Consensus on cruise RPM?

Post by **Admin** » Sun Jul 03, 2005 3:52 pm

So is there a consensus of what the most fuel efficient cruise RPM is for the 454s? I know there are general things to consider when answering this question, but one thing I noticed is that generally folks can cruise at about 20K @ 3000 RPMs, so I figured that a ballpark RPM would be better than nothing. I'm guessing 3000 RPM is not the most fuel efficient RPM, but maybe I am wrong. Anyone with flowscans or some other device have any input? David Sumich 33 Megabites Huntington Harbour, CA Forum Administrator

Post by **dougl33** » Sun Jul 03, 2005 4:56 pm

I think it all depends what you're running for transmissions and props. With my hardtop, enclosure, dingy on the bow, and outboard in the cockpit, I'm barely getting 18 knots at 3250 rpms. But, I'm running 1.9:1 with 4 blade 20X16. I have been tempted to cut down my spare 3 blade nibrals to 20x18 (which worked well on my old spares before I threw a blade last summer), but I know their current sizing (21x20) works well with a 2:1 ratio on the 8.1's (thanks to Tim's testings). Seeing as I'm probably looking at a re-power for the 2007 season (or sooner if I win the lottery), I'm reluctant to change them. Regards, Doug L. 1986 33 Bertram FBC Queen Elizabeth Marblehead, MA

Post by **Admin** » Sun Jul 03, 2005 8:18 pm

Douglas, this was an SF question my boy! Seriously, I do appreiciate the answer, I'm just kidding. I think the difference between SF and FBC matters as well as what you said, but again, if it seems like a good number of people can cruise at 20 knots @ 3000 RPMs, than I was just thinking that maybe the same good number all would have close to the same "fuel efficient" cruise RPM. Just a hunch. So I guess if folks starting chiming in, I might see a pattern and I can at least start from there. David Sumich 33 Megabites Huntington Harbour, CA Forum Administrator

photo finish · Post by **photo finish** » Mon Jul 04, 2005 4:15 am

David , Not that this relates to a sf with 454's but with my 496 crusaders ,My fuel efficiency range is from 3200-3600. It is a big range but the flowscan figures the mpg are within 2 hundreths mpg in this range...so i found that cruising at 3500 at 23 knots was where the boat felt great i could also cruise at 25 knots without a big jump in fuel useage and that is being fully loaded to the gills ...not to brag or anything[:)] also, are you suggesting that the sf is a faster more efficient boat than the fbc?[:0] Tim Stamm Photo Finish 1981 33' Bertram FBC

skyking · Post by **skyking** » Mon Jul 04, 2005 10:08 am

Tim,.......... Wow 23 knots cruise...man I'd love that...only problem is I'd be almost to the wall, well into the 4bbls and good for 40plus gals/hr. Dave,.......... I cruise at 3300 to 3350 rpms. Thats good for about 19 knots, but I'm very heavy. With a 13ft. dink, 15 hp outboard, genny, A/C, icemaker, and a ton of junk thats about 35plus gph.[:(]Other than that its a perfect boat. Ken 1982 33FBC "Blue Chip" w/454 Crusaders Westbrook, Ct. AIM: skyking4ever + 3 hrs. EST

Bompapen · Post by **Bompapen** » Mon Jul 04, 2005 10:31 am

Hello David If you have a Rochester Quadrajet the best fuel efficient cruise rpm is the top rpm you can reach with the secondary barrels disconected. The primaries triple ventury barrels of this things are by far the most fuel efficient of all carbs. You can also see vissualy when the secondaries open when cruise, but is safer an more easy the other way. David Vila

Mikedcabo · Post by **Mikedcabo** » Tue Jul 05, 2005 3:05 am

What do you see?

franklyprice · Post by **franklyprice** » Tue Jul 05, 2005 3:22 am

David, With my boat 3000 RPMs will get me about an 18KT cruise @ about 24 to 26 GPH. I can cruise faster but the problem is that to get the boat really on to plane the secondaries will open. I know that the boat would like to be at 20 or more but I really feel the drive train is happiest and uses less fuel at around 18. So what I do is set my throttles to 24GPH (about 2800 to 2900RPM)total and that usually will get me a cruise of 16 to 18 KTS, depending on other factors. I realize most people here will say that's a slow speed to cruise at but I feel it gets me where I want to go with some efficiency and my engines will last the longest. The extra 2 or 3 KTS just isn't worth the extra 6or 8 (or more)GPH to me. Frank Price

