Will swapping 3.0 heads gain me a few HP?

[craze1cars]

I have a boat with a 1992 3.0 L rated at 115 hp. I've seen mentioned on this board that this is likely a 2.5 head on a 3.0 block.<br /><br />I've seen many a newer 3.0 LX head for sale on Ebay and other places for $200 to $500 ish, some reconditioned, some used.<br /><br />Here's my question:<br /><br />If I buy a newer 3.0 LX head & gasket kit and stick it on my motor, will this change bump it up 20 horses as the LX was rated at 135? Whether the answer is yes or no, what else would I need to consider or change? Would carb or exhaust need to be different also to allow the new heads to breathe properly (or to fit properly), or would my original stuff just bolt on to the newer style head and give me some gain?<br /><br />Thanks for any thoughts or experiences with this idea.<br /><br />My dream? Buy a good LX head for a few hundred plus gaskets, bolt it on, sell my perfectly good used 2.5 head on Ebay for $100ish or better, and be out of pocket in the $200 to $400 range to get a 17% horsepower gain. Am I truly dreaming or do you think this could become a fairly easy reality?<br /><br />Thanks!

 

[trog100]

Re: Will swapping 3.0 heads gain me a few HP?

no i dont think u will gain what u think u will gain.. why cos its a "fib".. just to replace the missing 115 hp 2.5 engine in the line up that has just been discontinued..<br /><br />think about it.. 2.5 engine 2.5 head = 115 horspepower..<br /><br />3.0 engine 2.5 head = 115 horse power..<br /><br />if someone can come up with a good and valid reason why an additional 500cc counts for exactly zilch.. i will change my oppinion.. untill they do i stick to my "fib" theory.. he he he<br /><br />my theory goes thus.. to save money they stop making the 2.5 bottom end.. its cheaper to make just the one 3.0 and have it do two jobs.. fill two slots..<br /><br />they dont want to alter their power range (lose an engine of the bottom) so they tell a fib and call the 3.0 which was a 135 hp engine a 115 hp one.. they fit the real 3.0 with electronic ignition and call it a 3.0lx.. they of course would also charge more money for the LX version of what in essence was the same engine..<br /><br />there might be a slight power output difference tween the two engines but i dont think it would be much.. certainly not 20 hp.. swopping the heads would not gain u much if anything..<br /><br />trog

 

[Bondo]

Re: Will swapping 3.0 heads gain me a few HP?

Am I truly dreaming

Ayuh,..............

 

[craze1cars]

Re: Will swapping 3.0 heads gain me a few HP?

Thanks for the replies. Hey...a guy's gotta do SOMETHING while the snow's on the ground! I might as well dream! <br /><br />So we're saying that Mercruiser's marketing dept is a bunch of liars, eh? I'd love to see some dyno results from both the 115 and the 135 3.0's, but I doubtful anyone has been dumb enough to waste such time and money putting an entry-level motor on a dyno.<br /><br />Biggest reason I had the thought is that it's quite common to take an automotive V8 and swap almost nothing but heads to squeeze another 60 to 80 horsepower out of a motor with no change in displacement. Usually it's the result of more efficient air flow and higher compression. So I'm not quite as suspicious as you guys seem, that a head change wouldn't easily bump 20 horses out of an in-line 4. BUT, I respect your opinions that the LX head is likely not the one that would get it done, and to my knowledge there's really no other options out there.<br /><br />Oh well...just thought I'd throw it out for fun. Maybe if I can do it REAL cheap I'll try the experiment with before after stopwatch and top speed runs to see if there's any diff. Will report my findings if I do it, but don't hold your breath in the meantime! I do appreciate both your thoughts, and anyone else who'd like to chime in on the topic.

 

[Bondo]

Re: Will swapping 3.0 heads gain me a few HP?

