Powerboat Forums at SpeedWake banner
Cancel
Create thread New Forums

Bad start to the season, bad gas.

2 reading
Jump to Latest
61 - 80 of 98 Posts
#61 ·
Good info.

Looks like the issue is found.

And your question about how much power you lose by reducing the compression - "Who cares. The motor is way more likely to stay together." :shocked:

Technical answer: If the camshaft used is within the guidelines of the compression used then it can be figured approx 3% power loss or so for a point of comprssion.

580 X .97 = 562.6 .

Rough guidelines of course, but even if you lose 30hp or I don't think that it should be worried about.

The engine will still make real good power and not be so high strung.

Your present cam should be fine, if your exhaust can handle it I would consider a little less lobe speration (will cause some more overlap) and go to 112LSA from your present 114LSA.
 
#62 ·
cut/paste from some chevy dealership website


"BBC Mark IV Cast Iron, Open Chamber, Rectangular Port, 1968-90.
This is Chevrolet’s best cast iron cylinder head for high-performance big-block V8s. It offers exceptional power at an economical price. This casting has 118cc open combustion chambers that reduce shrouding at high valve lift and improve engine airflow. The seats are ground for large 2.19” diameter intake valves and 1.88” exhausts. This high-performance head has rectangular intake ports and square exhaust ports."
 
Discussion starter · #63 ·
10.5-1!!!. It looks like this is going to be the number, but still waiting on exact figure. This motor was 9.8-1 in my 19' jetboat and then transfered to this boat, amazingly it lasted 170 hours before I had problems. Last spring we rebuilt the motor going from a flat tappet jet boat cam to a full hyd roller marine cam, single plane intake and a goal of dropping the CR below 9.5-1. Obviously the wrong pistons were installed at that time.

The combination of ceramic coated pistons, a good dyno setup and Shell super allowed it to survive 30+ hours without a problem until the end of last season. That's when I filled it with an offbrand super gas and stored it for 5 months. When I ran it good friday I probably didn't have the octane needed and that was that. Hole in piston.

My plan is to unshroud the valves which should gain a couple of cc's and hopefully a few HP. Then cut the dome on the pistons down enough to get 9.4 or 9.45-1 CR.

Engine guy is at the dyno with 4 circle track motors and can't be reached.

We still havn't discussed who is going to pay for what. That should be interesting.
 
Discussion starter · #64 ·
Tinkerer said:
It doesn't look like detonation to me.
The hole would be under the plug.

it looks to me like the top ring stuck and it broke a piece off the top of the piston and that piece got in the quench area and punched a hole in the piston.

Did you say that this is a NEW or rebuilt engine.
Was this the first time out?

Was the top ring on that piston broken?
I am still leaning toward too tight of a top ring gap and it siezed in the bore and broke a piece off the top of the piston.

Was the engine built by a CAR engine rebuilder or one that KNOWS how to rebuild a MARINE engine???
Click to expand...
Tinkerer,

Top ring is complete, however I think you are right about it pushing a piece of aluminum into the quench area and hence the hole. The motor has about 34 hours on it.

Engine guy is primarily a race car motor guy, but has built a lot of boat motors. I think the CR thing was a major screw up as we were supposed to go from a 9.8-1 down to less than 9.5-1 at last rebuild.
 

Attachments

Discussion starter · #67 ·
CFM, 87pachanga22, Tinkerer, dominator scott, Aggressor Tom, simulatedjim, PropNut, wuped.

Thanks to all of you for taking the time to help me figure this out, I'll keep you informed.
 
Discussion starter · #68 ·
cfm said:
Good info.

Looks like the issue is found.

And your question about how much power you lose by reducing the compression - "Who cares. The motor is way more likely to stay together." :shocked:

Technical answer: If the camshaft used is within the guidelines of the compression used then it can be figured approx 3% power loss or so for a point of comprssion.

580 X .97 = 562.6 .

Rough guidelines of course, but even if you lose 30hp or I don't think that it should be worried about.

The engine will still make real good power and not be so high strung.

Your present cam should be fine, if your exhaust can handle it I would consider a little less lobe speration (will cause some more overlap) and go to 112LSA from your present 114LSA.
Click to expand...
CFM,

Not to worried about losing 30HP except that I don't want to get to the point where I have to re-prop. Hoping that unshrouding the valves will help a little. What do you think?
 
#69 ·
Interesting.

I can't even begin to tell you how many times this compression thing has bitten people in the butt.

Modern technology, in regards to cylinder heads, camshafts, intake manifolds, and cid's has made it possible to make huge power without having to resort to the old methods of big compression and huge cams.

Well, I'm glad you finally found what is eating your motors and I hope that you learn from this and go conservative in the next round.
 
Discussion starter · #71 ·
cfm said:
Priorties - I'd rather replace a prop than damage a motor.
Click to expand...
We are going down on CR, just wondering if unshrouding valves + shaving domes is a good way to go, or should I insist on new pistons to get CR down..
 
#72 ·
Some pistons can be cut a certain amt and some can't.

To find out you need to call J&E with the dimensions you need to remove.

Unshrouding the valves is a good way to improve flow and add chamber CC's. This must be done by someone that knows how to do it correctly - talking flow here.

I'm not one that likes material removed in between the valves. Makes it way to easy for the incoming intake gases to go right out the exhaust while at rpm. At idle and low speeds, the reversion (exhaust going backwards and into intake) can become greater because of the same thing.
 
#73 ·
Dont change the pistons - unshroud the valves-
You will gain the CC's that you need and add HP at the same time.

Any chance that you had an intake air leak at the intake gasket for that cylinder?

