G161 head + G200 block

Benjamin · backyard mechanic Joined: 05 Oct 2001 Posts: 525

Hi Geese and Gannets,

I would rather post this in R&D to get a decent answer, but the rules prevent. Please no stupid answers Smile

I have a TX G161 head with a mild cam grind. The head has been shaved close to its limit so I am told (been running it on G161 for 6 years reliably). The G161 has now developed a rather large blowby problem (80% loss on 3 cyl, 40% loss on the other). The head is still in decent condition.

I have just purchased a G200 for $250 and plan on installing that with the G161 head. I will probably reco the G200 in the process, something to freshen up, not looking to spend heaps.

I have searched/trawled the forums, and dug up some very interesting information, although there are mixed reports on the effects of putting G161 heads on G200 blocks.

Reports say anything from 10:1 to 10.8:1. Bearing in mind my head has been shaved close to end of service, I would be guessing the upper end of the figures is more accurate. I run only PULP, with an electronic ignition soon to be regraphed.

My main question is (multiple parts), what is a decent compression figure for a streetable car on PULP? What is the best method for taming what will most likely be a high compression?

Do I:
a) Source an extra thickness head gasket? Copper?
b) Source a new set of pistons with a slight dish? Can anyone suggest a brand?

I really need this sorted, I cannot afford, and do not have the time for failed experiments, this car is my daily driver and it needs to be reliable.

If anyone has more info that here, email me @ benjamin@fuckmicrosoft.com

Thanx and cheers
Me love you long time

rob_w · tinkerer Joined: 19 Feb 2002 Posts: 395

well, your best bet would be:

freshen up the g200 [rings bearings,hone ect], and if the pistons are good, then decomp it with a thicker head gasket [copper]. all you need to do is work out how much has been ground off the head in total, and add it to the normal thickness of the head gasket.

it the pistons are destroyed, then you could go dished pistons, or stick with the g200 head, and use domed pistons. but in this scenario, well, its entirely up to you.

hope this helps you some,
rob.

Benjamin · backyard mechanic Joined: 05 Oct 2001 Posts: 525

Cool thanx Rob.

Six of one, half a dozen of the other really. I know this head has been taken to its limit, so not a lot of meat is left. I would be more inclined to go for a copper gasket, but worry about reliability, how do these things last?

spud · petrol head Joined: 05 Apr 2002 Posts: 1106

10.5:1 is about right if it is a virgin head. you would be looking close to the 11:1 area if it has been shaved a couple of times. if you want to lower compresion you can get your pistons decked if they are still good. or a copper gasket or both.

Nick_H · tinkerer Joined: 12 Oct 2001 Posts: 76

i use a copper head gasket on my turbo gem. i got a 2mm for about $190 i run 14psi boost. i was told but the guy who did the machine work on my motor that he has never seen one fail. i used a 2mm to lower the compression and the block is "O" ringed. if your rebuilding your bottom end maybe you should do that too. mine cost me about $35 to "O" ring all 4 cylinders.

Nick

flatchatcharlie · rice boy Joined: 08 Nov 2002 Posts: 38

I just got my g200 back from the machine shop have not worked out the comp yet but the pistons are about 1mm down from deck height I'll be ccing the head tomorrow to see what the comp will be I'm fitting the g161 big valve head with 32/36 webber and I'm trying to make both barrels open together but its not as straight foward as it seams.I'll let you know about the comp once done . Cheers

Ben Wight · backyard mechanic Joined: 04 Oct 2001 Posts: 946

To work out what compression ratio you will end up with, you really need to measure the volume of your combustion chambers, or get it done for you.

Whatever you end up with, anything over about 10:1 and you will need to reduce your ignition timing to avoid detonation, which sort of counteracts the power benefits of high compression, overall though, you should make more power with more compression to a certain point.

You have a couple of options;

1. Keep the high compression, run it with reduced ignition timing on optimax, and then when you want to go a bit faster, put in some race fuel or octane booster, and turn up the ignition timing. This gives you a bit of a power increase with minimal work if required.

2. Reduce your compression so you get good power on premium unleaded, with decent amount of timing. You can do this a few ways, get the 2 litre pistons dished, run a bigger chambered head, or slightly thicker head gasket.

flatchatcharlie · rice boy Joined: 08 Nov 2002 Posts: 38

O.K just c ced my TX big valve head 43 cc's put that on a g2oo bottom end up 60 thou and thats 10.46 to one thats the figure I come up with any way