Lawnmower Carb on V8

[Noahfreak]

I didn't see anyone discussing this here yet, so I thought I'd post a thread to get it going. This guy just as an April Fool's joke designed a plenum to put this little lawnmower carb on his 302 V8. Because of all the attention it got, he decided to actually build it out and run it with spectacular MPG results. He added a custom IAC valve hack to the system to modulate AFR, which I'm assuming has a lot to do with how efficient it ended up being. I'm interested to see more extensive testing of this but so far this seems like a very promising design. Thoughts?

https://www.youtube.com/watch?v=1xHQ...MeE9aASoe1qUK_

[Isaac Zachary]

I jumped through it in parts so didn't see it all. But I do agree with him to a degree.

Carbs do have an advantage of getting fuel to atomize more due to the distance, and therefore time, that the fuel needs to evaporate in the intake. Direct injection leads to more particulate matter, a by-product of liquid fuel droplets in the combustion chamber.

But angles and turbulance actually tend to be enemies, not friends to atomization. Any liquid that spins tends to spin out of suspention and pool.

Perhaps the most ideal intake would be one that has a single straight runner into each cylinder with a single carb at the end. Also, with the intake designed more for average power, not full power. You want your intake runners as thin as possible to keep air speed up and therefore atomization. Big intakes with a central carb let the air thru slowly at cruising speeds which lets fuel fall from suspention.

The cross-plane V8 design also is more balanced than an inline 4 cylinder or a V-6. The balancing helps reduce vibrations without the need for balancing shafts. Both balancing shafts and vibrations sap away energy resulting in reduced power.

https://www.youtube.com/watch?v=f8XG9T8v-ng&t=789s

[pgfpro]

He should of went way more then only 36miles to get a more accurate test.

IMO the carb is doing better then the other carbs he tested based on a smaller jet and smaller single emulsion tube. The home built IAC valve is making a vacuum leak and taking away the signal from the main jet in which isn't good but hes getting away with it due to the small size of the carb itself pulling a strong signal, but this makes a higher pumping loss.

Hes also picking up more FE due to the small carb not really having much of a accelerator pump like a normal carb. Lots of raw fuel doesn't get atomized with the normal carb.

His A/F read 14.7 and this is where he and others like him don't understand. If you have a decent carb or EFI there will be no unburnt fuel going out the exhaust. So 14.7 A/F, the size of the engine 302CI, throttle angle, Cd, frontal area, mph etc. tell a math story. Plus when he said cruising at 70+ mph are not a great way to sell this.

I remember my first FE testing about 10 years ago I did the same thing went for a test drive under 40 miles went and filled the tank back up and thought I just made the most efficient engine man kid has ever seen. Then I join this site and got schooled and was told on here you need to go at least 300 miles or more to get a real true FE test. lol

EDIT: One more thing most carbs built for cars are from a long time ago (when was the last time a carb was OEM on an automobile) The fuel was way different then with a much higher VOC. Todays fuel has a much lower VOC and isn't really design for carbs. Go watch an classic cruise night when the cars are creeping along at a snails pace and you will see 90% of them with carbs smoking out the exhaust.

[Isaac Zachary]

Originally Posted by pgfpro

but this makes a higher pumping loss.

I generally disagree with intake pumping loss statements. Things like lean AFR, EGR, pulse and gliding, higher gearing and different cam profiles can reduce pumping losses. But an engine cruising at a steady speed needing x amount of fuel and therefore air cannot have greater or worse pumping losses from the type and size of the intake. If the intake really does have greater pumping losses then less air and fuel enter the engine, the engine produces less power and the end result is that the car slows down or can't maintain the target speed.

[ps2fixer]

Originally Posted by Isaac Zachary

I generally disagree with intake pumping loss statements. Things like lean AFR, EGR, pulse and gliding, higher gearing and different cam profiles can reduce pumping losses. But an engine cruising at a steady speed needing x amount of fuel and therefore air cannot have greater or worse pumping losses from the type and size of the intake. If the intake really does have greater pumping losses then less air and fuel enter the engine, the engine produces less power and the end result is that the car slows down or can't maintain the target speed.

I disagree to an extent. Pumping losses can be realized purely by intake design. Take the tesla one way valve design for example, if the intake was designed like this, it would have a very hard time getting much air even with the carb wide open.

How much of a restriction is it for the little carb vs the stock carb at the equivalent power output level, that's a hard one to answer. I'm sure it's measurable, but I don't know the math myself to give hard numbers.

My dad's told me long ago people racing would always swap 2 barrel carbs on their engines. If I remember right the concept was more for the fuel economy and have similar power levels. The larger carbs were oversized for the power output, I suspect accelerator pump generally is oversized, and the 2 barrel of a 4 barrel is smaller so the 2 barrel carb to start with was more of an ideal size. It's just hearsay but a lot of stuff he's told me over the years similar to that seem to be valid.




