Crazy Idea #51531205451351 - Direct Injection

Wrecked944 · 08-09-2006, 07:02 AM

I was reading up on the benefits of direct fuel injection and it really made me wonder if/how a 911 could be retrofitted to use DI. Then it dawned on me that many owners have their cylinder heads machined to include an extra spark plug hole for twin plug ignition. And I began to wonder if that hole would be better used for a direct injection nozzle. So here is the big question- Ignoring cost for the moment, what would be the best use for the second spark plug hole: Second spark plug or direct fuel injection? Which would provide the biggest performance gain?

Lukesportsman · 08-09-2006, 02:08 PM

Isn't the MFI setup already pretty darn close? I'd vote for MFI to EFI conversion with the second hole staying as a plug.

RLJ · RLJ's Garage

I "think" that compression = horsepower. If that is the case then I vote for the second plug so I can jack the compression up as hight as I can, and still use pump gas. The injectors on my system are very close to the flange between the ITPs and the head. IMHO I will take HP over a very small change in the mix ratio inside the combustion chamber. If I am wrong, flame me!

Randy Jones
1971 911 "Iris"

midlife · 08-09-2006, 02:19 PM

The MFI injector is before the intake valve. Direct injection is into combustion chamber.

MotoSook · 08-09-2006, 02:27 PM

It's not a crazy idea, but it is perhaps an expensive idea. You'll need to source injectors that will thread into the combustion chamber. The injectors will need to be able to work against the compression of the chamber and the heat of combustion. They'll then need sequential injection control. You wouldn't want the injectors to open during an exhaust stroke because you were batch firing the injectors. You'll need to make all this fit on your car and then tune it. That's about the bare minimim, yet you will have to consider fuel pressure/delivery, fuel rail design and mouting and injector mounting. If you need ignition control it'll add to the $$$.

In the end, you will have a very expensive and trick engine, which will make as much horsepower as something else (say a larger displacement NA engine with carbs or an available EFI system) that cost less to build.

midlife · 08-09-2006, 02:54 PM

Direct injection does have a higher horsepower potential than carbs or EFI. It's all about how much fuel and air you can pump into the cylinder. With direct injection you only need to get air past the intake valve, not air/fuel mixture.

But I agree that it is not cost effective. It is best left up to companies with huge development budgets.

MotoSook · 08-09-2006, 03:04 PM

Correct midlife. I was not eluding to any monumental gain in power. The direct injected engine will be more efficient and with the right engine management, it will allow for a higher compression since it can time the fuel delivery better. Take a DME or CIS engine. They are more efficient than carbureted engines, yet one is continuously injecting fuel into the ports and the other batch fires. Direct sequential injection will improve the engine, but it's not worth the investment to retrofit the technology to a 30 year old engine.

tbitz · 08-09-2006, 08:29 PM

You don't need spark plugs if you go that route. Crank the compression up to 40:1 and you got yourself a diesel engine