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Chops
01-19-2001, 05:31 PM
Hey SBC Donzi guys- I just got off the phone with Eddie Marine and I'm a little disappointed. I'm told that if I use a silent choice system requiring the shorter stainless risers, I can't run a cam with over 230 exh duration. Supposedly the water outlet in the short riser is too close to the manifold and long cam overlaps cause a lot of water draw back if you don't run through transom full time.
Does anyone know a way around this? I really wanted a little more cam.

GEOO and BigGrizzly- I know you guys are gonna know this stuff. Heck GEOO, judging by old posts I'm just building your old 383.

BigGrizzly
01-19-2001, 06:08 PM
I don't agree, bigest problem is backing up hard. and slowing down too fast and bouncing at idle. 230 is a healthy hit if done right. They gave you a round number Use a high rise exhaust then drop it fast and leave the choice open when stopped. HOw much hp do you want to get. As much as possible is the wrong answer. Give a real number.

Randy.

[This message has been edited by BigGrizzly (edited 01-20-2001).]

RickR
01-19-2001, 07:31 PM
Chops
It's called reversion. Do a search on previous posts.
Essentially moisture is drawn back into the cylinders because of cam overlap.Which leaks down into cylinders causing cylinder wall corrosion. It's a BIG problem with HiPerf Marine engines.

I plan on keeping the Silent Choice on my 22 w/454 Mag so I went with the Gil Magnum Choice Exhaust (from CP Performance) which has a steeper riser angle and Turbolators (a Gil product which aides in preventing reversion)

BTW The guys @ CP (Rick) have been very helpful, good prices and quick shipping. Eddie@ Eddies is good also.

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RICKR
mailto:riggerb@aol.comriggerb@aol.com</A>

GEOO
01-19-2001, 08:22 PM
Chops,
I wouldn't fool around with a chance of reversion. Salt water sucked back to the heads is bad news. Find an different exhaust or say with a smaller cam. GEOO

BigGrizzly
01-20-2001, 01:41 AM
Chops , I have been thinking about this for a couple of hours. Are you shore you want to go that long on duration. There are many things involved besides reversion. The better the hedder the less chance of reversion. I'm not going get into another reversion discussion, my views differ than some of the others, whom I have a tremendous respect.
Look at the CMI elbow top hedders the high elbow drops down sharply, the primary tubes are equal,a very big plus, and were designed for silent choice replacement exhaust. All other brands are cookie cutter types copies of each other(un equal primarys). Remenber I said equal,that means equal length and equal volume of the primary tubes. I had enough experience with big cams and inadiquit exhausts. I'm in this delema at this moment. I spent all my money on a engine mods and I have none for a really good exhaust.
If you use that big of a cam you need at least 10 to 1 compression and good combustion chambers. I got this bad feeling your looking at a bottom end cam!! Let me know what your looking at and the real purpose behind your endevor. I'll give you all the info. I can.

[This message has been edited by BigGrizzly (edited 01-20-2001).]

doug
01-20-2001, 07:51 AM
Big Grizzly,Rick or somebody
I have a 18 2+3 it's supposed to have been ordered with a higher X dim but I have yet to confirm it. The hatch cover has clearance mold
in it from the factory for the exhaust it came with a 300 tempest It has Gil cast exhaust manifolds into 4." steel tubes to the
transom the exhaust comes out just under the deck higher the most I've seen. My question is
should there be some kind of flappers? will water get in there? or are they high anoff on the transom?
The cam has 292.intake and 300. exhaust.
Thanks you guy are great.
Doug
Ps: Don't you guys think the price of the
hydraulic steering is to High.





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old Yeller

Chops
01-20-2001, 10:13 AM
Thanks everybody,

Grizzly,
Here's the deal: Notice Doug's response after yours on this post. That's what I remember about street cams from 15 years ago- Anything under 292 was strictly grocery procurement. I guess I just don't get why 230 seems like such a big number, especially given that roller is typically smoother. You are unquestionably correct concerning equipment matching. Comp. ratios, chamber sizes, port and valve sizes, and cam specs have got to work together. I've been a faithful follower of Smokey Yunick for many years and Dennis Moore has some solid sounding ideas too (a little conservative but solid just the same). Here's what I'm thinking: 383, 9.5:1 or 10:1, 2.02/1.60, 64cc chambers, w/200cc (or near) runners, Crane roller #119661 which is a 230int/238exh @ .539/.558. The exhaust thing still bugs me. Thanks for the tip on CMI elbow tops. Nobody seems to talk much about equal length primaries in marine circles but it's always been huge on street motors. If I'm all washed up on this stuff just tell me, I'm not proud (or rich- I can't afford to build an ignorant motor before the good one). I'd appreciate anything you've got. chops@snet.net

RickR,
Thanks for the tip on Gil. I'd forgotten about the riser height choices. Imco's got something similar.

