Will my lower unit take it?????

[gustafsonerik]

Hi i recently purchased a 125 force powerhead 88 which was bored out to a 135-140hp.. I'm going to attach my new powerhead to a 85hp lower unit force..Do you think that will be okay??? any pointers or anything will be appreciated thanks Erik

 

[Frank Acampora]

Re: Will my lower unit take it?????

Well, You need to understand that if the lower unit has 2 to 1 ratio gears, the teeth are smaller and the beam loading on the teeth is greater than with the 1.72 ratio gears. On the other hand, there are more teeth in contact at any given time. (not a lot, only about 3). Also, depending upon the lower unit, the weak spot is the splines on the dog clutch. I did snap a prop shaft there with a hopped-up 125.

The 1.72 ratio gears are the same for all engines; some 85s came with them and all 4 cylinder engines had that ratio. The 2 to 1 ratio will improve the performance of the 4 cylinder engines but you must go up 2 pitches in prop.

However, I have a 2 to 1 lower on my Chrysler 140 and that engine is bored, ported, has a higher compression head, and has the exhaust relieved. On my 21 foot cuddy, it swings a Michigan 13 X19 cupped stainless at 5000-5150 RPM, 38 MPH GPS. I guesstimate that it is delivering more than 150 HP. It pulls me at 170lb out of the water on a slalom ski. As of now, after two seasons, the lower unit still does not seem unhappy.

Boring out your engine in itself does not increase the displacement enough to increase the horsepower to 135-140. Your engine will most likely be delivering an honest 125-130 and that's it. In all likelyhood you will have no problems tearing out the gears, but don't come back and blame me if you do.

 

[steelespike]

Re: Will my lower unit take it?????

The first thing I notice is it appears the Force motors are all the same bore
50hp to 150 hp.All use the same rings.3.312.Three carbs are used on the 85 and 2 on the 125.I would assume jetting would be different and critical.
I believe the 85 and 125 shared lower units. I'm guessing the 135-140 depend on porting,port timing,manifolding etc for their hp boost.So it would seem you are within a safe hp range.

 

[Frank Acampora]

Re: Will my lower unit take it?????

Not quite, Steelspike. Some engines, notably the 75, 90, and 120 were 3.375 bore. Depending on year, some 85s and all the 90s had 2 to 1 gears but ALL four cylinder engines had 1.72 to 1. (prior to 1990) After 1990, some dual exhaust lowers had 1.95 to 1 gears. I think they were Mercury gears because they were cut in the opposite direction and thrust on the pinion was up instead of down like on Chrysler and early Force engines.

However, castings, dog clutches water pumps, and shafting were the same for all big engines.

 

[steelespike]

Re: Will my lower unit take it?????

I stand corrected but he has a 88 125 bored to 135-140 specs isn't the 135 the same bore as the 125,or for that matter most of the motors from 50 up. Being bored to 135-140 specs doesn't make sense to me.And it doesn't seem that boring to 3.375from 3.312 would make much difference by itself. It appears that he is putting a basically stock 125 on the 85 leg.
I'm short on specs but I guess what I'm driving at is you can't "bore" to 135-140 specs. There isn't such a thing.

 

[Frank Acampora]

Re: Will my lower unit take it?????

Well, you are correct. Force did not make a 135 or 140. let me qualify that. Later Force did not. It is possible that around 1984 some Chrysler engines were rebadged Force and thus it is POSSIBLE to have a 140 Force--not likely though. I don't think Brunswick kept them in their line.

Now, the 125 is a 2.87 stroke and 3.312 bore while the 140 was a 2.87 stroke and 3.375 bore. Honestly, I don't remember what the specs on a 135 are. They MIGHT have been 2.80 stroke and 3.375 bore. HOWEVER, the 140 was the ONLY engine with a 2.87 stroke and 3.375 bore for 104 cubic inches. Biggest displacement block Chrysler made. Even the Chrysler 4 cylinder 150 had less displacement.

It is not possible to bore a 3.312 engine to 3.375. (well, anything is possible if you want it badly enough) Even at .040 over, which is not recommended by the factory because the exhaust webs and cylinder walls are too thin, that is only 3.352. To bore further REQUIRES a larger diameter sleeve. At about 200 a pop for installation plus 50 or so for the part, it is not worth resleeving a block to 140 specs when a good used block is cheaper. Not many people would go for that. So: I think the block was bored .030 over and simply called, due to lack of information, a 140 spec.

However, back to the original question, the fact still remains that even a bored out 140 should not trash that lower unit. Remember, the 5 cylinder Force 150 used essentially the same lower unit.

 

[gustafsonerik]

Re: Will my lower unit take it?????

frank is this the prop u were talking about??? also if i change pitch to 19 will that hurt anything i'm puttin this 125 on a 16ft bayliner runabout how fast will it go


http://www.boat-props.com/propeller...id=176&**********=384650364&*******=504065785

 

[Frank Acampora]

Re: Will my lower unit take it?????

That prop will PHYSICALLY fit your engine---BUT---

If you take a look at posts in the prop forum you will see that picking a prop can be a complex task. You need to know the top speed of the boat with the existing prop, the maximum RPM and the average load it will be carrying.

Just looking at a boat and engine combination, you can GUESS an approximate prop, but due to various factors, that guess can be incorrect. You really need hard numbers to work with.

SO: Before you jump to confusions and buy another prop, run the boat with the existing prop. Then you will have an idea of which way to go (assuming you know the pitch of the existing prop and RPM) You need to have the engine running between 4500 and 5500 RPM at full throttle. If the RPM is too high, each 2 inch pitch increase will lower RPM about 400. Also, Force engines seem to like to run at the high end of the range so you would want to prop close to 5500 RPM.

That said, I could GUESS based upon experience that your 125 on a 16 foot Bayliner would use a 19 or 21 pitch prop. Now last season I repaired a 88-89 125 and originally on a 18 foot Bayliner bass boat with an unknown aluminum prop, it ran 40 MPH at 6000 RPM. A change to a 19 pitch stainless prop yeilded 5000 RPM with 47 MPH top speed.
Bass boats are designed for speed and your 16 footer would be a general purpose hull, so I would not expect the same top speed but above 40 MPH would still be a reasonable guess as to top speed with the correct prop.

Stainless props generally run about 400 RPM faster than the equivalent aluminum prop and generally yeild about 2-3 MPH more top speed. So, if you can afford a stainless prop, buy that instead of aluminum. If you have time, try to buy a good used one. Look on the auctions and sales lists before you consider buying a new one in either aluminum or stainless.

 

[Frank Acampora]

Re: Will my lower unit take it?????

Thinking about it, Bayliners seem to be reated low for horsepower. One of the things you need to do is check the capacity plate for maximum horsepower capacity. 125 seems a lot to rate a 16 footer. My 16 foot Glastron is only rated for 115.

Even if the hull is rated for less that 125, it does not necessarily mean that you can not power it with the 125 as many of the nay-sayers will proclaim. They will tell you about insurance, liability, tearing out the transom, etc. But, You do need to know if you are overpowering the hull and you do need to know that there can be consequences if the local gendarmes stop you for an inspection or in the event of an accident.

Now, I am not talking about say 5 Horsepower if the hull is rated for 120. But if it is rated for 90, that's a significant difference. So: check the capacity plate.

 

[Jack Daniels]

Re: Will my lower unit take it?????

My 16 foot bay is rated for 90 HP max, i would suspect yours is going to be similar.