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low power, 355 marine

2K views 17 replies 10 participants last post by  GoomBay 
#1 ·
I've reworked the 350-260hp engine in my boat recently, went to flattops,2.02's,480-232 hyd. flat cam, 1.6 on exh.rpm dual intake, and reworked Q-jet. The problem is that I've lost all low end, until about 3500+, which I figured I'd lose some, but I've gained nothing at the top either! It just doesn't have any *** to it? I checked the knowledge base, and have already done the carb. mod's. heads are cast hi- comp marine, 64cc, cr is 10.5. Runs a little rich at low end, but plugs are dark brown. Was getting 55mph @ 5000, now it's 50mph@ 4900. My first marine modification, I expected a few more rpm, and some power? What did I miss? Is the cam too big? I've only had it out twice, and haven't checked the total advance yet! base is 14. Doesn't miss, just doggy!
 
#2 ·
You will probably have to have you prop redone. Your engine is now making power at a higher RPM and you probably need more pitch. Best bet is consult with a good marine shop. The engine sounds like a good combination of parts. If I understand you correctly, you have 14 initial advance? Might try 10-12. Good luck,
Vern
 
#3 ·
Just a thought. I had to do some work for someone a couple of years ago who had just rebuilt his Mercruiser. His heads originally had rotators on the exhaust valves. When the heads were rebuilt the machine shop he went to put in standard aftermarket retainers across the board. Unfortunately the spring seats in the heads were cut deeper on the exhaust's to make up for the difference in the assembled spring height. The issue was that the exhaust valves came up about 50# light on spring pressure. Not too much of an issue at low RPM's but it prevented the engine from going over 4500-5000 rpm's. It ended up that I had a set of rotators from another engine. I went in and added them which cured his problem. From what I understand the deeper exhaust spring seats are somewhat common on Mercruisers used in the late 80's-90's. I'm not saying this is your issue, it probably isn't but it's some food for thought.
 
#4 · (Edited)
Hi Vern, I've talked to them before, they said 1mph per 100 rpm is fine! I didn't want to get into a prop discussion, but I tried a 22 from a 19, takes 2 blocks and lots of r's to plane. Shouldn't I have some low end?

Good point czar, My shop guy did the head work, and he says he's done lots of marine work, he has no thoughts yet, but did say 5000 is probably all it's gonna turn!
 
#6 ·
I thought boat guys loved to play with their props? I have rebuilt one boat motor, a 302 Ford. It ran backwards because of the v-drive. That was a trip. I have heard stories of Chevrolet intentionally running big blocks backwards in NASCAR. Not that any of this is any help, just thought I would share it. Let us know how it comes out.
Vern
 
#8 ·
Vern, it was Smokey Yunick who tried a reverse rotation engine to, as he said "create better handling and chassis loading"
I`ve read up alot about him, it`s fascinating to read about his ideas that most call "cheating" but I look at things the same way he did, "if the rules don`t say it`s illegal, I have to assume it`s legal" and he also said this "Those other guys were cheating 10 times worse than us, so really it was self defense" and I have to see his point, It`s the reason why I never got into NASCAR, it`s a circus of rules, and the france boy`s would bow down to whoever had the factory backing them.
 
#10 ·
just a thought did you check the spring pressures, if there stock springs 5K is about the limit anyway, may try heaver springs, a 480 cam needs heaver than stock springs anyway, maby even Z28 springs there cheep and fit right in, also advancing the cam a little will pull the low end in sooner, also going from a 19 pitch to a 22 pitch is the wrong direstion need to go to a 17 or 15 if you want the engine to get up into the rpm range, props are rated by how many inches they will travel threw sand per 1 turn and a 15 pitch will take 2 turns as verse 1 turn of a 30 pitch to cover the same distance, it the engine is having problems getting into its peak torque range your either gona have to make torque sooner by advancing the cam or letting the rpms come up by putting shallower prop if the engine will rev that high(the right spring pressures). and if the cam somehow got installed wrong like 1 tooth off or more that could be causing power problems as well allitle advancing or retarding can help a lot will usually hurt , if the engine really has around 10.25 or so any smaller cam is probably gona be a problem. good luck Mikey
 