RickR
01-20-2001, 10:19 AM
Doug

I am not familiar with SBC but I use flappers to prevent water ingestion due to stopping too fast or backing up.

Where is the water added to your exhaust? If water is added near the transom you should not have a "cam reversion" problem. That type of riser (Gil Offshore) is designed for larger cams.

If your Gil manifolds do not have Turbolators (CP $27 each)I would add them. I'd rather spend $54 than have to buy a new block.
The rule of thumb for BBC is 230 degrees also for short risers.

Grizz
I'd have CMI's or Kieth Eckharts if I could afford them http://www.donzi.net/ubb/eek.gif. Nothing like a little scavenging to build power! Sure worked on my ZX-10 Ninja.

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RICKR
mailto:riggerb@aol.comriggerb@aol.com</A>

BigGrizzly
01-20-2001, 01:25 PM
Doug, that cam sounds like the old but good Crane cam, if it is the one I'm thinking of it was a solid lifter with about 104- 105 lobe centers and around 540 and 560 lift. Tom, asprint car friend of mine sweares by this cam. If it is the same cam at 3200rpms it will push you back in the seat then leg out to about 5500 plus rpms. the real duration at .050" is around 250-260 duration. Best put flappers on that boy. nice unit thoe

BigGrizzly
01-20-2001, 02:29 PM
Chops, you got it right. when people talk duration here we normally talk at 0.050 lift. With rollers you need to talk at 0.050 because of the roller action. Most of the boat manufacturers make mild cams engines so customers don't beat the snot out of the motor before the warranty runs out. Marine users run fast and hard with the knowlege to save the motor. " It ran at 5000 all day yesterday and didn't break so what's the harm". I heard this many times just before the repower job. You actually did your homework. that cam is the marine version of the retrofit roller. they have done the same thing Isky has done, changed the lobe centers and did the Dual profile trick. All my ford stuff loves that and my new 502 likes it too! This broadens your powerband and idles better. Equal primaries belive it or not it keeps reversion down, just think about it un equal pulses, not good. I am not talking water but air reversion. Right now there is a reduced price on the CMI elbow top type. My opinion is a few hundred more now saves many hundred when you replace them later-you can't get a good price for used hedders.
No one that I know of has had water reversion that destroyed an engine that wasn't related to smothing else first. Here I go again, if you want more info on the subject e-mail me.
Rick you and I are in the same boat (no pun intended) we want the CMI headder but have no extra money. BTW my money problem is your fault. You said you were looking for a 502, sounded good to me. Sooooo I found one, that was only the beginning. I did the Randy thing just like my wife said I would. Oh well maybe next year, or maybe sell the Corsican, not. Just kidding I was going to do the engine thing the day I bought the Criterion.

Randy

[This message has been edited by BigGrizzly (edited 01-20-2001).]

GeneD
01-21-2001, 07:19 AM
Dudes, once again...you have me lost.
What is this about the Silent Choice requiring the "shorter" stainless steel risers?
What SST risers? Or should I say which?
Are you refering to the Mercruiser SST manifolds? If so, how are they shorter? They appear to be the same height and length as the originals.
Now if you are talking about a height "shortness", wouldn't a riser spacer do the trick?
I am concerned because I have the SST risers and a 1" spacer on 007. AND no flappers in my pipes, (long gone guys!). I have never noticed a problem with reversion, of course my cam is stock. And backing up is no problem either considering she hardly sees ocean, and when she does, she is not backing up very quickly!
What's the deal?

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GeneD
007
Melbourne, Florida

doug
01-21-2001, 08:20 AM
Griz,
Yes,that was advertized duration. At .050 lift on the profile sheet it is 231 intake
238 exhaust, IN. TDC .068, gross .351,
EX. .061 gross .364, 112.25 L.C. Hyd Roller.
At 5700 I backed off on it do to lack of seat
time and lake, it should turn 6300 but will
see if that's true.
Rick,
I dont know what a turbolater is, could it be
the 2 things that are some kind of muffler or baffle that bolt into the tips? The water comes in about 6." above the case manifold.
Looks like I will call Gil and find out what it is that I have. I might pull the cover of
and take a photo or two.
Doug



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old Yeller

doug
01-21-2001, 08:31 AM
Gene,
I dont think I could fit Silent Choice
with the risers I have there is about 2."
between them and the transom.
Doug

RickR
01-21-2001, 08:50 AM
Doug
Turbolator
http://www.gilmarine.com/products/exsys/access.html Top of pic on left. They fit in between the riser and manifold (Stainless) if you look closely you should be able to see it sandwiched between riser gaskets.
GeneD
The Silent Choice Valve Assembly is right behind the stock riser. To fit the Gil Magnum Choice riser you shorten the Silent Choice Valve Assembly . There is no room for the longer length riser needed to prevent reversion.

