Performance not as expected.....?

[95Vert]

Another queston Wyt, what are the consequences of running too big of a tb?

 

[wytstang]

Waste of money and making some one elses pocket fatter. Also there is what they call "tip in"

"this is the amount of open area the throttle sees compared the the amount of throttle angle or cable travel. A larger bore diameter creates a larger open area for any given throttle angle. On a street driven car, this "tip-in" can make the car a little touchy with the larger throttle bodies like may 75mm and up."

 

[wytstang]

Read all 2 pages you'll find something very intresting on page 2 post #52 2nd paragraph
http://bbs.hardcore50.com/vbulletin/showthread.php?t=26335&highlight=Cobra+intake

 

[WHITE94COBRA]

oh yeah i also have a Mac off-road h-pipe on that dumps right before the rear end

Are you running mufflers at all??? Because that can also be a reason for the power loss and the higher ET. If your dumping your exhaust straight from the h pipe you are getting no backpressure thus reducing the amount of torque your car makes down low, the only way to make up for this loss is one of two ways: 1. steep gears to get you up into the higher RPM range and keep it there through the gears. 2. Add mufflers, or high flow cats.

 

[Grn92LX]

what are the consequences of running too big of a tb?

If the tb size matches the inlet opening of the manifold there are zero issues. Been there/done that on both of my motors. Never had a "tip in" problem.

To answer your other question, No a 70mm is NOT overkill for him. Its ideal

 

[WhiteDevil]

If the tb size matches the inlet opening of the manifold there are zero issues. Been there/done that on both of my motors. Never had a "tip in" problem.

To answer your other question, No a 70mm is NOT overkill for him. Its ideal

Exactly that tip in is BS i have been running a 75mm for a while now and have had no problems with this "tip in". If the cobra opening is 70mm and his tbody is 70mm then that is perfect.

 

[wytstang]

How is it perfert to use an air valve that moves 726 CFM into an intake with an average flow of 198cfm???

 

[94tang]

that would be 198cfm PER runner wouldnt it?.....198x8=1584cfm.......the reason for the 70mm is that it came with the intake when i bought it....$300 for upper/lower and 70mm tb.....not a bad deal i thought..........oh and yes i have mufflers on it,just no tailpipes

 

[WhiteDevil]

How is it perfert to use an air valve that moves 726 CFM into an intake with an average flow of 198cfm???

If those flow number are really the case then i dont even need an upgraded Tbody with my 347 stoker! Going to a 75mm from my old 65mm with my H/C/I combo netted me like 50kg/hr at the top end and around 8-10 through the curve data logged on my tweecer.

 

[wytstang]

Are you guys just ignoring what Accufab is telling you??

Here are some CFM ratings on "an engine", based on displacement and RPM's, but without any "CFM losses" due to heads, intakes, exhaust, etc.

Disp. (in cu. in.)..........CFM @ 6000 RPM.........6500 RPM............7000 RPM
280.....................................486....... ..........527....................567
290.....................................503....... ..........545....................587
300.....................................521....... ..........564....................608
310.....................................538....... ..........583....................628
320.....................................556....... ..........602....................648
330.....................................573....... ..........621....................668
340.....................................590....... ..........639....................689
350.....................................608....... ..........658....................709
360.....................................625....... ..........677....................729
370.....................................642....... ..........696....................749
380.....................................660....... ..........715....................770
390.....................................677....... ..........734....................790
400.....................................694....... ..........752....................810
410.....................................712....... ..........771....................830
420.....................................729....... ..........790....................851
430.....................................747....... ..........809....................871

These CFM ratings are all "naturally aspirated" of course. I'm not suggesting that you only use a TB or a carb that is equal to these ratings, but (a), it won't hurt if you do, and (b), it won't help if you use one much bigger than what the engine can use.

If you check the CFM rating for the 350 inch engine (like a Ford 347), you can see why a street engine, peaking out at about 6000 RPM is happy with a 650 CFM carb. That same engine with a GOOD EFI intake will probably be best with a 70MM (787 CFM for Accufab's) TB. Installing a TB bigger than a 75MM unit (Accufab's 75MM flows 924) is NOT going to make more HP, everything else being the same.