Marine 273 build

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BillGrissom said:
Those cams don't sound big. I put a 0.422/0.444 hydraulic cam in my 1965 273, and it is termed "RV/Torquer", i.e. good for low-end torque. You can tame a big cam with Rhoads leak-down lifters, but they are "all in" by 3000 rpm, so probably not needed in a boat. The 273 excels on the high end, like 7500 rpm, but should give good power at your lower limit.
Click to expand...
The cam specs look tiny to me too... but my eyes are not calibrated to small block Mopars. I have a bit more experience building small block Fords and big block Chevies. It seems that both small block Chevies and small block Chryslers can make some big power at higher RPM even with small "looking" cams. Like the 5200rpm 235hp Commando with a .425 cam. As I said, the .429/.444 is the stock HO 340 cam. Of course, this is all neither here nor there, as those are the options we have- and thats it!

BillGrissom said:
Pistons are the killer price, especially if forged, which your friend likely found when buying the Egge's. You won't find a set for $90 like a Chevy SB. Your engines likely had the HiPo Commando pistons, since came with a 4 bbl. When installing them, note that most sets have a "left" and "right" bank. Some racers would swap the banks for better performance. You can read some posts here. Either way, keep the same bank set together.
Click to expand...
My buddy is doing a real rebuild and didnt have a problem justifying the cost of the Egges... I, on the other hand, am not planning any real engine work on this boat and am on a shoestring budget! If I can do heads and an intake cheap, then I will consider it.

Regarding the Commando pistons, Im afraid the marine engines were not so equipped. All of the manuals I can find from that period show lower compression, lower flowing, low RPM engines being the norm. Peak power was rated at or near 4000rpm, and even the mighty 340's got lowly 318 heads and dismal (250hp) power ratings. That is consistent with the rest of the marine industry at the time- 240hp Ford 351w's (8.5:1 CR and 4400rpm peak power) were the gold standard as far as small blocks go, into the early 90's.
 
Who's the lucky Marine that gets the engine when it's done???
 
Tr Benj

That Cylinder Head Chart is correct.

But those CC Volume Numbers are not what they left the factory at.

The #920 Castings {1967 Closed-Chamber} left the factory around 63 CC's.

The #675 Castings {1968 and later Open-Chamber} left the factory around 68 CC's.

Many guys will get the #302 Heads to run, because they have a tight 'Heart-Shaped'
Combustion Chamber much like the #920 Cylinder Head, but with a 'Swirl Port Design'
and a {62 CC Combustion Chamber}.

And the #302 Heads have induction 'hardened' exhaust seats.

I imagine that you're running a {340 Grind} Hydraulic Camshaft.

Unfortunately, the 273 loves a 'Solid Camshaft'.

More to come,,,,,,,,,,,
 
More good info... so the "real world" chamber sizes dont line up with that chart... ok. Are the published compression ratios also lower than advertised?

Someone help me here, as I must be missing something. Im playing with a compression ratio calculator to see the effect of the different chamber sizes, but I cant get a good starting point. What is wrong with these numbers?

bore = 3.625
stroke = 3.31
head gasket diam = 3.75
head gasket compressed thickness = .040
combustion chamber volume (cc) = 68
piston dome volume = 0
Piston deck clearance = .125
compression ratio = 6.81:1

Based on the numbers I have found searching this forum, the 273 pistons tend to be .125-.150 below deck. You cant even get small bore head gaskets anymore. The flat top pistons will technically have a little bit of dish due to the valve reliefs, right? What gives? I was expecting to get my stock compression ratio with the 675 heads (8.5:1) but Im nowhere close.

Edit: Nevermind... I think. It seems someone misplaced a decimal point when describing how far the pistons are in the hole on a 273... I think .0125-.0150 is probably correct, as that puts me at 8.24:1, which is basically in the ballpark I guess.
 
TR Benj,

If the 'Domed Piston' sits below-the-deck at {-.012"}.

My calculation comes up that you are 21 CC's below the Combustion Volume
of a 273 Commando with 10.5-1 Pistons, and with the Piston set at +.129"
above-the-deck.

That comes up to about about 2.3 Compression Points below a 'true' 273 Commando

10.5 - 2.3 = 8.2 {Net Compression}

And, you are using the #675 'Open-Chamber' Cylinder Heads to boot........
which have about +4 CC's larger chamber volume than the 'stock' #920 Closed
Chamber Cylinder Heads.

That's almost another {-.50} Compression Point.

??????
 
Ok, looks like I'm confusing people... I'll try to clarify.

