964 3.6L to 3.8L injector question

[silver911rdb]

Hello

When upgrading P&C from a 3.6L to a 3.8L on a 964, do the injectors need to be replaced with higher volume units? Or can the stock injectos handle the fuel delivery needs at full throttle at max rpm? This would be on a track car. I’ve heard of getting a new tuned chip, but a couple of people have mentioned upgrading the injectors. Any feedback would be appreciated.

Thanks

Rich

[Bill Verburg]

Originally Posted by silver911rdb

Hello

When upgrading P&C from a 3.6L to a 3.8L on a 964, do the injectors need to be replaced with higher volume units? Or can the stock injectos handle the fuel delivery needs at full throttle at max rpm? This would be on a track car. I’ve heard of getting a new tuned chip, but a couple of people have mentioned upgrading the injectors. Any feedback would be appreciated.

Thanks

Rich

The 964 injector flow 195cc per minute. nor recommended for 3.8
993 injectors flow 235 cc per minute. barely acceptable for 3.8 as the y are at 100% duty cycle at max rpm

generally a 320 cc per min injector is preferred

[silver911rdb]

Thanks Bill for the detailed info. We were planning on sending out the old 964 injectors for rebuild to Marren Injection in CT. I’ll have to ask if he can use these for cores and upgrade them. If not sounds like we’ll need to purchase new injectors. Would you happen to know the part number for the correct injector? Also, does the fuel regulator need to be changed as well? I just read a PCA tech article that the regulator needs to be changed but they didn’t give specifics on what to change it to

Thanks!

[nickd]

Unfortunately there is not a lot of hp margin in the stock 964 injectors. I’ve heard 300-305 hp mentioned by some shops. I don’t think I’m hitting more than 285 hp and I’m seeing well over 90% duty cycle at ~13:1 air:fuel. The recommended max is 80-85%. I haven’t monitored fuel pressure at the same time so maybe my fuel pump is getting tired.

Anyway, you definitely don’t want to run lean under power. You’re likely okay with a 3.8 (plus a little exhaust work) on the stock injectors if the rest of the fuel system is tip-top. A dyno pull with an air:fuel monitor would give you a rough guide. Even better if your shop knows how to use an oscilloscope to record the duty cycle during a pull.

I haven’t seen any vendors successfully modify the chip for larger injectors, but I have seen some bad attempts to try. The entire chip is essentially written around the stock injectors. The only other solution is moving to a stand alone fuel system, which is no small expense or effort.

[Steve W]

If you keep reading enough on internet forums, you will probably keep reading the same response from self proclaimed experts (i.e. Rennlist) that the 964 injectors are barely capable of supporting anything but a stock motor, well because heard/read that themselves somewhere from a guy who was a meister (hey gotta sell a Motec upgrade right?). The reality is that with hod rodded 3.8 combined with a modified exhaust (like header and an open exhaust) and cam like the 993SS, the injectors only start maxing out at full throttle after 6200 rpm on, and then you'd see AFR's around 13.4:1, where normally I'd ideally try target 13.0 to 13.2

I have dyno tuned more 964 3.8s on the dyno than I could ever count, both on chassis and Superflow engine dynos, and in some instances with injectors replaced with 993 ones. In the end what works every time and what I recommend is if you go beyond a basic 3.8, in other words, if you add a cam, then all you need to do is modify the fuel pressure regulator by 0.5 bar, (about 7psi), or you can get an aftermarket adjustable regulator. But modding your stock regulator costs nothing and looks factory. The net results are that the injector duty cycles come down to basically what a stock 964 run at, so any concerns of the injectors being pushed to max are not a concern anymore. If you build a 3.8 with the stock cam, there's no need to do this. On 3.8s I tuned where the injectors were changed to 993 injectors, such as engines built by Mirage or Singer, the results were very mixed and never consistent, so I'm not a fan of changing injectors. Keep it simple.

