Just CC'd my engine...

[Deadhead]

I just finished CC'ing my engine. I came up with 8.15:1 as my compression ration. It's a 2 liter. what does everyone here think? Should I lower it for longevity? It's going in my '74 412. All I've got for it is a single 2 barrel also.

Peace, Lee

[raygreenwood]

What is your deck height? Do NOT lower the compression. 8.1:1 is barely adequate for a 412. The lower compression engines tend to run hot and inefficient. The best runner of both of the engines in the 411/412 was the injected 1.7L with 8.2:1 compression. The lower compression 1.7 and 1.8"s were dogs in comparison. Also....the 1.7 ran even etter at about 8.5:1.

I take it from the photo, that those were once 1.7L heads. You hopefully went to 42x36 valves? Also...what piston are you using, what is your deck, with or without shims and gaskets?
8.1:1 sounds a tad low for enlarged 1.7 heads....espcially since they have a small chamber to start with. Ray

[Deadhead]

Hey Ray, I knew you'd come through. My deck hight with NO shims or spacers is .9 mm which equals 0.035433 inches, or 6.17cc. My pistons are dished and have a 15cc dish. My heads are stock 1700 bored out with no larger valves installed. What size are the stock ones? I'll measure my valves tomorrow. The heads measure right at 51cc. If I measured right, I get 8.14765 to one comp. ratio.

Thanks a lot, Peace.

[Steve Arndt]

If your RPM limit is low (sub 6000) you should be okay w/ .035" deck. That is cutting it close, so make sure to measure EVERY cylinder.

[raygreenwood]

Yup, good call. .040 would be a bit safer on the deck.. The stock 1.7 valves were a hair small even for the 1.7. They were like 39 x35. If nothing else, since you are going to a 2.0 liter, I would at least change the seats and add larger intakes.
51cc sounds just a hair large for a 1.7....but not out of line to where they could end up. It will depend a little on valve margin, plug etc. But generally they were 48-50cc on average.
The 15cc dish is whats getting you in the compression area. Those are bus pistons generally. Good...but low compression. If you had a choce, I would see if I could find some 7+cc sishes. there was a shallower dish available I believe.

Amother odd option is to chuck those pistons into a lathe and take about .050 off the piston edge. That makes a shallower dish. Then, either have the cylinder bases shaved in the lathe...or shave the case. That way you can adjust the deck to .040....and have higher compression using no shims and no gaskets. Ray

[vwbill]

Hey Ray, it looked like only the 914 2.0L 3stud had 3mm bigger valves?
http://www.tunacan.net/t4/reference/cylhead.htm
So, looking at the displacement chart it looks like 1871cc would be what he would have if the stroke was the same?
http://www.tunacan.net/t4/reference/displace.shtml
So does the dished area basically give him a 1800 EC type motor with 7.3:1 like this says?? http://www.porschedecals.com/history914wcodes.htm
So what is the stock deck height?
I guess I dont see the change to 8.2:1? bill

[raygreenwood]

There was no change to 8.2:1. It was there from day one one fuel injected 1.7's only. The 1.8 with 7.3 was a dished piston with 1.8 heads...which have larger combustion chambers as well keeping the compression low.
The 7.3 or 7.6:1 1.7 was with dished pistons, but the same 1.7 head. There was only one 1.7 head.
Since 90% of all type 4 heads need new seats...its just simple to replace them with larger seats and valves . It costs nothing more.

There was more than one dished piston for 2.0. The 15cc dish is for buses. Not that its bad...just that its a big hole in the piston. It makes it near impossible to get decent compression on anything without deep flycutting of the heads. Ray

[Deadhead]

I cc'd the next cylinder and came up with 8.3655 compression ratio. The only difference was the head cc. This time I could only get 50cc in. The deck hight and the piston notch were the same. I calculated the 8.14 with 52cc since it was close. I'm gonne re-measure number one tomorrow.

[vwbill]

Hey Ray, I was trying to understand how they got the comp ratio from that chart tuna had vs. the displacement? If the stroke was still stock 1.7L. Why do they show all the heads basically with the same size valves? It only looked like from that chart that the three stud was different? I see what you were saying about if you were doing the seats do the valve change to increase flow!! So if you increase the volume of the piston but the valves are still only a 1.7L movement you will still be at that limit it seems? It so funny how a dish vs a domed piston make such a difference when you add it up over the four cylinders! I wonder how that would effect the mileage over all? It funny how I always thought of increasing the jug size would make more power! I'm sure Jake has tried the combos and the results! It would be nice to have a dyno and try a bunch of different combos! I wonder how with gas so much and using reg vs prem. effects the performance? Bill

[vwbill]

