Emcon5 ? 3.4 numbers?

[rdane]

Originally posted by emcon5
What were your dome and head CC measurements?

What is your deck height?

Tom

Tom, I saw that you asked this a couple of times and hadn't had the chance to answer.

One of the things I just got clarified this morning was my new engine's compression and how I got there. I had bought 9.8:1 Max Moritz pistons for a 3.0 to a 3.4 swap. In the excitement of building a 3.4 I somehow ignored the fact that with the longer throw of the 74.4, 3.2 Carrera, crank I was going to up the compression a good deal from the get go. My compression ended up being 10.67:1 measured and will require twin plugs in the end.

Happens when an amatuer gets involved. I should have cut the piston tops back down to 9.8:1 or slightly lower instead of adding a ceramic coating is my take on it now. Good lesson as it could have been worse.

V1 = 561.2cc cylinder volume at BDC (which gets you a true 3366CC instead of the reality of a 3.4)

V3 = 58.0cc cylinder head volume at TDC

Compression ratio (actual measure, not calculated)

V1 + V3 divided by V3 = 10.67

exh valve clearence is 3.375mm
int valve clearence is 2.75mm
cam timing 1.9-2.2mm

Have yet to get the deck height.

[emcon5]

That is different than the way I measured my CR.

From Bruce Anderson's book:

V1+V2+v3 - V4
--------------------
V2+V3 - V4

V1= swept area
V2= deck volume,
V3= head volume,
V4= piston dome volume

V1, 9.8cm bore, 7.44cm stroke = 561.2cc
V2, I used 1mm, so 7.54cc
V3, My head was 90cc.
V4, ?

For your "V3 = 58.0cc cylinder head volume at TDC" how did you come up with this number? Did your CC it assembled at TDC, or subtract the dome volume from the head volume?

My old Moritz pistons had a dome of 27.2, Yours would be a little higher because of the valve pocket machining on mine. Unless you had the heads cut, yours should be similar, maybe 1 or 2 CC smaller because I had the machine work for the second plug.

It sounds like the big variable here may be the deck height. If the piston pin ofset is different and you are running close to zero deck (or a negative deck height), that makes the math come out to what you measured.

How did you come up with your V1? It doesn't look like the deck volume is factored into your CR at all. This doesn't seem right, as changing this number changes the CR.

Tom

[rdane]

Deck height with a 1.0mm shim was 0.99mm.

I am just repeating the numbers I have been given by the builder so I ma not sure how they came u with the numbers.

One cylinder was assembled. I also had the machine work done for a second plug.

[emcon5]

Ok, so assuming your head volume is the same as mine, which isn't unreasonable, and your deck vol is known, the only variable is the dome volume. To get the CR that high, the dome would have to be close to 40CC.

For comparison, my 98mm RSR pistons have a 37.3CC dome. Strange.

Now that I think about it, if your guy CC-ed the assembled cylinder, at TDC, that measurement would be the same as my V2+v3 - V4.

If this is in fact what he did, we can use the 58.0cc number and work backwards. 90cc head, + 7.54cc deck - "X" dome = 58cc. X = 39.54 Punching these numbers into my CR spreadsheet, it comes out to 10.672:1.

Still, that dome volume seems really high.

Tom

[rdane]

Tom your engine is a 3.2 @ 9.8:1, twin plugs and CIS correct? Have you dyno'ed it yet?

I have been told now by two engine makers that the crank made up the difference on the compressions. One of them mentioned the Pauter rods adding ot the crank. The othere engine builder who has seen a lot of these (both 3.2 and 3.4 form Max Moritz P&Cs) go together says that the 3.2 crank and the 98s are usually put together intenionally to get the higher compression (10.5 or better) for twin plugs. Not my original intention but it'll work.

But I agree the 39.54 cc seems pretty high when comparing a wedge piston to a dome shaped RSR @ 37.3cc. My take is the dome does't fill the head up as much as I might have thought and I suspect the burn front is better on the wedge.

Either way I am just guessing.

[emcon5]

Originally posted by rdane
Tom your engine is a 3.2 @ 9.8:1, twin plugs and CIS correct? Have you dyno'ed it yet?

Yes, with 964 cams and SSI exhaust. I haven't had the chance to put it on the dyno yet. It will have to wait until after I get it smogged.

What are your plans for twin plug?

Tom

[rdane]

Twin plugs? Down the road a bit......at the end of summer if I can deal with the new gas bill till then.

Right now, with no smog, it looks like a race gas blend at about 97/100 octane or $50 a tank