Mercruiser MPI V8 easy power gains

[QBhoy]

To my North American Friends on the forum....

given that these GM engines are only ever seen in marine applications over here in the U.K...tuning and general messing with these engines is not even a thing here...as far as I know.
When these engines like the 350 and 305 mpi are in a car....surely there are guys out there changing things like heads and injectors for a little more power ?
my own boat apparently has a cam from factory, as far as I know. I have no reason to doubt it, given it’s exceptional performance. But I’m presuming the fuelling was messed with to suit. Perhaps not, she is lumpy at idle, as some performance cam engines seem to be.
Without the expense of supercharging and replacing for more cc....are there easy bolt on options like bigger injectors or similar ?
dont want to mess too much, just a little perhaps and keen to know what’s out there.
Thanks

 

[alldodge]

Don't know of any "bolt on's" which actually provide any benefit which is worth the cost. If you want to know how well the motor is running, and how it changes, I would buy an AFR meter. This will tell you just how well the motor is running, and if anything added has any benefit.

Probably the easiest (while not easy) bolt on is a whipple charger. Cost is up there but the extra HP is also there

 

[QBhoy]

What would that cost AD and would the ecu require mapping? Or would it organise itself to suit ?

 

[QBhoy]

The LS3 and simialr are used a fair bit over here for transplants into land rovers and other crazy things. Plenty mods for them, not heard of anything other than a cam and exhaust headers for the 305/350

 

[QBhoy]

Ideally I would love someone to take my engine as a trade in and give me a 6.2 for not much extra. No one really does that here though. I’d need to take my engine out and sell it. Then buy a crate 6.2. Don’t like the idea of being boatless for any length of time.

 

[alldodge]

Whipple does have a MPI which includes remap, but install one on your boat and your going to go way faster then the hull was designed for

https://whipplesuperchargers.com/index.php?dispatch=categories.view&category_id=4

Cost is around 7 to 8K US

 

[QBhoy]

Thanks AD. Maybe a bit much, I’m thinking. I should just be happy with what I have. Once she is back in the water, my winter depression affliction gets better !
missing the sound of her badly now.
She is going in the water Easter weekend for sure.

 

[Scott Danforth]

Sell your current boat, buy a faster one

 

[QBhoy]

Sell your current boat, buy a faster one

Can’t do that Scott. Only 16 made as far as I know. Only one in the U.K. too special.

 

[Scott Danforth]

its cheaper and easier in the long run. however if you want, pull the motor, take it to a machine shop, open the bores up .75mm (.03") and go with a complete stroker rotating assembly for $900, add $1000 for cam kit, gaskets, bearings, timing set, etc. and build yourself a 6.2

 

[Bondo]

Ayuh,.... Bein' a 'puter motor is a drawback in my world,....

Just pull that motor, 'n drop in a 7.4l Mpi,....

 

[QBhoy]

Like your thinking Bondo...be a heavy lump I’d imagine ? Are they mane just about the same horse power as the 6.2 ?
Not sure what the hp of mine will be. Upwards of 260 anyway.
There doesn’t seem to be much power gains or difference between the 5.7, 6.2 and older 7.4 mpi engines. Only 20hp in it. That I can see. Perhaps the answer is the 496, haha. I’m dreaming. That would be ott.
I’ll maybe see if I can play with the heads a little. Clean up the breathing process.

 

[alldodge]

MPI no frills stock motors
5.7 - 300 hp
6.2 -350 hp
7.4 - 385 hp
8.1 - 370 hp or 420 hp HO
8.2 - 415 hp

 

[QBhoy]

Ah. That’s not what I thought at all with the 6.2 and 7.4. Maybe I was thinking of the old original 7.4. I thought both 6.2 and 7.4 were 320
there is a big scarab on the Loch here that has the 8.1 HO. The engine bay is a piece of art. Stainless 4 branch exhausts on each side etc. God knows where he got them.
The exhaust pulses can be felt in your chest from 50 yards away.
All that said...the wee Campion Chase Performance still runs neck and neck with it in calm water. He likely has hundred of litres of fuel in it all the time though, haha....and I get the impression it would do 60 odd mph in about any weather almost. Lovely thing.

 

[alldodge]

HP is great, but still needs torque with a heavy boat getting it up. Still no replacement for displacement in this area

 

[Scott Danforth]

A BBC with aluminum heads and aluminum echaust is less weight than a stock iron headed SBC with iron manifolds