[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?