Yet another 273 build advice thread!

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JDMopar

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I'm planning to build a 273 to put in a 65 Barracuda that I bought last summer. I'll add my thoughts on the plan in my head after reading dozens of 273 build threads I searched here. RRR, ToolmanMike and others, please tell me if I'm on the way to screwing it up! Here goes. 273 block, .020 or .030 over with the Egge 10.5 compression pistons, 340 steel crank, just because I have one laying around, 302 heads with better springs to match the cam. Howards cam Howards Cams Hydraulic Flat Tappet Camshaft and Lifter Kits CL711381-10 , stock rockers from a later LA, D4B intake with no more than a 600 Holley. Stock exhaust manifolds that came on the Barracuda, with dual 2 1/4 inch exhaust to the back bumper. Probably will use a 998 or 999 automatic...whichever one is the lockup version. The car is intended to be a driver/cruiser that I'm not afraid to get in and head to Myrtle Beach or Winslow, Arizona to go stand on a corner....lol. Probably 2.76 or 3.23 gears. I just want it to get out of it's own way, sound good at idle, and get decent gas mileage. Thanks in advance for your thoughts and suggestions. :)
 
Sounds like my 273 68 Valiant build.904 and 2.76 gears. Turned out to be a great cruiser,drive anywhere car.Not a powerhouse but still fun and sounds good.
IMG_2603.jpg
 
Why do want later LA rockers? The '65 adjustable rockers are to die for! I used them on my .58 lift 312 duration solid lifter camshaft.

65 Valiant4.jpeg
 
Why do want later LA rockers? The '65 adjustable rockers are to die for! I used them on my .58 lift 312 duration solid lifter camshaft.

View attachment 1715296693
Mainly because I don't have any. If I was going to use adjustable rocker arms, I'd probably use rollers. A motor like I'm wanting to build should be low maintenance and not need adjustable rockers with hydraulic lifters.
 
302 heads without cracks are tough to find, from my experience on the flow bench, i would run the 318 head last 3 is 675
 
When I was bracket racing my 273 '65 Valiant (pictured above) back in the '70's using the factory stock cylinder heads, I noticed after making a quarter-mile run a few drops of coolant scattered about the engine compartment without signs of a leak anywhere.

Later, when renewing the cylinder heads, we found a very minor crack in a water jacket. Back then, I never paid any attention to casting numbers, so now I can't identify the cylinder head we scrapped back then.
 
You lost me at hydraulic cam...

If nothin else, replicate the 273 Commando and use a solid lifter cam.
 
The one thing that jumped out is the Egge higher CR pistons with the 302 heads. Those pistons have a pop-up dome that protrudes above the deck and I just don't know of there will be any interference between those and the chamber shape of the 302 heads. That chamber is a bit differently shaped from the earlier closed chamber heads IIRC.
 
The one thing that jumped out is the Egge higher CR pistons with the 302 heads. Those pistons have a pop-up dome that protrudes above the deck and I just don't know of there will be any interference between those and the chamber shape of the 302 heads. That chamber is a bit differently shaped from the earlier closed chamber heads IIRC.

You are correct, you must shaved down the widow peak, unless you use a thick gasket. I used a very thin gasket and the cut had to be done.
 
I will more than likely use the Felpro gaskets, and I think when compressed they are at like .038 or .039. I'm not a bit opposed to using the 675 heads from a 318 with open chambers if it won't kill the compression too much. I'd like it to be at least 9.0 to 9.5 SCR. I have 2 sets of 302 heads, and know I will be lucky if 2 out of the 4 heads aren't cracked!
 
I would even use the Sealed Pro 8.5 pistons with the 302 heads if it would actually end up at 8.? SCR with thinner head gaskets. If I went that route, I would probably have to use a less aggressive cam.
 
The one thing that jumped out is the Egge higher CR pistons with the 302 heads. Those pistons have a pop-up dome that protrudes above the deck and I just don't know of there will be any interference between those and the chamber shape of the 302 heads. That chamber is a bit differently shaped from the earlier closed chamber heads IIRC.
You'll be hard pressed to get 9.5/1 out of the present day Eggies and 302's . The chambers are very close to the same as the 273 heads.
 
You are correct, you must shaved down the widow peak, unless you use a thick gasket. I used a very thin gasket and the cut had to be done.
Roger. Are talking about taking out part of the 'pointy thing' in the 302 chamber, that sticks out partway between the valves? Seems that would be pretty easy to just grind out.
 
