i need to lower my compression ratio

[borninabus]

i will open this for discussion:

the engine of reference is in my sig line and lives in a 71 bus.
it runs on 87 octane cheap shizz.
i bring this to you boys because i have a few options at this point and i need to decide where to go from here...

please keep in mind that this a $100 (cyl hone & rings / shims?) top end job that i will be doing myself.
i will not be splitting the case or springing for any NEW parts whatsoever
this is not a race engine! just a 'built' engine that gets me & family where we need or want to be every day in a fun, economical & peppy manner.

this is what i built:
AH 1600
69x85.5
stock cam, stock longblock basically, save the C/W crank.
040 heads w/ 48cc chambers
.053 deck
o19 (please, no timing questions)
zenith on an old EMPI manifold (isolated runner) ((jetted with an AFM))

in 20,000mi it always ran hot---head temp 350+ mounted on a head stud--and it started pinging when the weather turned hot.
i have it apart now and it turns out my heads have the step cut out and my chamber cc is only 48cc
plug those #s into CBs super computer and it comes up w/ 8.1:1 which is too much for a bus w/ a stock cam and leanish jetting, IMO.
so i want to lower my C/R to 7.5:1.

OK, i have some options:

1) shim cylinders .020 and have .073 deck and 7.8:1
2) add CC's by either dishing the pistons, sinking the valves, or manicuring the heads. i need about 5cc's.
3) all of the above.
it doesn't appear that i can 'restore' the stock step in the head because copper gaskets are not available in the size i need.

i think that i would like to keep a tight deck (.053, no shims) and possibly unshroud the intake valves a bit. but there is no way i'm gonna come up with 5cc's that way.
someone with deep VW lineage--the deepest, deeper than mine and yours combined, i promise; recommended that i hemi cut.

what says you
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88 Van WBX, A/T - 13 JSW TDI 6M/T - 2012 Touareg TDI Sport

[baked beetle]

stick a .020 copper shim on top of the cylinder and drive it. don't worry about your deck height. it' not a race engine

At least if it does fix your problem then you did good diagnosing.
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57' Theirs
63' Hers
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[borninabus]

i cant find copper gaskets in ..010"-020" or 85.5mm
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88 Van WBX, A/T - 13 JSW TDI 6M/T - 2012 Touareg TDI Sport

[Cusser]

On my 1835 engine (in 1986) I added shims underneath the cylinder, between case and cylinder, to decrease the compression. So I use regular unleaded.
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1970 VW (owned since 1972) and 1971 VW Convertible (owned since 1976), second owner of each. The '71 now has the 1835 engine, swapped from the '70. Second owner of each. 1988 Mazda B2200 truck, 1998 Frontier, 2014 Yukon, 2004 Frontier King Cab. All manual transmission except for the Yukon. http://www.thesamba.com/vw/forum/album_page.php?pic_id=335294 http://www.thesamba.com/vw/forum/album_page.php?pic_id=335297

[borninabus]

so VW thought that approx .050 deck was OK but they used a step to keep combustion in the chamber is the way i'm understanding it.

Cuss: your 1835 is in a light Baja or something, right?
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88 Van WBX, A/T - 13 JSW TDI 6M/T - 2012 Touareg TDI Sport

[modok]

most 040 heads did not have a step

grind a few cc outa the chambers, it won't take much

[craigman]

I'd bet $$$ that your running WAY hotter head temps than you think you are.
The temp probe does NOT belong on a head stud. You need to put it as close to the heat source that you can. And that's under the spark plug.
And, too low of compression can make it run hot.
What makes you think you need to lower the compression?

[DarthWeber]

[jfats808]

I agree with craig that you are running hotter th an you moght think. I read your post and know that you dont want to run a better octane , but that coupled with running a fatter jetting might get you running cooler. Sorry gotta mention also, might want to play with timing a bit. I wouldve tried these two options first before tearing down. My two bits.
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2276 IDA's 86C 11-1 DD !
2017 48 Trijet DRLA's W125

[craigman]

Also i just don't understand why people refuse to use higher octane fuel.
What extra does it cost at fill up time? A dollar or two? That's the price of a burger or a pack of smokes..

