Cam overlap degrees for marine cam selection

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CamaroMan
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Cam overlap degrees for marine cam selection

Post by CamaroMan »

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 posted this on a boating forum but wanted to get some of your input from here as well :)

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!)
http://wallaceracing.com/overlap-calc.php - 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

Image




GM Ramjet cam: -19 degrees overlap
Image
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Re: Cam overlap degrees for marine cam selection

Post by In-Tech »

Knowledge of the lobes is the key. One manufacture to the next might publish similar stuff and act completely different. The wet exhaust design of the boat is more of a critical factor than the engine, so they both are important.
Heat is energy, energy is horsepower...but you gotta control the heat.
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Re: Cam overlap degrees for marine cam selection

Post by F-BIRD'88 »

Merc Marine has a hyd roller from the Scorpion/Black Scorpion series ski boat engines. 350-383. vortecs, Tunnel ram efi 9.4:1
magnum manifolds. It works good.
Its a Crane cam.. Similar cam in the book.
GMPP also has it. wet ex good.
222-230 112 or 114 lsa .509-.528..
Its a hair bigger tham the GM LT4 hot cam
which will work also.
Merc tested.

We used to use the Crane hyd flat tappet version
Good results. Crane HMV278-2.
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Re: Cam overlap degrees for marine cam selection

Post by rustbucket79 »

What are you looking to accomplish with the different cam? Without any intended arrogance/malice, why do yo think your cam selection is an improvement over the untold research and development Mercury did on that engine?
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Re: Cam overlap degrees for marine cam selection

Post by F-BIRD'88 »

Is this boat thru the drive exhaust or thru the hull exhaust?
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Re: Cam overlap degrees for marine cam selection

Post by CamaroMan »

gr8 info - thats a fairly "large" cam ito duration but I have read some guy used the hotcam and had very good results.. ill save this on my data sheet-
F-BIRD'88 wrote: Sun Jul 18, 2021 3:09 pm Merc Marine has a hyd roller from the Scorpion/Black Scorpion series ski boat engines. 350-383. vortecs, Tunnel ram efi 9.4:1
magnum manifolds. It works good.
Its a Crane cam.. Similar cam in the book.
GMPP also has it. wet ex good.
222-230 112 or 114 lsa .509-.528..
Its a hair bigger tham the GM LT4 hot cam
which will work also.
Merc tested.
We used to use the Crane hyd flat tappet version
Good results. Crane HMV278-2.
CamaroMan
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Re: Cam overlap degrees for marine cam selection

Post by CamaroMan »

rustbucket79 wrote: Sun Jul 18, 2021 6:28 pm What are you looking to accomplish with the different cam? Without any intended arrogance/malice, why do yo think your cam selection is an improvement over the untold research and development Mercury did on that engine?
Hi - well for starters the "marine" cams are not always very different. I ended up going with the HT383 cam which in fact is merc marine cam, and very good results. Im building another 383 using the same cam- the torque hits perfect between 2500-3700 which is spot on for a marine 24ft boat..

Im also trying to gather some "math" on the numbers to id when a cam is in the green, yellow or red zone for marine use- there are so many options so part is research and part curiosity.. I want to experiment with some cams on my own builds and would be nice to see what performance the varying cams give..

no worries - i know merc knows their stuff, but they kinda do try make one engine for all.. the GM HT383 cam is 235 which is a great price, just the hotcam is pushing 400+ now!! so for that low of a price its a fantastic option for a marine 350 or 383 -

no offense taken and im sure not trying to come across smart or arrogant myself, just curious..
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Re: Cam overlap degrees for marine cam selection

Post by F-BIRD'88 »

Another cam that will work very well in this engine application.
Again similar to the GMPP LT-4 HotCam And that Crane/Mercruiser Black Scorpion Ski Boat engine cam

It is Melling Camshaft # 22280
283-287 220-224 @.050" .495"-502" 107/113° (110° LSA). Hyd roller
(Clevite- 229-2366) [Elgin- E-1871-P]
Use high performance valve springs.
The GM LS6 "blue springs" Beehive valve springs
Are just right.
You'll find the pricing on this Melling hyd roller cam very nice.
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Re: Cam overlap degrees for marine cam selection

