GT175 Mods

[Lockjaw]

I have an 89 GT175, that has been rebuilt, and came on a boat I bought. I would like to do some modifications to it to improve its low end response. Right now on an 18ft bassboat with a 5.5 inch jack, and a 24p raker, its slow out of the hole. I don't have the tach working yet, and neither speedometer is in agreement on speed, one says 54 and the other says 67.

What can I do? Will reeds help? How about an exhaust snout and taking off the cover to the carbs?

I know the engine has the hump on the back.

What can you tell me about the engine? How high can I turn it safely?

Can I swap heads or mill mine and still get away with 87 or 89 octane gas?

Thanks

[Forkin' Crazy]

What you have is a detuned 235.

As far as low end, tighter heads is the only thing that will help. It might require a rejet after that. I would check the timing too. Remember though, you will have to only run premium or better if you do that. I am running 125 psi cranking pressure and 26* timing on premium and it hasn't blown up yet, so I guess it's OK. :D Been together for 6 years now and with all factory parts.

I turn mine high 6,000rpms. I have had it to 7,300 for a minute or so and it didn't scatter.:eek:

A GPS is pretty much the standard now for an accurate speedo.

I think your power pack has a limiter in it.

[neveredge]

I responded to a couple of your posts on BBC but they don't talk much about hotrodding motors there so here goes with what little I know.

Your 175 has packs that are limited at 5850rpm. You need to get rid of them and find an unlimited set. You are going to have to get rid of your 35 amp flywheel and find a 10 amp or a flex flywheel which would be best. That heavy flywheel WILL BREAK YOUR CRANK.

Your stock heads are junk, throw them away and find a set of 638/9's. They cannot be shaved for more power, they are not designed for high compression.

Your 175 has lower exhaust ports than a 235 although they are shaped the same. Al Stoker wrote a report on porting the crossflow called the 9 page report. It is the gospel for crossflows.

If you want to really improve your low end you can have your pistons lightened. Most will say this is risky but hey, you want to go fast right?

Boysen reeds will not increase your rpm but will help in the mid range and at idle.

Find or make a set of intake stuffers for your reed box area. Stuff your wrist pins.

Take off the cover in front of the carbs, add louvers to the top of your cowling.

Ditch the VRO and go with an electric fuel pump.

You need more set back on your boat. 12" at least. put a Bobs nose cone on your lower unit and pin the carrier. Find a 24" chopper style prop. The raker will not run good surfacing and you won't get any real speed until you surface the prop. Relieve your exhaust, this will give you a little bit better low end but it will be much louder.

Now after you've done all this you get to jet the motor. Start really rich and work down one size at a time. My advise is to do all your mods at once or you will be re jetting for every mod you do.

Please note that after you have modified your motor it will no longer act like a stock motor. It will idle rougher and it will wear out much faster. Expect 100 hours on a set of rings, if you don't blow it up first. It will eat much more fuel and you must maintain fuel quality and use good oil. Good luck with it.

[chris_lacey]

I turn my 235 to mid to high 6's every time out. 2 years and it hasn't scattered yet. 110 cranking comp and 26 degrees of advance on 93.

[Lockjaw]

So where would I come up with alot of this stuff? I mean stuffers? How about heads, I guess I would need the whole part number(s). And they flywheel stuff, what about that? Power packs, where would I get that?

I guess I should say I don't want the engine to be a finicky, prone to breaking all the time engine, I just want to perk it up some.

Oh and where is this 9 page report?

[chris_lacey]

I don't have spare parts, but if you send me your email I can send you the famous 9 pages.

[fyremanbil]

I'd sure like a copy of that 9 page report. Probably pretty good info, judging from my (formerly) usual view of that yellow boat.

[neveredge]

I can get the part numbers you are going to need for the various parts but finding them is going to be hard. You can try ebay for the flywheel and unlimited packs. I have a pair of unlimited packs but I don't really want to sell them. The heads are going to be hard to find. The last set I had I sold for $350. I had tons of offers on them and should have held out for more.LOL

Chris got a set from me rather cheaply and I hope he is using them!!!

There are other heads you can use but they will need to be modified to fit. A set of GT150 heads will work but they won't fit without modification and they would need shaved for more compression. I can get a good set of these if you need them.

I had a GT175 and I can tell you that unless you raise the exhaust port it is not going to rev even with unlimited packs.

I guess the first question should be how much money are you looking to spend on this?

I have a 2.6XP block that needs bored, it needs 6 new pistons. You are looking at $1200 for this with the pistons and the bore job but you would pay more than that for the porting on your 175. The 2.6XP/GT's were the best ported factory crossflows ever made.

[Forkin' Crazy]

You can get by a bit cheaper and use standard big bore pistons and have the scallops cut in by a machine shop (a buddy deal is nice).

I am running rapair packs.

But don't the late 175s have single box for both banks?

[Rickracer]

And they are limited. We can get you the non-limited packs, jets, Boyesen reeds, etc. Your motor probably has timing from 28* to 32*. If you raise compression, be sure you go to 91 octane fuel, and back timing down to 26* or 27* max. If you are going to spin >6500, the lightweight flywheel is not imperative, but would help longevity. You might try contacting racer (Al Stoker) on this board for some heads. Even if he doesn't have them, he can recommend the best ones that may be available. :cool:

[chris_lacey]

I'd sure like a copy of that 9 page report. Probably pretty good info, judging from my (formerly) usual view of that yellow boat.
If anyone would like this, send me an email adress I can use with Outlook and I can send it to you as an attachment.

[BarryStrawn]

Lockjaw - The key questions you asked are "How high can I turn it safely?" and "Can I swap heads or mill mine and still get away with 87 or 89 octane gas?

