I have a pair of Cat C9 non Acert 503 HP engines on a Cabo 35' Express that are showing too much back pressure. Itrs limiting my PM to 2000 with normal at 2500. Have had heads rebuilt, new Heui Pumps and 12 new injectors. After testing I am showing between 3 and 4 PSI of Backpressure and the rated backpressure is 1.4lbs or 40" water column. I have not previously dealt with backed up mufflers and seeing if anyone had any suggestions. They will be very difficulty to access and remove. Thanks
You seem worried by not being able to reach the max rpm, but that's just a consequence. What a too high exhaust backpressure does, first and foremost, is skyrocketing the EGT, which is the #1 killer of diesel engines. I'd be surprised if Cabo built the boat with such inadequate exhaust system, but if they did, you'd better not use her at all before fixing that, no matter how hard it can be. Unless you don't mind rebuilding engines every other year, that is!
How are you measuring your back pressure? Mufflers usually don't go bad and ad back pressure. In the old days, We diverted water aweigh from the mufflers to reduce back pressure. A valve at the dump cans and a thru hull overboard at the engine room sides. The original Cabo crew are on YF from time to time, They may have some thoughts here.
I doubt it's the mufflers although I can recall removing two five gallon buckets of mussels from the exhaust tubes between transom and mufflers on a 50' sportfisher. Back pressure and RPM issues before and after new injectors? You've inspected the aftercoolers? Being over propped can cause excessive back pressure too.
Why/how? Of course if the boat can't reach the rated RPM because it's overpropped, she will run at higher than normal load, and this is another crucial EGT trigger. But if the exhaust is properly designed (as it should be) for the volume of air produced by the engines at their max RPM, I see no reason why it should create backpressure with the engines running slower, regardless of how overloaded and overheated they can be. I'm not suggesting that overpropping is OK, mind. Just that the reasons why it isn't aren't related to exhaust backpressure, AFAIK.
Swamp logic; Hotter exhaust = More expanding gasses. More expanding gasses = More volume of gasses. More volume of gasses in the same area with same outlet hole way in the back of the boat = More back pressure. So, my favorite question; What color is the exhaust? RER just reminded me of something; How much stuff has been added to the boat? Water line further up?
I see your point CR, but the temperature-driven volume difference is bound to be nowhere near high enough to compensate the lower gas volume due to the missing 500 RPM which the exhaust should be designed to cope with. Also because most exhaust systems of planing boats (including the Cabo 35 Express, I can easily guess even without knowing the boat) are wet. So, while a too high EGT can definitely damage valves, turbo, and the engine in general, by the time the gasses hit any restriction in the exhaust system they are much cooler anyhow.
Don't know what else to tell you, friend. It's not like it's top secret information. You could maybe look into it and educate yourself on the subject, or just keep guessing that you might possibly maybe be able to know something about it. It's up to you.
RER, I tried to explain why I couldn't understand your point. If the only explanation you can give for it is based on hearsay, you're the one who needs education, I reckon. The 8.8 liters displacement of Cat C9 means that at 2500 RPM, with charged air at 1.5bar, consumes 27.5 thousands liters/minute. So, the missing 500 RPM translates in 5.5 thousands liters/minute of air less, compared to what the exhaust should be designed to handle. Now, if you can explain how an excessive load due to overpropping can increase the exhaust volume anywhere near this number, I'll be more than happy to apologize. If not, no worries, I'll just classify your comments in the "useless internet BS" folder.
