LSX Efficiency: EFI vs. Carburetion

[Ralph LoGrasso]

Exceprt taken from the "Mileage!" thread, posted for Matt (DarkoNova).

One more question (I know, nonstop!), are the LSx engines so efficient that a carbureted version could still get good mileage (comparable to the stock EFI setup)? Reason I ask is, I want the EFI for sure, the superior tunability and the fact that you can drive them everywhere (my friend lives up in the mountains, around 5600 ft above sea level, and my car drives like complete crap up there), but I was just looking at the Car Craft LQ4 build last night and they got something like 480 hp with a stock LQ4, a hot cam "kit", and a carb conversion.

I'm interested in the pretty high power, but also, it'd be a lot easier and quicker for me to just convert it to a carburetor. The only thing is, if it won't get much better mileage than my 350, I don't want to go through the trouble of swapping all the parts.

Thoughts/opinions? I'll probably be picking up a 6.0 in the next week or two if it can get good mileage carbureted. Hell, I'll probably still get it even if it can't, but the carb would get the switch completed faster.



Matt

[Ralph LoGrasso]

In summary: how much efficiency is lost through the employment of a Carb on top of an LSX, as opposed to the factory EFI system.

Any ideas, guys?

Thanks,

[Ralph LoGrasso]

P.S. If we can get some good tech in this thread, I'd like to eventually make it a sticky.

[Andrew McBride]

I would like to hear some feedback as well on this. Is there anyone even running a carb on a lsx?

[parsonsj]

The boys from Wegner get more hp from a carb than a factory EFI.

Are we talking about engine efficiency (VE) or are we talking about gas mileage?

jp

[EFI69Cam]

They probably get more peak HP with a carb due to the air cooling effect of the fuel vaporization.

The factory efi system is going to do a better job maintaining the 14.7:1 ratio over all ranges than a carb, thus better fuel economy.

[68Formula]

EFI has more even fuel distribution which allows for a lower idle setting while maintaining idle stability. Also less wall wetting compared to a wet flow manifold. Faster starts (less cranking), and warm up (engine is more efficient when it reaches full operating temperature). Carbureted engines are not as precise in compensating for barometric and temperature conditions although OEMs tried (remember all the vacuum lines and "closed-loop carbs" in the 80s?).

P.S. Some of the stuff that makes it efficient is still present after converting to carb. For example fewer parasitic losses due to mechanical items (compared to old-school engines) such as mechanical fuel pumps, flat tappet camshafts, engine-driven fans, etc. I would think also the timing curves would be similar.

Are we sticky-worthy yet?

[parsonsj]

They probably get more peak HP with a carb due to the air cooling effect of the fuel vaporization.

Nope. Well maybe that contributes, but they get more area under the curve too. They believe they get more hp due to higher flow rates through the manifold. Better VE, in other words, due to uncompromised manifold design with a centrally located fuel and air source. Now, if you continue to use their manifold (which is the GMPP carb-style manifold) and a carb-style throttle body (such as offered by Holley or FAST) and use aftermarket EFI, the hp levels are roughly the same. But if you pull the manifold and use one of the factory (LS1/LS6/LS2/truck, etc.) manifolds and factory EFI, then you lose a little power -- across the board.

So what are we talking about here? Better fuel mileage? Better VE? Better driveability? Emissions? All of the above?

jp

[Andrew McBride]

Nope. Well maybe that contributes, but they get more area under the curve too. They believe they get more hp due to higher flow rates through the manifold. Better VE, in other words, due to uncompromised manifold design with a centrally located fuel and air source. Now, if you continue to use their manifold (which is the GMPP carb-style manifold) and a carb-style throttle body (such as offered by Holley or FAST) and use aftermarket EFI, the hp levels are roughly the same. But if you pull the manifold and use one of the factory (LS1/LS6/LS2/truck, etc.) manifolds and factory EFI, then you lose a little power -- across the board.