I would even use the Sealed Pro 8.5 pistons with the 302 heads if it would actually end up at 8.? SCR with thinner head gaskets. If I went that route, I would probably have to use a less aggressive cam.
Not familiar with that one.... got a PN?
 
My bad! I just glanced at the part number of another listing when I Googled Egge pistons, and it turns out the pistons I saw were for a stock bore 360. Part number was 273AP or something like that.
 
Why do want later LA rockers? The '65 adjustable rockers are to die for! I used them on my .58 lift 312 duration solid lifter camshaft.

View attachment 1715296693
Be glad for stock non-adjustable rockers. I'm trying to find info on changing my 273 to hydraulic flat tappet and every post I see says this REQUIRES custom pushrods because somehow there has never been an off-the-shelf pushrod made for this mod even though it's been done millions of times.
 
remember to add the cost of a B3 kit if you go roller tip rockers
adj iron are the way to go
or if your lift is in the .500 range or even a little higher just use stock stamped rockers
(depending on your spring pressure)
extra oil through the pushrods helps (AMC/magnum lifters and hollow pushrods)
or keep it solid cam which is a win win
 
Be glad for stock non-adjustable rockers. I'm trying to find info on changing my 273 to hydraulic flat tappet and every post I see says this REQUIRES custom pushrods because somehow there has never been an off-the-shelf pushrod made for this mod even though it's been done millions of times.

What are you trying to do? Hydraulic cam with 273 adjustable rockers? I'll give you stock hydraulic arms, shafts, and pushrods.
 
What are you trying to do? Hydraulic cam with 273 adjustable rockers? I'll give you stock hydraulic arms, shafts, and pushrods.
Yes. Hydraulic flat tappet and stock adjustable rockers in a 273. I had read so many post here where people had WAY overthunk this mod. So many insisted that custom pushrods were the ONLY way possible. Instead of starting yet another thread on the subject I commented on several threads hoping for some Real World part numbers that didn't involve special measuring tools and multiple phone calls to vendor tech help centers. Found that Summit Racing has a "House Brand" pushrod that many feel is Close Enough and several vendors make a 340 Six Pack pushrod that is a Direct Replacement for what I want. Going to order a set of Melling MPR157 pushrods and do it that way. Thanks for your offer though.
 
Call Jim at Racer Brown. We were talking at one point and he had measured the correct length. For something to think about, T/A heads exhaust rockers are on center just like any normal Small block. To get the most from any cam I'd measure and get exactly what I wanted. But, on the other hand, hydraulic lifters are pretty forgiving. I personally run solid lifters in everything I can though.
 
Call Jim at Racer Brown. We were talking at one point and he had measured the correct length. For something to think about, T/A heads exhaust rockers are on center just like any normal Small block. To get the most from any cam I'd measure and get exactly what I wanted. But, on the other hand, hydraulic lifters are pretty forgiving. I personally run solid lifters in everything I can though.
Yup. The plunger travel within the lifter itself will allow the pushrod to be slightly longer than needed. We are building a mild low maintenance engine. The two barrel carb, points and solid cam were at the top of the list of things that HAD TO change.
 
IMO;yur on yur way to screwing up
Wrong combo of cam,crank,gear, and log manifolds
Not a fan of converting to hydraulic lifters.
Didn't get past post #1
Forget the 340 crank; it's balanced for heavy forged pistons and heavy rods. I will cost you a lotta time to get the weight down.
The 273 has just 559 cc of swept area,
This makes getting the compression up very difficult.
If you stick a big cam in there with low compression, it will be a dog with those gears, no matter how sweetly it idles. The hydraulic cam compounds this by virtue of nobody knows when the intake will finally be on the seat and not leaking. The 273 absolutely needs a solid lifter cam, to exactly control when the intake is closed.
here's what I'm talking about; the only thing I changed in the following examples,was the Scr (Static Compression Ratio)
Static compression ratio of 8:1.
with that 255/261 cam in at 107.5 for an ICA of 55*
Effective stroke is 2.76 inches.
Your dynamic compression ratio is 6.83:1 .
Your dynamic cranking pressure is .............................. 132.22 PSI.
V/P (Volume to Pressure Index) is .............................. 91 very doggie

Static compression ratio of 8.5:1.
Effective stroke is 2.76 inches.
Your dynamic compression ratio is 7.24:1 .
Your dynamic cranking pressure is .............................. 142.90 PSI.
V/P (Volume to Pressure Index) is................................. 98 less doggie

Static compression ratio of 9:1.

