Twin turbo math question

[dcozzi]

I am calculating the proper sized twins for a motor (400cu) shooting for 780HP.

The peak HP has been calculated to require 75lb/min and the peak Tq requires 63lb/min (taking into account losses in plumbing, inefficiencies, etc).

Does that mean each turbo will have to provide 37.5lb/min(HP) and 31.5lb/min (TQ)?

When choosing a turbo, will the calculated pressure ratio to use on the compressor map stay the same (I calculated it to be 2.355)? I did not think it needed to be divided by 2 like the airflow.

And where is Mean69???? Anyone heard from him?? Mr. Rupp?

[RSX302]

I'm no means a turbo expert and I'm going by memory that's fading, but from what I've dealt with, turbo total volume will be split between turbos when running two. The pressure ratio will stay the same. Pressure ratio will be the max boost pressure the turbos can generate. example; 14.7psia x 2.355 PR - 14.7 to get pressure gauge of 20psig. I'm sure they'll go a little more then that because I've seen my 1.8 pressure ratio turbos do 20psig by accident. But then again my math could be off. :Alchy:

Check out this site for some good info: http://www.turbofast.com.au/javacalc.html

With 400cubes, 6800rpm, 10psi boost, 840hp the calculator comes up with a pare of Garrett GT25's with .82 turbines (77lbs per/min total) this will be a little different depending on engine parts used but should get you in the ballpark.

Stay with as small as you can to get the job done for best turbo response.

[dcozzi]

I used the equations on the Garrett website to do all my math. All 4 pages of it. Your link is great. Much faster and easier!

The best compressor map match I found for the ones listed on Garrett's site is a pair of 3071R. (with my math) It has HP and TQ in a great spot on the map. The 25 series says it is made for a lower HP. The link you gave me shows it working just fine.
I think I'll call Garrett and see how bad my math really is.

[Larry Callahan]

I don't have my numbers with me but had a lot of help picking mine. They too are small and spool up fast and make boost very early.

[GRIMMEY71]

found this on another site
http://nelsonracingengines.com/videos/nretvbillettbbchigh.wmv

[dcozzi]

Larry, I would like mine to spool early as well. My design involved making peak power at 6200RPM. Easy on the valvetrain, bearings, etc.
I had it mapped the turbo to be able to spin past 6200 for traction problems, missed shifts and "oops I let it really rev that time" situations. I did not want to overspin or "choke" the turbo if any of these happened.

Thanks for your input on this stuff guys. I think the more I learn about this stuff, the more I do not clearly know.

[dcozzi]

GRIMMEY71, that's a sick representation of what can be done with a twin turbo motor. Granted, it's a big block, but over 1000HP on 10lbs of boost running pump gas is pure heaven.

[GRIMMEY71]

GRIMMEY71, that's a sick representation of what can be done with a twin turbo motor. Granted, it's a big block, but over 1000HP on 10lbs of boost running pump gas is pure heaven.

Yeah I am real new to the turbo scene(just bought a csrt4 for a DD) and the more I learn the less I know. I like these type of informational threads. HAve you ever tried contacting acompany like Garret and asking them some of your questions? just an Idea

[dcozzi]

I am calling Garrett next week. Hopefully they are easier to deal with than their parent company Honeywell!
I also have a local shop I want to contact to see about a less expensive turbo. Garrett is the most expensive but probably the best unit.
Right now it is pure planning so I do not spend the $ twice.

[speedster]

I am calling Garrett next week. Hopefully they are easier to deal with than their parent company Honeywell!
I also have a local shop I want to contact to see about a less expensive turbo. Garrett is the most expensive but probably the best unit.
Right now it is pure planning so I do not spend the $ twice.

I recommend calling Tom Nelson at Nelson Racing Engines and ask him the proper size. He is the guru on this stuff. 818-998-5593 leave a message or email. It is tough to reach him by phone. He is a Pro-Touring sponsor and is on the cover of the new Hot Rod magazine.

[RSX302]

I used the equations on the Garrett website to do all my math. All 4 pages of it. Your link is great. Much faster and easier!

The best compressor map match I found for the ones listed on Garrett's site is a pair of 3071R. (with my math) It has HP and TQ in a great spot on the map. The 25 series says it is made for a lower HP. The link you gave me shows it working just fine.
I think I'll call Garrett and see how bad my math really is.

I looked up my turbos - CHRA 446179-32 (GT2871R) 46mm turbine
http://www.turbobygarrett.com/turbobygarrett/catelog/Turbochargers/GT28/GT2871R_743347_2.htm

Incon Twin system was tested on a 347 and generated 840hp. My 302 sees 750hp at 7500rpm and full boost by 2800rpm.

