2V or 4V

[Jayro]

Ok so going insane trying to work out exactly what headers I need to suit/fit my 73. Current hurdle is if I require to suit 2V or 4V heads. How do you determine what heads you have or is the only way to remove the exhaust manifold and see?

Cheers
Jase

 

[Doug's 73 Mach1]

For 351C engines 4V heads have "4" cast on a corner of the head next to the valve covers. 4V heads were used with 4 barrel carburetors, intake manifolds. 2V heads with 2 barrel carburetors, intake manifolds. If your engine has not been modified, the factory engine code is part of your VIN. It is a letter designation as the 5th character. That letter code can be used to determine what engine the car came with from the factory. Knowing the engine tells you weather it was a 2V or 4V from the factory.

 

[Jayro]

For 351C engines 4V heads have "4" cast on a corner of the head next to the valve covers. 4V heads were used with 4 barrel carburetors, intake manifolds. 2V heads with 2 barrel carburetors, intake manifolds. If your engine has not been modified, the factory engine code is part of your VIN. It is a letter designation as the 5th character. That letter code can be used to determine what engine the car came with from the factory. Knowing the engine tells you weather it was a 2V or 4V from the factory.

Ok cheers. Unfortunately the vin num is no use. Car was a 6cyl originally but running a 351c these days. I’ll go over it tomorrow and see if I can find a “4” on a corner

 

[71ProjectJunk]

The 2V heads will have a 2 on the corners and the 4V heads will have a 4 with or without a dot besides the 4. Here is a good video on how to identify what you have, you do not need to pull anything off, lust look at the corner of the heads:

 

[Stanglover]

I would also want to check the date code(s) on those heads. Date codes will be found between the valve springs, so easy to find. I can't speak for the actual exhaust port size on 72 or 73 heads, but one of my (M code) 71 head date codes for example is 0M1 1970 Dec 01 and therefore the LARGE exhaust port opening. From the date codes, you can cross reference to the port size for your engine.

 

[Jayro]

The 2V heads will have a 2 on the corners and the 4V heads will have a 4 with or without a dot besides the 4. Here is a good video on how to identify what you have, you do not need to pull anything off, lust look at the corner of the heads:

Legend, thanks heaps

 

[Jayro]

Thanks guys. So I’ve confirmed this morning mine are 2V. A set of alloy 4V heads and a new cam be much of a performance gain? I’m leaning that way if it’s a maybe say a 50hp gain.

Cheers

 

[71ProjectJunk]

It is so difficult to estimate HP gains, but a stock 351 2V engine had 240 HP and a stock 4V engine had 300 (285 in 71). So, if you upgraded your heads, cam, intake and carburetor, exhaust manifolds and dual exhaust, to the 4V factory stuff, you should see 50HP easily. Just the heads will not do it. Having said that the 2V heads are very decent street heads, if you engine is stock, you should easily get 50 hp by getting a good aftermarket aluminum 4V intake manifold for 2V heads, a good 4 barrel carburetor, some headers, a good dual exhaust system, and a bigger cam. The biggest issue that I see with the 2V heads is not the size of the ports or valves but the open chamber in the heads that will limit your compression ratio, and what you can do cam wise with the lower compression. I am sure someone that has a modified 2V headed engine will chime in. You can make 400 HP with basically stock 2V heads, they flow more than enough air to get there.

Here is a dyno run of a 351 C with 2V heads making 434 hp:

 

[Jayro]

It is so difficult to estimate HP gains, but a stock 351 2V engine had 240 HP and a stock 4V engine had 300 (285 in 71). So, if you upgraded your heads, cam, intake and carburetor, exhaust manifolds and dual exhaust, to the 4V factory stuff, you should see 50HP easily. Just the heads will not do it. Having said that the 2V heads are very decent street heads, if you engine is stock, you should easily get 50 hp by getting a good aftermarket aluminum 4V intake manifold for 2V heads, a good 4 barrel carburetor, some headers, a good dual exhaust system, and a bigger cam. The biggest issue that I see with the 2V heads is not the size of the ports or valves but the open chamber in the heads that will limit your compression ratio, and what you can do cam wise with the lower compression. I am sure someone that has a modified 2V headed engine will chime in. You can make 400 HP with basically stock 2V heads, they flow more than enough air to get there.

