Since my 968 has already been pretty thoroughly molested with aftermarket stuff I'm not being shy about upgrading more components to bulletproof it to the extent that I can and bring it into the 21st century. One of the parts everyone recommends keeping on hand is a spare DME relay, and the DME relay issues have been further addressed with aftermarket solid state versions that add additional features like fuel pump prime and built-in diagnostics.
From the factory, the 968 only runs the fuel pump when you actually turn the key to the "start" position. My car's in good shape so it starts well enough, but it does crank over a little longer than most other cars I've owned. With that in mind, I was going to go ahead and order one of these, thinking that there is basically nothing to lose by starting faster. Most other vehicles I've owned in recent memory prime the system when you turn the key to the "on" position so I don't know why it would hurt this one.
However, some guys in the Porsche community are absolutely convinced that you want the car to crank over for a while before lighting off as it helps circulate oil through the engine before starting. This kind of sounds like baloney to me, given that even my old BMW E28s primed the pump when you turned the key, and I had one well north of 300k and still going strong when I pulled the engine. Modern cars also do so much repeated lightning-fast starting and stopping that unless they have some kind of auxiliary oil pump system going on (do they?) I'd think this problem would be magnified quite a bit.
So, what say the hive? I'm not an engineer but based on empirical evidence I'd think that this wouldn't be a big deal at all, and a lot of these guys are just going on "Porsche knows best" mentality without recognizing that they were just working with the parts and knowledge that they had 30+ years ago. Does less cranking before starting = less oil circulating and thus more engine wear, or do you actually want to crank over for a bit to get the oil flowing on older engines?
Unless the oil system drains dry, there is oil movement as soon as the crank turns. I don't really think cranking long is really needed since there will be oil movement immediately.
Are you sure it's the lack of fuel prime with key on that is behind the slower starting? How slow are we talking about here?
IME, the biggest factor in how long it takes for a fuel injected car to fire is about how the ECU does crank angle detection. The more sophisticated the crank trigger system is, the faster the ECU can "sync" to the engine and start injecting fuel. Some systems want to see a full 720 degrees of rotation of the engine before they fire, others have enough resolution to fire the next time any cylinder comes around.
In reply to codrus (Forum Supporter) :
To be 100% clear. I'm not trying to troubleshoot anything here. My car starts in what I would consider a reasonable amount of time and similarly to other 944s/968s I've encountered at events.
I just don't know why I wouldn't want it to start slightly faster to prevent extra wear on the starter/charging system if I have the option.
People who have upgraded to the pump prime relay do indicate that their cars start faster than with the OE style relay, there's just debate over whether or not that is desirable.
The oil pump is a positive displacement pump so as soon as it turns it's pushing oil into all of the places it needs to be. That oil fills all of the supply orifices already so it's right there ready to go. I think quicker starting sounds just fine. Sounds like a misconception that's been spread around by the internet. If that's possible
In reply to pointofdeparture :
I've fully rebuilt a 2.2 911E, multiple VW bugs, Corvairs, a 2.9 Ford v6, and an '02 Audi S4 among others. The ONLY ONE that had any significant wear on the bearings or cylinders was the Ford v6, and that was cuz my daughter ran it with water in the oil from a blown headgasket. Keep clean oil in it and don't worry.
I don't think it would make that much of a difference (I also have an e28 BMW, with 220k on the odometer). Start the car, let it idle for a few seconds while you fasten your seat belt, adjust the mirrors and radio, etc., and then go about your day. If you wanted to be real picky about it, I suppose you could install an Accusump or similar product that would pressurize the oil system before starting the car.
I believe my truck (2015 silverado) will crank and not fire the injectors or spark before it sees oil pressure build. This usually happens in seemingly 1 or 2 revolutions of the engine.
I'd say it depends on your oil system. If you have external oil coolers you may have significant volume that needs to fill prior to the engine seeing oil pressure. You could need to pump a couple quarts depending on setup. In that case, priming before starting could help. Also there are oil accumulators. If you start with that open you'd divert a lot of pressure into it. More likely you'd use it to pre-oil your engine however. Also if you're running thicker oil that is cold, such as a track car you may want to take extra care that the oil system is pressurized prior to starting. On a stock car with oem weight oil I don't think too much about it though.
It is correct that the GM way around this was only firing the fuel pump when it sees oil pressure build. This was also the failsafe in the system to shut the fuel pump off in case of an accident or engine failure. Ford did the same thing with an inertia switch - their pumps run whenever the key is on, which is bad news in case of an accident. The inertia switch kills the fuel pump in event of an impact.
I cant imagine there being perceptible wear from your engine starting 1/2 revolution earlier, that seems like internet bologna to me. The only little bit of this theory that I buy into is I do disable the clutch switch on my manual transmission vehicles. Back in the DSM days, it was considered that a heavy clutch plus lack of oil pressure during startup could be a contributor to crankwalk, so guys disabled their clutch switches so you could start in neutral with no thrust load on the crank. This makes at least a bit of sense to me, especially with heavy pedal aftermarket clutches.
Building some pressure can be helpful but it's no big deal, the only exception might be if the oil has just been changed or the engine has been sitting for a long time, some extra cranking time could make a significant difference there. Modern cars with a start/stop engine setup have bottom-end bearings with a red coating that can take a ton of abuse so there's probably nothing to be gained with those. Aston Martin is a big believer in pre-cranking, they have powerful starters that will turn the engine over many times before some threshold/delay system in the ECU lets the engine start.
Also when I was setting up the 4AGE in my AE92, I got it to a point where it would start on the first or second crank and make some noises I didn't like, so I set up a fuel delay to force the engine to crank for 1-2sec before it would fire up.
codrus (Forum Supporter) said:
Are you sure it's the lack of fuel prime with key on that is behind the slower starting? How slow are we talking about here?
IME, the biggest factor in how long it takes for a fuel injected car to fire is about how the ECU does crank angle detection. The more sophisticated the crank trigger system is, the faster the ECU can "sync" to the engine and start injecting fuel. Some systems want to see a full 720 degrees of rotation of the engine before they fire, others have enough resolution to fire the next time any cylinder comes around.
A lot of modern (like in the last 10-15 years) cars can sync in less than one revolution. Doing DI, the goal was as fast as possible, and the sync was also needed for the DI pump. Once fuel pressure build high enough, it would fire off.
I don't think it matters if it cranks longer before starting.
Now some people like to drain their oil for an hour, you know so the pump drains. Some of my cars flicker the oil light after an oil change as it starts. That has to be 100x worse than a difference in crank time.
So you are better off never changing your oil because it won't have pressure when it starts for a second. Or just let most of it out then put the plug back in before the sump runs dry.
Modern oils (meaning anything produced in the last 2-3 decades) have enough protection film to protect the engine even for short run times without any pressure (like we forgot to put the oil back in and start it.).
If you have a big dollar motor build and need preoiling then a accusump is the way to go. Trigger it and wait for pressure in the system. ~30 seconds then fire the motor up and repressurize the accusump. They have button activated systems now, I used to do it manually with a big ball valve on my 671 blown 454.
