273/4 -318 debate

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NHRA specs are the limits to what you can machine parts to and be NHRA legal.... not what typically came from the factory. They give one an idea general differences, not exact production differences.
 
exactly
and even if they are not absolutely accurate,
they are probably relatively accurate.

Early teeners, when we still had good gas, were fairly stout, even in tank-cars.Of course tanks weren't tanks in comparison to the smog-era cars. A-bodies got to 3600pounds, which was formerly B-body territory. Eventually Ms got to formerly C-body territory. And the power kept falling and falling.
Now we were used to gutless cars
And then the K's came along.
Yahoo :(
 
Last edited:
the t
AJ/FormS said:
you know; we can always fall back on the NHRA factors. They give us a horsepower factor, and the blueprint to get there.
They give us;
67.... 273 2bbl@180hp with; 9.5Scr, 57.3cc heads, -.011 decks, .028 gasket
67.... 273 4bbl@235hp with; 11.5Scr, 57.3cc heads, +.129decks, .028 gasket
----------
67/69 318s not spec'd with; 9.5Scr, 57.3cc heads, close to zero decks.......
78/80 318 4bbl@230hp with; 9.0Scr, 65.5cc heads,, -.041 decks, .030 gasket
-----------
71..... 340 4bbl@330hp with; 11.0Scr, 63.3cc heads, +.045decks, .028 gasket
72/73 340 4bbl@275hp with; 9.3Scr, 64.7cc heads,, -.054 decks, .036 gasket
------------
74/76 360 4bbl@300hp with; 9.0Scr, 64.7cc heads,, -.067 decks, .032 gasket
78/80 360-4bbl@250/265 w/; 8.6Scr,68.4cc heads,, -.067 decks, .030 gasket

My Bulletin Book does not show the early Hi-compression 318s
Click to expand...

the thing about these NHRA specs is; they are a recipe.
They are saying if you build to such and such a recipe, regarding compression and the factory parts;then they are gonna factor you to the listed horsepowers, and there is nothing the racer can do about it; if you don't like it, build some other brand.
Now; if you look at the recipes, there is a buttload of interesting gems to be found, especially as regards compression.

Take a look at the 360s for instance, first the 74/76 ones;
To make 9/1 at stock bore,requires a swept of 737.2/8.0=92.15cc, total chamber volume.
and the decks are 13.8cc plus gasket at ~7.2 plus head at 64.7=85.7cc; leaving 6.45 in the dish.
Next, the later 360; requires 97.00cc; which being
68.4+13.8+6.75+6.45=95.4...... where's the missing 1.6cc?
Ok I get that 1.6cc does not seem like a really big deal;the point is this;
where did the; 300 less 250=50 horsepower go? As far as I know the cams where the same, and NHRA says they are both 4bbls..... so where is the 50hp?
Is NHRA saying .4 of a point of compression is good for 50hp? interesting gem that.

But it gets better
look at the 340s. From 71 to 73 they all had TQs and the same cam and heads.... so are they saying that 1.7 points of compression makes 55 horsepower? Sweet gem

What about the 318s?
My research shows the 69 318 2bbls to be rated at 230hp.. with 2bbls and either 9.2 or 9.5 Scr depending on the source.
NHRA says they can build a 78/80 4bbl teener at 9.0 and get the same 230hp. AFAIK the cams were the same. What are they saying? That a half a point of compression is equal to big-port heads and secondaries?.. What a gem this one is

Anyways, jus ruminating
 
‘74 318. Duster probably weighs about 3200. Has gone 13.69@99mph. I think that comes out to about 230hp.

20F3BD4F-7333-4494-88E0-009D332DCF38.jpeg
 
pishta said:
Were having a discussion on the merits of the 273-4 engine compared to the 318-2 of the following years, sometimes even the same year. Sure the 273-4 was a hot motor back in 65 when it was the best thing between the fenders of the early A's but you just can't deny that a 20% larger motor is going to make a difference. Performance test are all over the board, depending if the dufus can drive or if the dyno is set up properly, yadda yadda...but I found a site that has a lot of info on cars in general and has generated graphs (from factory data) on performance specs. So I tried to match 2 like cars (in weight, gear and transmissions) with our 273-4 and the 'new' 318-2. Results? Well, I think it would ultimately come down to the reaction time between these 2 motors as they are pretty close. You can the shift points are higher on the smaller higher winding 273 and the torque advantage goes to the 318 down low. Theoretical top speed goes to the high winding 273 due to the cam profile and possible higher RPM potential of its heavier valve springs and forged crank.
1965 273-4 Barracuda
reference weights: shipping weight 3029 lbs base curb weight: test weight 1430 kg / 3152 lbs
how fast is this car ? top speed: 126 mph (©theoretical);
accelerations: 0- 60 mph 6.9© s; 0- 100 km/h 7.3© s (simulation ©automobile-catalog.com); 1/4 mile drag time (402 m) 15.6
View attachment 1715247101







