Should I leave it Stock?

[thesiren74]

I've driven both a 2 barrel 273 and a 273 converted to 4 barrel with headers it does make a huge difference, like 18 seconds vs 15 seconds in the quarter mile.

 

[jau126]

It seems like the majority of people are not terribly offended by the modifications. I might have to find another mechanic though after I do them. (Yes I do as much as I can but I work in a parking space of my apartment garage with semi strict rules .) He is a great guy and owned one in the 70s but does not want me to touch the thing.
@cudamark Did you spec a symmetrical or non-symmetrical cam profile for the stock heads?
Also I know this is discussed all the time but is 500 cfm going to be over carbureted? I thought the tech of the AVS2 was cool and that is the smallest they size them.

 

[toolmanmike]

Yes and no.
It's not exactly a waste; it's just that;
the stock cam/carb/intake is good to say 3000rpm, so the 4bbl is not gonna do much until the 2bbl gives up..
and because of the small factory cam, headers are not a big help either. Taken together, you will get a power increase, but not until later in the rpm band. With a stall of say,1700 rpm, neither of those is gonna make a difference at zero mph when you hammer the throttle. With say 2.76 rear gears and those 15s, you might be looking at final drive gearing of 65=2200. If that is true then 2500 in first gear, where the power is just starting, will be over 20 mph. So from zero to 20ish mph, that 4bbl and headers is not gonna help you one tiny bit.
That 273 cam might power peak at 4000ish rpm, and the 1-2 shift might be 4400 so your power band is gonna be from no lower than 2500, to perhaps as high as 4400. So if you want rapid-transit to happen, you need to be in that band. The 4bbl, might not be fullopen until closer to 3000, and if you open it too soon or too fast, you will likely just bog down. Lets say you somehow make it happen at 2800. So then your 4bbl power begins at ~2800, peaks around 4000, and is rapidly falling off at 4400tops.
In first gear this is beginning at~24mph, peaks at 34mph and is falling away at 41 mph, The question is; does that help you?
In second gear, the power plateau might be at ~ 3600 to 4200. With 2.76s this is about 52 to 72 mph. Hey nice passing gear. But check out the hole from somewhere around 36 mph to 52 mph, where your engine is gonna be struggling.Even with the 4bbl, cuz she is off the cam.
But think about this;
How much power can you expect with the 4bbl and headers? and where is it gonna come in at?
I'll guess no more than 20hp at 4000rpm, more likely is 12 to 15
I'll guess zero at zero mph with a 1700 stall
I'll guess the power is linear from 2800 to 4000, so 1.67hp per 100rpm. It won't be linear tho, with less real power generated at 2800 than at 4000, but I just want you to see the point.
So;
if you are looking for more power to take off with from a dead-stop, then the 4bbl/header combo is a bust.
But if you want passing power from 52 to 72, it's hard to say no, to ~18hp
So

Yes and no.

Not as powerful as your combination but my 273 accelerates to 85 or so with authority. My 318-2 preforms about the same.

 

[thesiren74]

And there's nothing like the Whoomp sound of secondaries opening.

 

[273]

It seems like the majority of people are not terribly offended by the modifications. I might have to find another mechanic though after I do them. (Yes I do as much as I can but I work in a parking space of my apartment garage with semi strict rules .) He is a great guy and owned one in the 70s but does not want me to touch the thing.
@cudamark Did you spec a symmetrical or non-symmetrical cam profile for the stock heads?
Also I know this is discussed all the time but is 500 cfm going to be over carbureted? I thought the tech of the AVS2 was cool and that is the smallest they size them.

500 is fine could even go bigger just little reason too.

 

[jau126]

Yes and no.
It's not exactly a waste; it's just that;
the stock cam/carb/intake is good to say 3000rpm, so the 4bbl is not gonna do much until the 2bbl gives up..
and because of the small factory cam, headers are not a big help either. Taken together, you will get a power increase, but not until later in the rpm band. With a stall of say,1700 rpm, neither of those is gonna make a difference at zero mph when you hammer the throttle. With say 2.76 rear gears and those 15s, you might be looking at final drive gearing of 65=2200. If that is true then 2500 in first gear, where the power is just starting, will be over 20 mph. So from zero to 20ish mph, that 4bbl and headers is not gonna help you one tiny bit.
That 273 cam might power peak at 4000ish rpm, and the 1-2 shift might be 4400 so your power band is gonna be from no lower than 2500, to perhaps as high as 4400. So if you want rapid-transit to happen, you need to be in that band. The 4bbl, might not be fullopen until closer to 3000, and if you open it too soon or too fast, you will likely just bog down. Lets say you somehow make it happen at 2800. So then your 4bbl power begins at ~2800, peaks around 4000, and is rapidly falling off at 4400tops.
In first gear this is beginning at~24mph, peaks at 34mph and is falling away at 41 mph, The question is; does that help you?
In second gear, the power plateau might be at ~ 3600 to 4200. With 2.76s this is about 52 to 72 mph. Hey nice passing gear. But check out the hole from somewhere around 36 mph to 52 mph, where your engine is gonna be struggling.Even with the 4bbl, cuz she is off the cam.
But think about this;
How much power can you expect with the 4bbl and headers? and where is it gonna come in at?
I'll guess no more than 20hp at 4000rpm, more likely is 12 to 15
I'll guess zero at zero mph with a 1700 stall
I'll guess the power is linear from 2800 to 4000, so 1.67hp per 100rpm. It won't be linear tho, with less real power generated at 2800 than at 4000, but I just want you to see the point.
So;
if you are looking for more power to take off with from a dead-stop, then the 4bbl/header combo is a bust.
But if you want passing power from 52 to 72, it's hard to say no, to ~18hp
So

