.

The volume of the cylinder at BDC is much greater than at TDC, so be prepared to refill the burrett several times. We had to refill ours four times, in order to fill the cylinder. Here is a picture of the Vitara piston at BDC, with the cylinder filled with alcohol...



Performing the same start and end readings from our burrett for each refill and dispense, we calculated a cylinder volume of 418ml for the stock piston at BDC.

So now we have the cylinder volumes at TDC (4.2ml) and BDC (418ml). (By the way, subtracting TDC volume from BDC volume, will give us the "Swept Volume" of the cylinder. Some calculations will use this figure, but we won't need it for our calculation.) Before we can calculate compression ratio, we need a few more numbers. There are some additional spaces in the combustion chamber above the cylinder, and these volumes must be accounted for. These are in the cylinder head, and in the head gasket. Luckily, these volumes are static (do not change), and are easy to deal with. First is the cylinder head volume.

You can use this same CC'ing procedure to check the cylinder head volume, but we are going to use some standard numbers we found through online research. Yes, it would be more accurate to CC the heads (as we proved with TDC volume earlier), but for the purpose of this article, some commonly accepted numbers will be fine.

The A6 cylinder head has a 38cc head volume, and the stock gasket has a 5.4cc volume. Since these volumes are always there, whether the piston is at TDC or BDC, they must be added to both volume figures. So doing some simple math we need to add the following volumes...
  • Cylinder Volume at TDC - 4.2cc
  • Cylinder Head Volume - 38cc
  • Gasket Volume - 5.4cc
This gives us a total TDC volume of 47.6cc

Doing the same addition at BDC...
  • Cylinder Volume at BDC - 418cc
  • Cylinder Head Volume - 38cc
  • Gasket Volume - 5.4cc
This gives us a total BDC volume of 461.4cc.

Now we simply divide total BDC (461.4) by the total TDC volume (47.6cc)...

461.4 / 47.6 = 9.693

Rounded to the nearest tenth, this gives us a 9.7:1 compression ratio for our block, using stock A6 components. This is a little more than half a point different from what most online resources show. We are beginning to think that this block may have been decked before we got our hands on it.

This pretty much competes this Compression Ratio Calculations, the rest of the article applies to those using Vitara pistons in their setups. For them, we press on...

Vitara piston compression ratios.

Now that we have completed a full CC'ing procedure for the stock pistons, it is a simple matter to repeat the procedure on the Vitara pistons. We have included a few picture of that procedure, to highlight some differences between the Vitaras, and the stock pistons.



Of particular note, is the Vitara piston dish volume. Measured at TDC, there is 19.2cc of open space from piston to the top of the cylinder. This is 15cc greater volume than the stock piston, part of which is created by a lower compression heigth on the Vitaras. This is a part of the argument against Vitaras, that they sit 1.5mm down in the cylinder of our A6 block. The stock pistons come almost to the top of the cylinder (within 0.020"). Some very respectable Honda techies argue that this will cause the combustion to occur in the top of the cylinder, instead of keeping it all in the cylinder head chamber. This will put unneeded stress on the top of the cylinder walls, which is not desirable. I happen to agree with this point. However...if your goal is to build a budget motor, this risk may be mitigated with a simple addition to your build, a block gaurd. When comparing the costs of a Vitara-based build (with budget rods, no machining, etc), against a build using name brand components, the added cost of a block gaurd isn't that much. We've not built a Vitara motor before, so we are going to take the added precaution of a block gaurd. We'll do our best to make it fit as snuggly as we can to the area of the cylinder walls that will be exposed to the added combustion pressure. We are also going to install ARP Headstuds and a Cometic headgasket to help seal the tops of the cylinders.

So our CC'ing of the Vitara pistons yeilded the following numbers. Volume at TDC was 62.6cc, while volume at BDC was 474.3cc. This yeilded a compression ration of 7.6:1. Yikes! Seems like the lower compression heigth (Piston to Deck Heigth), has lowered compression to an unusable value. It's just way to low to even consider using these pistons in a bone stock A6 engine.

What about using a thinner head gasket? We ran some numbers on this using a volume values for a two-layer Y8 gasket. This gasket turns out to be about the same thickness as the thinnest Cometic head gasket, which does not require you to drill out any rivets. We'll prefer the Cometic gasket, to drilling rivets. The thinner gasket yields a 7.86:1 CR. Still pretty low if you ask me. I want to see some numbers up around 8.5:1.

Luckilly, we are not building a bone stock A6 engine... We are putting together a turbo MiniMe using a Y8 Head. This changes things as far as using the Vitaras use is concerned...

The Y8 head has a smaller combustion chamber than the A6 head. The A6 has a 38cc chamber, while the Y8 has a 32.8cc chamber. This 5.2cc difference may not sound like much, but it does make a difference. After running the numbers on the Y8 head, and the Cometic gasket, we arrived at 8.48:1 CR. Bingo! This is just within our desire CR range for this particular build.

Some might argue that additional decking could bring that number up, as well as milling the head. I agree, those actions could bring the number up quite a bit. But that really isn't the point of Vitaras. We feel that the point of Vitaras is to reduce the compression ratio of your stock engine, to a compression ration that is a little more forgiving under boost. If you start doing a bunch of machine work on your engine block and head to achieve a better compression ratio, you might as well buy a set of forged turbo pistons, at the desired CR, and be done with it.

