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Comp CS268XFI HR13 Reversion in SBC 383

Comp CS268XFI HR13 Reversion in SBC 383

Old 05-26-2019, 09:32 PM
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Question Comp CS268XFI HR13 Reversion in SBC 383

I have blown my head gaskets in my engine in the past. So this year am trying to make sure everything is copacetic

I just ran the engine on a stand and I'm getting water in my risers and exhaust manifolds. I ran it for about 3 minutes mostly at idle with a couple of light rev's to maybe 1800 or so rpm.
What's puzzling me is another guy who's active in here has that cam in a 350, so I took it as safe. Maybe it has something to do with my engine being a little more displacement, or maybe the compression has some effect. My compression is 10:1. I run high test fuel by the way.
I have looked through Comp cam's spec sheets and their recommended max cam for stern drive has one less degree of lobe separation, and one more degree of cam centerline. So in those specs the cams are darn close. What is different is that my cam has more valve lift. It has .570/.565 vs the .495/.503 of their max sterndrive cam.

I will happily change out the cam to be done with this issue. But I already have some performance parts and I hate to miss out on optimizing them. I have some EMI stainless extra height long tube risers, and I have some Scorpion 1.6:1 roller rockers.
I'm thinking of getting the Comp XM270 HR to reduce the lift, but using the 1.6 rockers since I have the better exhaust.


Roast me.
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Old 05-26-2019, 09:49 PM
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268XFI HR13

Advertised Duration at .006": 268 int./276 exh
Duration at 050 inch Lift:218 int./224 exh.
Lobe Separation (degrees):113
46 degree overlap at seat duration of .006"
-5 deg overlap at .050" duration
Lift with 1.5 rockers: .534 int./ .530 exh
Lift with 1.6 rockers: .570 int./.565 exh

COMP Cams Xtreme Marine XM270HR


Advertised Duration at .006": 270 int./276 exh
Duration at 050 inch Lift: 218 int./224 exh.
Lobe Separation (degrees):112
49 degree overlap at seat duration of .006"
-3 deg overlap at .050"
Lift with 1.5 rockers: .495 int./.503 exh.
Lift with 1.6 rockers: .528 int / .537 exh

-------------------------------------------------------------------------------------------

what do you have for exhaust on it now before those EMI w/long risers? I forget ? Or is this what it is reverting with ?

Last edited by SB; 05-26-2019 at 10:25 PM.
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Old 05-27-2019, 05:44 AM
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Look at the 270, it's effectively our cam. What can I be missing?

I bought the EMI pipes for the cam. They have always run together. 3 summers, 3 HG's. I have a stock exhaust sitting on my takeout 350 Mag, but using that doesn't make any sense.
I'm afraid I have to get a smaller cam so I can idle without reversion.

BTW I just put on a DUI ignition and it's stellar. The engine fires instantly and with authority.

Also, in the interest of full disclosure. I ran it yesterday afternoon. using city water pressure to the Mercruiser thermostat housing. Pulled the risers, they were wet by the manifolds. Then I looked down the manifold exits and they were wet. Then I removed the exhaust manifolds, and a few teaspoons of water fell out.

Around 2 hours later I decided I would blow out the combustion chambers. No water was seen.
Am I making a mountain out of a molehill? Don't think so.

