273 with 360 heads/valves

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MuuMuu101

MuuMuu101

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I've done some searching on this the past couple days and can't find anything for the 273 engine with 360 heads. I've found some stuff on the 318 with 360 heads. I do this as a brain exercise.

So, let's just say someone had a leftover 273 and a leftover set of 360 heads with the valves already installed. Let's just say that said person didn't want to hunt for a spare 318 or 360 (gosh, how lazy and dumb can this guy be?) because he liked the said 273 for being "different" (and he's a weirdo too).

Now, from my research, not taking into consideration the valves, if the 360 head were to be able to be used on the 273 it would lower compression significantly (like 8.8:1 to about 8:0:1 or less) unless you milled off some of it (which would require machining for the intake). Not only that but the valves are considerably larger (I'm talking about the 1.88" intake valves instead of the 2.02"). Would the valves interfere with the tiny 273 bores? Can they be installed in the 273 heads and clear? With/without machining? Is there any real benefit of larger intake and exhaust ports?

Let's just say the 273 is running stock pistons with a mild cam upgrade, Edelbrock performer or ldb4 intake, and a 400-600 cfm carb? Are higher compression pistons available for the 273?
 
I blew up a 340 in my 70' swinger and while a new 340 block was being built,i just threw in a 318 shortblock for awhile with all the 340 components. Block already had a stock 68' 340 4 spd cam in it. There was not a lot of difference in how it ran and it reved faster than the 340. I would say it will work but it is going to be down on compression. You would really have to pay attention to the valve/block clearance.

Glen
 
Legend has it that you can't get away with a 1.88" valve in the 273 as it will either interfere with the bore or will be shrouded if it doesn't.

2nd, compression will drop almost 1.5 points with the larger combustion chambers, you would need to get 10.5 pistons to keep it above 9.0 compression with the 360 heads..

Egge carries pistons for the 273.
 
krazykuda said:
Legend has it that you can't get away with a 1.88" valve in the 273 as it will either interfere with the bore or will be shrouded if it doesn't.

2nd, compression will drop almost 1.5 points with the larger combustion chambers, you would need to get 10.5 pistons to keep it above 9.0 compression with the 360 heads..

Egge carries pistons for the 273.
Click to expand...

I figured the compression drop would be huge, if possible, and that's what I was thinking. I noticed Hughes sells Egge's pistons, but they didn't really mention a compression height. The Egge's catalog online was hard to navigate.

When I get my engine back, I'm just thinking of doing a very minor build on it intake, carb, cam, gasket, seals, maybe an aluminum water pump and timing chain cover. I may end up with a set of 360 heads so I was just curious, but very doubtful due to the 273's small bores.

But even if the valves did fit, what would be the benefit of the larger intake and exhaust ports?
 
MuuMuu101 said:
But even if the valves did fit, what would be the benefit of the larger intake and exhaust ports?
Click to expand...


The larger intake and exhaust ports will flow better, but that will be restricted by the valve shrouding.

Stick with the smaller ports and take advantage of the increased velocity.... Velocity is torque...
 
krazykuda said:
The larger intake and exhaust ports will flow better, but that will be restricted by the valve shrouding.

Stick with the smaller ports and take advantage of the increased velocity.... Velocity is torque...
Click to expand...

So benefits may be close to none for a small cubic inched engine. And the 273 needs torque. I'm assuming it all has to do with the volumetric flow rate of the heads (or what I'm assuming they refer too when someone says, "my heads flow XX CFM."). If the volumetric flow rate is a constant and you increase the area than the velocity decreases.

I know my heads may need work/rebuild as one of they cylinders has low compression. So, I really don't know what kind of damage there is to the engine until it's torn apart. However, I do have a spare block lying around (assuming it's good but will need it checked), and if the pistons are good, they'll probably go back in. If not, I'll probably go with Egge pistons. I'm going to build the engine and probably the 904 later (on my free/bored time when I'm not working on the '68 Dart) and hope a decent/cool/cheap A-body shows up.
 
The bore size limits the potential from the 273. If you want more, go to a 318, 340, 360...

If you're going to build a 273, keep the smaller valves and runners and take advantage of the port velocity and let it rev...
 
krazykuda said:
The bore size limits the potential from the 273. If you want more, go to a 318, 340, 360...

If you're going to build a 273, keep the smaller valves and runners and take advantage of the port velocity and let it rev...
Click to expand...