Post by **Admin** » Tue Jul 05, 2005 9:04 am

Thanks for all the replies. It interesting how all the different factors out there can combine to create so many result. I usually do 3000 RPMs when I am not overly trying to conserve fuel, and easily hit 20K, sometimes 21K. So if I do 2850 RPMs, I should have pretty good efficiency and still be cruising and close to 19K. Seems to hold true for this 4th of July weekend that just passed. I made a long trip trying this RPM, and fuel consumption was at least what I was hoping for. The boat has a Marlin tower, but other than that, I try to keep it as light as possible. Seems to be paying off somewhat. David Sumich 33 Megabites Huntington Harbour, CA Forum Administrator

Bompapen · Post by **Bompapen** » Wed Jul 06, 2005 9:07 am

Frank,I find this accidentally. In my last trip to Virgin Islands my starboard engine dont pass 3000 rpm. If I try the engine drop the rpm and almost stop, have to back the throtlle for stay running. I remove the flame arrestor after the trip and see the upper flapers of the secondaries barrels (fuel needle) not open they are stuck, the mechanical open and the engine only receive more air no fuel. Thats the reasson my engine drops the rpms. I have a book for modify 454BB (Ebay) and in the chapter of carbs have the information I say about the Rochester QJ. They match with my fuel efficient in that trip, I dont know a GPH rate but know I complete the trip on one tank for the first time in 14 years on this boat. I reach 3200 in the last hours of the trip when the tank is 1/4 or less. I believe the tabs size of the B33 are too small. I change mine in 2002 to a same Insta Trim width but 4 inches long without any modification and let me plane about 200rpm less. Maybe you need a bigger too. David Vila

Rick · Post by **Rick** » Wed Jul 06, 2005 10:04 am

I'm about the same as Frank....18Kts @ 3,000 RPM. My Merc's are MPI's, so I don't worry about the secondaries but I do want to take it easy on the engines. Since I don't have Floscans, it's a guess how much I'm losing by cruising between 3,200 & 3,400. Depends if I'm in a hurry. You guys have me wondering.....am I the only one running with 21 X 19 props ?

Post by **dougl33** » Wed Jul 06, 2005 10:48 am

Rick, What transmissions are you running? I've seen a lot of different prop sizes mentioned, but never 21x19's. It seems that most of the guys with 87 or newer boats are running at least a 23 inch diameter. The props that were on my boat when I got it were 21x20's on 1.9:1 trannies. The boat wouldn't turn up over 3750rpms. I changed my spares props to 20x18 with a light cup. They worked great until I tossed a blade last summer. Currently running 20x16 4 blades. Regards, Doug L. 1986 33 Bertram FBC Queen Elizabeth Marblehead, MA

franklyprice · Post by **franklyprice** » Wed Jul 06, 2005 12:24 pm

David Vila, I agree that a little more tab wouldn't hurt a thing for planing. I probably won't be changing them though because I do know that the tabs on our boats are limited in travel specifically for the 33's . Obviously the designers opted to compromise the tabs in this way for a reason and I would guess that reason was so most people will get into less trouble with differing sea conditions with less tab in the water. I am not really happy as it is with the tendancy for these things to bow-steer and I've been on bill's with larger tabs and noticed with them down too far the bow steered it all over the place. I'm saving my money for diesels and in the mean time will play a little with the secondary activation on my carbs. Frank price

Bompapen · Post by **Bompapen** » Wed Jul 06, 2005 9:06 pm

Frank I use longer tabs and work well at low RPM 2600 and at hight RPM 3400 I can adjust them. I tell you about the tabs because I believe it can help you and not for miss the point of efficient cruise RPM for the 454s Sorry for that. In HP BOOKS BB CHEVY MARINE PERFORMANCE By Dennis Moore,compare the Rochester carb with other major brands for it eficiency. The most efficient cruise speed on a Rochester QJ is when only the primaries are open I dont know at what RPM in other boats but in mine is at 3000. David Vila

franklyprice · Post by **franklyprice** » Thu Jul 07, 2005 1:27 am

David Vila, I agree with you on all points. You are a very keen observer of things. Rick, I run 23x 24 props but I have 2.5 to 1 ratio gears to allow them. As far as I know, all the later gas powered boats with inch and a half shafts used the same gears, shafts and props. Frank Price