#12 ·
I didn't make myself clear on the prop statement! I don't mind changing em around, I didn't think anyone wanted prop talk on a hotrod forum? I've got enough $ in it, I'll change anything, short of the lower unit and gimble housing. Anyway, my goal was to try to get more top end, maybe 60mph? The springs are rated for 550 lift, and I will do the spacer. I ran the specs on dyno 00, and it showed peak hp 425@ 6000 and peak torque 412@ 4500. the 2 intersect @ 400 around 5500. Drop the pitch, and that will let it spin to 5500 or 6000? or prop it for the 412 @4500? or, keep playing with the prop until happy? Before I did the heads and 232 cam, I had 194's and 350hp vette cam, it would lay rubber if it was on asphalt, but only 55 mph, so.... I kept going. mistake? I really appreciate the help, most people don't like screwing around with marine applications.
 
#13 · (Edited)
sounds like you have enough spring then you just need to find out if your running out of torque or rpms then, if your running a 19 pitch prop try a 17 and see if the engine will come up any more, if it will then you arn't getting into your peek torque range yet, espically if you see major increases in power in the top end, if your having problems in the low end advancing the came will help greatly, you did not mention exact cam grind but some are ground streight up and some are advanced, if its ground streight up then advancing to some degree is almost a must, boats need torque to turn props, if you don't mind a little work then try advancing the cam about 4 deg, its more work than the prop change but a 4 deg key and gaskets will only set you back about 30 bucks instead of 300 and will help with bottom side problems and may even bring your peek torque down low enough to reach with the steep prop, also an open spacer plate will probably complicate your problem, they will fix breathing problems in the high end but in most cases kill bottom end power which you said you dont need any less of, look at the cam card see if the intake center line is the same as or different than the lobe seperation angle, if its the same then probably advancing would be a wise choice to start off with, usually the best aproach is to max the torque then prop (or gear) as nessessary
 
#14 ·
From what I have heard, the prop having more power will allow your engine to spool up and run the rpm's a bit higher.

I have a inline chevy six in my boat.. with a 19 pitch prop it has no bottom end. Up in the 4000 rpm range it'l scoot, acellerate faster, etc, but who needs the jump from 40-55 when you cann't get it up out of the water.

as long as your motor is all good and built well. When you go to a smaller pitch you will find that it'll jump out of the water rev quicker, and reaach higher rpms.

What I think is happening is that with that pitcha prop its just torqing the hell out of your motor, and boggen it down in the high end.

You should play with the props in my opinion. If motor is done rebuilt and running well, there's only one solution, and that's gearing.

A 19 prop is kind of like putting really high gears on a car, it takes forever to get there, and when you do, your still bogged down.

I hope this helps but I would bet with a lower geared prop you'll be very happy.

BTW I have a 19' tri hull (sea bus) With a 17 pitch prop it will stand up right, and reach about 55 mph.

With the 19 it wouldn't climb up out of the water without floggen the motor, and wouldnt go above 3200 rpm on a good day headed downwind.
 
#16 ·
Sorry I've been gone on vac.! Took the boat with me, found out, the electronic spark module was defective, I was only getting my 14 degrees of advance, replaced it with a 22 degree module, now it's at 36 total advance and runs like a scared rabbit! 63+ mph @ 5500 rpm, exhaust is thru hull, Thanks for all the help!
 
#17 ·
The classic case of matching motor to prop to boat weight. Theoretically, the top end speed should change about 3 mph for every 1 inch in pitch, if the WOT RPM stays the same. The theoretical top end, with no boat friction, for a 21" pitch, 5000 rpm, with a 1.50:1 gear ratio is 66.29 mph. If you are reaching that rpm,you should be seeing about 61-63 true mph due to drag factors. For a 19", the (mathematical) top is 59.97. Your drag factors will tell, but start with prop - your cheapest aproach to tuning a boat. Forget speed, get the WOT RPM in the right range for the motor by changing props first. Then you may want to think about how to rebuild that motor to where you want the baot to run. And like others have said, let it breath, but be darned careful about low rpm exhaust vacuum pulling water over the exahust risers. Sounds like a nice motor, though.
 
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