Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Large I-6 vs smallblock V-8 idle speed

  1. Jul 29, 2014 #1
    Why does a small V-8 have a slower idle rpm than a large diesel inline 6? For example, a Chevy 305 (5.0L) can idle at 500rpm but a diesel Caterpillar 3406 (14.6L) can't idle below 600 rpm? The Cat ,with a bore of 5.4 inches and a stroke of 6.5 inches, has a much faster average piston speed plus heavier pistons than the Chev with, a bore of 3.736 inches and a stroke of 3.48 inches. Compareable Cummins and Detroit engines also idle at the same speed, so it can't be just a Cat thing.
     
  2. jcsd
  3. Jul 30, 2014 #2

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    without getting into a lot of mechanical head scratching ..my Dodge cummins needs the min idle to keep up oil pressure..you got one very heavy crankshaft and inspite of super high volume of engine oil you still have to float the crank on the oil film..a spindly little small block chevy has a fly weight crank shaft compared to boss hog..and 500 rpm is about it on a V8..my Ford 4 cyl tractor can go down to dmn near 400 rpm but that is even lighter a crankshaft and you can hear every cylinder firing...oil pressure is 20 psi btw...
     
  4. Jul 30, 2014 #3

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    The diesel engine (especially the smaller ones) generally has a much higher compression ratio than a gasoline engine. After all, diesel engines are also known as 'compression ignition' engines.

    If you have a high performance 305, you might have a CR of 9.5:1. A racing engine running on gasoline might top out at 12.5:1 if specially prepared racing fuel is used.

    By contrast, the Cat 3400 series has a CR of 15:1 or 16:1 and is physically a much larger power plant than the Chevy. A Cat 3400 series engine weighs in at more than 3000 lbs, while the Chevy might tip the scales at about 550 lbs. or so. It takes a certain amount of power to keep the Cat turning, working against a high compression ratio while moving a lot of weight up and down and around, running a diesel injection pump and other accessories. The Cat is designed to run at a max. continuous speed of 1800 RPM, while the Chevy is capable of revving much higher.
     
  5. Jul 30, 2014 #4
    majority of standard production engines idles at 600-800rpm. does not matter if 1.0 four banger or 6.0 v12.

    I think it is about oil pressure, and bearings construction and load on them
     
  6. Jul 30, 2014 #5

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    There is also the issue of vibration transmission from the engine to the vehicle (maybe not so important for race cars at idling speeds, or for farm tractors). Keeping the idle speed above any large-scale resonances of the vehicle may be the easiest way to stop the doors and windows rattling.
     
  7. Jul 30, 2014 #6

    462chevelle

    User Avatar
    Gold Member

    We have a couple cars, one in the 15-1 range and one 13.5-1 range and the 15-1 motor idles at around 1500, and sounds like a tractor. The 13.5-1 one idles around 1200. Although its apples to oranges since that is mostly because of the amount of overlap in the camshaft. not having much to do with the compression.
     
  8. Jul 31, 2014 #7

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    I owned a 1984 Z28 with the 5.0 HO motor, which had a carburetor. Because of emissions regulations, the catalytic converters needed to start functioning right away after the engine started, and the only way for them to get going was to get to their operating temp. as soon as possible. The ECU controlling the engine was designed to go to a fast idle speed at about 900 RPM, and once its sensors told the ECU the cats were at temp., the idle speed would drop down to about 600 RPM or so. It took a while to do this at first start on cold mornings.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Large I-6 vs smallblock V-8 idle speed
  1. Torque vs speed (Replies: 13)

  2. 16 Valve vs 8 Valve (Replies: 4)

Loading...