L67 supercharged vs turbo gas mileage

[bs009]

So i know blowers are super inefficient across the board and I think that that's one of the main reasons why the L67 has let me down so much in the gas mileage category. Having a smaller pulley I'm sure doesn't help at all either since it's just beating the air even faster.

I know tons of people have converted from supercharger to turbo, but I'm not sure how many have cared about or reported on the gas mileage department. Any of you guys have stories to share about that?

I found a good deal on a really low mileage Regal GS that I'm going to nab this weekend. Seller says it needs a new engine but the mileage is so low and the price is so good that I can't pass it up. My guess is the engine is probably just fine too.
I'd like to build it as an amazing cruising car/DD so I'm not going to go crazy with this one. I'd like to see how much gas mileage I can get out of a 3800 while still having tons of power, so of course I'm thinking of going turbo.

I've been thinking about it a lot because it costs me just as much to DD my 2500 Silverado as it does to pay for premium gas and insurance on my Regal despite it getting better gas mileage. I've been driving about 30,000 miles a year on average too


So here's the idea I've been tossing around today:
- replace supercharger with a small quick-spooling turbocharger. keep the boost low to save the trans. Not trying to go crazy fast here for a DD/Cruiser car. I think I can do this for about $1,000
- If it needs a new engine, replace the L67 with an L36 for the higher compression. Would be a junkyard motor so probably $150.

Here's why I think this could work out well:
- When the cobalt SS went from SC to TC it saw a ~10% improvement in gas mileage
- L36's were rated at about 10% better mileage than the L67's despite having shorter gears

Between going SC to TC and switching from L67 to L36 compression in theory I might be able to see a 20% improvement in gas mileage.

I know it would take a long time to get my investment back if I did do this (~100,000 miles ish lol). Obviously there are other reasons why turbo seems like a good idea too: less heatsoak, fun noises, turbo would be better in mountains too, and because I think it'd be fun

Is this a dumb goal? lol

Click to read more...

 

[v6buicks]

I'm sure it's a bit difficult to do a truly fair apples to apples comparison. However, I would imagine that just removing the blower, adding an appropriately sized turbo, and tuning it would get you a little passed L36 MPG. Spinning the blower is only a giant parasitic loss when you're just cruising on the interstate where the turbo is only using energy that's otherwise wasted, but you knew that already. Sorry I don't have numbers! I'd be curious to know what you find out though!

 

[GTPpower]

If you are getting bad gas mileage with a l67 car, you have a problem someplace. They should be getting very close to 30 mpg, if not over.

A small quick spooling turbo still will not have even close to the response that a m90 does.

The m90 uses about 1/2hp to turn at cruising speeds. A turbo will most likely eat up more than that.

Swapping in a l36 lower end will help some due to the bump in compression, but don't expect a huge amount. Maybe 1 or 2.

 

[10sec]

Ultimately the best MPG and money savings you will get is with an l36 with no supercharger or turbo and use 87 octane, but where's the fun in that?

This idea sounds fun and you'd be able to get decent MPG, but long term a budget turbo and turbo kit will fall apart. If you made the kit out of stainless steel and bought a reliable turbo it would last a lot longer, but 100k miles idk... There's a lot of research and testing that goes in to factory turbos to make them last as long as they do.

I say go for it though, why not?

 

[Mattstrike]

You might have to start experimenting with timing and fuel strategies to see what works. Just have a look at the VW dieselgate debacle - they knew they could run a "tune" that resulted in higher emissions but also higher fuel mileage. Of course, diesel vs. gas isn't exactly apples to apples, but every manufacturer has to tune their vehicles to meet emissions - you don't. The thing to remember is that all engines are just heat pumps. Read up a bit on the Otto cycle (the physics bit) if you want to know why a turbocharger increases the overall efficiency of the system (basically harnessing waste heat energy from exhaust to do the pumping work - https://www.sciencedirect.com/topics/engineering/otto-cycle).

I would think the goal with a gas/turbo car would be to lean it out a bit and run more timing, hotter plugs, and keep it 87 octane safe at low throttle/cruise. The issue is you get more combustion heat - higher temps inside might stress parts, which leads down the EGT rabbit hole. As long as during cruise it's still pulling vacuum you don't have to worry about knock causing issues, so you can tune more aggressively there, but you'd have to keep close tabs on the timing and fuel as it transitions into boost to avoid knock.

The other thing to consider is revs/mile and finding a sweet spot where revs are low and being able to hold the gear (and TCC) up a fairly significant grade. I don't know how much control DHP has over shifting tables, but set 4th to hold to like 70% throttle before it kicks down kind of thing, or maybe the performance shift button can function as a trans eco button? Also trans ratio and tire size will play into that. Next is the obvious - manual trans swap but that might be more of a project than you want.

