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Never seen an AQ280 lower unit like this... is this a 290 lower or something?

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Walleye4Days

Contributing Member
All my 280's have had a singular exhaust flapper on the lower unit of the drive. This has a split flapper that kinda overlaps itself slightly, and it also doesn't have the hump in the center like my other 280's.... is this a 290 lower unit or something, or am I just completely missing something here?

PS: since having this lower unit flapper, can I do away with the restrictive flapper that's in the exhaust bellow? (I have a V8 and I feel it's too restrictive having that giant exhaust flapper in the exhaust bellow, but I don't want water surging up in my engine either...)
 
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Walleye4Days said:
All my 280's have had a singular exhaust flapper on the lower unit of the drive.
The 280 (with an external exhaust back flow prevention flapper) was only available as a production model. It is rare to see one with the flapper. And yes, the 280 flapper was a single piece (no split in the middle).



This has a split flapper that kinda overlaps itself slightly, and it also doesn't have the hump in the center like my other 280's....
Correct.... all 280s had the "hump" at the outlet area.

is this a 290 lower unit or something, or am I just completely missing something here?
Hard to say...... could be one of many AQ series Duo Prop lower units!

PS: since having this lower unit flapper, can I do away with the restrictive flapper that's in the exhaust bellow?
(I have a V8 and I feel it's too restrictive having that giant exhaust flapper in the exhaust bellow, but I don't want water surging up in my engine either...)
Absolutely!
The exhaust bellows interior flapper is way too restrictive..... especially for a V-8 engine. Additionally, it cannot be easily inspected.

The exhaust outlet back flow prevention flapper will work just fine.
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Walleye4Days said:
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Be sure to remove your Duo Props at least annually, and clean/grease the splines.
These props have an aluminum spline hub, and can become corroded.

Make sure that both prop spacer washers are in place!




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Thanks for the great info and help, it truly is priceless to me.

I see what you mean about lubing the props every year... I literally CANNOT get this rear prop off (the previous owner didn't really keep up on a damned thing on here, and I've definitely been having a few choice words for him in my thoughts while coming across some of the most neglected crap on this drive I've ever seen lol. Literally I don't think he's changed the gear oil even, except for 5yrs ago, and I only know that because it's on the receipts in the file he gave me with the boat.)

So, I literally am at wits end and have no clue what to do about this rear prop that's not coming off. I've tried heat, I've tried my prop puller I made (anymore torquing and I would've bent up the prop). So, I'm at a dead end right now with it and have no idea what to do about it to be able to reseal the lower unit.
 
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Walleye4Days said:
Thanks for the great info and help, it truly is priceless to me.
Glad to have helped.
Please send money...... I need a new lawn mower, new shoes, new jacket and a new 85" HD TV!

I see what you mean about lubing the props every year... I literally CANNOT get this rear prop off
Oh boy..... I've had to cut the FWD props off before, and it's no fun!
What a shame to see what looks like a perfectly good prop that now has to be destroyed.

(the previous owner didn't really keep up on a damned thing on here, and I've definitely been having a few choice words for him in my thoughts while coming across some of the most neglected crap on this drive I've ever seen lol. Literally I don't think he's changed the gear oil even, except for 5yrs ago, and I only know that because it's on the receipts in the file he gave me with the boat.)
If it's clean and debris free, run it until you need to re-seal the drive.

So, I literally am at wits end and have no clue what to do about this rear prop that's not coming off.
The rear prop generally comes off even if corroded a bit........... it's the FWD prop that is generally the difficult one!



I've tried heat, I've tried my prop puller I made (anymore torquing and I would've bent up the prop). So, I'm at a dead end right now with it and have no idea what to do about it to be able to reseal the lower unit.

In order to remove a corroded and stubborn FWD prop, you will need to make spiral cuts in the outer hub and just pull it apart piece by piece. Do this until you reach the inner hub.
Once you reach the aluminum inner hub, you can apply propane torch heat.
During the process, you will find yourself swearing at the previous owner.