Effective stroke is 2.76 inches.
Your dynamic compression ratio is 7.66:1 .
Your dynamic cranking pressure is.............................. 153.97 PSI.
V/P (Volume to Pressure Index) is............................. 106 still doggie

Static compression ratio of 9.5:1.
Effective stroke is 2.76 inches.
Your dynamic compression ratio is 8.08:1 .
Your dynamic cranking pressure is ............................165.15 max PSI. for best pump gas
V/P (Volume to Pressure Index) is ............................113 the best it can be with this cam, in at 107.5.

ok so lets say yur gonna limit the pressure to 160 from now on to use say 91 gas in your application, here it is;

Static compression ratio of 9.3:1.
Effective stroke is 2.76 inches.
Your dynamic compression ratio is 7.91:1 .
Your dynamic cranking pressure is .......................160.61 PSI.
V/P (Volume to Pressure Index) is....................... 110 the best safe-pressure number
Ok so 9.3 is the target Scr... so 559/8.3= 67.3cc is your minimum total chamber size. How you gonna get that?

Ok now lets go down two cam sizes, and again limit the pressure to 160ish.

Static compression ratio of 8.9:1.
Effective stroke is 2.89 inches.
Your dynamic compression ratio is 7.89:1 .
Your dynamic cranking pressure is........................ 160.08 PSI.
V/P (Volume to Pressure Index) is .........................115 instant torque!
Your new minimum chamber size is 559/7.9=70.76cc. Now that is easily doable.

Now
none of these cams are gonna be fond of 2.76 gears. with the A998 and it's 2.74 low-gear being a starter gear of 7.56
But 3.23x2.74=8.85 is doable, with a good TC having an internal Multiplier of at least 1.6, to get an instantaneous zero-mph ratio of 14.2 decaying to 9.29 as the car gets moving.

Next
if you want to spin the tires yur gonna need about 200 ftlbs at the crank to get her started. 200 is a lot for a 273. Ima thinking a 2800TC will cover it
for the torque monster 2-sizes smaller cam, than the 255/261/110 you got yur eye on. Ok 200 x 14.2=2840 ftlbs to the road, and this decays to 200 x 9.29=1858ftlbs to the road, which is not wanting to spin much longer, but with a rise in rpm that 273 might just keep it going for a bit in yur lightweight chassis with it's small-tire restriction.
If you insist on that cam, yur gonna need more gear than 3.23 or more stall, or a supercharger.

Or you could switch to a solid lifter cam and shut that pesky intake valve sooner, while retaining the 214@.050.
Course none of these combos are gonna like log exhaust manifolds, which will kill the overlap cycle, and it's attending horsepower bump, and the 214* cam will if yur not careful, suck gas.
It's just all wrong for your stated useage
.

And the final insult
lets say you installed that 255/261/110 cam and 3.23 gears. Ok that cam is gonna power peak around 5000 or more with logs,or less depending on the heads and stuff, at....42 mph with that 2.74 low gear. But at low rpm , unless you dress it up with the 2800TC, it will be a lil doggie.................
Whereas the 2 size smaller combo will be a powerhouse down low with maybe a 2200 stall TC, while peaking something like 400 rpm sooner or at 38 mph, altho giving up some power.

So the question is; where do you want the power to be?
IMO, this is more important to your combo than how much power it makes or how lumpy it idles.

I got an idea; figure out the total chamber volume without any machining, besides the boring. Then lets work the formula backwards and fit the cam into that Scr for the available gas, and the application you stated. Just keep your eye out for pistons hitting things and the final Quench-distance will be very important.
You'll be targeting a total chamber volume between 67 and 71 cc.
I think, with a solid-lifter cam, we can find you something to work with whatever you come up with...... it ain't rocket-science,lol.
 
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... We are building a mild low maintenance engine. The two barrel carb, points and solid cam were at the top of the list of things that HAD TO change.

Why spend money, just use the hydraulic setup. However Chrysler adjustables are not maintenance hogs. I have adjusted mine twice in 300,000 miles. Solid lifter cams also seem to last longer. Never had a solid lobe go away. Stock and relatively mild street builds. Anyway, good luck.
 
Why spend money, just use the hydraulic setup. However Chrysler adjustables are not maintenance hogs. I have adjusted mine twice in 300,000 miles. Solid lifter cams also seem to last longer. Never had a solid lobe go away. Stock and relatively mild street builds. Anyway, good luck.
Agreed. I can't remember the last time I adjusted mine. 5 years or so? Start it cold and it clatters a bit and quiets down when warm. Perfect! That's all part of the coolness of the HP 273.
 
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