see the image attached for the 225 HP engine rated speed RPM is 2300. High idle speed is 2680/2760. What is the highway travel speed engine RPM fro this engine?
I don’t think there’s a way to know the answer to that question without knowing the speed of travel and the gear ratio of the rear differential.
the fuller tanny top gear is 1 to 1 the rear axles are 4.56 ratio. I am wondering what is the maximum engine RPM I can operate the engine safely. 2680/2760 does not seam right to me.
According to your chart the peak HP is at 2300, you aren't going to want to exceed that in a sustained way. There wouldn't be much point anyway, it will run out of breath real quick once you get past that. That 2700 looks to be the redline.
Power will start to fall away real quick after peak power RPM is exceeded. So it will be kind of self limiting. Running at redline with no load will not hurt it. We used to run them at redline all day up against the exhaust brake. The faster it spins the better the brake works. Had to manually watch it though. Looks like 57 mph at peak power RPM though on 10R22.5 tires. I was not able to find a calculator that does tire revs/mile any more which is better than diameter which I put in at 38". This seems like par for the course for trucks of the 7.8 era to be geared to travel 55 mph with pedal to floorboard. https://wellertruck.com/road-speed-calculator
The 2680-2760.. let’s call it 2650-2750.. unless otherwise stated is Hi idle no-load speed.. that’s sitting in neutral reving the engine.. Deduct 10% for hi idle loaded.. Moving..Max..
We had the 210 hp version of the 7.8l, in order to obtain 90km/h we ran the engine at 2200-2300 rpm. As noted above these trucks tended to have low gearing with a top speed of around 55mph. Other than somewhat higher fuel consumption (well out of the torque curve no doubt!) it ran smoothly at this rpm. We had no issues on a 6000 km round trip to the Yukon. Currently we are running a 6.6l 170hp version (shorter stroke), it runs at 2600-2700 rpm to get 90km/h. We considered Re- gearing, however, the low gearing is really useful for us off-road. This truck has an Allison 545 and 2 speed rear diff (5.86/8.17). These “brazilian” engines have been very reliable for our use. I did put new Bosch injectors in the 6.6. It was a street sweeper and probably the main engine was never loaded properly…..
I think pumpguy is right. If that turns more than 2400 on the road without going downhill, that's going to be surprising.