1968 318-2 Barracuda
reference weights: shipping weight 1363 kg / 3005 lbs base curb weight: 1430 kg / 3152 lbs
how fast is this car ? top speed: 178 km/h (111 mph) (©theoretical);
accelerations: 0- 60 mph 6.8© s; 0- 100 km/h 7.3© s (simulation ©automobile-catalog.com); 1/4 mile drag time (402 m) 15.4© s

View attachment 1715247102



overlaid


View attachment 1715247103

<<<lower 75 from tq curve points to align 5250rpm intersection>>>
<<<the tq graph should start at 100 so the data points align SAE>>>

and the overlay of the HP/torque curves from factory data. increased displacement really shows in the torque curve but the smaller 273 can hold its HP peak longer. This is due to camshaft differences. Put a similar cam in a 318 and its curve would change.



Bottom line: in a 1/4 mile race all things being somewhat stock on these 2 cars, the 318 would edge it by .2 seconds..theoretically. Now on the highway having some fun with your buddy, the 273 would probably walk away after 4500 RPM. Now put the 4bbl cam in the 318 and it would probably be a 20% increase across the board on the 318 data. To say the 273-4 would walk a 318-2 in a drag race is not supported in this data. All data is from blueprint specs and we all know how well those are kept in production, especially compression ratios.
Food for thought...or to fling?
Click to expand...
Why don't we just get some one from the form with a stock commando motor and a stock 318 and go at it. Best 2 out of 3.
 
AJ/FormS said:
the t


the thing about these NHRA specs is; they are a recipe.
They are saying if you build to such and such a recipe, regarding compression and the factory parts;then they are gonna factor you to the listed horsepowers, and there is nothing the racer can do about it; if you don't like it, build some other brand.
Now; if you look at the recipes, there is a buttload of interesting gems to be found, especially as regards compression.

Take a look at the 360s for instance, first the 74/76 ones;
To make 9/1 at stock bore,requires a swept of 737.2/8.0=92.15cc, total chamber volume.
and the decks are 13.8cc plus gasket at ~7.2 plus head at 64.7=85.7cc; leaving 6.45 in the dish.
Next, the later 360; requires 97.00cc; which being
68.4+13.8+6.75+6.45=95.4...... where's the missing 1.6cc?
Ok I get that 1.6cc does not seem like a really big deal;the point is this;
where did the; 300 less 250=50 horsepower go? As far as I know the cams where the same, and NHRA says they are both 4bbls..... so where is the 50hp?
Is NHRA saying .4 of a point of compression is good for 50hp? interesting gem that.

But it gets better
look at the 340s. From 71 to 73 they all had TQs and the same cam and heads.... so are they saying that 1.7 points of compression makes 55 horsepower? Sweet gem

What about the 318s?
My research shows the 69 318 2bbls to be rated at 230hp.. with 2bbls and either 9.2 or 9.5 Scr depending on the source.
NHRA says they can build a 78/80 4bbl teener at 9.0 and get the same 230hp. AFAIK the cams were the same. What are they saying? That a half a point of compression is equal to big-port heads and secondaries?.. What a gem this one is

Anyways, jus ruminating
Click to expand...

A correction on the 71 to 73 heads the 71 used a 2.02 intake valve 72 and 73 used a 1.88 intake valve so they are not the same heads even though they may have the same part number on them.
 
Jadaharabi said:
A correction on the 71 to 73 heads the 71 used a 2.02 intake valve 72 and 73 used a 1.88 intake valve so they are not the same heads even though they may have the same part number on them.
Click to expand...

Thanks for the reminder; but I don't think the slow-compression engines care all that much.
 
AJ/FormS said:
Thanks for the reminder; but I don't think the slow-compression engines care all that much.
Click to expand...
.
The 71 is not a low compression engine. It had more compression more valve more cam and a 1 year only Thermoquad with backward idle adjustments.
72-73 are low compression small intake valves with a better Thermoquad.
 
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Jadaharabi said:
.
The 71 is not a low compression engine. It had more compression more valve more cam and a 1 year only Thermoquad with backward idle adjustments.
72-73 are low compression small intake valves with a better Thermoquad.
Click to expand...

AJ/FormS said:
71 also had the big valves
Click to expand...
.
That was the more valve part. lol
 
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