3800 V6 Series II Ported/Polished/Matched Heads
Grand Prix, GT & all W-Body
PLUS F
body Camaro Firebird) racers too!
Click on a
thumbnail for full screen view:
Flow
data - stock
Final
porting - stock exhaust valves
Final
porting - large exhaust valves
We're pleased to offer complete ported, polished, and
matched heads along with a matched intake.
When you bolt on a set
of these heads you can rest assured you are getting
exactly what you paid for. Each head is flow
tested, the data recorded, and the flow sheet is provided
with the head upon assembly. We have options
for new springs and keepers, and are currently testing
oversized valves for even greater flow!
The flow data
from a stock
head, a head with our intermediate
design, and our finished
product are presented to the left. Want
more? Check out the flow results when using
our custom larger
exhaust valves. Click on the thumbnail image
to view it full size. All flow testing
was performed on a Superflow 1020 flow bench at the
industry standard depression of 28" H2O.
1
1 All
our flow testing is done at the industry standard
depression of 28" H2O. When comparing flow
numbers be sure they get compared at the same valve
lift and vacuum. If you need conversions for
different levels of vacuum, drop us an email.
You cannot compare data taken at different
valve lifts.
First,
we baselined the stock head. After all,
without comparison data, the new numbers mean nothing.
As you see in the flow
data from the stock head, GM did their homework
as these heads are fairly good to start with.
Note the exhaust to intake flow ratio. (click
on the thumbnail of the flow sheet above.)
A good normally aspirated street head needs a flow
ratio of about 60% - 75%. (It is easier to exhaust
the gases as they are under some serious pressure
and tend to leap out of the chamber.) However, supercharged,
turbocharged, and nitrous assisted engines need
better exhaust flow because we are artificially
increasing the intake tract flow by using the blower,
etc. The stock head already has a 80% ratio.
While this is fairly impressive for an stock head,
there's always room for improvement! We measured
peak flow numbers for the stock intake of 186.5
CFM and 148.6 CFM for the exhaust. The average
flow numbers were 166.12 and 135.4 CFM, respectively.
After much R & D, flow
bench time, and $$$, the intermediate head
we came up with had a flow ratio of 82%, peak intake
flow of 206.2 CFM, and exhaust peak flow of 169.7
CFM. Average flow rates improved to 180.48
and 154.2 CFM. Pretty good gains, but we
weren't satisfied!
[Note: Intermediate head vs.
stock shown here.]
So... the air
grinder came back out again. Many metal shavings
later the finished product emerged with a peak
intake flow of 211.2 CFM and peak exhaust flow of 182.1
CFM! This seems to be the limit of the exhaust
port without getting somewhat exotic. (We broke
into a water jacket trying for even more flow:-)
Best of all, the flow ratio jumped to 86% with an average
intake flow was 181.4 CFM while average exhaust flow
increased to 156.84 CFM. The exhaust ports on
this head now flow incredibly well, which is very important
with a blown engine. You have a supercharger forcing
more air through the intake ports, but the exhaust does
not have this artificial help. Trying to force
more air into the engine will NOT make more power if
the exhaust ports are incapable of a corresponding increase
in flow capacity - the less backpressure the better.
These heads will now support an extra 38 HP on a normally
aspirated engine, but it's much more difficult to put
a number on the HP gain with an artificially aspirated
engine as various factors such blower over/under drive
and displacement, intake tract restrictions, and your
exhaust system will affect the power gain. Theoretically
speaking, forcing 25% more air into the engine with
a blower will generate 25% more power, if the other
engine components will support it. These heads
look extremely good on paper and on the flow bench;
after dyno testing, we will soon find out if our engineering
analysis and assumptions are correct.
Options [all prices for a pair (2) heads
on exchange (core) basis]
S&H
not included
All
port work, intake, exhaust and combustion
chambers (pair of heads)
Call
THRASHER
Assemble
2 heads, inside and out, balance valve
springs (corr hts.) Includes
flow sheets before and after porting,
intake port, exhaust port, and chamber
volumes
Call
THRASHER
angle valve job (pair)
Call
THRASHER
Subtotal for ported polished heads with
above 3 recommended options:
Call
THRASHER
Please
note the special requirements for
the options below marked with asterisks
(*)
1.60" stainless
exhaust valves (0.080" oversize,
11% more area)
Valve
Locks for titanium retainers for above
springs (12) **
Call
THRASHER
12 Bronze valve guides
installed
Call
THRASHER
12 Teflon Valve seals
Call
THRASHER
Machine for valve seals
(12)
Call
THRASHER
CC and match combustion chamber volume,
includes resurfacing
Call
THRASHER
Resurface
head only
Call
THRASHER
NEW
bare head casting
Call
THRASHER
NEW
Stock valve springs
Call
THRASHER
NEW
Valve Locks
Call
THRASHER
- The oversize valves
REQUIRE the use of the H11 Tool Steel springs,
titanium retainers and valve locks(marked
with **).
