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engine deck height on 78 bus
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LCOX
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PostPosted: Thu Jul 12, 2018 7:46 pm    Post subject: engine deck height on 78 bus Reply with quote

Does anyone know the correct piston/cylinder deck height on a 78 2.0 bus engine
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PostPosted: Thu Jul 12, 2018 9:55 pm    Post subject: Re: engine deck height on 78 bus Reply with quote

Depends on the volume of your combustion chambers.
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PostPosted: Fri Jul 13, 2018 8:03 am    Post subject: Re: engine deck height on 78 bus Reply with quote

busdaddy wrote:
Depends on the volume of your combustion chambers.


You want a deck height of no more than 0.060" and no less than 0.030 unless you are being very careful in your build in which case you could go to 0.025". With uncut stepped heads, uncut block, stock cylinders and pistons, and no cylinder base gaskets you will tend to end up around .040"
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PostPosted: Fri Jul 13, 2018 9:21 am    Post subject: Re: engine deck height on 78 bus Reply with quote

it is set using a compression calculator, and will depend on the compression ration chosen, the dish volume and the combustion head volume. VW used 7.3 on most of these engines. It allowed use of lower octane fuels. It also reduces power, efficiency and heat. A 2.0L engine makes a lot of heat and going up in CR is not as safe as it is on the smaller engines.
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PostPosted: Fri Jul 13, 2018 10:37 am    Post subject: Re: engine deck height on 78 bus Reply with quote

thnx to mayor ratwell
Compression Ratio's
http://www.ratwell.com/technical/Heads.html#gaskets
http://cbperformance.com/v/enginecalc.html
http://bobhooversblog.blogspot.com/2006/11/vw-compression-ratio.html
https://youtu.be/iLC_hKzJGio

Good luck
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PostPosted: Fri Jul 13, 2018 11:36 am    Post subject: Re: engine deck height on 78 bus Reply with quote

This engine has the original low compression dished pistons. There is a .025 shim at the base of each cylinder and a .030 head gasket in the combustion chamber. So I guess using a flat top 94mm p/c's isn't the wisest thing to do even with more deck height? Last question. How can I tell if the heads have been fly cut?
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PostPosted: Fri Jul 13, 2018 1:29 pm    Post subject: Re: engine deck height on 78 bus Reply with quote

LCOX wrote:
This engine has the original low compression dished pistons. There is a .025 shim at the base of each cylinder and a .030 head gasket in the combustion chamber. So I guess using a flat top 94mm p/c's isn't the wisest thing to do even with more deck height? Last question. How can I tell if the heads have been fly cut?


DO NOT use the head gasket. There was a TSB from the factory about removing the head gasket.

So install either one rod with its piston and no rings in one cylinder on the crank...or better yet all of them....and rotate the crank to bring each piston to its highest point in its cylinder. Use no shims between the cylinder and case while measuring.
Measure with a depth micrometer or a plate plate with feeler gauges...how far the piston is from the top of the cylinder. This is your deck height.

If your cylinder head has a little step ring inside of it where the cylinder seats....measure the height of that step. Add that to the deck measurement you already measured on the piston. This is your total minimum deck without adjusting anything.

Then...looking at Ratwells links...or others...fashion a plastic plate with a hole in it...and get a syringe to use so you can find out how many CC;s your combustion chamber holds.

With your deck height, combustion chamber volume, bore and stroke...you can go to any online calculator that will tell you your compression ratio.

As others have noted....really you do not want a "total" deck less than about .035"....and anything under .040" is on teh edge of advanced meaning you need to be sure your measurements are accurate.....and you do not want a deck at .060" or above. Really about .055" should be your limit. Ray
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Tcash
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PostPosted: Fri Jul 13, 2018 5:02 pm    Post subject: Re: engine deck height on 78 bus Reply with quote

How can I tell if the heads have been fly cut?
That is a good question. I can"t find any specs for the Spigot depths for new cylinder heads.
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PostPosted: Fri Jul 13, 2018 5:14 pm    Post subject: Re: engine deck height on 78 bus Reply with quote

The bottom head appears to be uncut. See what look similar to a gasket. That would be gone on a Flycut head.

