Pismo10

Here is a cut away picture of the stock manifold on the 496 HO. Clearly shows the restrictor that many are talking about here on OSO. I can see why the CMIs add 50-60hp. Restrictor.

Nordicflame

iTrader: (4)

Is it just me or is the restrictor actually just necked down to the size of the riser/collector.
From this picture it doesn't look like removal of the plates would really have that much of an effect.

What does come to mind is that maybe a set of better flowing 3+ inch inside diameter risers would wake these up a bit if increasing hp. You could also ward off reversion with a bit longer riser as well.

Just some thoughts...
Ray?
Dave

CigDaze

Doesn't look like too much of a restriction at all. Looks like it's just an exhaust guide to nozzle and smooth the flow. If you remove it, now you have a bumpy, jagged transition - might cause turbulence and other issues?

I've heard of people removing them....What are the real life gains?

JS232

I agree with you... The exhaust tube itself seems restrictive... Would explain why the CMI's produce the HP increase. From what I see, removing the plates and installing bigger tubes is the way to go...

t-islands

Below is a quote from Ray@Raylar on the subject.

"We have tested quite a few and we know along with various headers systems how they all tend to affect power output of the 496MAGS and HO's These plates can be removed in stock 496's and because they restrict the exhaust opening diameter from 3-1/4" to just about 2-1/4" in diameter and with this restriction they have a moderate affect on the power of the engine especially above 4000 rpms. We have seen about 10 to 15 addtional horsepower at 5000rpms on the dyno and there are several boaters who have removed them and seen measurable increases in power and speed."

http://www.offshoreonly.com/forums/s...&highlight=496

fbh-velocity

The reason why the restrictor plate are build in is to prevent water reversion. Under run it gets really hot caused to the thin sheet metal and non cooling area and will catch the water and steam it out. Just an insurance from factory

Kanookstr

Unfortunately they were not designed for water reversion. They are designed to trap any condensation that might occur after engine shut down. By trapping this condensation it prevents any chances that the condensation can reach the iron valve seats/guides. this problem is seen more in the northern states where the water is much colder. It is more of a problem for people who keep there boats in the water. A proper idle down time before stopping the engine will help reduce the chance of this condensation from happening, They will do nothing to stop or prevent reversion. The fact that they reduce the exhaust hole from 3 1/4 down to 2 1/4 is a definite restriction.

I plan on cutting mine down to maybe 3" so to have some protection , but better flow.
If you are worried about the problem by removing them, then going to an after market exhaust will still give you the same condensation problem, because the after market exhaust don't have the tubulater design neither.

The picture is also deceiving because the cut is not thru the center. So you can't really see the 1" difference in hole size.

Kanookstr

In this pic I cut away more so you can see the 1" difference in hole size.

CigDaze

Now it's clear. In the first pics it was deceptive.

Kanookstr

That's why I did that. with the outer tube not being cut down the center, it rapped around, and at that angle you can't see the big difference in hole size.