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A converstion about how much compression is too much for pump gas


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Zedyone, I am in Pearland, TX and am at the end of a similar N42/N42 w/flattops rebuild, albeit not with Enji, and with EFI. Initial startup is today with break-in over the next few days. Thanks for starting the thread and let us know how things go from here. I'll do the same.

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Great to hear form a TX guy. I may have to swing by to see your ride very soon!

I'll let you know when it's back home. Everyone says the stock Bosch EFI will not play well with anything but a stock cam, though Motorsport says it will with the milder Stage 1/2 cams. I've got a custom grind on a Nissan OEM cam at about .485/.270 with somewhere around 10.4 - 10.5:1 CR, so I'm about to find out who is right. I may be buying a case of octane boost soon:ermm:.

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"Holy horsepower Bat Boy".....don't ever say that! I run the .080 inch cut P79 head which is 10 to 1 CR just like yours. The P79 head is a more modern head, but shouldn't make that much difference. Z Doctor has told me that that set up won't have detonation because of the higher duration and lift of the Isky Stage II and Stage III cams that I run (don't know if that's true or not, but I believe what Doc tells me). Stage II is 280 degrees and .480 lift while the Stage III is 290 degrees and .490 lift. At 17 degrees timing at 1000 rpm, I can't make either engine ping, under any load. Seeing as your engine is close to my engine in just about every aspect, what is the lift and duration of your camshaft? Maybe that could be the difference, or maybe the increased deck height of your pistons are putting you closer to 11:1. If the engine runs good on an additive, so be it. It's your cheapest alternative. We can buy 100 octane race gas in Richmond, but it's $7.00 per gallon, which still makes the additive your best bet.

Guy,

Your engine is not just like Stephens- there are big differences. The biggest is the p79 head with high quench compared to the open chamber n42.

Combustion chamber design is everything when it comes to detonation. Stephen is running and old design open chamber more prone to ping . His pistons aren't to high either if you look at the numbers- there actually too short. All these little or big things depending how you look at it - add up to why one engine acts differently . The n42 is a great all out race head if lots of work is done to it, but the p79 and 90 heads are much better street modded heads .

The serious Z engine builders don't give out all their secrets, but if dig around over at hybridz you can pick up some good stuff .

Edited by madkaw
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Right Steve......I notice the plug location is right in the center of the head chamber. I guess that's what makes it better, plus the peanut shaped combustion chamber pushes the compressed gases to the plug. I'll have to take other people's (smarter than me) word for it though.....I just do what Z Doctor suggests (except for my L24/L28 combo) with my builds.....then I put them together. I'm picking up another P79 modified head from him in Roanoke next week.....I'll run Stephen's numbers by him and see if he can shed light on the subject.....he's driving the bus.....I ride in the back! :ermm:

Edited by Diseazd
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I know nothing about Datsun Spirit or the owner and he seems to be liked, but I have reservations about Stephens build . I'm not knocking anyone but this thread is about avoiding detonation or why we have it.

Of course not hardly anyone brings the important numbers to the table(HP) except 30 ounce. That engines sounds like a hoot!!

My L24 put out 155 rwhp on 2 different dynos, so I think the numbers are accurate . My early e-88 head , similar to the e-31 has 41cc chambers. CR is around 9.55 . My engine builder who builds spec Mazda race engines never did an L motor. Just by looking at the chamber design and knowing my smaller cam profile he advised staying under 10. The only ping I get, and it's not all the time, is around 3000rpm. This seems to vary by what has I use also- as in name brand.

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I have scheduled some dyno time on Friday morning to see what my car puts down. I will bring some jets and a timing light with me! Should be fun, but I am dealing with a shop full of Brotatoes who only work on LSX engines and Vipers.

Out of curiosity what are your reservations?

Edited by Zedyone_kenobi
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Blue.....When you shave a head .080 inches without shimming the towers, how do you adjust the chain slop? With the P79 shaved .080 inches, we use the early valves (.080 inches longer) and shim the towers and springs up .080 inches to end up right where we started. Just curious with how you make the adjustment on an N47 shaved that much? Guy

Edited by Diseazd
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I have scheduled some dyno time on Friday morning to see what my car puts down. I will bring some jets and a timing light with me! Should be fun, but I am dealing with a shop full of Brotatoes who only work on LSX engines and Vipers.

Out of curiosity what are your reservations?

-Is this meant for me?

Hopefully they will appreciate a very nice sports car!

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Blue.....When you shave a head .080 inches without shimming the towers, how do you adjust the chain slop? With the P79 shaved .080 inches, we use the early valves (.080 inches longer) and shim the towers and springs up .080 inches to end up right where we started. Just curious with how you make the adjustment on an N47 shaved that much? Guy

EZ PZ Just advance the chain a link and take up the slop with the tensioner and guides. The goal is to synch the cam with the crank. Using TDC and the marks on the cam, you can do this by lifting the cam with shims or by moving a link more on the timing chain. Works fine.

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