Yes and no.

I appreciate your thought out response and maybe there are some regions were I am not in my peak operating power range I would expect there to be an increase in torque through out the range from going from a stock single plane to the dual plane?Am I wrong on this?

 

[thesiren74]

I appreciate your thought out response and maybe there are some regions were I am not in my peak operating power range I would expect there to be an increase in torque through out the range from going from a stock single plane to the dual plane?Am I wrong on this?

Torque and overall power will increase, and that's the best intake for a 273.

 

[jau126]

Also how much stuff has to come out to put the cam in (in front of the engine)... radiator? I am having water pump replaced shortly. Should I just wait on the water pump until the cam? (I do not think it needs a water pump yet but I won’t hurt to do it.)

 

[SSVDP]

Our early A-bodies are cars that the real value is in using them. In other words almost nobody cares if it’s perfectly original, being stock doesn’t really increase the value.
So do whatever that makes it better for you.
Any performance increase will be appreciated by all.

 

[LO23M8B]

A stock 273 2 barrel car is not a high $ car so stock just doesn't matter. Put a stock 68-71 340 in it and if money is a problem leave the stock manifolds on it with dual exhaust and change out the rear axle to a 8 3/4 and you'll be the happiest man on earth. I know done it a lot and it makes a cool sleeper.

 

[jau126]

No room to do a motor swap right now and the 273 seems healthy. If it does ever die I will drop a bigger cub motor into it but for now the 273 is sufficient.

 

[LO23M8B]

No room to do a motor swap right now and the 273 seems healthy. If it does ever die I will drop a bigger cub motor into it but for now the 273 is sufficient.

If you do the headers and 4 barrel mill the heads the maximum amount it'll make a big difference. Have fun I just love the early a-bodies.

 

[toolmanmike]

If you do the headers and 4 barrel mill the heads the maximum amount it'll make a big difference. Have fun I just love the early a-bodies.

It would be interesting to build a zero deck flat top 273. The heart shaped combustion chambers would love it. It would run as good as a domed HP piston engine. May be better.

 

[toolmanmike]

No room to do a motor swap right now and the 273 seems healthy. If it does ever die I will drop a bigger cub motor into it but for now the 273 is sufficient.

Go for it!

 

[Ottmundr]

Even if you drop a LS in it, you should be fine. Just be prepared for the

 

[512Stroker]

Not in my dictionary. It's only original once, but, it can be stock any number of times if you return it to that state. I wouldn't hesitate to customize the car as long as it's bolt-on stuff. You can always pull the aftermarket parts off and put the stock parts back on if you plan to show or sell it. Even cutting and welding can be undone if proper methods are used. It's just expensive to do.

Boy, you aint right in the head.
It's only stock once, I can always tell when a car has been f--ked with.

 

[AJ/FormS]

The engine does not suck on the carburator.
The pistons go down and create a low pressure area, which the atmosphere then tries to fill.
Your tight-gap plasma-Moly rings and perfectly round/not tapered/ not ridged, 50plus year old cylinders (cough-cough,lol) are gonna set the stage for how low a pressure your engine can create. This sets the stage for how much air is gonna find it's way in there. Next are the restrictions of the intake valves , the runners and the throttle bores. And finally, the cam decides how long this column of air is gonna be allowed to flow. If your engine can physically only induct 300cfm of air, then it matters not in the least as to power, how big a carb you put on it.
Your 2bbl is good to ~3600, It will flow enough air, to about 3600, to not hold your 273 back.So 3600 in first gear with 2.76s is 33mph and say you are gonna shift at 4400 so that will be ~44 mph. At the shift the Rs will drop to 2600, and then,second will hit 60@3300. Wait what? That's right, with 2.76 rear gears, the only time the 4bbl is needed, is from 33 to 44 mph, in first gear. How often are you in that zone at WOT? Is the buy-in worth it to you?

 

[pishta]

I think a small 4bbl or an AVS would be a great carb instead of those relic 2bbl's we got stuck with and their single plane intakes. Many folks claim better MPG on the 4bbl transplant carbs. I dont think anyone will ask you "Aw, its got a 4bbl? Where did the stock BBD go? " when you sell it. You could always tell them the truth, "in the bin..."

