Anyone ever rebuild a 496?

[mnm99]

Thinking of my far future....LOL I'm not close to doing this, But was wondering what anyone did. Make it higher HP? Cost to do so ect.. I know Raylar makes a top end kit, but the bottom should really be built for it.

 

[hottoddie]

You can only squeeze so much HP out of a 496 (or any marine engine) without running the risk of what's known as exhaust reversion. In short your HP gain would mainly from changing the camshaft which could cause valve overlap to allow water in the exhaust system to be sucked back into the cylinders causing damage to the engine. A Google search of "marine exhaust reversion" will explain it in detail.

 

[WV 320 Dancer]

You are in a very good place to find a very good shop to re-build the long block or the short block. There are many shops in NY state that build HP GM engines all day long. In the hot rod world taking the 496 and making a 572 stroker is very common. The big question is do you want to add forced induction into the mix. If so then you would be looking at a whole different animal. But then you will need to start thinking about the upgrade to your outdrive to handle the additional HP.

 

[Joint Custody]

You would spend a-lot to gain 10 mph. To really see a big gain on a boat your size you would need to add forced induction just like WV said. Then you will be dealing with drive upgrades, prop upgrades, exhaust upgrades, overheating issues and over all reliability of your engines.

Talk to any of the go fast boat guys you know. They can give you fist hand how much 10mph at a time cost to gain is.

If your looking for speed in a cruiser type boat I would look at selling the searay and move over to a Formula 40 SS. They are built to run fast from the factory.

https://www.boattrader.com/boat/2001-formula-400-ss-7842527/

 

[sbw1]

Interesting question. I have read 8.1s can not be rebuilt. As the story goes there is not much space between the cylinder walls and the cooling channels in the block. If you bore them out, they overheat. Not sure if this is the case. Will be interesting to see what people do when the time comes for a repower. Merc of course builds a new 8.1 from the ground up, but they are very expensive. I would be very reluctant to do any thing to a perfectly good 8.1 that might shorten its life, and leave well enough alone.

 

[juggernaut1]

Might want to look at the torque/hp your ZF transmissions can handle as they will dictate your engine mods. Shafts and props can be upgraded.

 

[LG111]

Thinking of my far future....LOL I'm not close to doing this, But was wondering what anyone did. Make it higher HP? Cost to do so ect.. I know Raylar makes a top end kit, but the bottom should really be built for it.

Read the history of the 7.4 HO. That will halt your desire to HO the 496. Valve overlap via upgrading the cam will only vacuum water vapor back into your engine. Hottoddie is 100% correct. And Jugger is to dead on too. There's not much room to increase HP to the trany either. All components on the mains are set at an equal performance. Change one, change all.

 

[mnm99]

I know it takes$ to make HP. My motors are rated at 375hp and and like 490TQ. at 3200. Like I said just dreaming here, but who knows. I would just want like 100hp or so out of each motor more. The HO already does 50hp with a cam swap and ECU upgrade. Add a set of nice heads and I'm sure it's there. The trans can hold a stated 502ft tq so that would need to be upgraded too. The ZF85 would fit, but would be tight. Who knows, maybe the internals could be upgraded in the 63.

http://www.tadiesels.com/img/zf/docs/ZF_63_A.pdf
http://www.tadiesels.com/img/zf/docs/ZF_85_A.pdf

 

[sbw1]

I know it takes$ to make HP. My motors are rated at 375hp and and like 490TQ. at 3200. Like I said just dreaming here, but who knows. I would just want like 100hp or so out of each motor more. The HO already does 50hp with a cam swap and ECU upgrade. Add a set of nice heads and I'm sure it's there. The trans can hold a stated 502ft tq so that would need to be upgraded too. The ZF85 would fit, but would be tight. Who knows, maybe the internals could be upgraded in the 63.

http://www.tadiesels.com/img/zf/docs/ZF_63_A.pdf
http://www.tadiesels.com/img/zf/docs/ZF_85_A.pdf

Crusader 8.1s make 385 and 425 HP depending on which version you bought. Suppose you are able to approach those numbers. Are you willing to spend the money to achieve 425 and be done with it? The extra HP comes at high RPMs which is really not practically available without shortening the life of the engine. What do you do when the engine is at the end of its service life?

 

[hottoddie]

I know it takes$ to make HP. My motors are rated at 375hp and and like 490TQ. at 3200. Like I said just dreaming here, but who knows. I would just want like 100hp or so out of each motor more. The HO already does 50hp with a cam swap and ECU upgrade. Add a set of nice heads and I'm sure it's there. The trans can hold a stated 502ft tq so that would need to be upgraded too. The ZF85 would fit, but would be tight. Who knows, maybe the internals could be upgraded in the 63.

http://www.tadiesels.com/img/zf/docs/ZF_63_A.pdf
http://www.tadiesels.com/img/zf/docs/ZF_85_A.pdf

490 is a lot of torque. Take into consideration the hull of 340 is not designed to be a "performance" type hull. Your 496s should provide a cruise in the mid 20 knot range. At cruise speeds above that the hull will not be comfortable and may even suffer structural fatigue over time. Spending all those $$ for modest performance gains will not result in a higher sale price when that day comes. I'd leave well enough alone.

 

[390x]

I'm sure the hull will survive just fine, and some more hp will probably make it perform better, not worse. I do agree that the cost/value is the only real issue here.

 

[mnm99]

I cruise at about 28mph at 3450 rpm. Once the boat get to 30-31 it flattens right out and rides real nice. The boat wants to run at those speeds. If I can get to 32, that would be perfect. Just board at work and am just doing research that’s all.

 

[Brite Idea]

The thirst for more HP comes at a price and risk. Like everyone said most of the HP gains are from the cam being changed but the increased HP happens over 4000 RPM, how often do you run over 4000 RPM? I don't know what prop you're running but if it's a three blade you might achieve what you're looking for by switching to a four blade?

 

[HawkX66]

The thirst for more HP comes at a price and risk. Like everyone said most of the HP gains are from the cam being changed but the increased HP happens over 4000 RPM, how often do you run over 4000 RPM? I don't know what prop you're running but if it's a three blade you might achieve what you're looking for by switching to a four blade?

It's only over 4k rpms if that's what the cam and heads are designed for. No reason you can't build power below 4k anywhere you want in the rpm range.

 

[joeyleggz]

It's only over 4k rpms if that's what the cam and heads are designed for. No reason you can't build power below 4k anywhere you want in the rpm range.

Well said , the HO motors we’re not meant for cruises they were meant for go fast boats who put the hammer down all the way, the heads on HO motors are usually rectangle port meant for maximum air and fuel flow at high rpm‘s At cruising speed there’s not much of a difference between a standard MerCruiser motor and the HO motor in my opinion rectangle port heads are not an advantage at all for a cruising boat , you want the peak of your power to be somewhere in the 3000 to 3500 range Those rectangle port heads don’t really kick in until over 3800 RPM and above

if you’re running your boat at 4000 or better you will shorten the life of those engines significantly