Boost and EGT

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19 Oct 2019 10:02 #1 by Holmz
Holmz created the topic: Boost and EGT
My measured the boost under WOT:
RPM. Boost(psi)
1500. 5
1700. 7
1900. 9/10
2000+. 10

Cannot find any loose hoses and no IC is in it, so leaks there are not possible.

Question #1: What do others see for boost?

Question #2: is what EGTs do people see, and are their pyrometers installed before or after the turbine?

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19 Oct 2019 20:46 #2 by RobS
RobS replied the topic: Boost and EGT

My OKA has a waste gated turbo so I see 17psi for all of that rev range. I have fuelling set to limit EGT to 550C pre turbo.

With the original turbo I have no boost data. EGT peaked @1500rpm ~525C, that was measured post turbo at the time. Above 1500rpm the EGT would drop and if I remember correctly was ~480C @ 2200rpm again, post turbo.


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20 Oct 2019 14:06 #3 by Peter and Sandra OKA 374
Peter and Sandra OKA 374 replied the topic: Boost and EGT
My Perky pulled 17lb boost and 650 C pre turbo from the factory, never did anything to it, intercooled Cummins 6bt will go to 25lb and 620 odd pre turbo if you really get stuck into it but usually the Allison grabs another cog to get it back on max torque before it goes past 22lb/600.

OKA 374 LT Van, converted to camper/motorhome,
400ah Lithiums, 1100w solar, diesel cooking heating and HWS,
Cummins 6BT, Allison 6 speed auto, Nissan transfer.
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20 Oct 2019 15:40 #4 by Dean and Kaye Howells
Dean and Kaye Howells replied the topic: Boost and EGT
Hi Holmz, what you see is pretty standard for a non externally waste gated turbo. The boost increases with revs as a pretty linear function and for the Perky topping out at about 12 psi if you rev it hard enough :)
Whilst this is wonderful if you have a constant rev application for your engine it's pretty bloody ordinary otherwise. What was Oka thinking ?, probably bought the engines on the cheap.
The big disadvantage of this system in the Oka is that with a manual transmission you go off boost when you up-change gear which makes for pretty ordinary performance. Not as much of a problem with an auto as the torque converter acts like a big rubber band between the engine and the gear box but still not 'optimal'.
Mort imported some externally waste gated turbos which overcome this issue and to all accounts apart from being a bit 'fiddly' to fit they work well. For an explanation of how good this can be Rick Whitworths write up is a good read. I guess this is what RobS has (assuming a Perky).

It's not a lot different on #413 with with the 6BT Cummins. My engine came from a mid '90's Dodge Ram automatic and intercooled engine and has a non wast gated HC51 ? factory turbo. Whilst this may have been OK for the Dodge Ram application it is hardly optimal for the non intercooled manual Oka. Never the less it's still a hell of an improvement over the factory Perky :)
I'm looking to upgrade the turbo to a suitable Holset HX35W if I can find a good American made one. I'm a bit leery of the Chinese made turbos as quality varies greatly and it's too late after the engine has ingested failed turbo components.

For the same reason I'm a fan of the post turbo pyrometer. Apart from being easier to install on my Cummins there's little worry about dropping swarf into the exhaust when fitting the temperature probe. Similiarly there's no potential issue if the probe burns off and falls into the exhaust. For a pre turbo sensor any 'loose items' will enter the turbo and may cause damage on the way through. Pre turbo EGT's are typically 100-150 degrees C hotter than post turbo and I reckon the probe will have a longer life in this more 'benign' environment. :)
As to actual operating EGT's my experience has been that they vary greatly depending on a number of factors such as engine load, road surface, tyre pressure, ambient temperature and probably altitude.
My 'rule of thumb' for EGT's (post turbo) is up to 300 degrees (dreamland in my setup), 300-400 (cruising & OK), 400-450 (starting to work a bit but still OK), 450-500 (working harder but still OK), 500-550 (working hard, think about changing down), 550+ (danger Will Robinson change down), 600+ (oops, should have been paying more attention hope I haven't broken anything :(
Note. my EGT's rise exponentially/very quickly once above 500 degrees, it's VERY easy to get caught out.

An example of external issues affecting EGT's on our recent Kimberley trip was when we'd been running at reduced tyre pressures on rough tracks (45psi rear/40psi front) and returned to the 'main road' (a good quality graded loose stone surface road). We had 50Km or so to travel along this road before turning off on to a rough track so I decided not to drop pressures but to drive at reduced speed (75-80 kph) instead. My EGT's (5th gear @ 1500/1600 rpm) were in the 500+ C range necessitating change back to 4th gear. I stopped and put an extra 6psi in each tyre and could then easily hold the same revs/speed at 100 degrees C LESS EGT's. It's worth noting that a major contributing factor here was my diff ratio/wheel size not being 'optimal' but IMO a good example never the less.
So I guess Holmz my answer to your EGT question is "How long's a piece of string" :)

Deano :)
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21 Oct 2019 10:31 #5 by Greg Boyle
Greg Boyle replied the topic: Boost and EGT
Or in your case Dean
How long is a piece of Telstra Rope !!

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21 Oct 2019 18:30 #6 by Ralley
Ralley replied the topic: Boost and EGT
From my experience egt’s are affected mostly by the amount of boost for a set amount of fuel. Example I turboed my cruiser years ago it was a DTS turbo when towing my camper we would run about 6psi and 400c at 95. I’ve since changed to a Gturbo it now runs 11psi and 300c for the same situation.
The same thing happens when tuning the Cummins if you let it get to much fuel before it spools the egt’s will spike. With my Cummins there’s a fine line to get it spooled and not over fuel it before the boost rises, when it hits the sweet spot it’s fine right through to 28psi. This can be more of a problem with half throttle when you nail it it spools pretty quick. The Allison tends to short shift a bit much will low throttle input A program change would make it better.
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