[RSX302]

The 25 series says it is made for a lower HP. The link you gave me shows it working just fine.
I think I'll call Garrett and see how bad my math really is.

I think you're right after looking at my stuff. The GT25 will be too light. I think your right there with the higher side of GT28's or lower side of GT30's.

[WS6]

Weird. I used the link you posted Ron and it said I should use a GT42. I'm hoping to go single turbo and bring the boost in early but still pull to 6200 on my LT1. Maybe I did something wrong. I entered the following Engine: 5800, RPM 6200, Boost 10 and intercooled. I'm just building a simple 355.

[WS6]

I don't have my numbers with me but had a lot of help picking mine. They too are small and spool up fast and make boost very early.

Can you be more specific Larry? What RPM does boost start to come in at and when do you hit max? I really want to try and build my single turbo system to be as close to a positive displacement supercharger as possible. So I need boost to start by 2300-2500 and be full by 3000 so I can shift at 6200. I'm only looking to run say 10lbs. What does yours go up to?

Thanks

[RSX302]

Weird. I used the link you posted Ron and it said I should use a GT42. I'm hoping to go single turbo and bring the boost in early but still pull to 6200 on my LT1. Maybe I did something wrong. I entered the following Engine: 5800, RPM 6200, Boost 10 and intercooled. I'm just building a simple 355.

It appears that Ray Hall's site isn't updated with all Garrett's turbos. It jumps from GT35 to GT42. The GT35 could be too small and the GT42 seems to be too big. It looks like you'd be in the GT40 family turbo. Ultimately it will depend on the horsepower your motor will be set-up for. (Cam, heads, intake, etc)

[WS6]

Mine will be simple. For a long time it'll be stock everything except for a small cam to make noise and compliment the turbo. I'll probably home port my heads. More of a clean up really than a port job. Compression will be nearly 10.0:1 so it can remain responsive while not in boost. 500rwhp and as much torque as I can get is what I am shooting for. I really want my boost to come in early 2300-2500 and hold on til 6000 minimum. I'm even considering playing with a quick spool setup just to help ti spool even sooner. I know that's usually reserved for bigger turbos with lots more boost but every little bit may count. Besides, that would be a trick piece if I need to use one. Thanks Ron

[RSX302]

Mine will be simple. For a long time it'll be stock everything except for a small cam to make noise and compliment the turbo. I'll probably home port my heads. More of a clean up really than a port job. Compression will be nearly 10.0:1 so it can remain responsive while not in boost. 500rwhp and as much torque as I can get is what I am shooting for. I really want my boost to come in early 2300-2500 and hold on til 6000 minimum. I'm even considering playing with a quick spool setup just to help ti spool even sooner. I know that's usually reserved for bigger turbos with lots more boost but every little bit may count. Besides, that would be a trick piece if I need to use one. Thanks Ron

One turbo will always respond slower than two smaller. If your trying to get boost and quick response that low in the rpm range, you might want to go with smaller twins. Torque is not an issue with turbos, done right, it'll come on strong in your n/a torque curve.

A race motor is one thing, but 10:1 is kinda high if your running pump fuel and boost for the streets. An intercooler will be a must and start out with 5 psig boost and work your way up. I'm running ceramic coatings with 9.7:1. My EGT's went up with the higher static compression. I started out with 7.5:1 on the 302 motor and it was a little lazy off idle. (cubically challenged) EGT's were low and nice. Under full power, the EGT's were less then 1300Deg. I've then increased compression to 9.7:1 and now it drives sweet around town, but EGT's during cruise increased 300-400 degrees to 1000-1200deg. I see 1400-1450 under full power. I think 8.5 - 9.0:1 is the perfect range for a turbo boosted street motor. Your 355cu/in motor will have no problem with around town drivability at 9:1.

I've heard turbos can typically double your n/a horsepower. If the motor can't deal with that...Kaboom!

Always a dilemma....Design with a smaller turbo to increased response and protect engine under max power or install a little larger turbo for future engine mods but sacrifice response..... Decisions...Decisions:confused:

Hence Twins...

[WS6]

That's what I'm afraid of. I wanted to limit my turbos to one so I could save money. Of course, by the time I get around to them it may be easy for my to buy two of them. I'm still thinking about getting a couple small ones off junk yard cars. I'm cheap for the most part but I do my stuff right. Thanks Ron.

[MonzaRacer]

I found a compant in back of mechanics trade magazine and it tims turbos, ill have to look up the number, but according to them my 283 could run 2 TO4B V trim.