Twins vs. big single

[bigbird]

I am having some trouble deciding between twinning my silver66 with a GT47 or going big single ie. Phatshaft 71, GT4202/magnum, s400, s480,etc.. cost is an issue in this decision, but the major concerns are to keep egt's, drive pressure and rpm's reasonable. my truck has run 11.45 and the goal would be around 10.75. truck is a somewhat gutted crewcab s/b. motor has cut/coated pistons, rods, studs. no cam or heads. I know if money was no issue twins are a no brainer, but is the extra cost worth the additional performance to reach my goal. also i am not opposed to running a small shot of spray. thanks

Kelly

 

[Turbo Buick 6]

twins will no doubt run with lower EGTs

A twin setup with a 47 for a low pressure turbo would be a lot more air than a 42 single. In fact it would be more like running a 47 for a single but spool similar to what you have now

 

[ripmf666]

Same place i'm stuck at what to do lol

 

[bigbird]

twins will no doubt run with lower EGTs

A twin setup with a 47 for a low pressure turbo would be a lot more air than a 42 single. In fact it would be more like running a 47 for a single but spool similar to what you have now

Not sure if that's the case. the big turbo in twin setup is not as efficient as it would be if run as a single.

 

[Turbo Buick 6]

Not sure if that's the case. the big turbo in twin setup is not as efficient as it would be if run as a single.

That is true, but thats why I said it would be more like a 47

It would be far closer to a 47 single than a 42. According to my math it would be closer to a 47 than a 45.

And yes the single is more efficient at say 45 pounds of boost, but at 70 pounds the twins are FAR more efficient because the single would be way off its map where the twins could still be close to the sweet spot.


If you had heads and a cam you could get more air in at a lower boost number, but we run into a wall and boost numbers start to rise and the pressure ratio becomes higher than what the turbo is made to put out and that is where compounds truely show their advantage.

 

[malibu795]

Not sure if that's the case. the big turbo in twin setup is not as efficient as it would be if run as a single.

quiet the oposite

a big single effective usuable rpm is say 2,000-2,500rpm sweep IE 2,000-4500 if your lucky

twins can easily increase your rpm sweep by 50-75% or more

with twins lower egts, quicker spool of the big charger, less drive pressor. down side increase cost

 

[Turbo Buick 6]

quiet the oposite

a big single effective usuable rpm is say 2,000-2,500rpm sweep IE 2,000-4500 if your lucky

twins can easily increase your rpm sweep by 50-75% or more

with twins lower egts, quicker spool of the big charger, less drive pressor. down side increase cost

I belive he is talking about compressor efficiency. And it is true, untill you run the single way off the map...try and put out 70 out of a turbo that only goes to 4.5:1 PR. Then the twins become more efficient

 

[Turbo Buick 6]

All air compressors are less than 100% efficient. That means you spend more energy driving them than the energy stored in the compressed air. Where does the "lost" energy go? In a turbocharger, it becomes heat which is passed into the air it's compressing.

The best turbo compressors are 80% efficient, give or take. If you must compress the air twice, it would be 80% x 80% or 64% peak efficiency.

But let's say you want 75psi air. No single compressor we have access to is more than perhaps 50% efficient at that pressure, probably lower. But you could target a compound setup at 75psi and be 64%.

This explains what I am trying to say.

 

[malibu795]

I belive he is talking about compressor efficiency. And it is true, untill you run the single way off the map...try and put out 70 out of a turbo that only goes to 4.5:1 PR. Then the twins become more efficient

gues it is depending on the apliction it was built for...

getting ~120-140lbmin is alot easier with twin setup then a big single... plus less thing need change via tunning, fuel, timing maps, shift points and losser conveter

 

[bigbird]

thanks for the replies guys. trying to get everything straight in my head.

