compression ratio

[bulldog]

Hey guys, I'm getting my motor fully rebuilt and I have a question.

The compression ratio the previous owner ran was 10:1 which is pretty high for the old girl and I've been informed that this is fine if I want to run av gas all the time, which I don't. I talked to the previous owner and he informed me that he ran it on 98 and it was fine although the engine guys are saying looking at the pistions there was definately detonation going on.

So I have two choices, reduce the CR to 9 1/4 : 1 (reducing power and torque but be able to run on normal pump gas
OR
keep the 10:1 CR and good power and torque but have to run AV gas and or a mixture of all the time.

I don't have a problem with av gas if this was going to be purely a race car but I'll definately be using it occasionally for general road use too.

[zep]

reduce it. I doubt you'll notice a huge loss and av gas is ludicrously expensive / you're not alowed to fill your car up from the pump either.

Either that, or maybe your could use ocatane booster when you get some 98?

[bulldog]

Yeah I think you're right Zep. As much as I want to make the most power I can I simply can't imagine running av gas all the time...it's just not realistic.

[gordo]

Ah, but it's such an evocative scent ...

You may not need to worry about it, the block that ran the bearing had a negative deck clearance - the piston was higher than the deck - and a stock(ish) block will have them further down, reducing the compression, anyway.
Wouldn't be a bad thing to gain a few thou' clearance as the pistons were marked as coming VERY close to the head/valves.

[jaygti]

zep said:

Either that, or maybe your could use ocatane booster when you get some 98?

90% of off-the-shelf octane boosters do sweet Barry White all, so I wouldn't even consider that option either.

Just drop the CR, it'll still be plenty fast, especially compared to the 1256cc's of raw power it'll be replacing.

[bulldog]

I've got the dyno charts from when it was together. It was 170bhp and nigh on 700nm or torque! Barry white it's gonna move along nicely.

[zep]

Sheeeeit! Not bad. That's at the wheels? My turbo engine makes that at the fly at the moment!

[mikuni]

bulldog said:

I've got the dyno charts from when it was together. It was 170bhp and nigh on 700nm or torque! Barry white it's gonna move along nicely.

Ignore the torque figure. They are not acurate. I don't know why but its something to do with the gear ratio or some nuts. But thats quite good power, should keep up with me

Bring it to oldschool trackday ;D