Thru hulls vs thru leg back pressure test

[JHuck]

Okay, while my engine is out of my boat I thought about performing a test tied to a sometimes heated topic-performance gains of thru hulls vs thru the drive. I happen to have a left over Auto Meter gauge from a supercharged Mustang I once owned BK (before kids). It shows vacuum and up to 15 lbs of boost. I can tap into my Rex Marine diverters and read pressure both ways at WOT. Since the engine will be a free breathing 383, it should be a good representation of a hot SB/mild BB. What do you think, would it be worth seeing the results or has this test already been done? I figure it won't take long to drill and tap the diverter. Just seeing if there's any interest.

 

[TilliamWe]

Re: Thru hulls vs thru leg back pressure test

Your scientific experiment is one worth doing in my opinion. I'd love to help you!
As far as I know, noone has ever actually published data like what you are trying to collect. For years Mercruiser would say that thru hub exhaust actually was better than thru hull. But even just a month or two ago, Trailer Boats Magazine tested a boat with either a 5.7 or a 6.2 with switchable exhaust. And guess what, it ran faster with the thru hull. No surprise there, except that most "experts" say that a Small Block won't really benefit that much. Your 383 test would indeed be a good one, cause it's a small block, that thinks it's a big block!
Good luck, please post your results, if you do this experiment.

 

[JHuck]

Re: Thru hulls vs thru leg back pressure test

Do you think government TARP funding has grant money allocated for this type of research?

 

[TilliamWe]

Re: Thru hulls vs thru leg back pressure test

Witht he way the Federal Government is giving away our money these days, you should get some grant applications and give it a shot! He He

 

[windsors03cobra]

Re: Thru hulls vs thru leg back pressure test

No doubt a thru hull V8 breathe's a little better and will likely make more power but I think your being a bit optimistic with that PSI gauge.

 

[Tail_Gunner]

Re: Thru hulls vs thru leg back pressure test

I will be the first to say i am not even close to a authority on this subject...So i will try to relate what i have learned.....There is absolutey no benefit to be gained with with aftermaket exhasut under 500 hp and 5500 rpm.. with marine exhaust..None...Simply defined marine exhaust uses 3"tubing....cars use 1.75-2.5 step one.

Cams start back pressure known as reversion in marine application....simply said the kiss of death..water in the motor. Made even more simple you cannot use scavneging in a marine motor it tends to suck water into the motor.

So if i have this right you need to put a hot cam in a motor..mild in car terms....Isolate the water stream from the exhasut for at least 28" and the you can create back pressure to assist the motor in gulping air...but you need to make sure the negative intake pulses cannot influnce the water stream....

clear as mud huh...Bondo...Don...Sabbath.....TW ...be nice now..

 

[TilliamWe]

Re: Thru hulls vs thru leg back pressure test

Tail Gunner, what you say sounds logical, and I want to believe you. But why did the boat that Trailer Boats Magazine test go faster with the exhaust opened? It was a lot less than 500hp, and it most certainly benefitted from the open exhaust.

 

[JHuck]

Re: Thru hulls vs thru leg back pressure test

Regarding measuring PSI between the two set-ups, yeah I figure if I get any kind of variation in flow it will probably only be few PSI, maybe only one. You never know, I may be surprised. Going into it, I don't have an opinion on if there would be a difference or not. I have the set up on the boat because it was there when I bought it.

One thing I've thought about regarding auto vs. boat exhausts...performance auto exhausts are often dual setups (with a crossover) that might be 2.5" for both sides (5" combined). Yes they have mufflers and cats but many performance systems flow really well these days. Now compare this to a single exhaust bellows. I think they run 2.6". Does this make a difference, who knows? I've often heard size matters.

We of course can debate the heck out of this. I guess this means I'm going to Lowe's for some plumbing pieces. Now where is that US Treasury phone number?

 

[abj87]

Re: Thru hulls vs thru leg back pressure test

If you build a water manometer it may be a better test.

 

[wca_tim]

Re: Thru hulls vs thru leg back pressure test

This may not answer the question that you're going after, but was useful information when I was thinking about the same thing.

The fellow who built my current smallblock told me that they did a performance comparison (on engine dyno) of "off the shelf" performance engine components including marine exhaust set-ups working with a well known performance boat manufacturer in the area and that the manifolds / headers were the bottleneck.

soo your experiment might be a good one (any experiment that provides information is a "good" experiment) with the caveat that if the answer is no difference, you can only say that through hulls aren't a performance booster with stock or near stock exhaust manifolds...

Food for thought, respectfully submitted of course...

ps... I've got several manometers... including the nice ones filled with mercury...

any kidding aside, the manometer is a good idea. if done right will be much more accurate and sensitive than any gauge that can be had for reasonable cost...

 

[45Auto]

Re: Thru hulls vs thru leg back pressure test

Why waste time messing with all that mechanical stuff ?

Just use one of these:

http://www.musclecarsounds.com/boats/index.htm

Guaranteed to add 5 MPH to your boat!

 

[JHuck]

Re: Thru hulls vs thru leg back pressure test

If I only had a stereo that worked!!!

 

[Howard Sterndrive]

Re: Thru hulls vs thru leg back pressure test

has this test already been done?

similar:

http://www.sterndrive.com/board/show_subtopic.cfm?ID=9272&TopicID=41

 

[wca_tim]

Re: Thru hulls vs thru leg back pressure test

a powerful sounding engine will make those who don't know believe your boat is faster... those who really know, don't care much about a few mph one way or another...

btw... the problem is really a LOT more complicated than just backpressure... unlike an auto exhaust you have the many and difficult not so subtleties introducted by putting water into the system... water is cooling the gas... dropping pressure, but adding density and momentum... and so where you inject the water stream, what temperature it is, what fraction of the mix it is, all are going to have a potential impact... btw, that water thing is likely why there can be a difference between boat with 4" when might not be with car and 3".... omg that makes my head hurt! How about this... with no mufflers, and 4 inch pipe exiting above the rub rail, my mess is annoyingly, obnoxiously loud... but that plus removing the spark arrestor requires several steps richer jetting to get the plugs the same color... and adds in the neighborhood of 5-7 mph on the top end. However, i roll with mufflers cause i like it better that way, and i keep the flame arrestor on cause I don't want to go boom... (or get a citation).