The bottom is 2.0 liter head 022 101 372 G with SMALLER valves 37.5 and 33, a FI head.
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PostPosted: Fri Jul 13, 2018 6:30 pm    Post subject: Re: engine deck height on 78 bus Reply with quote

Tcash wrote:
How can I tell if the heads have been fly cut?
That is a good question. I can"t find any specs for the Spigot depths for new cylinder heads.


You can find the stock volume and compare that to the actual volume. If comparing OEM VW heads to AMC heads you would have to take into account the step in the AMC heads.
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PostPosted: Sat Jul 14, 2018 7:51 am    Post subject: Re: engine deck height on 78 bus Reply with quote

Ok thanks.
1. I know I don't have the AMC head with the stepped area.
2. I do have the FI heads with the smaller valves
3. I looked and felt closely at each head. I don't think these have been flycut. I can barely feel and see a light impression where the head gaskets seated in.
4. So far I can't find anyone who sells the head cc measuring tool for this bus head. Any idea how I can make one to continue with measuring the cc's ?
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PostPosted: Sat Jul 14, 2018 8:02 am    Post subject: Re: engine deck height on 78 bus Reply with quote

It says type 1, but can it be made to work

http://vwparts.aircooled.net/Cylinder-Head-CC-Measuring-Kit-Type-1-p/head-cc-measuring-kit.htm
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PostPosted: Sat Jul 14, 2018 8:19 am    Post subject: Re: engine deck height on 78 bus Reply with quote

Wildthings wrote:
Tcash wrote:
How can I tell if the heads have been fly cut?
That is a good question. I can"t find any specs for the Spigot depths for new cylinder heads.


You can find the stock volume and compare that to the actual volume. If comparing OEM VW heads to AMC heads you would have to take into account the step in the AMC heads.


IIRC there is a chart that my Machinist used that had the factory spigot depths.
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PostPosted: Sat Jul 14, 2018 8:28 am    Post subject: Re: engine deck height on 78 bus Reply with quote

4. So far I can't find anyone who sells the head cc measuring tool for this bus head. Any idea how I can make one to continue with measuring the cc's ?

Got an old CD
A 60ml syringe can be found at your local Hydroponic / Grow equipment supply store. Wink


Type 4 build
Here is how i check chamber volume - Anybody remember CDs?
Back in the days of CDs, a spindle of them would come with a plastic blank on top.
That gets cut down to make a CC kit.
Free is free, baby!
Image
Just like i always hit up my dentist for leftover pick/scraping tools,
A 60cc syringe can often be swiped from the hospital! Very Happy
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Link


Good luck
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LCOX
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PostPosted: Tue Jul 17, 2018 6:03 pm    Post subject: Re: engine deck height on 78 bus Reply with quote

Ok I have some measurements and numbers now.

-Following Rays' advice I measured deck height of each piston without rings on each cylinder with no shims under the cylinders and got a very small reading of .002 deck height at top dead center.
-I measured the volume of each combustion chamber in the heads and got 50cc on cylinder #2 and#3 and 47cc's on cylinder #1 and #4. I may be off a cc or 2 on the 47cc readings.
-The factory low compression pistons are used which have a 15cc volume.
-It is a stock 2.0 with 94mm bore and 71mm stroke.

Any more measurements needed before moving forward?
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PostPosted: Tue Jul 17, 2018 6:16 pm    Post subject: Re: engine deck height on 78 bus Reply with quote

Is that .002 INCH or MM?

.002" is almost flat with the top and is low.

The measurement is taken with the cylinders torqued down because that will change the measurement, and also taken at the wrist pin to avoid rocking the piston.

