Merc 4.3 Quench Distance

[adam619]

So , here's the deal , I have a merc 4.3 with 4bbl , fresh rebuild last year .030 over , .039 gasket , deck height is stock , so would of been 0.025 with stock pistons but now I had rebuilder pistons in it that I had realized after the fact had a compression distance of 1.548 as opposed to 1.56 therefore deck height is 0.037, the pistons in question are speed pro H345DCP flat tops with 4 reliefs -6.8cc , non vortec heads , my calc CR with 64cc Heads is now at 9.39:1 , stock is 9.3:1

this leads to a quench distance of .076 ,yeah I know not good right , but did the stock .064 have any quench effect is the real question I guess? Possibly the flat top may even help with that vs dish top???

Long story short I decided to run like that last year , and it is running quite strong , Larson SEI180 , 5 in boat pulls out two adult skiers on one ski no problem and does over 50mph. (prop quicksilver High5 21*)

I never heard pinging and ran it hard all summer , so far so good , what may be pertinent to know also is I only run 91 octane and have a 142 degree thermostat.

I now have vortec heads to put on it with a performer 2114 manifold , just because why not, and am now debating this quench thing again , will the vortec heads provide more quench effect with flat tops?? If I am not worse off than with the non vortec head then I think I will leave it at that , if not I need to disassemble to have it decked and change the pistons because at 0.045 clearance those piston will yield 11:1 CR so I would absolutely need dish tops. Alternate option is stock deck , change pistons to 1.56 comp. dist. and run a .027 non marine gasket the I will be sitting around 0.050 , either way need pistons.

What do you guys think , playing with fire,? (literally fire without ignition spark)? I am guessing it is probably surprising and informational to know it still running strong as is with 0.076 quench , or is this typical of what people are running, well for how long are they running is the more important aspect....

@Scott Danforth @Bondo

Thanks again for your help

Adam

 

[Scott Danforth]

Best you can do is go with a thinner head gasket

 

[Bondo]

I never heard pinging and ran it hard all summer , so far so good ,

Ayuh,... Have you inspected the spark plugs for signs of detonation,..??
Pingin' is rarely "Heard" in a boat,....

if not I need to disassemble to have it decked and change the pistons because at 0.045 clearance those piston will yield 11:1 CR so I would absolutely need dish tops. Alternate option is stock deck , change pistons to 1.56 comp. dist. and run a .027 non marine gasket the I will be sitting around 0.050 , either way need pistons.

Before I tore down a perfectly good runnin' motor for this, I'd find another long block, 'n rebuild it to what ya want, 'n after the swap, sell the 1st motor, to pay for the 2nd motor,....

'n yes, I believe 11:1 is to high,....

 

[adam619]

Thanks for your input ,

Ayuh,... Have you inspected the spark plugs for signs of detonation,..??
Pingin' is rarely "Heard" in a boat,....

@Bondo I have pulled them but to the untrained eye it is entirely possible I did not notice any signs of detonation, they just looked golden brown to me which I was happy with, I will pull the heads off tomorrow the pistons will tell the real story, I can show pictures. The second motor is not really an option for me now.

Best you can do is go with a thinner head gasket

@Scott Danforth it is starting to look that way to avoid a divorce...

Here is my current static CR @ .076 quench
Bore: 4.030
in.Stroke:3.48
Cylinder Head Volume:64
Effective Dome Volume:6.8cc
Deck Clearance:.037
Compressed Gasket Thickness:.039
Compression Ratio : 9.39 : 1
Total Displacement (in.3) : 266.34
Total Displacement cc's : 4366.23

With a thinner head gasket like this one https://www.cometic.com/products/C5739-027 ,(is a MLS stainless gasket acceptable???) Stock quench would be achieved at 0.064 but CR would go up to this...
Bore: 4.030
in.Stroke:3.48
Cylinder Head Volume:64
Effective Dome Volume:6.8cc
Deck Clearance:.037
Compressed Gasket Thickness:.027
Compression Ratio : 9.64 : 1
Total Displacement (in.3) : 266.34
Total Displacement cc's : 4366.23

So which one would you opt for? If the vortec head offers more detonation resistance, could .064 with a flat top piston arguably be better than stock?? Do you think there are longevity issues with this on 91 octane and a 142* stat? I would think there are other humans out there running a H345DCP with a 64cc head...

Adam

 

[Scott Danforth]

cometic makes the best head gaskets....... PERIOD. MLS is preferred

remember, stock motor with vortec heads came with flat tops with the 4 valve reliefs

 

[adam619]

 

[adam619]

Well tore it apart last night, above are the pictures, checked compression before taking it apart , it was cold and has not run since august so probably not much oil on cylinder walls got between 158 to 170psi on all but #2 @ 110 psi ..... I was expecting a broken ring or something such , what I found is exhaust valve seat as seen is picture does not seal and has heavy amount of deposits , I cleaned it up after and the valve seat is not burned and looks good??? The valves themselves look mint, as seen in pics. You can also see there is a good amount of carbon deposits on the pistons as well, I am not sure what is going one here. Cross hatch pattern and cylinder wall and piston look good , about 20 tanks of 91 octane since rebuild.