Obviously I am not well experienced with exhaust backpressure and while originally I mentioned muffler backpressure I should have said exhaust. To see if this helps will add a few more comments related to what has been found and done related to the boat. First I purchased the boat knowing there was some issues with the power although not expecting quit what has been found at this point. Both engines would only turn up about 1100 RPM WOT and would blow black smoke like a coal locomotive until reducing throttle to about 1000 RPM. At this point but I have too much in it now to quit. Using a Cat service tech and his computer they decided to replace the injectors and Heui pump, While that stopped the smoke it didn't help the RPMs. We pulled the heads and found 6 cracks as well as pitted valves and seats. Cat didnt have the injectors or heads and had no idea when they would arrive so after calling over 100 dealers I found a shop that repaired the heads and we used aftermarket injectors. With the heads off we had the after coolers cleaned and tested. In these processes we also changed all the filters and fluids and tried a separate fuel tank and fuel lines. We also replaced most all of the sensors as well as tried different ECMs borrowed from another similar engine along with the electronic controls. With issues remaining they "tricked" the boast sensors using compressed air so they through they had more boast and got some improve RPM up to about 1600. After one shop having the boat for over 6 months and running up more than 75K in charges, I brought it back home. I got a couple of friends who have a diesel truck shop in Georgia to come down and they started over. They found the injectors we for the wrong C9 engine, (a much lower HP acert engine). The boxes they had been shipped in showed the correct part numbers but they were not right and were not checked when installed. Thankfully I had pictures of the boxes and part number so after several days or calling the supplier finally agreed to replace them. We swapped them out and readjusted the valves. On the next trial the engines continued to improve slightly but would still not reach but about 1800. They had also reflashed the ECMs but the computer indicated the environmental smoke controlled was holding back fuel. They had another program that for testing that bypassed the smoke control and finally we got one engine to jump up to 2000 RPM with no smoke while the port engine smoked and took a couple of minutes to get up to about 1900 and where the smoke totally cleared. Before reaching the 1900RPM the boast pressure was cumming up much slower than the other engine that practically jumped up to 27lbs. boast. Both engines were showing 100% load and throttle. We didn't have a water coulmu or low pressure gauge but this is when these guys said they felt the exhaust was restricted on that one engine and with it struggling due to that to make enough boas as well as it was likely was holding the othe rengine to reduced RPM with the extra load. With a 5 PIS gauge I see the port engine has 3,75 LBS of back pressure where specks call for 40" WC or aut 1.5 lbs. allowable. I'll note the boat's bottom and gear were clean and the props were the original according to 2 different older appraisals that were on the boat. I had them checked and balanced. I also spoke with the prior owner and his captain whom I have know for years that assured me in the past the boat was running 41 MPH at 2500 RPM WOT with a good load. We have been testing with 3 people, 1/2 fuel and little fresh water so definitely not loaded. My hope was that someone might have some cleaning, acid or other thoughts related to this challenge that might be less of an task than removing them or cutting them open in place, repairing and re fiberglassing them back up. Clearly while I have been boating for well over 60 years with boats from 12' to 70' and in years past could crawl around, over and under doing many repairs myself those days have passed my by now at 72. I any case I appreciate this opportunity to become more educated from all the great experience that is is and can be offered here and I appreciate it all very much.
If you're showing 100% load and throttle at 1800/1900 rpms your issues are elsewhere. IS the bottom and props clean??? I would have a good diver or haul the boat and rule this out. is the boat over propped. All of the signs you have, point to a dirty bottom, vessel deeply overloaded with weight, or over propped. 3-4 psi of backpressure is the least of your issue.
I don't think the boat could perform that way with cracked heads etc. Do you know what happened, since then? And how many hours had the engines clocked? Fixing engine problems without knowing/understanding what caused them always leaves you wondering if there isn't something else that can creep up again... Besides, did your Cat service check what max RPM both engines can reach with gear not engaged? Can't tell by heart, but it should be a bit above 2500 - they should know, anyway.
Let me mention the issues started with the previous owner but before they started the boat with the same props ran very good and with this the case, it doesn't seem to be a design issue. I had them checked when I had the boat pulled to verify they hadn't been damaged or changed, so not over propped .Total engine hours are right at 2500. At this point the boat load is not loaded at all, 1/2 fuel, no water, and 3 of us on board with no gear etc. When the heads were off we inspected the sleeves and they looked excellent. I've been told by different Cat shops that the issue with the valve and seat pitting is related heavy condensation in the engine room and no heat maintained on the block and not operated for extended periods. Apparently a common issue on the cat c9 marine engine. They recommend using block heaters year round if not run very regularly. Why they had cracks I don't have specific answers. In neutral engines will turn 2575 - 2600 RPM. With all that has been done the mechanic feels the backpressure is causing reduced and slower boast on the port engine. My using a gauge confirmed the pressure is almost double the speck. He says if air cant get out, it cant get the boast it needs. Again I'll mention that I knew there were issues when i purchased it just was hoping they were not as many as we have corrected and still having issues. The mufflers are fiberglass so thinking about cutting an access hole in the muffler to inspect and potentially repair while it is in place then repair the glass. I'll do some more looking for access but initially it looks like to remove it might require cutting access in the back deck or pulling an engine which I really want to avoid. Again always thanks for the thoughts and help.
Pull air filters, run the boat with the air filters off. I've seen old air filters do exactly what you're experiencing on 2 different boats. They can only be cleaned so many times. Check turbo's, bad or wrong turbo's will do this also, IF they put the wrong injectors who knows what else is wrong.
