Chevy 305 upgrades

[greg82255]

Hello,

I currently have a Mercruiser 305 engine (198 horsepower) with a Rochester 2bbl carburetor. I am planning on installing an Edelbrock performer intake manifold and a 600 cfm Holley 4bbl carburetor. I am also planning on upgrading the standard log manifolds and through prop exhaust to center rise manifolds with a silent choice system. Does anyone know approximately how much horsepower these upgrades will add to my engine?

Also, if I buy exhaust tips with internal and external flappers, will this be enough to prevent any water from flowing back into the engine? My boat sits very low in the water and the exhaust tips would be about half in and half out of the water when the boat is idling.

Any comments/suggestions would be much appreciated. Thanks.

 

[Bondo]

Re: Chevy 305 upgrades

Does anyone know approximately how much horsepower these upgrades will add to my engine?

Ayuh,... About 20hp...

My boat sits very low in the water and the exhaust tips would be about half in and half out of the water when the boat is idling.

As long as it's Downhill from the risers,+ the risers are atleast 13" above the waterline,....
Proboably......

 

[180shabah]

Re: Chevy 305 upgrades

Ayuh,... About 20hp...
As long as it's Downhill from the risers,+ the risers are atleast 13" above the waterline,....
Proboably......

Ditto - carefully measure your exhaust hight above the static waterline.

 

[greg82255]

Re: Chevy 305 upgrades

If I install a through transom exhaust, the risers would only be about 10 inches above the water line. Is it better to just keep the through prop in this case? or is there something else I can do to keep the water out of the engine?

 

[180shabah]

Re: Chevy 305 upgrades

Install a 3" or 6" riser between the manifold and elbow.

 

[ENSIGN]

Re: Chevy 305 upgrades

If your going for the transom exaust you can always use the 3''or6'' riser spacers to raise the height.

 

[Utahboatnut]

Re: Chevy 305 upgrades

Other than it sounding cool the thru hull exaust won't net you any gains in hp. You could take all the soup up money and find a good low hr 350 vortec and be way ahead of the game..

 

[180shabah]

Re: Chevy 305 upgrades

Other than it sounding cool the thru hull exaust won't net you any gains in hp. You could take all the soup up money and find a good low hr 350 vortec and be way ahead of the game..

True enough -

Remember that 198hp is at the crank, so really only 168hp at the prop. A 2bbl 350 vortec put out 250hp to the prop, a huge gain from what you have now. Throw an intake and 4bbl at it and you are in the 280-290hp range. All for the exact same weight as your current 305. For that matter 4.3s make more power the old 305s. Old style heads w/ 4bbl = 175hp, vortecs = 190-210hp, all at the prop and for 100lbs less weight on the stern.

I am NOT suggesting that you replace your engine with a 4.3, that v8 just sounds too cool. That's worth a couple of cool points. just don't go racin' and 4.3's in light hulls. Stingrays are killers, in that department.

 

[frank246]

Re: Chevy 305 upgrades

A 4 bbl carbed mercruiser 305 is the 228, this would be some extra 28 theoreticaly horses resulting in 2 or 3 mph speed increase.

 

[greg82255]

Re: Chevy 305 upgrades

Other than it sounding cool the thru hull exaust won't net you any gains in hp. You could take all the soup up money and find a good low hr 350 vortec and be way ahead of the game..

Are you sure about this? I thought that a through transom exhaust provided more free flow and therefore increased horsepower. I've even seen some through transom systems that are guaranteed to add 10 horsepower to the engine. Doesn't a through-prop exhaust restrict flow?

 

[Bondo]

Re: Chevy 305 upgrades

I've even seen some through transom systems that are guaranteed to add 10 horsepower to the engine.

Ayuh,... Wherever you saw that,... They were lying to you...

Thru-hulls make Noise, not Horsepower...

 

[Utahboatnut]

Re: Chevy 305 upgrades

By all means do the thru hull and find those ten extra ponies, you may need them to get you back to shore when the motor is filled with water and trashed......

 

[frank246]

Re: Chevy 305 upgrades

Through hull exhaust will increase horsepower on big cubic engines and high flow/ hp applications. At the end, the outdrive exhaust diameter has its restrictions.
Running a double 2" exhaust flows better than a single 2" exhaust resulting in less ristriction for the piston to push the exhaust fumes thus more hp. I know in standard or slight modified engine boats it is not noticeable but I do like the sound and the looks though.
So, no hp increase for the 305 but if you like looks and sound, hey its your boat.
As said in some the above posts a step up to a 350 would be the easiest way to get more hp.
For the sound you can add mufflers as I did.
Good luck!