Compression Ratio

hambone · Post by **hambone** » Sat Feb 28, 2009 8:42 am

Hi, advice needed - I need to buy shims today.
Here's the numbers:
1. 49cc, .070 deck height = 7.7:1 compression ratio (I gotta recheck that deck height)
2. 50cc
3. 47cc
4. 49cc

Hal is recommending 7:7:1 c/r which would need a .015 shim. Whattya think?

Also, do I need to calculate c/r on each cylinder, or is 1 ok for all?

Thank you Jasan BTW for the use of your tools! And thank you Hal for getting me so far ahead in just one night.

tristessa · Post by **tristessa** » Sat Feb 28, 2009 9:22 am

hambone wrote:Hal is recommending 7:7:1 c/r which would need a .015 shim. Whattya think?

No, I tend to be *biased* towards 7.7:1 .. for certain engines that have a different induction system. For yours I'd aim for 7.3:1

Post by **Amskeptic** » Sat Feb 28, 2009 10:06 am

tristessa wrote:
hambone wrote:Hal is recommending 7:7:1 c/r which would need a .015 shim. Whattya think?
No, I tend to be *biased* towards 7.7:1 .. for certain engines that have a different induction system. For yours I'd aim for 7.3:1

If you have flat pistons and a vacuum distributor, 7.7:1
Colin

hambone · Post by **hambone** » Sat Feb 28, 2009 10:12 am

If I ever "upgrade" distributors, will that compression ratio cause problems?
Can I use that value for all 4 p&cs, or is that not the way to fly?
thank you!
I will err on caution and measure all.

tristessa · Post by **tristessa** » Sat Feb 28, 2009 12:44 pm

I tend to be a lil' conservative with C/R on single-carbed engines .. but at least I don't go as far as Berg with that semi-hemi nonsense he concocted all those years ago.

Post by **Amskeptic** » Sat Feb 28, 2009 1:28 pm

hambone wrote:If I ever "upgrade" distributors, will that compression ratio cause problems?
Can I use that value for all 4 p&cs, or is that not the way to fly?
thank you!
I will err on caution and measure all.

The only reason 7.3 comes with the later buses is all that VW did for emissions. The later lower compression ratio was to reduce combustion temps a tad for NOX and pinging reduction with the soon-to-come unleaded fuel. Your vacuum distributor has a good anti-ping sensitivity, the centrifugals are sometimes stupid. . . i.e. they know about rev advance but cannot know about hot days at full load where a vacuum distributor advances a tad less due to less-dense hot air. All very small stuff for the conservative engine.

The engine will be fine with an "upgraded" distributor.
Colin

hambone · Post by **hambone** » Sat Feb 28, 2009 5:51 pm

Excellent.
Halsey doesn't have shims in stock, and only sells them in mm special order.
Based on http://www.aircooled.net/gnrlsite/resou ... /calcs.htm
.070 is the deck height I'm shooting for to get 7.7:1 so I'll need .01" shims. Not much, a shame they're $10 each...
It seems that with the variations in cc volume, it's almost impossible to get the same exact C/R for each cylinder, unless yer crazy. Even ol Hoooover says a variation in 3CCs is ok.

spiffy · Post by **spiffy** » Sat Feb 28, 2009 7:52 pm

I would bring the deck height down to between .040 and .060 and then even up the CC's on your heads to achieve the CR you want. Shims are the quick and dirty (read WRONG way) to do it.

I had to switch my heads to get a tighter deck height and still achieve the CR I am shooting for. Also, don't be afraid to go to a higher CR. Sure there are folks that will argue with that but there is absolutely nothing wrong with a 8.25 CR as long as you have a tight deck.

spiffy · Post by **spiffy** » Sat Feb 28, 2009 8:05 pm

Oh, and I would even up the CC's on the heads....why? You spent good money on balance work why not get the CR in each cylinder spot on?

Post by **Amskeptic** » Sun Mar 01, 2009 12:17 am

spiffy wrote:
there is absolutely nothing wrong with a 8.25 CR as long as you have a tight deck.