Here is how I hold mine down when I take that measurement. The aluminum blocks came from a metal surplus store, and this dial indicator is only to be sure I am at TDC. Th e first time I tried it with some sockets etc but could not get a stable reading - also didn't want to break a fin so I made these blocks. you can stack feeler gauge blades together to get close to what the deck is. There is not one measurement that is perfect. A slight difference isn't going to get you into trouble. If you think it is say .080" and it is actually .078" you won't feel the difference nor will the engine. What ever shim you decide on will end up being used on all the cylinders. Sounds like one head has been flycut a little more than the other. 47 CC is way below average which I think is around 53 to 54 CC.

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PostPosted: Wed Jul 18, 2018 7:55 am    Post subject: Re: engine deck height on 78 bus Reply with quote

Thanks for the input. I went back and checked again and got a new deck height of .005 INCH up from .002 inch. I made the first measurement at the top of the piston. This 2nd measurement was made at the wrist pin. The piston must have rocked alittle
-I guess I may need to take my heads to a machine shop and have them checked along cc's measured again I may have made a mistake in measuring the volume but thought I was close.
-Any advice appreciated. Thanks everyone.
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PostPosted: Wed Jul 18, 2018 8:21 am    Post subject: Re: engine deck height on 78 bus Reply with quote

LCOX wrote:
Thanks for the input. I went back and checked again and got a new deck height of .005 INCH up from .002 inch. I made the first measurement at the top of the piston. This 2nd measurement was made at the wrist pin. The piston must have rocked alittle
-I guess I may need to take my heads to a machine shop and have them checked along cc's measured again I may have made a mistake in measuring the volume but thought I was close.
-Any advice appreciated. Thanks everyone.


Sounds like you just need a cylinder base shim of about 0.030" and you will be fine.

In my opinion, a tight deck and a bit higher compression will give you an engine that is less apt to knock than a looser deck and less compression. The tight deck may also allow you to get by with a degree or so less timing advance which will let the engine run a tad cooler.

FYI the timing for the original Type 4's was 27 BTDC at 3500+ rpms, hoses off and plugged, while the compression was much higher with either flat or domed pistons. As with most every other automobile manufacture on the planet VW dropped the compression on their engines in the early 70 for air pollution reasons.
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PostPosted: Wed Jul 18, 2018 8:36 am    Post subject: Re: engine deck height on 78 bus Reply with quote

use a compression calculator. VW chose 7.3:1 as their 2L CR. They detuned these engines to reduce heat. Heat is the enemy. Making max HP is not the goal here - survival thru the heat is. Keep that in mind as you make decisions on rebuilding.

using a quick online calculator, and not double checking the math, I get that a .030" shim would raise your compression ratio to 8.0:1 using 50 cc as the average combustion chamber size. That will require use of premium fuel and generate more heat. 8.0:1 is what VW used on European 914's and 912's. In a bus that may cause additional heat problems a 914 or912 would not encounter.

A .070" shim sets it at 7.3:1. I am using about a .070" shim also and my bus engine kicks ass. Often when we get on the freeway these mid-grade sedans try to out accelerate the bus so they don't get trapped behind it, but I easily pull away from them. The engine is using a Webcam #142 stock European 914 (100 HP) engine cam. It is smog legal as well - passes smog easily.
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*You can do it right the first time, or do it over, the choice is yours. It is a free country. As to brakes, steering, tires, axles etc. - you may only get one chance at doing those right.

* VW Buses and VW cars share many of the same systems but if you haven't owned a bus and maintained it for awhile, you really don't know how different it is.

Bus Engine Cooling Issues? https://kentcomputer.com/77VW/BusEngineCooling.pdf
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PostPosted: Wed Jul 18, 2018 9:55 am    Post subject: Re: engine deck height on 78 bus Reply with quote

Your loose deck causes the flame front to have to travel about 20% further than my tight deck which means your engine is running out of tune because of insufficient advance when compared to mine, the lessened turbulence of a loose deck also slows the burn making knock even more likely. Heat just has never been much of a worry for me with ACVW engines, but I can see where it would be for those running a loose deck that throws the whole engine out of tune.
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