By the way..... I have two complete sets of OEM B-6s, brand new in the original boxes.
I have no need for them and they are up for grabs.

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Hahaha! They really should have a "tips/donations link" so people can opt to tip others or etc. Hehe.

The props currently on it are: B3 854832 and B3 854824. Now, I know ZILCH about prop specs, dimensions, and matching up effective power. So, are yours swappable with what I'm currently running now? (I'm afraid of the price tag too, as I've seen how crazy expensive some of these prop sets can be lol.) But, it's looking like I may need to get a set for when I cut that guy off when I have to seal them eventually. Probably sooner than later I'd like to if course.
 
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According to my measurements, it's not the exhaust flaps that restrict exhaust flow but rather the small holes through the transom at the "Y" pipe. The Mercruiser install manual specifies that for a 5.7L engine the exhaust ports should be either a single 4"dia port or dual 3" dia ones. When last I looked, the holes thru the transom did not add up to a single 3" dia hole, furthermore, the back pressure is waaaaay higher than max specified ( as I measured it, 6.5 psi vs the 4 psi specified). I ended up converting my 280 drive to thru transom with dual 3" fiberglass mufflers.
 
sandkicker said:
According to my measurements, it's not the exhaust flaps that restrict exhaust flow but rather the small holes through the transom at the "Y" pipe. The Mercruiser install manual specifies that for a 5.7L engine the exhaust ports should be either a single 4"dia port or dual 3" dia ones. When last I looked, the holes thru the transom did not add up to a single 3" dia hole, furthermore, the back pressure is waaaaay higher than max specified ( as I measured it, 6.5 psi vs the 4 psi specified). I ended up converting my 280 drive to thru transom with dual 3" fiberglass mufflers.
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Yeah, there's definitely WAYYYYY too much back pressure with the exhaust flaps, bellows flapper, AND relief holes in the transom (I'm assuming that's what you're talking about, the little black stubbies hanging out the lower part of the transom shield).

Im gonna do away with the bellow flapper, and just rely on the exhaust reliefs and the flapper on the lower unit to prevent serious back surge. I do like the idea of the through exhaust though, that's what I did with my last 24'er - 3.5" fiberglass exhaust tubes shooting straight out thre back with exit flaps.
 
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Walleye4Days said:
Yeah, there's definitely WAYYYYY too much back pressure with the exhaust flaps, bellows flapper, AND relief holes in the transom (I'm assuming that's what you're talking about, the little black stubbies hanging out the lower part of the transom shield).
The little black stubbies are the "rubber hooks" according to Volvo Penta.
They are low RPM exhaust pressure reliefs to prevent fuel load-up during trolling RPM.

Im gonna do away with the bellow flapper, and just rely on the exhaust reliefs and the flapper on the lower unit to prevent serious back surge.
That will work!

I do like the idea of the through exhaust though, that's what I did with my last 24'er - 3.5" fiberglass exhaust tubes shooting straight out thre back with exit flaps.

The sound of a straight out V-8 engine exhaust is great for a while.

Years ago you could buy a Salisbury (sp?) rubber muffler affair that connected to the thru-transom exhaust outlet tips.

They looked similar to an outboard fuel primer bulb that was on steroids.
shopping


They were not very restrictive, yet they brought the DBs down quite a bit.
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RicardoMarine said:
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Actually, this current Lancer is my second one. My first I reengined with a MOPAR 318 @ 240 HP and used those Salisbury mufflers. Great setup. I reversed the elbows so the faced forward and then ran exhaust line down and had the mufflers hanging on the transom facing forward under the manifolds. Easy peasy setup. Unfortunately, when I went to do this pn my current boat, Chrysler wasn't making marine engines so I went with a fresh crate MERC. Also, Salisbury was out of business. Soooo, I went with a pair of 3" fiberglass "shorty" from Centec/Vernatone. Routed exhaust lines from standard dry joint elbows back thru a 4/3" 70 degree reducer elbow, with mufflers parallel to transom, and then right angles out thru the corners of the transom just above the waterline. Sound great at idle and you can actually talk at the helm while at cruise.
 
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