Cam only MPI 383?
 

I’m new to boating but not to engines. I have a 383 Mag Stroker that is unmodified, but was wondering if it would accept a COMP 218/224 cam without other upgrades. So far, technical details are hard to find. I found the part number for the valve springs that come in the engine, but no details about them. Does anyone know if the MPI 383’s valvetrain is built stout enough to handle a cam like this? Thanks!

Carb or EFI?

You would need to run wide band to check AFRs and modify the fuel tables if EFI. That would be your biggest hurdle. That cam, without knowing the LSA or lift, is likely not going to give you any noticeable improvements for the amount of trouble you will have tuning the motor if it has a Merc 555 controller.

Ayuh,..... The thread title says MPI,.....

What are the specs of the current cam

So I don’t know the specs of the current cam other than .497 lift on both intake and exhaust. The COMP cam has .495/.503 with a lobe separation of 112°. It’s actually meant for a 305-350 with an operating range of 1200-5200, so it may be on the small side for a 383. I’m just afraid of putting too much cam to it because I know it’s not as easy to get away with as it is with an automotive engine.

Not exactly sure what your plans are, but if this is a marine motor with water jacketed exhaust manifolds, you need to be careful with cam lobe overlap, because it can cause reversion where your engine will suck in exhaust water... Obviously this isn't a consideration on automotive engines. I'm not exactly sure what the limits are for overlap before reversion becomes a problem, but I'm sure lots of people here could chime in on that.

As Trash mentioned above - not worth it.

Enjoy it for what it is.

then.......:D its a 3/4 race cam for you.

Assuming the stock valve springs can take it with no coil bind, And you have the clearance under the valve covers, And there's no issue with piston to valve clearance, ..it would be faster and cheaper to just upgrade the rockers to a 1.6 ratio from the stock 1.5
That would increase the lift up by about .030. So in essence you would be getting a higher lift cam without actually changing the cam.. :)

Not worth it. It would have to be re mapped as well.

Simply offered as an FYI reference, I have been using a 221/230 @ .525/.545" on a 114 LSA hyd roller cam in a 377 (Crower cam) with no reversion issues. In my estimation this cam is near, but not at the limits for stock wet exhaust in a SBC.

Hell yeah! Will that make my boat jump a Coke can?! 🤣

Honestly, I had planned on swapping an L92 in. But the LS swap community agreed that the 383 is an excellent marine engine, so I thought I’d explore options for getting more out of what I have. I know how quickly a build can rack up a bill, so I really wanted to leave the rest of the valvetrain alone, if at all possible. There are also some cost friendly aluminum heads local to me, but I have no idea how much the machine shop bill would be to marinize them.

If in the end there’s no way to do it without dropping massive amounts, then perhaps a custom calibration would be the only thing I’d do in order to make sure it’s running its best.

I know most of the 350's came with that GM 395 cam (288 int./308 exh,, 0.431 int./0.451 exh., 109 LSA , and the 377 Scorpion had the 222/230 .510/.530 114 LSA. The cam for the 383 MPI likely falls somewhere in between these two. I can't find the part number but it is possibly around these specs Cam Specs: 215/220 @50 .457in/.474ex 112LSA

Small block chevys don't make any power in marine applications. You're just wasting your time.....

Not sensing sarcasm, but if you are looking for 400 ish HP SBC will do the job. More than that gets problematic if naturally aspirated.

Trash,
The sarcasm skipped past you then... Do a search of OSO with my username if you want some marine SBC tech.

Nonsense. Sounds like a friend of mine who swears it’s big block or nothing. I smoked him with a naturally aspirated 346” LS.

My boat came with a 350. The 383 is plenty for it, even if I leave it alone.

If you didn't pick up on the sarcasm, you might want to check out this thread.

https://www.offshoreonly.com/forums/...light=94sylvan

Having been down this road... a light boat and a 383 will get it done. My Chris Craft Stinger 222 went 77 on GPS (outrunning a nasty thunderstorm at that) with a 385, TFS heads, and a Comp hydraulic roller. We even ran a poker run with when the Bullet ate one of it's engines.
https://cimg0.ibsrv.net/gimg/www.off...cf654e96e.jpeg
https://cimg1.ibsrv.net/gimg/www.off...d6845bfda.jpeg

Okay, looked up the OP’s engine to get cam specs.
click here :
https://www.mercruiserparts.com/bam/...2006/13933/120

Then click on the #11 part #.
scroll a touch and you’ll see all the different engines it comes in.
The notables - 350mpi’s.

Here are the valve springs. Can’t find the specs but they come on lots of nothing special motors (4.3 tbi’s and etc)
https://www.mercruiserparts.com/483471-spring

Free shipping!
https://cimg5.ibsrv.net/gimg/www.off...6a4ead873a.png

Yeah probably not a good bet to try running them with an aftermarket cam. How is the rest of SBC valvetrain? Pushrods are cheap enough, but is the timing set up to the job?

Also, thank you to everyone for your contributions here. I may forego any modification, but then again, a 6.2 LS swap has been on my mind. Not sure how much I could sell a healthy 383 for, but I thought it may cover the cost. I don’t know about boats, but in cars, these engines not only run smoother, but I’ve recorded noticeable fuel economy gains.

Ai ^^^ ????!

Yuuup

Ten posts and most read like AI.

Yep that’s 100% AI based on my experience having the same conversation with ChatGPT. And actually, it seems like an aluminum 6.2 swap wouldn’t be terribly expensive or difficult given my experience and parts availability. I’m beginning to think it’s better than modifying the 383 since I already have the engine on hand. It’s fresh too…probably 2-3k miles. I’d just have to change the cam. 🤷‍♂️