-Engine did not burn oil since first 2 tanks maybe, even out of the water with the ears one it did not smoke
-Engine did not have blow by when running
-Intake valve has stem seal and o-ring seal , exhaust valve only has o-ring seal, valve guides are also sloppy and out of spec. It seems odd to me the failed exhaust valve itself is very clean tho.
-The divorce choke has been known to stay on for extended periods could this cause that much carbon?
-Could the 142* stat cause incomplete combustion when running low load because temp too low?
-Would the carbon on the piston be blasted off after a sustained high rpm run?

The piston are between 0.038 a .042 in the hole

 

[Scott Danforth]

No need to run 91 at 9.3:1

140 T-stat is normal for a carbed raw water cooled motor

Have your machine shop upgrade the seals

 

[adam619]

This amount of carbon flaking off normal? Would the exhaust valve seat cleaned itself up on its own and what would be the cause of this? With those flat top at .045 quench it would be at 10:1 if i have it decked, is this ok on 91?

Sorry about all the questions?

Thks again

 

[Tail_Gunner]

No need to run 91 at 9.3:1

140 T-stat is normal for a carbed raw water cooled motor

Have your machine shop upgrade the seals

Mr Danforth why 140 I am rather curious, that would make the combustion process rather poor. My latest tinkering with engines has been with big block rv motors, they are heavy duty cycle engine's and operate at optimum effenciecy at 185/200 degrees.

Make a note here i am not challenging your thoughts but very curious as to why so cold. Saltwater engines would be another story however.

 

[Tail_Gunner]

This amount of carbon flaking off normal? Would the exhaust valve seat cleaned itself up on its own and what would be the cause of this? With those flat top at .045 quench it would be at 10:1 if i have it decked, is this ok on 91?

Sorry about all the questions?

Thks again

I would stay away from anything more than 9 something..low 9 compression ratios. There is very little gain and a very high risk of detonation, now getting your quench right is much like the happy wife happy life adage. Make no mistake

From 9 to 10 you might gain a 3% gain in power and only if your timing advance is set for optimal combustion timing. Marine motors run very low timing to prevent detonation. Think about that for just a moment..or two if needed.

 

[Lou C]

You have the 140 stat in a raw water cooled motor because:
If in salt water when temps get above 160 or so calcium can drop out of suspension and coat cooling pasages
Raw water cooling is not pressurized like closed cooling so you can have localized boiling in hotter parts of the system like the cyl heads, this can cause localized overheating that won’t show on the temp gauge. So you want to keep engine temps well below 190-200 and the 140 stat allows that. Some engines came with 160 stats like later EFI Mercs and most OMCs and Volvos carbed or EFI.

The rust on the exhaust valve seats could be from a leaky exhaust elbow, esp if you have wet joint exhaust.
The coating flaking off the pistons looks unusual to me; it’s a grey color not black like carbon usually is.
Your plugs look good.

 

[Lou C]

Mr Danforth why 140 I am rather curious, that would make the combustion process rather poor. My latest tinkering with engines has been with big block rv motors, they are heavy duty cycle engine's and operate at optimum effenciecy at 185/200 degrees.

Make a note here i am not challenging your thoughts but very curious as to why so cold. Saltwater engines would be another story however.

Open or raw water cooling is not pressurized therefore the water in hotter parts of the system like right behind the exhaust valves can start boiling at around 200*F. Usually the overheat alarms are set to go off at about that temp.
A closed system with 50/50 water/antifreeze & a 15 lb pressure cap won’t start to boil till about 265*F. See the difference?

 

[adam619]

@Lou C thanks for your insight, the pictures may be misleading.. the top of the pistons are most certainly a black coating that is flaking off, i can take it off with my fingernail. Maybe it is there because of long idle time before storage, however is not refelcted on the plug?? Should I opt to a 160* thermostate like the gen+ 4.3 that comes with vortec heads and a 4bbl? As for a water intrusion causing valve seat damage, my exhaust valves on the same side are mint , and my exhaust flaps are intact and functional , kind of impossible to pressure test the wet exhaust as it is open to atmosphere but i cant see any visible signes of damage. Maybe water made its way back to that valve from wet exhaust at one point somehow?

