Combustion Chamber volume

[DDS]

Almost ready to reassemble my 2056...

I had the machinist cc my heads and he measured 60.2, 60.2, 60, 58.6 cc.

Bearing in mind measurement tolerances, Is that close enough for a mild street engine?

Does that seem accurate? I've played with a CR calulating spreadheet and it seems that I'll be unable to get my CR to 8.5:1 as hoped.

Any thoughts or experiences?

Thanks!

[turboteener]

You should have the chambers matched a little closer. What deck hieght are you going to run? Have the heads surface after you match the chambers. THen you should have no problem reaching 8.5.

[DDS]

I really appreciate the response Turbo. This is very new ground for me.

I used both .045 and .060" in calculating the CR. 96 mm bore, 71 mm stroke, no head gasket. I don't have the pistons (being balanced) to check but they are NPR either 7cc or 4cc dish.

When you say surfaced, you mean flycut, correct?

If I flycut them after matching the volumes, don't I risk mismatching them again if the chambers are irregularly shaped?

Your name intrigues - do you have a turbo-teener - any web info on same?

Thanks again.

[Special K]

I recently cc'ed my combustion chambers and came out with 60 cc's, give or take a drop. Used the "Plexiglass disk and Big ass syringe" method. On your heads you're only talking about .6 cc's between the largest and smallest chambers. I'm no Jake Raby or Type IV expert, but I can't imagine that less than a cc difference would make that big of a deal (Unless this is some kind of Indy car that you're trying to sqeeze that last bit of HP out of). Did you run the calculations on the "compression ratio" program and notice any significant difference? I doubt it.....but there's bound to be someone out there that totally disagrees.

Now that you're back at the "top of the BBS", let them speak out!

[DDS]

Thanks K.

I get a range of CRs from 7.805:1 to 7.952:1, or a variance of 1.8% What I don't know are the tolerances of the cc measurement - If its to within 1% (+ or - .6cc) my chambers are identical. the accuracy is probably better than that but I'll guess that the differences we're talking about aren't all that significant. (I'll try to compensate when assembling - assuming the jugs aren't 100% etc...)

I do have a problem though - with a .045 total deck ht (assuming I can achieve that) and the 7cc piston dish (NPR 96mm P&Cs) mr CR maxes at 7.8-7.9 :1 - considerably below my target of 8:5. I really needed flatter pistons. (Got these really cheap...)

My heads have been welded, including the sealing surface, so a fly cut is probably called for anyway. it seems that 55 cc would get me close enough. (as would increasing the stroke to 78mm heh, heh)

Anyone have a ballpark of what a given depth of flycut does to the chamber volume, or what I would have to remove to get the chamber volume down to 55cc?

Anyone know of a shop set up to flycut VW heads in the Toronto Area?

PS - As a total novice, I wouldn't trade this learning experience for anything as I don't intend this to be my last engine, but If someone only wanted to build an engine once, a Raby kit would be the way to go, despite the apparent high up-front cost. A good engine build takes lots of planning and lots of dis and reassembly and measuring. Or experience.

Jake, are you L(ING)OL? ;-)

Dave

[yoh!!!]

Hi,

I just finished my CR calculations and I know how much effect a few 1/100 mm here and there have on the result...
Regardin the combustion chamber volume... my 2.0l heads needed to be resurfaces due to age and wear. according to the machine shop they machined 0.012 inch off the sealing surface. when I measured the cumbustion chamber volume after the flycutting was done I measured 58ccm +/- 0.2 ccm
I think that the original combustion chamber (not cut) is 60 ccm. By cutting 0.012 inch off, the chamber lost 2ccm of it's original volume. I hope this helps...
In case you decide in fly cutting check where your cylinder seals with the head. In my case the cylinder did not touch the sealing surface in the combustion chamber, it "sealed" with the top of the outer ring around the combustion chamber. I noticed that when I tried to hand hap the cylinders in the heads. The outer ring around the combustion chamber needed to be machined ase well, in my case about the same as my fly cut...
It would have worked with out any cutting if I would have decided to use the head gaskets, but the combination of no gaskets and the fly cut almost caused problems...
At this point I have a CR of 8.2:1 with a healthy deck height.
Keep playing with the CR calculator, it helped me to find out what had to do in order to get to the specs that I wanted to have...

[DDS]

Thanks Yoh!

I may dremel the smaller chamber to get it to 60cc.

