Camshaft and reversion?
 

Would this camshaft create reversion in a 454 w/ EMI and stainless risers?

Comp Cam 11-420-8 Hydraulic roller
215/215 @.050, .566/.566, 110* LSA

The cam is already installed in a recently purchased boat

Thanks for the input.

Paul

You should be just fine. Stock 454 May is 224/224 at 0.050 with 114 LS degrees if my memory serves me.

Mine are 232/234 with 114 LS. Stainless Marine with Captain's call.

Your cam seem pretty mild.

I'm not so sure...now I don't know my cam specs really well, but 110* lsa should give it a really good lope. I'm thinking that the cam will revert.

I'm with bajalion, he doesn't overlap at 0.050 (-5*) with that little duration. Shouldn't be a problem.

I asked this same question a couple of weeks back.

Lots of people responded, and I got quite an education on cam duration, lobe separation angle, and overlap.
I also spoke with two camshaft manufactures on the subject...

bajalion is correct on two counts:
A 454mag or 502mag Cam is 224°/224°, 114°

With a duration of 215°/215° even with 110° lobe separation, you should not have any problems.

Overlap Calculation
 

Just to set the record straight:

Overlap is proportional to Reversion

Overlap is the time that both intake and exhaust valves are open at the same time.

LC = Lobe center
TDC = Top dead center
CL = Centerline (from TDC)
IND = Intake duration @ 0.050 tappet height
EXD = Exhaust duration @ 0.050 tappet height

LC = (Exhaust CL + Intake CL)/2

Overlap (at 0.50) = ((IND + EXD)/2) - (2 x LC)

So at 224/224 with 114 LC = negative 4 degrees overlap

215/215 with 110 LC = negative 5 degrees

228/232 with 114 LC = 2 degrees

Please correct me if I'm wrong.

No problem!!!:)

Re: Overlap Calculation
 

You are quite categorically WRONG!:D

(That was fun.:D :D :D )

Reversion is a function of overlap AND the distance from the cylinder that the exhaust mixes with the water. In cars and marine dry exhaust setups, there is never an issue with reversion. Some wet exhaust systems will mix the hot gases further down the line than others. You can also modify some exhaust setups to mix further from the engine.

For instance, EMI suggests for their Thunder system you keep duration under 230 with a 114 LSA. They get a little more liberal with the stainless riser setup. BTW -- this sounds like you would be fine, but I wouldn't trust my memory and you shouldn't either. Give them a call, and their tech support people will help you out.

If you end up with negative 5°, is there any overlap?
Without both valves open, how you going to suck water in?

May I join in? I think you most certainly can have intake/exhaust reversion in any dry system....it's just not water reversion. I read this in Chevy HiPerf Mag about 45years ago.-non-member smilie-

Re: Re: Overlap Calculation
 

:D

WELL I EXCUSE ME (quote from Steve Martin)

I held the exhaust as a constant and the 454/502 Mag cam as the reference.

I almost went on about exhaust but I figured some intelligent soul would do this.

FYI Look up proportional

:)

How about 226/235 with 108° and wet exhaust... runs and sounds great!!!

As LC decreases and duration increases the overlap increases My last example (my cam I had to look it up) has 2 degrees of overlap at 0.050. At 0.050 is where the lift become significant

Overlap can be calculated in the overall sense. My cam is as follows:

Intake duration @ 0.050 = 228
Intake duration overall = 280

Exh duration @ 0.050 = 232
Exh duration overall = 284

Overlap @ 0.050 = +2 degrees
Overlap total = 54 degrees

I also run Stainless Marine Captain's call (short)

14.5 You better have long (kingsize) risers!

And then the cam timing comes into play too.

I'm lost.:(

What about 216/228 112 Lsa?

Thanks for the replies,
First off, I did talk to Comp Cams.... not much help, more interested in selling me a new cam. B.S.

Camshafts to me are like an unknown science. I have water in the oil and just trying to rule out variables. Any more info would be great. Keep it coming!

Bajalion,
I am an engineer(propeller head) and very interested in the origin of your formulas for cam overlap.

If there is negative overlap, I assume the chance of reversion is eliminated?

Paul

If it is coming from reversion you will see it on your spark plugs. Pull one right after letting the motor idle and see if you see residue.

:eek: OUCH!!! I'm busted!!! Technically, I think you are correct, but as a practical matter, it isn't likely. In a dry exhaust system, I have heard that the danger is sucking a bit of cold air into a hot cylinder and warping a valve. The chances are fairly slim if you have a properly working exhaust system. However, if you have an exhaust leak near the cylinder or a great deal of overlap and a large amount intake vacuum with a short exhaust system such as a set of "zoomies" it could happen.

I stand (sit) corrected. :drool: :drool: :drool:

By the way, Bajalion -- overlap CAN be proportional to reversion. Your statement was "IS proportional". But then I guess it depends on what your definition of what the word "IS" is.:D :D :D

(Someone please kill me -- I just quoted a Democrat.:D )

hey guys I'm not here to make judgements just a little education on Cam profile and what it is.

KAAMASCARAB30 and Formula31 are both correct you have other factors. I understand engine attitude and exhaust attitude are also involved. Formula31's knowledge of boats and engines makes me look like a dunce.

Cuda, Unless you have standard merc exhaust I bet your OK. Even if so your still pretty mild.

The best way to check for reversion is to pull off a riser after idling as low as she'll go for a while. Turn off, remove a riser. Do you see water down in the bottom of the manifold? If not no problem.

