Logo
FULL SITE MENU

1990 454 longevity / proper cruising RPM questions...

B

bman440440

New member
I have a 1990 32' Luhrs with twin 454's and I want to know what is the best way to run the engines... Should I run them at 3000-3200 RPM's for cruising or should I run them at 3400-3500?
The reason I ask this is because my thoughts are lower RPM's are better but when I run at 3000-3200 I get about .8 to .9 NMPG at 18-19 knots but when I run them at 3350-3450 I get 1.1 to 1.2 NMPG at 22-23 knots, so my thought is that the engines are not working as hard at the higher RPM's and evident by the increase in fuel economy. Is this a correct assumption?
I don't want to unduly over work the engines that is why I'm trying to find out the best RPM's to cruise them... FYI speed is measured by GPS and fuel consumption by properly calibrated flowscan. Also at WOT I get 4150 RPM's and 28.5 knots fully loaded.
Thanks for any help!!
 
A lot depends upon how you define "best"....

From a longevity point of view, lower RPM is better.

From an efficiency point of view, you want the boat on plane with the best manifold vacuum (largest number on the gauge) which will indicate lowest load....many times this aligns with the MPG calculation. and is at the torque peak the engine produces.

Based on your data, the 1.2 NMPG target is a good one.

Do the secondaries open on the carb at the 3300-3400 range?

which gear ratio are the velvet drives?
 
makomark said:
A lot depends upon how you define "best"....

From a longevity point of view, lower RPM is better.

From an efficiency point of view, you want the boat on plane with the best manifold vacuum (largest number on the gauge) which will indicate lowest load....many times this aligns with the MPG calculation. and is at the torque peak the engine produces.

Based on your data, the 1.2 NMPG target is a good one.

Do the secondaries open on the carb at the 3300-3400 range?

which gear ratio are the velvet drives?
Click to expand...

I don't "think" the secondary's are open but I'm not totally sure (and sorta think they are not being that my NMPG is what it is)... I plan on checking that in the near future... as far as the transmission ratio un fortunately I do knot know that either... and same for the ratio on the props... it is coming out next Tuesday for bottom paint and I will get the pitch on them and the trans ratio then. So what would your opinion be even though you don't have all the info as far as it loading (working less) less at the higher RPM's?
And thanks SO much for your help so far!!!
 
my guess is probably have the 1.88:1 gears and your secondaries are likely closed.

again, i'd cruise at max efficiency...
 
Back
Top