revving an engine to "warm it up"

I recently had a discussion on this topic. I have heard that revving the engine may do harm and does little to "warm" the car up. This is an older car with an engine that has been modified. This guy revvs the engine at the house and still seems to have trouble making it down his driveway (about 1/4 mile steep downhill) without stopping and revving his engine halfway down and then again at the bottom. Is this type of "warm up" accomplishing anything or is he just destroying his car?

Reply to
huts
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It's probably not good for the car; however, if the engine is "modified" it may have issues with a cold start, esp. if he has a chokeless carburetor (sometimes people who don't live in particularly cold climates will mill the choke housing off the carb for better airflow) therefore he may have to play with it a little to keep it from stalling.

I know a friend of mine has an old Studebaker with a 383 Chev "crate motor" in it; he doesn't have the specs on the motor (bought the car pretty much as it is) but it is pretty hairy (and fun to drive) even after both of us threw all our tricks at it to get it to start and run without a long period of sitting still and revving the engine, the "fast idle" had to be set at about 2000 RPM and it still doesn't always start on the first crank. (of course, it also has dual AFBs on a low rise Offy intake, probably not the hot setup for anything but WOT but it looks cool)

I am guessing the PO didn't ever plan on driving the car in cold weather; neither carb had a choke on it when he got it. We did install an electric choke on the rear carb and that did make a lot of difference, although it's still not the kind of car you'd want to hop in and drive to work (well, I might, but I'd still probably have to leave it sit and "idle" - using the term loosely - for a minute or two in the AM before driving off.)

But yes, your basic premise is correct - the best way to handle a normal cold start, unless it's bitterly cold, is to simply start the car and drive off, but drive "easy" until the engine is fully up to operating temperature.

nate

Reply to
Nate Nagel

Both! Yes, that can reduce the time to warm the engine. But, WHY are we warming the engine? Primarily to ensure proper lubrication from the oil. If we rev it up before that, we are not getting proper lubrication, so why do it? This is kind of self defeating.

We ordinarily think of warming the engine before stressing it as contributing to reduced wear. But if we wear it excessively WHILE warming, we gain nothing.

Admittedly, if this is an older car with a carburetor, drivability improves when the car is warmed up. But again, drivability is bad when he is revving it anyway. Better to ease it along and warm it that way. If it tends to stall, he should play with fast idle rpm setting.

Reply to
Don Stauffer

Not the best for it, but likely it is stalling out so needed.

I have a carb engine that came stock with a 'timer' choke. The sucker goes open according to how long the engine has been running with no reference to how 'hot' the engine actually is. These types of chokes are a pain in the ass up here in Canada. I would stall every time I let off the gas pedal in the first 1/4- 1/2 mile unless I revved it until the engine had really warmed up. 3 foot driving style or heel toe on the gas brake for a while.

I fixed it's case by putting a manual choke in. I now can keep the fast idle at 1200 until it is actually warmed up and drive away in any temperature.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail >
Reply to
Mike Romain

Exhaust is often shut off, and other things are closed for quick warmup. shouldn't push the engine under these conditions as you can cause stuff to wear out more quickly. Probably not as hard on newer cars, but also even less beneficial.

Reply to
dnoyeB

He will minimize the wear on the engine during warmup by just getting in the car and driving it, keeping the engine at a steady and low rpm. If the engine doesn't idle well when cold it is probably in need of repair. Much less likely in my experience is that the car is a highly tuned street racer. The many cars I've driven over the past 15 years all use an engine computer to make warmup and overall running very smooth. If taken care of there should be no problem with a modern engine holding an idle when cold.

Reply to
John S.

It depends on the car. If its an older car that is unmodified or lightly modified, he's got a PROBLEM that needs fixing (choke flooding the engine or not applying sufficiently, etc.) Even fairly heavily modified engines can be made to behave 'tolerably well' when cold if all the systems are working right.

If its an extremely highly modified car, being horribly cold-natured is just part and parcel of living with a high-strung engine. Its hard on it, but its probably not designed with longevity anyway. If its a question of stopping to rev up and blow the unburned fuel out, versus stalling and restarting over and over- then revving it a bit is the only choice.

That is without a doubt one of the sillier things I've ever read on an automotive tech newsgroup.

The BIGGEST risks for a cold engine are:

1) fuel dilution in the engine oil and "washing" down the cylinder walls with raw fuel. Mostly a problem with carb'd cars, though EFI does this to s limited degree as well. Prolonged idling to warm the car makes this much worse than just driving gently.

2) Excessive wear because clearances are larger when cold. This isn't normally a big deal as long as you give it time to get the oil circulating before putting a full load on it. Even audible piston slap doesn't really cause excessive wear most of the time. BUT, the car is more sensitive to how HARD its driven when cold. Romping on a stone-cold engine will wear it more than driving it gently for a few minutes until things come up closer to operating temperature.

Reply to
Steve

Do you mean that incoming air is first routed over the exhaust manfold to warm it and thereby speed engine warmup?

Reply to
John S.

