I am port fuel injected (1995 EFI/MP).

I know SB states it is mild sounding but I would characterize it as slightly more than mild. EFI and Mercs spark stabilization do miracles on taming the idle down. On first light off when the motor is cold it has a bark and is fairly choppy (you know it isn't stock) but after about 3-5 seconds the EFI/Spark magic smooths things out quite a bit. AFR also plays a huge part on how tame it idles. My first light off with stock tune and bigger injectors it was quite rowdy, but at 10.X AFR that was to be expected.

When I went through cam selection I went to the Crower headquarters and manufacturing and described what I was doing. Old guy came to the counter listened and pointed to this cam and said "don't go any bigger" referring to the next cam up in the line which I believe was the 484S cam. We are using the 483LM cam.

Cam selection based on overlap vs LSA
 

Great to see some info and questions that dig deeper into the cam black holes.

Seems large LSA suggestions mostly come without good mechanical descriptions. Just the reversion scare.

Some puny GM TPI Camaro cams had 109* LSAs. So LSA does not by itself determine reversion safety, as the OP mentioned.

So total overlap is the decding factor, as far as the cam is concerned, but then that has to be applied to the engine. The CID, the head flow, the valve size, the intake, carb, single or dual plane, the exhaust system and backpressure....where the cam is dialed in...etc.

Not to mention, where in the exhaust you inject the raw water...as well as the exhaust system complete.
​​​​
There are a few vids online with clear exhaust tubing post elbow, showing the water flow, and the snap of the exhaust pulses. It tends to turbulate and atomize the raw water, and can pull it back with exhaust pulsing. They key is to have as steep of a run per ft of merged exhaust. Merc has a formula for this. Having a lip at the elbow to prevent water coming back is also a need...and the elbows were re-designed back to the older shape, to help prevent the reversion.

Larger CID engines will be able to eat up more overlap than their small siblings. Simply dude to the reduced signal through the heads and valves that feed the larger CID.

Anything that can reduce exhaust back pressure, will help with reversion.

Getting the elbows higher, and having a steeper approach to the outflow system, either thru hull or prop, will benefit.
​​

Apologies for reviving this thread, but it appears to be the correct place for this question.

I am getting ready to build a stroked 400 SBC (~409ci) for my 1979 Hallett Mini Day Cruiser. I will also be replacing the Alpha stern drive with a Bravo 1 and keeping the stock wet manifolds and exhaust through the stern drive. The roller cam that is being suggested by Jones Cams for this build is 232/236 @ .050" 576/576 on 114LSA, which gives about 6deg of overlap @ .050". He said that the engine won't have a problem with reversion, and the restrictive exhaust won't be an problem until about 5600rpm. Does this cam sound reasonable or a bit too big?

I would extend the inner “pipe’ prob 10” or so. Take a good google search for oso threads on how to do. Very common on sbc’s with decent cam sto
ck/near stock exhaust

Thanks for the quick response! I'll look into the extension suggestion.

OGHallett,

I'm by no means an expert on cams, and anyone can correct me if I'm off base, but I did get a crash course in cam math during my recent 496 rebuild. It's not so much the overlap, but WHEN the cam events actually happen. Look at exhaust lobe CENTER degree number and the lobe duration. Do the math. If the exhaust valve is closed by the time the crank passes over TDC, and thus the piston is NOT yet dropping, there pretty much can't be any reversion. There is technically still some open duration, due to the fact that cams are typically measured at .050" off seat (why, I don't know), but the piston doesn't actually start dropping significantly for several degrees of rotation, due to the rod/crank geometry relationship.

Thanks. Brad.

When I plug the cam specs into the MGI cam tool with a 4deg advance, it appears that exhaust valve close occurs at TDC, so I think I should be good to go. Thanks Brad!

So that’s that ? No research as suggested ? :)

Haha, I did pull the elbow portion of the manifold off this evening. The elbow runs directly into the boot and then the Y-pipe. The down leg of the elbow has four ports on the inner diameter that dump water radially into the exhaust flow, so unfortunately no pipe to extend. I'd post some photos, but it appears I am not allowed to post links until I've reached 10 posts.