The Harley-Davidson ® High - Performance Cylinder Head Design and Engine Modification Shop
Complete Engine Rebuilds - Fuel Injection and Carburetor Tuning - Chassis Work - Parts - and More ...
Very competitive pricing
Harley-Davidson® High-Performance & Reliability Specialists
J.E.T. Modified Stock Throttle
|OUR CUSTOMER'S SAY:|
Our goal when we started our project was to offer our customer’s a
high horsepower throttle body option for a lower cost than what is currently
available. Right now, big throttle bodies are available from Screamin’
Eagle and other aftermarket manufacturers at costs between $400 and $800.
They all produce good results, but the total cost of installation and tuning
can be over $1000. For just a small increase in performance, this can seem
a little expensive to many people. Instead of having to purchase an expensive
new unit, we set out to see what we could accomplish by some simple inexpensive
modifications to the stock unit.
All ’06 and later Twin Cams (except for ’08 FLH’s) come with a one piece 46 mm throttle body. Upon testing we found that this 46mm unit was limiting customers to about 92-98 HP depending on displacement and the level of performance work. We found the overall design of the part to be a little lacking (it was clearly designed without performance in mind). The port runner increases in diameter from 1.900” to over 2” after the butterfly and still gets larger in area until the port exit at the cylinder head which is 1.640.” Any intake manifold that is designed to increase engine performance has port runners that are smaller then the butterfly, which also has a constant taper to the cylinder head. This is done so that the air in the throttle-body is constantly increasing in speed (this reduces reversion and increased throttle response, it also helps prevent detonation by keeping the fuel in suspension (low velocity and reversion cause fuel to fall out of suspension which increases the likely hood of detonation).
We flow tested the stock throttle body and found out that it flowed 219 CFM at 28.” We then bored it out to 48mm (.090” bigger than stock). The manifold was flow tested with just the bigger 48mm butterfly and we saw an increase of 13 CFM (232 @28”) We then did a little bit of clean up porting and re-flow tested the throttle body with results showing an increase of 22 CFM (241 CFM @ 28”). After flowing the throttle body we installed the unit on a 103 cu” TC with a mild camshaft and 9.8:1 compression ratio (baseline power of 92 HP and 104 ft lbs torque). Upon installation of the 48mm High-Flow throttle body and proper tuning; the HP increased to 96 HP and the torque increased to 106 ft lbs. SEE DYNO
Seeing as the 48mm worked so well, we thought we would push it a little further and try going to 51mm. After re-boring the throttle-body to 51mm (an increase of .196” over stock), we flowed the throttle body and that’s when the bad news started. The 51mm throttle body only flowed 243 CFM @28” an increase of only 2 CFM over the 48mm unit. Seeing as the flow bench is not always a tell-all in performance, we then installed the part on the same 103 cu” TC as before and proceeded to test and tune the 51mm throttle body. The results were not good; the HP and Torque both went down about 3-4 units in the low and midrange RPMs, but the power above 4750 RPM increased by about 4. SEE DYNO
All in all I would say
that the 51mm ported stock throttle body was a failure, because not many
people spend a lot of time riding above 4750 RPM, but we were determined
to make the 51mm throttle-body work. We thought about what we could do
to increase the velocity behind the butterfly. Increasing the velocity
and the CFM would increase the power everywhere and make better use of
the big butterfly. The port runners were then cleaned up and some very
high-tech epoxy was used of fill the ports (to replicate the port runners
of our J.E.T. intake manifolds). After the manifold was epoxied and ported,
we tested it on the flow bench and it flowed a lot more with the epoxy,
257 CFM @28”. Now we were faced with a manifold that was about 15%
smaller, had the proper taper that an intake should have, and it flowed
14 more CFM (38 more CFM than stock!!!!) This could only mean
that the port velocity was much higher in the manifold with the epoxy.
So we took the 51mm High-Velocity throttle-body to the dyno.
The HP went from 100 HP
to 107 HP and the torque went form 110 ftlbs to 115 ftlbs of torque. We
also noticed the power increases were produced throughout the entire RPM
range and not just in the higher RPM areas. These results were very encouraging
find and could only mean one thing; we accomplish our original goal.
As a result, Johnson Engine
Technology, Inc. can now offer a service that will take the stock throttle
body of a potential customer and modify it for optimum performance without
the high cost of the aftermarket part.
-Written By: Robert Johnson
Email us or call (401.596.9507) for more information or to order your modification today.
Picture submitted by Tony Johnson
J.E.T. Modified Stock Throttle Body: 51mm High-Velocity
As of 12/13/07