8.1 Horizon WOT/performance help.

G

gdavis67

Active Member
Mar 1, 2017
137
Saugatuck, MI
Boat Info
1990 390ec
Engines
Twin 454 Mercruiser T 340 with Hurth drives
Looking for some help from other owners of 36-38' Dancers with the 8.1 horizon motors. I was helping a friend this weekend with some tune up items on his 99 370DA. Installed new plugs, caps, wires and checked/set the timing on both motors. Also checked the idle speed prior to and during all timing adjustments. The reason for doing all of this was that the owner felt the port engine was lagging a bit compared to the starboard motor and that it wouldn't develop the RPM that the starboard motor would. After all tune up process was complete, we took the boat for a run. All RPM information was recorded with a digital tachometer connected to the engine while running, not with the boat tach's. At full throttle, max RPM achieved was 4700 starboard and 4520 port. This was better than the pre tune up rpm he saw as best RPM at WOT was around 4400. The boat did not seem to have any lag and both motors responded well and quickly to throttle input. His concern is the small difference in max RPM. To my thinking, 120 RPM difference is nothing. This could come from load, seas, hull drag, any number of items not related to the motors. Also, these number fall a little short of "rated" max rpm. Again, I don't see this as a concern, but would like input from other owners of similar boats with the 8.1's and see how this compares. FWIW, a full compression test was recently done (warm) and two cylinders on port motor were at 105psi and the others all around 120.
 
maybe to port prop has a small bend from a strike....when was the last time he had the props inspected?....

cliff
 
RPMs should match closer, 50 RPM+/-. If it isn’t a prop issue then something is most likely going on with the engine. How are the throttles at cruse? Are they even or one more then the other?
 
With my 2006 38 - in perfect conditions with half fuel, full water, gear and tender on the bow, only myself onboard, my port hits the rev limiter at 4950 and starboard about 70 rpms behind and peak speed is just over 31 knots. I've never had the props checked.
 
Both RPMs seem in range for WOT on the 8.1s. It could simply be a minor prop tuning / change is needed. I know my Port 8.1 get slightly higher WOT RPMs than my starboard but I never stay at WOT long enough to notice or worry about it. Again both of mine are in range.

-Kevin
 
Thanks for the replies. Some answers to questions: props were checked and serviced last fall after pulling for the winter by the best local prop shop so the props should not be an issue. Hours on the motors are fairly low (around 1K) so I wouldn't suspect they are truly tired yet. Yes, at higher rpm (3200 and above) one throttle needs to be more advanced than the other, but not by much. I have yet to be on any boat that both throttles are constantly at the same position with twins in order to keep the sync gauge happy. Maybe just my experience, but this seems to be "standard". We didn't leave them at WOT long, just enough to see what the top RPM we could get was and then backed down to about 3600 to check cruise speed. BTW, bottom is clean with fresh paint this year, we had roughly 1/2 tanks, and two adult men (420# total) on board during the testing.
 
I have 8.1s in my 360, there are no caps or rotors. 8 coils per motor. Just want to make sure we're talking apples to apples.
 
gdavis67 said:
...props were checked and serviced last fall after pulling for the winter by the best local prop shop so the props should not be an issue...
Click to expand...

Not necessarily true - the props could be 100% perfect but just not perfectly matched for performance. A 1" increase in pitch would drop RPMs by approx 200 (https://www.boats.com/how-to/what-to-do-if-you-have-the-wrong-propeller-pitch/).

So if one engine is at 4700 and the the other at 4520 then a 1" change to the first would put them both at approx 4520/4500. Now this may not be ideal but it demonstrates how while a prop can be perfect for it's spec that changing it would affect the RPMs.

-Kevin
 
Agreed. I was more referring to the condition, not the optimum prop for the boat. I will have to ask him if he has the specs on the props that are on the boat.
 
juggernaut1 said:
With my 2006 38 - in perfect conditions with half fuel, full water, gear and tender on the bow, only myself onboard, my port hits the rev limiter at 4950 and starboard about 70 rpms behind and peak speed is just over 31 knots. I've never had the props checked.
Click to expand...


unless you have 8.1 HO's you are 400 rpms over max RPM for those engines.. you should check your props.. because everything else seems to be in order
the 8.1S Horizons are 4200-4600 RPMS max..
HO's 4400-4800 this is per the Service Manuals PDF.. there are newer versions that show the HO's at 5000 MAX..
 
Pony Racer said:
unless you have 8.1 HO's you are 400 rpms over max RPM for those engines.. you should check your props.. because everything else seems to be in order
the 8.1S Horizons are 4200-4600 RPMS max..
HO's 4400-4800 this is per the Service Manuals PDF.. there are newer versions that show the HO's at 5000 MAX..
Click to expand...

Just have the standard 8.1s and agree the boat is underpropped - will have them checked next time the boat is out of the water.
 
SeadawgVB said:
I have 8.1s in my 360, there are no caps or rotors. 8 coils per motor. Just want to make sure we're talking apples to apples.
Click to expand...
Seadawg, the 99 model does have cap and rotor ignition, not individual coils. Not sure what year that changed, but this is the setup on a 99.
 
KevinC said:
Both RPMs seem in range for WOT on the 8.1s. It could simply be a minor prop tuning / change is needed. I know my Port 8.1 get slightly higher WOT RPMs than my starboard but I never stay at WOT long enough to notice or worry about it. Again both of mine are in range.

-Kevin
Click to expand...
Kevin, what do you typically run at for "cruising" speed and at what RPM? I think the cruising RPM should be around 3450-3550 based on 75% WOT for cruise. I have the speed written down somewhere from this weekend at 3550.
 
gdavis67 said:
Seadawg, the 99 model does have cap and rotor ignition, not individual coils. Not sure what year that changed, but this is the setup on a 99.
Click to expand...
Maybe your not working on an 8.1. To my knowledge they all came with a coil per cylinder. Engine numbers began 0M for the earliest models
 
gdavis67 said:
Kevin, what do you typically run at for "cruising" speed and at what RPM? I think the cruising RPM should be around 3450-3550 based on 75% WOT for cruise. I have the speed written down somewhere from this weekend at 3550.
Click to expand...

Lately I have been doing a lot of short runs and my on plane time I have be cruising at 3600-3650 which seems to run well and puts me at around 29-30 mph SOG for my 340SDA - of course weather, seas and loading will affect that. If it's a longer cruise and I am not in a rush I may run it at 3450-3500.

-Kevin
 
gdavis67 said:
Seadawg, the 99 model does have cap and rotor ignition, not individual coils. Not sure what year that changed, but this is the setup on a 99.
Click to expand...

The 8.1's were not introduced to Sea Ray until 2001. The engine's setup has never changed since its introduction. No distributor and individual coils.
 
Jimmy Buoy said:
The 8.1's were not introduced to Sea Ray until 2001. The engine's setup has never changed since its introduction. No distributor and individual coils.
Click to expand...

Yep! 2001 and all Smart Craft ready even with today's technology! BOOM!
 
You must log in or register to reply here.

Forum statistics

Threads
113,122
Messages
1,426,616
Members
61,037
Latest member
wojozobl
Back
Top