 

[273]

Know not a 273 but here’s a header and 4bbl on a 292 straight 6 Chev and it gained power, and would of gain a bit more with a proper intake. Can’t see why yours could do similar.

 

[273]

Also how much stuff has to come out to put the cam in (in front of the engine)... radiator? I am having water pump replaced shortly. Should I just wait on the water pump until the cam? (I do not think it needs a water pump yet but I won’t hurt to do it.)

I’d go cam to some 250 .450 ish cam either 110 or 112 since your gonna have it half apart, I’m sure a few will disagree.

 

[RustyRatRod]

No room to do a motor swap right now and the 273 seems healthy. If it does ever die I will drop a bigger cub motor into it but for now the 273 is sufficient.

I am SLAP happy with my 64 with the stock 170. It's an absolute hoot to drive. Were it not for the worn main bearings, I'd be leaving it alone.

 

[67 gt]

I have a ‘65 Dart GT with a 273 2 barrel. (It was built the last week of production for that year which I am not sure actually matters.) It is a nice fairly clean example and am in the debate in my head of if I should make some changes come winter. I don’t want to ruin the car for whoever the next owner is but would like to have a little better performance then what the factory 2 barrel had to offer. I already picked up a D4B as I am not sure what I am going to do but they seem hard to find so when one was listed on eBay I scooped it up.
Is it a waste of money to put an intake, 4 barrel, and headers on a stock 273? Am I going to ruin the appeal of the car by taking away from its originality? If not should put in a mild cam as well?
It is mostly a street cruiser but I do want to have a little excitement getting on the freeway. Thoughts?

I felt the same as you do did some small mods to the little 273 which responded well than got hungry for more punch . The car represents factory exterior interior besides the wheels but I changed the heart and running gear and have it where it puts a smile on my face every time I’m in the saddle. As mentioned life is short enjoy it !

 

[thesiren74]

Boy, you aint right in the head.
It's only stock once, I can always tell when a car has been f--ked with.

Everything is reversible, it's just an intake and headers geez! Sometimes you do need to rebuild engines, reseal intakes you can't tell me after 50+ yrs nothing has been removed and put back on.

 

[jau126]

The engine does not suck on the carburator.
The pistons go down and create a low pressure area, which the atmosphere then tries to fill.
Your tight-gap plasma-Moly rings and perfectly round/not tapered/ not ridged, 50plus year old cylinders (cough-cough,lol) are gonna set the stage for how low a pressure your engine can create. This sets the stage for how much air is gonna find it's way in there. Next are the restrictions of the intake valves , the runners and the throttle bores. And finally, the cam decides how long this column of air is gonna be allowed to flow. If your engine can physically only induct 300cfm of air, then it matters not in the least as to power, how big a carb you put on it.
Your 2bbl is good to ~3600, It will flow enough air, to about 3600, to not hold your 273 back.So 3600 in first gear with 2.76s is 33mph and say you are gonna shift at 4400 so that will be ~44 mph. At the shift the Rs will drop to 2600, and then,second will hit 60@3300. Wait what? That's right, with 2.76 rear gears, the only time the 4bbl is needed, is from 33 to 44 mph, in first gear. How often are you in that zone at WOT? Is the buy-in worth it to you?

How often am I in first at 33 to 44 mph... admittedly almost never. (I thought I was in second by that time.... but I bet the previous owner did not adjust the speedometer for the taller tire setup. Hmmm...).
Would I at least be getting better "throttle response" either at part throttle or going to WOT with smaller primaries that won't be dumping as much fuel in...?
I understand that an engine can physically only induct so much air but the mixture in the cylinder also maters. To me that is where the headers come into play for some scavenging reducing the residuals left in the cylinder. Yes the exhaust valve will be the limiting factor but I would spec cam with a slightly longer exhaust duration to compensate for the unequal intake and exhaust valve size. This should allow for a slightly more aggressive timing. There is no way the factory manifolds can be doing any good here especially the drivers side.

I understand that the other components will end being the restriction but an additional reason for me to go to a 4 barrel set up is for tuning. Right now I have no clue what the afr/lamda is but I am guessing it is off as I live a 4000ft+ elevation and the car came from sea level. (To me all the more reason to try and get more air into the engine.) I do not think I have options to re-jet the stock Stromberg unit to get my mixture in check and advancing the timing a couple degrees to increase cylinder pressure is just a band aid. Where as an after market carburetor has plenty of support from the manufacture. The reason to go 4 barrel over a two barrel for this application is the better intake. (Plus, have you seen the price of a holley 2 barrel.) At this point I already have the manifold.

 

[jau126]

It would be interesting to build a zero deck flat top 273. The heart shaped combustion chambers would love it. It would run as good as a domed HP piston engine. May be better.

I am tempted. I have considered pulling the heads and doing a port/polish job and some decking. Once I start taking away material though is when things become non reversible. I can see myself sending the heads off the machine shop for a simple clean up and "accidentally" having some other work done.