"the 383 builds 10 KPA of vacuum at 5,500": did you mean hpa?

 

No I mean KPA. It pulls 4 kPa at only 3,600 rpm. In the screen shot I posted you can clearly see it already has 6 kpa drop at 4,500 rpm. The 383 SBC made 502 hp @ 5,600 and 510 tq @ 3,600 with an 850 cfm Q-Jet and a Performer RPM. The 850cfm Q-Jet had the secondary air doors on the stop and still had nearly 2 in/hg of manifold vacuum at 5,500+ rpm. I wish I had remembered my 1" coned venturi Q-Jet spacer when it was engine dyno'd. Same spacer installed upside down on my 350 gave it 10+ tq across a broad rpm range and 15 hp above peak.

I do not fault the 4" air inlet, rather the TB being too small. I am about to swap an Indmar manifold on to the 383 with a 92mm throttle body. Should pickup power nicely on the top-end.

 

check for a dead rat in the system.

 

No dead rat in the system.
For test purposes, I temporarily ran the engine with an open throttle body. Roughly half the restriction disappeared instantly, up from 89 KPA to 94 KPA at 5,500 rpm on a 99 KPA key on barometric pressure reading. The Vortec intake air bonnet that attaches to the throttle body is to blame for 5 kpa of restriction. Removing the intake tract ahead of the bonnet makes zero difference to the MAP at WOT. The other half of the restriction is the 80mm throttle body itself. The 80mm throttle body has 5 kpa of restriction in itself on a 500 hp 383.

 

o.k., i see you are serious about this. i can assure you, that your maesurements are not correct, whatever the reason my be. iv done this often enough.

example: stock 99c5. pressure loss throught that air box (remember the little holes?), filter, maf, tb etc. is barely measurable. my best guess would be 1.5kpa. removing the box and filter makes no measurable difference. thats when logging map and map sersor voltage.

 

I assure you it is correct. MAP voltage and MAP readings both drop. The pressure loss is real. With the 80mm throttle body wide open there is still a 5 kpa restriction. With the complete intake tract there is 10 kpa restriction at 5,500.

For the record we are not talking about some stock C5. I have an engine that makes over 100 hp more than that Vette.

 

i give you another example, then im out: look up racing series that use a 40mm restrictor. how much power do they make?

 

Honestly does not matter. Restrictor plate engines are built to maximize the power output using the restrictor plate in place. Many could not be operated without them in place because of the sky high static compression ratio.

I am talking a run of the mill small block or LS, not some highly developed restrictor plate racing series.

The MAP drops in my manifold, it is a fact. When I get the engine more airflow it will make more power.

You can spare me with your non-sense on something that does not apply to a basic SBC or LS build, that is not at 16:1 static compression with an optimized cam profile to make decent power pulling against a high intake vacuum. I have run a 2bbl restrictor plate type camshaft in GM TBI engines to help the power production with the small 2bbl TBI. They can make power but not as much as an engine with ample airflow.

Key On Engine Off Barometric pressure was 100.4 KPA. MAP progressively drops the higher the engine revs. Means the intake tract is restricting airflow. Not exactly rocket science. 6.4 kpa is 1.89 in/hg of vacuum and that is only at 4,500 rpm. The MAP pressure is right in the screenshots I have posted. Next thing I expect to hear from you is the engine will make more power with more exhaust backpressure too.

 

i was trying to help. your measurement is flawed. over and out.

 

The measurement is not flawed. The data tells what is going on. The MAF also confirms that the flow is restricted when it basically hits a flat line at about 320-330 gms/sec and holds there for 800 rpm.

As I also said an 850 cfm Q-Jet on a dual plane manifold pulled close to 2 in/hg vacuum under it with the secondary air doors open to the stop on the tab, it was flowing all the CFM it had available.