3.8 RS style rebuild and injector size

[Tbone1209]

Does anyone have an informed opinion as to if the stock 3.6 injectors can support a 3.8 RS style rebuild?

I believe my 3.8 is running stock injectors and in an effort to try to maximize efficiency and power, I wanted to see what others we're running.

[rsscotty]

There are 2 different injectors for the 3.6 engines.

The 964 injector flow 195cc per minute.
993 injectors flow 235 cc per minute.

The 993 injector in a 3.8 engine @310 HP will be 100% duty cycle...so about 20% over duty compared to what we would use with aftermarket EFI systems.

[faverymi]

What is that AFM you are running?

Stock motronic, chip or aftermarket ecu?

[Mahler9th]

When I was running a hot cammed 3.8L with Haltech, I use 944 turbo the greenies.

I still use them with my more tame 3.6 with the same EFI. Often can be found used, and easy to get refurbed.

[Tbone1209]

It's a 95 993 motor running a stock 95 Motronic/OBD1 with Steve Wong custom chip (Ron 91)

RSScotty- So you're saying, in effect, my injectors are too small for optimal performance...

What style/size injectors are others running in similar style engines?

[Mahler9th]

One trick with a 3.6-based engine is to run a lighter and less power draining alternator/fan set up. Mine is set up that way, as are quite a few of my friends.

You can also rig up a larger throttle body to get more air in, as long as you can add more fuel with a chip change or EFI tuning. I ran RSR-style hot cams at 3.8 with a 993-style intake for years. I ran 1 3/4 OD racing headers.

I switched out the 993 TB and installed an aftermarket BBK 80 mm throttle body-- about 10 mm bigger in diameter than the 993 TB. Made more power. Not very difficult to fab up. You need to have a way to interpret throttle position from the BBK's generic TPS or fab to use a 993 TPS.

I still have that TB and I am going to sell it. My current race class does not allow me to use it.

[Tbone1209]

Originally Posted by Mahler9th

One trick with a 3.6-based engine is to run a lighter and less power draining alternator/fan set up. Mine is set up that way, as are quite a few of my friends.

You can also rig up a larger throttle body to get more air in, as long as you can add more fuel with a chip change or EFI tuning. I ran RSR-style hot cams at 3.8 with a 993-style intake for years. I ran 1 3/4 OD racing headers.

I switched out the 993 TB and installed an aftermarket BBK 80 mm throttle body-- about 10 mm bigger in diameter than the 993 TB. Made more power. Not very difficult to fab up. You need to have a way to interpret throttle position from the BBK's generic TPS or fab to use a 993 TPS.

I still have that TB and I am going to sell it. My current race class does not allow me to use it.

I just DM'd you Mahler

[rsscotty]

Originally Posted by Tbone1209

It's a 95 993 motor running a stock 95 Motronic/OBD1 with Steve Wong custom chip (Ron 91)

RSScotty- So you're saying, in effect, my injectors are too small for optimal performance...

What style/size injectors are others running in similar style engines?

Depends on the horsepower the engine is putting out. Your stock injectors may not supply enough fuel at peak power levels and will be too lean. For a 325 hp 3.8 we would use a 320cc injector. This is with Motec, ITB throttles and performance camshafts & exhaust system.

[Mahler9th]

I responded to the PM. I cannot remember the specs on the green injectors, but I think they flowed enough for my ~360-370 bhp 3.8L config back in the day. Guys usually sell them used in sets of four.

Of course the injectors are part of a system.

If I had a 3.8 that I wanted to hot rod and was not constrained by smog, I'd consider ditching factory ECU and running something like Haltech, with ITB's. And for ITB's I'd likely use the new products from Reno Rennsport-- shaftless butterfly technology is I think the closest flow to slides that we have ever had.

I wish I could use these ITB's, but ITB's are not legal in my racing class. Nor is the BBK.

[Mahler9th]

I should of course add that injectors need to be considered part of a system. In addition to other specs, their impedance can be important depending on what other components are in the system.

[Raceboy]

Stock injectors are barely adequate even for stock power leveles despite higher system pressure. Couple that with old style, slow response and there is no reason imo to even consider using them in any high power build.

Why fork out 4000-5000usd for just P&C's and then skimp on few hundred from good injectors?
Weird is that people are not hesitant to put tens of thousands into engine rebuild only to use some AFM system with questionable driveability and totally off fuel and ignition...

[Tbone1209]

So I wanted to revisit this topic as I've learned a bit more re the particulars on my engine and have a clear idea of the type of driving I like to do.