Depends on what "safe" means to you, but my opinion is the risk starts increasing rapidly when you exceed 6,000rpm and hold it there for long periods of time. Which I expect you might want to do.

To run the higher compression stock heads safely, you will have to run 93 octane.

Your engine makes all the horsepower it is going to at 5,800rpm and there are plenty of propellors that will run well in that speed/rpm range. So there is no reason to switch ignitions to raise the rev limit. Unless you build an engine with modified ports.

The stock reeds should last just about forever at stock rpm and there isn't much, if any, performance to be gained from the aftermarket reeds. Unless your current reeds are bad, there is no reason to change them.

If you are willing to always run 93 octane, the best bolt on heads are #325638 and #325639. Original to 1980 235. You can find them at a good price if you look and are willing to wait. I've bought a few sets both here and on eBay ranging from $125 to $200 for a pair. I've seen individual heads from salvage places go for $25 on eBay.

The unlimited ignition packs are from a 1979 to 1983 V6 or inline 3. These have different connectors than yours so you have to rewire or replace the harness. #582138 is the 1983 part number. Taking the whole working system off an early engine would be the easiest way to make the switch. The 1979 to 1983 V6 would also have the 10 amp stator and flywheel if your bass boat can live on limited current. Again, no reason to do this unless you have an experienced engine builder modify the porting.

[Lockjaw]

I would turn the engine into the mid 6k range without to much worry, however, that would mainly be for blast off on tournaments, and the like. I am not planning on driving it wide open all the time.

My boat has 3 batteries, and a 3 bank charger, so I am sure it would handle a 10 amp charging system no sweat.

I am not looking to make this into a race motor, and don't want it to be one. I just want to bump the response and power and keep things somewhat sane. I have a 175hp engine limit on the boat, and since I insure it, the only real way around that is to have a 150 or 175 lower section, and mod the powerhead, or drop on a different one. (Provided they don't find out)

Ideally if I could get the thing into the 220hp range or so, I bet that would work out fine.

I think opening up the intake tract by removing the carb cover and relieving the exhaust will help, I know that my friends former GT2.6 did not run very good with the carb cover on, or with the relief holes in the foot plugged. Of course his engine was also ported. I also think Boyesens would help, especially if I put on the 235 heads.

What if I just put on the 235 heads and did not cut them, would that bump the compression enough to matter?

Wiring power packs is not a problem.

[BarryStrawn]

For the 235 heads with your described use, I would run them at stock depth to provide some margin for a load of marginal gas. Although uncut ones are the hardest to find. If you are sure of your 93 octane, use a set milled to establish .040 squish clearance. Measure with soft solder down the spark hole. Depending on the heads and your engine, that probably means .030 to .040 milled from original depth. Most of the used heads are probably already milled some. I use head gasket #329774.

If you find a set of uncut heads, think about posting them for trade before you have them milled. Someone probably has some cut heads that would trade.

In my experience, opening the exhaust just makes noise. Fine for a race boat, not for recreational use.

Taking off the airbox cover should help and so will louvers in the cowl. Both will also add to the noise.

Whatever else you do, spend time tuning the engine and working on props and boat setup. Blueprint the boat bottom. There is more speed in these efforts than by changing parts on the engine.

[Lockjaw]

I want to work on blueprinting the hull, and right now, I am running a 24p raker, that has been re-worked. I assume that means someone chopped a rock with it, and they sent it off and had it cleaned up and polished, and replaced the hub.

It seems to work well, much better lift then the shooter I was running, and more top speed according to the speedometer. I still think it needs to be tweeked for what I want, and that would help.

The engine already has louvers in the cowl from the factory, and you cannot talk in the boat when running anyway, at least when its strung out. I was going to get a set of louvers from Bob's and put them on the top of the cowl, and I ordered some stainless mesh to replace the carb cover. I was thinking maybe getting one of his snouts, it mounts on the back of the engine, so it might be a little more tolerable.

I am definitely putting Boyesens in it. They made a noticable difference on my 70hp OMC. Helped out on the bottom and with mid range punch.

I am also going to play around with props, my friend has several he says will fit it, and see what they do.

What do you use for a thrust washer if you run an over the hub chopper or something like that?

[BarryStrawn]

I make my own thrust washers. But you can use the regular thru hub OMC washer for traditional flat faced overhub props unless it is a chopper with the line/weed cutters.

[neveredge]

Lockjaw,

You have to understand that if you don't port the motor you are not going to see any significant gains other than the heads. The heads will give you the bottom end you are looking for and a little on top end too. Everything else for your application (safe, reliable operation) is a waste of time. That motor makes power at 5800 rpm and raising the limiter will not change your top speed but will definatly decrease your reliability. The crossflows have a harmonic vibration in the mid 6000 rpm range that is a motor killer. If you spin this motor in that rpm range with that heavy 35amp flywheel you will kill it.

If you don't want to believe me then call Al Stoker. He will tell you the same.

Al is the absolute king of crossflows. Loopers too for that matter. No one on this board will deny this. Ask him, he is Racer on the board.

[Lockjaw]

Well if gas keeps going up, I may not be using the boat. HAHA

I sent Al a PM asking him about heads, so we shall see what he says. I have to get out on the water and see what kind of rpms the thing is running anyway.

I will see about coming up with the light flywheel and all that stuff as well.

Perking up the bottom and midrange would probably really help. I figured the boat would come out of the hole better then it does, it was awful with the shooter, and dropping an inch in pitch with the raker did not help it much at all, except once it was on plane. I get way better lift with the raker, and can feel the boat get up on the pad about 45 mph or so.

I may need to enlarge the vent holes a little, wonder if I could get someone to fit it for the Merc plugs. Anyone ever done that?