1. All my comments are based on my experiences with many different diesel, manual trans 4x4s.
2. I would choose the diesel over the gasser for offroad stuff any day of the week, and twice on Sundays.
3. I don't know how to do a meaningful calculation to scientifically compare the two.
In the diesel, each compression stroke is compression 700cc of air at 22:1
4. In the gasser, you'll get mininal compression because it's under vacuum. Under vacuum, it's still drawing some air into the cylinder, but not the full volume? Static compression ratio of 10:1, but partial vacuum drops the actual compression ratio.
5. No sure how vacuum helps?
6. Are you saying the resistance of the gas engine piston pulling a vacuum on the intake stroke outweighs the resistance of a diesel engine piston compressing a full pot of air to 20ish:1
7. The difference is notable on the street too.
Driving the fzj80 manual, you back off the throttle, it decelerates, but gently.
In a diesel, back off the throttle, they decelerate hard!
I added some numbers to your comment so I can address individually. My comments match up below.
1. I am not trying to detract from your experience in any way
2. I would too, that’s why I’m doing a diesel swap in mine.
3. Calculations aren’t even needed for this, it’s higher level ideas that govern what’s going on here.
4. Compression ratio isn’t playing a role here in any way, shape, or form (for the engines we are talking about, if we were talking about 14L Cummins or Volvos that would be different because they have
compression brakes added to them).
5. it’s not the vacuum itself that’s helping, it’s the work the engine has to do to
create that vacuum that matters in this situation.
6. There are outside forces at work on the gas engine (in this case a restriction to air being allowed into the engine by the throttle plate, or whatever they are called) and none on the Diesel engine. It literally does not matter what the engine is doing inside of it in this scenario, with “this scenario” being one where no fuel is being added to the engine and your foot is off the skinny pedal.
Think about this example for a minute: say you are looking into a room with an electric outlet and a fan. Said fan is plugged into outlet, and fan is on. Work with me on this when I say “imagine this room is perfectly insulated”. This means ZERO energy of any kind can leave the room. What happens to the temp in this room? Up? Down? Same?
7. Here, I have to disagree with you. I have owned diesel vehicles for over 20 years, worked in them, worked with them, worked on them, set world records with them, watched them runaway on the dyno from turning the fuel screw too much, hauled many a trailer over mountain passes, etc, etc
Coming down a mountain pass with a 5.9L diesel powered rig with manual transmission and a trailer in tow, if you take your foot off the
brake, the vehicle will accelerate until there is so much inertial energy in the rotating components of the engine that it will come apart, all on its own. No if’s, ands, or buts. Does not matter in the least what gear you’re in. This is why semis are
required to have
compression brakes installed on them.
Come down the same pass with same trailer and a 6.0L cheby V8 in it with manual trans, drop a gear so you have high rpms, take your foot off the go pedal and that thing is going to slow the hell down.
This is why companies like PacBrake are in business. They make lots of things, but consumer grade exhaust brakes are what they are known for. This device is literally the same throttle plate from #6 above, but put in the exhaust pipe of a Diesel instead of the intake pipe of a gas engine. Instead of the gas engine sucking air through it, the Diesel engine has to push exhaust through the exact same device. Then,
and only then, will you be able to take your foot off the brake pedal going down a hill with weight behind you.
en.wikipedia.org
