How to De-Tune a Z

[JimmyZ]

Stoichiometric ratio (sp?) is 14.7parts air to one part fuel if memory serves me. (Modern engines can stretch the ratio due to design) Proper Stoich ratio gives us cars that run right and have the nice by-product producing lowest emmisions.

Rich = Change the ratio to more fuel per unit of air and it will foul the plugs. (Black soot on plugs/Black smoke from exhaust)

Lean= Change ratio to less fuel per unit of air and things will run hot. (Lean is very bad on a high compression engine) Going excessively lean can soften/melt pistons to the point of failure. Hot spots can lead to preignition or detonation. Normally fuel expands at a certain curve but when the mixture detonates the expansion becomes more of a jarring/slamming effect on the engine's parts. In race engines the parts are already loaded and the above average heat decreases their load carrying ability. Throw a few shocks to the system and something will break. Torsional vibration from detonation can trash a crank.

It is interesting that some people in the past used water injection as a means of curbing detonation during peak power. Some combat aircraft in WWII used it. They had a minutes supply of water but that was often all they needed to outrun the bad guys etc.

The beauty of EFI is that when it runs right the mixture is appropriate for the engine's demands. Modern engines have a sensor in the block which "hears" detonation and automatically retards the ignition timing ten degrees or so to keep from hurting anything. In our Z's we don't have this luxury.

Jmortensen knows what he's talking about. It might not hurt to have the FI system looked over. There is also a great book on modifying Bosch fuel injection. This book explains many testing procedures and how things work. If I had a 280 I wouldn't be without it.

http://www.amazon.com/Modify-Bosch-Injection-Motorbooks-Workshop/dp/0879385707

[geezer]

After reading through this entire thread a few times, let me ask if I have this right. This engine was not built with the intentions of being a daily driver. There are probably enough mods done to this engine to make it economically unfeasible to tone down for practical street use. That does not mean there isn't someone who would be interested in buying outright or trading/upgrading engines with you. I agree, it is a pain in the azz & you would be better off swapping out engines so you end up with something more enjoyable to drive. I'm sure that engine was built as a complete package and never intended to be detuned. Fun toy but not a good driver.

[turbo 260]

Vicky,

Can you bring Zrush to the August event over at Jimbo's house? I have the factory EFI analyzer for Nissan fuel injection. I also have an innovate wideband that will tell us exactly what your car is doing. I'm sure we can get you on the road with pump gas.

Jim

[JimmyZ]

True geez!

It would be easier to have a "show" engine and an everyday engine.

[Zrush]

After reading through this entire thread a few times, let me ask if I have this right. This engine was not built with the intentions of being a daily driver. There are probably enough mods done to this engine to make it economically unfeasible to tone down for practical street use. That does not mean there isn't someone who would be interested in buying outright or trading/upgrading engines with you. I agree, it is a pain in the azz & you would be better off swapping out engines so you end up with something more enjoyable to drive. I'm sure that engine was built as a complete package and never intended to be detuned. Fun toy but not a good driver.

How True, How True,

I just don't want to open the engine and replace to stock parts. Funds are a problem, but so is $9.50 a gallon for a Z that MIGHT get 10 mpg.

Truly sucks for me now. I love the car but can't afford to drive it.

Z Painz in the wallet .

Vicky

[JimmyZ]

Swapping engines isn't so bad. (My Z takes less than 2hrs to swap) Just have a few people handy to help keep from scratching when moving engines in that beautiful Z of yours. Not being able to enjoy such a fine ride is a crime.

These cars can get you plenty of tickets and enjoyment with the stock motor. I almost lost my license twice when I first got the Z. My worst nightmare was coming home through Georgia doing 110-120 and seeing my ENTIRE rear view mirror consumed by a tailing state trooper. (Roads were entirely clear at 5AM where I was.) I was screwed and wondering how soon I'd be making my "one phone call" but then he passed me up!

Dropping a few hundred for a spare motor makes good sense to me.

[jmortensen]

How True, How True,

I just don't want to open the engine and replace to stock parts. Funds are a problem, but so is $9.50 a gallon for a Z that MIGHT get 10 mpg.

Truly sucks for me now. I love the car but can't afford to drive it.

Z Painz in the wallet .

Vicky

There is NOTHING you can do to an L28 that should make it get 10 mpg while driving casually. If you're getting 10 mpg, something is WRONG.

[Tony D]

I have to agree with JM on that last point! Well....maybe with Webers. Mine got 12mpg routinely, but admittedly it was not driven 'casually'...

But my Triple Mikuini Blowthrough setup in my 73 averaged 17mpg being hammered through Carbon Canyon to and from work in Brea from Corona every day for three years+ That car had 350 RWHP, and on open-track events could get as low as 5mpg when actually USING 350HP on a consistent basis. Hard daily commuting the mileage was 17...

I would say something is wrong if 'casually' driving it the thing is only getting 10mpg.

From what I am reading here, it's a mix of gasoline formulation incorrect for the carburetion---that will cause you to run with more throttle to get the same acceleration over the stumbles.

And this 'black smoke out the tailpipe' : Air Fuel Ratios in the 9's or worse. 10:1 AFR's will not puff black out the tailpipe from my experience, and with the fuel mileage I'd say something was seriously wrong with the Carburetion. My guess is there was a lot of overjetting to cover transient flat spots, or other bad manners. "Fat is Driveable" was the axiom, but it's not right from a serious tuning standpoint.

[jmortensen]

Tony if you look at that webpage with the description of the car, it says 60mm throttle body is one of the mods. It's still running stock injection with a larger cam. My guess is that someone put a RRFPR on the thing and has it cranked up way too high...

[Tony D]

"Carburetion" "Fuel Delivery System"--tomato tomatoe...same series of things affect it.

I somehow missed the website. But if it's puking black, and it's EFI, then fueling is definately incorrect---and ditto again for the mileage!

"Stock Injection with Larger Cam" is a recipe for terrible drivability. They don't work well together at all. That would go hand in hand with the drivability issues...possible injector swap there overfueling and the typical Bosch Band-Aid approach to fueling something not-stock.

Sure you can drive it, but it's nowhere near 'right'...

[Tony D]

Oh, this is 'Scott from Florida'---I found the post. I have seen this car, firsthand, in the flesh.

This car should NOT need anything other than regular pump gas, without a doubt!

This is a problem in the EFI system dealing with mismatched components or a sensor that has failed...the typical old stock EFI stuff.

"BOG" in these EFI Cars can be as simple a matter as misadjusted AFM causing a delay in the throttle response.

But the 460 Cam is not the friend of the Stock EFI system.

I'd suggest trying Megasquirt (programmable EFI) but at this point it may just be compounding a situation based on inexperience/unfamiliarity with the EFI systems as a whole.

This car is 'not' built so radically as to 'need' anyting but pump gas.

[sblake01]

Those '40 over flat top pistons' would raise the c.r. about 1.5 points by themselves. Add in the 'Head shaved' and it climbs even more depending on how much it was shaved. That cam might not work well with EFI, though Schneider says it will, but it's the same cam they recommend for carbed cars. It's the mildest L series grind they sell but the duration seems a little hign for basically stock fuel injection. A 60mm, probably 240SX, throttle body won't work well with a stock AFM. I'm assuming the AFM is stock since there is no mention otherwise. I see no mention of larger injectors either. It might just be that all this engine needs is a stock throttle body, a stock fuel pressure regulator, and a milder if not stock cam. I would be a little concerned about the compression ratio possibly creating the need for higher octane gas, however.