Revs and cold engines
 

We all know (well, most of us do) that revving a cold engine is the best way to wear it out quickly. An modern fuel injected engine will zing up to around 2000rpm at startup and then takes a little while to go down to what (I think) is an acceptable level. I hate that, but there's nothing I can do about it!
Question 1: Why do manufacturers program the efi to do this?
Question 2: What is a realistic maximum rpm to use on a cold engine
Question 3: How long should the revs be kept down (or what water temp / oil temp)?

Cheers,
Glen

That zing up to 2000 is to establish oil pressure and flow.

After pressure and flow is established, the best thing for the engine is to put a light load on it to get it up to operating temperature.

You're going to get a lot of "2 cents worth" comments on this. Although a fast idle of 2000 rpms sounds a little high, I don't think a static fast idle is considered damaging to a cold engine. Not sure but I suspect most EFI fast idle programs are for building up catalyst heat quickly, promoting a clean burn and minimizing emissions on start up cycle.

For my two cents a static fast idle (1500 - 1600 rpm) on a carbureted engine is fine to keep the engine running reasonably clean on a rich choke setting until the intake and carb warm up enough to promote good fuel dispersion and permit a reasonably decent idle. You don't want it chugging along at slow idle, unable to burn the rich fuel/air mixture and fouling the cylinder walls, rings and oil with excess fuel. But as soon as it will pretty cleanly fire the cylinders then gentle driving and cycling the engine without gunning it or lugging it will help get all the mechanical bits warmed up quicker, keep the oil moving and allow tolerances to reach their normal range.

I would be more concerned about gunning and reving a cold engine up and down in an attempt to get it to warm up enough to hold an idle. That's why I have a functioning choke and fast idle on all of my old cars - because I don't like having to sit there and constantly feed throttle to the engine and try to keep it running on a lean mixture while it's cold. But most of these cars are kept in fairly warm garages and a choke is probably not a real necessity on them. My thinking is that most of the wear on a cold engine is from actually starting it while it's trying to get the oil circulating. For that reason I usually prime the carb on my engines if they haven't been run in a few days so that they fire and run on the first turn of the starter. I don't like the idea of sitting there grinding on the starter for a long period with marginal oil movement and pressure while trying to get fuel to the carb.

But there will be other theories I'm sure.

Hi Brent,
I always drive off straight away. What should be the rev limit on a stone cold engine? I keep my daily driver down to 2000 max until the coolant is up to temp. Too conservative?

Cheers,
Glen

You can change all the start up parameters, if you chip the ECU and get into tuning. There is a whole new world out there.

This is the tuning parameters for a 5.0 Mustang GT with manual transmission, from 1989 to 1993, which is commonly called the EEC 4 A9L Box Code.

Idle RPM is set to 672

The start up idle adder is table based on engine temperature. The maximum adder is at 76 F and that is 432 RPM. If the engine is colder or warmer the adder is less. At 50 F or less the adder is 48 RPM. At 100 F or greater the adder is 48 RPM. When the temperature is between 50 and 100, linear interpolation is used to calculate the adder. So a 63 F (which is half way between 50 and 76) the adder would be half of 432 or 216 RPM adder.

So at most (when the engine is 76 F) the engine will idle at 1104 rpm, at start up.

The start up adder time controls how long it stays at this idle and it is set to 25 seconds.

With my chipped ECU, I have changes all of these parameters.

Now when you first crank the engine the manifold is full (at atmospheric pressure) as if the throttle is wide open. So when it first fires, it revs higher than the idle set point until the air is sucked down (vacuum in the manifold). So for a few seconds it will rev a bit higher.

There are other parameters that might impact this. There is a goose idle that is used to test the IAC valve, which is 250 rpm. There is an idle increase when the AC turns on. Another when the power steering pressure increases. I do not think Ford adds any of these in on start up, but it is possible. If you did have the AC on or was turning the steering, these would add in.

That all said if you are hitting 2000 rpm for more than a few (5) seconds, that seems excessive to me. You might want to have it looked at.

Thanks Dan. I suspect you're correct about warming up the cats quickly.
The logic of "zinging" to 2000rpm to build up oil pressure quickly doesn't sit well with me....that sounds like a cross your fingers and hope for the best strategy.
My '76 Alfa twin cam always starts from cold easily (never use choke) and runs cleanly, and I hold it at around 1000rpm on the hand throttle while I'm buckling up.
Pressure comes up quickly, but no doubt its emissions are crap ;)

As far as priming the carbies when it's been sitting for a few weeks - as mine does - that's probably a good idea as it would save the starter motor ....but then it's nearly 40 years old and the starter hasn't suffered noticeably so far. The oil pressure DOES come up on the gauge though as I'm cranking it over.

Cheers,
Glen

Thanks olddog. That's really interesting to an 'efi Luddite' like me to know what parameters are being fed in.
What concerns me is that when something stops working on a car, it's likely to be electronics related. To me simple is good, which is one of the many, many reasons that building a car like a Cobra replica is so appealing....if something breaks, I can fix it easily.

Cheers,
Glen

On the same subject of revs and cold engines......something I've wondered about regarding hybrids (like a Honda Prius): if the car is running slowly in city traffic, so I assume it's running predominantly on battery power....and then the driver wants full power. The petrol engine is stone cold and it's being told to go flat out. It sounds like the sort of treatment for a cold engine that makes us car guys wince.
Is that what happens...or is there an electric heater that keeps the petrol engine's coolant hot, or what?
Just wondering....and no I'm not going to buy one.

Cheers,
Glen

The petrol engine in a lot of hybrids is a "generator set" and not connected to the driveline, so it won't be commanded to go flat out from cold.

Haha, "Honda Prius" .....shows how much I know.

....but, from the Toyota Prius web page "The intelligent
management system seamlessly synergises the car’s
petrol engine with its electric motor so they support
each other or take over completely, depending on the
driving situation.
The electric motor in the Hybrid Synergy Drive®
system generates enough power to drive Prius on its
own. The petrol engine only kicks in when you call for
more power, accelerate hard or cruise at high speed" ....so it doesn't appear to be a 'gen-set'

I'm probably misinterpreting something....

Cheers,
Glen

Obviously another one of those subjects we all have an opinion on. Mine always was and will be is never punish any engine till it is up to temp. Air cooled billet engines are especially vulnerable at the cylinder/engine block interface.

Totally agree. Hence my wondering what I'm missing with design philosophy of a hybrid. The average hybrid driver probably isn't considering if the IC engine is up to temp before calling on max. power.

Cheers,
Glen