It is a tad more complicated than that, spiffy. Yes, we like most of the compression/power event within the chamber, but compression ratios are more tied to camshaft profiles. A mild performance cam will have more overlap between the exhaust and intake strokes, so some of the piston travel is "wasted" with open valves. These engines can have a higher static compression ratio. A stock cam, however, is using more of the piston travel, so I strongly recommend stock compression ratios on a bus engine, particularly with today's alcohol and other exciting blends of gasoline, like no-name "we sell it as 89 octane, but it is actually 87 because YOU have knock sensors so we don't care". Well, our VWs pre-ignite in a silent but deadly fashion hidden under all of the clattering of the engine.

I don't think 8.25 is outlandish or anything, but it is a step away from reliable pedal to the metal all day early bus motoring. More compression = more heat both compression induced and power induced. Law of physics. It is also a step away from the smoothness you like to have with an engine that lives at 4,200 rpm on the freeway.

Bob, if you had the original B case, I would recommend against anything but stock compression, but a fancy new case, nottabiggadeal.
Colin

hambone · Post by **hambone** » Sun Mar 01, 2009 8:20 am

Interesting.
Well if 3CCs variation is OK for airplane engines then I'm fine with it for bus piloting. Besides I've got enuff to do.
Is there much difference between 7.9:1 and 7.7:1? I suppose so or you wouldn't be able to buy .010 shims. I want cool reliability for those 30 min/3000' summertime climbs. Conservative seems to be the name of the game for a decent stock engine. Weird that I'm going that route...Spiff we've changed polarities with each other again! First it was the Olympia and now this...soon I'll be living in an old fridge.
Well I'm going to check the deck height for 1 & 2, hopefully it's the same.

Engine Volume of the VW Workshop manual gives the following specifications for these specific engine types:

"Compression (with engine warm, throttle open, all plugs out, gauge in plug seat and engine turned by starter):
Engine code B, AE (up to 558 000) [Compression Ratio = 7.5]
New part: 114-142psi Wear limit: 100psi
Main difference between cylinders: --

Engine code AE, AH, AK, AM [Compression Ratio = 7.3]
New part: 107-135psi Wear limit: 85psi
Main difference between cylinders: 28psi"

hambone · Post by **hambone** » Sun Mar 01, 2009 6:23 pm

Hey, check it out. I meticulously re-measured the deck height via feeler gages and the plastic calipers (which are pretty accurate actually).
So:
deck height is .065
49/50ccs chambers
which makes a c/r of 7.7:1 or 7.8:1 without shims. I think I'm gonna run it, any objections?

I should check the deck for #2 just to be sure...

spiffy · Post by **spiffy** » Sun Mar 01, 2009 6:36 pm

I think yer golden with that. But I'm crazy

Personally, learning all this deck height, CR stuff has been the funnest part. Plus you could play around and try different things if you wanted to. Of course that would require a ton of work

For reference, I'll be running a .050 deck with a CR of 8.25. I told the guy building the heads that I wanted those two numbers and whatever the head CC's ended up being is what they ended up being. IIRC around 55 or so CC's.

hambone · Post by **hambone** » Sun Mar 01, 2009 7:06 pm

That makes sense with a larger engine like yer running. A balanced system needs all bits equal.
I just want a reliable camper engine that will run in 95 f. with 87 octane.

(I just launched the piston circlip across the garage into the last colony of spiders and filth. And FOUND it somehow, a miracle)

P.S. #2 DH is @ .067

Post by **Amskeptic** » Sun Mar 01, 2009 7:28 pm

hambone wrote:P.S. #2 DH is @ .067

As an absolute number, .067 is no problem. In relation to the others, does it affect overall cc's more than 3%? Play with the AirCooled.Net calculator with your individual combustion chamber cc.s and deck height measurements, i.e. average out the other three and insert the variables on just the one cylinder you are measuring to see if the overall changes more than 3%.
Colin

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Compression Ratio

Compression Ratio

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