I suspect there's 2 major things at play for better econ on the lawnmower carb, his self adjusting carb setup is actually a very good idea for carb'ed engines. It's a self adjusting carb that doesn't suck. The secondary effect is I think floored he might be dumping the same amount of fuel as 1/4 throttle on the stock carb. I think since he's under a power restriction, it's forcing him to drive more ideal for mpg. It sounds like his shift points aren't half bad for mpg to my ears at least.

I suspect if that engine was converted to EFI and had a similar throttle limit imposed on it, he would see similar mpg. Either case It's a neat project, not too many people would think of running a tiny carb on a v8. I've wished my Lexus LS400 had finer control for the throttle at times so I could better control the engine for mpg. Almost need 2 throttles, one for 0-25% throttle, and the other that works like normal.

[Isaac Zachary]

I still stick to my theory. If the Tesla valve intake produces greater pumping losses either the engine has to be modified to pull the air in with more force or the car will simply not go faster.

It's basic math. If the engine needs 4 fl oz of fuel per minute to cruse at 60mph then it needs about 3 lbs of air per minute. The amount of air that gets sucked in is determined by the design and the total intake resistance. If the intake produces so much pumping losses that it can't suck in more than 3lbs of air per minute then it can't produce enough power to cruise at 60mph.

If it cruises at 60mph it doesn't matter what intake it has. It has the same suction and the same intake resistance and therefore pulls in the same amount of air with the same amount of pumping losses, tesla valve or not.

Putting on a low pumping loss intake won't reduce pumping losses unless you pulse and glide. That's because the throttle valve will make the pumping loss the exact same. If you really did have less pumping losses then that same engine trying to pull in 3lbs of air every minute would start pulling in more air and therefore more fuel which would mean more power and the vehicle would accelerate instead of hold a steady 60mph.

[oil pan 4]

I found about 20% more mpg to be had when tuning for lean burn.

[Isaac Zachary]

Originally Posted by ps2fixer

if the intake was designed like this, it would have a very hard time getting much air even with the carb wide open.

That's the thing. If we're talking about racing or pulse and gliding or accelerating in general then yes, the intake design will affect pumping losses.

But if we're talking about cruising at a steady speed like 70mph in the video, then the intake doesn't matter all that much.

As you said, the engine will have a hard time getting much air even with the carb wide open. If you can cruise at your desired speed even with the carb wide open you wouldn't benefit, in that state, from an engine that has a huge, low restriction intake because the throttle would not be wide open. It would be shut to the point you end up with the exact same total intake restriction and therefore pumping loss.

[ps2fixer]

The concept in my head is the air has to be physically moved, the effort for that moving is what I'd count as pumping losses. If the intake is very restrictive, then it's harder to move the air, so more work is required to move the air. Basically the vacuum past the throttle plate is pumping losses that could be avoided. A plugged air filter vs a new one would have similar effect and if I recall correctly most owner's manuals says lower mpg can be a result from not changing the air filter.

Change air for engine oil, for the same flow of oil, a smooth pipe vs a pipe that's 100ft long will take vastly different amounts of effort to move. Air acts like a fluid, it's just very thin. Testing how much efficiency is lost purely by a restrictive intake would be an interesting test. I suspect it's measurable but not a massive effect.

The same concept is on the exhaust side, it's a well known fact on diesels if you delete the muffler (and cat if it has one) that mpg will increase for a diesel. That should be effectively pumping losses as well. If intake didn't make no difference, then there wouldn't be dual length intakes for low vs high rpm. Long tube headers vs short tube effects where the peak torque is a bit.

I'm not an engine engineer, so very possible I could be wrong, but nothing is free, every little design change should help efficiency.

The only real pumping losses I'd think of in the lawnmower setup is the wild size difference from the carb size to the intake size. If that is a smooth transition, or I'd think ideally smaller intake tubes, it would have less pumping losses with the same lawnmower intake.

Another point, individual throttle bodies when no air filter is ran has a bell mouth to the intake. Of my understanding it's to reduce pumping losses to achieve higher max air flows. Now I wonder if the mpg effect could be measured for stock intake vs the ITB setup just for a proof of concept. The same areo effects are happening inside the intake/engine as the outer body of the car. Clearly more air = more fuel needed to be dumped. Defo an interesting thought experiment though.

[R.W.Dale]

The 30 some odd mile tank fill using gps is so far from a valid or even halfway done mpg test the results weren't even worth posting.

But if course the people who believe everything on the internet are it up and got him clicks

Did the car get better mpg than before. Probably.

Did he get 41mpg?

No