trophy

I think it really is the CFM flow that I amthinking about, and that all of these mods do improve to some extent the in and out of these engines. (i was just dont want the discussion to go down the ..."This exhaust adds 300hp and you loose 400lbs off the rear end' path...)

Most people are seeing some gains in HP with exhaust and Intake mods, assuming the injectors are in a perfect state of tune and we are moving more air into the engine then the computer will add more fuel.

There is going to be a point where there can be no more fuel added as the Fuel Injectors could be operating at 100% duty cycle. This could then cause a lean condition, resulting in the ecu bringing back on tming to compensate (thus reducing HP).

In the figures in the chart above I am not sre what our engines BFSC is, if we knew that we could have accurate figures. If BFSC is higher than .4 (which is very possible) then we are out of range of the injectors very quickly..... And therefore could be reducing the potential output of our engines.

NineMeister

In my experience (from many tests on the 9m chassis dyno) the stock 964 injectors will (just) support 300-310hp and the stock 993 injectors are good for 350-360hp (i.e. flywheel corrected DIN hp). This relationship is roughly proportional to the fuel flow numbers quoted by the OP, but you also have to allow for the increased running pressure of the 964 & 993 fuel systems which run at 3.8bar/55psi against the 43psi quoted, so the hp numbers quoted need to be factored accordingly.

There are a lot of new multi-pintle injectors which offer better atomisation and more accurate fuel delivery, however in my opinion it is pointless in changing injectors on a 993 engine unless you are considering a substantial engine upgrade due to the complexity of remapping the Motronic engine management system properly. Obviously there would be no point in changing injectors and not remapping the ecu.

In the case of the 964, we have found that with minor exhaust and intake system modifications, the engine will move more air than the fuel system can support, which is why we always replace the injectors as part of the 9m Motec upgrade package. The modern multi-pintle injectors we use flow a similar amount to the 993tt injectors.

trophy

Thanks Colin,

So when doing the Motec upgrade to the 993's do you also change out the injectors for larger ones or do you find the stock ones are suitable?

What overall improvement does the multi pintle style injector give over our older style injectors?

I have updated the second chart from the first page with the fuel pressure at 55psi, at 95% duty cycle it is showing 355hp, my other concern for a DE car that will spend quite alot of time at WOT, what is the potential damage that could occur to the injectors and ecu when running these duty cycles?

NineMeister

It's rare that we add Motec to a stock 993, but when we do I would undoubtedly fit the same injectors as we would in a 9m 964 Motec conversion.

erm, "better atomisation and more accurate fuel delivery".....

... which in theory means that you should get a more homogenous fuel/air mixture and as a result be able to run the engine leaner during part load and full load conditions without losing performance (or increasing head temperatures).

trophy

Sorry, should have been clearer, I got the 'better atomisation and more accurate fuel delivery" I was after your next comment..... So realistically this would provide better fuel economy rather than an increase in performance, and an additional benefit of better temperature control (which is very good in air cooled engines)

I also added a chart above...

NineMeister

... not quite. Realistically this could provide better fuel economy and an increase in performance, but only if the ecu was up to the job of making the best of the improved characteristics. In all likelyhood, changing injectors for a different version with no other modifications will give no benefit whatsoever.

trophy

Thats great information, thanks Colin.....

Stealth 993

I don't really think most new injectors would be a upgrade. If you choose the wrong spray pattern you can loose HP from it. I've tested it in Audi 2.7t motor when upgrading the turbo's. There was a super cheap injector that a lot of guys wanted to use. Over the more expensive one it was a 35hp difference, & drivability.


If we maxed out our injectors with intake & exhaust we would be on the fence stock. Just about every OEM has their cars running rich, especially under WOT. Adding more CFM will lean it out a bit & give even bigger gains. Also don't forget the MAF can see the added airflow & adjust for it.

95% of the time even on the track you are not running WOT in the last few hundred RPM's, this is the only area that would stress the injectors. I bet most of the time even under track conditions they are in the 40-80% rate.

trophy

Great points Stealth,

I keep thinking that we change our shocks because technology improves, we change brake pads because material improves, we change tires because of the same. I was using the same logic (slightly flawed it seems) on other components of our cars, and in this case fuel injectors.

I am always looking for other ways to improve the performance of my car....

Stealth 993

It can improve performance on a modified car, but it takes a good plan to get it right. You would need to test several injectors to get the right flow & spray pattern. Then tune the ECU properly. Then we are talking 1-2% HP increase. But may improve drivability & throttle response.

The problem is time & money.

If you want more HP, why not just build a 3.8l, or add a super charger. That's one of the biggest HP per-dollar things you can do.

trophy

I really like the idea of a 3.8, 3.9 or 4.0 motor.... One of my initial ideas of this thread was to see what changes could be made along the way to the ultimate goal. Assuming I go to an increased capacity in the future, why not put in new injectors now and then know they will be suitable for the planned upgrades.

I love turbo engines, but for some reason I am drawn to NA at the moment.....