Sticky?? Cam overlap degrees for marine cam selection
 

Hi all - so im trying to get some usable numbers for folks (like me) who have little idea what they are talking about when it comes to cams! Since there are some incredibly bright folks around here I thought Id post what ive researched and see if any of you can chime in and confirm/correct me and my results -

I am in the process of camming an otherwise stock 6.2 for a customer. I have found through many hours of research that merc used the ramjet cam in many of there 90s-2000s 6.2 motors so ordered one. So I searched online for a overlap calculator and found a very nice one here (no affiliation) and here (again - no affiliation) - the numbers add up on both.

• https://mgispeedware.com/camshaft-calculator/ (this one is great cos you can enter 4 profiles and compare!)
• Camshaft Overlap Calculator - Wallace Racing (very basic)

So I compared these two cams for my example:

• Ramjet cam (PN 14097395) - great cam option for the 6.2, tremendous torque, works on stock heads, great price and OEM made
• Comp XM270HR requires machining, springs, more expensive.

The comp cam shows -4 degrees of overlap despite having a larger LSA and shorter duration numbers VS the ramjet - longer adv duration and tighter lsa but -19 degree overlap.. is lower better, and what is the safe threshold? Im sure the IVC and EXC play a role in reversion but I am not skilled enough to even begin a conversation on where the safe threshold is-

Please excuse my lack of expertise if I have missed anything critical or come across misleading but I really think some overlap numbers would really help us in cam selection options - would be wonderful if vizard could/would chime in on this :)

Here are my results:

Comp XM270HR: -3 degrees overlap

https://cimg5.ibsrv.net/gimg/www.off...a46c407b0c.jpg




GM Ramjet cam - 19 degrees overlap

https://cimg7.ibsrv.net/gimg/www.off...1ca3dd641b.jpg

Where'd ya get the Ramjet cam info as far as that's what Mercury used?
Seen your other thread. I guess you degreed the current one?
What year is the 6.2 you are working on?
Not disputing anything, just FWIW, the lift specs are off with it compared to this manual 31 snip I posted in this thread here: https://www.offshoreonly.com/forums/...ml#post4701334
I'm not sure what year 6.2's the manual covers.

The '395' cam was used on a bazillion marine small block motors. It is mild. Don't expect a screamer motor with that cam. Not saying its a bad cam but don't expect big numbers from it. It will also fall on it's face around 4400 rpm. If you want to bring a little more life into the motor something closer to the 270 would be appropriate. If you want a rock solid basic cam the 395 is a good choice.

2001 boat- No screamer here - this is a 40ft twin 383 fishing cruiser, low down torque is key and the 395 cam is a torque monster, 444ft lbs at 3000.. I was mostly interested in the overlap calculations - hard to believe the GM cam has less overlap despite a tighter lsa - which is what most ppl tend to quote ito marine cam selection and reversion - i would say cruising rpms are around 3200-3500, and the 395 cam fits nice, i was more concerned with reversion..

https://www.offshoreonly.com/forums/...0-a-print.html

I found more references to the ramjet being the merc cam - found it elsewhere too, ppdiesel or somewhere.. I basically used the comp cam to compare the overlap as a factor in making a safe choice.. fwiw i also bought a CRANE 2032 cam Hydraulic Roller P/N - 104224 which was cited as the merc 6.2 cam elsewhere. Its closer to the comp 270 cam but for the size and use of the boat i opted for the ramjet cam instead.. i dont think the customer would care much for anything over 4k..

and fwiw - this one came with a kellog crank-

Is this 6.2 (377 cid) fuel injected ? If so, it will need reprogramming for many of the more typical marine sbc performance cams that would be good for 383’s. GMHot Cam, Comp Cams Extreme Marine 218/224 at .050”, etc/etc

What kind of exhaust ?

runs through regular dry joint manifolds, then thru the chambered inboard exhaust and out the back via flappers.. this ramjet is a mild cam - im hoping the stock ECU will work ok with it- im specifically trying to avoind any kind of programming and im not building a race boat! This is a heavy cruiser-

Okay, so I’m lost. The stck 300hp 350mpi/ 320h 6.2 mpi cam is the vortec truck 196/206 109 lsa cam right ? The svorpian 6.2 and merc reman 383 used bigger cams , right ? Didn’t you ask about bigger cams ? Sorry for being confused with your wants and needs. Straihten me out, thanks

Nope - ive been trying to get specs for the motors, stock basically.. I guess I wasnt aware the scorpion variants sued bigger cams - im also trying to get some overlap numbers down so people who are looking for marine style cams can perhaps get a hook.. thats part of what I wanted to find out - a: stock cam specs (as confirmed there are various ones - i thought there was one or 2 with slight variances), and b: get some overlap numbers down ito reversion because alot of ppl drop LSA like it solves reversion but unless ive made a mistake or the 2 calculators are inaccurate, lsa alone has little to no bearing with overlap I referenced bigger cams because it was suggested they might have been stock cams-

Okay, well I'm a pretty good source of cam info. If you look back under my name, I've done most cams in .004, .006, overlap at such, .050", overlap at such, and ,200" durations...most marine performancer bbc cams and some sbc cams, but know them pretty well too.

