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Intake Swap
#1
Hello all, So I swapped Intakes from an RPM air gap for 351C, to a Blue Thunder and seemed to have lost some bottom end. I am running a Pro-Systems carb. Is there anything I can try doing to get some of the bottom end back or is this just nature of the beast?
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#2
Omie01,
The bottom end may be there but you will have to re- find it.
Looks like you lost a bunch of air flow velocity due to port sizing differences in the two intake manifolds.
I would start by looking at accelerator pump nozzle and cam tuning on that carb and then take a good look at power valve tuning and possibly the power valve channel restrictor sizing.
Did you happen to take note of vacuum readings of the two manifolds at warm idle ?
What are you running for ignition advance ? mechanical only or vacuum/ mechanical ?.
I am not a huge fan of double pumper carbs on a Cleveland, in my opinion you have a top end carb with a low end intake manifold on a top end engine.
Just a guess here, but I think most of us that are running blue thunder intakes are probably running vacuum secondary carbs or probably should be.
Any chance you have wide band o2 monitor ? that would really help in re tuning that double pumper.
  Boilermaster
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#3
I do not have the 02 tool, unfortunately I can only tune by trial. My vacuum did not change at warm idle. I am using a stock dizzy that was re-curved for my engine, but does have the 10L slot. And is all in by 2800-3000 RPM. I do have a 750 Vac secondary carb I can try. I just didn't feel that is was enough carb for this engine. I have no issues going back to Air Gap if that gives my power back, or I could send my Pro Systems carb back to be re-calibrated.
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#4
Omie01 Wrote:I do not have the 02 tool, unfortunately I can only tune by trial. My vacuum did not change at warm idle. I am using a stock dizzy that was re-curved for my engine, but does have the 10L slot. And is all in by 2800-3000 RPM. I do have a 750 Vac secondary carb I can try. I just didn't feel that is was enough carb for this engine. I have no issues going back to Air Gap if that gives my power back, or I could send my Pro Systems carb back to be re-calibrated.
Omie01, bolt the vacuum secondary 750 carb back on there and see where you stand.                                       how about posting the specs of the two different carbs, to see where the 2 differ.                                                 I am running a vacuum secondary 750 on my mild 408 and it pulls hard until I shift.                                             BTW what axle ratio and tire diameter you running ?                                                                                           Boilermaster
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#5
I am running 3:50 rear gears and 15 by 8 magnum 500's. I think I am just going to put the Air Gap back on and save the BT for a later build. My VS Holley was tuned for Open Chamber heads so it is probably not enough Carb. But thanks for your replies!!!
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#6
I’ve got the Blue Thunder intake on my 351c and just had my motor rebuilt. I can easily spin the tires with just a half stab of the throttle. I have a Tru Trac Posi with 3.50 gears, so nothing wild. I have a 735 HR Quick Fuel carb, on the BT intake and no spacer. My cam is a Crower flat tappet 292/302 with a mild .538 lift, so nothing wild there either. I also went with the crane gold aluminum rockers. Have a c6 trans with a TransGo shift kit and a 2500 stall converter.
Now I have never tried the Air Gap intake, but my combination I have really rocks pretty good. I just got it all broken in and still haven’t put the pedal all the way to the floor, yet. But from a standing start and about a 3/4 punch of the throttle smokes the tires and will break loose at 45-50. It really runs great!
So maybe you had the perfect combination with the air gap and everything else. I have heard great things about both intakes.

run_horse Run Horse Run!
John 72 Q Code
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