Alpha 1 gear ratio

[robjq5i]

How can I tell what gear ratio I have? 2013 Tahoe Q5i 4.3 MPI. Looking for a prop change. I think I have the 14x23 and looking to go with a 4 blade black max in 14x21. Looking for a little more bottom end for wake boarding.

Thanks!

 

[Starcraft5834]

you should have a serial # on your drive...look here---> https://www.seilermarine.com/info/me...atio-chart.php

 

[alldodge]

Going from 4 blade 23 to a 4 blade 21 will give you more low end. Doesn't matter what the drive ratio is unless you have trouble reaching WOT and the drive has been changed. Just remember, if your current prop allows you to reach max WOT rpm's, going to a lower pitch prop will cause you to over rev

 

[Scott06]

What's your current WOT rpm? Normally a 4.3 would have a 1.82 ish ratio, but as said above if you're already at WOT limit going down in pitch will bring up the rpms.

 

[Starcraft5834]

How can I tell what gear ratio I have? 2013 Tahoe Q5i 4.3 MPI. Looking for a prop change. I think I have the 14x23 and looking to go with a 4 blade black max in 14x21. Looking for a little more bottom end for wake boarding.

Thanks!

what Dodge said...

you need to know your WOT/RPM numbers with your current prop before messing with a change......

 

[robjq5i]

Thanks for the replies. I looked closer at the prop and am pretty sure I have a 1.8 ratio based on the stock prop being a 14x21p(some v6 q5's I read had a 3 blade 14x23p on them). So I popped on a 14x19 4 blade. $150 Turning Point from Amazon(prop and hub kit). No SS as to the really high costs. What a difference. Jumps out much better and going from about 20mph to WOT really pulls. Turns also seem to hold the line better in turns. I usually cruise with trim about 1/4 on the gauge. It'd jump to over 40 very quickly. Never got a chance to wring it out with the passengers I had on board. Also no WOT/GPS tests. Losing top speed was not a big issue. The prop was an easy install and I keep the old one in the boat just in case.

 

[Sea Rider]

What about max wot revs achieved as usually loaded with 4 blade prop, combo seems not to count with a tach, right ? if so, won't know at which min-max wot rpm range is that motor revving, could have serious consequences in the short run specially if lugging the motor.

Happy Boating

 

[robjq5i]

Not sure what you mean

 

[Bondo]

Not sure what you mean

Ayuh,.... Yer motor has to run at 'bout 4400 to 4800 rpms at Wot, or yer gonna hurt the motor,.....

Aka: ya Need a workin' reliable Tach,....

 

[robjq5i]

Ok thanks....but I rarely run at WOT. Mostly easing along at 2500-3000 rpms that's usually around 23-30mph with 1/4 trim. Definitely a noob here but I understand over revving the engine(mpi I would think as a limiter? maybe not). But the lugging the motor as mentioned above has me stumped. less pitch prop "should" be like lowering the ratio on a truck axle, motorcycle, etc. Quicker vehicle and engine acceleration, lower top speed.

 

[alldodge]

You need to run the motor at WOT to determine IF it can reach 4400 to 4800 rpm. If it CAN rev more then 4800 under normal load and use, then you need to increase prop pitch to lower the rpms. But IF WOT is under 4400 rpm then the prop pitch needs to be reduced to get the rpm up higher. The motor is lugging IF it cannot reach the rpm range.

A motor which turns at higher rpms for a given speed is not necessarily burning more fuel, just like one running lower rpms is burning less, its a balancing act to achive best all around performance

 

[robjq5i]

Thanks, so if I pretty much never run...then no issues? Just looking for more bottom end. With the stock prop I've gotten the boat to well over 50(lowrance) and no need to do that again. We pull wake board, skiers and tubes and non of those need WOT.

 

[alldodge]

Thanks, so if I pretty much never run...then no issues?

Ah, no
Sorry, guess I'm not being clear enough. You don't need to run at WOT except one time. The one time is to see IF (the big IF) your motor will turn 4400 to 4800 rpm your good, if it is below or above something should change.

My guess with being able to do 50, you should be fine, so maybe don't bother with it.
Happy boating

 

[robjq5i]

Finally smooth enough with my wife on board to open it up(she doesn't like speed). I replaced the OEM tack with a mercury smart gauge, maybe better but it should read decent enough on a MPI motor. but around 4500 rpm and 47mph on the gps. river was smooth and into a little headwind, two on board and 16 gallons of fuel. 1/4 trim and I didn't try and trim it any. Most of the day spent cruising at 3000 rpm and about 32-33 mph with 1/4 trim. Just needed a jacket as it was coolish down south. Thanks for the input.

 

[QBhoy]

Perhaps a spanner in the works.....
I'm pretty sure that most MPI engines are meant to run between 4600-5000 rpm...not 4400-4800 rpm. That is usually for older carb variations.
This being the case, you may just be lugging a bit below the recommended.
I'm not sure I totally get all the input figures to your set up.
If it was a 1.8-1 gear ratio... and a 19"p and 220 hp...I would have thought your set up would be well under propped with a 19"
I have a similar weighted boat with a 5.0 MPI and a 1.47-1 ratio. Currently run a 19" laser and she will turn at 5000rpm.
As they say...each boat setup is a different case.

 

[QBhoy]

Correction.
4.3 MPI is indeed between 4400-4800. Must just be 5.0 MPI and others that are the aforementioned 4600-5000.
Other point about prop and gear ratio still a strange one to me though.
All the best.

 

[robjq5i]

Well I would think this 4 blade prop would let the engine rev more so the oem 3 blade would(maybe) lug more. But now I have a clear baseline with the 4 blade 14x19 and I can pop the 3 blade 14x21 on in 5 minutes, then see. Pretty sure the 3 blade is faster but no clue on the rpm. Thanks

 

[Sea Rider]

With current load and 4500 wot revs you're at the lugging side of the engine. Need to know precise factory min-max wot revs range and work from there. If you dial a prop that will make the engine to rev at near or max max wot range as currently loaded, once there can throttle at the speed you like best. Will achieve good hole shot, good speed along fuel consumption. If at an emergency that needs full throttle, engine will rev and respond happily.

Happy Boating

 

[jkust]

So on the actual ratio here...4.3mpi is always 1.62 and modern 4.3 carb is 1.81 is it not?

 

[robjq5i]

No Title

I went back to the oem prop the other day. Results. 3 on board, about 425#, 15 gallons of fuel. Not much difference in rpm vs the 4 blade smaller prop. 100rpms or so. I know the indicated speed has be a bit higher on the speedo and gps in the past. I trimmed it up trying to get all I can out of it. The boat still plays fun!!