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First engine run (vid) - exhaust reversion help needed!

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First engine run (vid) - exhaust reversion help needed!

Old 04-08-2012, 09:11 AM
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Default First engine run (vid) - exhaust reversion help needed!

So I'm fairly new to all of this, and got a project Donzi 24 Black Widow with dead engine.

I bought a built engine from a friend, which had already been run-in on the dyno (flat-tappet cam) so don't worry about the revs.

It sat for a long time and we just finished the accessory stuff and finally got it started.

In any event, some guys over on the Donzi forums all noticed that exhaust reversion will be a problem based on the video and that it might have been what killed the previous motor (which had water in the oil when I got it, but the main problem seemed to be a stuck exhaust valve with everything else relating to that).

I didn't take measurements yesterday but I'm worried about my options at this point. It might be possible to extend the risers - does anyone know if SM makes taller risers at a different angle? I've also seen other solutions while searching such as x-pipes, introducing water at a different location, or running thru-hull water dumps.

Some info on the build: assembled by Gardener Performance @ .060 over to 468. .510 lift and 226 deg. intake and 236 deg. exh. @.050 on 112 centerline, roller rockers, all forged internals, dimple rods, SRP 10:1 pistons, heads are GM Perf w/ inconel valves, Sea Demon 830 carb, stainless marine exhaust.

[YOUTUBE]http://www.youtube.com/watch?v=WXJyYFpTplg[/YOUTUBE]
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Old 04-08-2012, 09:34 AM
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If reversion is an issue, Stainless Marine can lengthen your risers. I have a set on a SBC that have been lengthened by 11 inches.
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Old 04-08-2012, 09:42 AM
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Originally Posted by Sutphil
If reversion is an issue, Stainless Marine can lengthen your risers. I have a set on a SBC that have been lengthened by 11 inches.
Thanks, if you don't mind me asking - how much did they charge you? I just sent them the video for opinions and what they can do to help or if they'd suggest changing the cam.

It looks like it was most severe coming OFF revs into idle at the end of the video. Idle looked decent?
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Old 04-08-2012, 11:07 AM
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That camshaft spec. does not seem like a reversion prone problem. The engine does not seem to have a unsettled idle that would be a problem with a standard exhaust like this. I think what you might be seeing here in this video is that you appear to be running water from a household hose hookup where the water pressure could be 40-50psi or so and you might have been trying to push too much water through the exhaust to begin with in this out of the boat run test. Also remember that in the boat the engine with boat in water that especially at lower rpms and idle the angle of the engine and the exhaust will be down more than in your floor test where the engine is flat. I am not sure how those experts on the Donzi forum can see reversion in the exhaust anyway, I kinda doubt they can really see it in these videos. Did you find any moisture back in the engine after this test?

You might also make sure the engine idle is kept @650-700rpms and that you can also install a thottlestop damper to keep the throttle from snapping shut to quick which tends to cause the engine to nearly stall when it comes back to idle to quickly. It did not appear in this video that someone was using a tach to establish idle, but maybe that was done earlier.

You can also have Stainless Marine make you some new tips where the water will exit at the tip at the transom and this will lessen the issue of water being to close to the engine.

Best Regards,
Ray @ Raylar

Last edited by Raylar; 04-08-2012 at 11:13 AM.
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Old 04-08-2012, 05:51 PM
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Originally Posted by Raylar
That camshaft spec. does not seem like a reversion prone problem. The engine does not seem to have a unsettled idle that would be a problem with a standard exhaust like this. I think what you might be seeing here in this video is that you appear to be running water from a household hose hookup where the water pressure could be 40-50psi or so and you might have been trying to push too much water through the exhaust to begin with in this out of the boat run test. Also remember that in the boat the engine with boat in water that especially at lower rpms and idle the angle of the engine and the exhaust will be down more than in your floor test where the engine is flat. I am not sure how those experts on the Donzi forum can see reversion in the exhaust anyway, I kinda doubt they can really see it in these videos. Did you find any moisture back in the engine after this test?

You might also make sure the engine idle is kept @650-700rpms and that you can also install a thottlestop damper to keep the throttle from snapping shut to quick which tends to cause the engine to nearly stall when it comes back to idle to quickly. It did not appear in this video that someone was using a tach to establish idle, but maybe that was done earlier.

You can also have Stainless Marine make you some new tips where the water will exit at the tip at the transom and this will lessen the issue of water being to close to the engine.

Best Regards,
Ray @ Raylar
Thanks for the thorough post Ray, it's nice to get some expertise looking at the problem!

All of your explanations make sense to me - do you think I should just continue with the build, put the engine in the boat, and see what happens on the water? The cam thing isn't much of a solution for me anyways so I'm glad you think the profile looks okay - sounds like the engine is going in!

Worst case I have a few options to help the issue once it's in there. The engine uses a standard mercruiser style water pump with a thru-hull pickup, so I'm not sure how that will change the pressure or the water - at idle as you mentioned, this should be less of a problem. The engine is indeed at a more optimum angle IN the boat and I'd imagine this will be further enhanced by how the boat sits in the water as a whole.
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Old 04-08-2012, 06:26 PM
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I agree with raylar on all counts. The cam is close I believe to the stock 365hp mag flat tappet that came with stock merc cast iron exhaust and it was fine. I'd be a bit more concerned with the compression ratio and cast iron heads. You need to be conservative with all in timing and run good gas. Also make sure you are running a zddp oil additive to keep that cam from going flat. After that run it. It sounds healthy.
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Old 04-08-2012, 06:46 PM
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According to the gauge on my dash (I'm sure it looses atleast a couple psi in the length of the hose) at idle I have about 3psi and at WOT I usually have some where between 15-20. Hope this helps.
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Old 04-08-2012, 07:34 PM
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I have a set of extended risers for SM exhaust. I had the internal pipes extended to keep any chance of reversion down with my HP500's As it turns out I hacked off all but about 2" of the extension and they work great with no sign of reversion on the HP500. I still have one untrimmed set that I would let go for a reasonable price, with or without the lowers.
Attached Thumbnails First engine run (vid) - exhaust reversion help needed!-parts-014-large-.jpg   First engine run (vid) - exhaust reversion help needed!-parts-012-large-.jpg   First engine run (vid) - exhaust reversion help needed!-parts-013-large-.jpg  

First engine run (vid) - exhaust reversion help needed!-parts-015-large-.jpg  
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Old 04-08-2012, 08:14 PM
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Run it at idle for a few minutes, then shut it down and check every spark plug. Reversion will clean some spark plugs so they look brand new, also, bad reversion will make some cyl's miss and you'll hear it stumbling trying tp idle. like Ray said, that cam isn't too bad and shouldn't be sucking back too hard. get over 239*ish @ .050 and it's like a hoover coming back up the pipe at idle.
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Old 04-08-2012, 09:24 PM
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Originally Posted by blue thunder
I agree with raylar on all counts. The cam is close I believe to the stock 365hp mag flat tappet that came with stock merc cast iron exhaust and it was fine. I'd be a bit more concerned with the compression ratio and cast iron heads. You need to be conservative with all in timing and run good gas. Also make sure you are running a zddp oil additive to keep that cam from going flat. After that run it. It sounds healthy.
Hm I'm not sure about the cam vs. stock. Original motors were designed by Dennis Moore for a twin, and were sold on here a couple years ago. Apparently the original engine build was at 420hp and at 10:1 on the initial dyno runs they saw 490 with this setup.

Ted - I'll ask the guys to take some measurements and I will get back to you, thanks!
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