I just got my boat back ready for the season. The mechanic and I discover good news. I don't need a new prop. But I called the prop company and they said I still need a new one becasue the oppsite has happened.

Long story made longer: Last year I blew my engine ( 502 m1 procharged engine in my 1997 24 ft outlaw )and had it rebuilt and had all the corrections made by my mechanic, (fixed the original owners mistakes). But problem when we first took it out was that even though the rpm gauge was showing 5000 to 5200 and the msd box had a 6000 rpm chip in it, my mechanic assume that the rmp gauge was off because it was sputtering alittle at 75 mph. So he thought the msd box was kicking in even though the rpm gauge was not reading 6000 rpm. So with that thought, we decide ( since it had a bigger cam in it ) to go smaller cam or higher pitched prop for next season. Prop idea is cheaper then the cam change so i called a prop place and told them I need to get a prop higher pitch to lower my rpms. I Was already to go and get a new prop and we discovered something new. Two weeks ago at the marina I had the mechanic change my fuel pressure regulator becasue it seemed that the fuel pressure was on the low side for my m1 procharger. We changed it out to a areometer one. Pressure was still low. As a result we discovered the way the fuel pump was set up was that we were still running off the original mechanical fuel pump and the add on electrical one was not doing much. And the original add on holley electrical pump could not handel the job even if put on the system correctly. So we put a areomatic 700 to 750 one on the boat. After that everything seemed to run perfect. Rpms never went over 5000 or so and we hit 75 mph (my speed gauge is accurate to my gps on phone). So our assumption was that the false sense of the msd box kicking in last year at a 6000 rpm was not at all the msd box and a false rpm reading but ( becasue of my mechanic's findings in the begining of this season was that the fuel pump was starving the engine alittle bit and spudering the engine becasue the slit lack of fuel pressure at top speed. With the trim tabs down all the way , or the trim on the out drive all the way up we could not get it to go mover 5200rpms and trip the msd box at all, The box is set for 6000 Rpm . Good news right. I was excited
mechanic said the engine was desinged for max and safe running at 5200 rpm ( friends told me the bigger the engine the lower the rpms should be for safty of the engine and longevitiy

I told the prop guy this and he now said the complete opposite, "that now my engine is running under rpms and i need a lower pitched prop 2 or 3 pitches instead of a higher pitched prop, becasue NOt sure if i am saying this right but Under running the engine with to low of rpms (loading the engine) will cause more heat to the engine , out drive ect and it will also decrease my power and torque and will do more damage to my engine and I should be running 5600 to 5800 becasue it is a 502 and has a m1 procharger. i thought lower rpms is safer for any engine to have it last longer and not damage the engine. He said that the procharger will make it run alot more higher rpms then i am running. This prop guy has confused my new found excitement for not spending more money. I am confused. Please help with insight. Sorry for the long duration of info. don't know how to explain it in a short version due to my lack of experience here in the field. I thought slightly lower rpms is better then higher ones from what i learend
ps: (it is a labbed 4 blade spinelle prop unknown pitch) no markings

Please help
please tell me he is trying to sell me something i don't need right now. All i want is to enjoy it and take my son tubbing off it.
and wake board off it
please help

I can't really help you with your situation but I'm curious as to why your engine blew. I have a Gen IV 502 with an M-3 procharger on my 25' Checkmate. I'd like to make sure I don't make the same mistake (don't take offense please) that you did. Just got done rebuilding my outdrive and don't have the $ to rebuild the motor.

Your story is kind of confusing what RPM's do you want the motor to turn at WOT and what do you have now? Give us speeds at RPMs and then we know what advice to give. You should get a real GPS and make sure your tach is accurate first.

Sounds like he doesn't KNOW what rpm's he NEEDS to run because he has the mechanic telling him one thing and the prop guy telling him something else.
Heres my 2 cents-when I first suprcharged my original 502 in my Baja I used the stock cam and turned it 5200 or so and it was happy running there, BUT the stock cam is SMALL-around .500 lift and around 228 at .050 duration. As I upgraded my motor further and used bigger cams the hp peak went UP and the rpm's that I propped it for also followed. What your prop guy is trying to tell you is in order to keep motor and drive from being "torque loaded" to run a prop that frees u the motor a little. There is no mention of a dyno so I am going to assume you don't really know where the power band of your motor really is, thats not the end of the world though, if you post your cam data-.050 duration, max valve lift, L/S, even just the brand and part number we can TELL you pretty much where your motor is going to like to run. If it hp peaks around 5500 you want it to turn near that, when you try to load a boat motor closer to the tq peak then the hp peak you will stress the drive more and greatly increase the chance of detonation. and on the opposite end if it peaks at 5500 and you prop it to turn 6000 you are making your boat slower and reving motor higher than it needs for no reason. Post more info and we will all take turns throwing our opinions around (just kidding), Smitty

My mechanice said the head gasket blew primarily becasue i didn't have arp studs in it. The engine 4 years and then blew. The original owner used it one season. The idiot i bought it off of used it for 2 years and I used it for 1 year. Also it did not have a msd box on it and no rev limiter.

Originally on 1st lake trial we assumed it was running 6000 rpms at 75 mph because the motor was acting like the msd box was kicking in with the chip (chip is 6000 rpms) Even though the rpm gauge was not dictating it on the dial. But the rpm gauge never showed it. But after changing the fuel pump we didn't have the same reaction with the motor so we new the tach was right and it was reading 5200 rpms at 75 mph gps speed. I didn't want to be running at 6000 rpms at wide open speed . But in the end i wasn't. The mechanice said the engine is built for max of 5200 rpms and that is what we achived. The reason the engine was acting up was becasue the fuel pump that cmae on the boat was not the correct one for he job, After switching it out we had no issues. Sorry for the confusion

jj

No I didn't get it dynoed, and i don't know the peak power band is. I am trusting what my mechanic says, becasue he is not the one selling me the prop. He built the engine and he knows the max rpms the engine should have . So I am trusting him, I know nothing about engines. I am learning quickly and the expensive way. I will never be an engine guy. i just want to enjoy the boat with the family and and take advantage of the extra pick the boat has becasue of the procharger and enjoy. I am not going to be drag racing the boat or going on 10 hours poker runs with it

thansk for the insight.
I will try to get those numbers for you from my mechanic

jj