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CFM calculations for /6 Turbo
https://slantsix.org/forum/viewtopic.php?t=58761		 Page 1 of 1
Author:  gtb123 [ Mon Nov 30, 2015 4:02 pm ]
Post subject:  CFM calculations for /6 Turbo
Hi,

I'm doing a lot of flow estimate calculations and boost curve plotting on compressor maps, etc. right now, and I just wanted to let some of this information out into the public to see if I'm way off or not. And give people an opportunity to give me feedback on my build of course! (thanks ahead of time) 8)

So, I know the stock slant from '76 has an out of box bhp of around 100, and with the following upgrades I'm doing, I'm just wondering if these flow rates/bhp additions line up and make sense.

So far I've made or will be making these improvements:

-270hp max Garrett GT2554 Ball bearing turbo .80AR/60 Trim comp - .64AR/62 Trim exhaust
-500CFM Holley blow-through modified carb
-3" Cold air intake to the turbo compressor inlet
-2" piping between compressor outlet, intercooler, and carb
-2.5" Straight pipe exhaust coming off of turbo turbine outlet
-Stock exhaust manifold with 2"ID J-pipe to turbo turbine inlet
-Conservative intercooler size to prevent lag (yet to be determined)
-New head gasket, valvecover, carb, exhaust/intake, and plenum gaskets
-AFR sensor system for safety


So far, I have this calculated:
@ 65% volumetric efficiency with 10PSI (theoretical) boost at intake plenum.

RPM
1500 - 107 CFM
2000 - 143 CFM
2500 - 178 CFM
3000 - 214 CFM
3500 - 250 CFM
4000 - 286 CFM
4500 - 321 CFM

Giving me a max base horsepower of just over 200 at redzone
or approximately 150-170 at cruising speeds.


Now, I'm scared because I'm using these CFM calculations on the next big purchase phase here, and if they're incorrect, I may be purchasing the wrong thing. :shrug:

Just wanted to get some advice before the $$$$ starts coming out.
Author:  bmimken [ Mon Nov 30, 2015 7:23 pm ]
Post subject: 
I would really encourage you to reconsider a draw-thru setup. A blow-thru setup has more advantages than a draw-thru. I am currently running a blow-thru setup. I can honestly admit that if I had to do it over again, I would go the fuel injection route instead. Especially if you are planning to do a lot of street driving with the car.

Brian
Author:  Junior [ Mon Nov 30, 2015 8:22 pm ]
Post subject: 
like already posted. at least run a blowthrough setup. drawthroughs work dont get me wrong but they arent as good. you cant run a conventional intercooler (air to air or air to water) with a drawthrough setup either.

im assuming your red zone is 4500 because you dont list any flow figures after that? a 225 will rev to and over 5k pretty easily. not necessarily with a stock cam but boost has a habit of changing things.

the other thing that stands out is the turbo. thats pretty small. they are popular with the import 4cly guys for lighting off quickly on the lower displacement motors. it would most likely be straining to reach your projected 200hp.

last thing. you have the head gasket as something to be changed? why? the stock steel will hold up to more boost than you are looking to push. are you changing the cam or pistons, o- ringing the block or doing anything that would require a new head gasket?
Author:  ProCycle [ Mon Nov 30, 2015 9:36 pm ]
Post subject:  Re: CFM calculations for /6 Turbo
Quote:
@ 65% volumetric efficiency.
65% seems a bit low to me. Using too small a VE% will point you at too small a turbo. That will be fine for quick spooling and good low-mid range but not so good for top end.

When I look at VE table data from folks who have injected these motors they tend more towards 75-80% VE.

Maybe someone with experience can chime in and say if that's a good number or suggest something different.
Author:  gtb123 [ Tue Dec 01, 2015 2:58 am ]
Post subject: 
Haha you'll have to excuse me,

Draw through was a typo! It's a blow through system.

Corrected above.
Author:  gtb123 [ Tue Dec 01, 2015 3:05 am ]
Post subject: 
The only reason I was considering a head gasket was if I decided to port the head or change the cam, and also, because I have no history on the motor whatsoever.
Figured an inspection and gasket swap wouldn't be a bad idea so I could see if there were any horrible surprises lurking and ready to make this system fail before it even starts. Haha

I've read here that well tuned carbureted /6s only get VE around 70? Maybe that's a conservative estimate?
Author:  gtb123 [ Wed Dec 02, 2015 2:54 pm ]
Post subject: 
Well, anyway.

To add, I have a 3.55 rear end and a 727.

The other additions I left out was HEI. So I can get rid of the ballast resistor and improve ignition energy.

Aside from the turbo being possibly too small, are there any other tips or criticisms that I might be able to gather that would put this build a step or two in the right direction?

I'm a damn amateur with a reading addiction and I'm sure there is a lot I'm missing in the equation. :oops:
Author:  bmimken [ Wed Dec 02, 2015 4:02 pm ]
Post subject: 
Well, in order for us to give better feedback, it would be nice if you could elaborate on what your goal with the car is.

Is this a daily driver, part street/part strip, weekend cruiser, etc. ?

Brian
Author:  Junior [ Wed Dec 02, 2015 4:51 pm ]
Post subject: 
turbo slants like being held back. 3.55's would be slower than 2.73's. its the best of both worlds. hang a gear long enough to get some boost and get the best fuel economy as far as rear end gearing goes.

the hei is a solid choice for an ignition upgrade. if you are going to get serious about the setup a better choice would be the msd 6AL2 6530. just going with the hei you will have to lock the distributor and lose your timing curve. with the msd box you can program normal timing under normal conditions and set it to pull timing as the boost rolls in.

like previously states knowing your intentions for the use of the setup are key to requisites.
Author:  gtb123 [ Thu Dec 03, 2015 2:14 pm ]
Post subject: 
Thanks for helping me sort this stuff out.

It's going to be a daily driver for the most part, aside from the winters.

I drove the motor 2000 miles across a good part of the country and it never skipped a beat, and started everytime on the first try.

I ran into problems in the appalachian regions and could barely get up a rather steep hill with full traction. Tranny had pretty much the entire vehicle's weight on it and I thought it was going to give, not going to lie. Haha but it didn't

So, to summerize the goal, I want to be able to get up hills, have some decent acceleration response at cruising rpms (60-65mph 3500-4000 rpms) in case I have to make a power move, and it would be awesome to bring the max speed up to 85mph or something, because right now it struggles at 70. lol
Author:  Junior [ Thu Dec 03, 2015 6:04 pm ]
Post subject: 
i would start looking for a buick grand national turbo. they seem to work out well for people not going all out. the rest of what you have listed seems ok. might wanna add a electric fuel pump and regulator to that list. its just safer and works better than the referenced mechanical pump. make sure to pay close attention to the timing curve. if you are keeping the boost low then you can run some timing. the vacuum advance will help with cruising but on boost you wanna be considerably lower. other then that, slap it together and have some fun.

i used this site to help with the basic setup of the carb for boost.
http://www.hangar18fabrication.com/blowthru.html
Author:  the_engineers [ Mon Dec 07, 2015 11:47 am ]
Post subject: 
This website used to be THE reference for turbo selection, but it seems to have been abandoned. Nonetheless, the calculator for cfm, etc still works.

http://web.archive.org/web/201410241536 ... turbocalc/
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