Johnson 175 and 235 powerheads interchangeable?

[jonnyspeed]

I have a 1989 175 GT. I believe it is a 90 degree cross-flow design. If I need a rebuild could I just find a rebuilt 235 powerhead and drop it on the 175's mid/lower ? What would be the 'real' HP diff between them? What is different between the 175 and the 235? They are both 2.6 L I think... is that right?

What can be done to pump up the GT 175 if I stay with this powerhead? What kind of gains can be expected? It's mounted to a 1990 Checkmate Pulse 186.

[graemers]

I beleive the 235 depending on the year is a looper. It may fit with an adapter plate. I'm not that knowledgeable, hopefuly someone else has a better answer;)

[dixtover]

The 235 is a crossflow and puts out 235 hp at crank not propshaft. I think it only puts out around 190 in stock form. I believe in 1984 they switched hp ratings from crank to propshaft so any engine pre 84 is measured at crank which means knock off say 15% or so.

[wrechin2]

The 235 will bolt right up. They are both bubble back X-flows. The 2 are basicly the same engine except for port timing/carbs. You can "Copy" the ports of a 235 (it is like .035 )and you have turned your 175 into a 235. There are alot of improvement to be made with the X-flows. Cuttin the transfers, cleaning up the flash, Opening the exhast, squaring the ports, and scalloping the pistons/block. The Gt may or may not have squared ports already. If anyone would know would be racer! Where are you located?

[Lockjaw]

I have an 89 GT175 with a dead cylinder. You would have to swap electronics too, in other words, you would need a complete 235 power head.

Also the GT175 is prop rated and only will turn about 5850 rpms, the 235 should turn 6200. The GT175's also were rumored to push the 10% envelope on power, so a healthy one should be putting out close to 200HP at the prop.

Even if you modified the power pack someway to get around the GT175 rev limit, I have been told the engine would probably not turn much higher unless way underpropped.

You might find the 235 to be a little spunkier depending on year. It would have intake filler blocks which the GT175 would not have. You can retrofit them and find them at an outboard salavage yard. They help with response. The 235 might also have the really good heads, which part numbers end in 638 and 639. Those are 93 octane only heads, and if you run those on the GT motor, perk it up considerably. You need to rejet it though.

Mine has the higher compression heads, boyesens, and intake filler blocks, and it ran well until it dropped a cylinder. The difference was 5 or 6 mph and much stronger acceleration and punch, especially down low and through the midrange.

[Forkin' Crazy]

If it has the exhaust bubble, it'll swap. The heads on the 235 and the carbs are both better is what I am told. Not sure on the ports. '85 is when the HP changed from the flywheel to the propshaft. I have read that the dif is about 7%.

[Lockjaw]

The GT would not have squared exhaust ports. Unless someone else squared them. ;)

[Forkin' Crazy]

The GT would not have squared exhaust ports. Unless someone else squared them. ;)

Are they plunge milled or CNCed? You had better add some numbers with that "GT" ;) You add the right ones, they will be squared. :cool:

[EMDSAPMGR]

Just as a heads up, be sure what year 235 you are looking at. The early ones (78-79) had the small pistons (149 cubic inch.) The later ones were the larger pistons (160 cubic inch)-similar to the GT175. Changed around 1980. Look for a 1980-newer. Also, the 235's used two power packs, so you will need to change the ignition system (as previously mentioned.) Some 235's did not have VRO-if you intend to keep VRO you need to check this out. You could look for a 235"S" model-the old 2.6, an unrated hp production model made 83-85. They were in the 240-245 propshaft hp range, with a 6200 limit pack.

[jonnyspeed]

Thanks for all of the great info everybody! I'm very new at performance outboards so it is going to take a while for me to digest everything that has been said. So if you were me and the 175 GT isn't enough to quench the need for speed :) what would be the easiest upgrade? Bolt on parts for the 175 or find a 235 powerhead? I plan on adding a nosecone with low water pickups and a hydraulic jackplate so would I be correct in thinking that the increased RPMs of the 235 would be benificial if I'm going to be runing it up high?

Is there a primer or something I can read to give me the 411 on some of these parts and mods some of you have talked about? Outboard mods for dummies or something?

Thanks.

[neveredge]

Just use the search function and you'll find more info on here than anywhere else in the world. That's no exaggeration, it's the truth.

But first you might want to determine what it is exactly that you want out of your motor.

[Lockjaw]

Having a GT175 I would go with a 235, because it won't limit you to the 5850 rpm limit, due to packs and porting. If you port the GT175, you still have to get around the limiter in the pack.

Get a 2.6 liter is you are stuck on a crossflow model, if not get a looper. I went to a 200 Looper instead of rebuilding my GT engine, and while the looper doesn't have the punch my GT motor had, due to the high compression it has a lot more torque, and it gets my pig boat moving out of the hole better.

neveredge is right about a search though, you can find all sorts of info.

[graemers]

I'm new too....:eek:

[wrechin2]

If money is a object, I would build what you have and modify it. Then you will know what you have and that is has been done correctly. Do a search for x-flow mods and you will be amazed at how much you can find out. If money isn't a option got to a looper engine.

[Action04]

jonnyspeed,

I have a fresh '91 175XP that has the high compression 325638/325639 heads. It's on a rebuilt '85 lower/mid with nose cone & LW pick up. It's on a 2004 Action I'm trying to sell. I've had some interest on the boat, but not the package yet. I don't want to part out the motor yet, but would sell it for $1500 if you're interested.

[jonnyspeed]

jonnyspeed,

I have a fresh '91 175XP that has the high compression 325638/325639 heads. It's on a rebuilt '85 lower/mid with nose cone & LW pick up. It's on a 2004 Action I'm trying to sell. I've had some interest on the boat, but not the package yet. I don't want to part out the motor yet, but would sell it for $1500 if you're interested.

Action04,

Do you have pics? What is the compression like? Do you know what it would cost to ship it to Cleveland?