Compression Ratio

Maybe someone has done this before but I searched and didn't find anything.

I decided to omit my head gaskets while my engine was apart but was worried that it might raise compression ratio (CR) too much. So I needed to make a comparison of compression ratio with head gaskets and without.

These are the tools I used.

Click to view attachment

In the picture is..

1. Straight edge (blue-point tool made for checking flat surfaces.
2. Plexiglas disk link type 4 store
3. Syringe with CC gradients
4. Digital Caliper
5. Feeler gauge
6. Short and long spacers that I made to hold down the cylinder (optional)

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First
You’ll need to measure Head Chamber Volume. Lay the head on a flat surface. The rockers/valve springs make it uneven so I used 2x4’s on the valve cover surface to make the head sit flat. Put the Plexiglas disk on the sealing surface of the head and measure the amount of fluid it takes to fill the chamber. I used ATF. My Head Chamber Volume was 60cc.

Click to view attachment


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Second
You’ll need to measure Piston Dish Volume, or the size of the valve pockets in the piston using the Plexiglas disk again. Mine measured 4cc.

Click to view attachment


If you have flat pistons skip this, your Piston Dish Volume is zero. If you have domed pistons you’ll need to fill a container with fluid and measure the amount the dome displaces and use a negative number as the Piston Dish Volume.



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Third

You’ll need to measure Deck Height. You’ll need to “mock up” the engine with at least one cylinder and piston. I reused the original cylinder shim for the mock up but you could just add the thickness of the shim to the deck height. If for some reason you were going to use head gaskets you would add their thickness to the deck height as well.

I tried two different ways to compare which was more accurate. Both ways were close enough that I think either way is acceptable. My deck height was .040 inch.

Deck Height with feeler gauge (easier)

Click to view attachment


Deck Height with Digital Caliper (Looks flashy with fancy tool but don’t forget to subtract out the thickness of your straight edge)

Click to view attachment


Now for the math...

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Calculate Volume of one cylinder = Displacement / 4

My engine example…

My 2.0 liter is actually 1971cc.
1971cc / 4 = 492.75cc
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Convert Deck Height (inch) to Deck Volume (cc) with this formula

Bore (mm) x Bore (mm) x Deck Height (in.) x .01996 = Deck Volume (cc)


My engine example…

94 mm x 94 mm x .040 inch x .01996 = 7.0546624 Deck Volume (cc) (round to 7cc)

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Calculate Combustion Chamber Volume

Head Volume + Piston Volume + Deck Volume = Combustion Chamber Volume

My engine example…

60cc + 4cc + 7cc = 71cc Combustion Chamber Volume

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Calculate Compression Ratio with this formula

(Combustion Chamber Volume + one cylinder volume) / Combustion Chamber Volume =Compression Ratio

My engine example…

(71cc + 492.75cc) / 71cc = 7.9 Compression Ratio
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I think I can live with 7.9 but how does this compare with the compression ratio with the head gaskets?

Most numbers are the same except I’ll need to add the thickness of the head gasket to the Deck Height. My head gaskets were .030 inch making Deck Height .070 inch.

So…

Deck Volume
94 mm x 94 mm x .070 inch x .01996 = 12.3456592 Deck Volume (cc) (round to 12cc)

Combustion Chamber Volume
60cc + 4cc + 12cc = 76cc Combustion Chamber Volume

Compression Ratio
(76cc + 492.75cc) / 76cc = 7.4 Compression Ratio

I think I can live with the 7.9 CR. Maybe I can gain a little power without too much additional heat??

Maybe use this link if you don’t like math.
http://www.race-cars.net/calculators/compr...calculator.html


Thoughts…
1. Without too much work I could have shimmed the cylinders the thickness of the head gaskets and ended up with the same CR as before with no extra time spent on all this. But isn’t it better to know?
2. By omitting the head gasket I changed the rocker arm geometry. Didn’t realize this till after the motor was assembled. I’m going to go forward with it the way it is.
3. If I would have done number 1 then number 2 wouldn’t be an issue.
4. How does my Head Volume compare with what others have seen? Does this seem high. Does this mean my heads have never been cut? OOOHH! Untouched heads???


Comments welcome. If I missed anything or gave any misinformation please correct me.

Dangerous proposition clamping down the cylinders with pressure only on the fins. How did you verify exact TDC? I use a dial indicator

Other than that looks good.

Just to make sure and since you have it apart. do the cc's of all 4 combustion chambers you will gain free HP if they are all EXACTLY the same.
i don't think .010 head gasket change will affect your rocker geometry much.
Your deck hieght also has to match each other EXACTLY so you will need to repeat this step for all cyliinders.
your deck of .040 is nice and safe. so strive for the same measurement all around. it may require different shims.

yup, looks about right at a quick glance. as joe said, you're gonna want no more than like an 001 difference in deck height between cylinders.

