High Speed OKAs

02 Nov 2012 20:48 #1 by dandjcr
dandjcr created the topic: High Speed OKAs
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Dean & Kaye Howells
Posts: 79
In a couple of recent threads there have been some amazing claims as to the engine speed of the Perkins Phaser 110 engine. Cruising engine speeds of 2800 and 3400 rpm have been mentioned. I don't think so. On paper #413 cruising at 2800 rpm in 5th would have me doing around 140 kph and an incredible 170 kph at 3400 rpm. I don't reckon the OKA'd go that fast if you dropped it out of a plane!
The Perkins Phaser as used in the OKA is an old style diesel engine with a comfortable cruising speed of around 2000 rpm. Whilst it is capable (in theory) of 2800 rpm, so I'm told, I can't actually find any power or max. rpm settings in any of my OKA literature. But 2800 rpm max. is about right for this type of engine, though I wouldn't like to hold this engine speed for too long. The most I've had #413 reved to is 2600 rpm in low 4th when starting an ascent of sand dunes and that was adventurous enough for me. At 2800 rpm I'd be scared of pushing a piston through the seat and as for 3400 rpm it reminds me of the old Mission Impossible shows..........................'Mr Phelps this (OKA) will self destruct in 5 seconds'.......After a short period of valve bounce the engine would most likely drop a valve and turn itself into a boat anchor.
What is most likely is that the alternator has been changed at some time for one with a smaller diameter pulley. As the tachometer reading is dependant on pulses from the alternator, having the alternator spinning at the correct speed is essential for an accurate reading. A faster spinning alternator (smaller pulley) will read high, a slower spinning alternator (bigger pulley) will read low. The tacho will also read low if its v belt is loose. This happened to me on the CSR. The tacho wouldn't read when the engine was started from cold but when the slipping belt had 'warmed up' would read OK up to 2000 rpm but no higher. There was no screeching slipping noise associated with this as I would normally have expected from a slipping belt.
I just measured the dia. of #413's alternator pulley at 2 7/8" or for those more metrically inclined about 74mm and reckon this is about right. Generalising, I reckon 1900 rpm - 2100 rpm should give 100kph in 5th gear depending on wheel diameter.
An even scarier scenario for high reving is a slipping clutch. In hindsight this also happened to me on the Tanami Road between Bililuna and Halls Creek. I couldn't work out why I was doing 2500 rpm in 5th and was only going 97kph. That's 97kph on the speedo and the GPS. At first I thought it was wheel slip on the corrugations or a tacho fault. Used half a litre of oil on this section, the same as I'd used on the whole CSR. Replaced a leaky rear main seal and worn ceramic clutch when I got home and havn't had a recurrence of this fault. This was potentially damaging to the engine as it was over reving and could have been damaged.
The OKA speedo is easily calibrated by setting the DIP switches on its rear, and can be set very accurately. There is another trap here though. I set #413's speedo to read accurately at 100 kph and it's within 1% of the GPS at this speed. Unfortunately the odometer is out by 5%. I only discovered this on our latest trip when I had unexplained errors in navigational distance measuring. This is a big issue for us as we rely on accurate distance measuring to locate tracks, turn offs etc. What is interesting here is that at 60 kmh on the GPS the speedo shows 57 kph, a 5% error. The moral of this story is to re-calibrate the speedo at half scale, 60 kph, and hopefully the odometer will be correct. At 100kph the speedo should then read 105 kph, not a problem.

December 6, 2011 at 3:32 PM Flag Quote & Reply

Peter & Sandra James Oka 374
Posts: 413
As I said in the other thread I suspected the tacho was out and will check it with a totally seperate unit when I get home from this trip.
I actually checked the tacho revs at 100kph today and it says 3400 rpm. As max power is produced at 2850 it is obviously wrong.
I have a later model VDO electronic speedo with odometer and tripmeter! which is calibrated over a measured kilometre by various pushes of the tripmeter button via the internal menu so the odometer should be spot of but the speedo is about 2kph high.
Oka 374 LT Van

December 6, 2011 at 8:06 PM Flag Quote & Reply

Ewart & Vivian Halford
Posts: 117
I have an old spec sheet for an LT max power. 81 KW @ 2800 rpm, 354 Nm @ 1600 rpm, max engine speed is 3150 rpm.
December 6, 2011 at 9:40 PM Flag Quote & Reply

Dean & Kaye Howells
Posts: 79
Hi Peter, it might be an idea to look at the alternator pulley first. If my questionable calculations are correct I'd expect a pulley diameter of around 44mm for the sort of readings you're getting which are around 70% high. A problem here could be that the alternator may be spinning too fast for it's own good and could fail prematurely. It might just be a pulley change or spacer change between the pulley halves that is required to fix the problem ?
Another issue, though I don't know if it's a problem or not is that as the alternator delivers full output (assuming small pulley) at a lot lower speed than normal the dual battery coupling solenoid will probably operate at close to idle speed. This solenoid is fed from the same pulses that drive the tacho. This can result in excessive battery charging load on the engine / v belt at low engine speed. On #413 the solenoid pulls in at around 1000 prm.

December 9, 2011 at 10:38 AM Flag Quote & Reply

Dean & Kaye Howells
Posts: 79
Thanks for the info Ewart. I looked in the OKA operators manual, the Perkins Phaser User's handbook and the Service and repair manuals to no avail. The cutting edge performance of the Perkins must be why this info is such a closely guarded secret!
I'd be way too scared to run the Perkins at anywhere near 3150 prm though.

December 9, 2011 at 10:40 AM Flag Quote & Reply

David Hallandal
Posts: 133
The Tacho is calibratable through a small hole in the rear of the unit. Most Diesel workshops will have a tacho they can plug in to get an accurate reading of the engine RPM and then adjust the Tacho to suit, My OKA ran at 2800 to 3200 all the time.

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December 9, 2011 at 12:03 PM Flag Quote & Reply

Posts: 2
I am interested in any comments re revs and perhaps advice on driving when using gas assist. Our Oka really gets up and goes when the gas is switched on. When not on gas the motor sounds as though it's doing it comfortable.

December 10, 2011 at 7:54 PM Flag Quote & Reply

Hal Harvey
Site Owner
Posts: 510
Hi Peter and welcome along. I see you bought David Rosenzweig's LT when he took delivery of his new NT. I can't offer any advice on driving with diesel/gas as I don't have the gas and have never driven a vehicle with it, but others may have; Frank Mercuri in Victoria has it and may post information here; there was also a brief couple of discussions back in the day here and here.

Re real revs, the calculator here is one that has been referred to a bit. Any common XT/LT with original gearbox and diffs will have a 0.79 fifth gear and 4.88 diffs. It tells you that at 100km/h, real revs would be:
2150rpm with 325/85R16 (37.8")
2300rpm with 285/70R 19.5 (35.2") (what you probably have, and the diameter Frank has but his are on 18" alloys)
2475rpm with 285/75R16 (32.8")
for example.

December 10, 2011 at 9:12 PM

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