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RICKR
mailto:riggerb@aol.comriggerb@aol.com</A>

[This message has been edited by RickR (edited 01-21-2001).]

BigGrizzly
01-21-2001, 09:00 AM
Numbers are wierd could you scan the spec sheet. also whar rocker ratio are you using?
GeneD, if you use Silent choice your tail pipe is short (it sits at the riser of most boats). When you close the diverter your exhaust hits the door and backs up before heading down to the Y pipe and is restricted at the lower unit. Kind of like putting uour hand over your exhaust pipe. At overlap there is a chance that water could go back to the cylinder. So the theory goes. I you really wan't to know my true though we will discuss it at happy hour in Miami.

Randy

Chops
01-21-2001, 02:42 PM
OK, I priced the CMI elbow tops. Outdrives.com wants $2,500.00 and that's supposedly on sale. Anyone know a cheaper source? I love the equal length primary deal but that's a big chunk O change! Gil offshore sounds like it might be the next best thing.

BTW the advertised vs .050 lift on cam specs was a huge short circuit on my part. I never bothered to look at the advertised on the roller conversion cams.

RickR
01-21-2001, 09:52 PM
Chops
I am not famaliar with CMI pricing, check Rex Marine @ http://www.rexmar.com.
Don't forget to add in the price of Oil Filter, Shift Cable and Electronics Brackets.


Price for Gil SBC Magnum Choice with brackets is $1900 through CP. When i installed the Gils on my Silent Choice I had to modify (shorten) the Silent Choice Diverters.

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RICKR
mailto:riggerb@aol.comriggerb@aol.com</A>

ALLAN BROWN
01-22-2001, 09:56 AM
I AM IMPRESSED WITH THE GENERAL KNOWLEDGE OF THE MOTORHEADS ON THE BOARDS. HERE IS THE SCOOP! REVERSION ONLY OCCURS AT IDLE RPM. IT IS A FUNCTION OF THE DURATION OF THE CAMSHAFT (MEASURED AT .050"). OUR NUMBER FOR STANDARD LENGTH RISERS IS 224 DEGREES MAXIMUM DURATION. FOR A FREE-FLOWING EXHAUST, YOU WILL DEFINITELY REVERT AT IDLE SPEED IF YOU EXCEED THAT. THE REASON THAT EVERYONE HAS A DIFFERENT IDEA OF THE SITUATION IS THAT, A. MERCURY RISERS ARE SO RESTRICTIVE THAT GAS VELOCITIES ARE DOUBLE, HENCE NO REVERSION, AND B., SOME PEOPLE DON'T IDLE AT ALL, AND NEVER KNOW THAT THEY HAVE IT. REVERSION IS SNEAKY! IT COMES IN IN MICRO-DROPS, AND, IF YOU IDLE FAR ENOUGH, THE TINY DROPS GET TOGETHER AND FORM PUDDLES. IT ALSO OCCURS AT THE TIME THAT BOTH GAS VELOCITY AND TEMPERATURE ARE LOW. IT IS AVOIDED BY DUMPING THE WATER FURTHER DOWNSTREAM, SO THAT THE EXHAUST PULSES, WHICH JERK BACK AND FORTH AT IDLE SPEED, CAN'T JERK IT BACK FAR ENOUGH TO THROW IT DOWN THE THROAT OF THE MANIFOLD, AND INTO THE CYLINDERS.

ALLAN BROWN
DIRECTOR OF PRODUCT DEVELOPMENT
STAINLESS MARINE

Len
01-22-2001, 12:37 PM
Anyone,...This is probably a stupid question but one that I don't know. Is the (reversion)water that we are afraid of sucking into the engine coming from OUR cooling water that we dump into the exhaust inside the boat OR the water that is available for pick up on the outside of the transom through the exhaust thru-hull exits? (eg. the ocean water at an idle with tips close to/under the waterline outside of the hull?)

ALLAN BROWN
01-22-2001, 02:43 PM
COOLING WATER