I am about to plan a 4.3 build up, just the cam and valve train and a new tune for a mefi-3 computer. As to what i am looking for is increasing the motors power and efficency to certain power bands... that being 2500 to 5500....Ever see anyone try to plane out and hold there @ 2000...

I am not even concerned about getting out of the hole...a 3.0l can power the hull out of the hole and i never slam out...always build or slowly get up and out. Having watched my own boating habits amd many others cruise speed is generally about 3500-4000 rpm. With occasional runs @ 5000 for maybe 10 minutes.

I have no egr to worry about and gearing is really simple..(1) that is. (props not included). The reality is i run 80% of the time @ 3500..Reminds me of almost of circle track racing...with some long straight aways.. cut off @ 5500

Below is all factory and what i am starting with...


1. 2000 model year 4.3 w/bal shaft
2. vortec heads (113's)...has scew studs not pressed
3. marine 4 barrel intake with tbi adapter
4. 454 throttle body and injectors..This has been verifyed....
5.Duration @ .050” lift = In 202, Ex 213, Valve lift = In .403”, Ex .409”, Lobe sep 112 hydr roller with fulcrum rockers
6. Mefi-3 controller
7.Bore (in.) 4

Stroke (in.) 3.48

Compression ratio 9.41:1

Displacement (cu. in.) 262

Power (hp) 205

Max engine speed (rpm) 4400-4800

And here is where i think i am going


1. XM270HR cam ground for a v6
Cam Style Hydraulic roller tappet
Basic Operating RPM Range 1,200-5,200
Intake Duration at 050 inch Lift 218
Exhaust Duration at 050 inch Lift 224
Duration at 050 inch Lift 218 int./224 exh.
Advertised Intake Duration 270
Advertised Exhaust Duration 276
Advertised Duration 270 int./276 exh.
Intake Valve Lift with Factory Rocker Arm Ratio 0.495 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio 0.503 in.
Valve Lift with Factory Rocker Arm Ratio 0.495 int./0.503 exh.
Lobe Separation (degrees) 112

2. LS 6 springs with compcam retainers
3. replace the fulcrum rockers with true roller rockers
4. Intake 1.6 EX1.5...
5.Adjustable fuel pressure reg
6 custom tune
7.Clean up the Intake on the heads and polish it...No work of the EX..


Im not worried about water reversion and the mefi 3 should be able to tune that cam or so im told by the tuner. But the question is does anyone think the 4.3 can make 275hp doing this mod....its cheap about 600 gets me there.

 

This forum is really not the place for V6 engine tech, so you likely won't find much help here. 275HP sounds feasible with that cam and a little head work. You need to verify clearance between the valve stem seal and the bottom of the retainer at the lift you will be running. You will likely need a machine shop to cut the guides down and install the smaller style seals. Also the roller rockers increase stress on the pushrods(especially the 1.6 ratio) and will require a good hardened piece. I have learned from experience that the stock 4.3L pushrods will bend like pretzels, so figure some good ones into that budget.

 

Thank you for the reply,i had not given much thought to push rods. I will take that advice. I hope i have not offended anyone with this v6 post

 

you might consider tightening up the lobe center on that cam to better match your intended powerband. something closer to a 108 or 110.

Also, with the intake rocker, you'd be getting .528 lift, which will get a little close to the intake valve stem seal. I've head .550 lift is good with beehive retainers, but, I would definetly take a good look when you get the retainers.

Also, I would look at the edelbrock performer vortec with a nice adapter made for your BB TB. Should flow better than a gm intake.

EDIT: Just noticed you're not planning on working the exhaust? why? that is one of the weaker parts of the vortec heads, although not bad, they can be a lot better.

 

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Are there really any gains to this...gases are expelled under force and i was hoping the extended duration would aid...Here is what i can tell you marine application that do work seldom benefit from exhaust work below 6000 rpm that is ..Im limiting the motor at 55 or 5700 r's...Outdrive's heat up considerably above 6000

Again thank you for the reply

 

112 isn't a set amount apart. since duration of both plays a big part of this. With your cam a 112 on a 108 ICL (4* advance ground in) you get -1* at .50. with a 108 on a 104, you get 7* of overlap at .050. with a 110 on a 106 the overlap is 3*. I'm not thinking there will be much reversion there, but there are many more factors involved in reversion than just overlap numbers.

As for the beehive/vortec being good for .550, its something i would look at closely in your buildup. You'll probably be ok, but with it being that close to the suggested limit, it can't hurt to do an extra measurement. Better to find out early than later.

You could try using spacers on the gm intake to up the plenum size to see if it helps your engine in your powerband. trying diff spacers would be a good idea in itself.

You're increasing your intake duration by running the 1.6 rockers. It depends how much based of your lobes, but I'd say you're getting closer to 220-222 degrees. Not much extra duration for your slightly weak exhaust. A good valve job goes a long way on these, both intake and exhaust.
Anyways, heres a good thread to look at when you start.
http://speedtalk.com/forum/viewtopic.php?f=1&t=22571