Undoing a twin plug

[mcfoo]

Sorry if this post is redundant since it appears my first one disappeared.

I have been looking for and finally found a race built 6cyl 914 under 2.5L

Problem is, its twin plugged and the org I want to race with does not permit twin plug motors.

Seller says he's got a single plug distributor and non-drilled lower valve covers which would appear to solve the problem.

What is the downside (if any) to running the motor in this configuration other than having six spark plugs not doing anything.

Thanks
Tom B.

[AlistairWallace]

How high is the compression? The big reason for doing dual plug is compression.

[mcfoo]

Originally Posted by AlistairWallace

How high is the compression? The big reason for doing dual plug is compression.

Seller states 13:1 which seems pretty high.

Running this motor with single plug would cause what problems?

[winders]

Originally Posted by mcfoo

Seller states 13:1 which seems pretty high.

Running this motor with single plug would cause what problems?

It's a good bet that the engine was designed with twin plug in mind. You can't just get rid of half the plugs and expect the engine to run without destroying itself. Detonation is what you have to worry about.

What octane fuel has the seller been using in the engine? Leaded? Unleaded?

You might be able to get away without the dual plugs if you back off the timing a lot and run much higher octane fuel. Might..... But, the performance would suck compared to an engine designed to be single plug.

You should change pistons to reduce compression....and different cams may be in order as well. I would check with an engine designer about your options.

[Walt Fricke]

While the super high (for an air cooled engine) compression ratio seems like a big problem, if you plan to run in SCCA, consider this: 25 + years ago a friend lost an autocross national championship over this. Stupid ruling (had unconnected plugs under the lower valve covers, which had no plug holes in them), but you face that kind of thing. Perhaps if my friend had used a short bolt with the right threads and a sealing ring under it he could have won the argument? See, can't be twin plug because no extra six plugs.

Best to check with whoever interprets the rules for the organization you are thinking of running with.

[RobFrost]

The explosion detonated by the spark spreads best when the chamber is ball-shaped and the spark at its dead centre. The wave of combustion spreads out in all directions.

At high compression such as 13:1, the combustion space is very flat and constricted when the pistons reach top dead centre, restricting and slowing down the spread of combustion, leading to incomplete combustion and reduced power.

Incomplete combustion is likely to lead to unburnt fuel entering the exhaust and exploding there. Additionally, fuel may still be combusting in the chamber when the next stroke of unburnt mixture is drawn in, igniting it and everything in that intake manifold.

Additionally, the extra compression generates heat which is itself likely to ignite the fuel before the spark even occurs (pre-ignition). This makes the engine happier at higher revs when the faster moving piston is more likely to get past top dead centre before any pre-ignition becomes an issue.

Higher revs exacerbate the first problem, i.e. of ensuring the entire charge has had time to explode during the piston stroke. Twin spark mitigates all these problems by initiating the explosion in two places, each having less distance to travel than a single explosion in order to combust the entire charge faster despite the cramped chamber and short stroke duration.

If you take twin spark out of an engine designed to have it you are likely to have a mess. You might be able to mitigate it by reducing the redline, using very high octane fuel or reducing compression. The tops of your pistons might be machinable or your cylinder heads, or you may be able to shim your cylinders, to reduce compression but performance would be down.



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[colingreene]

Im just going to say it. You cannot run that motor single plug period.
Not only will RPM be a factor but overall cyl pressure so its effectively going to limit how much throttle you can give it.

If you buy it plan on changing the pistons and potentially the cams and heads.
Might want to pass on this one.

[stownsen914]

I’ve heard of running very high compression like this on single plug for racing use only on leaded racing fuel, but as suggested it won’t be optimal.

Paging faapgar 🙂

[manbridge 74]

E85 legal?

Not pump E85 but as in VP fuel in cans/drums.

[Henry Schmidt]

Originally Posted by mcfoo

Seller states 13:1 which seems pretty high.

Running this motor with single plug would cause what problems?

First, dynamic compression is your concern, not static compression. Calculating dynamic compression is key.
Second, 13:1 on a short stroke 2.5 is not very likely. Being a twin plug race engine means high lift cams (deep valve pockets) and extra material is removed from the chamber for the second plug.
Even if the heads are stock "ish" the chambers should be around 69+cc.
That means the dome volume must be over 40cc. That is a hard number to find.
That said, you're thinking about buying an engine that doesn't really suit your application. As such, it's basically a core with some cool parts.
If it's priced that way, you win. If you're paying for a built race engine, I'd pass.