Upgrade of 260 Mercruiser

[mohead1]

I talked about doing this on the forum some time ago....but Im back to it again for the winter project possibly. Im working with a 1988 5.7 260 Mercruiser. I want to bump the power to the 300hp level or so in the easiest method possible. I was looking at a CompCams or Isky mild cam for better mid range, and possibly go to a Vortec cylinder head. I was thinking more on the lines of mild port work on the heads, upgrade them to match the cam, and a better intake. I want to go to a highrise manifold and maybe getting rid of the Quadrapuke?? Go to a Holley? Or leave the stock quadrajet and rework it......

Any thoughts on this? I thought getting a stock cam from Mercury for a hotter engine, but I think an aftermarket marine cam would be better. Before I start buying a bunch of parts I want some input from yall......
Im not sure if the current cam is a hydraulic roller stock or just a standard hydraulic. But the replacement would be a hydraulic roller.

 

[RogersJetboat454]

Re: Upgrade of 260 Mercruiser

I talked about doing this on the forum some time ago....but Im back to it again for the winter project possibly. Im working with a 1988 5.7 260 Mercruiser. I want to bump the power to the 300hp level or so in the easiest method possible. I was looking at a CompCams or Isky mild cam for better mid range, and possibly go to a Vortec cylinder head. I was thinking more on the lines of mild port work on the heads, upgrade them to match the cam, and a better intake. I want to go to a highrise manifold and maybe getting rid of the Quadrapuke?? Go to a Holley? Or leave the stock quadrajet and rework it......

Any thoughts on this? I thought getting a stock cam from Mercury for a hotter engine, but I think an aftermarket marine cam would be better. Before I start buying a bunch of parts I want some input from yall......
Im not sure if the current cam is a hydraulic roller stock or just a standard hydraulic. But the replacement would be a hydraulic roller.

Swapping to Vortec heads/manifold will be the single best improvement you can make. Even if you retained your stock cam, you would be looking at netting 20-30 hp.

Chose what ever flavor cam you would like, just as long as it's a marine grind, and as long as you realize the engine will need to be pulled (or at least lifted) to swap your cam. Unless the engine has been replaced, an '88 vintage is probably a standard hydraulic flat-tappet.

A healthy Q-jet is a fine carb as long as you know what your doing. If you intend on retaining it, I would purchase a book on tuning it so you have some idea of the tips/tricks to set it up properly.

 

[John_S]

Re: Upgrade of 260 Mercruiser

Given your boat year, you need to identify the 350 vintage. Is it a 2pc oil seal?, which should have flat-top pistons and 76cc heads, or the later vintage with 1pc rear oil seal, recessed pistons and 64cc heads, and factory roller cam capable. A good indicator is the valve covers. If you have center bolt, probably the latter. If perimeter bolts, probably the older vintage. You would be best to do a partial tear-down to varify, and would also answer what type cam is there.

If the older (flat-top) pistons, the compression with 64cc heads can get a bit high for a marine engine, especially if this is not in a light or fast planning hull. The vortec heads you want, are the ones with the exhaust valve seat. This seat is used in truck and marine engines to disapate heat. The non-valve seat version is what is commonly available at Jegs/Summit. To go with vortec heads and a cam a little larger than the stock Merc roller, the vortec heads will need to be modified for more lift. I know you can buy the non-valve seat version, pre-modified, not sure with the valve seat. If you are planning to port or go to larger valves in the vortec, look around at some aftermarket heads. They might be a better value. If you are rebuilding the bottom half, then you can select new D shaped recessed pistons to keep compression in check and still have a good quench.

As far as vortec intakes, you will have to look around to see if you can find a high-rise dual plane (like Edelbrock RPM) that has speadbore. Most only have squarebore. The mid-level Performer series does have one that is spreadbore. It might be time to ditch the Q-get. it would certainly need to be recalibrated for the head and cam changes.

The Edelbrock 600cfm 1409 usually requires little or no tuning out of the box, but a Holley makes 600 cfm squarebore marine carbs, in case you are more familiar with teardown and tuning of them.

 

[mohead1]

Re: Upgrade of 260 Mercruiser

I found this on Jegs catalog for the heads......looks like the hardened exhaust seat already there....is this what yall mean? And it has dual bolt pattern so I could use readily available intakes, not a Vortec specific. I have built high horse small blocks and big blocks for my race cars over the years so Im not a virgin there. Yes, the motor has center bolt valve covers, so i figured it might have the hardware for a roller cam stock (spider plate and the lifter directional retainers). I built souped up Quadra-craps for stock class racing many years ago, still have alot of parts laying in boxes, so I know that carb can be modded for this type of low performance. I mean we aint talkin about a monster motor here, just a little more umph for the ol Wellcraft.
I do like the ease of tuning of the Holley, and would probably do that route with a decent highrise manifold. Maybe just not do a cam change right now, and concentrate on the top end and see where that goes. I have the Alpha I so I dont want to hurt it....but it was rated at 300 or 325 I believe.

Heres the info on the Vortec heads from Jegs:

Cast iron small runner or large runner heads
66cc combustion chamber, .450" deck thickness
No heat risers
Machined for 2.00"/1.55" valves
Hardened exhaust valve seats
0.530" max valve lift (without modifications)
Drilled and tapped for screw-in studs (7/16-14)
Dual bolt patterns for perimeter bolt and center bolt valve covers
Dual bolt patterns for both Vortec and early model intake manifolds (use early model P/N 10051103 or Vortec design P/N 12366573, P/N 12496820, P/N 12496821, P/N 12496822, or P/N 12499371
Use production intake gasket P/N 12529094 for Vortec intakes or dual bolt pattern intake gasket P/N 12497760 for either early model or Vortec design manifolds
Vortec logo (intake port roof), GM logo (intake port floors) & Bowtie logo (exhaust side) are cast-in

 

[John_S]

Re: Upgrade of 260 Mercruiser

I think you are refering to the bow tie vortec heads. GM Performance 25534421. Those are higher performance heads than the standard iron vortec heads that come on marine engines. I haven't seen anyone report on using them here at iBoats. Offhand, I don't see where they would be an issue. They would want to run some above 5K rpms to use that extra breathing capability. One thing to check would be spark plug clearance to marine exhaust manifold.

While they use both intake bolt patterns, given the 185cc intake runners, you probably want some port matching on the intake.

While the blocks were roller ready, Merc didn't jump on them right away. They phased them in over time. First the 350 Mag, then LX, and finally the 5.7L in '97. Guessing you have a flat tappet cam.

 

[mohead1]

Re: Upgrade of 260 Mercruiser

Ok, thanks for the info John S. I figured I had a regular hydraulic. But no worries, I can upgrade to the roller with no problem. On the heads, yeah, I see they are the larger ports, so its kinda overkill cause I will never rev the motor obviously, lol. That was just the only heads that had the hardened exhaust seat plus the sodium filled valves.....you think there are other "small" heads that have the same option? I cant find any info in regard on GM Performance site, so I guess I need to call Jegs or others.......

 

[John_S]

Re: Upgrade of 260 Mercruiser

There are two casting numbers for the vortec heads, one of which the hardened seat could come on. It was originally believed that these heads w/seats severly restricted flow and the hotrod club shuned them. While these heads have been discussed at iBoats, not sure if anyone provided part numbers. Here is info on GM's marine 350 and you can see the seats are used: http://www.gmpowertrain.com/Libraries/Marine_Engines/Vortec_5_7L_V-8_Marine.sflb.ashx

I installed the non-seat version on my boat (before I knew there was a difference). I haven't had an issue, but I'm on a small lake and do not make any long wot runs.