Mercruiser 5.0 backfire up intake

[ibrw1]

So, I'm out looking for gaskets to rebuild top end. I need valve cover, head, exhaust, intake. I read in forums about automotive. Beings I do not have a radiator and its a fresh water boat. Automotive is ok? Except exhaust?

 

[Bondo]

I need valve cover, head, exhaust, intake. I read in forums about automotive. Beings I do not have a radiator and its a fresh water boat. Automotive is ok? Except exhaust?

Ayuh,..... Valve cover, 'n intake gaskets don't matter,....

The head gasket should be marine, or atleast ssteel based, rather than steel,....

The exhaust, most automotive gaskets are multi-piece, so ya can't hold 'em in-place,.....
Ya need 1 piece exhaust gaskets,....

 

[ibrw1]

Then that is that. Thank You sir!

 

[ibrw1]

Ok, waiting for gasket set to arrive. Got new aftermarket heads with bigger intake valves, got matching 4bbl intake manifold, got a Rochester Quadrajet 750 CFM
As I understand it, if stock 5.0 then 600 CFM 4bbl is correct. I asked several individuals about my 5.0 and they say with the heads I have with the big intakes, a 750 should be just fine, especially since the secondary's are vacuum actuated. Any truth in the 750 CFM is ok? Also, wondering if there is a rule of thumb on 16 year old risers. Chuck em? It's a river boat. No salt.

 

[alldodge]

The 5.0 has a 3.736 bore and a 3.48 stroke. The standard 305 has a max 466 CFM with standard bore, going to .030 increases to 474 CFM

Based on WOT of 4800 rpm

 

[ibrw1]

Sorry, I was unclear, the heads have larger intake valves.

 

[alldodge]

Sorry, I was unclear, the heads have larger intake valves.

Larger valves allow air to move in faster into the combustion chamber, they don't increase the CFM. Things that effect CFM is bore, stroke and RPM. Things that reduce the ability to bring in the volume is the valves, valve lift and carb. The standard CFM for the 305 is around 360 CFM. Using the racing side increases the volume to the max it can, Here is summittracing calculator

http://www.summitracing.com/expertadviceandnews/calcsandtools/cfm-calculator

 

[ibrw1]

I've read online that putting on the 4bbl intake and carburetor increases the hp by 20, I read online that ""World's S/R Torquer 305 cylinder heads come with 58cc combustion chambers, and are ideally suited ... features 170cc intake runners and can provide up to a 30 HP increase over most OEM castings."" That last came from jeggs, World S/R Small Block Chevy Cylinder Heads, Straight Plug, 1.94/1.50 Casting ID Number: I-052, I-058. I understand the calculation for volume in CFM, but my calculation does not include valve, lift or duration, and without, is inaccurate. I was unclear earlier, my fault. If you see anything wrong here, feel free to point it out. So, Where I'm going with this is. The power of an engine, or difference (before and after) is a product of gas/air in the cylinder. Before and after. If values given, 20 + 30 = 50 hp. Increase, You calculated 466 CFM. Then volume per HP 466/220=2.118 CFM per hp. Then calculations solving for CFM at expected new hp would put it at 571 CFM. This nuts? I have a question. The only difference between Mercruiser 5.0 220 hp 2 bbl and 240 hp 4bbl is the intake and carburetor?

 

[ibrw1]

I thought there was a direct relationship between fuel/air in the cylinder and hp. Without increase in fuel/air where is the increase in Hp coming from? Thanks again. I understand that I still have a good deal of learning ahead of me. I appreciate any and all information.

Having said that, at higher RPMs I thought that within the time allotted. At faster RPMs the easier the flow, was increased volume. Anyway, last big question was absolute differences between 5.0 220 and 240hp. I was reading that it was just carb and intake. If so, the 600 CFM. May be my best bet. The engine rebuild can be delayed another week if I missed something. Thanks again.

Thanks for the calculator all dodge!

 

[alldodge]

HP increases by how much fuel can be burned efficiently, the key is efficient. If more fuel is dumped in and not burned efficient it looses power. Agree that the intake, heads, exhaust, cam all play their part in the equation. The 20 HP increase from intake and carb is a calculated expectation during the old days, there is a HP increase but exactly how much would need to be measured using a dyno.

features 170cc intake runners and can provide up to a 30 HP increase over most OEM castings

Note the phrase in the statement "up to" meaning from 1 HP up to 30 HP. This is a sales phrase and in order to get the 30 HP you may need to put a blower on the motor. Not saying this is how the manufacturer came up with the number, just a comment.

Everything done to help the motor breath and burn fuel efficiently is HP increase. Increase the runner size, port and polish heads, valve size, lift and duration all play in the final number. A 1 HP per cu in motor is a good running motor, going to 1.2 HP per cu in takes a lot of things to work together.

Example: my 502 started as 415, then built to 535 HP, there was a lot of stuff used to get it there.

 

[ibrw1]

Yes, I understand fuel air ratios must be maintained. I watched show recently about btu,s and hp in an engine. They claimed that the more energy, the more btu's the more power. Difference between 2 and 4 is extra 2 at higher CFM which allows more fuel/air due to reduced restriction. Actual Volume at atmosphere, vacuum or pressure is substantially different. It in fact goes against my thinking. I'm not arguing at all, you guys fill in the holes here, and I appreciate all of your efforts. This stuff is very interesting. Thanks

 

[ibrw1]

So, I finished my boat engine repairs and had it back in the river a few times, newer aftermarket heads with over sized intake valves, 600 CFM upgrade from 2 bbl. 200 Amp alternator, new carpet throughout, added dock lights, 4 1000 CCA batteries for the 2000 watt stereo. As far as performance. To be brutally honest. The performance is more than noticeable. BUT is that because of the engine issues? Anyway 4th year with this boat and the 19" 14.3 diameter prop is no longer enough. WOT was 4600-4700ish. But same prop, rebuilt top end engine exceeded 4800 accidently. Cleared 5200. Yikes, no clue where engine comes apart, glad I use proper grade oil, and never intend to over rev. I have a 21" but it was too much pitch at the time. I ran 60 gallons through it, going to put the 21" on it for next trip. I have a fuel minder and GPS so I'm interested in fuel economy and top end. Last year before I noticed the occasional backfire, I got 3.1 mpg. At 3k on tach. Max speed was 46 mph. 1999 202 BR 5.0 2bbl. Fuel minder is best thing I ever bought. I know to the gallon how much I got and got to go.

 

[alldodge]

Rule of thumb is 200 rpm decrease for each pitch of prop increase, so 21 should bring you down to 4800 if 5200 was top end.

 

[ibrw1]

Yup, thanks. Hopefully back out Sunday. Glad to have it running, and running so well. I'm going to try my hand at gel coat. Been working on a hitch for my riding mower. Tandem axle is difficult to squeeze into garage. Those two are about all I got left on to do list. Thanks for all previous help.

 

[NHGuy]

The ignition might have stopped at 5200. They are supposedly limited there. So definitely put on the 21 pitch prop!