Exhaust system size

[ndutton]

We started talking about increasing the size of exhaust systems over on the troubleshooting section so I thought I'd move the discussion over here.

The more I think about larger aqualift exhaust systems on our little engines and their ability to expel water, the less I like 'em.

As an example, the difference between the cross sectional areas of 1 5/8" exhaust hose and 2" exhaust hose is about 50%. In terms of exhaust gases, going bigger reduces backpressure allowing the engine to breathe better. No quarrel from me.

In terms of expelling water though (the whole purpose of an aqualift system), it takes a greater volume of water to 'seal' off the exhaust hose so the exhaust pressure will push it along, especially push it up. Also, there's less exhaust velocity to do the job with a larger system. We need to factor in how much exhaust force we have available in the first place, most critically at idle. In sort of a twisted way, I can see where a larger exhaust line could result in more backpressure when factoring in the added volume of water in each 'batch' as the aqualift system batches out the water.

I'm sure there's a sweet spot somewhere between too small a system and one too large. We have pretty good historical data given the number of A4's with aqualift systems but I'm not sold on the 'bigger is better' strategy. The fact we're dealing with wet exhaust systems changes things.

Thoughts??

[hanleyclifford]

Neil - I guess you knew that I would be among the first to respond to this new thread. I like the way you have framed the issue. Let me begin by saying that I have been using 2" ID pipes and hoses all the way to the transom for several years. But your discussion about "diminishing returns" is sound and should give reason for pause about going any larger. The engine must always be able to expel the water. This requirement of course is going to vary with each boat's configuration. The greater the "head" against which the engine is working, the more dangerous larger wet sections would be potentially. All this of course needs to be quantified. Just one final thought for now: the manifold exit diameter is 1 1/2" - comments on why the manufacturer would reduce immediately to 1 1/4"?

[Dave Neptune]

Exhaut options

I pretty much agree with you both about the additionl preassure and or "flow" to force the water out up & over and so on. However after the water is injected the exhaust gasses will cool rapidly and require less diameter than an exhaust system without water injection and we still need to evacuate the water through the same crossection.

Perhaps the exhaust back preassure guage will give some answers as to how the A-4 responds to back preassure. Many engines due to there cam profiles require a bit of back preassuure to "breath" properly under load conditions.

I actually have two riser hoses so to speak, I take it up under the cockpit then back down to the bilge then up to the transom. Lotsa pipe and hose maybe 14~15 feet or more.

Dave Neptune

[hanleyclifford]

Dave - I think as we move forward in this discussion it might be useful to post some exhaust system pictures. We have seen quite a few Catalina 30 pix. I would like to study your "twin risers". Attached is a view of my hot section. 4000 hrs on this configuration.

[rigspelt]

I like Hanley's two 90s and one 45, but can't arrange that in our boat. Ended up going with the design it had when I bought the boat, after much head-scratching and research, except for more height in the dry stack, adding height to the red coolant line (added antisiphon valve in the red hose later -- this photo was taken during dry-fitting) and shortening the exhaust hose run to the transom. Haven't measured the back pressure, but seems to run just fine, with no seawater backup threat. After running the engine, there is about an inch of water in the muffler. Boat's level waterline is just below the top of the muffler.

[hanleyclifford]

Rigspelt - Nice neat set up. Are we looking at the MM aqualift unit? I like the fact that you appear to have your injection point about a foot above the water line. One of the weaknesses of my system is that I inject just about at the line. For this reason I am obsessive compulsive about shutting off the thru hull every time I shut down the engine - even at a refueling stop! Regards, Hanley

[Dave Neptune]

Headers?????

Guys, I think this may work out to gather some useful information albeit that there are so many ways to get it to the transom or out the side.