I haven't done alot with the 3.0l/ 2.5l motors,....<br /><br />But,.....<br />I know there have been Atleast 6 different heads used on these motors,.........<br /><br />The Biggest Ports I've seen on Any of them were the Old 140hp heads.......<br /><br />I don't rmember if it's the last series of heads that used the 18mm based sparkplugs,..... Or the 1st generation of the heads that use the 14mm based sparkplugs,.......<br /><br />Ya Know,.. The Pre-3.0l, 181cid engine........

 

[craze1cars]

Re: Will swapping 3.0 heads gain me a few HP?

Thanks Bondo. And if they're old, I'd think they may also be very cheap and easy to find in rotting hulls sitting in driveways all across the good ol' USA...<br /><br />In the off chance that I try to screw around with this little experiment (which I'm sure most of you find to be ridiculous!), do I need to be concerned with various components fitting together or would all of the 6+ heads you mention be direct bolt-ons for my 1992-vintage block and it's outer components? I'm referring to any possible differences in maybe water passages, intake & exhaust manifold bolt locations, pulley interference, are different cams needed for some, squish clearance problems, whatever...<br /><br />This can't be a difficult project, I've removed & rebuilt a lot of heads in cars before and done some of my own porting & polishing work. But never in a boat...so tips from those of you with extensive experience with the Mercruiser stuff would be appreciated if there's any quirks or potential roadblocks I should know about...especially the ones that might cost more money since I'm a cheap 'ol bastage.

 

[Bondo]

Re: Will swapping 3.0 heads gain me a few HP?

do I need to be concerned with various components fitting together or would all of the 6+ heads you mention be direct bolt-ons for my 1992-vintage block and it's outer components? I'm referring to any possible differences in maybe water passages, intake & exhaust manifold bolt locations, pulley interference, are different cams needed for some, squish clearance problems, whatever...

Well,..........<br /><br />All the Bolt Holes wll be Where they otto be............<br /><br />Water passages will be where They otto be......<br />Intake,+ Exhaust Ports will Move,.. not so much Location,.. But Size.....<br /><br />The Only Squish Problems Might be with dropping a 3.0l head on a 2.5l,.. there Could be Valve to Cyinder wall interferences....<br /><br />Cams have never changed very much,....... Not enough to Matter......<br /><br />Have Fun,+ Good Luck,.......<br />I Hope it's a Labor of love,..........<br />'Cause the Pay-Off will be in Personal Satisfaction,....... Not MPHs.........

 

[trog100]

Re: Will swapping 3.0 heads gain me a few HP?

the practsie of making one engine fill two price/hp slots is common with outboard boat engines i think..<br /><br />if u can find a website that lists all the specs and prices for the mercury line up it kinda shows up.. makes a lot of sense from a commercial point of view as well.. making half the number of different parts costs less money..<br /><br />they seem to go up in pairs.. each pair being identical in weight.. size.. dimentions.. cubic capacity.. in fact everything listed is the same except the hp rating and the price..<br /><br />a simple method of doing this would be just to fit a simple more restrictive carb on the lower hp version of the pair..<br /><br />i have suggested this theory before and folks have said nooo there is far more to it than that.. none of em have said exactly what that "far more" was thow.. he he he<br /><br />in todays "marketing" world things are never quite what they seem..<br /><br />trog

 

[Mile-High Mariner]

Re: Will swapping 3.0 heads gain me a few HP?

Come on, guys, the answer is fairly obvious. An internal combustion engine is just a big air pump, and the more air you can get it to ingest and burn to push those pistons, the more power you will end up with. The porting on the 2.5L heads is appreciably smaller than that of the 3.0L heads, which means less air getting in and out of the combustion process, and thus less power being produced. Trog and Bondo - whassa mattah you? You guys know what I'm talking about. Give 5 guys 5 of the same block and 5 different sets of induction and/or exhaust gear and you'll end up with 5 widely variant sets of power curves and output specs.<br /><br />Craze - it's more than just a fib, dude, and you will see power gain, but you will have to replace more than just the head and gaskets. The manifold is different for the 3.0L head, as well as the specified carburetor; the plugs specified for the 3.0LX are longer and there are a few other things you'll need to consider. There should be no reason why you can't do it, tho' - I'm actually toying with the same idea, myself. I, too, have a '92 3.0L and I'm pulling the head to have it reman'd (low compression in #2, probably a stuck or burned exhaust valve), so I'm looking at the 3.0LX head/etc. retrofit. I'm also looking at a forced induction option, not to dramatically boost my output, but to allow my motor to produce at least sea-level power up here at over a mile high in western Colorado, like the turbo-normalizer in the motor in my plane. Maybe we can compare notes as we progess.

