new mufflers for diesels

[scurvy dog]

Hi guys, this is the first thread I have posted on the site although I have been a member for a while. I have two brand new Vernalift 3" mufflers I would like to use for my new beta marine 25 hp diesels. the exhaust leaving the motor is 2". I would like to use the 3" mufflers because, well, I have them. I can easily fiberglass 2" tubing into the 3" openings. But will they work right? Will the size of the 3"exit pipe going to the bottom of the silencer affect the back pressure even though the rest of the exhaust system will be run in 2"? Thanks for any help. Rob

[GordMay]

Greetings and welcome aboard the CF, Rob.

I can’t think why it would be a problem.

➥ http://www.centekindustries.com/inst...ft_install.pdf

[a64pilot]

Having a 3" muffler on a 2" system ought to be better as the backpressure would be lower, and it might even be quieter as the amount of exhaust is lower than it was designed for. I wasn't gong to post anything though as I didn't know if it would flow water through it as well.
Drawback of course is cost and space, but you must have the room and yours is already bought.

[glenn.225]

It should not present a problem, however all engines have a recommended back pressure range. If your install is at all different than the manufacturers recommendations you can check the back pressure with a manometer and alter system to bring in line with manufacturer specs. You can borrow or make you own water manometer. It's just some tubing and a measuring tape.

[Jim Cate]

For a four stroke diesel I don't know of any downside to lower back pressure. The exhaust specs that I have seen have an upper limit, but not a lower limit.

Cheers,

Jim

[a64pilot]

I hate to say it, but the whole backpressure thing is a myth. Properly designed high performance exhaust systems actually have a neg pressure at the moment of exh valve opening. These are the equal length "tuned" headers, not sailboat exhausts. Of course this neg pressure only exist for a short time and then only in a narrow RPM range, but ideally a slight neg pressure evacuates the cylinders more fully than a positive pressure.
I think the backpresure myth came from the damage that can be caused by having too short of exhaust stacks, too short and cold air can be drawn back on the exhaust valve causing damage.

[Jim Cate]

Originally Posted by a64pilot

I hate to say it, but the whole backpressure thing is a myth. Properly designed high performance exhaust systems actually have a neg pressure at the moment of exh valve opening. These are the equal length "tuned" headers, not sailboat exhausts. Of course this neg pressure only exist for a short time and then only in a narrow RPM range, but ideally a slight neg pressure evacuates the cylinders more fully than a positive pressure.
I think the backpresure myth came from the damage that can be caused by having too short of exhaust stacks, too short and cold air can be drawn back on the exhaust valve causing damage.

Uhh, Pilot, how many waterlift systems have you seen with tuned headers associated into the design?

Excessive back pressure ain't no myth, mate! Many engine manufacturers will void the warranty if pack pressure is not within specs. This is especially true with turbp engines...

Jim

[a64pilot]

Never seen a water lift tuned header.
By myth I mean many peoples belief that there is a min among of back pressure needed or harm will come to the motor if there is too little back pressure.
Obviously too much back pressure is bad

[HappyMdRSailor]

Originally Posted by a64pilot

Never seen a water lift tuned header.
By myth I mean many peoples belief that there is a min among of back pressure needed or harm will come to the motor if there is too little back pressure.
Obviously too much back pressure is bad

Mr. Pilot....

Goes against our knowledge of tuned engines and higher reving stuff, but there is a fundamental difference with low reving turbo'd diesels...

The spinny parts are the things I think the mfr's are worried about...

I'm no expert, but I have lots of experience with positive pressure intake engines.... balanced and polished to the hilt from intake to tip turning 8-10k and....

Something doing 2800 with a giant turbo that sounds more like a ghost.... Or creeeeeepy ghost digs....

[leftbrainstuff]

Originally Posted by Jim Cate

For a four stroke diesel I don't know of any downside to lower back pressure. The exhaust specs that I have seen have an upper limit, but not a lower limit.

Cheers,

Jim

Scavenging and tuned length is most important in the primaries from each cylinder. Backpressure in the secondaries will impact torque and to a lesser extent tip in throttle response. Back pressure in the exhaust and mufflers will impact max revs. Note that these comments relate to high performance, high revving gasoline engines.

For a low revving diesel auxillary engine in a sailboat i wouldn't bother trying to tune an exhaust to this extent. You need some backpressure to make a water cooled exhaust work of course. Too much back pressure will act like a blocked exhaust reducing power output and max usable revs.

I would benchmark your usable rev range, exhaust outlet temps and fuel consumption before the change because this gives you a good baseline. Then if you can still discharge water from the exhaust then you are probably good to go. A check of the afore mentioned parameters will confirm. Without some complicated testing you might find that your engine might not be that sensitive to back pressure variations.

Back pressure is not a particularly useful tuning parameter except in fixed speed engines and for diagnosing lack of revs.

[scurvy dog]

Thanks guys. I think I'll give it a try. If the water output doesn't look right I can always get the 2" Vernalifts and swap them out I guess. Nothing that another couple of hundred bucks won't fix.

[Nicholson58]

I believe you will have no problems with this. Watch and record your engine temp, ex temp, boat speed, rpm etc. and compare to the new installed data. I expect little or no change. Possible slight improvement.

[scurvy dog]

great, thanks for all the advice from everyone....i feel a lot better about using them.