[silver911rdb]

Thanks Steve! So what you're saying is that by increasing the fuel pressure, the stock 964 injectors will increase the cc delivery for a given duty cycle compared to stock. And this should be enough to feed the engine at sustained high RPM. I'll need to search around to see how the fuel pressure regulator is adjusted as I've never seen that done before.
Just to give a little background on the engine. The car was recently purchased and it had a 3.8 conversion done. The injectors and tune were never changed from stock. The previous owner ran it for a day at the track. It seemed to run fine. On the next track day the engine blew. He said it smoked and just stopped working. He said he thought it hydralocked. My buddy recently purchased the car and I'm working with him to go through the engine. Compression was really bad on most of the cylinders so we pulled the engine. We found the engine definitely ingested a ton of oil (possibly due to an overfill) and the mufflers were full of oil. Most of the cylinders had broken piston rings. At this point we're not sure if it was an overfill that broke the rings or the injectors/tune. Do you think the injectors/tune would cause the rings to break the way they did? They were broken into small pieces around the piston. This is the 2nd time the previous owner had an issue with the rings breaking so I'm guessing it's the tune. Either way we'll work with you to get the tune correct once we get the engine back together again.

[nickd]

Broken piston rings are almost certainly from detonation. The stock chip will pull timing to help avoid/minimize destruction, assuming all is working as intended.With the rings as bad as you describe, oil will get sucked past them into the cylinders easily. Bummer, these are expensive motors to keep rebuilding!

I’ll leave out whatever the snarky self-proclaimed experts thing is all about. I gave numbers that I’ve measured along with tips on what to check. Cranking the fuel pressure is not my idea of a good solution, but if wot tuning is the only goal then it can work.

[Steve W]

Originally Posted by silver911rdb

Thanks Steve! So what you're saying is that by increasing the fuel pressure, the stock 964 injectors will increase the cc delivery for a given duty cycle compared to stock. And this should be enough to feed the engine at sustained high RPM. I'll need to search around to see how the fuel pressure regulator is adjusted as I've never seen that done before.
Just to give a little background on the engine. The car was recently purchased and it had a 3.8 conversion done. The injectors and tune were never changed from stock. The previous owner ran it for a day at the track. It seemed to run fine. On the next track day the engine blew. He said it smoked and just stopped working. He said he thought it hydralocked. My buddy recently purchased the car and I'm working with him to go through the engine. Compression was really bad on most of the cylinders so we pulled the engine. We found the engine definitely ingested a ton of oil (possibly due to an overfill) and the mufflers were full of oil. Most of the cylinders had broken piston rings. At this point we're not sure if it was an overfill that broke the rings or the injectors/tune. Do you think the injectors/tune would cause the rings to break the way they did? They were broken into small pieces around the piston. This is the 2nd time the previous owner had an issue with the rings breaking so I'm guessing it's the tune. Either way we'll work with you to get the tune correct once we get the engine back together again.

An increase in fuel pressure from 3.8 bar (55.1 psi) to 4.3 (62.4 psi) nets an injector flow increase of 6.3%. You can use any online fuel injector calculator to compute this. A 3.8 conversion is actually only a 3.75, so the net displacement increase of such a conversion is only 4.2%. Assuming your different cams and exhaust actually add to the flow requirement, you still have a couple of percent to left which bring you back to the injector duty cycle where a stock 964 is at.

Without knowing the background of that motor, it would all be speculation on what happened to detonate that motor. Obviously the quality of the build is important as there are many things that can go wrong on a 911 motor if not done correctly. But more fuel is a is must - using a stock chip would net a motor that would run 6-8% lean which will cause an engine under load to run too lean and detonate, breaking the rings and melting the pistons.

So, a little background on what has been a major issue with these 3.8 conversions for who knows how long. Back in 2015 Randy Greff of Greff Motors built a 964 3.8 that had problems first with excessive oil consumption, and second with pinging and knock, running on 93 octane. The pistons were the standard 964 3.8L p/c kit that Mahle sold for the 964 and were listed at something like 11.4:1 CR. The oil consumption was something like a quart per 400 miles. The typical answer you'd read online or heard from suppliers were 'oh, everything is new, break in the motor with dyno oil and drive it like you stole it until the rings break in'. But that didn't address some of the various other issue we were dealing with. So after the second time around with the motor, Mahle said to send the pistons and cylinder to them to evaluate. In addition Randy cc'd the pistons to measure something like 12.4 CR! So after Mahle received and evaluated everything, they said, 'oh yeah the pistons actually are 12.4-12.6 CR pistons, the catalog is wrong, we just never fixed it. Oh and by the way, half of the rings are installed upside down because the rings are a specific version of Total Seal rings that have a slight taper face that's supposed to scrape the oil back to the case on the downstroke. Installed upside down, the reverse occurs. So Randy asks, well why aren't there any notes, instructions, or branding telling a user any of this? To which there really was no answer, other than we'll send it back to you with a new set of rings. Randy and I know pretty much many of the same of old school engine builders and suppliers around the US, and when the above was asked, apparently no one knew either about the high compression or the taper of the rings. So I don't know how long this problem has been going on but I'd have to assume from the beginning of time as there is no other 3.8 p/c kit for the 964 with any different compression. And I was surprised that no one AFAIK ever CC'd the kit and discovered this. And it's a 50/50 shot that the piston rings would be oriented correctly in half the cylinders right? Anyways I can say there are a lot of 964 3.8 motors that were built over many years prior to 2015. And many shops and owners still don't know about this problem as Mahle has never addressed this problem AFAIK.

