Emi exhaust still reverting
 

Engines: Twin 509 500 HP carb'd. Crane cams Hydraulic Flat Tappet. 534/551 lift 224/236 @ .050 112 LSA .Over the last several years I had issues where the engines would seem to load up at idle while in the opened loud mode and run smooth while in the quiet mode with no loading up. That setup was with the Mercruiser silent choice and the Eddie Marine EMI-512 short SS risers.Many people told me it was reversion. Then I switched the exhaust tips to Drew marines shotgun III which then i thought cured the reversion issue because I had no loading up issues. Those spring loaded tips added back pressure to the exhaust. Little did I know it was still reverting. Last winter I removed the 2 inner exhaust manifolds to get to the starter and I noticed rust spots in every exhaust port on both motors. These heads are 2 years old. Just recently done over. So I decided to get rid of the short EMI risers and the Merc silent choice. I sent my EMI risers back to Eddie Marine and had them extended to the standard length EMI-111 which comes with the thunder series manifolds. The water comes out at the end of the pipe. I was told any big cams must have a longer than stock pipe to prevent reversion. Missing the quietness of the merc silent choice I decided to go with Gil select sound clamp on switchable mufflers. Last weekend my first test run. Gil mufflers in quiet mode. Runs smooth, sounds awesome. Gil mufflers in loud mode..Runs not so smooth, sounds loud and seems like its loading up again.Close the mufflers and it smooths out and idles fine again. Wow I'm in shock!!!..What an expense I just went through. Had 4 pipes extended and bought new switchable mufflers AND the same problems as before....HELP!!
My camshaft isn't that radical...friends of mine are running HP500's , Big blocks with a lot bigger cams than mine ..Exhaust systems like Gil, stainless marine they are all about the same in exhaust length before the transom........Any input would be appreciated

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https://www.youtube.com/watch?v=MRMGLcQKbzY

We're the cams degreed when installed?

I don't know. I bought the boat with these engines. The only upgrade i did was MSD ignition and the timing is locked @ 34 degrees. Please explain why you asked about the cams being degreed?

Hmmm, dual plane intakes cut down on reversion a little too.

I'd say quite possibly, because of the mild cam (however, it does have a lot of exhaust duration) and etc, you could very well have too much initial timing.

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As far as exhaust being 'closed' having less reversion, than open, it is probably because the design of your mufflers dampens the exhaust enegy waves (back and forth) and thus not reverting. I have not seen your mufflers, but this does and can happen with some designs. Mecruiser even has an internal baffle one can install that does the same thing ie: dampens exhaust pulses.
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Are you sure it's not condensation that's not falling back in and causing the 'rust' ? Some companies use 'turbulators' that install between the riser and manifold to help catch this.

Again, I have a feeling you have too much initial timing. A motor like this will probably do better with somewhere between 16-20 initial.

BTW: other than adding water much later, Eddie Marine exhausts do not have any other safe guards against reversion like welded in dams, steps, or etc. They are just smooth with a smooth tranisition between the inner and outer pipes. Their risers don't raise up high like the late model HP 500's neither, which creates a larger rate of fall.

http://www.bullockmarine.com/mercury...500Bulldog.gif

I called Eddie Marine yesterday and they told me to send the 4 risers back and they will add a few more inches in length and also add a dam to stop the water from creeping up the pipe.

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Thanks for your reply. I believe you are correct about the exhaust pulses. As long as there is some kind of back pressure it stops reverting. Does this look like condensation or reversion? I was told in previous posts last winter it was definitely reversion. Can't change the timing. Those msd weights and springs screw up and rust. That's why they are locked out. no moving parts for advance at all. I guess I'm going to run in closed mode at idle then open them after 1000 rpms for now.At least to the end of boating season here. Eddie marine said they would add a dam and extend them longer but it will take at least 3 weeks. Thanks

If the cams ended up getting installed a few degrees retarded it would change the valve timing events, most notably EVC ATDC. With the longer exhaust duration that it already has, this would amplify low rpm reversion pulses.

I just ordered 2 MERCRUISER EXHAUST RESONATOR KIT that is supposed to stop the reversion by changing the pulses in the exhaust. Lets see what happens

https://www.perfprotech.com/blog/art...ust-resonators interesting info

The exhaust resonators (provided with the engine package or available separately) are designed to provide increased resistance to water ingestion due to the tuning effects of the exhaust system. Quicksilver resonators are designed to break up the exhaust pulses.

If this works, I could have saved a bunch of $$ this past month:mad:

35Fountain, the Merc resonator looks interesting, but first I would try installing some salisbury type flappers on the ends of your tips to see if this dampens the back pulses enough to tame the reversion. I have a long thread on here about reversion with a mildly cammed engine when I was running mine on a test stand, and I found that simply installing the flappers significantly cut down on the reversion. Before doing this I could hold a $20 bill in front of the tailpipe and it would suck it right out of my hand - proof positive that boats eat money! :)

It is strange how one engine with what appears to be a mild cam setup will revert, while others that seem to be more on the ragged edge run happily along without apparent problems. My suspicion is that there are a lot of boats running around out there with some degree of reversion occurring, and no one knows about it.