My buddy and I are each looking at 273 builds. We're starting from the same point (200hp marine 4bbls with 8.5:1 CR and 675 heads). He is doing a full rebuild and going back together with the egge 10.5:1 pistons, ported 315's, the ld4 intake and the 340 reverse rotation cam.

My build isn't touching the short block, so keeping the stock pistons (which I presume are the 2bbl auto flat tops). Id love to get as close to the mid 9's CR as possible, and based on the numbers above I think I'd be close with a set of 302 heads. The cam will be the same 340 reverse rotation unless someone out there knows of something better that's available. Intake, I was thinking ld4b but am open to suggestions there as well.

Clear as mud? Ha.

Do my compression ratio numbers look correct for a stock 2bbl auto with 675 heads? I assume mine is the same... Just trying to get all the baseline numbers correct and then see the effect of the 302 chamber size once cleaned up, along with a thinner head gasket. I have not looked at any CR numbers for my buddy's build with the domed pistons and 315's.
 
One thing I didn't see mentioned that some folks might not be aware of...counter rotating crankshafts are different than those in cars. When I worked at a Chrysler dealership, we received a bulletin that a few hundred 318 marine cranks had been inadvertently installed in car engines, resulting in excess oil consumption. Apparently the knurling is different and instead of forcing oil back into the crankcase, it draws it out past the rear seal.
 
Welcome TR - Cool to see something different and also from CT! Some of my earlier tuning jobs were CC ski boats. I did a Ford out on the lake and the locals took notice (Crystal Lake in eastern CT).
I think if you want to keep the shortblock then there's only so much you need to really sweat. First - the compression. As you notice it's hard to get compression in a short stroke, small bore engine. Normally I'd say pistons but you want to leave them alone. You have the "later" 273 open chambers - going closed will help. Milling the heck out of a set of closed will help more but brings more issues to deal with - namely intake fitment and pushrod length.
So knowing the technical limitations leads to the next two questions:
What are you capable of doing yourself?
In real dollars what can you spend for this?
 
Now it's coming together,,

TR Benj,

Sounds like a 1968 273, with Flat Top Pistons {w/Notches} and 'Open-Chamber
#675 Cylinder Heads.

As per your 'post', the Pistons are sitting at {-.012"} 'below' the Deck.

If you want to stay with the current 'Pistons', you will undoubtedly need the
#302 Cylinder Heads to get the compression up for more power.

That's the only thing that will work.

The 'more' Compression, the 'better' that 340 Camshaft will work...

Below {#4323302} Cylinder Head

0706_mopp_24_z+small_block_overhaul+used_heads.jpg
 
OK, so I think Im close with my compression ratio calculator, as long as my assumptions are fairly close.

I assume I would be going from:
bore = 3.625"
stroke = 3.31"
head gasket bore = 3.75"
head gasket thickness = .030"
combustion chamber volume = 68cc
piston dome volume = 0cc
piston deck clearance = .012
compression ratio =8.4:1

And assuming I can get a set of 302's down to ~60cc, either by being lucky and finding a set with small-ish chambers, or a bit of milling, Id be going to:
bore = 3.625"
stroke = 3.31"
head gasket bore = 4.08"
head gasket thickness = .021"
combustion chamber volume = 60cc
piston dome volume = 0cc
piston deck clearance = .012
compression ratio =9.4:1

If that is in the ballpark, I'd be happy. Of course, I wont know some of those numbers until the old heads come off and new ones cc'd.

Yes, it definitely is tough to get compression out of a short stroke, small bore motor!

Yes, I understand that milling the heads has consequences in terms of valve train geometry and intake fitment. No sense in getting too far down this path until I get a set of heads (302's or otherwise) in hand, to determine the chamber size and what they need for clean up- and then I will need to have a conversation with my machine shop.

My capabilities? Everything short of doing the machine work, pretty much. Assembly is no problem. I am not afraid to try porting on my own, but time would most likely be a limiting factor.

What do I want to spend? As little as possible! I know what most of the pieces will cost- heads, springs, cam, intake, bolts, gaskets, etc. I want to say my shop charged me about $300 for the last set of heads I had done (clean up, valve job, etc). I assume this set would run similar or slightly more, depending on how much extra machining was required. I do not know what it would cost to have a set of heads ported, if I decided I didnt have the time. Are we talking $200? $500? $800? How many hours would it take me to tackle the heads on my own? Say just the basics- port matching and general smoothing?

