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The Mitsubishi engineers initially choose a high octane fuel strictly to meet the parameters of a mathematically derived theoretical stochiometric mixture. When this was shown to be empirically correct, they used their hard-won knowledge to create this amazing vehicle. With the 999cc IL3, these superb engineers dared to push the conventional design envelope beyond what was previously known to work. The 451 was a revolution.

"To reduce friction, the tension of the piston rings
was made 60 % lower than that of conventional piston
rings. To permit the low-tension piston rings to be
used, steps including minimization of the cylinder-head
bolts’ tightening force and deep location of the
bolt threads were taken to minimize bore deformation.
Reduction of the contact surface area of the piston skirts
was also implemented to cut friction. Each piston has a
30 % smaller contact surface area and is 25 % lighter
than a conventional piston with the same bore diameter."

"For combustion enhancement, cooling of the cylinder
head was optimized to permit a high compression
ratio."

All this advanced technology necessitated a meticulously calibrated and injected fuel/air mixture that would burn evenly and progressively from the initial spark ignition near Top Dead Center, smoothly and as coolly as possible, down to the very bottom of the piston stroke, gaining maximum efficiency out of every drop of premium gasoline and protecting these magnificently-engineered combustion chamber parts from debilitating shock and improperly focused heat. That's what high-octane premium does. Low octane regular blows up all at once, placing more stress on these vital parts and creating higher, out-of-spec, damaging temperatures at critical junctures such as the tiny valve seats.
:)

Thanks to SCoA member "Ptero" for providing this.
 

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I am lucky Florida still has 93 octane available most everywhere. Other than the gas in the car at time of delivery, my car has never had anything but Shell 93 octane in just over nine years.
 

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My 2 cents... I've found the crux of the biscuit to be the damned spark plugs... The stupid projected nose ir"idiot"ium plugs make the engine gas/octane sensitive. I use non-projected cut-back electrode race plugs. These ignite the fuel charge close to the head and the flame front clings to the head and spreads much slower than igniting (or more like detonating) the fuel charge in it's center. Ir"idiot"ium plugs are for smog. They'll make sure the fuel burns completely, but what use is peak cylinder pressures at top dead center? The piston can't go anywhere and you're just stressing the engine. A slower burn gives you peak pressures when the crank is past TCD -- at an angle where you can extract some torque from the burning fuel. I mean, an engine is designed to do work -- that's what it's for, yes? These make a big difference when adding a turbo -- for that I ran a colder non-projected cut-back electrode racing plug. The smart ECU is always trying to advance the timing to the pinging point. Non-projected race plugs gives you some margin to avoid pinging when adding boost (I was able to add 12+ PSI on Kalifornia Pee Pee gas). Works well without boost too. As always, your mileage may vary.



These are the spark plugs I use:



https://www.briskracing.com/?keywor...uemart&view=category&virtuemart_category_id=0


I've got 2 smart 451's. One is a '09 that had a turbo on it for 6 years until it needed smogging, the other is new -- got it "New Old Stock" a month or so ago with 62 miles on the odometer...
 

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Mine were conventional plugs, too. Maybe that's a California thing?

The plugs were changed on mine at 3 years, maybe 9 or 10k miles on the odometer, per the service manual. So did you mean the spark plug type, change interval, or both?

The Ir type can last 100k, some reporting up to 200k miles, on Gen 2 Toyota Siennas as I see on SiennaChat. What’s so Ir(idiot)ium about them???

EDIT- My car was a Cali car too...
 

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I had iridium plugs in my 451 up until the point it caught fire. I didn't buy replacement iridium plugs but I have no complaints from the time I was running them.
 

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Most people running unmodified or normally aspirated 451 gas engines won't notice much pinging (note that not all detonation makes an audible pinging sound). Out in the OC, one of the Tollroads I drive on (the 73) has a long steep upgrade. Everyone uses the Tollroads like an Autobahn out here as it's rare that there's any CHP patrol on one, so with a normally aspirated 451 Smart it's pedal to the floor most of the time. And even with the pedal pegged, you're still being passed by gardener/landscaper mini pickup trucks -- their rakes and week whackers flapping in the wind. Normally aspirated you'll get audible ping with ir"idiot"ium plugs. I get this on that long sustained upgrade. With a turbo, you'd get audible pings any time you enter sustained boost. With non-projected nose race plugs on a normally aspirated 451, you can keep the pedal to the floor without any audible pinging. But, you'll still be passed by pretty much everything else on a long sustained upgrade...
 

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As always, there are many variables... Try running an aftermarket turbo on Kalifornia Pee Pee gasoline -- or even just a stock engine... Like most State-Mandated fuels, the stuff is great for smog but bad for engines. "Knock-Knock" "Who's there?" "Valve Job..." But, irrespective of the gasoline (turbo or not), switch to cut-back non-projected race plugs and you'll notice more torque...
 

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The non-projected cut-back electrode plugs I use on the Smart 451 are Brisk MR-12S silver electrode plugs... I typically gap these down to 18 thousanths. On a turbo, given the limited coil-on-plug ignitors, you need a small gap to light-off the much denser mix. But, even normally aspirated, I use this smaller gap to make life easier for those coil-on-plug ignitors. Jegs and others have these race plugs on their web pages, but at this time of year they're typically on backorder... This plug fits a myriad of BMW's and the Nissan 300Z (among others), so when the ambient temps get up into the 90's and the pingfest starts you end up waiting a few weeks to get them. I've tried looking for other compatible race plugs by Autolite and others, but so far this is the only one.
 

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The non-projected cut-back electrode plugs I use on the Smart 451 are Brisk MR-12S silver electrode plugs... I typically gap these down to 18 thousanths. On a turbo, given the limited coil-on-plug ignitors, you need a small gap to light-off the much denser mix. But, even normally aspirated, I use this smaller gap to make life easier for those coil-on-plug ignitors. Jegs and others have these race plugs on their web pages, but at this time of year they're typically on backorder... This plug fits a myriad of BMW's and the Nissan 300Z (among others), so when the ambient temps get up into the 90's and the pingfest starts you end up waiting a few weeks to get them. I've tried looking for other compatible race plugs by Autolite and others, but so far this is the only one.
Thank you "Gear head" I just ordered three Brisk MR-12s plugs.
I should have them installed by next Saturday.
Thanks again
Take care.
 

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The "cap" on the top of the spark plug should be screwed on. The MR-12S Brisk plugs have the same thread and thread reach as the stock plugs, but you'll need a different spark plug socket to install it than the one used to remove the stock plugs... From memory, you'll need a 14mm spark plug socket for the MR-12S plugs. Make sure the engine is dead-cold before removing/installing spark plugs. I typically use anti-seize on the spark plug threads, which makes future removal easy. They have "O2 and catalyst friendly" anti-seize now-a-days, but I've used regular anti-seize without issues.
 

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Thank You Gearhead,
I left the caps on and yes I bought a 14mm thin wall magnetic spark plug socket for the install. I always prefit new plugs to the sockets I have just in case.
I installed the iridium plugs last year so I knew I had a socket that fit them.
Thank you again for the timely reply.
Take care
JL.
:smartje.bl.zl:
 
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