Here is a dyno run of a 351 C with 2V heads making 434 hp:

Ah that’s awsum info thank you. Mate I’m not after a heap of hp. She already has an upgraded Holley and manifold and the headers and new exhaust get done next week. I might see how it is after that and a tune and go from there.

 

[73inNH]

I might see how it is after that and a tune and go from there.

You going to plug the tuner into the obdc port?

 

[Superbond]

If that’s a 72-73 2v. It has 164hp @4000
I too have one I changed to a 4 barrel. Did what your talking about doing. I put mine on a Dyno and she only makes 214 hp at the tires. I little disappointing. So I changed the rear end from a 2.75 to a 3.50. Little bit of a kick in the butt on take off now

 

[Superbond]

Oh
I used patriot ceramic long headers. Fits like a charm! 2 or 4 barrel heads

 

[Stanglover]

If that’s a 72-73 2v. It has 164hp @4000
I too have one I changed to a 4 barrel. Did what your talking about doing. I put mine on a Dyno and she only makes 214 hp at the tires. I little disappointing. So I changed the rear end from a 2.75 to a 3.50. Little bit of a kick in the butt on take off now

On the 72/73 HP, correct me if I'm wrong ( I didn't live this side of the pond then) but wasn't there a change in how HP was calculated, from gross to net HP? That would be why the HP rating seemed much lower than 71.

 

[Michael O’Harran]

It is so difficult to estimate HP gains, but a stock 351 2V engine had 240 HP and a stock 4V engine had 300 (285 in 71). So, if you upgraded your heads, cam, intake and carburetor, exhaust manifolds and dual exhaust, to the 4V factory stuff, you should see 50HP easily. Just the heads will not do it. Having said that the 2V heads are very decent street heads, if you engine is stock, you should easily get 50 hp by getting a good aftermarket aluminum 4V intake manifold for 2V heads, a good 4 barrel carburetor, some headers, a good dual exhaust system, and a bigger cam. The biggest issue that I see with the 2V heads is not the size of the ports or valves but the open chamber in the heads that will limit your compression ratio, and what you can do cam wise with the lower compression. I am sure someone that has a modified 2V headed engine will chime in. You can make 400 HP with basically stock 2V heads, they flow more than enough air to get there.

Here is a dyno run of a 351 C with 2V heads making 434 hp:

So I'm gonna totally botch my attempt at regurgitating something my mechanic told me about my motor build. I have a 351C, with aluminum Edelbrock performer intake manifold and heads. The heads are 2V. The piston compression is 10:1 and its bored .30 over. It has a 4 barrel Demon Carb. Went back from headers to stock exhaust manifolds. I am hoping it will do between 360-400hp, but unsure and not gonna get it dynoed. But I was told that when it came to the 2V or 4V, some people would intentionally use 2V heads with a 4bbl carb, something about the restriction adds pressure or something. Its kind of like mine has a 4 bolt main, but I'm told it strong than what I need so its kind of not important, and likewise if I had 4V heads, I'd be using a lot more gas for little end difference. I don't know something like that. Now's the time for people who know what they are talking about, unlike me to have plenty of comments I'm sure. Ultimately I keep trying to learn as much as I can so that when I talk about my car to people who ask about it, I can speak intelligently about it and what I've had done to it, etc.

 

[71ProjectJunk]

On the 72/73 HP, correct me if I'm wrong ( I didn't live this side of the pond then) but wasn't there a change in how HP was calculated, from gross to net HP? That would be why the HP rating seemed much lower than 71.