The nice thing is that I have no time table for this project... it could happen this winter if I really wanted to push it, or I could slowly start collecting parts and do it next year. Its never too early to start planning.
 
I don't think those Egge domed pistons make the same compression as the stock hipo pistons. I did a side by side comparison of the Egge piston against an old TRW forged commando piston which is no longer made. The Egge had lower compression height and bigger valve reliefs. I think the TRW's were closer to the factory commando pistons.

http://www.forabodiesonly.com/mopar/showpost.php?p=1969625909&postcount=44

Besides the lower compression height of the pistons, deck height tends to be greater than spec and all the 315 heads I've cc'ed have come in around 63 cc's instead of the spec 57 cc's. Off the shelf head gaskets are thicker and have a larger bore size than the original factory gaskets. Add all this together and you have to do quite a bit of milling to get to 9.5:1.
 
69 Cuda 440 said:
Now it's coming together,,

TR Benj,

Sounds like a 1968 273, with Flat Top Pistons {w/Notches} and 'Open-Chamber
#675 Cylinder Heads.

As per your 'post', the Pistons are sitting at {-.012"} 'below' the Deck.

If you want to stay with the current 'Pistons', you will undoubtedly need the
#302 Cylinder Heads to get the compression up for more power.

That's the only thing that will work.

The 'more' Compression, the 'better' that 340 Camshaft will work...

Below {#4323302} Cylinder Head

0706_mopp_24_z+small_block_overhaul+used_heads.jpg
Click to expand...

I'd look for a set of 302 heads. They were used in the 80's on 318's.

Junkyard hint: Dodge probably built a lot of vans in that era with those heads. Also the rear wheel drive Chrysler and Dodge cars. They have the more efficient swirl heart-shaped combustion chambers as shown above. Someone here will know the model years they were used (85-89?)

A few years back, I came across THIN head gaskets at the Mopar Nats and bought a set. Don't quote me but I think they were 0.028" compressed. Sounds like you are trying to get every little bit of compression. Check with Mancini Racing?
 
Yep, sounds like 302's are the most readily available, small chamber, decent flowing head out there... so I will start shopping. Not many junk yards around here carry stuff more than 10 years old, unfortunately- so finding stuff thats 25+ years old might be a challenge.

There are a few head gasket options listed on Summit with a compressed thickness <0.030"... though theyre all larger bore. Still, better than a small bore, thick gasket I suppose.

65Dart, I did read that thread you linked a while back and will be forwarding it along to my buddy who already has his Egges in hand. I have a hard time seeing how going from 675 heads and factory flat tops to 315's and domed pistons wouldnt make it a piece of cake to gain a point of compression, but I suppose the math is what it is! Im sure some milling is in his future, as he was planning to have the 315's ported anyways.

For my build, Im keeping the factory flat tops and trying to break into the low-mid 9's for CR, if at all possible.
 
I don't know the true story on the head gasket bore size. I bought an old Felpro gasket set off ebay that was for a 273 only, according to the label (assume it pre-dated the 318 LA). However, the bores were the same >4" D as today's gaskets. I posted a few years ago w/ photos. I recall that others said the original factory gaskets did have the correct bore size. Of course, someone could have swapped gaskets in the kit since the box was opened.
 
TRBenj said:
65Dart, I did read that thread you linked a while back and will be forwarding it along to my buddy who already has his Egges in hand. I have a hard time seeing how going from 675 heads and factory flat tops to 315's and domed pistons wouldnt make it a piece of cake to gain a point of compression, but I suppose the math is what it is! Im sure some milling is in his future, as he was planning to have the 315's ported anyways.

For my build, Im keeping the factory flat tops and trying to break into the low-mid 9's for CR, if at all possible.
Click to expand...

If your buddy is willing to repeat my measurement of the compression height for the Egge pistons and post it here, it would be greatly appreciated. I'd like some confirmation that I did it right and that they don't vary from batch to batch.

Factory flat tops might be taller than currently available flat tops. If your engine is apart, maybe you can let us know how far below zero deck they are. Or if they're out of the bores perhaps you could measure the compression height directly.

EDIT: Just noticed your earlier post where you state that your pistons are .012" below deck. That's really good and that's where I keep having trouble on my build. My block deck is .010" above spec at 9.610". Even if I mill off .010" to get the deck height on spec, those Egge's will still be .072" in the hole. I would love to know your deck height and/or your piston compression height.
 