Looks to me like that piston got a lot hotter than the others. That extra heat may have caused the top ring to sieze in the bore and then take off the top of the piston in the weak areas.

All of the times that I have seen a hole get burned in the top of a piston due to lean condition or detonation it is always under the spark plug.
 
Discussion starter · #74 ·
Tinkerer said:
Dont change the pistons - unshroud the valves-
You will gain the CC's that you need and add HP at the same time.

Any chance that you had an intake air leak at the intake gasket for that cylinder?

Looks to me like that piston got a lot hotter than the others. That extra heat may have caused the top ring to sieze in the bore and then take off the top of the piston in the weak areas.

All of the times that I have seen a hole get burned in the top of a piston due to lean condition or detonation it is always under the spark plug.
Click to expand...
Tinkerer,

I thought about an air leak, it was too late for a spray test, however I looked at the intake, gasket and head in that area and didn't see any obvious problem. All the intake bolts were snug when I took it apart.

The bore was damaged in that area which supports your theory of the ring hanging up and peeling a piece of the piston up. The river water temp was very cool, I am going to check the thermostat for proper operation today.

It seems possible that there was more than one thing going on here. The water temp guage showed 160F or so when I felt the misfire. I am running a crossover system with a bypass and don't really trust this system until it is up to temp. I have had it installed since the engine was rebuilt last year.
 
#75 ·
Wobble said:

It seems possible that there was more than one thing going on here. The water temp guage showed 160F or so when I felt the misfire. I am running a crossover system with a bypass and don't really trust this system until it is up to temp. I have had it installed since the engine was rebuilt last year.
Click to expand...
Hi Wobble:
Just to throw another few thoughts out there...

Are you running a thermostat with your crossover?
If so, are you running a pressure regulator?
Are you running a water pressure gauge?

If the answer is yes, no, no, then I wonder if this had something to do with the hole?

If the answer is no, don't care, don't care, then I wonder if the super cold front cylinders under major load had an effect? At least my front cylinders slapped even with temps, before I ran a thermostat.

If the answer is yes, no, yes, what were your water pressure readings?

Just probably irrelevant thoughts...
Kent
 
Discussion starter · #76 · (Edited)
It's official guys 10.45-1 :( , wrong pistons, chambers are 116cc, engine is a 30 over 502 Given the above and piston info how many cc's have to be added to get to 9.4-1.?

Mudball the answer is yes-no-yes. Peak pressure around 30-35 when I'm looking. have been considering pressure releif valve, just waiting for CFM to start mass production :laugher:

got to go
 
#78 ·
Good job Mud.

Wobble - you do need to run a regulator. I've been testing on the dyno and Mud has been testing in the water.

Crossovers need:
1) Bypass from crossover to top of t-stat housing
2) Psi regulator valve to keep psi's under 30

Now, with that said, it doesn't appear water has been leaking internally or externally, therefore this wasn't an issue - yet.

Wobble - get yourself a regulator.

Real issue - compression.

I answer e-mails, and posts on this subject all the time.
Why so big compression?
I don't know.

It needs to be followed up by the builder and the purchaser. Many builders do not calculate actual compression ratio for some reason and let it fly with an off the top of the head figure.

Everyone needs to know the piston part#'s, headgasket thickness, and head camber size so that they know what the close estimate on compression is.

Factors the builder must measure are how far down/up the piston is from the deck.

On the car/truck side of things I get maybe 5-10 calls a week from owners and builders wondering why their engines are breaking. The answer is (on small blocks) is usually " I have flat top pistons. I don't know what the combustion chamber is." I then look it up and explain that they have over 10:1 compression. :dead:
 
Discussion starter · #79 ·
cfm said:
Good job Mud.

Wobble - you do need to run a regulator. I've been testing on the dyno and Mud has been testing in the water.

Crossovers need:
1) Bypass from crossover to top of t-stat housing
2) Psi regulator valve to keep psi's under 30

Now, with that said, it doesn't appear water has been leaking internally or externally, therefore this wasn't an issue - yet.

Wobble - get yourself a regulator.

Real issue - compression.

I answer e-mails, and posts on this subject all the time.
Why so big compression?
I don't know.

It needs to be followed up by the builder and the purchaser. Many builders do not calculate actual compression ratio for some reason and let it fly with an off the top of the head figure.

Everyone needs to know the piston part#'s, headgasket thickness, and head camber size so that they know what the close estimate on compression is.

Factors the builder must measure are how far down/up the piston is from the deck.

On the car/truck side of things I get maybe 5-10 calls a week from owners and builders wondering why their engines are breaking. The answer is (on small blocks) is usually " I have flat top pistons. I don't know what the combustion chamber is." I then look it up and explain that they have over 10:1 compression. :dead:
Click to expand...
CFM

I am going to get the following info from builder.

Bore, Stroke, rod length, Cylinder head voume, Deck height, Head gasket bore, Head gasket Thickness, Piston to cylinder wall clearance, Top ring land height. Piston dome volume. And double check his calcs using http://www.smokemup.com/auto_math/compression_ratio.php

I'm so ticked off about this I couldn't sleep last night
 
#80 ·
I understand your being pizzed.

Not my intention to cause this bad emotion, my intentions are to help figure out issues - as much as I can without being there.

Sorry it came to this but the good news is you found out what was eating your motors.
 
61 - 80 of 98 Posts
About this Discussion
97 Replies
10 Participants
Last post:
Wobble
Powerboat Forums at SpeedWake
posts
921K
members
19K
Since
2002
A forum community dedicated to Powerboat owners and enthusiasts. Come join the discussion about performance, modifications, classifieds, troubleshooting, maintenance, and more!
Full Forum Listing
Explore Our Forums
General Boating Discussions Powerquest News Articles