Yes, the horsepower rating went from "gross" to "net" in 1972. So, the 240 HP gross engine went down to 164 net because of the lower compression in 1972 and mostly because of the change in how HP was measured. In 1971 some manufacturers rated their engines in both net and gross hp. Just the change in how engines HP was measured easily took out 20-25% of the advertised power. Net HP measures the engine as it is run in the car with manifolds, full exhaust, all emission equipment, alternator, water pump, PS, AC, engine fan, air cleaner, everything as the engine runs in the car. Gross HP is how HP is usually measure on your local dyno room, engine with no accessories, water pump run by an electric motor, no emission, no air cleaner, no exhaust. There is a huge difference in power when you have no accessories, air cleaners, exhaust etc...

As for wheel HP like Superbond measured on his car at 214 HP. Wheel HP is even lower than net HP, because that takes into account ALL parasitic loses including the transmission, and rear end. No manufacturer has ever used rear wheel horsepower to rate their engines. Rear wheel HP on an automatic transmission car will be anywhere from 15-20% less than what net HP is. On a Mustang with an automatic transmission and a 9" rear end you are looking at closer to the 20% mark. So, that 351 which made 214 HP at the wheels, probably was making close to 260 HP at the flywheel (net HP as all engine accessories and exhaust are hooked up), and easily 330-350 Gross HP as HP was measure in the muscle car era. To put that into perspective, in 1971 Chrysler rated all their engines in net and gross HP. The 426 Hemi was rated at 425 gross and 350 net HP, that is a 75 HP drop in just how the engine HP is measured. Chevrolet did the same thing the 1971 350 LT1 was rated at 330 gross HP and 255 net HP, that is a drop of 75 HP, just on the way HP was measured.

 

[cjstaci]

The info provided here is good. The main thing to know is to use parts that compliment each other for your intended use. 2v heads are great for street use. 4v heads are great for higher RPM use. Then there is open chamber and closed chamber heads, the difference is 2 to 3 points of compression ratio. Open chamber heads typically provide around 8 to 8.5 to 1 ratio, while closed chambers provide around 10.5 to 11 to 1 ratios. You can run closed chamber heads on pump gas but tuning is critical to avoid pre-ignition. Cam selection is very different depending on your compression ratio. Cam grinders can optimize a cam if you provide them with accurate info of your engine and your intended use.
In the past, I’ve known people that have used the wrong cam on low compression engines, they sounded okay but didn’t make the power.
1971 was the last year of high compression engines. 1972 all manufacturers lowered compression ratios, ******** ignition and valve timing to meet emissions standards. They also changed from brake hp ratings to net hp rating’s.

 

[Stanglover]

Yes, the horsepower rating went from "gross" to "net" in 1972. So, the 240 HP gross engine went down to 164 net because of the lower compression in 1972 and mostly because of the change in how HP was measured. In 1971 some manufacturers rated their engines in both net and gross hp. Just the change in how engines HP was measured easily took out 20-25% of the advertised power. Net HP measures the engine as it is run in the car with manifolds, full exhaust, all emission equipment, alternator, water pump, PS, AC, engine fan, air cleaner, everything as the engine runs in the car. Gross HP is how HP is usually measure on your local dyno room, engine with no accessories, water pump run by an electric motor, no emission, no air cleaner, no exhaust. There is a huge difference in power when you have no accessories, air cleaners, exhaust etc...

As for wheel HP like Superbond measured on his car at 214 HP. Wheel HP is even lower than net HP, because that takes into account ALL parasitic loses including the transmission, and rear end. No manufacturer has ever used rear wheel horsepower to rate their engines. Rear wheel HP on an automatic transmission car will be anywhere from 15-20% less than what net HP is. On a Mustang with an automatic transmission and a 9" rear end you are looking at closer to the 20% mark. So, that 351 which made 214 HP at the wheels, probably was making close to 260 HP at the flywheel (net HP as all engine accessories and exhaust are hooked up), and easily 330-350 Gross HP as HP was measure in the muscle car era. To put that into perspective, in 1971 Chrysler rated all their engines in net and gross HP. The 426 Hemi was rated at 425 gross and 350 net HP, that is a 75 HP drop in just how the engine HP is measured. Chevrolet did the same thing the 1971 350 LT1 was rated at 330 gross HP and 255 net HP, that is a drop of 75 HP, just on the way HP was measured.