 

[trog100]

Re: Will swapping 3.0 heads gain me a few HP?

David.. let me put the equation a different way.. take that 2.5 block.. thats the basis of yer pump.. bore it out or stroke it to increase its capacity by 20%.. in other words make it a 3.0.. leave everything else the same.. explain to me how u can do this without at least some power increase and i will change by fib theory..<br /><br />i aint saying u cant increase power outputs by swopping bits about.. just that in the example mentioned u cant..<br /><br />i just upgraded my computer.. new £170 amd 64 bit cpu.. its now runnng faster than the amd £750 cpu.. if i was to tell u the difference tween the £170 one and the £750 quid one is more to do with AMDs marketing strategy and the label on the box than any real difference tween the two products.. u wouldnt believe me there either. he he he<br /><br />trog

 

[craze1cars]

Re: Will swapping 3.0 heads gain me a few HP?

Originally posted by Mile High Mariner:<br /> The manifold is different for the 3.0L head, as well as the specified carburetor; the plugs specified for the 3.0LX are longer and there are a few other things you'll need to consider.

Glad to see I'm not the only one here who's willing to think outside the box a bit...your air-pump speech is drag-racing 101 yet so few people seem to grasp that more efficient airflow = more horsepower. Period. And Trog keeps rambling on about displacement. Displacement has some miniscule effect on HP, but really is secondary. CFM is what makes power. That's why many 2.0 L sports car engines make more horsepower than 4.0 L truck engines. They FLOW more air, even if they don't hold more air. So you can take a fixed intake, head and exhuast systems, bore them out as far as you want to, and they won't be able to pump significantly more air than they did with smaller bores. <br /><br />The big question is does the LX indeed flow air more efficiently than our L heads to provide more CFM's to the fixed 181 cc's? If the answer is yes, then it WILL make more power.<br /><br />I've stared at part #'s long enough to recognize that the carbs and manifolds are different on the LX's. The big question for me is "do they NEED to be different?" Or will the 1992 manifold & carb I already have be able to provide sufficient flow to feed the "better" heads (if indeed they are better heads) enough flow or will the motor be choked for air if I don't change them as well?<br /><br />The reality, which is why nobody ever messes with this stuff much, is that if ALL those items need to be changed it really doesn't make any economic sense. I'd have to find one HECKUVA deal on a LX setup in a wrecked boat or something to give me everything I need...in which case I might as well just swap motors entirely...in which case it would make a hecukuva lot more sense to just focus my "junk boat" search with the idea of doing a 4.3 upgrade.<br /><br />BUT....if the reality is that I can gain something with head and gaskets ONLY...and turn around and sell my old stuff to someone who simply needs a new head, then a cheap experiment could be fun since NOBODY mods the 3.0.<br /><br />I feel kinda like today's new hot-rodders probably used to feel in the early 90's when they started modding their Honda Civics to make them into drag-racers. The old, closed-minded, grey-haired farts surrounding them with modded V-8's just shook their heads and thought they were morons. BUT...in the end, power-to-weight ratio means EVERYTHING, and those front drive Civics now put a lot of 60's musclecars to shame every weekend at dragstrips around the country.<br /><br />Sounds like you may beat me to the punch...as it sounds like you need head repairs now. Mine's perfectly functional and I don't really want to mess with it till the ice comes off the lakes and I get a good "pre-mod" test in. So if I do this at all it won't be 'till summer. But I'd love to see what you find for yourself if you go forward. Please share your experiences if you take the plunge...<br /><br />I have considered alternative # 2 being doing my own porting and polishing on this existing head....but flying blind on something like that may not be a good idea as I'm sure it's mostly uncharted waters and I may have a detrimental effect on airflow. Would rather use Mercruiser's R & D (I'm sure they did some) and find a head THEY produced that will flow better than this one. Oh well...as usual, I ramble on.<br /><br />Hope all head a Merry Christmas!

