Compression

[QldRobbo]

G'day
I am planning on taking my 1600 and boring & stroking it out to 2L specs (same bore and stroke as a G200). I have a 1600 big vavle head that I will put on the block. My question is, how much compression would be optimal to run with PULP? Such as, will putting flat-top pistons make it too high, or will I need to shave the head to bring it up to an optimal level?

Also, will I need to do something with the head to run on unleaded?

I searched the forum before posting this, but couldn't find any useful answers, other than identifying the head in my shead as a big valve head (has 00 markings on it).

Thanks heaps

[Frederick Stienheld]

You will have to change your valve seats to run unleaded and set your compression to suit unleaded also

[QldRobbo]

Hmmm, funny, I thought I asked *what* I would need to do to get the compression that would be optimal for PULP (Premium Unleaded Petrol).

Any help anyone?

[daCoupe]

fuks me,i run pulp,or optimax,in my coupe

[NUT161]

big valve heads can also be identified by no isuzu symbol near the front hose and they also have 7, 14 or 21 stamped on them above where the engine No. is on the block

[Ben Wight]

AS far as 2litre internals in 1600 block, you may know more about this than I do, but I would guess that due to the shorter block height of a 1600, that you would maybe run 1600 rods with 2 litre crank and pistons? I am pretty sure I have heard of a 1800 crank with 2 litre pistons in a 1600, cant remember what rods.

Anyway, with the shorter rods, it will generate a lot more stresses than the actual 2 litre block would, tending to want to bend/break rods and not rev as high. But I guess you want to keep a 16oo block for insurance/legal purposes.

As far as compression ratio, you need to know the size of the combustion chmabers of your head, only then can you work and what compression ratio you will have with flat tops. Compression ratio is simply the volume of the swept path of the piston plus the volume of the chamber + head gasket, all divided by the volume of the chamber and gasket.

Depending on the size of the cam you run, (as this effects the dynamic compression ratio of the engine, ie the bigger the cam, the more compression can be run) you should be able to crank it up to 10.5:1 whilst using PULP and still gettting decent ignition timing.

To work out how much to shave the head, well thats what you have to decide after doing some measurements. Keeping in mind that the average head gasket has an internal volume of 6 cc, and are approximately 1mm thick, so 1mm off the head might yield 4-5 cc, as the chamber diamter is not as great.

You can physically machine down to the valve seats, but this will stuff up cam timing and a vernier cam timing set will need to be used. Also keep in mind that valve reliefs in the pistons will need to be cut.

AFAIK, all alloy heads have a hardenned valve seat just like any other motor that runs on unleaded from the factory.

Ben Wight

[Gold TX Sleeper]

I was speaking to the guy that just re-did my head for me, and i asked him if i could still run unleaded or premium after he did the 3 angle valve job on my head. I wasn't sure if the valve seats were still hardened after a mod like this, he advised me that he could recut the valve seats a dozen times and they would still be hardened. And i also asked if running unleaded originally was a problem, he told me that i could run it 100% of the time without any problems at all and he guarenteed it. I should also point out that this bloke has been in the cylinder head business for almost 20 years, so he definately knows his business. So to anyone else that wants know if a Gemini can run unleaded - for the ten thousandth time YES!! And that's from the mouth of a professional head specialist.