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What really happens in a *modern car* is the thermostat opens gradually, and in cold weather might take 20 minutes to open fully. It doesn't *snap open* like a switch. It starts out fully closed, but as soon as the engine gets to about 170 degrees F, it opens a little, and as the coolant becomes hotter, it opens more, until at around 200F it is fully open and oscillates back and forth allowing the water to become a little warmer, and than a little cooler, producing something like a sine wave with respect to the temperature signal coming from the engine heat sensor.

I don't see how that would make any sense. True that the coolant is circulated through the heater core according to dashboard settings, rather than the thermostat.

Lg

Reply to
Lawrence Glickman

Go back and look at cars with carbs in the '70s and you will see this air ducking from the exhaust manifold to the air cleaner. You talk about "What really happens in a *modern car*" well computers and EFI are a different world from a modified engine with not exhaust heat to the intake manifold, no choke on the 2 4 bbl carbs and that has a cam that when the engine is warmed and running normal will not idle at less than

1200 rpm and best of all was high test leaded gas for 36 cents a gallon. Stan

Reply to
Stan Weiss

He is talking about a heat riser valve that shuts off one bank of a V engine's exhaust and routes it across the intake for warming the fuel/air mix.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06
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Reply to
Mike Romain

I believe this is what he is talking about. Scan from an old Chilton's.

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Reply to
Stan Weiss

No, all carb engines have heated air cleaners, that has zip to do with the exhaust except to suck warm air from a box over it. Modified engines tend to have aftermarket air cleaners missing this so can ice up easy. That heat pipe opens and closes as the temp of the air filter changes so you can drive without the carb turning into a block of ice.

They also need to heat the air in the manifold for a more efficient warm up. This is done various ways. Some older Jeeps for instance used a heat riser like the V8's but the newer carb'ed Jeeps used a heater element at the carb base with engine coolant flowing through the manifold.

The older V6's and V8 used a manifold cross over and shut off one bank of the exhaust with a weighted flap on a bi-metal spring.

I must be showing my age here.....

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06
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Reply to
Mike Romain

its not good to "rev" a cold engine.

the heat "riser" routes exhaust heat....off the manifolds up to the air filter housing. Its not exhaust...its heat. There is a valve in the air filter housing that controls the heat (it often wears out over time. THe valve regulates the amount of cold air through the regular air pathway, and the amount of hot air coming off the exhaust. Thats the way it works on the older GM vehicles. I don't imagine that any car re-routes exhaust....its just the heat coming off the manifolds. No connection to turb equipped cars and trucks.

Elbert snipped-for-privacy@me.com

Reply to
Elbert

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I believe it. If you're into antique automobile, there are better n/g's than this one.

Yes, that's my point. Modern car.

What a waste of fuel. My engine idles at 700 rpm.

There was nothing *best* about those old tanks.

Lg

Reply to
Lawrence Glickman

Heat riser or more correctly, EFE (early fuel evaporation) is often a vacuum or bi-metal controlled flap in one of the exhaust manifolds that re-directs hot exhaust gas to a chamber on the bottom side of the intake manifold underneath the carburetor. This passage often serves as the feed to the EGR valve also. Another method is to use an electrically heated grid that mounts under the carburetor, i.e., early 80s GM "X" and FWD "A" bodies.

It's both.

You're thinking of the Thermostatic Air Cleaner (Thermac). Entirely possible to have both a heat riser -and- a Thermac.

Here's an example of bi-metal spring operated one:

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Here's an example of a vacuum operated one:
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That would be the thermostatic air cleaner.

Reply to
Neil Nelson

I don't think so, if you say all all carb engines have heated air cleaners. In the '60s check out the muscle cars and show me a heated air cleaner. Stan

Reply to
Stan Weiss

When someone asks about a modified engine telling them about your factory stock / modern car does them little good. The point is to answer the question that was ask. Stan

Reply to
Stan Weiss

Nope, you are talking about the thermostatically controlled air filter and it's heat pipe. The GM V8's and V6's of the 70's and 80's with carbs and TBI's used a heat riser valve to shut off the driver's side exhaust manifold forcing all the exhaust to cross over under the intake manifold and exit out the passenger side pipe until it warmed up. I haven't checked the newer ones specifically for that.

Here is a link that shows both:

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The first photo shows the air filter heat pipe setup and the last photo shows the heat riser valve to shut off the exhaust on that pipe.

Mike

Elbert wrote:

Reply to
Mike Romain

Hair dryer. Is what I use on my landscaping equipment in wintertime to get engines/carburetors warm enough to start. It is the only thing that works for me 100 times out of 100.

No iced carbs, no frozen fuel lines, just vrooom putt putt putt

Hair dryer. For my landscaping equipment. A car? Lift the hood, use remote switch, blast hairdryer on carb. vrooom putt putt putt

I still say, 1.8 liter is little more than a lawnmower engine where I live. Not to sound like a snob, I owned a Renault R8, and a Fiat Sardine Can for many years. I know the problems that go along with these *more economical* vehicles. That's why I will never own one again.

Lg

Reply to
Lawrence Glickman

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