I'm agreeing with you at this point. A high reving small block would definitely suite the bill. :D
 
Really, the deal with a 273 is, the factory 4bbl Commando/Charger version took it almost as far as it can go. There aren't any bolt-ons that will really take it any further than the factory set-up (well, headers, sure... maybe a different intake). And the only thing you can do to help the 2bbl low-compression version is to add the stuff from the high compression version. The valves and ports are as big as they can be given the small bore. The only cure for the small bore is to start with a different block -- 318, or 360.

I mean, I love my Commando engine, but if you are starting with a 2bbl 273, it's just not worth the effort to hot rod it -- the same parts will all work better with a bigger bore.

Krazykuda has the right idea, but to get there, you need the high compression. There's no room to shave the 273 closed-chamber head -- the only way to get it is via the expensive and hard-to-find Commando pistons. Which brings me back to the idea that buying hi-comp pistons for a 318 would still put you ahead.
 
smaller ports give better low end, bigger ports better top end, like smaller and bigger cams. the 1.88 360 valve is not much bigger than the 1.78? 273-318 valve, I think the 1.88 will fit the 273 up to a point-depending on cam size and bore size-call egge and check comp distance available and oversizes-bore that 273. use the thinnest/smallest bore size head gasket
 
Well, if you're gonna run 360 heads on a 273 I'd run a large port intake instead of being caught up with an aluminum- last time I swapped from one of those economy aluminums it was a big difference even with a broomstick doing a cam impression. Between 500 and 600cfm's not a lot of difference so I'd let price be your guide, you'll need to adjust it anyway.

Compression drop's gonna depend on the change from your old heads to your new ones. I'd think it'd end up around 8.1-8.4 with the majority of gaskets now. I wouldn't overestimate the compression drop to the point of relying on it to run cheap gas, that's a recipe for a for sale ad that you won't enjoy.

The Commandos had decent cams in them, a lackluster intake, and horrible heads. Sure, they might have been good for 1964 economy small blocks but there's a lot left to be desired. The Magnums OOTB and especially the Magnum replacements put all of those old heads to shame on the smog motors for a stock bottom end build.

When you're talking about high-revving, that's why all these new motors have large valves and ports to go along with their small bores and large cams. Port velocity's for the opposite. It's for the low range and has been gone away from with tight chambers being used to keep CR up.
 
Years ago in "Mopar Action" was a write up of a 273 powered Dart called "Darn Dart". It had a set of "X" heads, .509 hyd cam , don't remember intake.........it went 10's, I don't think that example was suffering from small bore syndrome.

Yes, you have to notch the block for valve clearance and de-shrouding, SO?? If doing so can be a path to power, why not do it? Too hard?? Too difficult?? Too much of a pain in the a$$??

Hot rodding is about doing something different, so why not do it.

In the other camps......Ford and Chevy..........you don't hear much complaint about small bore sizes in the Mod motors or LS series engines, they still seem to make crazy power for what they are.

Pistons and compression ratio are the hold up to the swap, but not a deal stopper,. Those issues can be worked thru, it just take the willingness to go a road less traveled.

Yes, it is a swap much more suited to the race track than to the street, thou you could drive it around. Not much different than the tunnel rammed, supercharged, things we drive now.
 
JBurch said:
Years ago in "Mopar Action" was a write up of a 273 powered Dart called "Darn Dart". It had a set of "X" heads, .509 hyd cam , don't remember intake.........it went 10's, I don't think that example was suffering from small bore syndrome.

Yes, you have to notch the block for valve clearance and de-shrouding, SO?? If doing so can be a path to power, why not do it? Too hard?? Too difficult?? Too much of a pain in the a$$??

Hot rodding is about doing something different, so why not do it.

In the other camps......Ford and Chevy..........you don't hear much complaint about small bore sizes in the Mod motors or LS series engines, they still seem to make crazy power for what they are.

Pistons and compression ratio are the hold up to the swap, but not a deal stopper,. Those issues can be worked thru, it just take the willingness to go a road less traveled.

Yes, it is a swap much more suited to the race track than to the street, thou you could drive it around. Not much different than the tunnel rammed, supercharged, things we drive now.
Click to expand...


Why go to all that bother if you can just bolt the "good" parts onto a 318/340/360 and get better results for less money/effort???

