Dave WM

Trip stats, gas milage, oil consumpion, operating temps.

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    Just wanted to document a recent 800 mile trip (central fl to central al) driving at night, mostly AC off about 27mpg, speeds were 55-75 with mostly 65-75. none alcohol gas. a little less than a Quart of oil (28 oz approx. over 1400 miles on the trip odometer set at the last oil change) so about 1500 miles per quart. Temps pretty much right at the E of the temp gauge, guessing this is 175f since that is what I use in the T stat, and it would get there and hold. It will get hotter (so T stat is no longer controlling the max temp) if I drive in the heat of the day with the AC on, then it will get just a little over the middle of the range. this is 95-98f with high humidity and AC on. I think I am going to try the Alum rad I have on the test stand to see if it is able to maintain the max heat of 175f.

    This also is with the use of a ZX 5th gear overdrive use much of the time (anytime I am over 65 mph). with that setup I get 80mph at 3k rpm.

    I have the skinny tires at 30psi. They are 185/75 R14 Hankook brand. I noticed after putting this on vs the wider recommended size 195R70 that the car rolls easier.

    I don't know the effect on MPG with the AC on constant.

    Overall very happy with the stats, but would like the oil consumption to be closer to 3000 miles per quart (my standard distance between changes). I have good overall compression, can see no signs of oil burning (blue smoke) have no external leaks, so I assume its just the oil control rings letting a bit too much past. the plugs do not present any obvious excess oil, and the book say excess is something like more than 1Q per 600 miles (that seems like a lot) so I suppose from that point of view my oil consumption is not excessive.

    The  oil consumption was based on about 800 miles of the trip and 600 miles of non trip around town (the total of 1400 miles since the oil change). I don't know if highway use vs around town use impacts the oil consumption.

    lastly oil pressure about 2/3 indicated on a 90 psi max scale after warm up at anything much over 2500 rpm (I would say 90% of the time I am around 2500 on long trips). On a cold engine its more like 3/4 indicated. This is cold as in mid 70's as the cold temp on VR1 20/50 racing oil (high zinc).

     

     

     

    Edited by Dave WM

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    ah forgot to mention that, I did replace the intake valve guide seals. I passed on the exhaust guide seals, as I read that they do not account for oil burning. the old seals were "slightly" stiffer than the Nissan OE replacements, the new ones also seemed "slightly" tighter fitting.  I get no noticeable smoke on startup.

    Forgot to mention this is with at least 105k miles, as I owned the car starting with about 85k. Maybe its 205k... If so it has very good compression (about 160-165 psi warm across all cylinders).

    Edited by Dave WM

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    I used to drive an old 911 that, according to the technical specifications,  would consume about 1 quart per 500 miles.  Aftet reworking the motor, yep, it used 1 quart about every 500 miles. 

    The car was so fun to drive though, I didn't care.  I also didn't ever compute fuel economy,  which I would always do as a mental exercise after leaving the pumps.  I just didn't want to ruin my fun by knowing how bad it was. 😳

    The info you've posted will serve as a great point of reference for monitoring engine condition. 

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    9 hours ago, heyitsrama said:

    dave what rear end are you running on the car? 3.7?

    3.54 stock rear end that was in it with the 4 speed. 80mph at 4k rpm in 4th.

    with the OD 5th gear Its a bit tall as so I cant really use it until get to at least 65mph (5th gear), would be a problem if I had hills to deal with, as that would make the engine lug even at that speed, but in Florida its flat everywhere. I have a 3.7 spare diff I could install (would prob be a nice upgrade) to make that OD gear more useful around town or on trips to other places with actual terrain to overcome.

    Edited by Dave WM

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    I think those post-trip numbers are fine.  I'd be happy with those too. 

    If you put new rings in you'll probably use less oil, but there's no way I would go that deep in just for that.

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    At 105K, I wouldn't be bothered by 1500 miles per quart of oil.  That's about where my 1983 280ZXT is, and has been for the last 20-30K miles. I had the valve seals replaced recently but don't have a new number on oil consumption to compare with yet.  

    Agreed, with a ring and valve job you could reduce oil usage but spending a couple thousand or so to save on $9 quarts of oil seems quite extreme. You can buy a lot of oil for a couple thousand bucks.

    Edited by Pilgrim

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    I still have the spare engine with the MN42 head. I may after retirement try that one just to see how it goes (just curious and with retirement time is in abundance). As is the engine pulls nicely, so agree, no hurry to mess it up. My goal has been to see how well I could set up for MPG since I like to take long road trips and prefer to cruise rather than blast off.

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    As for the trip, I recall driving one of these (a 1974 260z) when it was new, and my 75 seems to have aged well, It has low road noise, and suspension was soaking up the minor bumps in the road well. The biggest improvement was the new tires, while my old ones looked fine, they caused a wheel shake, I assume it was the tires could have been the rims as I replaced both tire and alum slotted rims with the new tires and steel rims.

    Edited by Dave WM

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    On 6/30/2020 at 9:31 AM, Dave WM said:

    This also is with the use of a ZX 5th gear overdrive use much of the time .... with that setup I get 80mph at 3k rpm.

     

    Yup, me too.

     

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    That 5th gear makes a big difference in highway cruising, and the drop in RPM does provide a bit more quiet.

    My daily driver is a 2008 BMW 328ix Sportwagon with a six-speed tranny and about 130K miles. When I get out of it to drive my 83 ZXT I'm amazed at how much louder the ZX is. It doesn't help the noise level that I had the rubber bushings changed for poly, and that also made the ride much rougher on sections of bad road.  

    I added a radio with bluetooth phone capability, but if I'm traveling over about 50 MPH there's no point answering the phone since it's hard to hear and the mic picks up as much car noise as it does my voice.

    Edited by Pilgrim

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    I have heard that a lot about PU bushings, why I bring it up when folks start talking about upgrades to suspension. Really depends on what you what, for me since my plans are 0% performance driving, and mostly long distance trips, I wanted to keep it a quiet as possible. I replaced the OE cat back with the cheapo standard size one sold for something like 80$ delivered. it was a tad long, but other than that an easy bolt on operation and keeps the exhaust noise to a very low level. New window rubber keeps the wind noise out. Its no lexus but its not that far off from one either.

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