[borninabus]

[craigman]

8.8:1 compression is NOT making your engine run hot. You have other issues.
Lowering your compression is NOT going to help you.
I'm willing to be you have no idea what your A/F is. Too lean can run hot, as well as too rich can run hot..
Find out what is making it run hot and fix the problem. Masking the problem won't do your engine any good.

[borninabus]

[craigman]

Not pissing at all, just trying to help!
John at aircooled.net recommends (and i agree) that:
range of 16:1 for light throttle (progression circuit), and 12.75-13:1 from 1/2-Full Throttle.

But you seem to have it figure out already.
Good luck!

[baked beetle]

i run 10.9:1 (2234cc) on Shell 92 octane fuel with no pinging and heads temps of 375 (which is probably low considering the vdo gauge) after 3 hot lap runs in my race car going full throttle. I have simple fans cooling the heads but have not even seen 400 on the gauge.

I too, do not think .5 of a drop is going to fix your problems.
_________________
These days people like me are hard to find- we don't give a damn what you think, about what we think. - JR
____________________
66' His
57' Theirs
63' Hers
62' Drag Bug - Theirs 13.1 @ 101 mph (GONE)

[borninabus]

[jsturtlebuggy]

Volkswagen keep dropping the compression ratio for stock engines as the gasoline changed.
The last engine in the 1970s where at 7.3 to 1 and recommended 91RON.
You have a stock camshaft with very little overlap and want to run 87 octane rated gasoline with 7.5 to 1 compression ratio. Not a good idea unless you think you are smarter than the VW engineers.
For example the VW Bugs in Mexico drop the compression to 6.6 to 1 and recommended 88 octane as minimum.
Even the 36hp engines use less than 7 to 1 compression ratio in the 1950s.
What you are asking to do with using the cheapest fuel and higher than stock recommended compression ratio is not possible for an engine to run cool and live very long.
Drop the compression ratio, yes you will notice a lack of performance, but your engine will run cooler and live longer.
_________________
Joseph
Fair Oaks/Orangevale, CA
Elrod Motorsports
Motion Tire Motorsports
Having fun with Dune Buggies since 1970
Into Volkswagens since 1960

[jfats808]

Cheapest route at this time is stick shims/copper gaskets in there and see. Head shaving is next. Guess .070 + deck is your next option. What about plumbing in an oil cooler-fan?
_________________
2276 IDA's 86C 11-1 DD !
2017 48 Trijet DRLA's W125

[yamaducci]

How was the carbon build up in the chambers and piston tops?
Was it measurable?
You say it started pinging which leads me to believe either carbon increased your compression ratio or mearly the ambient temp changes your air density just enough to mess with the engine tune. I say given your take on this with not wanting to spend excessive money on new parts and not knowing your skill level to do chamber work or have the time / tools to do it; just go for the $6 shims under the cylinders that CB sells and give it a shot.
Don't get me wrong; if you wanted to do it right I would have a whole 'nother answer for you.
Don't under estimate carb and ignition tuning as has been mentioned. I understand it can be trivial but a lot cheap on time and money to correct.
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-John Cox
My 2498 Turbo Re-Build Thread: http://www.thesamba.com/vw/forum/viewtopic.php?p=5578697#5578697

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[Gary Massin-Ball]

No to the Hemi cut! it's best to leave the "quench" area in the heads for proper combustion.

If you cut a dish in your piston 1.5mm deep and 70mm diameter this will give you 6cc of extra combustion chamber and still leave active quench area on the piston..

What wieght oil are you using? what is your cruising speed and RPM?

Deep sump? 2.5 qts in the stock sump is too little I think.

Could be at hwy speeds you are overwhelming the stock cooler.

Thinner oil could help and a 3.5qt deep sump is a no brainer.

3x on trying the shims in the head to check your theory.

These are steel head shims avaiable at cip1.com

C13-21-5008 - EMPI -CYLINDER SPACERS - BETWEEN HEAD & BARREL - SET OF 4 -.030 INCH THICK - 15-1600cc BEETLE ENG.

Website Discount Price US $7.95

C13-21-5009 - EMPI -CYLINDER SPACERS - BETWEEN HEAD & BARREL - SET OF 4 -.060 INCH THICK - 15-1600cc BEETLE ENG.

Website Discount Price US $7.95
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