Are you guys saying that you would put different size shims under the piston to get exactly the same deck height for all cylinders? I always thought that you would need to have the same shims on adjacent cylinders to make the tops of the cylinders in the same plane. I would think that different size shims would make it hard for the head to seal against the cylinder. I could see different sizes on opposite sides of the engine, but not on adjacent cylinders.....Jake, where are you?

What would you recommend as a minimum deck ht?

ooops...I meant to say "different size shims under the cylinders"

some cylinders/cases might be a 'little' different, so yes -- it's my understanding (and in fact, my practice) to use different size shims under (even adjacent) cylinders. this shouldn't be very much, though. this, in fact, ensures equal compression between cylinders and level head sealing. ymmv, best to confere with an expert.

If you are running carbs and some sort of cam your engine will run like crap. You really should look at some other pistons or milling the heads. That engine should have more compression. Low compression generates more heat in the engine. 9:1 is a good place to start.

Bull shit Brett

I am currently running those exact pistons cylinders in my car.
with a deck of .037 62cc combution chamber
OK more compression would be good but it will be a good motor as it stands with a medium same .435 lift 270 duration cam.

My cam is bigger at .500 lift 300 duration.

9 to 1 will work fine if you have a non stock cam, otherwise too much heat! So 8 to 1 is nice and safe.

What cam are you installing?

That is a great write up on how to measure compression ratio. I think many people really aren't sure how to do it.

Joe you are saying you have less than 8.1 compression ratio? If that is the case you really need to run the engine with more compression ratio and you will see how much better it runs and how much easier it is to tune the combination. What is you cranking compression? 140-160psi?

Compression ratio does not make heat. In fact it does the exact opposite. When you have a low compression engine you will not burn all of the mixture in the combustion chamber. Some of that mixture will continue burning as it leaves the exhaust ports thus causing more heat to be forced into the heads. Basically the same effect as retarding the timing.

When you run a higher compression ratio the engine is much more efficient. More of the mixture is burned and less escapes out the exhaust valves, plus you can run less timing, thus creating less negative work, and freeing up more power.

The goal is to reach maximum brake torque with proper timing adjustment. A low compression engine will require much more timing, to make the same amount of torque.

With a modern camshaft design, 9.0-9.5 is a much better option. Your engine will be infinitely more efficient and more fun to drive.

A stock 914 cam is not a modern cam so running 9 to 1 wouldn't be good.

We are down to weeks now and the new motor will be mocked up with that snazzy cam I bought from you. 10.5 : 1 Yea I am looking for lots of GO FAST.

Not enough CR makes for an untunable engine, especially if lower CR is coupled to a cam with longer duration or more overlap..

Joe, your parts will be ready to ship this week- I just need to finish them up (damn holidays!)

BTW_ Guys the special deck height plates and other tools have never been easily attainable for the TIV engine before. This has made it more difficult to do jobs like this accurately and correctly. I now offer these tools bothe for rent and for sale through my store- we tooled up to make them and did a run a few weeks ago.. From the syringe to the head and deck plates we have them all, with detailed instructions for prioper use!

I'm running a 2056 that I built with Jake Raby parts. I have carbs and a Mallory Dist. I calculated 9.5 to 1 and am running 27 deg timing. I couldn't be more pleased. 125 Ft lbs (over 120 from 3500 to 4500 RPM) and 106HP at the rear wheels (over 100 HP from 4300 to 5800 RPM). My Air/Fuel ratio is 13.5 from 3400 to 5000. I could probably get more by leaning out a bit and maybe adding some timing, but as I said , couldn't be more pleased ....Oh it is in a 912E.

I didn't really torque them down they were just snugged. I dont think anymore than that is necessary.



I forgot to even say to set it to top dead center! I used a magnetic base dial indicater. That would have made a nice pic. Doh!



I didn't do this and the motor is back together allready. Just reassembling a used engine anyway. When I build a 2270 turbo monster I'll make sure and do this.



Hot Vw's and Tom Wilsons How to Rebuild Vw Engines both say .040



I have no idea. Probably the stock cam. I'm not sure if anyone changed it before. I'm reusing D-jet. So I think I'd prefer nice & safe.



Makes sense. Cant wait to get a modern camshaft. Why does Hot Vw's always recommend a lower CR? Is the type 4 less likely to detonate?



I was gonna drop a link to those tools when I made this thread it but I couldn't find them in the type 4 store. Probably just need to call??

These tools are another one of the items that we have not gotten added to the store just yet and probably won't until the first of the year.. I have a whole bunch of new tools and other products coming out that we have come up with in 2006, I spend all the time developing and making them and then don't have the time to list them on the store! You can submit a question about them over on the Type 4 Store Support forum and get pricing. (FYI-Telephone is the worst way of reaching us)

As far as Hot VWs, well they have liability issues to consider AND they are NOT engine builders. They do not know the importance of proper CR being coupled to the camshaft for the operating elevation, hell most builders of these engines don't even know that!

In a properly designed engine the camshaft choice will be heavily impacted by the CR you'll be using- the two go hand in hand like two virgins sneaking out behind the barn!