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Datsun#1

Stock cam high CR.

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Datsun#1    0

Hey, had a thought and ive searched but found nothing in regards to my thought.

 

Say you have a 280z, stock engine with a carb swap on it. Access to 100oct fuel and someone to dyno tune the car.

 

What would be possible as far as a high compression ratio with a STOCK cam. No timing retard.all things being equal,just flat tops and a slightly shaved head (say to straighten it). What would be the results as well as the final CR? just curious

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Chickenman    4

I have an F54 with Flat-tops and an N47, slightly shaved. CR is in the 10.3 to 10.5 range. With the stock camshaft the engine was very prone to detonation and was hard to run on anything but 93 or 94 Octane. Timing at 32 degrees. 

 

Car has headers, 60mm TWM Throttle body and 2.5" exhaust.. After engine ate itself on a trip down to Cali, I had a new one built in Portland. Same specs only this one had a Schneider 280 cam. That really woke the engine up and it will happily pull past 7,000 RPM ( Head was completely rebuilt as well with new springs ). Stock cam ran out of breathe around 5,800 rpm. Can now bark tires in 3rd gear and it is so much more fun to drive now. Has that lovely 6 cylinder " Big Cam " Lope at idle ( 1,000 RPM ). You're giving away an easy 30HP by running the stock cam. 

 

I can now comfortably run 33 -34 degrees of timing on 91 Octane gas. I do get some audible  pinging between 4,500 and 5,500 RPM on 91, but that is a slight  lean spot in my fueling. Really hot days it likes 94 Octane better though. 

 

High CR ( over 9.5 ) and a stock cam is a waste of time on Pump gas. You can get away with 100 Octane fuel, but the question is... why?  Unless it is a temporary thing there is no point. You won't make any more power than a 9.0 - 9.5 CR engine ( Stock camshaft is the limiting factor ) and you are just throwing money down the drain buying 100 Octane fuel. 

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JMortensen    235

Totally agree with Chickenman. I'd rather have the engine built with dished pistons if the cam is going to be stock. If you're going with flat tops, get a bigger cam and do the work to make it function (shorter valve stem seals, different springs and retainers). Only disagreement I have is that I'd have gone bigger with the cam than he did. Otherwise, like he said, you'll be retarding the timing to the point where you're leaving a lot of power on the table to prevent pinging.

Flat tops and N42 with a minimal shave just to get it flat would put you in the 10.5:1 range or close to it. N42/N47 has a crappy combustion chamber. It's going to ping unless you run a big enough cam.

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clarkspeed    10

I agree with above. At 10:5 the stock N42/N47 is on the hairy edge of 93 pump gas no matter what cam you choose. Unless you can get 100 octane for free, not worth it. Otherwise leave it stock and just work on freeing up intake and exhaust.

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