Post by Geoff2 »

I would think max engine rpm, & where max tq is reqd, would be the first & foremost parameters to decide, THEN find a cam that suits.
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Re: Cam overlap degrees for marine cam selection

Post by F-BIRD'88 »

The Comp cams Durations are measured at a different height reference than other cams
.006" lifter rise VS typical SAE .004" lifter rise (.006" @valve)
Therefore you cannot compare the two based on the published "advertized" durations. There is typically about 8° ± difference.
You are better off going by the .050" lifter rise specs to compare various cams by application "shoe size" duration.
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Re: Cam overlap degrees for marine cam selection

Post by CamaroMan »

thanks fbird - on that cam recommendation, is there a dyno to view torque production? I had a similar cam in my 350 - GM hotcam 218/228 iirc - was incredible, smooth power and 16" idle vac- fantastic hotrod cam.. just the price is almost double the ht383-

another nice thing about the HT383 cam is stock valvetrain. Its a perfect match for a vortec build- no spendy parts up top.. id say for now its my go to cam at this stage-

I might throw together a 350 with my old roller cam and see how it performs in my 22ft searay-

I build a very nice little 350 using the 192 tbi heads (64cc swirl port?) - everything else the same, man that little bump in compression woke the whole boat up - effortless cruising and getting on plane. Huge difference between the nasty smog 70+cc heads of the 70s..

only issue is the whacky intake bolt pattern. Lucky I have 2 iron qjet manis i bought for 10 bucks each-
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Re: Cam overlap degrees for marine cam selection

Post by F-BIRD'88 »

The cams I suggested work great in I/O V8 sbc boats.
Yes cams with more than .470" valve lift need valve train mods on OEM stock vortec heads. The guide boss may need shortening the springs must be upgraded.
There are drop on spring+ retainer sets that allow .500"+ lift without guide machining. The GM and like Beehive valve springs with comp 787 retainer work great too.
Comp has a similar "blue spring" 918 series beehive spring.
GM puts these on their perf crate motors now.
The engine will make more power and torque especially at high rpm. More boat top speed.
Thus better valvesprings. Still very smooth power band with lots of torque.. Re-curve the diz. May need boat prop tuning to max tune the boat performance.
MORE Power and speed than the 383HT cam.

Melling has a copy of that cam too.

These cams work best with high perf Magnum ex manifolds, high rise dual plane intake, 750++ cfm carb and thru the hull high perf exhaust.
9.4 and up compression.
If you want to run 10:1 cr in your boat use strictly 92++ octane gas. The marine pump gas is usually only 91 octane at best.
Not the same as the good good 94 octane up here.
YA it matters, in a boat.
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Re: Cam overlap degrees for marine cam selection

Post by F-BIRD'88 »

Short of using vortec heads the 350 magnum mercruiser engines came with the GM "217" head. same as the 083 head from the 350 L-98 camaro TPI motor.
These gain a LOT of POWER when fully ported and upgraded to 2.02x 1.60 valves.
Way more power than a TBI swirl port head.
Comparable to vortecs when generously ported.
Same cams, high rise intake - 750++ cfm carb,magnum ex, valvetrain upgrades.

The 305 "081" head can be used also with porting + bigger valves chamber deshrouding... 58 open to 64cc.
That Melling 22280 cam will make a hair more power than the GM LT4hot cam. That Crane "Black Scorpion" 222-230 hyd roller will make a bit more yet on a built boat 350-383.
They all make great power and torque in a high perf 350-383 boat motor with high perf heads.
Last edited by F-BIRD'88 on Sat Jan 15, 2022 12:00 pm, edited 1 time in total.
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Re: Cam overlap degrees for marine cam selection

Post by CamKing »

You need to look at overlap at the seat duration, not the .050" duration.
If you look at seat duration, the Comp cam actually has a lot less overlap.
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Re: Cam overlap degrees for marine cam selection

Post by F-BIRD'88 »

None of the cams I suggested in no way cause water reversion problems on 350-383 sbc Marine engines with the Magnum ex manifolds and thru the hull high perf exhaust.
They don't have too much overlap.
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