Very well explained, thank you. That's is pretty much how I understood it to be, now I know, as do others.

 

[Stanglover]

The info provided here is good. The main thing to know is to use parts that compliment each other for your intended use. 2v heads are great for street use. 4v heads are great for higher RPM use. Then there is open chamber and closed chamber heads, the difference is 2 to 3 points of compression ratio. Open chamber heads typically provide around 8 to 8.5 to 1 ratio, while closed chambers provide around 10.5 to 11 to 1 ratios. You can run closed chamber heads on pump gas but tuning is critical to avoid pre-ignition. Cam selection is very different depending on your compression ratio. Cam grinders can optimize a cam if you provide them with accurate info of your engine and your intended use.
In the past, I’ve known people that have used the wrong cam on low compression engines, they sounded okay but didn’t make the power.
1971 was the last year of high compression engines. 1972 all manufacturers lowered compression ratios, ******** ignition and valve timing to meet emissions standards. They also changed from brake hp ratings to net hp rating’s.

Good info also, thanks.

 

[Mister 4x4]

Lots of good info here. Depending on how far you're willing to go with things, any of the hard numbers provided are able to overcome. For instance, the 8.0-8.5:1 compression for the 2V heads - yes, that's a factory number used with the factory dished pistons. However, a set of aftermarket flat-top pistons can get you whatever compression you're after (For instance, I have some .060" KB Hypereutectic Flat-Tops in mine with 9.5:1 compression with the stock 2V heads). Using an engine calculator (CamQuest) found on CompCams website when putting together my list of go-fast goodies, I was able to come up with a recipe to get 406hp (at the crank) using the 2V heads. Granted, mine was seized, and I had to rebuild everything anyway... so, instead of spending a big chunk of money to rebuild back to stock-ish, I decided to spend a bigger chunk and warm things up a bit (since it's a restomod, anyway). The things I went with to warm-up my '71 H-Code:

• Bored (.060"), balanced, blueprinted, mild port work on the stock 2V heads, stock crank
• KB .060" 9.5:1 flat tops
• CompCams K32-421-8 full kit (Hi-Energy roller cam - 270/270 @ .566" lift, roller lifters, push rods, double roller timing gear/chain, etc.)
• Crane Cams 7/16" screw-in studs
• CompCams Hi-Energy aluminum 1.72 roller rockers
• Edelbrock Performer 4bbl intake
• Edelbrock Performer 1406 carb (to be replaced with Edelbrock E-Street TBI Fuel Injection)
• Holley Mighty Mite electric fuel pump
• Duraspark solids state ignition system (fits perfectly with factory Ram Air air cleaner)
• Accel 45KV Super Coil & 8mm spark plug wires
• "Pantera Mod" 185 degree themostat and restrictor plate
• Hooker Ceramic-coated Competition Long Tube Headers
• Pypes Street Pro 2.5" stainless exhaust w/X-pipe
• AOD from '89 5.0 Mustang

Again, it all depends on how far you're willing to go with things. Even with 3.00 rear gears, I can do burn-outs all day, and comfortably cruise the highway at around 1800 rpm thanks to the AOD's overdrive. A lot of people will look at the .060" over and say that my engine is doomed to overheat and explode, taking out a city block, half the power grid, and disrupt beer distribution for the next 3 years. But it's running at a solid 190-ish even on the hottest triple-digit days here in West Texas. If it was a race car, I could see survivability being an issue, but not the case for a fun car for casual driving.

 

[steves73]

Cam and compression are the most important to move to the next level. I couldn't afford a roller cam, just a high lift short duration. The 2v head has better torque than the 4v for regular street use like mine, and still flows better than other small block heads.