If you have blown out your stock pistons, but the rest of the engine is good, Vitaras might be a good way to go if you are on a budget. Put in a set of Vitaras, with some inexpensive H-Beam rods, and you have a cheap solution to a lower compression motor. Just be sure that your gasket and head combination will work with the lower compression Vitaras. If you get your CR number too low, the car will have no real usable power. Do your homework. Vitaras are appropriate for only a limited number of situations. Don't take the Vitara route just because you are on a budget.

Keeping in mind that the block used for this write-up may not be completely stock, here are some of the compression ratios we calculated during this write up....
  • Stock A6 Block/Head with stock Pistons and stock Gasket - 9.69:1
  • Stock A6 Block/Head with stock Pistons and two layer Y8 Gasket - 10.2:1
  • Stock A6 Block/Head with Vitara Pistons and stock Gasket - 7.58:1
  • Stock A6 Block/Head with Vitara Pistons and two layer Y8 Gasket - 7.86:1
  • Stock A6 Block with Vitara Pistons, two layer Y8 Gasket, and Y8 Head - 8.51:1
  • Stock A6 Block with Vitara Pistons, Cometic .027 Head Gasket, and Y8 Head - 8.48:1

That pretty much wraps up this article. We are doing a companion article on the actual build up of the engine itself. We'll replace this with a link to that article, when it is placed online. If you use this article on another site, please keep the embedded links intact, and send us a link to where it has been posted. That way we can send updates to the original article.

See this article in it's entirety at How to Calculate Compression / Vitara D16A6 Build Up Also check out the site for more articles, or for updates to this article.

Send us an email regarding this article - [email protected]

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Old 01-23-2006, 01:23 AM
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Default Re: How To Measure for Compression Ratio Calculations / Vitara Evaluation

Excellent write up as it simplifies it for most of us.You still doing that article on turboing a CRX as well?
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Old 01-23-2006, 02:21 AM
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Default Re: How To Measure for Compression Ratio Calculations / Vitara Evaluation

i vote sticky it answered some questions i had about vitaras
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Old 01-23-2006, 07:22 AM
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Originally Posted by tr4cti0n.i55ues
i vote sticky
+ 1

Very good info!
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Old 01-23-2006, 10:29 AM
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Default Re: How To Measure for Compression Ratio Calculations / Vitara Evaluation

Do you plan to address the piston to wall clearance and wrist pin fitment issues? There has been quite a bit of debate around these topics with the vitara slugs.
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Old 01-23-2006, 03:09 PM
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Default Re: How To Measure for Compression Ratio Calculations / Vitara Evaluation

Originally Posted by Foowee
Do you plan to address the piston to wall clearance and wrist pin fitment issues? There has been quite a bit of debate around these topics with the vitara slugs.
We used the TT H-Beams (of course) and had them honed to fit the floating pins on the Vitaras. Our machine shop charged us $40 for this.

For the piston to wall fitment, I was initially that the Vitaras were going to be a bit sloppy in the holes, but they appear to be okay. Since this is my first real hybrid of this type, I wanted a professional to verify my work. We used the same machine shop for milling the head mating surfaces of the block, and I had them hone the cylinders while they had the block. Ray (Basko Machine) stated that the bores were not round, and did a rigid hone, to correct it. He also had me bring down the Vitara pistions, to ensure the fitment, and said there would be no problem.

If the standard size Vitaras had not been usable, I had imagined that I could just get some oversize Vitaras. But once you start down the oversize/boring/machining path ($$$), you might as well buy some forged pistons. On that path, you'll already have a lot of money in machining the block, so why put in bargain basement pistons...

Craig
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Old 01-23-2006, 03:15 PM
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Default Re: How To Measure for Compression Ratio Calculations / Vitara Evaluation

Originally Posted by 90dx
Excellent write up as it simplifies it for most of us.You still doing that article on turboing a CRX as well?
Actually the turbo project is complete, I just need to complete the write-up now. This engine is going in to that same car. (We blew a head gasket with the turbo project, and prolly some other damage.)

So now the turbo set-up will end up on the A6/Y8/Vitara Mini-Me, which is currently in progress. The bottom end is done, with the head install next. (And another write up...)

Craig
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Old 01-24-2006, 01:06 PM
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Default Re: How To Measure for Compression Ratio Calculations / Vitara Evaluation

Originally Posted by TunerToys
Actually the turbo project is complete, I just need to complete the write-up now. This engine is going in to that same car. (We blew a head gasket with the turbo project, and prolly some other damage.)
ive never seen someone so happy to mess up a motor. It was just an old tired motor, 10.5 afr's, a degree of retard per pound of boost @ 6 pounds. We should have compression tested it be4 we started. Criag i cant wait till this one is ready.
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Old 01-26-2006, 01:16 PM
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Originally Posted by Rx7toCivic
ive never seen someone so happy to mess up a motor.
It was fun! Thanks for your help, I hope to put the Y8 head on this weekend. Lotsa other parts are on the way.
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Old 01-28-2006, 09:55 PM
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Default Re: How To Measure for Compression Ratio Calculations / Vitara Evaluation

GREAT article!!! A little curious now though. You are using 2 seperate Y8 HG or the Cometic and reading your compression numbers makes me wonder about if you would have use a Y8 block what would have happened seeing how HIGH your compression was just with a Y8 head and gasket. Stock Y8 is like 9.5:1 so I can only imagine the block on the Y8 and how much different it is. Wow. I learned SO much in this write-up. Never knew that's how you get the compression ratio; really cool. I used a burret just last week...in Chem Lab though for an Acid-Base Titration. Anyway, good stuff and keep it going and I might be looking into to doing one someday. Must say though, a LOT of work JUST to find/get the right compression. Not exactly plug and play, but you DID make it easier and more understandable. Kudos to you.


JP
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