How do twins work?

[J.Davy]

I can see how they make more power than a single by operating with less back pressure but what if you have a big single running at the same boost and same back pressure as the twins? Do the twins still make more? If so how much more and how so?

 

[Forrest Nearing]

you can't have a big single running at the same boost and same drive pressure... that's why we run twins... multiply the pressure ratios to keep the compressors in their efficiency range.



Forrest

 

[J.Davy]

Some guys were just dicussing how that J. R. Adkins little 55 runs at 10 pounds less drive pressure than boost @ 50 lbs. I called him today and that is what he said as well.

 

[banshee]

One of the keys to making good power is keeping the turbo(s) in their efficiency range. Most turbos are most efficient in the 2 - 2. 5 : 1 "Pressure Ratio" range. In other words, they work best when multiplying atmospheric pressure (14. 7 psi @ sea level) by 2 - 2. 5, or roughly 28-36 PSI. When pushing past this the turbo is "off its map" and this causes the compressor to become very inefficient thus overheating the air. A big compressor can run at higher pressure ratios (more boost) efficiently, but you run into lag issues. To combat this, we run twins to get two main advantages. First, using wastegates you can set both turbos to run at their optimum pressures to maximize their efficienies to produce the coolest air possible. Secondly, you can size the turbos, housings, and wastegates to give the spoolup of a smaller single, but still have the airflow of the larger turbo. When looking at the systems on paper & theory or in the real world with R&D twins can't be beat.

 

[NPloysa]

Twins can be beat! Triples! Ok, so no triples are in real standard use yet, and twins are probably all most people need



Nick

 

[J.Davy]

Banshee,so pressure ratio and drive pressure ratio are 2 different things completely. I had not heard about the multiplying atmosphere comfort zone but it makes sense. Thanks for the help. Putting the bottom end of the map aside,if you have a big turbo that falls in the map from 2000 to 3000 rpm say even a variable geometry design, and drive pressures are the same, is there a difference in H. P. ,The only benefit I can see is that the big turbo would sucking the air but not running as hot because it is a little farther down the line.

 

[SRadke]

The heat in the intake air doesn't come from the exhaust (well, some does but it's negligable), it comes from inefficiency in the compressor. A single compressor operating at 3. 5:1 will make much warmer air than two stages of compression to create an overall 3. 5:1. Your engine really likes cool air.



-Scott

 

[banshee]

Yes, Pressure Ratio and Drive Pressure Ratio as two different things. The first is basically a way of measuring how much boost you are creating. You can either say I have 30 psi boost or I have a PR of 2:1... they are one in the same. It's very similar to the relationship between humidity and dew point... if you know one it's very easy to calculate the other.



About the HP difference, IMO the high number will always go to twins. A single is going to have a really hard time making "good" boost over 45-50 psi. After that the air gets really hot and power can drop instead of increase. An average size twin setup will still be making efficient boost in the 60-70psi range. Higher boost means you can cram in more air while the intake valve open, and it's cooler, denser air (more Oxygen) to boot.



The drive pressure ratio is important because you don't want the exhaust pressure in the exhaust manifold highter than the boost in the intake manifold. During the overlap period (both valves open at the same time) the exhaust can "revert" back into the intake manifold and displace the cool intake air. You're basically getting EGR without running an EGR system it drives up EGTs and power will drop. Drive pressure is, IMO, harder to deal with in a twin system, but it isn't a problem if they are set up right.

 

[6naline01]

How do twins work

I use my truck for a daily driver and towing a 35 ft Triumph 5ver are twins workable for this application,thanks in advance Mike

 

[Forrest Nearing]

Some guys were just dicussing how that J. R. Adkins little 55 runs at 10 pounds less drive pressure than boost @ 50 lbs. I called him today and that is what he said as well.

what kind of drive pressure does it have at 80psi?



also, driver pressure isn't everything... compressor efficiency is key as well... better compressor efficiency = lower EGT's and more power



and, twins will spool up sooner than a big single.



Forrest

 

[J.Davy]

How come when they twin turbo gas motors typically they don't run them in series?I suppose it would be along the lines of them running within their pressure range and running less boost. Back in 1988 when F1 cars were still turbocharged they would run over 100 psi boost but I am not sure if that was with a single or twin setup. I guess I might have answered my own question but if there is more to it than that feel free to let me know.

 

[Forrest Nearing]

gas motors can't run really high boost or the charge will pre-detonate... you can get away w/ more boost w/ toluene or methanol... never heard of 100psi on a non-diesel though... I thought the F1 cars were limited to 40psi.



Forrest

 

[J.Davy]

They can if they run really high octane fuel or really high rpm. Honda proved that detonation is time dependent and typically does not occur above 10500 rpm. The old bmw f1 qualifying motors made 1500h. p. out of a 1500c. c. motor.

 

[Forrest Nearing]

that's just sexy

 

[drawson]

I ran Piers HX-50 and made 556 HP @ 55+ pound of boost. It would still run over 1600 deg EGTs. Now this isn't a great comparison because I have a newly rebuilt motor that is cammed, balanced, pistons coated, ect but at this years Edge Dyno days I ran 598 HP @ only 43 PSI because of a hole in my inter cooler. With the new intercooler and 65+ PSI I have never seen over 1400 deg.



The twins spool faster and my intake air temps is down 60 deg even though now I suck under hood air instead of Scotty 2 air.



PDR Street (HX-40)twins are a great turn key set up!!!

 

[Extreme1]

that's just sexy

ROFLMAO! Oo.



(and it is)



I never thought about it (the time dependancy of detonation) but that makes sense. I wonder how many other pesky problems we can overcome if we spin the motor fast enough?

 

[Evan A. Beck]

With the new intercooler and 65+ PSI I have never seen over 1400 deg.

How much drive pressure do you have?

 

[drawson]

It has 60 PSI drive pressure at 65PSI boost. I used my old boost gauge as a drive pressure gauge for a while this summer. Now I have it hooked up to the big turbo as a boost gauge.

 

[J.Davy]

What do you use for tubing to the gauge?

 

[drawson]

I used the old Pyro hole that is now in the bottom of the manifold with about 1 1/2 feet of copper tubing then into the old boost plastic hose. I had plans on putting a switch in and monitoring both boost from the big turbo and back pressure but the plastic mettle at my union. I still might do it again using the copper tubing and a big alum junction block to take the heat out if I get some time.