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Discussion Starter #1 (Edited)
There is a very common tuning trick in the world of small engines/go-kart racing, which is to run a longer-than-stock-thread spark plug, which protrudes further into the combustion chamber, effectively reducing chamber volume and causing a bump in compression. I have seen several sources claiming a dyno proven increase of .5 to 1 full hp on an otherwise stock 5hp engine. So it would seem that IF the same trick could be applied to something like the TiVCT 2.0, an increase of up to 5hp could be achievable.

My concern with attempting this has 4 parts.
First, is it even possible to get a longer plug that will screw into the cylinder head of the focus engine?

Second, will a longer plug risk making contact with the piston, or the valves?

Third, would a longer plug cause valve shrouding and airflow interference, which would negate any gains in compression?

Fourth, would a longer plug create a localized hot-spot, or result in compression that is just too high for the engine to handle, causing premature detonation and reducing power or even causing damage?



So.....Has anyone over tried this or given it any thought? Suggestions? Comments? Trying to start a discussion here, on something that may never have been considered as an option...
 

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The 2.0L TiVCT is already 12:1 compression, I would think any plug stuck in there that is longer than stock is going to strike the piston, I wouldn't risk it. I'm definitely no engineering expert, but I also think that the gains would be so minimal it would go unnoticed.
 

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think about hte size of the plug vs cylinder volume.

on a tiny engine, a bigger plug takes up a larger % of the cylinder increaseing compression as you say.

on a bigger engine the same size plug would have much less effect. even without the possible problems you mention.
 

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IIRC that 5 hp class for 4 stroke go carts is pretty limited for modifications, so ANYTHING allowable that might make a difference will be tried.

Isn't that still based on a flathead engine? With low compression and the plug located in the transfer area (not directly over the piston).
 

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Discussion Starter #6
Sailor,

No, most are running overhead valve honda's/chinese honda clones, which is primarily the engine I have seen this used on.
 

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Discussion Starter #7
thenorm,

Actually, the chamber volume on the honda clone engines is usually between 45 and 55cc. I'm not sure what the chamber volume is on the TiVCT, but I can promise that it is MUCH smaller, I would guess 35cc MAX.

So if anything, this trick would have a MUCH GREATER EFFECT on the Ti VCT than it does on the single cylinder engines.
 

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Discussion Starter #8
Tom,

You say you wouldn't try it for reason #4. Would you care to elaborate at all?

I know from reading this forum that the TiVCT is capable of handling a respectable amount of boost, so I don't think that a half a point bump in compression would have any negative effects on an unboosted engine.

However, as far as causing a localized hot-spot, could running a colder plug help? Have you ever heard of anyone trying this on anything other than a kart engine?
 

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The spark plug tip IS a hotspot relatively regardless of where it ends up location wise and no way you can get away from that, it even being useful to a point. Why it gets regulated by the plug insulator as a temperature control device.

That mod has nothing whatsoever to do with increasing compression, it works because the combustion chambers are archaic in design and anything that gets spark further out in the wind increases burn speed and the evenness of the burn. The compression increase is negligible. BTDT. It's about putting the spark deeper into the main mass of material you want to burn. I used to do it on race 2 strokes all the time. On Honda DOHCs when I used to race them shortening the plug tip to 'get rid of the hotspot' added a solid tenth (bad) to 1/4 mile dragstrip times. Why extended tip plugs were created. Look at some of the odd emission ones (mid '70s) back when they ran engines super lean, some of those tips were 1/2" long easy, to get spark deeper into the lean mixture which was harder to ignite. Depending on the intake valve location fuel/air sweep across a plug tip deeper into chamber can cool it off a bit too.

On smaller volume chambers the biggest issue will be physical striking of plug end. A plug end too close to piston material can tend to overheat it too.
 

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If a plugs hangs inside the combustion chamber its going to glow red hot and cause detonation , It will also mess with the timing altho slightly the 2 togather would cause issues
I wouldnt do it for the small gain it MIGHT get

Tom
 

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when i ran thunderbird super coupes i liked a range colder shorter plugs always ran better. if i could find colder motorcraft plugs for my focus i would be glad to try them . i also seemed to notice a good increase in gas mileage when i opened up the spark plug gap from 36 to 52 the last two tanks of gas i averaged 47 and 42
 

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I have to install plugs here pretty soon. I ordered the Motorcraft SP535’s and read they’re gapped at .037 Is that correct?
 

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2 liter 4 each cylinder swept volume 500cc.
12 to one compression 1/12 of 500 cc is 41cc

Naturally this a poor way to guess at the TDC piston area in the chamber..
SO you can say the most you could jam in there is about 1CC of spark plug before you F*K you engine good and proper..

1cc of 41cc is 2.5% increase in compression ratio. And I bet the most possible space used is more like 0.5cc of volume..
So a ONE PERCENT increase in ratio

So instead of 12:1 you will have 12.01 :1

When the very first time you crank you hole a piston. [bigcry]
When the very first time you crank a valve jams and bends hitting the spark plug [bigcry]

And if those problems do not happen.
Then when you hole a piston due to the white hot plugs blowing up you cylinders during a fast high RPM drive. [hatchet]
and finally if all those bad things do not happen.. when you cannot get that carboned up burnt up dead spark plug out ever again..'Cuz you left it in there for a year... and have to take the head off [smackbum]

Added: the small engine power increase from ignition advance (due to the plug being more centered in combustion chamber of a fixed advance in a small motor) will be negated by the automatic spark plug advance/retard of a modern engine using the knock sensor...
 

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I pulled these out today at 72k miles and replaced with the previously mentioned motorcraft plugs. Let me know if anything looks suspect.

Oddly enough, I found a piece of black plastic or rubber on the last plug (driver side):
 

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I don't know if it's the angle or if it's the camera. But that second plug from the top looks darker than the others.

I replaced my plugs @ 103K and they looked good. Didn't even check the Spark Plug Gap on the SP535's, just placed them right in.
 
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