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THRASHER Engineered Performance

3800 V6 Series II Ported/Polished/Matched Heads

Grand Prix, GT & all W-Body PLUS  F body Camaro Firebird) racers too! 

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Click on a thumbnail for full screen view:

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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.]

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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)

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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

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angle valve job (pair)

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Subtotal for ported polished heads with above 3 recommended options:

 

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Please note the special requirements for the options below marked with asterisks (*)
1.60" stainless
exhaust valves (0.080" oversize, 11% more area)  

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THRASHER

H11 Tool Steel increased pressure valve springs (12) 

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Titanium Intake retainers for above springs (6)

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Titanium Exhaust retainers for above springs (6)

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Valve Locks for titanium retainers for above springs (12)  **

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12 Bronze valve guides installed

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12 Teflon Valve seals

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Machine for valve seals (12)

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CC and match combustion chamber volume, includes resurfacing

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Resurface head only

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NEW bare head casting

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NEW Stock valve springs

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NEW Valve Locks

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- 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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