Other supporting mods: opening up spark gap, porting and polishing (flow restrictions add to pumping inefficiencies), polish the entire combustion chamber to a mirror finish (reduce heat transfer from the combustion cycle to the engine), equal length exhaust primaries (mainly to reduce heat transfer variation between cylinders, improved flow characteristics reduce pumping work), not having an intercooler on the turbo (hot charge helps with atomization), running the 192 thermostat (experiment with hotter ones?), enlarge the coolant crossover in the LIM (again, consistency in temperature between each cylinder), put the IAT in the manifold to use it as more of a charge temp indication (more consistency with the tune).

I'm not suggesting that there aren't practical limits. How hot can the intake get before the charge temp actually causes issues with knock, or are you better off heating the fuel in the rail? Go the Northstar route and add oil squirters to actively cool the pistons? What else can I think of that goes against the grain of KISS it's just a pig iron 3800?...

 

[Ron Vogel]

Easy 30+ mpg on an l67. With e85 it’s going to be bad. Add timing everywhere you can, and keep intake temps cool. Avoid piggybacks “mini-afc”.
Intercooling is going to help tuning for mileage.

 

[bs009]

Most of the threads I've read through had reports of mid to low 20's in average gas mileage. Typically on longer trips cruising between 75-80 mph I would get somewhere around 26mpg even with some timing sprinkled in the low airflow ranges of the timing table. I don't think I've ever gotten 30mpg at least where I drive.
My car did have about 230,000 miles on it though and that might've been a factor. Typically I would average under 20 mpg every week. I do drive to work downtown in Detroit though and that's gotta be another big factor. I was also running 245-45/18 tires though and I'm sure that wasn't helping. Might consider going with a thinner tire this time then.

I never considered that an intercooler could improve fuel mileage, that might be something worth looking into...

E85 is also on the table because of the huge difference in cost here vs premium. It would end up paying for injectors within the first 10,000 miles from what I've calculated. E85 is pretty consistently around $2.25/gallon or less and premium will swing between $3.25 and $3.75 a lot

I know I'll be losing a pretty good amount of throttle response by switching between SC and TC and I'm okay with it I think. I figure it'll at least be better than my truck especially with some tweaking in the shift tables.

... long term a budget turbo and turbo kit will fall apart. If you made the kit out of stainless steel and bought a reliable turbo it would last a lot longer, but 100k miles idk... T

Yeah definitely something that I thought of since it will see winter use. Stainless will be a bit more expensive but should be worth it. The most expensive parts will be the little things: tubing, wastegate, bov, oil lines, exhaust pipes, etc. If I need to replace the turbo at some point I'd be okay with that since it should just be bolt-in and go at that point.

The other thing to consider is revs/mile and finding a sweet spot where revs are low and being able to hold the gear (and TCC) up a fairly significant grade. I don't know how much control DHP has over shifting tables, but set 4th to hold to like 70% throttle before it kicks down kind of thing, or maybe the performance shift button can function as a trans eco button? Also trans ratio and tire size will play into that. Next is the obvious - manual trans swap but that might be more of a project than you want.

DHP can absolutely do all of that. It's usually the place where a canned tune will make the biggest difference for L67 cars with the downshift to 2nd. The shift points will be a huge factor in gas mileage and getting the turbo to spool quickly and I'm sure it's going to take some tweaking.
I would definitely go manual if there was a cheap route to go too, but there aren't a lot of budget parts out there for that.

I'm trying not to go too overboard here with some of the tiny tweaks. I'm definitely not going to be hypermiling the car in Detroit traffic, and probably not too much on roadtrips either. If I can just find ways to make what's there more efficient though that'd make me feel better at least

 

[Mattstrike]

The intercooler itself won't help with mileage, unless there's something else going wrong or not dialed in like the charge temp is getting too high and causing pre-detonation, or if it would get you a few degrees more timing before seeing knock (in which case you'd want something smaller and specifically sized to deal with it). Maybe there's a sweet spot on a blower car where you could lug the engine at a lower rpm with a little boost up a hill where the extra power per stroke pays off by keeping a lower rpm. I could see it situationally helping. But I don't see it doing much while just cruising, and I'd think you could gain more in adjusting the tune itself rather than need the intercooler.

I'm trying not to go too overboard here with some of the tiny tweaks

Why not go all the way with it? Run the car as is for now and on the side build up the test rig and swap them one day. Could be fun to test these theories out.

 

[Ron Vogel]

Anything smaller than a stock pulley is going to make a heat pump. You need an IC to add any useful timing

 

[PANDA]

come on Ron. what do you know about intercoolers and 3800 tuning?