I think you'll find the pistons are down further in the bore than you think, and even the 302s will have larger than advertised chambers, and any head gaskets will have a 4.10 bore and be billed for 273 to 360s... It's a struggle.
For clarification - milling does not affect geometry on a Mopar with shaft rockers. I'm going to suggest something different tho. Get aset of Enginequest Magnums and use them. The 5 bolt valve cover will work, an AMC and most mopar lifters are configured for pushrod oiling, you can get cheap rocker gear and valve springs, and they will outflow the 302s in stock form (Magnum based). You may still need to mill them to get the compression ratio up and run the MP .028 gasket.
 
About 2-years ago, we built a 1968 '273 Marine' for a customer, and he had
a set of 'factory' 1968 +.040" over 'Flat-Top' Pistons w/notches.

He got them from some old Marine Boat Shop in Michigan, and they were
much taller than the 1967 Pistons.

Without decking the block, they sat {-.015"} down the cylinder bore.

Original factory replacement cast Pistons. Not sure how long they were sitting
on the shelf, but the box was old.
 
69 Cuda 440 said:
About 2-years ago, we built a 1968 '273 Marine' for a customer, and he had
a set of 'factory' 1968 +.040" over 'Flat-Top' Pistons w/notches.

He got them from some old Marine Boat Shop in Michigan, and they were
much taller than the 1967 Pistons.

Without decking the block, they sat {-.015"} down the cylinder bore.

Original factory replacement cast Pistons. Not sure how long they were sitting
on the shelf, but the box was old.
Click to expand...

Since the '68/'69 273 uses the open chamber heads, and the 2bbl version has the same 8.8:1 comp ratio as the '67-down (w/closed chamber heads), shouldn't using '68/'69 273 pistons with the earlier '67-down small chamber heads give an increase in comp?
 
65 Val,

Correct,

A 1968 - 273/190 HP with Flat-Top Pistons w/Notches.

Just a swap with 1967 273 {#920} Closed Chamber Heads, providing the Pistons
are 'below' deck.

Worth about a +10 HP gain.
 
69 Cuda 440 said:
65 Val,

Correct,

A 1968 - 273/190 HP with Flat-Top Pistons w/Notches.

Just a swap with 1967 273 {#920} Closed Chamber Heads, proving the Pistons
are 'below' deck.

Worth about a +10 HP gain.
Click to expand...

...or, just swap '68/'69 273 /190HP pistons into a '67- down block w/early or "302" heads will accomplish the same thing?
 
The following is excerpted from the press release on the D/Dart.

The "D/Dart" features a modified 273-cubic inch engine with a special four-barrel Holley carburetor. Other features include:

A special Camcraft camshaft (284 degrees with .495 intake and .505 exhaust), Racer Brown valve springs, a modified, full-breathing intake manifold, a low-restriction air cleaner, and a Weber clutch. Other standard items are: a heavy-duty 4.86:1 Sure-Grip 8-3/4 inch rear axle, free-flow exhaust headers by Doug of California, a four-speed manual transmission, heavy-duty suspension, and 6.95x14 B.S.W. tires.


The Doug's headers might not work in a closed engine bay marine application. However, they're probably good for 20-30 hp over the marine exhaust manifolds. Just for a talking point, call a marine D/Dart a 250 hp engine.


If you're interested in improved cylinder heads, you may want to take a look at Indy Cylinder Head's LA-MAX. It's an iron head with Magnum valves and a rocker shaft. I've had a look and think it will fit the 273 without having to notch the block. I have not confirmed this with Indy, however.

FWIW: The early 273 heads had the smallest cc among LA Mopar heads.
 
The Boat Owner 'TR Benj'

Is on a 'fixed budget', so he won't be going for any high-priced after-market
Cylinder Heads.

He's on the prowl for #302 Heads.
 
Correct... Budget is tight on this one. If it weren't, Id be looking at bottom end work and more cubes (318 or 340). I do think trying to extract some more power from the small engine without breaking the bank will be a fun project though.

Another Chrysler guy I know suggested magnums a while back but I want to keep this looking original- factory valve covers included. So, I'd prefer to try and work with some earlier small chamber heads (302 or otherwise).

I have not so much as touched my engine yet, heads are still on. No idea how far down in the hole the pistons are. The only way I can get the compression ratio calculator close to the advertised CR numbers is to plug in a below deck height of .010-.0125. Much more than that and it's down in the 7's. No one has told me whether my other assumptions are wrong, or if some 273's really did come from the factory with a sub-8 CR. Either way I guess it didn't really matter, as changing the pistons or block are not on the table. With the head change I'm contemplating I stand to gain about a point over whatever I have now, so that will have to be good enough!
 
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