 

[craze1cars]

Re: Will swapping 3.0 heads gain me a few HP?

[double-post....oops.

 

[Mile-High Mariner]

Re: Will swapping 3.0 heads gain me a few HP?

Trog - I may be mistaken, but didn't the 2.5 put out 120 hp? Part of the problem may be in the way marine engine output measurement has changed; they used to measure it at the prop, then decided it best to measure it at the shaft, or something like that (or maybe even vice-versa). Anyway, see Craze's comment about airFLOW for the reason that the two motors produce "roughly" the same horsepower - it's not all about displacement. Combustion chamber volume has far less impact on power output than how much air you can pack into the chamber; it's the reason forced induction can turn small displacement engines into big-block ball-busters. Turbo- and superchargers allow the introduction of a much higher-density air charge; add the proportionately correct amount of fuel and you get a kind of "loaves and fishes" effect.<br /><br />Craze - mismatching the ports of the LX head and the L manifold will introduce some major flow issues. They're just not the same size, if they're even in the same places. I don't think you want to mess with that particular marriage, if it will work at all (I don't think it will). Don, are you listening in here?

 

[Mile-High Mariner]

Re: Will swapping 3.0 heads gain me a few HP?

Oh, and by-the-way: it's not just turbos and supers that allow a higher density air charge into the comustion chamber - larger ports, smoother, less turbulent induction pathways and stuff like that can improve airflow well enough without having to resort to forced induction. Craze is thinking about porting and polishing - that's simply cleaning up the induction side to let more air in.

 

[tommays]

Re: Will swapping 3.0 heads gain me a few HP?

i have a 1995 3.0 LX and on my series OFXXXXXX of motors there are 3 different styles of manifold in that series alone<br /><br />who knows which one of those is best ?<br /><br />with end caps without end caps ect they even take different water hoses from the t-stat to the manifold<br /><br />i really dont think they put to much R & D into this motor beyond keeping it a cheep base motor<br /><br />and if my experience on motorcycles is the same heads that have the same part number have will have widely varying flow differences and putting them on a flow bench is the only way to tell<br /><br />tommays

 

[craze1cars]

Re: Will swapping 3.0 heads gain me a few HP?

Originally posted by Mile High Mariner:<br /> Craze - mismatching the ports of the LX head and the L manifold will introduce some major flow issues. They're just not the same size, if they're even in the same places. I don't think you want to mess with that particular marriage, if it will work at all (I don't think it will). Don, are you listening in here?

THIS is exactly my concern. And is a very helpful comment for me. As many car motors as I've rebuilt and tuned for racing, I have absolutely ZERO experience seeing the inside of these 3.0 motors. And I don't have a bunch of old ones laying around to compare parts against each other. So this is the sort of mish-match stuff I was sort of hoping someone here has some personal experience with (exactly which 3.0 Mercruiser head/manifold/block combo flowed best), but appears few do beyond replacing damaged parts with the same parts that were there before. Bondo's remark about the 140 heads being largest is a step in the right direction. Looks like if I'm gonna do this cheaply I'll just have to open it up and start studying to try to clean up what's already there for the cost of labor & gaskets alone.<br /><br />To anyone who happens to stumble across this with real-world experience in this arena...I'm all ears.<br /><br />People may be interested that I did find this engine for sale on-line:<br />"This performance 3.0 liter inboard marine engine is bored oversize, has two stage Sunnen plateau honing, 3 angle competition valve job, performance camshaft and lifters, seamless cam bearings, alloy pushrods, oil pump, mildly pocket ported head and has moly Sealed Power rings, new oversized and balanced pistons, Clevite 77 main and rod bearings, gear drive, roller rockers and more! (can also replace the 2.5 liter Mercruiser and OMC)"<br /> http://remanufactured.com/Inboard_Marine_Engines.htm <br /><br />It appears the site doesn't give a HP rating, but it gives me a clue that SOMETHING is available to improve these things. For only $2,690 outright! More than I'm willing to pay for sure. But it proves something CAN be done to the 3.0.

