type 4 Compression Ratio calc question

[malcolm2]

I am shooting for 7.3 CR. To get there I need 0.075" deck height. That is out of spec. My searching here tells me that too much deck height can cause engine problems, poor mileage, etc....

The heads are Camper Specials with a 55.2 total cc. The pistons have a 10 cc dish. 2.0 engine.

As the picture above says: 0.065 deck height would be at the top of optimal range and the CR would be 7.4.

Are all my numbers correct? Any ideas to get me to 7.3 and stay in deck height optimal range??
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My Toys: '75 Porsche 914, '92 Ski Nautique and now a 1972 VW Transporter Deluxe (punch list fixing)

[Glenn]

You might want this in the Engine forum. If you do, PM me and i'll move it.
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Glenn
74 Beetle Specs | 74 Beetle Restoration | 2180cc Engine
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[SGKent]

These were an engine I built that runs fine and smogs legal as well.
Cylinder 1 Cylinder 2 Cylinder 3 Cylinder 4

Chamber 49 cc 49 cc 49 cc 49 cc
Piston Dish 11 cc 11 cc 11 cc 11 cc
Deck 0.105 0.105 0.105 0.105
Bore 94 mm 94 mm 94 mm 94 mm
Stroke 71 mm 71 mm 71.mm 71.mm
Calc comp 7.3 7.3 7.3 7.3
Spacer 0.071 0.071 0.071 0.071


who cc'd the heads and was the step part of that measurement?

Are you measuring the pistons at the wrist pin at TDC?

Are the cylinders bolted down with a plate or something like this?

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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[malcolm2]

I am guessing that Hoffman CC'd the heads. Cause someone used a sharpie and wrote on a very special piece of paper in big letters..... 55.2

yes, I bolted the cylinder down. If pictures were not such a bitch to add, i would show you..... I have a magnetic mount just like you, but mine is black.

So let's assume I did it exactly like you would have..... now can you help?
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My Toys: '75 Porsche 914, '92 Ski Nautique and now a 1972 VW Transporter Deluxe (punch list fixing)

[malcolm2]

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My Toys: '75 Porsche 914, '92 Ski Nautique and now a 1972 VW Transporter Deluxe (punch list fixing)

[malcolm2]

so your 0.105 deck height vs. my 0.075.... All will be well? Why are you ok going over spec and out of optimal?


I am assuming that you measured the CC's on the dish? Cause the published CC on my pistons is 10. If mine are also 11, I am creaping toward 0.06....
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My Toys: '75 Porsche 914, '92 Ski Nautique and now a 1972 VW Transporter Deluxe (punch list fixing)

[malcolm2]

double post
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My Toys: '75 Porsche 914, '92 Ski Nautique and now a 1972 VW Transporter Deluxe (punch list fixing)

[airschooled]

If you're confident in your measurements, I would run it.
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Learn how your vintage VW works. And why it doesn't!
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[malcolm2]

[SGKent]

.075" with a .030" step is the same as a deck of .105" on the smaller combustion chamber number of 50.2cc.

My heads have the step cut out, yours don't. The difference between 7.3CR and 7.4 CR is nothing in the world of power or efficiency. You'll want to choose based on a good steel shim set that is available. I'd run 7.4 if that is what your shims give you.

In other words - if one shim set gives you 7.1, and the next gives you 7.4, and the next gives you 7.8, run with the 7.4 on a 2L. Type one buses I would be comfortable at 8.0:1. Not on a 2L bus. 2L Porsche yes. 411/412 yes, but not on a bus because of the heat at freeway speeds. Around town and up small hills around town, sure. Sustained freeway, especially in the heat of summer - no.

Here is the reason I don't care about performance as one thinks of it in a bus engine. I care more about longevity. It is a different thinking cap. Bus engines get beat to death on long freeway drives.



.
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[airschooled]

I am enjoying .065” on my Type 1 these days.

Realistically, no human being will feel a .2 difference in CR in a stock Volkswagen bus.
Robbie

PS- a bay bus has a lower drag coefficient than a bug
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Learn how your vintage VW works. And why it doesn't!
One-on-one tech help for your Volkswagen:
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[malcolm2]

[telford dorr]

[SGKent]

[malcolm2]

OK let's say I am gonna shoot for 0.069 deck height. That is gonna require a shim.

The gasket kit came with what I suppose are sealing rings. They are ~0.008" thick. What is best?

1. Use the sealing ring and a 0.030 barrel shim

2. Don't use the sealing ring and use a 0.040 barrel shim
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My Toys: '75 Porsche 914, '92 Ski Nautique and now a 1972 VW Transporter Deluxe (punch list fixing)

[SGKent]

you use 1 shim at the base. They cannot be stacked. No sealing ring at the top. Without knowing what measurement you are getting above the cylinder wrist pins now, it is impossible to know what size to put at the base. What is your measurement with no shim?
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[malcolm2]

Cylinder lightly wrench tightened, and no shim under the barrel, piston at TDC, I get 0.027”.

Just like Tom Wilson’s book, I slid feeler gauges under a ratchet extension in several places.
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My Toys: '75 Porsche 914, '92 Ski Nautique and now a 1972 VW Transporter Deluxe (punch list fixing)

[SGKent]

if your goal is .069" then you would need a .042" shim. .027" + .042" = .069". Do the measurement above the wrist pin to prevent the piston from rocking.

But you won't find a .042" shim. Your choices will be .040" or .050". A .040" will give you 7.39:1. A .050" will give you 7.25:1 which when everything tightens will probably be 7.3:1. Either would be fine. The "compressed gasket thickness" below is the step. It is nice to see you so involved in your engine build. In general. I'd say that the average engine build standard here in the Bay forum has greatly improved with engine builds where one pays attention to small details. 10 - 15 years ago many were just buying crate motors from folks who did marginal work.

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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[malcolm2]

So what is the deal on not stacking shims?? I have read some posts with folks putting multiple shims. Others say multiple shims can give you another place for a leak. Is that your reasoning?
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My Toys: '75 Porsche 914, '92 Ski Nautique and now a 1972 VW Transporter Deluxe (punch list fixing)

[SGKent]