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Another 8V-92 Boost concern
I know this issue has been beat to death but I have a concern about the boost on our 1992 Prevost 40′ and if it is “normal” or not. It has been upgraded to 500 hp by Detroit Jax.
I say normal due to my several calls to Detroit/Allison and Prevost that left me as confused as I started. Our bus had a 15 lbs. boost gauge in it when we bought it and it pegged it so I installed a 25 lbs. gauge after reading about all the 20 to 25 lbs. readings others were seeing. Still showing 15 to 16 lbs. on new gauge.
The only performance issue we had was initial off the line power, but I have cured that issue with adding a boost module
inline with the boost sensor. Now I am wondering if our mileage would be improved if the boost was higher.
I keep reading posts on different forums that say boost on a late 80’s early 90’s 8V-92 Silver TA should be in the neighborhood of 20 lbs. to 25 lbs. At least that is what they say they are showing on the dash. I think Jon said his 8V-92 was also in this area.
Mine is showing 15 to 16 lbs. @ 2100 RPM’s and the pyrometer is reading 1100 F.
Now my questions to Detroit Tampa and Prevost Jax. was should 15 lbs. be enough ? Does the pyrometer reading increase with more boost and if so, should I persue more boost ?
Their answers were 15 lbs. sounds low but it depends on the engine configuration.
Before I really dig into it, I thought I would ask if other 8V-92 TA’s are really reading 20 to 25 lbs.
Also do other Prevost in this vintage come with 15 or 25 lbs. gauges? The Detroit tech asked me that and I was not sure if the 15 lbs. gauge in ours was original.
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35 Replies
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Our 1992 Beaver #34 has a boost guage that has notated hash lines up to 25. Our coach had Silver 8V that was rated 500 hp for model shell year 1992. At full throttle, our boost is 21-22 and pyros are roughly 1100
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A very minior exhaust leak on a 8-V 92 will cost you boost. Been there done that..Look for any carbon traces before turbo..
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What Bob said.
The 8V92 was notorious for blowing the exhaust manifold gasket or cracking the cast iron manifold or developing a crack in the tubing ($$$$$) from the manifold to the turbo.
If the engine is dirty it is very hard to see the small exhaust leaks so if needed clean it up and maybe even put a fresh spray coach of silver so you have some contrast to see the sooting from a leak. The hardest leaks to see are at the back end of the manifolds although the leaks can occur anywhere.
I don’t remember if all the engine configurations generated 20+ pounds of boost. Early models DDEC I and maybe early DDEC II that were 475HP might have only developed 15 to 16 PSI.
I would not worry about sluggishness coming from a stop. The way the 8V92 is designed it is an absolute dog until you can get some boost and the engine doesn’t develop boost at an idle. It really comes alive from 1500 on up and it is an engine that likes to be driven like you stole it as long as you keep the coolant temps low. It is not an engine to be lugged.
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I wrapped all the exhaust from the manifold to the turbo when we got it so I am now removing the wrap to see if there are any leaks.The engine is actually very clean and the silver is in great condition. I do not see any soot on the battery side manifold so I will go under and check the radiator side manifold as well.
I am also going to pull the turbo inlet hose and see if it spins free and smooth.
By the previous post, it sounds like the boost should be up around 20 lbs. and if the pyrometer readings don’t go higher than 1100 at that boost, that is what I am going to try and achieve.
Jon, the low end power use to be an issue,,, so much that the light would almost change before we could get through it. But now that the performance module is installed inline with the boost sensor it comes alive off the line. It feels like it has more low end performance than a series 60. At least the two series 60 40 footers I test drove.
Thanks for the replies, hope I find a leak.
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Don’t have a boost gauge on my 1991 Silver 8V92TA and wonder how often your pyro hits 1100 degrees? Never seen mine over 900 when pulling these hills around Central Texas and thought that was high. Keep the RPM’s around 1850 when cruising. Plenty of power from take off to cruise. Just wondering. Also pull F150 super Crew Ford behind. Now that I said that if will go though the roof.
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I appreciate the reply,
Mine will go to 1100 if I put it on the floor on a hill, and I think it would stay there if I did not lift. I usually let off enough to stay around 900 but I do lose some speed on a hard hill.
I too stay about 1700 – 1800 cruising and about 400 – 600 on pyrometer.
Sure hope I find a leak,,,,,,,, had stuff to do today but will hit it hard in the morning.
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I would thrash my 8V92 and my pyros would peg at around 1100 when I drove hard. Under normal cruise, level driving it would run between 500 and 700.
Foot off the gas pedal, coasting down it would drop to the bottom of the gauge and using the jake down a long hill I would see 300 on both if I used the full jake or just 300 on one side if I was using the low setting.
