Air cooled compression question?

[polkat]

Hi guys, new here, and while I built Mopar performance engines for years, I'm relatively new to VW's.

I understand that higher compression in air cooled VW's is touchy because of heat considerations. I'm presently doing a somewhat mild performance build on a 1600 for (mostly) street use, and I am wondering just how high I can go with the static compression ratio? And how to acheive it? I have read that some builders leave the rings out between the cylinders and case to get a bit more compression.

In the water cooled Mopar arena, we often build what is called a "Quench" engine, which among other things cuts down on detonation. It requires the piston tops to come just .045" below the chamber, but it appears that attempting this on a VW engine is not recommended? Or is it?

What are you performance builders doing about compression on engines that will be mostly street driven?

Much thanks!

[66brm]

Squish is good, the .045" is about as close as most want to get in a performance acvw engine, with expansion etc. Compression is widely debated on here, as camshaft selection and heads will determine what you can get away with and what you can't, longer duration will allow higher CR's to be bled off during valve overlap. Finding the balance in the whole package is the key.

Are you looking to go up in stroke and bore? What kinda budget did you have in mind? Remembering that speed costs, how fast do you want to go?
_________________
Aust. RHD 66 Type 1
Aust. RHD 57 Type 1 Oval

[polkat]

I was thinking about keeping the stock bore and stroke, basic head porting, headers, single 2 bbl, and not sure about the cam (I understand that the factory cams were all the same). But I'm quite new to these engines and not firmly sure about any of this. I'm not looking to build a high HP strip engine, but more like something for a street sleeper. Yes, quench is a strong thought, but I'm unsure how to acheive it with these engines (parts?).

Any websites around describing builds like this? Or detailed books I should be reading?

[modok]

As the factory built them the piston should come up to .060 short of the top of the cylinder with no gasket under the cylinder (nobody uses gaskets there anyway since the invention of silicone sealant!).
If the case has been resurfaced where the cylinders sit you might have no deck height at all, it is common to need shims.
The cylinders do sink into the case over time, so chances are your case ought to be resurfaced unless it is new or it has been done already.
The majority of stock heads have a step cut in them so the actual deck height was .120", you must cut the step out or use heads with no step to get close deck. most engines have been rebuilt many times by now so you'll just have to take it apart to know what you have.

With the stock cam you better stay below 8-1CR
If you put a cam in it like an engle 100 or 110 then you might get away with 8-1, 8.5-1 or even a little higher if you use fancy stuff like dual carbs.

[polkat]

Well, this is good as I was thinking in terms of about 8:1, but I'm still unsure of how to acheive it. So, assuming the cylinders have not sunk into the case, you are saying cutting the head steps out will get me about 8:1, and to use an Eagle cam and higher octane?

Can you name a good book out there that covers these kinds of mods?

[miniman82]

[modok]

I could tell you exactly what it would all be like on a stock 70's engine.
But 95% chance that somebody has rebuilt it and put diferent heads on it that might be flycut or might not, or might be aftermarket, or might be junk, hell they probably don't even match, I'm not kidding.

Step one: get two good heads that match.
Step two: take them to a machine shop that works on vw stuff and have them flycut to the chamber CC you desire.

I'm just warning you friend, it's not like working on normal engines, if half the parts in the engine match factory specifictions it is a miracle.

[Bruce]

[polkat]

Thanks guys, all good info here. Bruce, you seem to favor quench in the VW's. Here's my concern with that. A quench engine is basically built for a near miss between the piston top and the flat area of the combustion chamber. As might be expected, this dramatically raises compression, with 10:1 being common in water cooled engines. The advantage is that the resulting very high charge volicity across the chamber actually cools the chambers and piston top, to the point where, if done right, can allow a quench engine to run higher compression then a non-quench engine and actually get less detonation.

But even with quench, 10 to 1 compression seems very high for an air cooled engine as I understand it. And I'm still not sure what 'quench' pistons (which usually result in zero deck) are available for the VW's. Do the books mentioned discuss this?

[Bruce]

[polkat]

Okay, this is where my VW ignorance comes in. Perhaps the small stroke of these engines is the reason the compression on a quench built engine won't reach 10:1 even though the piston is coming all the way up to just short of the chamber. So deck height is acheived by machining the heads...right?

I am seriously listening here, but I had in mind another approach to this as well, and that is keeping the engine basically stock and running a turbo. How much would a 1600 have to be beefed up for a turbo (I guess depending on boost amount), and are we talking just aftermarket parts here (case, pistons, etc.)? or is this just a bad idea overall?

