Advantages to bigger turbo on pump gas?

[GN and Sho's]

I'm sure you guys are sick of this topic, but I keep debating what combo to go with. I don't go out looking for races and don't drive the car very often. I just want the car to be as fast as possible on pump gas. I currently run 94octane with 15lbs boost. Would a bigger turbo help if I'm still on pump gas?

Or is the only alternative to run an alky set-up?

Thanks for any help in my confusion.

 

[azgn]

very little, if any, advantage to a larger turbo under 20 lbs boost...spend your money on an alky system

 

[QuickWE4]

You're in the same boat as me. I am currently stepping down from a 51 toa 49 as I don't drive mine a whole heck of a lot, but when I do, I mainly just want to have fun on the street.

Sweet spot for the 51 is like 22psi +, but most of the time, I'm just gettin on it a little around town, so the bigger turbo really does me no good.

I did a Mease perf. intercooler (stock core, bigger neck), THDP, exhaust, no cat, a chip, all the fuel mods, injectors, and a good chip, and I found that it was plenty fast for me. And I did all that when I was running the stock turbo, and it was a beast on the street.

However, I run 100+ octane in there all the time. Since you don't drive it that much, splurge on the race gas (Sunoco GT100, for example) if you don't want to be worried about tuning for alky, etc.

Someday, when I'm more skilled with tuning these cars, I'll probably jump to alky.

Rich

 

[Reggie West]

It really does not matter if you use a big turbo or a small one.

A big turbo at 15 on the street is the same as the stock one running 22 psi. It is airflow that counts. That is why you have to have the higher octane fuel.

Alcohol kits do the same thing, they raise the octane of the fuel so you are able to run the amount of air you are looking for on the street.

 

[Red Regal T]

I'm really puzzled when people talk about "tuning" alchy. What the hell are they talking about????? You basically install whatever kit you desire, fill it with alchohol, set it up in a few test runs, and your done. It's about as difficult as changing your oil.

 

[Intercooler]

From what I see at the track and on this board people have problems getting the correct alchy on setting to not bog the car early, jet size that may be too big and some are running way too rich with them. Probably more work for someone that knows little about tuning than someone that has been doing it for years.

 

[ST. RACE]

Bigger turbo

Does a bigger turbo make (push out compresser) less heat, than smaller turbo at same psi. Resualting in lower intake temp were you can run more boost/same with better I/C.
A ??? as much as any thing.

 

[ijames]

In general the bigger the turbo the more efficient, which means the cooler the compressed air at any given boost. That means that at the same boost you will have cooler, denser air, and will be able to turn the boost up some at constant octane, so yes, to me there is a bigger advantage to a big turbo with pump gas than with race gas. I can run 22 psi boost on 93 octane with no alcohol injection, with a PTE54 turbo and a CAS V2 frontmount with 20 degrees of timing, with no knock. With my previous turbo, a little TE34, I could only run 18.5 psi, and with the TE34 and stock intercooler I could only run 15-16 psi. Car went from 13.20's to 12.60/105 to 12.30/110 typical with a best of 12.18/110.5, all on 93 octane. Naturally there is a price, and that is spoolup. You have to decide if you can live with a somewhat soggy bottom end (my choice, so I don't instantly blow away the tires), or live with a 3000 rpm stall 9" converter. I give up a few tenths in the 1/4 but the car is great to drive, the power comes on smoothly and controllably, and passing on the highway is such fun .

 

[RealFastV6]

Re: Bigger turbo

Originally posted by ST. RACE
Does a bigger turbo make (push out compresser) less heat, than smaller turbo at same psi. Resualting in lower intake temp were you can run more boost/same with better I/C.
A ??? as much as any thing.

Precisely. A puny inefficient stock turbo wont run 19psi on pump gas because its putting out air thats HOOOOT, on the other hand a larger turbo will not only be able to squeze a couple more psi out on pump gas due to the efficiency of the compressor side, but the 19psi of boost that you can run will be more efficient in itself... cooler air=more air.

 

[azgn]

normally, when the combustion charge gets more efficient (cooler or more dense) more fuel or octane is required.....in other words, a big turbo will detonate much sooner than the "hot" air stocker simply because it is more efficient!

if your experience was otherwise, I would suggest you did not have the stocker maxxed out.......put a bigger turbo on, turn the boost down or add some octane!

 

[RealFastV6]

Originally posted by azgn
normally, when the combustion charge gets more efficient (cooler or more dense) more fuel or octane is required.....in other words, a big turbo will detonate much sooner than the "hot" air stocker simply because it is more efficient!

if your experience was otherwise, I would suggest you did not have the stocker maxxed out.......put a bigger turbo on, turn the boost down or add some octane!

Uhh, It'll require MORE fuel of course, yes, but certainly not more octane as long as the fuel supply is there. The cooler your intake temps are, the cooler your combustion temps will be, and the LESS octane you'll need.

