Hey everyone,
Finally got Corky Bells "Maximum Boost" in the mail yesterday. Very good
book! Really covers the everything, but I'm having a not so easy time
understanding exactly how to select the turbo I need.
The basic formula for flow rate (cfm): cid X rpm X .5 X VE
1728
279 X 3500 X .5 X .75 / 1728= 212 CFM
I want to run a pretty serious 7-10 pounds boost through my 270, so figuring
for 10 PSI, the pressure ratio ((14.7+boost)/14.7) is 1.68. Yes, I will be
doing some serious intercooling.
1.68 X 212 = 356 CFM
So I need a turbo with a compressor that will operate most efficiently @ 10
PSI @ 356 CFM. It seems as if using the redline RPM in the formula would mean
my turbo's gonna be working most efficiently, creating the least heat, when
the engine's at it's redline. Is this the way it should be?
If these specs are right, selecting the right compressor shouldn't be hard,
but what about the turbine? Corky focusses entirely on street and racing
applications and doesn't say much about truck applications other than "they
might be different."
I'm gonna have a 3.19:1 final drive ratio with the 30% overdrive and 4.56
gears running stock 17" split rims and big truck radials. The RPM's will be
right in spec for cruising at 75.
Do I want to lean one way or the other towards more top end torque and later
boost with a larger turbine or more low end torque and less top end with a
smaller turbine or is right in the middle the perfect solution for my truck?
What's gonna make this truck the funnest to drive?
I guess one of the reasons I'm not so confident making decisions is because
my application's only gonna see 3500 RPM. I'm thinking that means I will need
a smaller turbine than most engines because I will need the boost to come on
at a lower RPM, at about 1000.
FYI, the engine's a 1948 270 GMC six it has/will have:
279 cid
4" stroke
stock head
stock 7.5:1 compression forged pistons
TPI
water intercooling
dry sump lube
water cooled bearing, remote wastegate turbo
Please, please let me know if I'm on the right track here or just
constructively criticise.
Dustin
50 & 53 GMC 1/2 tons
48 GMC 3/4 ton
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