@Tailguner thks , Sounds like i need new pistons for low 9:1 with .045 quench, although I would like to keep those pistons as there were fitted to 0.0015 clearance to each cylinder last year, now new ones will require new rings , possibly different clearances as well depending if hypers or not. I could get to 9.8:1 with .055 quench with these flat tops, better than stock quench with slightly higher CR , running on 91 should be good??? 87 available here is shitty and ethanol blended ,and 89 not available anyways. Possibility is .064 stock quench @ CR 9.64:1.

 

[Tail_Gunner]

@Lou C thanks for your insight, the pictures may be misleading.. the top of the pistons are most certainly a black coating that is flaking off, i can take it off with my fingernail. Maybe it is there because of long idle time before storage, however is not refelcted on the plug?? Should I opt to a 160* thermostate like the gen+ 4.3 that comes with vortec heads and a 4bbl? As for a water intrusion causing valve seat damage, my exhaust valves on the same side are mint , and my exhaust flaps are intact and functional , kind of impossible to pressure test the wet exhaust as it is open to atmosphere but i cant see any visible signes of damage. Maybe water made its way back to that valve from wet exhaust at one point somehow?

@Tailguner thks , Sounds like i need new pistons for low 9:1 with .045 quench, although I would like to keep those pistons as there were fitted to 0.0015 clearance to each cylinder last year, now new ones will require new rings , possibly different clearances as well depending if hypers or not. I could get to 9.8:1 with .055 quench with these flat tops, better than stock quench with slightly higher CR , running on 91 should be good??? 87 available here is shitty and ethanol blended ,and 89 not available anyways. Possibility is .064 stock quench @ CR 9.64:1.

You must decide what you want to achieve. Both Bondo and Mr.Danforth have given you a way to go.

 

[Scott Danforth]

Mr Danforth why 140 I am rather curious, that would make the combustion process rather poor. My latest tinkering with engines has been with big block rv motors, they are heavy duty cycle engine's and operate at optimum effenciecy at 185/200 degrees.

Make a note here i am not challenging your thoughts but very curious as to why so cold. Saltwater engines would be another story however.

salt and minerals come out of suspension at about 145 degrees, and that will extrude hone your water passages and crust up the heads. hence the 140F T-stat used for decades since the builder doesnt know what water the motor will be run in.

if pure, clean fresh water, then yes, 160F

you are correct, a higher temp is preferred for emissions. I run a 160F T-stat with my motor that is running a heat exchanger. My manifolds are usually about 130 when under way, however will creep up to about 160 at idle.

If the motor was EFI and heat exchanged cooled it would run about 185

 

[adam619]

Thanks for your input gents...

So I have ordered a 160* stat and electric choke that will hopefully deal with the carbon deposits in the future , will re-calibrate the weber carb for vortec head and intake also.

So about the quench distance , I am really stuck between a rock and a hard place , I know it is likely important in high performance build , no doubt , but I don't know how much it really applies to my situation.

Here is why I am really confused about it all , I have spoken with 3 different machine shops , (maybe they don't specialize in high performance applications) but all are seasoned veterans that have seen tons of engines. They feel that comp ratio is more important than quench distance and they don't think me going to a dish top to make 0.045 piston to head will make a noticeable difference. I heard the point that some stock vortec had flat tops with 4 reliefs had appox .060 quench. One shop presented me with 2 options without decking the block.

He found 4.3 gasket @ 0.022 , and engine tech pistons 1.560 comp height , 12cc dish (just a regular dish not a D-dish) , this would put me at ~ .047 quench at 9.4:1

Alternatively I could go with the 0.022 with my flat tops .038 in the hole for ~0.060 quench and 9.65:1 , which is what Mr @Scott Danforth suggested. Is this safe with 91 Octane?

Sorry to ask about this again but I am really confused about it all, in my view , albeit not backed by any experience , I would think the flat top although @ 0.060 matches really nicely with the vortec quench pad as it matches 100% of its surface with a flat portion , it is hard to comprehend that a tighter .047 for a much smaller area around the outer perimeter of a dish piston will provide better quench & squish at 0.012 closer than such a larger area of the flat tops , most surface of the dish top will be ~0.080 away from the quench pad. I understand a D-dish as best but not in the budget.

Is this making sense to anyone else? Am i worrying about it to much?

Adam

 

[Bondo]

Ayuh,..... I'd skip the full dish pistons, 'n go for the next best option, the flat tops, 'n thin gasket,.....
So long as yer runnin' hi-test, 'n ya keep the fuel filter clean, you'll be safe,.....

'n btw, you never completely stop the carbon build up,.....

 

[Scott Danforth]

'n btw, you never completely stop the carbon build up,.....

nope, not even with an Italian tune up

 

[Scott06]

Ayuh,..... I'd skip the full dish pistons, 'n go for the next best option, the flat tops, 'n thin gasket,.....
So long as yer runnin' hi-test, 'n ya keep the fuel filter clean, you'll be safe,.....

'n btw, you never completely stop the carbon build up,.....

This is what you should do