Apart from helping to identify which side of the engine had a misfire or had a bad cyclinder I never did figure any other beneficial purpose to the pyros other than to see if something had changed. There is no known pyro limit that I could ever find in any Detroit literature.
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Well, this is what I like about this forum. I called myself looking for exhaust leaks last year and again this week and did not see anything, the silver on the sides of the engine looked clean.
Then after reading the replies I felt sure my boost was low and pretty sure we must have a leak somewhere.
Sure enough, I had to climb under the engine compartment and there along the passenger manifold was lots of black soot but it was on the step of the block preventing it’s detection from above looking in from over the batteries.
Two of the studs came out and I had coolant everywhere. Found a couple of 7/16″ bolts to screw in while I removed the stuck nuts from the studs and resealed them.
Now I am resurfacing the manifold to remove the deep pitting.
Can’t wait to reassemble and see what the boost looks like.
Thanks very much for all the replies,,,, definately poked me in the right direction even though I thought I had done a thorough job before, the reassurance of what the boost should be and the probable cause inticed me to look again.
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When putting back on;notice step it sits on–bottom– it is slight and easy to get under manifold surface=cracked manifold when tightened in the wrong position. Fwiw. Bob
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Another critical thing to remember when assembling the manifold to the block is to be very careful to torque to the proper values. Those manifolds are notorious for cracking and the gaskets are subject to blowing out. If everything is right life is good.
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Well, finished resurfacing the manifold and reinstalled, thanks for the concerns about the steps. Been there and done that with an 8V71.
We only gained about 2 lbs. of boost,,, but it makes the boost much sooner and the difference in acceleration is significant.
Waiting to see what it did for MPG. next month.
Thanks again for all the input.
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Having read this thread and others, is there great vale to having a boost guage? Looking in my original manual it showed the guage as optional. Needles to say I do not have one! Not knowing the expense to purchase one and the possible pain to install it, what are the benefits? Have owned the bus for just a year now and am getting more famillar every day with the unit. Just turned 100,000 miles and all seems to be good.
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I think it is a great tool. My issue was the lower readings were from the day we bought it (used) and did not know any different.
After reading several posts about the boost others were seeing I decided to look harder for any reason our boost was 5 lbs lower, and found it.
If the boost had been higher from the start I would have noticed when the reading went lower and looked immediately and continuously until I found the cause.
To me the boost is just like the pyrometer gauges, If you monitor them like the oil pressure and temp they can tell you if there is a problem arising.
Being involved with aircraft and streetrods I am a believer in gauges, and pay close attention when they differ from normal readings.
As far as trouble installing, other than the tubing you have to run all the way to the back, you would also need to have room in your dash for the gauge.
If it is running well you probably don’t need it. And if you have owned it long enough you can feel any loss of performance.
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I’d like to echo Robert’s comments. I also got used to the gauges as a pilot and no matter what I drive a gauge scan is second nature. We pilots tend to burn “normal” gauge indications into our brain and when something is different, even a half needle width we are not satisfied until we find out what has changed and why.
Our coaches are the same way. I have a couple of problems with them however. First, there is no real definition of “red line” or limits to be observed. I would feel significantly better knowing what Detroit, Allison and Prevost consider both normal and an upper limit. I have a real good idea but I would like a listing of “limitations”.
My second gripe is I think certain gauges are left off to minimize questions or concerns expressed by customers. I had pyros on my first bus and by knowing what to expect under various conditions I was able to discern a change in my engine for example. But I have not had one since. I know on our buses the EGT which is what the pyros measure is out of our control and is what it is because of DDEC, it is still a piece of information that might help with the diagnosis of an impending problem. The boost gauge is the same thing. We step on the gas pedal and it goes up, we lift off the gas pedal and it goes down. But how high is up? Or at least how high is the maximum when we are at sea level at 59 degrees F and does it reach that maximum?
My solution is simple, not cheap, but effective. I have a Nexiq reader that I will plug in and that my wife will read for me while I drive. The minute I suspect anything changes I plug it in to see what I can see. Others have used the Silverleaf which also reads the data.
I do like the fact that the newer coaches do have a message center, do allow for more detailed readings, and do report codes.
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Raveson,
Check out the Speedhut website. I changed out all but my Twin Pyrometer gauges at the end of 2012. I had a Boost gauge as you describe that “pegged” at 15. Put the new gauge in and checked for Boost readings on some trips. New Gauge read 17 -18. The only gauge I had problems with was the fuel gauge. It took 3-4 back and forths to get it right. A knowledgeable Prevost person suggested purchasing a new fuel sender, remove the existing sensor wires and connect them to the new sender, and then “program the gauge using the new sender at full, 3/4ths 1/2, 1/4, and of course empty. When finished he suggested I return it for full credit.
There are other sites that may have a gauge to fit your style.
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