Deck height, after a flycut.....?????

[Special K]

Time to poll the "Brain Trust". Just got my 2.0 heads back from the shop (looking good!), and have brand new piston/cylinders to mount them to. The guy that rebuilt them had to "flycut" the heads .040" and gave me shims to install between the heads and cylinders. Okay, this has to have some kind of impact on my C/R. I want to keep the C/R around 8.0:1, and all the books I've read say to keep the deck height to around .060" (to avoid the valves and pistons from getting too intimate), but fail to mention if this measurement should be adjusted due to the flycut. Should I reduce this figure abit to make up for the fly cut, or leave the deck height at spec.? Any suggestions?

Kevin Jung
'73 2.0L (gonna-be)
'75 2.0L (has-been)
St. Louis, MO (MO RUST)

[DDS]

I see no responses yet, so I'll chime in. I'm not an expert at all, but maybe as a quick and dirty check you could use a piece of graph paper determine the approximate area of the oddly-shaped valve area (correct term please!) in the combustion chamber and multiply that by the depth of the flycut. That would give you an approximation of the decreased volume, so you could calculate the relative CR. One problem might be the change in the valve train geometry, I don't know if that .040 is significant. If so, I understand that you can shim either the base of the cylinders or the rockers to bring it back to spec.
"How to Hot-Rod VW engines" is an interesting read because it discussed a lot of the issues involved in VW (mostly type 1) tweaking. It covers cc'ing the cylinders, CR calculations, setting deck height, etc....
Good luck!
Dave

[Special K]

Thanks Dave,

I've got the "How to rebuild Volkswagen Engines" by Tom Wilson, I'll have to check out the book you referred to. I just thought that this might have happened to someone before, and was curious as to what they ended up doing about it, if anything.

On an unrelated note, I was up in St. Charles,MO on some "Boeing" business (I work on the F/A-18 Super Hornet)and there was a SCCA autocross going on in the parking lot. Went down to check it out, hoping to see some Porsches. The only Porsche out of about 50 cars was a rough looking 924.

[Jake Raby]

The entire equation of deck height, and cylinder head volume must be adjusted after the machine work you have described.

I use .050 deck all the time, and never go below .045 on the 914 engine.

You will need to "cc" the cylinder heads, then install one piston on each side of the engine. You can measure the deck height various ways, just make sure you are exactly at TDC..

Have the cylinder heads "cc'd" and e mail or post the results here, I can give you what the CR will be without shims, and with shims.

THIS IS VERY IMPORTANT..

[Dave at Pelican Parts]

If you can measure all of the specified quantities, you can use the following Excel spreadsheet to calculate your compression ratio:
http://www.pelicanparts.com/914/technical_specs/914_compression_ratio.htm

Don't approximate the combustion chamber volume as DDS suggested; very small errors can have large effects on the ratio! Tweak the numbers in the spreadsheet to get some idea there...

--DD

[Special K]

Dave and Jake,

Thanks for the replies. I downloaded the worksheet (emphasis on "work") and it appears that it could get the C/R down to the "gnats ass". Bear with me on this next theory, because I just made it up.
Is it feasible to set the head, combustion chamber up on the bench, lay the shim I received from the rebuilder in the head, and use a calibrated syringe to fill the chamber level with the top surface of the shim with something like alcohol (less surface tension than water).
If I started with a predetermined amount in the syringe, and the graduations on the syringe are in 10th's of a CC, I should be able to get it to within 10ml of accurate. Then, if I found the deck heights (as described in the "How to rebuild VW Engines" book) and added the volumes together to get a total volumes at TDC, and then gave you guys the numbers (Damn it Jim!! I'm a mechanic, not an Engineer!)could you tell me where I stand as far as C/R goes? Or is this a total "caveman" approach, requiring more accuracy than this.

Again, thanks for the replies!

Kevin
'73 2.0L (give or take a few CC's)
'75 2.0L (victim of canibalization)

[This message has been edited by Special K (edited 04-08-2001).]

[This message has been edited by Special K (edited 04-08-2001).]