Last edited by NHGuy; 05-27-2019 at 06:46 AM.
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Old 05-28-2019, 11:55 AM
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OK, time for confession. Hopefully someone else can learn from my mistake.
I had my Cloyes 3 way crank sprocket in at 4 degrees retard. I'll be installing it straight up and will report back.
What a friggin dope.
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Old 05-28-2019, 01:07 PM
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Do you have a degree wheel ? Top of my head cam should be installed so intake centerline is 109 icl and exhaust centerline is 117 ecl.
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Old 05-28-2019, 01:30 PM
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BTW: Why do you feel this hurt the headgasket ? I would think something else did that. And what part of the headgasket was hurt ?
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Old 05-28-2019, 03:47 PM
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This is theory til proved. But here's my conjecture.
Retarded cam timing allowed reversion, water in combustion chambers caused compression to be too high. High compression overcame head gaskets seals.
My engine use could have contributed too. I spend 10 or more minutes idling in and out of the marina. Since my brother bought my dad's place I have been racking the boat at West Alton Marina where there's a few hundred yards run out to the lake. Plus there are some times we'd just putt.
I did not know I had this issue and changed my distributor...I was theorizing that the Thunderbolt V knock sensor may have been listening for a different frequency than the one my forged 383 would produce.
If the knock sensor was wrong it would send the timing ahead which would cause preignition and/or detonation. No proof of that though. I never did confirm which sensor is used in the later Mercruiser 383's and 377's.
I'm going to work Wednesday and Thursday, but I'm thinking of taking a personal day Friday to do a run test on my engine, that way I can install it Friday and Sunday.
I don't see a way to get it installed On Friday unless I can get a helper. Setting up my gantry takes time.
Maybe I'll pay a tow truck to install it to save some time. Or maybe the marine repair up the street will drop it in for me.
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Old 05-28-2019, 04:11 PM
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I was trying to degree the cam by using a piston stop to find TDC and I could not feel the piston hitting my POS piston stop. I was partway through degreeing when I came across the cam timing mistake. My heads are on and they are a pain to remove and reinstall.
To get my quench I use steel shim gaskets. Year 1 I put the shim HG's in dry and ran it, failed. Year 2 I put in cometic mls gaskets and gave up I think about half a point of compression. I did not get the heads surfaced, although I did check them with a (non professional) straightedge. That put my quench outside of ideal, but not horrible. Maybe they were imperfect then. Can't go back now. This year I had my heads surfaced by a machinist we use at my work. They took, I think he said .003" to get flat. I installed them this time with copper coat, and retorqued them after a heat cycle. So I want to leave them on.

So I jumped over the degreeing. Hope I'm not being dumb.
The gaskets looked like the heads got pushed up. The heads had wet marks on them. I'm an administrator and sales guy at a repair and tire company. I'm not a pro, so I can't say where the HG's failed
BTW the heads are Dart Iron Eagle. I did some porting to them this spring since they came with small ports that had a little bit of shrouding So I removed the shrouds and opened the runners a touch while port matching them to my intake.
Fingers are crossed.

Last edited by NHGuy; 05-28-2019 at 04:19 PM.
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Old 05-28-2019, 04:27 PM
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I can tell you I have the short length Eddie Marine risers and we do long put put's at times too. In your area We used to eat at Sandy Point all the time...and still Pop's Clam Shack since the day we hunted Dinosaurs. LOL.

We go to Braun Bay all the time and have to idle under long island bridge which can be a while......we also fit under the short bridge in Wolfeboro so we do the long idle under that to the Wolfetrap restaurant.

I bring it too 800-900rpm in gear....I think ? No wake at all though....anyway, still super slow speed wise...small prop....

I have TBIV. Set to 12* initial and has 34 total.
As you know 5.7, same cam as you. I used a timing wheel. It came in at 110* ICL (1* from where it was supposed to), so I left it.
GGB mufflers slid into the barely out of water tips. They go under water a lot. He made me custom sized since my tips are only 3" diameter. Boat too small for 4" tips....had to cut the 3" tip's flanges as they where. He.He. SO, yes, risers are 4" so I have 4" to 3" reducers before the tips. I thought this and the ugly mufflers would bottle up water but they don't. Maybe the mufflers dampen exhaust pulses enough ? I dunno....tons of people run the XM270 with extended inner risers using stock or glm and have no issues.

It just maybe your TBV having to much timing initial...as far as the reversion......but causing headgaskets ??? I have seen a few engines with bent connecting rods from too much water getting in......but we did not find that until rebuild times on those motors...Doh !

Only time I have had to replace headgaskets on any marine engine for any person on the lake where for engines overheating and burning the part of ringland that runs in between cylinders. On small blocks this is usually in between the center cylinders because of the exhaust ports being siamesed and this part overheating before rest of motor. Or, detonation of course, typically from supercharger or bad injector or etc.....

This is my exact manifold/riser but color. Mine are black.


Last edited by SB; 05-28-2019 at 04:40 PM.
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Old 05-28-2019, 04:41 PM
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Okay, made a few edits above ^^^.
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