** - The H11 Tool Steel springs REQUIRE the
titanium retainers and valve locks, but not
the oversize valves.
Grand
Prix GT and 3800 V6 F body (Camaro/Firebird) racers!
Yes! You can have Thrasher ported
heads too! We not only service the supercharged
racer, we also provide the same services listed above
(same prices) for normally aspirated (NA, non-supercharged)
heads. In fact, our intake flow increases are
even greater on NA heads! Why? On the supercharged
(L67) engines, the fuel injectors are mounted in the
heads, due to lack of room in the intake manifold because
of the blower's presence. On 96-98 Series II
NA (L36) engines, the fuel injectors are not mounted
in the heads, but the injector bosses are still present,
just not drilled. Thus a stock NA head flows pretty
much the same as a stock intercooled head. Without
having to worry about keeping the injector boss, the
intake port can be optimized even more! Exhaust
ports on L67 and L36 heads are identical.
A
Common Question: "I installed
a set of ported heads with no other changes and my
boost level dropped. What is happening?"
The
Answer: In
this instance, the improved flow of the exhaust ports
on the new head allows the engine to more efficiently
get rid of the burned gases. The blower is not
building "psuedo boost" trying to push out
the remaining exhaust gases. Although the boost
pressure is higher, less air is actually being forced
into the engine because of the restricted exhaust
and power is limited. With a better flowing
exhaust, more air is forced into the engine because
the blower is not also trying to force out the exhaust
byproducts. The boost pressure is lower, but
the mass of the air entering the cylinders is actually
greater.
Stated another way, boost pressure
is a measure of how restrictive your intake and exhaust
system is. The more restrictive your system
is, the higher the boost number will be. Restrict
your exhaust flow and you may see your boost rise,
but your power will actually fall off as the blower
uses more energy tryng to push against this back pressure.
Trying to spin the blower faster
in an attempt help "push out" the spent
gases may help slightly, but it may actually hurt
power. As a Roots type blower spins faster,
its adiabatic efficiency drops. This means that
you are heating the incoming air more than compressing
it. As the air is heated it expands and although
the apparent boost pressure rises, there are actually
less air molecules per unit volume and less air means
less power.
This point is outlined in the
October 1998 issue of Automotive Industries,
a highly respected automotive engineering industry
trade magazine in an article titled "Eaton Boosts
Engine Aspirations" by Don Sherman.
"Compared
with supercharged engines offered by the imports,
GM's Series II 3800 V-6 uses lower boost pressure
and no intercooler, note GM Powertrain engineers
Bill Owen and Robert Gardner. The GM
engineers regard boost as a measure of inefficiency
- GM uses 7.5 psi, only 2/3 as much as Jaguar.
Boost is the pressure required, above atmospheric,
to push the charge through the engine to make
power. Less boost for a given amount
of power means more efficient flow, because
heat and parasitic losses both rise with boost
pressure."
Another important
point made in this article is this:
"Superchargers
are highly tunable. By adjusting the
crankshaft-to-supercharger drive ratio, OEMs
can schedule air delivery low in the rpm range,
for extra surge from a stoplight, or up-register
for enhanced passing performance.
What this means is by changing
the size of the supercharger pulley, one can optimize
the power output to favor high RPM horsepower, or
low RPM torque. We've seen this in testing on
the dyno where decreasing the size of the blower pulley
beyond a certain point results only in more low end
torque with diminishing gains in upper RPM horsepower
- the smaller you go, the more the high RPM horsepower
is sacrificed. Therefore, the optimum pulley
size is one that balances the low end torque and high
end horsepower, and is very dependent on how efficiently
the exhaust can be expended.
See our Intercooler
page for more detailed discussion of a roots blower's
adabatic efficiency.
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