Should I go to a 331 over a 306?

[Andres2882]

Planning to put a crate engine in my car soon and I was trying to decide whether I should get a 306 or step up to the 331. Question is, I don't really know which would be better for a mostly street vehicle. Will a 331 have a much rougher idle? What would be the pros and cons?

 

[89CopCoupe]

Power is about getting higher volumes of air/fuel in & out of an engine in the least amount of time. Cubic inches = volume .... RPM = time.

Based on simple physics, a 331 cubic inch engine has a stronger (higher volume) pull through the intake tract ... as well as push more exhaust out of the exhaust tract through 1 cycle or RPM.

Think of the piston/cylinder as just a big syringe. A 331 will take in 331 worth of air/fuel in the same RPM as a 306 takes in only 306 worth of air/fuel (as a general example).

If you were to bolt the same stock 302/306 parts onto a 331 it will make a bit more power based on the above principle. A larger engine will mellow out a camshaft because it desires more than a smaller engines camshaft provides.

As far as daily driver is concerned, it mostly about camshaft selection. The cam is basically the brain of the operation. Other factors play a role as well. Such as heads/intake/tranny/gears to compliment the desired driveability goal. It's most important to match all of these parts to work well with eachother.

Most big-name manufacturers have tried to simplify the matching process by providing 3 simplified stages of performance:
1- mild (street ... example - Edelbrock Performer)
2- medium (street/drag ... - Edelbrock Performer RPM)
3- hot (race ... - Edelbrock Victor)

A rough idle all depends on how you build it and mostly what cam you choose.

331 pros : better power (faster than a car with an equally built 306) ...
331 cons : gas mileage not as good as a 306
306 pros : better gas mileage than a 331
306 cons : less power (slower than an equally built car with a 331)

This is of course assuming the cars weigh the same. Hope that helps a little .... anything I missed

 

[Venom4.6]

Just like the saying goes, "There's no replacement for displacement".
Like 89copcoupe said, Your H/C/I combo is the key.

 

[tjm73]

Defintely no replacement for displacement. Yes go 331 or even better 347 (it'll cost the same money).

331 is not going to inhearintly idle rougher. That's a carateristic of the cam profiel you choose.

If going carb don't rule out the Weiand Stealth Intake. It's good to 6800 rpm and IMO it's a better intake than the Edelbrock Performar or Performer RPM or even the Air Gap. And it's much less expensive.

 

[bln92cpe]

I have a blown 306 and couldnt be happier..If your going the blower route talk to siresureshot about his blown 331.

 

[Andres2882]

I figured as much. I want to be able to run super low 12's injected and was previously thinking about just putting a blower on my 302. I have only recently started thinking about going the bigger displacement route. How much more gas are we talking? Any more than a blown 306? Also, someone had told me that 331 and 347 were less reliable and wouldn't last as long as a 306. Any truth to that?

 

[T_5.0_N'works]

If you are THAT worried about gas, why do you even have a v8 guzzler? Go 331, you will be fine.

 

[tjm73]

I figured as much. I want to be able to run super low 12's injected and was previously thinking about just putting a blower on my 302. I have only recently started thinking about going the bigger displacement route. How much more gas are we talking? Any more than a blown 306?

Fuel usage is not so much about displacement vs power adder. In use, it'll be pretty similar probably, but there is no way to know for sure. There are too many variables.

Also, someone had told me that 331 and 347 were less reliable and wouldn't last as long as a 306. Any truth to that?

There used to be some talk about 347's consumming some oil and not lsting as long, but that has almost completely gone away. I've never heard that the 331 is not a long lastign engine. I plan to build a 347, but instead of the common 5.4 rod I am planning on using a kit with 5.315 rods so that the bottom opil ring and the piston pin do not encrouch on each others space. This should eliminate the alledged oil consumption issue.

As for how long they last, it depends on how you use them and how you build them. Built and cared for right, the 347 will last just as long as any garden variety 306. Same with the 331.

 

[90mustangGT]

Good info on this thread. Unless you have the H/C/I or at least the H/I you will gain plenty of tq, but say a highway pull there won't be much difference. Same thing with a high revving 306, more rpm means more pull on the intake and with the right supporting components, alot more power, but top end power, say goodbye to your low end TQ. A V8 has enough tq to sacrifice.

It's all about getting more air into the engine, so you can get the right about of fuel and compression/heat to make power and then getting it to the wheels/ground more effeciently.

I'd like to do a stroker, when I finish paying for this tranny, but I might do an engine stand buildup. Since I would have to replace pretty much everything, and it cost about the same, I would do a 351W stroker, maby a 408/392. The wait would suck but once I got done I would be very happy.

 

[NKau]

Still haven't got a combo figured out yet Andres? If you're dead-set on running LOW 12's (and still maintain streetability, keep a/c, etc.) you will need a stroker engine with well matched h/c/i, or go with a blower or nitrous on the smaller motor. I'd be half tempted to put together a cheaper 306 with GT40 heads/intake, put a nice vortech or procharger on it, and run the thing 'til it pops. Just be advised that you will want to at least put in torque box reinforcements and subframe connectors before running DR's or slicks and not be surprised if you pop your 8.8 or tranny. If you don't powershift or launch hard, I wouldn't imagine you will break too much.