So what are we talking about here? Better fuel mileage? Better VE? Better driveability? Emissions? All of the above?

jp

mileage-this is something that I am curious to hear the differences? I would think that the carb would not get the same fuel economy that the efi set-up would get, but with the carb not being extremely popular among the lsx guys, its hard to tell the true differences.

thanks for your input John

[WS6]

Nope. Well maybe that contributes, but they get more area under the curve too. They believe they get more hp due to higher flow rates through the manifold. Better VE, in other words, due to uncompromised manifold design with a centrally located fuel and air source. Now, if you continue to use their manifold (which is the GMPP carb-style manifold) and a carb-style throttle body (such as offered by Holley or FAST) and use aftermarket EFI, the hp levels are roughly the same. But if you pull the manifold and use one of the factory (LS1/LS6/LS2/truck, etc.) manifolds and factory EFI, then you lose a little power -- across the board.

So what are we talking about here? Better fuel mileage? Better VE? Better driveability? Emissions? All of the above?

jp

TPiS showed the same type results with their spider manifold. Same idea too, the air path is superior because it's not coming in the front like a factory LSx manifold.

Fuel economy should be better with EFI simply because it has the capacity to adjust to everyday changes in conditions better than carb. However, you can tune a carb for that particular setting and it should get the same mileage. A carb is quite capable of running 14.7:1 as well.

Also as far as overall power goes, remember most drag cars run carbs still. Sure it has to be tuned for those exact conditions at the track and even be adjusted as the day goes on, but it still produces power with the carb.

High Performance Pontiac just finished up their article series on a 455 SD that they put EFI on. The carb made more power and it wasn't liek they tuned the EFI wrong or anything. The carb setup was simply superior in the power making department. With that said, I'll still take EFI simply because in everday changing conditions it is superior.

[Ralph LoGrasso]

So what are we talking about here? Better fuel mileage? Better VE? Better driveability? Emissions? All of the above?

jp

I'd like to see discussion mostly concerning fuel efficiency (mpg), emissions and driveability.

A discussion of volumetric efficiency (hp) as it relates to EFI and carburetion would not be a bad thing, however.

[Zedzag]

I run a carb on the LQ4 that I built and put in my 81z. I've been tuning with an LM1 wideband and I'm getting right around 14.2 - 14.5:1 AFR at cruise. IMO you need a tuning aid like this to even try to dial in a carb for good fuel economy. Pulling a plug is not precice enough. I've also logged some driving sessions and compared them to a buddies 2000 camaro AFR logs and I'm on the right track- trying to shoot for similar AFR numbers.

I can't tell you what my MPG is because the car still has the 3000k stall and 4.10 gears with no overdrive but it does get much better mileage than the old SBC which was down around 100 hp.

I run the carb now because I bought a long block which I rebuilt. Factory EFI and overdrive are my next hurdles but I'll most likely do the overdrive first and I'm pretty sure I can get into the very low 20s mpg...maybe better.

WOT power is fantastic and as soon as my foot goes down, all thoughts of mpg disappear for a while LOL.

[DarkoNova]

Good to know Zedzag. One question, though, how are you supplying the fuel? I've seen some cheap $50-ish fuel pumps at Summit. I thought about getting one of those when I got the 6.0, but I'd like to know how you're doing it.

Matt

[Zedzag]

Fuel is supplied by a Holley Blue pump. A little noisey but the exhaust drowns that out once she's running.

[parsonsj]

I would think that the carb would not get the same fuel economy that the efi set-up would get

I'm sure you're right. Computer control and direct combustion feedback via O2 sensors allows for very sophisticated algorithms to maximize fuel economy and reduce emissions, especially if we mean OEM EFI. There is just no way that a carburetor with an idle circuit, steady state jetting, and a mechanical acceleration enrichment method can compete in terms of fuel economy and emissions.

Is anybody disputing that?

If we want to talk about volumetric efficiency (horsepower), then we have a whole different set of things to consider.

jp