• Welcome to the Checkmate Community Forums forums.

    You are currently viewing our boards as a guest which gives you limited access to view most discussions, articles and access to our other FREE features.
    By joining our free community you will be able to:

    » Interact with over 10,000 Checkmate Fanatics from around the world!
    » Post topics and messages
    » Post and view photos
    » Communicate privately with other members
    » Access our extensive gallery of old Checkmate brochures located in our Media Gallery
    » Browse the various pictures in our Checkmate photo gallery

    Registration is fast, simple and absolutely free so please, join our community today!

    If you have any problems with the registration process or your account login, please contact support by clicking here or by using the"contact us" link at the bottom of the page.

intake Exhaust upgrades SBC Merc 5.7 260 hp

C

cvx16cvx

Member
Okay so last winter I planned on upgrading my 1987 21' Enchanter. I added a land and sea stern jack to my stock 5.7 260hp alpha drive set up. On gps I am getting 65.1 with a Mirage 23p at 4800rpm.
now I am planning on adding a free flow exhaust, I already exit thru the transom (factory set up) and I plan on using an Imco marine powerflow or a Stainless Marine system. Manifolds and risers. Will I see any gain?/
I have a new Edelbrock Performer Air gap Intake manifold, was planning on using a Edelbrock carb or a Holley 600cfm marine unit. Any suggestions?? At this time I do not want to change the heads but I do have a line on a cheap set of Edelbrock Performer Aluminum heads.. I am fresh water only. HELP!!
So would the exhaust change with the intake I have be a worthwhile swap. this would reduce some weight and let the exhaust flow!
I do not want to change the cam or heads as I may swap the motor next year. Thanks Chris:welcome:
 
Anytime you can increase airflow will help your performance. However, airflow means just that, FLOW. That doesnt mean that bigger ports and cam, is necessarily the answer. Ports or cams that are too large will hurt you more than help. The reason being, your port velocity will drop. Again, your old friend airflow drops. After all, what is an engine? The answer... an air pump.
Getting back to your specific engine. Exchanging the stock Merc exhaust is always high on my recommended list. While a stock engine doesnt need the extra exhaust flow, here is the one example where it wont hurt. Notice I didnt include exhaust above with cams and ports. There is no down side to upgrading the exhaust. Well, there is one, your wallet takes a hit. The best part is that your chances of reversion is much less than with stock exhaust. Afetrmarket exhaust will greatly widen your camshaft choices. I would suggest you limit your search for manifolds to ones that incorporate a by-pass tube, not ones that allow water to flow from manifold to riser though a gasket. Gaskets can leak, destroying your engine. A by-pass hose is much more reliable. The one downside, dont expect to see any significant speed increases, because you wont.
Swapping just the intake wont net you any increase either. The stock Merc intake on your engine is adequate to supply that engine. If you want to make more power, you have to look at the entire combination. You would have to at least change the cam, heads, intake and carb to see any real performance gains.
Be aware that on small blocks, there are 2 different bolt patterns for intake manifolds and valve covers. Later style heads, with the valve cover bolts that run through the center of the covers require a different intake and covers over the older style heads with the bolts on the outer edge of the covers.
 
Don't use the Edelbrock carb if you want true performance. That carb is good for one thing good starts. Or if your 90 years old and have no carb tunning ability. The fuel bowls are smaller on a Edl. then on a holley, and that causes problems sometimes. I only use a Edelbrock carb to put on something I am going to sell.


Big Red N.C.
 
wow.. you took the playing field away.. i'm pretty certain that you'll see very little to no gain, just by changing your intake manifold and carb.. the exaust.. still wont gain you much if anything.. now if you start getting into the cam, and doing a valve job, matching all the intrical pieces.. then yes.. you will see a gain.. one large gain you can do is getting over into a roller cam.. but just adding pieces wont do much.. its kinda like a baloon.. if you only let i have so much air.. you'll only get that much back
 
WOW

So no real gain if I put a high flow Imco exhaust? The powerflow wont give me a noticeable difference?? What if I went with some Edelbrock Aluminum heads with matching intake, say Performer and then upgraded the cam?
I see alot of 383 short blocks on Ebay , no heads just block crank rods pistons.....
But really no noticeable gain with the High Flow exhaust and a high rise with a Holley 650????:banana:
 
better be careful with aluminum heads and an iron block, two disimilar metals that don't play well together, no big deal in a car that has closed cooling, boat a boat with freshwater cooling has a potential to cause problems, thats why the 470/488 mercs were closed cooling. if you want to go that why you may want to consider a closed cooling set up and parting with the dollars that will go with it.
 
Fresh water

I dont think I will go with a closed cooling set up. So let it fly what should I do , cast Iron Vortec heads?? How about starting from scratch with a forged short block set up.. I am on a budget!! Or maybe just some heads cam intake exhaust?? I know the street vs marine argument, so I wont be buying any street crate motors. I do have a great deal on a GM 385hp Fast Burn street crate motor all internals are forged and built for extreme conditions with brass freeze plugs. I never run in the salt.
So all you motorheads steer me in the right direction, I jut want to make enough power to gain about 5 mph to run a 25' prop with no issues.
Unfortunately I am not willing to give up my low end performance.....:brickwall:
 
The street vs marine engine issue has been hashed out here many times. I am one of the guys who remains adamant about using an engine designed for its application. That said, I am not going there right now.

