Dustin,
From this and your previous post where you outlined the engine plans, I
would suggest you re-group a little bit and reconsider what you are
expecting from this combination. 350 BHP is pretty ambitious for an engine
that puts out about 130 in stock form.
First off I am no authority on turbocharging but have owned a couple of
turbo cars and have done a good bit of research on what goes into a turbo
project. I haven't read Bell's book but have a couple of others. Your basic
plan sounds good, I wonder about the dry sump, but I guess you have your
reasons for that.
You didn't mention what you are planning to use for a camshaft. I can
tell you that with a stock small port GMC head, valves and cam, that 270 is
going to be hurting in the airflow department. Since the whole idea of
turbocharging is to increase the available mixture by force feeding the
cylinders, you will gain performance with any turbo but you have a
pre-existing bottleneck in the very area that you are trying to enhance. The
270 is rated (not red-lined) at 3500 RPM for the simple reason that running
it any faster does not increase power. It was designed as a truck engine
with low-end torque. The stock components are strong enough to withstand
much higher RPM, and with the proper aftermarket valve springs, more lift
and duration from the cam, bigger valves and adequate port size, a 270 Jimmy
will safely take 5500 - 6000 RPM and suck all the mixture your turbo can
supply! That's assuming you can control detonation and not burn up pistons.
I would strongly suggest that you look for a 270H head which has 1/4"
larger intake ports (there is one for sale on the Inliners site - it won't
be there long, they are rare as hen's teeth), install V8 Chevy 1.94 / 1.60
valves, and talk to some cam grinders about a suitable cam grind for a turbo
application. The Bell book should give some rough guidelines on what to plan
on with duration and especially overlap which I understand is very important
on a turbo engine. Also consider an HEI or other high performance
distributor. One final thing - you mentioned "7.5 stock forged pistons". I
could be mistaken but I am pretty sure that stock GMC pistons are cast, not
forged. This is a potential weak point although GMC pistons are pretty hefty
chunks. BTW, stock 270 CR is 7.75:1, a little higher with an overbore.
Cylinder pressure in a turbo engine really stresses the pistons and rings, I
would question whether they will be adequate.
I think it's great that you are so enthusiastic about your turbo 6, I
really wish you well with it! But, don't get ahead of yourself. Give some
thought to the basics - especially airflow - of the engine you're going to
hang that turbo on, and you'll be much happier with the end result.
Jack / Winter Park FL
----- Original Message -----
From: <CLLLSLS@aol.com>
To: <oletrucks@autox.team.net>
Sent: Wednesday, March 15, 2000 10:25 PM
Subject: [oletrucks] 270 redline help (was selecting a turbo)
> What's a safe redline erring on the conservative side for a 270 Jimmy? At
> what RPM would I expect accelerated engine wear? I have sorta assumed
> spinning this thing no faster than the 3500 RPM shop manual stated redline
> was the smartest way to go about turbocharging, but 300-350 bhp is my
target
> for this engine, so finding out I would be squeezing 320 @ 5000, the
figure
> at 3500 is gonna be considerably lower. Help me find the happy medium!
>
> Dustin
> 50 & 53 GMC 1/2 tons
> 48 GMC 3/4 ton
> oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959
>
oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959
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