So my 3.8 has solid rockers, RSR cam, some minor port and polish to the heads and the aforementioned 3.8L Mahle kit. It's a 95 so my SW chip, could be re-flashed (OBD1). BB headers also. I don't want to go down the expensive ITB /standalone ECU rabbit hole as it's expensive, a street car and will see very little if any true track time. I do like canyon carving and additional umph if it's available with just a injector upgrade and chip reflash.

Is that plausible?

Thx, for the opinions guys...

[Steve W]

The 993 injectors will be more than adequate for a highly modified 3.8 993 motor, even a race motor with headers and an open exhaust. It's the 964 injectors on some 964s that combined with a 3.8 and a hot cam like a 993SS, that run out of duty cycle above 6000 rpm full throttle, needing something like 103% duty cycle to meet optimal AFR target. In such instances the option is to either mod the fuel pressure regulator to increase fuel pressure by about 0.5 bar, or fit in 993 injectors which will leave a ton of headroom, more than any 3.8 will require, because as Rothsport notes the difference is 195 > 235 cc/min @ 3.0 bar and remember 964 and 993s run at 3.8 bar.

[Tbone1209]

Thanks for posting Steve! There's so much conflicting information on this topic. I'm still scratching my head, though. If you use any number of online fuel injector calculators, the calculations for a 320hp (at the flywheel) hp engine w/ an 85% duty cycle ceiling will require ~260cc injectors. That puts the stock 220cc basically at a ~15% shortfall.

Are all these calculators just so much marketing fluff to get people to purchase larger injectors?

[Steve W]

I'm guessing online calculators err on the conservative side when computing injector sizing (i.e. Lambda 0.82-0.85, low BSFC, 20% headroom ...) The reality is an AFR of around 12.5 is a bit too rich for a Porsche motor and 10% injector headroom is all that's really needed.

Combine that with the fact that the 993 injectors at 235cc/min @3.0 bar actually flow about 266cc/min at the 3.8 bar fuel pressure the 964 and 993 run at, and they are more than sufficient for a heavily modified 3.8 and likely a 4.0.

[rsscotty]

Originally Posted by Steve W

I'm guessing online calculators err on the conservative side when computing injector sizing (i.e. Lambda 0.82-0.85, low BSFC, 20% headroom ...) The reality is an AFR of around 12.5 is a bit too rich for a Porsche motor and 10% injector headroom is all that's really needed.

Combine that with the fact that the 993 injectors at 235cc/min @3.0 bar actually flow about 266cc/min at the 3.8 bar fuel pressure the 964 and 993 run at, and they are more than sufficient for a heavily modified 3.8 and likely a 4.0.

I would have to disagree with your above statement.

This info is based off a number of years of data accumulation and mapping on a Superflow engine dyno. A recent 4.0 engine we did used 535cc injectors based on 4.0 bar fuel pressure. The data log shows right at 80% injector duty at 7600 rpm. That is the injector headroom as you call it that we have always shoot for. This is the same injector that Porsche put in the 993 RSR engine..

The OEM 993 235cc injector at 3.8 bar is done at 320hp on a 3.8 RSCS engine.

[Steve W]

No doubt you're reaching those flow rates on high hp custom ITB motors. I should perhaps predicate that I was referring to injectors sufficient for the single plenum setups on both the single flap and VRAM Motronic manifolds, which is what I think the OP is referring to, which make max power around 6200 rpm and rev limit max at around 7000-7200. The data accumulated here is from years of dyno tuning 3.8 motors built and tested, some of which go into track cars or cars like the Singers, which were also dyno tuned and tested on Superflow engine dynos at Aasco's or Ed Pink Racing, and/or Dynojet chassis dynos, including all the Motronic 3.8s your shop has built and sent over the dyno data for custom programming. It could be that none of these 3.8s I've seen ever exceeded 340 hp, but so far I've never run into a Motronic 3.8 with 993 injectors where I've run out of injector flow to achieve target fuel AFRs while still leaving sufficient headroom.

[Sboxin]

Originally Posted by Tbone1209

So I wanted to revisit this topic as I've learned a bit more re the particulars on my engine and have a clear idea of the type of driving I like to do.

So my 3.8 has solid rockers, RSR cam, some minor port and polish to the heads and the aforementioned 3.8L Mahle kit. It's a 95 so my SW chip, could be re-flashed (OBD1). BB headers also. I don't want to go down the expensive ITB /standalone ECU rabbit hole as it's expensive, a street car and will see very little if any true track time. I do like canyon carving and additional umph if it's available with just a injector upgrade and chip reflash.

Is that plausible?

Thx, for the opinions guys...

We are running stock 964 injectors in a 3.8L Steve W dyno tuned race engine for 5 years . . . after spending a whole day with Steve and our car on the dyno I agree with his comments on the injector size.

All the best to you and your Porsche experiences with the 3.8L hot rod . . .

Regards,


Steve working his programming craft . . .