 

[10sec]

Intercooled cars were always the easiest ones to tune, they loved timing and stayed dead consistent. I remember Ron Vogel saying it's like driving on a cold night but it's all the time and that is 100% true.

 

[GTPpower]

They don't create heat with the bypass open though.

 

[bs009]

Found an interesting PDF from GM about the superchargers from way back in the day on one of the old 3800 sites. Looks like GM found that it does only draw 1/2 hp with the bypass valve open but mentions that it's still not a big deal because you're using a smaller engine that can put out the power of a bigger engine.

1/2hp is pretty small but if you guesstimate that at 70 mph you're probably using 20hp to maintain that speed, you're losing about 2.5% of your power to spin the supercharger. Not a big number but I'm sure in the engineering world it's a big deal. I'm sure with a smaller pulley that number would increase exponentially. I have no idea what that would look like for a 3.4" pulley where the supercharger is spinning 10% faster though.
Of course you're almost never consistently maintaining a speed though. Almost anytime you accelerate moderately you'll build boost and create extra heat and that's probably where most of the benefits of intercooling might come into play.

People want too much for an M90 intercooler for me right now though so I think that's out of for now. It would probably cost me about $500 to piece that all together since everyone wants like $200 for the stack itself. A2A intercoolers are a dime a dozen in comparison, I'm even finding A2W heat exchangers around $100 too if I did want to go with that for the turbo route- it would probably help decrease lag and would be easier to plumb buuuuut idk yet

 

[GTPpower]

I had a chart once that showed loss per rpm. The highest it got was around 1.5hp. I'll have to try to find that.

 

[Ron Vogel]

If you guys ever want information about the development of the l67 or the cobalt ss ...my boss was on the development team from eaton. They were planning on intercooling them, but cost was an issue

 

[GTPpower]

It would have been interesting to see where the car would have ended up if they had been intercooled.

 

[Ron Vogel]

It would have been interesting to see where the car would have ended up if they had been intercooled.

Well, the cobalt got the intercooling. It’s pretty weird I ended up at a place working with this guy. We both have BMW’s with n54 engines too. We talk a lot about cars at work.

 

[GTPpower]

That must be fun. It's interesting how we find people that mod weird cars.

 

[bs009]

Picked up the car over the weekend. Looks like I should be able to save the motor. Last person who worked on the car installed new LIM gaskets wrong apparently and also didn't clean the old gasket material from the intake. It was basically drinking coolant because of it. I'll know soon if the bottom end was damaged from having coolant in the oil, but I think I might be holding off on going crazy here with swapping things on this for a few more months. The whole car only has 74,000 miles on it so it should be a pretty good example of what kind of mileage these cars should get.

I did go ahead and get injectors so the car is going to run on E85 though. It'll save some money and help cool down the charge temp too as a bonus.

 

[Ron Vogel]

Pretty sure I was running 50’s, positive I had to use a boost a pump to augment it. I was right on the edge of fueling at 19ish psi.
I’d get bigger than 60cc for sure. Just don’t go crazy in size or you’ll never get a good idle. A lot of the bigger injectors have shit metering at low duty cycles

 

[GTPpower]

My 80's idle like stock and I changed nothing more than the ifr table. My buddy was running 95's on pump gas and those idled well, but he had to adjust the latency some to help idle fueling. He's now on 125# injectors and e85, and it's being a bit more difficult.

 

[Ron Vogel]

My 80's idle like stock and I changed nothing more than the ifr table. My buddy was running 95's on pump gas and those idled well, but he had to adjust the latency some to help idle fueling. He's now on 125# injectors and e85, and it's being a bit more difficult.

Yeah I admit it’s been few years since I’ve had to deal with injectors so there had to have been improvements in construction. I’m DI now, it’s a whole different world

 

[bs009]

I've had pretty good luck with the Siemens injectors and idling so far. My GP is running their 80 pounders so I went and got the 60's for this car for now. I found them used for a pretty good price so I jumped on it.
I figure if I need more down the road I can always sell these and go bigger.

Good to know about the pump though. When the car goes turbo I will definitely be upgrading the pump too then!

 

[Ron Vogel]

I've had pretty good luck with the Siemens injectors and idling so far. My GP is running their 80 pounders so I went and got the 60's for this car for now. I found them used for a pretty good price so I jumped on it.
I figure if I need more down the road I can always sell these and go bigger.

Good to know about the pump though. When the car goes turbo I will definitely be upgrading the pump too then!

Watch the fuel pressure/duty cycle like a hawk. When you start hitting 15+ psi boost it’ll shelf off and fuck with the tuning. There’s a little wiggle room with base pressure, but not much.

 

[Yaj Yak]

did not read

it's gonna be shit

turbos sound fun so youll like to make the sound then youll drink gas