 

[trog100]

Re: Will swapping 3.0 heads gain me a few HP?

"That's why many 2.0 L sports car engines make more horsepower than 4.0 L truck engines"<br /><br />displacement.. rpm.. and how well u fill that displacement with the correct combustable mixture is what governs power..<br /><br />displacement.. well a 2.0 engine is a 2.0 engine..period.. the swept volume measures 2000cc of combustable mixture space.. a 4.0 engine is 4.0 engine.. assuming u fill its cylinders the same as u fill the smaller engines cylinders it will put out twice the power per power sroke as the 2.0 one will.. <br /><br />rpm.. how many power strokes per minute.. assuming the cylinders are being filled the same.. the more power strokes per minute the more power the engine puts out.. the 2.0 car engine operates at more power strokes per minute than the 4.0 truck engine.. which is why it can put out more power.. so we have displacement + RPM = power output.. one on its own means bugger all.. <br /><br />How well u fill that displacement.. well the more combustable mixture u bung into that displacement each power stroke the more power each power stroke produces.. also the more u compress that mixture before firing the more power it produces.. these two kinda go together.. u fill the cylinders better u also compress the mixture more..<br /><br />a none blown engine could at best only fill the cylinders with combustable mixture at atmospheric pressure.. in real life this never happens.. this ultimately governs the power gained from each power stroke..<br /><br />with a blown engine the cylinders can be filled with combustable mixture at whatever pressure u choose to pump it in there at.. the limit here is just how much u can compress the combustable mixture on the compression stroke without it pre-igniting or detonating..<br /><br />soo how do u get noticably more power out of a given capacity/displacement engine.. assuming u are gonna run that engine at the same RPM.. without blowing/compressing it u dont..<br /><br />now back to that 115hp merc 2.5 litre engine.. by increasing the capacity by 20% and making it into a 3.0 engine even thow its still got everything else the same no way on gods earth are u still only gonna get the same 115hp out of it at the same rpm..<br /><br />my marketing policy "fib" theory still holds.. the 89 to 94 3.0L mrec engine aint a 115hp unit.. and the 2.5 head used is probably more to do with merc having a lot of old stock they needed to get rid of.. <br /><br />if that 2.5 head plus its attached bits enabled the smaller 2.5 engine to generate 115hp it probably isnt any more restrive than the later 3.0 head which permits the bigger 3.0 engine to generate 135hp.. the power difference between these two engines (2.5 and 3.0) came from the capacity/displacement difference nothing else..<br /><br />as for the real power difference tween the 3.0l and the 3.0lx.. i dont recon there is any..<br /><br />trog<br /><br />ps.. something can be gained from the 3.0 merc engine no doubt.. but without blowing it or increasing its operating rpm significantly .. not much..

 

[paulie0735]

Re: Will swapping 3.0 heads gain me a few HP?