Maybe we can re-invent the wheel...
 
krazykuda said:
Legend has it that you can't get away with a 1.88" valve in the 273 as it will either interfere with the bore or will be shrouded if it doesn't.

2nd, compression will drop almost 1.5 points with the larger combustion chambers, you would need to get 10.5 pistons to keep it above 9.0 compression with the 360 heads..

Egge carries pistons for the 273.
Click to expand...

This ^ is 100% correct.
 
I think a re-read of the original post is in order, the OP wasn't talking .650" lift 8000rpm
package mods here,street cam & DP intake was the proposal.Which make this swap totally
ridiculous,AND pointless. Mopar themselves dyno'd this very thing,.450" lift MP cam,9.0:1 CR,
DP 4bbl.,elec. ign.,and small headers.On a 318. 1st up they added stock 360 heads w/the
1.88/1.60 combo-275hp,next ported version of the same-304hp,finally the newer swirl-port
"skinny"318 fast-burn head ported w/the same valves-332hp!
How do you think the smaller bore 273 would respond?The other small bore examples
above have heads designed for the task and are taller and more efficient w/smaller valves
and better up-to-date engineering.Believe me,if mopar had built a larger bore slanty, every
/6 fan would be pitching the small bore one,and be building them a better running six-gun!!
Unless you insist on running the OE engine(and there is nothing wrong with that if that
makes you happy),there is no logical reason to do that swap,unless you just like a challenge
(nothing wrong w/that either)!
 
The 273 with 'J' Cylinder Heads with 1.88" Intake and 1.60" Exhaust Valves

Will the Valves clear the Cylinder Wall {Bore 3.63"} ?

The problem is not will they clear, but will they give you the required .080"
Valve-to-Cylinder Side Wall clearance.

NO !!
-----------------------------------------------------------------------------------

Comparison

With a 318 {3.91" Bore}

When you install a set of 'J' Heads {1.88" Intake and 1.60" Exhaust} you
will have damn near {.080"} clearance.

It is close, but it will be adequate. And 'notching' of the Block Deck will
not be required, unless you install a 'radical' Camshaft.
 
MuuMuu101 said:
I've done some searching on this the past couple days and can't find anything for the 273 engine with 360 heads. I've found some stuff on the 318 with 360 heads. I do this as a brain exercise.

So, let's just say someone had a leftover 273 and a leftover set of 360 heads with the valves already installed. Let's just say that said person didn't want to hunt for a spare 318 or 360 (gosh, how lazy and dumb can this guy be?) because he liked the said 273 for being "different" (and he's a weirdo too).

Now, from my research, not taking into consideration the valves, if the 360 head were to be able to be used on the 273 it would lower compression significantly (like 8.8:1 to about 8:0:1 or less) unless you milled off some of it (which would require machining for the intake). Not only that but the valves are considerably larger (I'm talking about the 1.88" intake valves instead of the 2.02"). Would the valves interfere with the tiny 273 bores? Can they be installed in the 273 heads and clear? With/without machining? Is there any real benefit of larger intake and exhaust ports?

Let's just say the 273 is running stock pistons with a mild cam upgrade, Edelbrock performer or ldb4 intake, and a 400-600 cfm carb? Are higher compression pistons available for the 273?
Click to expand...

I have actually done this, 72 340 1.88 intake valves on a commando 273 in a 4 speed 64 Barracuda. It was really fast and quick. If you go this route, use at least a 340 intake and AVS, TQ, or QJ carb. Why put great heads on, and choke it with a small port intake. Most people will tell you not to do it, but I have done it and ran it as a daily driver for years. The heads were milled .040 and cc'd to min NHRA chamber size, and the intake side milled .038 so I could swap to any intake. The block was bored .040 and the chambers centered on the bores using offset head dowels. Forged TRW 10.5 pistons were used. I did not notch the block. No need for a killer cam, I just ran the used 340 cam from the low mileage 72 340. A 71 340 intake and TQ were used on top with a chrome box and MP vacuum advance electronic ignition kit. I ran the stock exhaust manifolds and custom dual exhaust using Turbo mufflers. There is no comparison how it ran with 273 heads. It went up to a whole different level. The really wild thing is I could get mid 20's for mpg cruising on the highway.
 
Id use magnum and notch the block EQ'S would be a good choice.
 
I agree when people say building a 273 doesn't make the most economical sense and a stock low mileage 5.9 would probably smoke most built 273. But going by that reasoning building anything less than a 440 wouldn't make sense. I'm still on the fence on building my 273 when I can get back to building my Cuda.