If you have water passing through the manifold to the riser (no hose bypass). You may need to drain the manifold first.

PB-reversion probably would not cause water in the oil. A leaky head gasket will.

Of course a rod through the side of the block will do it too:D :D :D but I guess that's my problem

Vette,
I did pull the plugs and look at them, however the engine was cold.
There was an indication of water droplets on a few of the plugs. I also cranked the engine with the plugs out and about a 1/4 teaspoon of water came from a port side plug hole. It shot all over the side of the engine compartment.

When I pulled the starboard side manifold, the back two exhaust ports were wet inside. Port side manifold was dry. Also my engine guy had relayed to me that the intake valves and seats were horribly pitted and need replacement.


Paul

I have run cams as large as 228/235 @ .050" on 112 LSA and have had no problems with stock manifolds. Where is your idle set? If it is just barely running the possiblity of reversion is even greater. Even on a 110 LSA 215 duration is really short and shouldn't be pulling back very hard on the exhaust. It is amazing how much condensation is produced when a motor is first fired. You should double check the plugs once the motor is up to temp and run for a while.

Sutphen30,
Heads are already off being magnafluxed and pressure tested.

Paul

Not wanting to beat a dead horse, but overlap is the time in crankshaft degrees that both the intake and exhaust valves are open. It is not an absolute number, but is measured at a specific amount of lift. For example, overlap at 0.050" lift, .006" lift, .004" lift. Its very hard to measure overlap at 0" lift because there is no overlapat 0", but a fraction off of 0" is your largest overlap. See Bajalion's example.

I have no hard evidence for this, but I have heard that marine engines should have less than 7* of overlap at 0.050" (assuming riser to waterline height is correct). If you look at the .050" overlap of stock Merc cams, you'll find that they don't overlap at 0.050". I'm shooting for 1* at .050 for my 383 thru prop: 218/224@110

Cam Profile
 

FYI attached is the sheet PB requested.

I have always run a 224 , 232 at 114 cam, #1 intake at 106 which is about +4 timing, with no reversion on stock Gil magnum choice risers BBC which are as close as a stock Merc. As was already stated, if you are close, you can increase your idle speed. Dont go too high if you have a Bravo, it wont like it.

Two cams using the same LSA can have very different overlap numbers because overlap is a function of the duration of the intake and exhaust lobes. Here is an article that may help clarify the subject.
http://www.chevyhiperformance.com/te...98/index1.html
Hope this helps,
Bob

Thanks, cool site!!!

As you increase intake/exhaust duration and/or decrease LSA you increase the number of crankshaft degrees that both the intake and exhaust valve are both open. Reversion occurs when both valves are open and the pressure area (pulse)downstream of the exhaust valve is greater than upstream (combustion chamber, intake port, manifold) The longer a period this condition occurs (increased duration/overlap) the greater the opportunity for water in the exhaust passage (mostly from manifold cooling water exiting into the exhaust stream) to be drawn into the combustion chamber. This is an extreme simplification but hopefully it will help explain that reversion occurs due to a "spring effect" beginning at ambient above the carburetor and ending at ambient at the exhaust exit.
Hope this helps,
Bob

LOL!!! Dont ask me how this cam works,,, but it is wonderful thing...It was made by a very well known cam mfg for his family boat,,,said he had to have a cam that sounded "good" and just happen to make two of em, Just running stock exhaust...I can tell you that the exhaust closes 7° before TDC and intake opens 8° after TDC ,,,,, intake closes 38° BDC and exhaust opens 48° after BDC.

And, its not just how long the intake and exhaust valves are open together, its also what the piston is doing at that time. Some cams and timing will actually create a cloud of gas vapor above the carb at idle. Its also a condition called reversion.

Just a little more info from the cam card,

Valve timing at .006:
open close
intake 29 BTDC 61 ABDC
exhaust 69 BBDC 21 ATDC

These specs are for cam installed at 106 center line.

A little lesson regarding this info would be appreciated.


Paul

This could be an online ITT tech course. Great info, but one question. Are we just splitting hairs by trying to push the cam specs just below reversion levels or is the HP gains that significant. Is there a HP calculator for cams and doesn't intake, carb and heads have as much to do with cam characteristics as exhaust? Aren't they all support each other. How much HP gain can be achieved from stock cam to performance?

Hey Chris, it doesn't matter what cam you put in it if it's gonna be a trailer queen!:D :D

When ya comin' back out to play?

nautical ILLUSIONS,
Because reversion is inherent in nearly all engines the object is not to eliminate the effect but to keep it within acceptable levels depending upon the environment or application. When a passenger car is idling smoothly essentially you are "hearing" very low levels of reversion due to a limited number of degrees of crankshaft rotation where the intake and exhaust valve are open at the same time. However when you listen to a sprint car or pro stocker that " choppy" idle is caused by camshaft with a generous amount of overlap. The difference here is the sprint/ pro stocker ( cammed primarily for relatively narrow power bands) doesn't have to idle with water being introduced into the exhaust path providing that water an opportunity to "revert" back into the combustion chamber. The challenge in the design and selection of marine camshafts is managing the additional variable in addition to the proper component selection.
Hope this helps,
Bob

cuda, how are ya. Ya this has been a dismal boat season. A total write off actually, but next year I'll be ready. I got a pair of Bravos on the way and I think I'll throw a pair of ext. boxes on them. That with a few engine massages and that shouldl be 80+ for me. You'll have to take a rip over this way some time and we'll rip up to Daytona.