My combo is a lower Metco 9.5" and an upper Whipple 3.25" .

Engine is a JDM aluminum GT-40, JDM 3V ported Heads, Dynatech long tube headers etc...

The S/C is making 9lbs of boost ONLY !
I think this combo was supposed to make at least 15lbs of boost.
Are the ported heads and long tube headers the reason for a loss of 6lbs of boost ?

One more question, is there any lower pulley bigger than 10" ?

Thanks a lot, any comment will be trully appreciated !

 

You can have a larger lower made if you want. I run a custom built 11" lower I got from Charles at Magnum Powers. I believe some feel that this large a pulley is a bad idea on our cranks. The boost you see may be correct for your application. Hard to compare your ported 3 valve setup to the 2 valves. You have much better flow #s.
Drop the upper pulley size down and see what results. Should be a lot of fun once you turn it up! I would also step up to the 10" lower if you have an interchangable hub setup.
Jim

 

But anyway Ct.TOPGUN, if my boost is right and i am making 9psi only with a 9.5" lower and 3.25" upper i think i will never pass 20PSI if i keep the 9.5" lower even if i use a 2.75" upper.

Thanks.

 

The only thing that matters is the airflow, and power made. The only reason a pulley combo will give higher boost #s is if there is enough resistance to flow(restrictions). You are already making some good #s at low timing and boost.
I would go into some Mustang forums to see how ported 3 valve motors compare to the 2 valves. A much bigger sampling to be found in the 'Stang world. Yours is the first modified 3 valve motor I have heard of in our little world.
Jim

 

Your heads and exhaust create much less back pressure than on a stock motor. The boost #s typically supplied are taken from stock motors. This is why the #s are lower than you'd expect (provided correct install w/ no leaks, belt slippage, etc.)

Whipple has the 2.75" 8-rib upper pulley, and Jim mentioned the availability of the custom lower pulley.

Since you now have established a ballpark mathematical relationship between your lower/upper pulley ratio and the boost that it provides, it should look like this:

9.5/3.25 = 2.92 pulley ratio for 9 lbs. boost

15 lbs/9 lbs = 1.67 times more boost

Therefore, pulley ratio must be 1.67 times higher to get 15 lbs. of boost.

2.92 X 1.67 = 4.88 (necessary pulley ratio for 15 lbs. boost)

Now, let's say you use the Whipple 2.75" upper:

4.88 X 2.75 = new lower pulley diameter = 13.42 inches

Of course, that's huge! The one confounding factor is that your engine doesn't always behave according to this simple model.

The problem with using "linear" math like in this example is that your engine's flow characteristics are not linear as your approach its limits. The restriction develops back pressure at a geometric rate as you near the total air flow limits of the design. So, you may be running a certain pulley combination and getting "x" lbs. of boost. If you change a pulley to achieve more boost, you might get even more than you calculate.....if you're getting close to the limit.

For simplicity, why don't you slap the 2.75 Whipple upper on your engine and see what boost #s you get? (Agree with Jim here) This will give you a good indication if you truly need to have a large custom lower pulley fabricated, before you spend all those $$$$. You'll have a much better idea of how to proceed then.

Good luck,

Lee

 

Also, ultimately, it is really about the HP you wanted to make. The boost # is secondary. Your motor is certainly well-built, so get the HP you want and don't worry about what boost # you have. You may well make big HP with less boost due to the overall excellent componentry you've used.

Lee

 

Rafa26, one quick question for you. I remember reading your pump gas thread,and that you had a torque converter issue at your power level. Did JDM detune it for safety? Are you sure the lower pulley is 9.5 inches(8#s)?
Thinking they took some power away to keep you safe while the torque converter issue is resolved. They are usually very conservative with a new setup until sure all is right.
Jim

 

Good thoughts.

Lee

 

Yes, it's detuned, Jim Sr. said the TB is closing earlier and that is blocking some TQ and HP.
I was charged a 9.5" Metco lower fo Lightning and i am sure that's what they installed.

The good news about the TC is that JDM will have a Precision triple plate for me tomorrow and Jasper shipped a new transmission and should be there by monday.

 

"boost" is a measurement of backpressure. more boost does not equal more power.
more boost equals more restriction.


(I'm sure someone will argue that more boost increases the PSI in the cylinder, which is true, but that doesn't necessarily mean more power as you reach a point of diminishing return)

 

Why did you ask if i am sure that my lower pulley is a 9.5" ??

Do you think S/C is making little boost too ?

 

Be sure that your boost bypass valve is functioning correctly, too.

Lee

 

I just checked with JDM and my Lower is a Metco 8.5" and not a 9.5" as i thought. They did it that way to keep the power down.
Next week after they install new transmission and TC they will install a 10".

Thanks .

 

Kinda thought that may be the case! With the 10# lower you should be about 14#s on a cool day. That will be a nice increase in power and especially torque. Combined with a converter that doesn't slip it should be a big difference SOP.
Jim

 

You're right man !
Thanks.