I am not a fresh water guy. Unfortunetly, I only run salt and only know salt water issues. I have never heard of any problems associated with running aluminum heads on a cast iron block in fresh water. Does that mean that anyone running an aluminum intake in fresh water without closed cooling is bound for disaster as well? I cant see why it would create a problem, but I am not sure. Dont get me wrong, I love the idea of closed cooling, in fact, I have it on my engine. Then again, I run salt water, I have no choice.
You say your goal is to gain 5 mph. At the level you are at, that is not alot to look for. Providing that your lower end is not old and tired, you should be able to achieve your objective with a simple head, intake, carb, exhaust and cam swap. You mentioned your budget, but not it was. The parts described above are not cheap. I would say you are looking at about $5k, thats if you handle all the work yourself.
 
I am running aluminum heads with stainless steel head gaskets I personally never had a problem in 233 hours my brother with the same set up has 467 hours no problems.

Big Red N.C.
 
I went to dyno with a problem child merc 350 a couple of weeks ago.. the biggest problem with the engine is the boat had bad fuel, roasted all the valves.. we went a head and did a freshen up on the lower end, and a competition 3angle valve job.. stock merc flat tappet cam, and automotive headers, rpm intake manifold, holly 650 at 4800 rpm built 290 hp.. 36Deg timing..
at 32 degrees timing.. 283 hp, the stock flame arrester made no impact, as the #s remained the same.

after we knew the engine was running right, we went ahead and hooked up the marine fuel system including the filter, added the marine exhaust and then did a few more pulls.. wound up at 279 hp... not bad for a stock 260 hp engine. again this was at 32 deg timing..

we weren't looking for all the performance we could get, as this engine goes into a cruiser, we were looking for reliability.. i'd bet if you change the flat tappet over to a roller, comp valve job, decent intake, you'll be able to produce a solid, reliable 320 hp... now you'll be looking at your alpha drive issues..

what we have found in our shop.. one of our clients went and got him a 5ive slamma jamma, custom built, wanna be marine engine, that was supposed to be 325 hp small block.. wond up running slower than the wore out 260 hp engine.. we had to go down in prop size to get the engine to spin up to 5000rpm.. turned out to be a wrong cam.. so be careful on what you get!


we had to tweek with the carburator,
 
i had to put the thinking cap on again, besides the dissimilar metal issue, the other reason for closed cooling was that cast iron and aluminum expand at a different rate with heat. closed cooling keeps the coolant at a more consistant temperature than freshwater cooling does. this may be more of an issue where i live due to the fact that water temperature can be 45 in may, 80 in august and 50 in october. this info was provided by a merc rep a few years ago made sense at the time and still does, i went to the merc site but i couldn't find anything to verify what i was told.
 
Here I go Now I need some real help!

:shakehead: okay so I bought a stainless Marine free flow exhaust setup with the stainless risers. Now I am trying to pick some heads intake manifold and carb. I really want to run a roller cam but may end up with a flat tappet. Sneaky Pete has a set up I think would work for me.
Cmon I need the truth I am no mechanic let me hear your suggestions... Thanks cvx16cvx
I just bought a heavy duty engine stand , I may just buy a short block set up and with some overtime and patience will build a complete set up and leave my stock 260hp in the Checkmate until it is complete. :drool:
 
cvx16cvx...

Now that you got the weakest link covered (exhaust), Look into the complete Eldebrock RPM set-up for alloy heads & the RPM intake if you have the room for the extra height. Run a holley 750 marine.

The motor I did up for a guy had 9:1 copression, forged pistons (steel crank/4-bolt/pink rod lower). With a Comp-Cam marine grind hyd. it was dyno pulling 412hp @ 5500rpm through the Stainless Marine manifolds on 91 octane pump gas. Probably would have seen 430hp with a roller cam and tuning! Total motor build start to finish with ALL parts and machining was $4500... (and that was buying a Vette shortblock to start with).

One note I do have to add, after putting it in his boat, we played around and propped it to run at 5500 rpm, but he fried the old stock Merc Alpha drive. He changed over to the Alpha SS, added a good drive shower set-up, runs synthetic gear lube, and has had no drive issues since... boat hauls a$$ for a mild small block!

P.S. You can SELL you running 260 and spend the cash towards a nicer HD shortblock... the stock 260 is a cast crank, 2-bolt, cast piston, small rod motor, it is NOT capibal of the 5500 rpm safely. Like anything, invest in what works and your goals. That 260 can easily see 325+hp with the right steel heads/cam/intake/carb (like $2000ish after freshining up your shortblock).
 
You must log in or register to reply here.
Back
Top