Port question...

[442_Mustang]

Ok, My engine is finished and dynoed. Olds 455.460 hp and 538 tq. over 500 ft/lb at 3000rpm when the pull started and 500 ft/lb till after 5000 rpm. Kinda Dieselesque...lol.. But my engine builder was talking about the ports being too large on the Performer RPM heads. He explained and I pretty much understand how smaller more efficient ports are better than large lazy ones. Makes sense. While we were dynoing it we put a 2 inch carb spacer on it and picked up another 28 hp. I really didn't care much about that because it won't fit under my stock hood. May as well test nitrous. That isn't going on there either but it makes for tasty bragging rights.
Ok... Here's the question. Why do smaller, more efficient ports make better power but the plenum area under the carb makes more power with the spacer? Wouldn't that be a big lazy intake area?
Thanks 4 reading my ramble:-)

[WS6]

velocity. It's always about velocity. The valves are only open for a very short period of time. During that time, you must cram as much air into the cylinder as possible. Flow tests only show volume with an unlimited valve open time so velocity doesn't show up.

As for the spacer, grab a copy of High Performance Pontiac's June 09 issue. They talk all about Wilson spacers and how they work. Very good read.

[442_Mustang]

I understand how velocity works to increase cylinder filling. I just don't understand how the increased plenum volume can make power. Seems like a contradiction to me. My engine builder and dyno guy can't explain it in a way I can understand. I will look into that article.

[jackfrost]

could it be a ram effect? sort of like a small tunnel ram? where was the power picked up?

[442_Mustang]

Power picked up across the graph. I'm also running a 750 Mighty Demon. I should put on an 850 but what are you going to do. I'd rather not overcarb. If spacers are so good, why don't the manufacturers build them in by making the carb pad higher? I hope when I get it fnished I can at least put one of those 1 inch Wilson spacers.

[Pro Stock]

Ok, My engine is finished and dynoed. Olds 455.460 hp and 538 tq. over 500 ft/lb at 3000rpm when the pull started and 500 ft/lb till after 5000 rpm. Kinda Dieselesque...lol.. But my engine builder was talking about the ports being too large on the Performer RPM heads. He explained and I pretty much understand how smaller more efficient ports are better than large lazy ones. Makes sense. While we were dynoing it we put a 2 inch carb spacer on it and picked up another 28 hp. I really didn't care much about that because it won't fit under my stock hood. May as well test nitrous. That isn't going on there either but it makes for tasty bragging rights.
Ok... Here's the question. Why do smaller, more efficient ports make better power but the plenum area under the carb makes more power with the spacer? Wouldn't that be a big lazy intake area?
Thanks 4 reading my ramble:-)

High velocity is highly overrated, low velocity as a result of to large of port will not completely fill the cylinder resulting in a lazy engine, the easiest way to fix this is to raise the rpm band of the engine through camshaft change and and carburetion, if this is not possible due to not having strong enough engine components to support the increase in engine speed then you would need to head the other direction in port size, to high a velocity will cause the cylinder to fill to quickly and cause the column of flow to momentarily stop which will cause loss of inertia in the air column, this starting and stopping of the column results in poor cylinder filling. Velocity is not determined by a fixed amount of depession on
a flow bench, it is a result of engine size and engine speed. An example is, 400ci at 5000rpm will have less air speed than a 455ci at 5000rpm. The bottom line is I don't think that you have too large of head on your combo, I think that your power curve goes flat because you need the bigger carb, see if you can borrow one from a friend and pay attention to what your butt tells you. as far as the plenum spacer goes, natural tendancy would be to assume that you are seeing hp gains because of increase in plenum volume alone but another big part of the equation is that you are also changing the distance from the carb butterfly to the intake valve which also is a tuning aid, just the same as header pipe length and diameter. The bottom line is that there are no pat answers, especially in the mags.


Dale

[Skip Fix]

Spacers besides adding plenum change the length of the intake tract and that affects the ram effect based on harmonics. This can help or hurt. Unlike the magazine articles spacers do not always help. Personal example the 400ci Pontiac RAIV head 72 HO intake 73 SD Q jet motor I built picked up 5 hp with a 1/2 spacer(all that the Shaker would fit). A friend's 72 455 HO with the same casting intake and same model carb LOST hp with my spacer. Different cam, CR and stroke made the difference.

Spacers are a test and tune addition for every individual combination.

[TraviZ28]

I went as big as I could with everything, valve size, runner size and intake size, throttle body too! But I am forced induction and I was told to let that motor breathe as much as possible because air is being forced in, not sucked in, and its a totally different ball game.

[Pro Stock]

I went as big as I could with everything, valve size, runner size and intake size, throttle body too! But I am forced induction and I was told to let that motor breathe as much as possible because air is being forced in, not sucked in, and its a totally different ball game.

This advice is correct, don't overlook exhaust port flow on a supercharged or turbo engine, they like larger exhaust ports and lots of flow and a larger header.

Dale