I got my ported rebuilt heads back and did a rough guesstimate by measuring the area of the cc, and I got about 1.8cc / .012 cut, so your post confirms the ballpark. Great!
I did the calcs and I think a .060 flycut or the entire bored area will do it, with a little leeway for deck height.
What do you consider a safe deck height?

Thanks again.

[Special K]

Seems like the general concensus on deck heights is .045". I'd have to double check, but I believe engine rebuild book said something about "at least .040". Again, I'm not the "expert" on engines, but I did stay at a "Holiday Inn Express" last night :-)

[yoh!!!]

Hi,

1.0 mm to 1.5 mm (0.04-0.06 inch) for the deck height seems what people are shooting for. I remember the Wilson book (is it Wilson... hmm the red book on rebuilding VW engines) recommends 0.05, G. Berg recommends a deck height in the range from 0.05 to 0.06 depending on the quality of your piston, rod, crank, ...
In my case I ended up with a deck height of 0.051 and a calculated CR of 8.2:1. The CR would have turned out higher, but the kind if piston (2.0l Euro Spec ?!) I used had some miniatur dishes (2ccm)... so this is my setup.

Hmm, I forgot that...
Grinding the combustion chamber with a Dremel tool is a good idea... If I remember right, you would have to remove 1.4 ccm from the smaller chamber. That's quiet a bit of material.
You might want to concider geting the fly cutting done first. I think the machine shops use the surface where the valve cover mounts to mount the cylinderhead on their mill. It is possible that this surface on the cylinderhead is not super streight, or I could imagine that the shop could correct or compensate for that while they do the fly cut...
... Would keep you from dremeling... at least you would not have to dremel 1.4 ccm.
I remember I read something about a guy who rebuild his crovair head... he had similar problems with an untrue valve cover surface... I will try to find the link.
Here is the problem he faced...
"When the air bubble disappeared completely, I had released 48.3 ccs into the chamber. The second one was 48.7 ccs and the third one (number 5) is 49.1. That's in line with my theory that GM didn't bore this head the same depth on all three cylinders. Maybe the valve cover surface isn't in the proper plane assuming that's the flat surface they use when milling the chambers for the cylinder bores. The other head is 47.5,47.8, and 47.8. Now all I have to do is find the largest one, and remove a little aluminum from the five smaller ones so that they all match within 1cc.
I wanted to get an idea of how much 1cc of volume looks like, and this is a picture of it. Right now I have a spread of 1.6 ccs, so I have some work to do. I may only do one more iteration, but the result will be combustion chambers of similar volume, which will give me similar power from all six cylinders. And more importantly, I will KNOW what my compression ratio is, rather than guessing. This is part of the "blueprinting" process of eliminating manufacturing tolerance stackup. You might have noticed that my cylinder number 5 (about 10 pictures ago) either wasn't machined to the same depth (by the factory) or the casting wasn't quite as tall, so there is a definite need to see which is the case."
Check this link for the whole article (it might take a while to lode it due to the pics) http://home.hiwaay.net/~langford/corvair/valvejob.html

MfG
Patrick Koch

[DDS]

Great link! thanks.

an update now.

I found a shop in TO that is set up and knows their VW stuff.

I hadn't checked it before, but I found that there is only .018 clearance between the head cooling fins and the cylinder fins when they are assembled without a head gasket. This means that if I am to flycut the chambers greater than that (less a bit of course) the sealing surfaces would not mate.
The obvious answer is to 'deck' the head or counterbore for the cylinder fins to ensure adequate clearance, but what strikes me as odd is that I've never heard this mentioned before when fly cutting is discussed. The guy at the shop said that it is common to remove the bottom cooling fin on T1 heads for this reason.

Is this typical of T4 also? TIA

[Dave at Pelican Parts]

I don't think I'd call it "typical", but it is certainly not unheard of to remove one of the fins. I forget if it's more usually the bottom head fin or the top cylinder fin--I would think the cylinder one, but I'm far from an expert in this...

I don't think it's common unless you're going for some interesting combination oof compression and deck-height, or you're using somewhat non-standard parts. (I.e., 96mm pistons designed for the 66mm stroke on a 2.0 motor with 71mm stroke.)

--DD

[DDS]

The machinist is decking the head .060 to allow for the flycut.

Also, that link posted by Yoh! is a great read if anyone is int'd in this subject.

[Otto]

If you have rebuilt as many blown/dropped seat heads as we have you would know how to solve this problem. For years we have welded in material to conform more closley to the configuration of the piston top. We have to reshape/reprofile most of the heads we refurbish. How do you think WE get a 13to1 compression motor out of a set of 10 1/2 cr Mahles????????OTTO