 

[Turbo Buick 6]

gues it is depending on the apliction it was built for...

getting ~120-140lbmin is alot easier with twin setup then a big single... plus less thing need change via tunning, fuel, timing maps, shift points and losser conveter

I totaly agree, I think we were just talking about 2 different kinds of efficiencies. I was speaking of efficiency on energy and you seem to be speaking of efficiency meaning they work better and over a wider range, which i couldn't agree more:hug:

 

[malibu795]

Originally Posted by
All air compressors are less than 100% efficient. That means you spend more energy driving them than the energy stored in the compressed air. Where does the "lost" energy go? In a turbocharger, it becomes heat which is passed into the air it's compressing.

The best turbo compressors are 80% efficient, give or take. If you must compress the air twice, it would be 80% x 80% or 64% peak efficiency.

But let's say you want 75psi air. No single compressor we have access to is more than perhaps 50% efficient at that pressure, probably lower. But you could target a compound setup at 75psi and be 64%.

This explains what I am trying to say.

boost=restriction=heat=decrease efficency

reduce boost eliminates ALOT of problems right there..

how does one do such a thing.... by increaseing air flow through the restiction areas. IE inter-cooler tubes, CAC, Y-bridge, intake manifolds, cam, cylinder heads and exhaust.

IMO Boost does not equal power... air flow = more power... and normally runs wiht boost.

IMO just becaus .8x.8=.64 does not mean you have decrease said efciency of both chargers... i dont think that math can effectily say what the effciency point a both chargers in a compound setup. math looks right.... and looks good on paper.... doesnt mean it is effective in a real world scenario....

 

[Turbo Buick 6]

But if someone wants to get crazy amounts of air through a mostly stock induction system the only way to do that is through crazy boost numbers. Hense why cummins run over 100 pounds of boost. If we did that with worked a worked top end we would be over 2000hp

 

[malibu795]

this is true and you would see a drop in boost as well

 

[SIKDMAX]

this is true and you would see a drop in boost as well

Just needed to quote this as I think its the first post Ive ever seen by adam with all words spelled correctly! :rofl:

Carry on. :angel:

 

[Dan@PPE]

lol

 

[Turbo Buick 6]

IMO just becaus .8x.8=.64 does not mean you have decrease said efciency of both chargers... i dont think that math can effectily say what the effciency point a both chargers in a compound setup. math looks right.... and looks good on paper.... doesnt mean it is effective in a real world scenario....

The math doesn't lie. The assumption people make when they see this is that 64% is a bad number. It is actually a pretty good number concidering many people run singles at a lower efficiency than that. And it is compressor efficiency....not turbo efficiency, meaning it doesn't take into account for drive pressure and other factors that can change HP. You could have a turbo opperating at 80% efficiency and make 500hp and swap that turbo out for one that is running at 70% efficiency running the same boost and the one running at 70% could make more hp if drive pressure was less. Basicly saying a lower compressor efficiency can still make for greater volumetric efficiency of the engine.

 

[Subman]

Just needed to quote this as I think its the first post Ive ever seen by adam with all words spelled correctly! :rofl:

Carry on. :angel:

This is true all 13 of them, two even had five letters. :thumb::smart:

 

[Killerbee]

IMO Boost does not equal power... air flow = more power...

this is the most valuable (and misunderstood) concept statement in this thread. Excellent statement.

You can have higher boost, same rpm, and less air mass. It happens with our trucks frequently.

To the OP, since you mentioned N2O, I would definately consider the single with spray, pre-compressor. The cooled charge will be sensational for compressor efficiency also. I am developing a controller that will aid spoolup of this configuration also, and allow nitrous to be fed via 5V reference multisignal inputs/outputs.

 

[Diesel power]

this is the most valuable (and misunderstood) concept statement in this thread. Excellent statement.

You can have higher boost, same rpm, and less air mass. It happens with our trucks frequently.

To the OP, since you mentioned N2O, I would definately consider the single with spray, pre-compressor. The cooled charge will be sensational for compressor efficiency also. I am developing a controller that will aid spoolup of this configuration also, and allow nitrous to be fed via 5V reference multisignal inputs/outputs.

Pre compressor allways seems to destroy the turbo(blade temp differential), we have been haveing better luck post turbo injection...
Whats your take on this?