Intake Swap

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Joined
Oct 4, 2014
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Location
Minnesota, USA
My Car
1972 H code fastback Boss 351 clone
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?

 
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

 
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.

 
Last edited by a moderator:
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

 
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!!!

 
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.

 
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