Well you can't really build a 911 motor with 12.4 CR and run on 93 octane pump gas, it's way too high and you can really only use racing fuel. It has been a pain to try and detune the programming to salvage such motors, but you can only do so much because you can't stop predetonation which is when low octane fuel detonates on it's own due to excessive compression. Since then some of these motors have been disassembled and rebuilt to address the excess compression issue. On some, the pistons would be brought to somewhere like Randy Asse at ASSCO Motorsports where he would mill some of the compression off the top of the pistons. On many others the whole piston kit is just junked and replaced with the only other proper option, which is to use the 993 3.8 kit, which is a 11.4 set. Using those you must either use aftermarket connecting rods or modify the stock 964 rod by milling a mm off the width of the small end to fit into the piston. The 964 rods are actually a pretty good rod because they are forged, rather than sintered like the 993 rods. There are two versions of the 993 sets, one which is a slip fit, and the other is the RSR set which require boring the case to fit the cylinders which addresses a piston pin offset that the slip fit doesn't. When someone says they have a Mahle 3.8 conversion, but didn't address the rod differences, that's a clue on what the CR is.

Originally Posted by nickd

I’ll leave out whatever the snarky self-proclaimed experts thing is all about. I gave numbers that I’ve measured along with tips on what to check. Cranking the fuel pressure is not my idea of a good solution, but if wot tuning is the only goal then it can work.

I'm not sure if you're implying that when we tune a motor we don't tune and map idle and part throttle, or if you're saying that the physics of increasing fuel pressure by 7 psi doesn't also properly increase the fuel flow from at idle and part throttle, but if the latter, I'd like to know why? Do Bosch injectors on a Porsche react differently because it's a Porsche?

[silver911rdb]

Originally Posted by Steve W

An increase in fuel pressure from 3.8 bar (55.1 psi) to 4.3 (62.4 psi) nets an injector flow increase of 6.3%. You can use any online fuel injector calculator to compute this. A 3.8 conversion is actually only a 3.75, so the net displacement increase of such a conversion is only 4.2%. Assuming your different cams and exhaust actually add to the flow requirement, you still have a couple of percent to left which bring you back to the injector duty cycle where a stock 964 is at.

Without knowing the background of that motor, it would all be speculation on what happened to detonate that motor. Obviously the quality of the build is important as there are many things that can go wrong on a 911 motor if not done correctly. But more fuel is a is must - using a stock chip would net a motor that would run 6-8% lean which will cause an engine under load to run too lean and detonate, breaking the rings and melting the pistons.

So, a little background on what has been a major issue with these 3.8 conversions for who knows how long. Back in 2015 Randy Greff of Greff Motors built a 964 3.8 that had problems first with excessive oil consumption, and second with pinging and knock, running on 93 octane. The pistons were the standard 964 3.8L p/c kit that Mahle sold for the 964 and were listed at something like 11.4:1 CR. The oil consumption was something like a quart per 400 miles. The typical answer you'd read online or heard from suppliers were 'oh, everything is new, break in the motor with dyno oil and drive it like you stole it until the rings break in'. But that didn't address some of the various other issue we were dealing with. So after the second time around with the motor, Mahle said to send the pistons and cylinder to them to evaluate. In addition Randy cc'd the pistons to measure something like 12.4 CR! So after Mahle received and evaluated everything, they said, 'oh yeah the pistons actually are 12.4-12.6 CR pistons, the catalog is wrong, we just never fixed it. Oh and by the way, half of the rings are installed upside down because the rings are a specific version of Total Seal rings that have a slight taper face that's supposed to scrape the oil back to the case on the downstroke. Installed upside down, the reverse occurs. So Randy asks, well why aren't there any notes, instructions, or branding telling a user any of this? To which there really was no answer, other than we'll send it back to you with a new set of rings. Randy and I know pretty much many of the same of old school engine builders and suppliers around the US, and when the above was asked, apparently no one knew either about the high compression or the taper of the rings. So I don't know how long this problem has been going on but I'd have to assume from the beginning of time as there is no other 3.8 p/c kit for the 964 with any different compression. And I was surprised that no one AFAIK ever CC'd the kit and discovered this. And it's a 50/50 shot that the piston rings would be oriented correctly in half the cylinders right? Anyways I can say there are a lot of 964 3.8 motors that were built over many years prior to 2015. And many shops and owners still don't know about this problem as Mahle has never addressed this problem AFAIK.