I understand overlap well too. BTDC and ATDC. The one we are more concerned with is exhaust ATDC as that's when the piston is going down and doesn't care where the air comes from - intake or exhaust.

Crane used to state Overlap at .050" passed +8 degrees probably needs dry exhaust. That's a pretty good spot to ponder. Also, all the crane blu motors with crane cams, had the cams installed + 5 degrees. So, 114LSA cam was installed 109ICL/119ECl.

Enough of that.



If you are not going to program the motor, then don't touch the cam. All These mercruiser (black and race blue) motors are speed density.

Good info thanks SB! so looking at the graphs i posted - the overlap numbers - are they the numbers you specify @ 50? or are they perhaps calculated differently? I want to know at what point reversion becomes an issue - i might want to build some hotter engines for my boats (and friends) and would like a scientific based approach :) I have a few good cams here and would like to punch numbers in so I can get some idea of what might cause issues -

sorry cheech for not reposnding sooner - nope havent degreed anything yet - im swamped with engines to rebuild. Its still on the lathe where I was attempting to polish out the rusty sections (unsuccessfully) -

I watched an episode of Redline Rebuild on Hagerty YouTube channel. Guy sent an old Cadillac cam to this place in link to get reground.
Never heard of them until then.
Have no idea if it's an effective solution in your situation or not, cost wise or else, mainly longevity. If you are determined to run the OEM specs.
And what's the audience reading this take on reground cams anyways?
Delta Camshaft - We manufacture horsepower!

yes regrounds not really an option - if the merc cam goes for $650, and a GM Ramjet cam goes for $230- and both deliver the goods, id be inclined to say its quite obvious where my thinking is headed - there are alot of cams that can work as marine cams - im trying to get some kind of science behind it - too much talk of LSA and nothing about actual overlap in degrees (for starters) and as SB pointed out, other valve timing events-

fwiw - if anyone is possibly interested in collaborating - I can code almost anything - would be nice to have a wet exhaust marine cam selection tool for the general folks that feel upward of 600 for a 350 roller cam is on the spendy side- esp for a cruiser.

Not sure if coding really needed. Here are basics from real world and we can get deeper into it later.

For sbc:
comp extreme marine 212/218 at .050” 112 lsa installed 108icl/116 ecl pretty much max with stock mercruiser manifolds. Good power though, especially with vortec or good aftmkt heads.

conp extreme marine 218/224 at .050” 112lsa installed 108icl/116 ecl good with eddie marine long risers /glm or merc with 10” to 12” inner extensions welded in. Comp extreme marine 224/230 at .050” 112lsa teally needs 10:1 compression 350 or very kight fast boat with 9:1, but more 383’s use this cam. Same exhaust rules apply. Stock merc even with mods not good here at all.

ok - so for a 9 to 1 standard 383 the ramjet is fine yes? im not looking for exotic power levels.. im trying to make this as reliable and long term as possible.. its a big heavy boat- is my question is if i punch in those numbers, will the overlap degrees give any sort of indication? appreciate the info!

I'm looking at getting a crower cam similar to the cam in the 377 scorpion for my current mpi build. I currently have IMCO powerflow exhaust with 12 inch risers. Would I be looking at reversion issues?
https://cimg0.ibsrv.net/gimg/www.off...8bb9506e20.jpg
https://cimg2.ibsrv.net/gimg/www.off...b71bcf7843.jpg

I run this exact cam on a 377 stroker (standard bore ) with GLM exhaust, stock Merc elbows and 4" through hull. I have a wide band sensor running full time and have been doing so for the last 11-12 years. Reversion is not an issue with my setup.

Good to hear! Man i Would love to hear what it sounds like! How do you like the power? Are you injected or carb?

Will sound mild, with that duration and 114lsa. Pretty much just Like a 365hp 454.

would more duration than this be safe with a 114 LSA and wet exhaust?

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.
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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.
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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.

OGHallett,

For the record, my comment was not for the purpose of an alternative to the research that SB suggested. He's one of several VERY reliable voices here on OSO. He's one of our "E.F. Huttons".

Pics.... Just spam this thread with 5, 4, 3, etc, until you get to ten posts. Then post your pics.

Thanks. Brad.

SB, your number of posts did not go unnoticed. I figured you were probably the real OG on this forum!

I'm sorry to spam a thread, but here we go...

Post 6

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Ok, now that that is out of the way...

https://cimg7.ibsrv.net/gimg/www.off...b863f9d156.jpg

https://cimg1.ibsrv.net/gimg/www.off...8950ca5dc4.jpg

https://cimg0.ibsrv.net/gimg/www.off...bf61fa58ca.jpg

Okay, good thing about posting pics…. Those are the really old style logs… so I have no idea, at least they pointed down. More common style are the center rise where the risers go up and then down a little bit and those need extensions. The old style is even more restrictive as far as exhaust gas but I have no idea how much I never dealt with them before. Maybe the guys that dealt with the old stuff what time in?

Eddie marine had some under 1800 center rise manifolds supposed to help scavsnge as well as getting the water level to prevent inversion a bit higher up
the liberator i am to pickup sat...
Has the old flat logs and short rise at end
New manifolds are on my list along with new water circulation pumps during winter lay up
maybe not from summit...but they popped up 1st 🤣
https://cimg2.ibsrv.net/gimg/www.off...4fea5a60ba.jpg