If you've got a stock turbo pushing 19psi and 300 degree intake temps, then your combustion temps are going to be... ta da... 170 degrees more then a larger turbo pushing 19psi and 130 degree intake temps.

The only reason you'd need more octane is to band-aid a lack of fueling.

 

[azgn]

well, I guess we'll all go out and buy TE-70 turbos and run 25 lbs boost on pump gas?

You are forgetting about cylinder pressure and knock...if you wanna do it, go for it!

nobody ever said a bigger turbo is less efficient.....it is just that you will run out of octane before you ever max a stock turbo........on pump gas

 

[cool 84]

Originally posted by azgn
normally, when the combustion charge gets more efficient (cooler or more dense) more fuel or octane is required.....in other words, a big turbo will detonate much sooner than the "hot" air stocker simply because it is more efficient!

if your experience was otherwise, I would suggest you did not have the stocker maxxed out.......put a bigger turbo on, turn the boost down or add some octane!

Finally, someone I agree with. I've never bought into the bigger turbo= more boost for a given octane. When both me and my dad had no intercoolers and the stock turbos we could run 19-20psi with no knock on 92. He converted over with a big FM and a TE62 and I ran a FM IC and a TE64 and both cars would detonate in the 16psi range on 92 octane. And yes, we both had plenty of fuel. You can make more hp for a given octane but not run more boost.

 

[RealFastV6]

I'm saying that you will probably be able to squeze a couple more psi of boost with the same octane gas with a larger more efficent turbo as long as sufficient fuel volume is there.


Just my .01 though...

 

[cool 84]

You're reducing backpressure and getting a cooler intake charge but at the same time you're increasing cylinder pressures which negates the cooler intake and less backpressure as far as boost level is concerned.

 

[REDS HOT AIR]

weird nobody has tooken into consiteration of backpressure at a boost level..

from what i understand is the stock turbo has a 2 to 1 ratio meaning at 18# of boost you have 36# of back pressure in the exaust ...then you take a more efficient turbo with say 1.5 to 1 ratio and at that same 18# of boost you have 27# of back pressure

better exaust flow is a cooler cylinder which turns to less knock on a given fuel octane....

if people would quit thinking stuff more in and turn to letting more out then alot of people on here would be much faster than they are.

 

[UNGN]

At 18 psi of boost on a stock motor, the compressor outlet temps of a stock turbo aren't much different from the biggest turbo you can bolt on. The heat is due to the fact air is being compressed, and it doesn't make a great deal of difference how it is compressed or with what. A stock turbo is efficient at 18 psi with a stock long block. It may not be at 22 psi, but 22 psi on pump gas is not normal. With any turbo.

The exhaust back pressure side of the equation as Red pointed out is likely where any gains would be made. Any horsepower that is freed up would come from here and not from a cooler turbo outlet temps, which isn't all that different in reality.

 

[strikeeagle]

A larger turbo will:

1.) Exacerbate turbo lag.
2.) Provide a HOTTER charge air temperature.
3.) Be more likely to surge.

 

[RealFastV6]

Originally posted by strikeeagle
A larger turbo will:

1.) Exacerbate turbo lag.
2.) Provide a HOTTER charge air temperature.
3.) Be more likely to surge.

1.) Yeah. Enter ball bearing turbo.

2.) No, the stock turbo is getting on the hot side at 18-19psi. A slightly larger compressor will provide a cooler charge at the same psi. Also of course as Red pointed out the decrease in backpressure from the larger turbine would free up some horsepower as well.

3.) If your turbo is built correctly then you shouldn't have to worry about this. Surging is often times a sign that either a) the turbine section is too small and the motor can't move what the compressor section is pushing, or b) you're trying to run way too much psi with a small turbo. The pressure in the intake becomes greater then the pressure coming out of the turbo and the turbo literally wants to turn around and spin the other way. Very bad things can/will happen.

If I were picking out a turbo that would go fastest on pump gass then I'd be looking for a larger turbine anyhow with only a lightly larger compressor. The more exhaust gasses you can scavenge out of the cylinders then the more cool mixture you can get in.

 

[strikeeagle]

I didn't mean as compared to a stock turbo running way off its design point. I meant that a stock turbo running at its design point will produce a cooler charge that a larger turbo running well off its design point. Evidence the compressor curves. Lower efficiency = more heat.

So far as surge goes, for a centrifugal compressor stage, surge is the condition where there is insufficient flow for the particular head (boost, if you like). If the compressor associated with a large turbo can't deliver sufficient flow into the motor for the boost you're running, trust me, it will surge.

Also, I think we're confusing the relationship of turbine and compressor. It is the expansion of hot exhaust gases across the turbine which provides the power to drive the compressor, not the other way around.

A bigger turbine wheel and cover are wonderful from a pressure drop point of view, but not if you need a 4000 stall to get the motor to make enough air to drive the turbo to design speed.