new engine build or buy long block ?

[lively]

hey guys my 351 is wearing , the compression is about 80 across the board and im wanting to see how much HP will these velvets hold ?
what was factory HP on the 351w? im wanting 300-325 @ the crank , what can i do to make this engine more reliable and still have some ass behind the throttle ..

long blocks go for 1k around here with reman stock heads and a diff cam for the boat ..

what this noise about GT40 heads ?

just wanted some insight might bite the bullet in a couple weeks and throw a new power plant in ..


help !! lol

[UKandH]

Factory HP on the standard 351W ranged from early engines circa late 60's to late 70's were about 220 HP to a standard 260 HP in the early 90's boats (most PCM motors). The HO BOSS was the next rated at 285 HP with GT heads, then comes the GT40 engine with FI at 310HP.

A properly rebuilt 351 with very little work will bust about 270hp with a quality manifold and carb set up and a little head work on the valves and inlet / exhaust. GT heads will be your best option and with a bit of decent work will top 295 HP, put a good quality manifold and carb should see 300 HP and globs of torque.

I was building a spare motor for a car project a few years ago which still sits in my garage and was pumping 360 at the crank, a 351W with a ground up rebuild, wiseco pistons, comp cam, solid lifters and roller set up, head work by TTS, larger valves and a GT-P head set. Its still sat in storage as i went down the LS6 route instead, unfortunately i dont think its suitable for marine use as the cam is a fast road/rally style. I was thinking of tearing it down, fitting a more suitable cam, lowering the CR and fitting twin Rotrex superchargers and slapping that in the TS6, now that would be a hoot, it should pump 400-450 HP easy with twin superchargers on regular pump gas.

Anyway back on track, personally i prefer to build engines myself, but crate motors are a great option. A crate short engine and search for a pair of GT-P heads (the hot rod boys change them out as they are hard to change the plugs on in a road car) You should be able to source them easily in the USA. Good quality components, build slowly and double check everything as you tear down and rebuild, dont scrimp and save money. One thing i would do is have the bottom end balanced with the flex plate as well, get the pistons weighed and balanced as well. Shot peen the rods and balance. Once the bottom end is blueprinted it will give a much improved throttle response and be a lot smoother running, rev much quicker and the hole shot will be bang on and a huge grin factor.

Next thing i would do would be to source a new Cam, stay with a stock marine profile, marine cams are unique and are the result of a lot of compromise, unless your going blown / force induction i would stay stock but new. Replace the lifters at the same time and lubricate well with compcam lube when assembling. Also fit new bearings in the block for the cam, get it done professionally and line bored / checked for perfect true, a lot of people screw that part up in rebuilds.

Head work is where a marine engine benefits the most, nearly all 351's have poor gas flow due to design compromises. My advice is to source GT-P heads and sent them away to a specialist for rework, they already have the larger valves fitted but can still benefit with modification in the seat cut. Get the heads CC measured, skimmed and crack tested.

Next is piston choice, most would go for stock CR, and i would tend to agree, but spend some money here and get QUALITY pistons as they will be drop forged and better, lighter weight and higher strength and inherently better balanced from the factory, i like Mahle but Wiseco, JL and many others are great pistons. Get the squish as good as you can for optimum compression and adjust the CC in the head chamber per cylinder so they are all equal, and good engine builder can do that work for you as it means dry fitting the heads and measuring a lot of things, stripping adjusting and refitting again and again but the benefits outweigh the expense.

The thing to remember here is that engine design is always a compromise of cost vs performance, if you build an engine that is not always true.

Last but not least is the top off, the inlet, we cant do much about the exhaust as its a stock PCM block item, but the inlet is seriously flawed as in most ski boats the top of the engine needs a flat profile for the dog house. I have looked at a lot of different options here for my marine rebuilds, and on my TS6 there is a few inches clearance, so an Edelbrock performance inlet would be my choice topped off with a new Holley 4160 or 4150.

A well built and put together motor will not always be full of huge horse power due to marinised cams and exhaust limitations but you can build it to be ultra well balanced, and HUGE torque gains and this is where we notice the difference as its torque that drives us through the water.

Obviously if money is no object then i would fit Rotrex superchargers as they are small and very efficient and as they sit at the side of the block they take no room from the top of the motor housing.

Need any build advice then im always here, and yes, i have built a lot of American V8 engines haha, my current project car is fitted with a LS6 pushing 640 HP and so much invested in it i dare not look at the receipts

[crystal waters]

I looked at changing out to GT -40 heads several years ago.
A forum member responded saying i should look seriously at using Dart Heads which are half the price and in fact better performers than the GT - 40 heads.
I never did the work but thought i would throw this notion out to you at this point in time.

[wotan2525]

UKandH sounds like he knows what he's talking about. But what he's describing is going to require you to write a blank-check to your machine shop. These boats are not high-performance machines and don't need to be blueprinted and balanced to the extent that he's describing. Is it better? Absolutely. Will it result in more power and smoother operation? Yes. Will you ever get your money back out of that type of build when you sell? Never, never, never.

If you can buy a longblock with a warranty for $1k, I wouldn't think twice. You'll have twice that into rebuilding yours with mild upgrades (and the minimum in machine work.)

[Cusefan78]

I agree. Go the long block route. You can always sell the crate heads and get gt40 heads. Then I would look for a screaming carb and bigger intake. If that's isn't enough slide the cam out and get a bigger cam. Building your own will never he the cheaper route. I have built a few 5.0 engines and I sware my mouth is bigger then my wallet and you end up going overboard with little return.