As for the bore size it would probably limit the power to the 400-425 hp mark cause of air flow and rpm requirements. 300-350 hp is probably more in the realm of most would get out of their build.
 
273 said:
I agree when people say building a 273 doesn't make the most economical sense and a stock low mileage 5.9 would probably smoke most built 273...
Click to expand...

Not even close, a High Performance 273 would easily beat a stock 360. It would not even be a race.
 
273

Not always the choice for Mopar Small Block performance, as the 273 has
limitations.

Biggest problem, no doubt > Cylinder Heads.
 
66fs said:
Not even close, a High Performance 273 would easily beat a stock 360. It would not even be a race.
Click to expand...

All I was saying is a stock 5.9 puts out gross hp of 300 hp and 320 with headers. I doubt there's too many 273 making that, not saying that they can't do better.

But saying that I can see why someone might still would build one and when I get to my engine build it still might be the 273.
 
If I would ever find myself doing a 273, it would be turbo'd.

FWIW, I twice put 2.02X heads on a stock teener, with the 340 goodies. It worked pretty good on the 69 motor. But the LC(low compression) 73 was somewhat of a disaster. I did NOT notch any bores.Both of them lost torque, out of the gate, and required gears to bring the off-the-line performance back. Both were automatics. The 73 woke up, a little, with a higher stall TC. I was very unhappy with the performance of the 73, and felt it wasn't worth the time I had invested.The parts were left-overs, just reconditioned. One thing I can say, the redline was much extended, and they pulled up there, where the stock parts would not go. The biggest problem was getting there.
A turbo, even a little one, would solve that.

In a lightweight early A, the 273 can be built to be fun; it just requires careful parts selection, and a decent cylinder pressure. Keep in mind that the ratio of engine displacement to total vehicle weight, is very similar between early,lightweight As with 273s, and later As with 318s. Unfortunately any go-fast parts you might choose to install on a 273, will gain more on a larger-bore engine; assuming the cylinder pressure is optimized for either/both.
There's nothing wrong with wanting to be different. But when displacement limited, building horsepower will require trade-offs. The first thing to go will be low-speed torque. Normally aspirated; that's just the way it is. A small loss of torque can be compensated for with gears and/or hi-stalls. Too much loss of torque and it leaves the realm of streetability, and the car just ends up a showpiece, parked in the garage most of it's life.
.8 hp/cid is a nice, fun, almost no maintenance, cruiser. Say about 220hp on that 273
1.0 hp/cid is a nice street bomber. This is now 275hp, and could still be a hydraulic-lifter engine.
1.1 hp/cid is pushing the edge for a streeter. Now 300hp. A solid lifter, big cam engine.
1.2 will be pretty much a Saturday afternoon car.Now 330hp. Lots of maintenance.Loud. Raunchy. In-your-face-wicked.
And after that you are in race-car territory
The sticking point will be cylinder pressure. After 275 hp, you will be looking at custom pistons, and a custom cam, and lots of machining.
If you really like the 273, ask yourself; why. You can dress up a 360 to look like a 273,build it to .8hp/cid(basicaly stock), which comes in at 288 hp(super low perf., low maintenance, drive-it-anywhere/anytime,burns anything 2bbl yet ), and be thousands of dollars ahead. Just yank almost any 360 at the pick-a-part.......................Can you say Free Las Vegas vacation!
 
MOPEkidD-3 said:
Stock 5.9 Magnum, not a stock 360 4-bbl LA... big difference
Click to expand...

The only thing that might give a run for the HP 273 would be a HP 360 from 74-76 and it would have to have the 340 cam and valve springs. I also thought we were talking stock? Once you upgrade a 360 to the same level or higher, all the same goodies, the 360 should win. A lot of people discount the 273 but don't realize that 235 HP at 6,000 rpm will kill a 318 with 230 HP at 4,000 rpm. Same for a 360. They made 360's since 1971, mostly 2 barrels. A 5.9 Magnum with a cam and good intake and carb, should, but not a stocker in a truck. I've had or built everything except magnums, an RB, and a Hemi. I know what they all do and where their strengths and weaknesses lie. We all decide how far we are willing to go and how much time, money, and aggravation we are willing to put up with. Knowing what it takes to run and survive without breaking parts is big with me. I never had the luxury of a race car, I always had to drive my car the next week.
 
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