speedtigger

iTrader: (16)

I have been toying with the idea of CNC ported heads on my LQ9. The ported 243s from Texas Speed looked attractive in flow and price but I was concerned about the smaller combustion chambers because it is a regularly driven street vehicle. I want to make sure the compression does not give me detonation problems with pump gas.

I called Texas Speed to ask about the heads and they said it should raise me about a half a point. This did not seem right to me sound a I found a handy dandy online static and dynamic compression calculator and plugged it all in. Here are the numbers:

LQ9 cylinder head volume 71.06cc
243 cylinder head volume 64cc
bore 4"
Stroke 3.622
piston volume 1.2cc
gasket thickness .052
gasket bore diameter 4.040
deck clearance -005 (.005 above the deck)
rod length 6.098
intake valve closing 68 degrees after bottom dead center (222/224 comp cam)

I came up with 10.08:1 static compression ratio for the factory LQ9 which is what it is advertised. With my cam this yielded a dynamic compression ratio of 7.83.

When using the 64cc combustion chambers of the 243 heads the compression jumped to 10.94:1 yielding a dynamic compression ratio of 8.48:1.

From what I have read, 8.5:1 dynamic compression ratio is the upper limit and hairy edge for pump gas.

To my thinking the ideal situation would be about 10.5:1 with a 67.5cc combustion chamber yielding a dynamic compression ratio of about 8.14:1.

So, if I am going to do this, I am thinking I would be better off either porting my factory 317s or maybe working the 243 combustion chambers up to 67.5cc

What say you?

Here is the calculator that I used: http://www.kb-silvolite.com/calc.php?action=comp

ss454327

iTrader: (10)

It would be easier to change your cam honestly...

intenseblue

iTrader: (1)

8.5 from what I've read should be fine on premium. I have also heard that the TSP heads are around a 67cc head anyhow after the cnc work is done on the combustion chambers

speedtigger

iTrader: (16)

If that is the case, we have a perfect scenario.

Bumpers

iTrader: (10)

I don't know if this helps you but I have. 243s milled down to 59cc chambers on my lq4 should be around 10.8-11 to 1.. Cam is a 226/230 .595 .600 113+4.. It runs just fine on 93 haven't had any problems yet (bout 500 miles)

01ssreda4

iTrader: (96)

You'll be fine, like already mentioned, the chambers should be opened up a bit with the port work.

outkast6991

iTrader: (34)

using that link my motor comes out at 11.2 with 64cc heads and the pistons out .008. it's also 9.4dcr!!! car runs good and made decent power so i guess the 8.5 deal isn't the limit

speedtigger

iTrader: (16)

If you have a stock shortblock lq9, your numbers should be the same as mine for static compression.

outkast6991

iTrader: (34)

it's a stock lq9. all info was measured during the build

speedtigger

iTrader: (16)

Interesting. Plug in the stock 71.06 combustion chamber volume and see it you come up with 10.08:1 which is the factory spec. If not, I would assume the difference is in the piston volume. What did you enter for piston volume?

outkast6991

iTrader: (34)

0 for flat tops. pistons were measured at .008 out. running a .051 gasket

speedtigger

iTrader: (16)

While I understand that they are flat tops. Their volume is not really 0cc. The area around the ring land has to be accounted for when figuring the compression ratio.

Since we know the final compression specs are supposed to be 10:08:1. You could probably put it whatever piston volume number in that makes your compression on a stock motor come out to 10.08:1 with a 71.06cc head. This is what I did because the only specification that I could not find online was the actual piston displacement. To actually measure this, you have to put a little grease in the top ring land and fill the ring gap with grease so the measuring fluid does not leak out, then physically measure it.

Using the method above to make sure that we had the proper compression ratio with a stock head, I then just changed the head volume to 64cc and came out to 10.94:1. Not exactly ideal, but I think it is a good alternative when you don't know the actual piston volume and, in my case, the precise deck height.