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Author Topic: Optimizing 8v71 High Altitude Performance  (Read 3051 times)
Lin
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« on: July 06, 2013, 08:12:33 PM »

A lot of the trips we have been taking involve mountains. A non turbo 8v71 is really doing its best at around sea level, so we are getting poor performance a good part of the time.  We are not going to turbo it, so forget that option.  My question is whether there is a way to tune the engine's performance at some altitude, like 3000 feet so we would get some better power higher up, but not cause problems lower down.
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HB of CJ
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« Reply #1 on: July 06, 2013, 08:21:51 PM »

Not much you can do, other than going to a smaller injector and the rare high altitude blower gear train that speeds up the blower.  I am not sure if that is going to help at ALL altitudes; setting up the N engine for high altitude helps up high, ( a little) but MAY not improve things down low.  HB of CJ (old coot)
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RJ
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« Reply #2 on: July 06, 2013, 08:43:41 PM »

Lin -

The BEST way to get the most performance out of your 8V71N while playing around on Rocky Top is to remember which character you're driving based on Aesop's Fables "The Tortoise and the Hare" - and it's NOT the Hare!

 Grin
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RJ Long
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« Reply #3 on: July 06, 2013, 09:06:14 PM »

Since we opted not to turbo our 8V71 during our rebuild...  we're making sure our music and podcast collection is well stocked for helping get through the high altitude roaming Smiley

 - Cherie
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Cherie and Chris / www.technomadia.com
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Lin
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« Reply #4 on: July 06, 2013, 09:43:33 PM »

It is not so much the speed of the ascent that I am concerned about.  On the last trip, the bus did not have the power to pull the Jeep through a hairpin turn at 6000 feet.  We had to disconnect to continue.  This is always an option, but it would be nice to have been able to just keep on going.  Of course, as mentioned it is not worth the cost of a turbo for me.
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« Reply #5 on: July 06, 2013, 11:41:37 PM »

Not much you can do, other than going to a smaller injector...

I'm curious how the injectors affect performance at altitude.  In the next day or so we need to decide what injector to use in our 8V71 rebuild, and I am still weighing whether to go N70 or N65... 

Will the N65 outperform an N70 driving at 6,000 - 8,000 feet elevation?  How about doing a pass at 10,000?

  - Chris
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« Reply #6 on: July 07, 2013, 01:15:10 AM »

 If given the choice, I'd prefer turbo with smaller injectors over non turbo with bigger ones. Being a newly rebuilt engine in the hot summer weather, I'd go with N65's. You could always upgrade down the road when you decide to turbo it or desire more power and/or tune-up. You can use a lower boost turbo with standard compression pistons.
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« Reply #7 on: July 07, 2013, 02:26:07 AM »

When we have been in not enough power spots, we put a non steering driver in the toad, start her up and actually help push the bus. A few hundred feet will usually do it, stop, reset toad, retrieve driver and off you go. Amazing how much this helps. Just the ol' country boy way.     06 Bill
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bevans6
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« Reply #8 on: July 07, 2013, 03:47:48 AM »

Power is roughly based on fuel and air, so when you put more fuel in than the engine can burn based on availability of air, you run out of steam.  You kind of hit a wall where more fuel doesn't add much power.  So optimizing the airflow with the turbo is always the first option, second might be over-speed the blower, always have the intake filters and exhaust optimized, and then at the end of the day turn the engine faster.  More RPM equals more bangs per minute, equals more power until you hit the intake and exhaust limits.  Mostly this doesn't work in a bus, you only have a few gears, the road tends to dictate speed so your rpm is what it is.  You could usefully raise the high idle setting a few hundred rpm, maybe to 2400 or 2500, and get to stay in a lower gear longer with good effect climbing a hill but there are other impacts from that change.  Different injectors will add more or less fuel per stroke, maybe change the curve of how they add fuel as the rack opens or closes, maybe change the atomization of the fuel in the chamber and affect burn speed but at the end of the day when it starts to roll coal (smoke heavily) from too much fuel for the amount of air you have, you are getting all the power your combination can make.

Brian
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« Reply #9 on: July 07, 2013, 06:43:40 AM »

Before I turboed the 4-71 in the Courier 96, I built a ram air scoop for the air intake on the roof. Remarkable how well it worked starting at 20 mph: a lot less smoke out the tail pipe.

JC
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JC
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Lin
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« Reply #10 on: July 07, 2013, 07:31:56 AM »

I like the idea of the Jeep giving the bus a boost in a jam.  I do not know if it would have worked in last weeks situation, but I'm sure it could help sometime.
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« Reply #11 on: July 07, 2013, 07:56:54 AM »

When I turbo'd my engine, I went all the way by having a custom air to air intercooler made and went with larger injectors. I know you don't want to turbo, but consider this-if you do install what we call a "smoke turbo" it will put out about 5psi, you keep the same injectors, don't have to change anything on or in the engine and it keeps the same power from sea level up to altitude. It is the only real way you'll get the same performance. Yes you could go with a different gear ratio on the blower to spin it faster-but then you'll have a hit on fuel mileage all the time. The nice side benefit of installing a smoke turbo with the same injectors is you'll have more power coming from the same injectors. For instance, with the same N65 injectors, the non turbo engine will be 304hp and 800lb/ft torque. Add the turbo and you'll be at 325hp and 975lb/ft torque-just more efficient.  Talking from experience-turbocharging really wakes up the 8V-71 even without changing the injectors. Good Luck, TomC
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« Reply #12 on: July 07, 2013, 08:11:13 AM »

I don't think Don will work on a bus now but find someone to install a high capacity blower they made 14 different blowers for a 8v71 just for different operations and altitude they do fine with the different blowers at lower altitude turboing one is &&& even with out a after or inter cooler 


The best way to go is the turbo if you need or want all the power
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technomadia
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« Reply #13 on: July 07, 2013, 08:45:18 AM »

Thanks everyone for the great tips!

I never knew there were different blowers for the 8V71. Are they just geared differently, or do they have different lobes as well? Our Detroit book makes no mention of any options other than "Blower Speed is 2.05 Times Engine Speed". If there are 14 different types, is there a way we could tell whether or not ours might already be different than stock? Where can high capacity blowers be found?

I am still a bit confused though as to the tradeoffs between injectors. All things being equal (standard blower, no smoke turbo, no full turbo) - are there places/cases where an N65 will outperform an N70?

The shop here is happy to install either - we just need to figure out the tradeoffs and pick one.

Cheers,

   - Chris

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muldoonman
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« Reply #14 on: July 07, 2013, 09:29:46 AM »

Hey Cris, just tell the shop what you want, the most power you can get safely and reliable. If they are Detroit 2 stroke guys they should have a solution for you. 
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