Are new injectors necessary on a rebuild for 3.6?
 

In the planning stages of my rebuild and will be doing minor upgrades including 993 supersport cams and steve Wong chip. Keeping everything else factory. Are new injectors due at this point as well or will the 964 injectors be appropriate for these upgrades? Thanks for suggestions.

Unless the chip tuner tells you that he is running into a ceiling where the stock inejectors can't flow enough fuel into the engine you are fine.

When you drastically enlarge displacement or add a Turbo you usually need to increase the injector size but in your case you should be good.

Ingo

Open ended question..

Stock rebuild:

Unless the injectors have some sort of problem... damaged, clogged...

You don't need new injectors

As mentioned above... if you run out of duty cycle because your new 3.6 rebuild is getting extreme you may need new injectors..

It all depends on how much fuel your engine needs to provide power safely...typically you don't run injectors over 85% duty cycle.

Also stock tune was very conservative... lots of headroom in injector capacity..

What are you building?

oops... re-reading your post... stock should be good..

Probably be able to just send them out for cleaning and testing.

Emerald

Thanks for the suggestions. Cleaning and testing is in order.

JonT,

I will take the other side here, if you plan on doing any power adds you may be in trouble. If you check the flow rate of the stock injectors they are very close to maxed out on the bone stock motor. If you want more RPM you will be beyond 90% duty cycle.

Here is a good link including the flow rating on the injectors you most likely have. Stan Weiss' - Electronic Fuel Injector (EFI) Flow Data Table .

Stay tuned,

PFM

JonT

The 3.6 L injectors are rated at 18.45 lbs/hr, at 95% duty cycle and a reasonable AF ratio around .42 BSFC you are looking at 43 to 46 horsepower per injector, 258 to 276 HP.

Stay tuned,

PFM

Thanks PFM--The cam and chip change to my engine reportedly produces an additional 20ish hp on the factory engine. I realize the early 964 3.6 factory ratings are 247hp but I also have 993 headers which improve hp modestly as well. I should remain just below 300hp. This will put me over the 85% duty cycle per TimT's comment. Can anyone recommend the next step up in fuel injector size I should look at? Brand?

I know several 3.6s that are pushing 280-300 (exhaust, chip, MAF) with stock injectors, but that must be at the edge of the injectors.

We buy a lot of injectors from Marren Fuel Injection

They also have a handy injector calculator available..In your case you estimate you are going to have 300hp...

Injector sizing calculator

Plugging the numbers into the Marren calculator indicates you need a 27 #/hr injector.

I have seen the Stan Weiss site before and it makes me cross eyed... And I dont think it is 100% accurate...

Finally I am drawing a blank regarding the stock 3.6 injector flow rate. however 19#/hr only support 217hp, a 3.6 is 247 stock?

A 19 #/hr injector would have to run at 108% to make 247 hp...

Don't think Porsche would send cars out like that?

Best bet would be to call up Steve W explain your mods, and ask him if he thinks you need new injectors

TimT

Check out the injector part number, I could be wrong, the part number I have says the 3.6 injector is a 18.45 pound per hour injector. And yes I was surprised that Porsche ran it so close but that is what I saw and have had others tell me the same thing. Keep in mind the small injector helps with idle quality and emissions, you can turn up the fuel rail pressure and make some gains.

JonT,

A 24 pound injector sounds good to me too, some of the new multi hole injectors will help you out a bit.

Stay tuned,

PFM

Keep in mind that the DME and its chip containing the maps "don't know" about the injector flow rates. The maps and factors in the DME assume a certain flow rate and injector characteristic. A modern DME needs to correct injector opening times depending on supply voltage.

The maps are used as look-up tables and eventually the DME calculates an opening time in milliseconds to achieve a certain amount of fuel to be delivered to the engine. The shortest time modern injectors can open is around 1 ms. Shorter times can not be reliably realized.

If you simply change the injectors without making any other adjustments to the maps and/or fuel pressure the mixture will change according to the new flow rate divided by the old flow rate. For example if the stock injectors are rated at 18lbs/hour and the new injectors are rated at 25lbs/hour you will increase the fuel delivered into the engine by 25/18 or by 39% when the DME commands the same injector opening time. It will make the engine run like crap. On top of that the new njectors might have different response to supply voltage changes.

During closed-loop operation (part throttle) the O2 regulation will constantly try to lean out the mixture. This might or might not work depending on how much the O2 regulation is able to modify the mixture. This is typically +/- 30%. So the regulation is constantly pegging and thus not very effective.

During WOT the mixture will be way too rich and you will leave power on the table. The chip tuner has to redo the WOT maps with the new injectors installed on the dyno to account for the increased flow rate. Better yet, depending on how well the chip tuner has reverse-engineered the chip contents he can adjust the injector correction factor directly.

Typically, as a first approximation all fuel values in the maps need to be divided by the value (old_flow/new_flow) assuming the injector responses are completely linear. To make things right a real tuning session is required were the A/F ratio is monitored during WOT dyno runs.

And as others mentioned the higher-flowing injectors will deteriorate idle performance since the dynamic range is now further away from the idle fuel requirements.

Ingo

ischmitz- so if you are tapping out the injector duty cycle on a built 3.8L running a 1995 OBD1 DME, is it possible to adjust the chip to work effectivly with moderately larger injectors? I have a SW custom chip so would I have to send it back and forth and revisit the dyno a few times to get things right?

I understand that the stock injectors can handle 300 BHP at which point they’re maxed out, I’m going to be rebuilding my engine soon which will be standard with the exception of Schrick cams, decat and possibly 993 headers which might well be asking a little too much from the standard injectors so I’ve been looking at this setup:

https://www.specialist-components.co.uk/index.php/shop/engine-management/posche-engine-management/porsche-911-964-management-kit.html

This is a UK company and costs £1650

I would imagine you’ve something similar in the US, the bonus is that it uses the standard Motronic plug and wiring loom,comes with 6 new injectors and gets rid of the standard barn door metering unit.
If my memory is correct I think it gives something like 25 bhp gain on a standard engine and is fully programmable.

Yes, you should get bigger injectors, nice to be on the safe side.
I used some injectors from a Volvo V70 turbo cheep from the scrap yard in my 3.6. I think they were 40-50% bigger than the stock injectors.

I bought a VEMS PnP system that fit the stock wiring harness. Works pretty good, but I had to fabricate a proper TPS adapter. A proper dyno tuning was necessary, but it always is no matter what system you get....

OP, at a minimum if not mentioned already for safe measures, have them inspected/tested/organized.

My buddy recently sent his 3.2 injectors to RC International. 4 of 6 were clogged. The cleaned and replaced exterior parts the inner strainer. Tested flow rates and even organized it so they would be balanced in the engine. $125, not bad to have peace of mind.

Good thing is the flow test to show you if it is adequate for your rebuild.