While you're at it, how about a temp sensor into the lube in the outdrive... I had someone tell me that a good reason to go with through hull was to keep from adding heat to the drive by running exhaust gas through it... Sure makes a lot of sense, but does it really make a difference in lube oil temp when you're leaning on the throttle for a while?

Alpine head unit, polk audio db series speakers and add the amp of your choice.

 

[Tail_Gunner]

Re: Thru hulls vs thru leg back pressure test

Tail Gunner, what you say sounds logical, and I want to believe you. But why did the boat that Trailer Boats Magazine test go faster with the exhaust opened? It was a lot less than 500hp, and it most certainly benefitted from the open exhaust.

Could you post a link...this should be intresting...I do have a extreme intrest in this subject.

 

[Tail_Gunner]

Re: Thru hulls vs thru leg back pressure test

a powerful sounding engine will make those who don't know believe your boat is faster... those who really know, don't care much about a few mph one way or another...

btw... the problem is really a LOT more complicated than just backpressure... unlike an auto exhaust you have the many and difficult not so subtleties introducted by putting water into the system... water is cooling the gas... dropping pressure, but adding density and momentum... and so where you inject the water stream, what temperature it is, what fraction of the mix it is, all are going to have a potential impact... btw, that water thing is likely why there can be a difference between boat with 4" when might not be with car and 3".... omg that makes my head hurt! How about this... with no mufflers, and 4 inch pipe exiting above the rub rail, my mess is annoyingly, obnoxiously loud... but that plus removing the spark arrestor requires several steps richer jetting to get the plugs the same color... and adds in the neighborhood of 5-7 mph on the top end. However, i roll with mufflers cause i like it better that way, and i keep the flame arrestor on cause I don't want to go boom... (or get a citation).

While you're at it, how about a temp sensor into the lube in the outdrive... I had someone tell me that a good reason to go with through hull was to keep from adding heat to the drive by running exhaust gas through it... Sure makes a lot of sense, but does it really make a difference in lube oil temp when you're leaning on the throttle for a while?

Alpine head unit, polk audio db series speakers and add the amp of your choice.

I cannot post all the links right now on this suject..but you have just nailed a very big part of the puzzle...Water in the exhaust slowing down velocity...yep mass in the stream slowing things down.

Take the time to ponder a bit on the history of engines and exhaust.... Only DOHC bikes really benefit from from exhaust scavenging..aka a bike doesnt get hot until 5000 and cut's off at 12000...That Velocity or kinetic energy..


Question ever see a top fuel rail with collector's...there just dumping fuel with huge cam overlap.

CFM if you are lurking how about a small apperance in this forum...

 

[45Auto]

Re: Thru hulls vs thru leg back pressure test

My personal experience has been in line with Tail_Gunners:

I will be the first to say i am not even close to a authority on this subject...So i will try to relate what i have learned.....There is absolutey no benefit to be gained with with aftermaket exhasut under 500 hp and 5500 rpm

My 22' Crownline with a 496 HO (425 HP) has Captain's Call switchable exhaust (side exit). It makes absolutely NO difference on the GPS speedometer or fuel flow gauge whether I am running through the outdrive or through the open exhaust.

Only benefit I've seen from the open exhaust is that if you accidently have it open when you crank the boat tied to the dock, small children will almost jump in the water in surprise when it lights up ............. :redface:

 

[JHuck]

Re: Thru hulls vs thru leg back pressure test

Only benefit I've seen from the open exhaust is that if you accidently have it open when you crank the boat tied to the dock, small children will almost jump in the water in surprise when it lights up ............. :redface:

Ha, now that's something I can relate to. My wife usually puts the boat in the water and tends to give it a bit too much gas at start up (partially due to carb being out of tune last year). I've seen more than one person on the ramp nearly sprain their neck in an attempt to see what is making that sound.

 

[JHuck]

Re: Thru hulls vs thru leg back pressure test

Not having a job that requires knowledge and use of manometers, I did some research on this instrument. The ability of this tool to measure down to minute pressures would make it useful. My question is, if I can't detect a pressure difference any higher than one PSI, is it even worth doing the test in the first place. My thoughts with using the PSI gauge would be to see if something more significant could be detected. Any exhaust flow experts know at what point does back pressure start to have a measurable impact on performance (i.e. 5-10 hp)?

I once had an extreme case where the cats were junk on the supercharged Mustang. At WOT it would buck and sputter. Took a while to find the cause but a smart Ford tech finally plugged a PSI gauge into the one of the O2 sensors and found the problem. I never heard what the reading was.

 

[MikDee]

Re: Thru hulls vs thru leg back pressure test

On a point here, I was told by a mechanic at the marina that he found by putting thru hull exhaust on a boat, actually slowed it down, because they found that blocking off the Y-pipe during installation created enough vacuum at the prop to lose speed. I believe it! I'm sure this is seriously something to consider. there must be a way around this though?

In my own experience, I had 3" rubber marine mufflers on my old 24' SeaRay with the 350/260hp Merc. and it had a nice mellow glasspack sound, no sharp rap noise,,, and I did nothing with the Y-pipe, just left it open (I know now it's not a safe idea though ) but, I was footloose, & fancy free back then