Well you can't really build a 911 motor with 12.4 CR and run on 93 octane pump gas, it's way too high and you can really only use racing fuel. It has been a pain to try and detune the programming to salvage such motors, but you can only do so much because you can't stop predetonation which is when low octane fuel detonates on it's own due to excessive compression. Since then some of these motors have been disassembled and rebuilt to address the excess compression issue. On some, the pistons would be brought to somewhere like Randy Asse at ASSCO Motorsports where he would mill some of the compression off the top of the pistons. On many others the whole piston kit is just junked and replaced with the only other proper option, which is to use the 993 3.8 kit, which is a 11.4 set. Using those you must either use aftermarket connecting rods or modify the stock 964 rod by milling a mm off the width of the small end to fit into the piston. The 964 rods are actually a pretty good rod because they are forged, rather than sintered like the 993 rods. There are two versions of the 993 sets, one which is a slip fit, and the other is the RSR set which require boring the case to fit the cylinders which addresses a piston pin offset that the slip fit doesn't. When someone says they have a Mahle 3.8 conversion, but didn't address the rod differences, that's a clue on what the CR is.



I'm not sure if you're implying that when we tune a motor we don't tune and map idle and part throttle, or if you're saying that the physics of increasing fuel pressure by 7 psi doesn't also properly increase the fuel flow from at idle and part throttle, but if the latter, I'd like to know why? Do Bosch injectors on a Porsche react differently because it's a Porsche?

That's certainly good information to have, thanks for the write up. These pistons and cylinders were JE's and Nickies. We're looking at replacing the pistons with new ones and having the cylinders inspected and possibly rebuilt. We'll cc the pistons and cylinder heads to calculate the CR on the current setup. It'll be interesting to see where they are. I feel the best way forward is to keep the CR the same as the stock 3.6. It's pretty high anyways and there's no need to increase it anymore causing the need to buy race fuel.

[nickd]

The advertised stock 3.6 compression is reportedly wrong too Factory says 11.3 but stock motors seem to measure in the 10.6 range. Not my measurement, but from multiple reliable first-hand sources.

"Do Bosch injectors on a Porsche react differently because it's a Porsche?"
Are these silly comments necessary? The motronic on any make/model makes many fuel adjustments beyond the idle, part throttle, and wot maps you mention changing. The stock fuel flow is hard-coded throughout the chip, which I already mentioned. This only one of many reasons why cranking the fuel pressure is not a good approach. I'm only offering my direct experience and chip knowledge for the author's decision; not selling anything.

Best of luck!

[Steve W]

Originally Posted by nickd

"Do Bosch injectors on a Porsche react differently because it's a Porsche?"
Are these silly comments necessary? The motronic on any make/model makes many fuel adjustments beyond the idle, part throttle, and wot maps you mention changing. The stock fuel flow is hard-coded throughout the chip, which I already mentioned. This only one of many reasons why cranking the fuel pressure is not a good approach. I'm only offering my direct experience and chip knowledge for the author's decision; not selling anything.

Best of luck!

I have no idea what you are talking about, but unless you have a proper explanation, it is best not continue to perpetuate false misinformation.

[nickd]

I don’t know what brought on the creepy threat and weird responses. The motronic fueling being more complicated than just the idle, part throttle, and wot maps isn’t exactly controversial. I’m not interested in whatever drama this seems to have inspired.

Originally Posted by Steve W

I have no idea what you are talking about, but unless you have a proper explanation, it is best not continue to perpetuate false misinformation.