Hey when you measure deck height do you go by the edge of the piston to the top of the jug edge? Do you have to worry about the dome going above the deck of the jug? What about the cc for getting the compression ratio do you subtrate the dome area from the cc number of the head? What would the dome equal in cc?? 15cc? So far I have .044 for the piston edge and the top of the jug and seems to figure out at 8.099 compression ratio? The pistons are 89.97 and the stroke is 66 and I the head cc was 51cc to 52cc? bill

p.s. Well I get piston to jug edge of .044 to .o467 and 51cc for the head;
the dome crosses the jug plane for that circle into the head valve receases?? I used between 5cc and 7cc for the dome and deducted in the equation. What number should I use for the bore, the piston lable or 90mm for a 1.7L?

[raygreenwood]

The domes are an easy way to mess up the calculation. First, you need to get a volume....for what the dome displaces . You can get about 98+% accurate by just measureing the height that the dome rises over the rest of the piston. You will notice that there is a "fillet" or curve at the edge of the dome where it tapers out to flat. Don't get too hung up on that.
Just measure the height of the dome. It is usually about .079" high and around 70mm in diamter. Treat it like a small cylinder of its own....with a height 0f .079" (2.02mm) x whatever the dome diameter is.

Now....once you have the volume for the dome....just write it down and set it aside. Now.....think about the piston as a flat top. You measure the deck...from the edge of the piston...not the dome. The stock 1.7 with 8.2:1 compression had a deck measurement of around .075" to about 079"....when using the standard .010" spacer shim between barrel and case....and the stock head gasket ring...which starts at .030 and compresses to .027-.028". So getting rid of the head gasket and shim...leaves you with a deck of about .035"

When you measure the cc'c of the combustion chamber and then add the deck volume of the 1.7L piston as a flat top to that chamber volume....you then subtract the dome volume from that...before finishing the equation.

You will notice the dimple on the edge of the quench area of the 1.7L combustion chamber. That is made to work with the dome to help "steer" the compressing mixture into the quench area. When you remove the gasket and shim...use clay to check and make sure you have proper valve to dome clearance wit hteh new .035" deck......which you should. This shoudl give near 8.5 or 8.6:1 compresion. I did the calculations in a thread somewhere. Ray

[vwbill]

Ok, so I'm confused? I get a deck height of .044 to .0467 at the flat spot of the jug and that's with the .010 base gasket? I get 51 to 52cc on the heads but have to guess with the dome area is for the subtraction? So what is the limit of the gap?? Will I hit a jug or head or valve?? Also what did I lose in the lapping?? Should I have added a head gasket? I thought we should not use a head gasket? Also can someone tell me the cooling door setup adjustment and what way is closed? So if I take the all slack out of the cable, is the door closed for the oil cooler and will open or should it be set in the middle of the travel arch? Which way is right if the thermostat has failed? bill

[raygreenwood]

Actually you are on the right track. There will be some variations from engine to engine.
Remember...on AVERAGE.....WITHOUT the shim and the head gasket you should have around .035"-.038" deck (give or take). So right now you have about .045" deck....WITH the shim. That shim is .010". Take that out...and you have .035" deck.
Don't guess on the dome. Its a really easy calculation. Take a straight edge and place it across the dome of the piston. Now...with either a caliper or a feeler gauge stack...measure the gap between teh straight edge and the rim of the piston. You may get a range of between .075"...to .081". Generally most KS pistons are right around .079" for dome height above the rim of the piston. Now...with a dial caliper...measure the diameter of the dome.

Here is the calculation example:

lets say the dome height is .079" (.079"/.0391"= 2.02mm)

So the dome height is 2.02mm . Lets say the diameter of the dome itself is 65mm.

(pi) x radius squared x height = volume. So....

3.141 X (32.5 x 32.5) x 2.02 = 6701.716 (divide by 1000)= 6.701 cc's

So a dome of 65mm by 2.02 mm....would give you a displacement of 6.07 cc's. You calculate your displacement by acting like the piston is a flat top...using the deck height around the edge of the piston...not measured off the dome. Once you have the chamber volume......51cc? Then simply subtract the dome volume from the chamber....because it protrudes into that chamber volume.

So you have a chamber volume of 51cc - 6.71cc= 44.29cc.
Then use your deck based off the piston diameter with a deck height of .035" (90mm x .895mm). Ray

[coaster]

raygreenwood
You sound like the right guy to ask this question ? 2 liter heads with 42&36 mm. valves. What is the combustion chamber volume? If you have answered this question 1M times I apologize, but I am new to these engines and want to build an 8.0/8.2 2liter with a flat top piston for my bus. I have an aversion to dished pistons from my small block days.
Thank You