It would seem most people here have not been able to get past the addiction of 'peak horse power rating', and have never actually compared numbers on a dyno graph with 'real world' applications. Squeezing power out of a small engine is great for bench racing, but put that high horse power 2.0lt in the truck and see how could it isn't! It all gets down to the designed purpose of the engine. It’s not just about a peak horse power reading, so much more makes up the character of any engine. As an example, try bolting a 400HP Drag Bike engine into your 30’ cruiser. I have no preference for small and high tech or old school and big, each has their place and their fans. My only point is that to compare one against the other all things must be the same or there is simply no worthwhile information in the outcome.<br /><br />As for the original question, is your 3.0lt 115hp engine really just a 135hp 3.0lt with a 2.5lt head on it? I seriously doubt it but I’ll stand corrected if anybody has the absolute answer!! It’s more likely that your 115 is a detuned 135 its likely most of the changes are in the head but rather than using a 2.5 head it would more likely be a 3.0lt casting but giving a lower compression, smaller valves and perhaps a smaller carb. Again I’ll stand corrected but generally speaking, as has already been mentioned manufactures will design an engine with more than one application or power rating from the start. In doing so they are able to use a lot of common parts and castings and provide different markets with a power unit that will suit their needs.

 

[trog100]

Re: Will swapping 3.0 heads gain me a few HP?

"It’s more likely that your 115 is a detuned 135 its likely most of the changes are in the head but rather than using a 2.5 head it would more likely be a 3.0lt casting but giving a lower compression, smaller valves and perhaps a smaller carb"<br /><br />#####<br /><br />i could fish up the absolute answers they are pretty much in the manuals.. i have researched the subject.. my commments are based on a little more than what i "assume to be more likely"...<br /><br />what u assume to be more likely isnt correct.. plus what u assume to be more likely would cost money.. spending money to detune (redesign) an engine that no one is gonna dyno coupled with the fact if they did they aint exactly gonna complain that their cheaper engine actually puts out more power than its claimed to..<br /><br />we all assume things.. as long as our assumptions are based on some kinda sensible logic it dosnt really matter.. your assumption seems based on the logic that they wouldnt tell "fibs".. mine is based on the logic that saving money was the name of the game making one block and standard parts fill two price performance slots and useing up a whole bunch of of 2.5 heads they already had laying about with no 2.5 blocks to stick em on..<br /><br />plus the absolute factual knowledge that valve sizes and every other part in the two engines except sparkplug reach is identical.. the same manual covers both engines.. everything checks out identical except sparkplugs and the valves for one engine have an extra oil seal.. nit picking kinda things but in the entire manual its the only part differences i could find.. <br /><br />its trying to find out what the diffrence was/is tween these two engines that got me interested enough to poke about and have look.. all it needed was for the link to the discontinued 2.5 engines to be discovered and it all made sense..<br /><br />i have a 89 3.0L (change over period) and wanted to find out exactly which version mine was and what the difference if any there was.. he he he.. otherwise i wouldnt have given a rats wotsit..<br /><br />in 94 they saved a little more money and got rid of the so called 115hp engines altogether and just kept the 135hp one..<br /><br />trog

 

[paulie0735]

Re: Will swapping 3.0 heads gain me a few HP?

Trog, Why the backhanders? Are you the only one with a theory that can hold water? Did I provoke you? If so, my apologies. <br />We are both saying the same thing, that being; same engine different head. My theory is that they modified the 3.0 head your suggesting they used excess 2.5's. I don’t know the particular engine very well and I certainty don’t know the engineer who designed the 115 -135 HP engines, but my opinions are based on 27 years in this trade. In that time I have seen manufacturers do lots of weird things, as new engineers are employed the first thing they always want to do is change somebody else’s idea’s, under the guise of ‘improvement’. Many parts are out sourced and as the OE parts suppliers change so to the design and the look of the parts they supply. I have heard manufactures tell lots of ‘fibs’ including talking down the HP rating of a marine engine. Yep it was and still is very common in muscle cars but that’s for very different reasons. I'm still happy to stand corrected if you want to state your theories are fact! I did find your previous posts interesting and I agree with most of your comments on what makes HP. But I’m struggling with why you want to turn this into a competition or belittle others for putting up a theory that might not sit inline with your own. At the end of the day our idea’s and theory’s are just that and while I haven’t been on board this forum for long it would seem that there are a lot of people here who are very quick to shoot people down just for having an opinion on things but very few will actually put up their opinions as fact!! What I find ‘likely’ is based on my experience just as yours is based on yours!!!