New guy on this block... RPM and fuel burn questions

[sehord]

Hey everyone... I just joined this site and love the info... Looks like a lot of activivity here, which will be good for everyone! I have a 1983 10-meter (33') sedan cruiser with twin 454 gas cruisaders. I have had the boat about a year, and usually cruise at 3000 rpm, showing 17gph and roughly 18knts. Recently I started loosing top end rpms and got down to a max of about 2500 on both engines +/- 100rpm. I changed all fuel filters and got a very minor increase in rpm, then thoroghly cleaned my flame arrestor on the top of the carbs and that got me back to a max of about 3200 rpm. I believe this boat is stock and think I should be turning a WOT close to 4000. I do not believe the weels have been changed. Can anyone tell me what I should expect in WOT, and does 3000rpm, 18knts, 17gph seem normal? Thanks in advance!

[rickalan35]

Sehord

I think those 454's, without a reduction gear will rev out top power at around 3800..... whereas a 350 cid, can get up to around 4400 rpm.

Don't know why you're losing your top power, although electronic ignition and fuel injection come to mind. Obviously, the good fairy didn't put bigger props on it without telling you.

R

[sehord]

I'm sure the props are the original, I just don't recall a top end on it when I bought it... possibly 3200 was all she had, but that seems low.... I have holly carbs and the ignition has been updated, but as both engines are fairly equal all signs point elsewhere. I pulled the anti siphon valves off and ran without them to test, but no increase (i did put them back on later)... so I need to get a pressure gauge and check my fuel pressure and then probably have a mechanic come and do some PM work and give me some ideas... just looking to see if someone here has had that problem or has the same boat and could compare numbers with me... thanks for the response...

[RWS]

You SHOULD achieve 4400 RPM WOT.

Otherwise there is a PROBLEM.

Is the bottom clean?

Props dinged?

Here is a link to the performnce chart on my 1983 express cruiser International when it had the 454's.

These engines would do 4400 RPM after we adjusted te props, however for the purposes of the test, I was uncomfortble running much over 3600 due to the age and questionable condition of the engines.

Hope this chart and the additional info helps.

Other than your increased windage and weight, your numbers should be similar.

We did a few minor modifications to get there which I can cover at a later post

RWS

[sehord]

Yes something is definatley going on.... props are good as is the bottom...I keep it on a lift and pull her out every after every trip..... I start up nice and idle smooth. Power up is good, just max out at 3100 or so.. I have all the original manuals and they say top speed of over 30 which I couldn't touch now for sure (and don't care to burn the fuel to get it) so.... I'll try to get some answers as to where to look next.

[MattSC]

Sounds like you're over propped. A crusader 454 should be in the range of 4200-4400 rpm. Just as an example, when I repowered my F-26 with a 5.7 fuel injected engine, I have an WOT rpm range of 4800-5200. When the yard ran the specs with the boat they gave me a 14x14 prop. I was only able to obtain 3400 rpm. I ended up with a 14 x 10" prop with a full cup and I now reach 5100rpm, with full fuel/water. The common rule is 1" of pitch equals 200rpm or so. However 4" of pitch gained my 1700rpm, not to mention better performance. To start, I'd check your props to make sure they are the right diameter and pitch for your model, it sounds like every thing else is good. Keep us posted,
Matt

[ltbrett]

I ran into a similar issue with my 454's. Apparently, the cam lobes for the mechanical fuel pumps wear down, inhibiting the pumps from taking a full stroke. I was topping out at 3200 rpm. I put on electrical fuel pumps and got back 4400rpm, just as the books say.

Brett