[Dave at Pelican Parts]

Yes, you could indeed do that. Or you could measure the volumes of the combustion chambers the same way, and measure the thickness of the "head gasket".

Either way, all you have to do is plug the numbers into the spreadsheet and it should do the number crunching for you. Isn't that why we have spreadsheet programs?

--DD

[Special K]

Dave,

I don't have Excel. I was able to open the file by converting it in Wordperfect 2000. All that allowed me to do is print the three "Worksheets" associated with the program. Some of calculations in the printed version must be calculated down to the billionths (not kidding),and it's not very interactive (unless you consider my head banging against the wall doing the Geometry/Trig. [whatever it is] calculations "interactive";-). I will, however, try to locate a copy of Excel and give it another try. I definitely want to get it right the first time.

Thanks again,

Kevin

[SteveStromberg]

Try: www.aircooled.net/gnrlsite/resource/specgeninfo/calcs.htm
Steve

[Special K]

Ah Yes Steve, this is more my speed! I believe I can handle it now. I'm wondering if DD's program would've worked similar to this one if I had the proper software to run it?

I'd like to sincerely thank everyone for their input!!

Kevin
'73 1999cc
'75 doesn't matter

[Jake Raby]

Al these spreadsheets and CR calculations are fine for static CR, but dynamic CR will change with valves, cam and etc..I have made my own table for this. Tell me the specs, and we can go from there, otherwise the excel spreadsheet should do it for you.

[Dave at Pelican Parts]

True, but with the stock (very mild) cam, or even one that's pretty close to that, the static one will tell you pretty much what you need to know.

--DD

[DDS]

Ok, a newbie question here - should I start another thread?
I presume you refer to dynamic CR because the intake valves do not snap shut at precisely BDC and that the CR really should be calculated based on the volume in the cylinder the instant the chamber is sealed. This is very interesting - please correct me if I'm wrong.
Would that suggest that a 'wild' (longer duration?) cam would have a lower dynamic CR in the identical engine than would a milder cam? If so, the would-be tweaker rodder would want to raise the static CR accordingly, right?
Which CR is quoted by manufacturers ie porsche in specs? Thanks for sharing your expertise!
Dave




[This message has been edited by DDS (edited 04-09-2001).]

[Dave at Pelican Parts]

As far as my understanding goes, Dave, you've got the right idea on "dynamic compression". That's one of the (many) reasons that the early 911S cars (the high-powered "S" models) run very high compression for the time--they had lumpy enough cams that the dynamic compression was somewhat lower than the static ratio.

As far as I know, all compression ratios quoted anywhere are static. They serve as a good rule-of-thumb indicator for what kind of fuel you'll need to use to avoid preignition (pinging). But that's only for one particular engine design--the 911, for instance, runs higher compressions on lower grades of fuel than even the Euro-spec 914 motors.

Jake can probably correct me if I'm wrong, but I don't remember seeing any "dynamic compression ratios" anywhere. My guess is that, when you take the dynamic effects into account, then you start talking about cylinder pressures, rather than ratios.

--DD

[Special K]

Jake,

Okay, as for the "dynamic C/R"- I'm carbed (dual 44IDF Webers), with a "Weber 3/4 race cam" as the P.O. put it (DD said that he believed that 3/4 meant 3/4 of the way between a "stock" and a "full race" cam). Not sure where that falls in the lumpyness catagory. But if I find out the static C/R, can your C/R program figure out the dynamic C/R?

Sorry Jake, I should have read your last post a little more closely, it answered the above question. But as far as the cam goes, are you familiar with the Weber term "3/4 race", and know what the lift and duration might be for future reference?

Kevin

[This message has been edited by Special K (edited 04-10-2001).]

[DDS]

Thanks Dave. Intriguing stuff for the would-be engine builder!

[Jake Raby]

The question is 3/4 of what?????

The grind needs to be specific...Otherwise you are wasting your time...

[Jake Raby]

The question is 3/4 of what?????

The grind needs to be specific...Otherwise you are wasting your time...