I'll try to give a better description of mine then I'll borrow a camera. I don't think I can get pics of some sections. My engine sits amidship and is entirely below the water line. From my exhaust flange 1 1/4' it's straight out back and facing down hill slightly, then a 90 down (my water injection is here well bellow the water line) to the bilge and another 90 and about 2 1/2 feet to the water trap muffler beneath the floor (I made it of F/glass ). Out of the muffler it goes up about 4 feet under the front of the cockpit then two 90's and right back down to the bilge, then uphill to the top of the transom another 90 down to the exit under the transom. I have an old Ericson 35 with an overhanging transom and while motoring the exhaust stays out of the water in most conditions. I'm quessing that I have a run of about 18~20 feet. My hot section is less than a foot long and I made the mistake of using galvanized pipe as I had it in stock at work. The engine box is under the settee and the settee bunks are below the water line. I have had many an issue with salt water getting back into the engine and have since gone to a manual syphon break. And 3 times I have forgot to open the valve and filled the beastie at anchor. It's about an hour to get the water out from under the oil and I'm running easy until I get the oil changed a couple of times.

I'll try to get some details with photos soon.

Dave Neptune

[hanleyclifford]

Dave - Yeah, headers! I remember that dollar for dollar they were the best thing you could do for a street racing machine. But we didn't have to worry about water backing in on us. Sounds like you do (me too). Also, remember why Neil started this thread - we are going to try and figure out just how big one can go with an A4 system before the danger of being unable to "batch" the water out sets in. I'm hoping some of our young college members (Trumps, where are you?) will step in and correct me if needed. I think we need to look at how much work has to be done by the exhaust gases to lift the water up and out. Work=force x distance. Force=mass x acceleration. Ok, all you engineering students, take it away! Regards, Hanley

[Dave Neptune]

Wow!

Guys, I stopped by the boat this morning on my way to work and checked a few things including my compression for the first time in probably 8~10 years. I was a bit surprised as the numbers are now better than they ever were.

I checked my exhaust so we know what I am dealing with. I have a hot section that the total length is 8" (3 straight out of the manifold and 5" straight down) and another foot to the bilge, then 4' to the muffler. from the muffler straight back 1' and then up 4' 90 straight 90 back down 4' then up hill 9' to the top of the transom and back down 15" to the atmosphere under the transom. Approximately 25' and it's all 1 1/2" hose and 1 1/4" pipe fittings and a shut off 1 1/4" ball valve at the transom. I WAS SURPRISED AT THE LENGTH~~ WoW. That'll make some back preassure I would think.
Last weekend I was cruising at 2100 rpm's @ 7 1/2" of vac with my Indigo.

Dave Neptune

[67c&ccorv]

Here is mine - original copper jacketed exhaust installation on 1967 C&C Corvette:

(I have not measured but am pretty sure the exhaust is 1&1/4" from the exhaust flange to the muffler.)

[rigspelt]

Here is mine - opriginal copper jacketed exhaust installation on 1967 C&C Corvette

Nice engine room! I continue to be mightily impressed by the seamanship displayed by the owners of so many of these old A4s, chugging away and well cared for. I can see why Don got hooked on them. Must be gratifying for him to see them carry on this way.

[hanleyclifford]

practical exercise

While still mulling over how to come up with some formulas (formulae?) that will predict exhaust performance of various pipe sizes, I decided to run the back pressure test on the hot section according to the instructions from Don Moyer. My system consists of hot section 2" ID with two 90 degree turns and one 45 degree turn, thence to Vetus waterlock (2"), out to transom via 2"ID wet hose. The rise from bottom of waterlock to maximum hose height is about two feet. The pressure gauge oscillated with the "batching" cycle between 0 and slightly less than 1 psi. I had never paid much attention to this matter until this issue came up in this thread. I recommend that any owner with a long or otherwise suspect exhaust system consider ordering the pressure test kit from the MM on line catalogue.

[rigspelt]

Rigspelt - Are we looking at the MM aqualift unit?

Hanley, yes, that's the stainless MMI waterlift muffler.

[hanleyclifford]

Copper jacketed system

67 - That exhaust system you have could very well be the least restrictive of all. It appears to eliminate the need for a riser and all the 90 degree bends associated with most systems. Can you give us the ID numbers from engine to transom? Also, do you have any sort of muffler or waterlock anywhere in the system? Regards, Hanley

[lat 64]

I think I have nearly the same as 67'.
Here' some "before" shots from '08 that might give a clue. I don't think it gets any smallr than the fitting at the flange.

Russ