Best rev range for Isuzu 4.6 litre engine.

Beaver7 · Member

Best rev range for Isuzu 4.6 litre engine.

Possum3 · Guru

A great looking unit Andrew - If Isuzu produced it as a non-turbo 4.6 I would be trusting their Engineers on this - I would certainly wait until you get a few Klms under your belt before changing the design specs. 6 tonne GVM doesn't need Turbo, particularly with less than 80,000 on the odometer.

Beaver7 · Member

Thanks for your input, I just drove 1200 kms up from Vic and thought I could have a bit more up hills and cruise at lower revs if I turbocharged it.Factory fitted turbo was an option on similar model.

KJB · Guru

Beaver7 wrote:
Thanks for your input, I just drove 1200 kms up from Vic and thought I could have a bit more up hills and cruise at lower revs if I turbocharged it.Factory fitted turbo was an option on similar model.

You would have to change the Diff. Ratio if you wanted to drop cruising revs. (but then probably need more torque - via a Turbo ? - to maintain the cruise speed ...) KB

yobarr · Guru

Beaver7 wrote:
Hi everyone, today myself and my beautiful wife are the owners of a 1998 Swagman Signature 25 ft motorhome, we have been waiting 3 mths to go to Victoria from Sydney and pick it up. Very happy campers, lovely motorhome, just a question about ideal and max revs for the 4.6 liter motor and if we get it turbo charged will we get more power but lees revs?
thank you
beaver

These people may be able to help.They're somewhere near Wollongong,I think? Cheers

Jaahn · Guru

KJB wrote:
Beaver7 wrote:
Thanks for your input, I just drove 1200 kms up from Vic and thought I could have a bit more up hills and cruise at lower revs if I turbocharged it.Factory fitted turbo was an option on similar model.
You would have to change the Diff. Ratio if you wanted to drop cruising revs. (but then probably need more torque - via a Turbo ? - to maintain the cruise speed ...) KB

Hi Beaver smile

Hmm have you driven a heavy vehicle before. They do not drive like a car ! You could just sit back and enjoy a more leisurely drive and arrive half an hour later.

I had an Isuzu NPR Winnebago about the same size, that had the same engine and it had been turbo'd before I bought it, as the person towed horses. It went OK. The turbo will give more torque through all the rev range but the revs stay the same if in the same gear and speed. If you want to cruise at lower revs then as KB said you will need to change the diff ratio. No problem, where is your wallet ? aww

I suspect I would have been happy with mine if it was standard. hmm I do not flog along at the speed limit no matter what, and as we venture further afield we go slower on the back roads. But you might be different. The fuel gauge goes down a lot faster as you go faster too. I must say I do not miss putting that $250 of fuel in regularly.

Jaahn

-- Edited by Jaahn on Monday 8th of November 2021 01:09:23 PM

Peter_n_Margaret · Guru

As a start point, I would choose the RPM that the engine produces maximum torque at or a tad over, for economical cruising.
For the hills, increase revs to that appropriate for maximum power.
If you turbo charge it, you will also need an intercooler.
Cheers,
Peter

SoloMC · Senior Member

If it was me, i would contact the manufacturer for details.
I would also contact the turbo people to get their advice

Aus-Kiwi · Guru

Isnt there a tachometer on your Speedo cluster ? Turbo Diesel engines are less stressed than naturally aspirated! IT WILL pull way better up hills ! But real steep ? It pays to come off throttle and just cruise at low speed till you get to the top ! The governor will stop rpm ! But I dont suggest using that as rev limit . You will feel torque drop off as rpm increases anyway . Plus efficiency of turbo falls off .

deverall11 · Guru

Diesel with less revs = hotter engine.
Confirm using an EGT.
Not the benefit you may think.

yobarr · Guru

deverall11 wrote:
Diesel with less revs = hotter engine.
Confirm using an EGT.
Not the benefit you may think.

Good comment Larry.Many people seem to think that towing in a high gear saves fuel because the engine revs are lower.Not so,as lower revs under load mean the engine runs hotter,so the water is hotter,the oil is hotter,the air through the radiator is hotter and the fuel is hotter.At 6800kg GCM, with 40c outside temp,my car's pyrometer shows 280-360c,and on hills it has,on occasion,reached 450c.This is good,I reckon,and I still get 5.8km/litre, or about 17 litres/100km.Happy with that.Cheers

Beaver7 · Member

Thanks very much for all the input, many good ideas and advice. Ill take it all onboard.I am learning a lot about heavy vehicle driving. Thanks Beaver

Aus-Kiwi · Guru

Well the same can be said for higher rpm and boost also ? Some common sense ? However its not very common ! Diesels are generally low revving anyway !! Heat mainly increases with extra or too much fuel ! Taking things to extreme is bad !

deverall11 · Guru

Aus-Kiwi wrote:
Well the same can be said for higher rpm and boost also ? Some common sense ? However its not very common ! Diesels are generally low revving anyway !! Heat mainly increases with extra or too much fuel ! Taking things to extreme is bad !

Yes, however some are of the opinion that a diesel torque is produced at a lower revs range then the equivalent petrol engine, therefore

it must be laboured/lugged and therefore use less fuel. In fact this is a recipe for overheating and a fallacy. The 200 series Landcruiser has very

clever electronics that prevent you from using too low revs. Eg: the majority of the time, it will not hold 6th gear under or at 100kph.

Yet the rev counter is in the beginning of the fat portion of the torque band. People invariably try unsuccessfully to do so. With a modified

ECU or Chip, it is possible to do so, however the increase in EGTs should discourage you.

Aus-Kiwi · Guru

Yes . At low revs to a point the turbo wont spool much anyway. I have converted a few large turbo engines . I found in general up hills they can be driven one gear higher and possibly 10 kph faster ? Due to better torque, efficiency of turbo
Engine . Its a combination of gears, rpm and road speed ! Being in the middle, in the s w e e t spot . The last thing you want is the ECU to cut in with DBW . (Drive by wire ) isolating throttle !! Leaving you dead on the road !