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L18 comp ratios for street motors


slowlearner

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Seems like the easiest thing in NSW (the nanny state I live in) is to stick with an L18 in my 510. I think turbos are out for now. So I'm thinking of internal mods to make the engine more efficient and peppy. I've got EFI going on it and a decent cam in one of them, so I'm wondering how much compression I can safely run without needing to use E85 (which is not easy to get here). Are L18 heads given to hot spots? What sort of ignition timing would I want? Is there anything else I should know?

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On a stock open chamber head such as the A87...

 

The L18 pistons stop 0.55mm below the top of block so mill the deck 0.5mm for an 8.84 compression

Mill the head an additional 0.5mm and the compression jumps to 9.7

 

L heads are not known for hot spots. I would use sand paper and round off any corners and edges inside the combustion chamber. This reduces the likely hood of a sharp edge becoming a hot spot.

 

Higher compression will likely require a cooler plug. NGK B5ES?

 

Higher compression shortens the burn time so less ignition advance is needed. How much? No way to tell, just keep backing it off till the pinging is gone.

 

You may find that the increase in efficiency when used with a premium grade of gas can increase mileage enough to off set the extra cost.

 

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It really depends on the cam and the tune. I have run 12:1 in L motors on pump gas, but it's not advisable. If you're willing to spend the time to tune it to the max, go big, but for the average guy, anything over 10:1 is high.

 

10:1 is the target for your average hot street Datsun motor.

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Thanks guys. I do wonder what my club race motor has in it. Whether he bumped up the comp ratio or not is unclear. I don't think he could remember but it's got a SSS head with the big ports.

 

I've got another A87 head that's milled down to the letters. So it might be worth having a look at. 

 

Fuel wise we've got E10 (94 octane), 95 oct and 98 oct. I'd like to keep to E10 if I can. I know with NA motors you can run lower octane fuel for better power. 

 

 

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4 hours ago, slowlearner said:

Thanks guys. I do wonder what my club race motor has in it. Whether he bumped up the comp ratio or not is unclear. I don't think he could remember but it's got a SSS head with the big ports.

 

I've got another A87 head that's milled down to the letters. So it might be worth having a look at. 

 

Fuel wise we've got E10 (94 octane), 95 oct and 98 oct. I'd like to keep to E10 if I can. I know with NA motors you can run lower octane fuel for better power. 

 

 

 

A87s were open chamber heads but some were closed in Japan. You can port a closed chamber head to make it almost a 219 head. A87 heads were used on L18 engines and the very first L20B application the '74 610 in North America. None would be closed chamber on the 610 because of the compression and emissions requirements.

 

SSS heads are again a 219 'number head' used only on the L16SSS. Later L18SSS may be 219 or likely A87 one the right side but with 219 cast on the left front of the head near the thermostat housing. A closed chamber head such as the 219 on a stock L18 will produce a compression of 9.0 where the 'stock' compression is 8.43. If this is a true SSS engine the additional flattops with the closed chamber head produces 9.68.

 

NOTE:

True SSS 219 heads were available from Nissan as an after market head but were all used up for racing. The demand was high enough that they re-issued a new casting called the 912.

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The head "milled down to the casting numbers" - that doesn't mean anything. Typical casting shift caused those numbers to move periodically, so just because half the numbers are cut off, it could be an un-cut head. The only way to tell for sure is to measure the head casting thickness.

 

 

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