building a 331 vs. 306

foureyed86 · Mar 29, 2010

i'm currently restoring an 86' mustang and the first step is the motor. i'm wondering whether or not its worth building a 331, or just to go with a 306. In your opinions what would the hp difference be between the 2. Also is that a practical build if the car is going to be driven not a daily driver, but driven often. give me some ideas.

tannerc91gt · Mar 29, 2010

ok, so heres how i look at it

347 > 331. for the same money why not take the extra cubes?

if you have a 302 block, have it cleaned up and get it honed, no sense getting it bored over if you dont need it, and then buying new pistons when all it needed was cleaned up. i dont care what other people say about 4ci of "high performance racing super power" its 4ci.

you will be able to produce more tq with a stroker motor obviously, but you can have all the displacement you want and if youve got a crappy H/C/I combo, you still arent going anywhere fast.

in terms of being practical, thats whats great about these cars. you can build them to be whatever you want them to be. Whether or not its streetable is up to you and depends on the parts you choose and the amount you want to blow on "go fast" parts. there are guys who DD 10 second cars and guys who have relatively stock sunday drivers. just a matter of preference really

7upstang91 · Mar 29, 2010

I would go for more cubic inches if you are going to rebuild. 331s are a more streetable motor than a 347, more ofter used for daily driven cars. The 331 is more likely to put down more torque, where the 347 puts out more horsepower.

SpartaPerformance · Mar 29, 2010

I thought this was a debate of 306 vs 331, where did 347 come in? I guess it all depends on what you plan on adding for power. A forged 306 with proper H/C/I and a nice shot of juice will rev to sky and you get you in the 10's, so will a 331 so basically it's really about budget and convenience. Like was said in a post before the best part about these cars you can build them what ever way suits you best (money & time) wise and it can be a as streetable or not depending on combo.

Grn92LX · Mar 30, 2010

7upstang91 said:
I would go for more cubic inches if you are going to rebuild. 331s are a more streetable motor than a 347, more ofter used for daily driven cars. The 331 is more likely to put down more torque, where the 347 puts out more horsepower.

Wrong. You increase the cubic inches, you increase the power potential EVERYWHERE given the same parts

foureyed86 · Mar 30, 2010

the motor is a 69' 302, so thats why i was asking about 331vs. 306, cause i know the bore is the same. someone told me not to bore it if i didn't have to. so basically like i said b4, i'm debating on 331vs. 306, but on the other hand will just honing the cylinders and keeping the btm end, w/cam upgrade be powerful enough to get into the 11-12sec. mark. will just honing and not messing with the bore make an engine last longer? what you think the max hp is on just a 302 w, bolt ons be? oh yea and im running with a carb not EFI.

NIKwoaC · Mar 30, 2010

7upstang91 said:
I would go for more cubic inches if you are going to rebuild. 331s are a more streetable motor than a 347, more ofter used for daily driven cars. The 331 is more likely to put down more torque, where the 347 puts out more horsepower.

Now, come on, man. That was just a silly statement.

foureyed86 said:
the motor is a 69' 302, so thats why i was asking about 331vs. 306, cause i know the bore is the same. someone told me not to bore it if i didn't have to. so basically like i said b4, i'm debating on 331vs. 306, but on the other hand will just honing the cylinders and keeping the btm end, w/cam upgrade be powerful enough to get into the 11-12sec. mark. will just honing and not messing with the bore make an engine last longer? what you think the max hp is on just a 302 w, bolt ons be? oh yea and im running with a carb not EFI.

Ooooh, first off, the early blocks are supposed to be tougher than the later ones. So you're off to a good start with the '69 block. The problem is, that block has little differences here and there, like it is NOT a hydraulic roller block, it should be flat tappet. I believe there are roller conversion kits out there for the older Ford engines, and a roller cam would really be ideal for streetability.

Second, if you have the cash, there is no reason not to go stroker. People like 331s over 347s for reliability reasons, but all else equal, a 347 will always outperform a 331 just because it has a 16ci advantage. 331s just have less rod angularity and taller compression height pistons, which is easier on cylinder wall loading and oil consumption.

Lastly, a GOOD H/C/I 302 will make 360-390 HP at the crank, and will easily push your car into the 12s providing you have the right supporting mods. Lots of guys out there running 11s on N/A 302s, but those are usually more extreme setups.

7upstang91 · Mar 30, 2010

Grn92LX said:
Wrong. You increase the cubic inches, you increase the power potential EVERYWHERE given the same parts

This is what I have been told. Typically, not that I have seen results. Based on the stroke. Again, this is what I have heard. Sorry for the wrong info.

90lxcoupe · Mar 30, 2010

7upstang91 said:
This is what I have been told. Typically, not that I have seen results. Based on the stroke. Again, this is what I have heard. Sorry for the wrong info.

cant always believe what you hear, and whoever told you the jiberish about 347's being unreliable is wrong too. Anything can be unreliable if the right dumbass builds it.

Go for the 347, you wont regret it

306notchback · Mar 31, 2010

347! Got all the parts for mine alone with a bare block sittin in my computer room waiting to be installed!

mwood · Mar 31, 2010

foureyed86 said:
the motor is a 69' 302, so thats why i was asking about 331vs. 306, cause i know the bore is the same. someone told me not to bore it if i didn't have to. so basically like i said b4, i'm debating on 331vs. 306, but on the other hand will just honing the cylinders and keeping the btm end, w/cam upgrade be powerful enough to get into the 11-12sec. mark. will just honing and not messing with the bore make an engine last longer? what you think the max hp is on just a 302 w, bolt ons be? oh yea and im running with a carb not EFI.

Ok, I think I see a goal, an 11-12 second street strip car (please correct me if I'm wrong). Second I see a question about honing rather than boring over. Third I see carb.

Here are my thoughts. First there is alot more to quarter mile time than just the motor. Getting a fox to run 11-12 with the right clutch, suspention, tires, ... shouldn't be rough. Second, I'd never skip the machine work. Let a good machine shop tell you what to do to the block and listen. Third, I like carb and it is easier as long as you are going naturally asperated. If you want a power adder you might reevaluate that decision. I"m not saying
it can't be done, but it is my understanding that power adders are easier to tune with EFI.

Now to the question of 306 or 331, start with the heads. If your willing to pop for good heads, than take the extra cubes. If not, ...

Cobra912 · Mar 31, 2010

well, big thing is budget. If you can afford the accompanying parts or not will determine the direction of your build.

old_blue · Mar 31, 2010

I think people all have a different degree of what they are willing to put into a motor (HCI) versus what they would sacrafice to get the build done (just doing heads then adding cam intake later).

I dont like it when people say "building a 302 and 331 cost the same so why not just build the 331" While the machine work will probably cost the same the 331 rotating assembly will cost ya some dough and more than a 302 would have. I agree with the above it all depends on cash. I wont build a 331 with stock 302 parts. I just know after doing all that work to it I am not going to tear into it again for heads and what not. But if you have the money for new heads and what not then I say do it.

I also agree that you can build a stout 302 for the money versus building a mild 331 with the same money. I have seen many 302's in the past that walked 331's due to the 331's not being properly planned right.

Many people build their engines different ways. To me the heads are what is going to tell which direction to head. Most 165 Heads can be put on a stock motor without issues but you go up to 185 and 205s then you started running into PV clearance.

Just depends on the funds my man. good luck and keep us posted.

building a 331 vs. 306

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