[steve34]

You could actually turbo a stock 1600 without any mods. Most would say the minimum is C/m head studs, Hbeam Rods, and forged pistons.

AA pistons would be fine with even stock rods...As long as you tune it appropriately

[polkat]

Okay, thanks, lots to sit back and think about.

Last question for a while. Would the stock 1600 tranny hold up to a turbo'd engine?

Thanks for all the advice guys!

[66brm]

It all depends on how the engine produces the power, and how you engage it, the stock trans has been known to last on high performance engines but MUST be driven accordingly. Start doing drag race starts and look in your mirror for your diff components. If you intend building up to a stroker engine later then its good insurance to have a quality trans built.
_________________
Aust. RHD 66 Type 1
Aust. RHD 57 Type 1 Oval

[modok]

[polkat]

Well, it's well known in the water cooled arena that anything above about .050" looses any 'quench' type benefits. A .060" space is too big for effective quench. That's why I was asking for specifics.

[leaker]

Check out some of the older tech articles on Cal-look.com,there's a couple of good reads on there you might like.

[vgajames]

Are you aware of the VW Engine calculators?
http://www.cbperformance.com/enginecalc.html
Also moving cylinder up and down with shims under the cylinder to change deck height volumn? As well as using copper shims between head and cylinder...And reshaping head combustion chambers
to increase head CC volumn to lower CR...

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[mark tucker]

I like .035 -.040 max.remember the cylinders grow quite a bit on these things.I try to build mine about the same as my mopars but with 10.2-10.5 cr on any thing smaller than 2050cc but I did have 11+cr on my 1874.all were street DD motors.mild cam (fk8) lotsa port work(wedge ports) if you are just getting into vw.s do some research.if you are going to hot rod it you need a counterweighted crank & balancing is a must.so when you are looking at a counterweighted crank look at the price dif between a stk stroke and a stroker crank. get the stroker.&get the 90.5 P&J or 94's. the cam is up to you but a mild cam starts around 244 @.050 witch is a thumpy one on a v8 but stil kinda small.I ran a [email protected] in my 1874, and ran great.top & bottom.better on top. below 2000cc 40mm idf webber or empi hpmix ( china clone), above 2000 44mm. heads need a lot of work & big valves.or you can buy them ready to go hand or cnc ported in just about any configuration.raito rockers,many to choose from, some cheep some not so cheep. have fun and dont get hurt.and you will need to get some vw type tools to do the job.there is more out there for this motor than a small block chevy,but you wont find any parts for it at wallmart, the chevy superstore.

[AlteWagen]

I like to use the engine calculators on aircooled.net

http://www.aircooled.net/gnrlsite/resource/articles.htm

good articles to get used to the aircooled engine.

You will need several specialty tools as well when calculating compression

A deck height tool. This flat plate holds the cylinder down as the head would under torque so you can measure actual deck height once assembled.

A head cc kit. It consists of several plastic disks for various size head bore sizes, and a 60 cc syringe. You use brake fluid or ATF as the measuring liquid and find the cc of your chambers.

Many late heads have a step that modok mentioned which add to the volume of the chamber. Many builders now use copper head gaskets in the same location to lower compression on large stroke engines. Some love them, some hate them, but its the cheap and easy way to lower compression without using larger shims under the cylinder.

I built a mini stroker engine with a 74mm crank and stock bore and built a zero deck engine. I used one large shim under the cylinder to make the positive deck to "0". I used new VW mexico heads with the step, an engle 110 cam and have been VERY happy with its performance.

Matching compression to cam is VERY important

common grinds with accepted compression as follows

Engle Cam grinds

E100 - 7:1
E110 - 8:1
E120 - 9:1
E130 - 10:1

Again as mentioned earlier more duration allows cylinder pressure to bleed off and becomes acceptable "dynamic compression"

There are several ways of adjusting compression:

Case decking
Shims (between case and bottom of cylinder)
Copper head gaskets (between head and top of cylinder )
Combustion chamber shaping (deshrouding the valves and changing shape of chamber adds ccs)
Aftermarket "long" cylinders. You can have these custom machined so you do not need shims between the cylinder and case. Mainly used on long stroke engines.

Bore and stroke. Changing components will require a total recalculation.

If you really want some hope for building a stock block turbo read this

http://www.thesamba.com/vw/forum/viewtopic.php?t=271444

I would suggest a built trans if you do go turbo just for sanity sake.