[lively]

wow ! lots of insight !! thank you for the responses guys !

what intake ? i have a marine 650 cfm edlebrock carb now .. stock

what about a 97 351 EFI ? i can get them cheap from LKQ or wrecking yard but would still have to possibly rebuild and change cam ..

long block sounds good but i need details on what works for these engines, just any after market heads? what intake to be paired ?

im no engine guru but i have rebuilt a few engines with stock parts "R&R" just never any power adders ....

WOT 3600 35 knots . timing 10 * BTDC.

[UKandH]

The old adage, *there is no substitute for Cubic Inches* holds a lot of water for marine engines.

A crate motor will give you instant gratification and be up and running with a weekends work, going for a new crate big block is a great idea if money is the prime concern.
My personal choice is to build myself (its an engineer's thing) and doing it right is never cheap, and of course, if your intention is to sell it then you will never get the money back. Take my boat as an example, a 93 TS6 and i have just spent over $6k on upgrades to the interior, electronics and other things. Will i ever get that cash back, hell no, of course not. Does it make it a better wake...no, do i get a better grin factor / more pleasure / less stress when driving it? hell yeah. Its all about where you want to spend your dollar my friend.
I get loads of satisfaction building hot rod motors for other people as well as my projects and its all about what floats your boat!

Words of caution here though, throwing a profile cam that is not for marine use will make it undriveable in many respects so use great caution. If your going to build the motor for drag boating, ski racing or high speed work then yes thats fine, but for skiing or tow sports i would choose wisely, talk to Crane Cams or Edelbrock for advice as they produce re-profiled marine cams and they will tell you the same as I...USE CAUTION! Just my 2 cents worth.

Next thing to be careful of is over carbing, there is a lot of misunderstood advice floating about the internet on carb choice. The first things to calculate are RPM, Volumetric efficiency, compression ratio, temperature range, cam profile, flow rates etc.
All of this equates to required volumetric flow through the carb at any given rev range. Most of our marine engines work OK with a 600 cfm carb. Restrictions on rev and torque curve requirements dont call for high flow fuelling. Most 700-750 cfm carbs are designed for high RPM engines, ours peak at 4800ish, so do we need better airflow? the answer is no we dont. Our ultimate goal is TORQUE, nothing else really matters to a boat engine, more torque equates to a bigger prop surface area which equates to speed or pulling power.
The maximum realistic CFM should be a 650 or at a push a 700 in larger CC engines, a 351 is suited with a 650 max CFM, the key to better performance is how that mixes the air fuel ratio and delivers it to the engine at certain RPM. That is the job of the manifold. High rise manifolds are designed for high RPM and maximum direct unrestricted air flow, as the engine increases speed it becomes more efficient, high RPM = fast air delivery, the downside is the low and mid range torque disappears. Engine designers and racers describe this as *bogging down* or poor hole shot. The lower the profile manifold the better the torque delivery (to a point) A good manifold encompasses the firing order mode of the design so that one port is not robbed of air fuel mix by the requirement of another at the same or similar time (port timing) You will see many ideas floating about by Edelbrock, Holley etc, each has its advantage and disadvantages but ultimately look for a manifold that delivers low RPM torque for marine use, this is why the cam profile is so important as this has a direct influence on the carb / manifold choice.

Also remember my point about exhaust efficiency and how poorly designed our marine manifolds are, this also has an impact on your given choices...ther is NO POINT in throwing on a high CFM carb if the exhaust gas speed is low, it will simply create a restriction to gas flow in the heads and combustion chamber. This is in effect a giant pump, air in has to match air out, if one is restricted and creates back pulses and therefore back pressure it just wont work unless we increase one part of the efficiency (or RPM) to gain the flow rate desired.

Right, the next point is that of fitting after market FI to a marine engine, our biggest problem is of closed circuit monitoring, the use of HEGO and UHEGO sensors in the exhaust gas flow to calibrate and monitor the stoichiometric ratio (the ideal fuel air mix)
You need to get the sensors in a dry location in the exhaust gas flow, thats quite a problem in the usual marine header as there is water being introduced down stream of the manifold, any water getting on to the sensor will destroy it in seconds.
Its a problem !! As long as the system is a closed loop monitoring system were completely screwed It will take a complete manifold redesign or a bit of innovative modification (something i am currently looking at as a little design project at home)

I hope that helps clear a few things up haha.

[CJD]

It costs more than $1k to rebuild a 351. As with all things, you get what you pay for.

My father gave me an old truck that burned oil faster than gas. To save time, I bought a cheap long block. First block had sticking valves. I removed the engine and took it back. They replaced it for free, but the second engine leaked as much oil as the original burned. Removed the engine for a 3rd time...this engine never ran right. I suspect the cam timing was off.

After putting double the time into engine swapping than I would have put in if I rebuilt it myself...I sold the truck.

The lesson I learned, if they sell you a long block cheaper than the price of the parts it would cost you to do it right...then you will always wonder where they cut the costs. Of course, you will find out shortly!

[Cusefan78]

Yeah a bargain basement long block is not the way to go. Do you're research on the engine companies and make sure they are building engines that last. The 351w that's in our boats is a converted truck motor not one built for a car. Huge difference.

[Cusefan78]

Here you go. Gt40 heads on a long block. 285hp. You can boost that with carbing and intakes http://m.summitracing.com/parts/hme-dma1-gtp.