Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - my 671 isn't making boost?
View Single Post
  #9 (permalink)  
Old 04-24-2008, 11:45 PM
F-BIRD'88's Avatar
F-BIRD'88 F-BIRD'88 is offline
Yada Yada Yada
Last photo:
Join Date: Oct 2005
Location: Ontario, Canada
Posts: 9,285
Wiki Edits: 0

Thanks: 6
Thanked 400 Times in 394 Posts
What you're seeing on your boost guage is pretty normal.
"boost" is a reading of what is not getting into your motor.
Its what is left over. You camshaft valve timing will effect the boost curve and boost response.
Normally with the drive ratio you are using you will see boost start at 2500rpm +/- and build as the motor revs higher. Pobabily not peaking out until 5000rpm or higher.

If you erwe to make your eninge breath more efficiently, say a cam change or less restrictive cylinder head ports then you would see less boost on the gauge and make more engine power. Its how much power you make that matters.
not how much boost ( manifold back pressure) the gauge reads.
if you were to make the motor less efficient (smaller cam, more restrictive heads smaller valves and ports and/or more restrictive exhaust, you would see the boost gauge go up (and power go down) because less of the air available in the manifold is getting into the motor.
Cam timing (overlap can have a big effect on the boost curve. if the camshaft has too much overlap, some of the boost will go right thru the motor and out the exhaust i the low/mid rpm range resulting in a slow boost curve that all of a sudden comes on.
Just remember that the boost is not an indication of how much power you're making, its an indication of the amount of air that is being pumped by the blower but not consumed by the engine.
Your carbs are not too big or too small. Changine the blower drive ratio will increase peak boost and quicken the boost response ( more mid range boost) but just be aware that there are practical limitations on how much boost you can run without detonation on pump gas. For 92 octane fuel with the cr you have, you're set up pretty well right on. More boost ( blower drive ratio) will require higher octane fuel or lower engine compression ratio or water injection to avoid engine killing detonation.
how much boost do you have at 2500rpm,3000,4000,6000rpm with full throttle?
Should be a smooth progressive building boost curve and power curve. If it has little or no boost (and power gain) in the low/mid (3000ish) rpm range and then all of a sudden at 4500rpm you have a ton of boost then the camshaft may be wrong for your motor. (too much overlap) causing overscaveging.
Like I said you have to take the boost gauge reading relative to how much power the motor is actually making.
Do you know the cam grind that you have in your motor.
I don't suspect a problem, just asking.
Reply With Quote