302 - 306

[LxVert]

Hey everyone,

The 302 started began to get tired so I brought the motor into the machine shop for a simple hone, clean and all new bearings, well they told me that one piston was cracked and the cylinders were going to need a bore. The shop is ordering pistons and boring it 30 over, I'm staying with the factory compression ratio. The question I have is will there be any power gain in this 4ci increase, I know its going to be a noticable difference from the tired 302 compared to a fresh motor, but wasn't sure if I have a couple extra horses with the 306

Any input would be great

Thanks for the help

 

[5spd GT]

You might see a couple HP, after it frees up and "breaks-in."

Remember, with all else being equal, the 306 will make more compression, with the larger sweeping volume.

 

[NIKwoaC]

Like said above, you might see a very slight increase in performance, but probably not anything you will really feel. I did some quick and dirty calculations that show you might get 0.1 unit increase in compression ratio, but likely less than that because your combustion chamber volume will be increasing with the larger bore. Anyway, you will benefit from the overbore, likely about 1% of power potential beforehand. The real benefit is having lost performance back.

For reference:

Compression Ratio=(Displacement Volume+Combustion Volume)/(Combustion Volume)

Edit: That was post no. 150! Woo hoo!

 

[90topdown]

As long as it is apart, why not go 347 in the short block??????

 

[85_SS_302_Coupe]

You're talking about 1/2 cubic inch per cylinder increase....any gains are going to be from increased compression and new rings.

 

[LxVert]

As long as it is apart, why not go 347 in the short block??????

I wanted to go to a 347 but it was way out of my price range for building, I was trying to stay in the $1000 range for the entire short block and the 347 was goin to put it up to $2000+

 

[Strype]

done it.

If you notice the difference it's in your head. the only difference is tired vs new and you should drop a the et just a hair. anything else is bs


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[LxVert]

Alright thanks for the help yeah I didn’t think it would be any gains over the lost power of the tired motor.

Next question I have is about pistons, how bad of an idea would it be for going to a hyper piston vs. a forged piston? I have a low budget because I’m still in college, so I was looking at putting in Federal Mogul hyper pistons in.

 

[NIKwoaC]

If you ever want to go with a forced induction or nitrous application, you want to stay away from hypereutectic pistons. Other than that, you can get away with using them. They are actually better for emissions, if that concerns you at all.

 

[ID89GT]

Ya im kinda in the same boat, Im pullin my motor tomorrow to do a rebuild on it, and was wondering if I should just leave it as a 302 or bore it out, but then wouldn't I need new pistons and such, Idk lol.

 

[LxVert]

Would something small like 5 - 6 lbs of boost even be to much on the hypers and just destroy them? As far as emissions I don't have to go through them.

 

[NIKwoaC]

I've read articles about people doing light boost with hypereutectic pistons. The idea is that you have to be spot on with the tune from the get go, because its really detonation that kills hypereutectic pistons. Forged pistons can handle a bit more abuse, thats why they are preferred for boost.

 

[Stopsign32v]

Would something small like 5 - 6 lbs of boost even be to much on the hypers and just destroy them? As far as emissions I don't have to go through them.

No. Anyone that says a power adder will kill a hyper piston is a moron. Your tune will determine how long a set of crappy pistons will last.

Also if you are rebuilding the motor and changing the pistons IMO you are stupid for keeping the compression ratio the same as stock. If you are going all motor or nitrous bump it up to 10.1:1-11.1:1. If you plan to put boost through the motor bump it up to 9.5:1-10.1:1.

 

[LxVert]

I thought stock compression was 9.5:1? If not what is stock compression?
While I have been going back and forth with the machine shop they told me that with the set up im I have it will be 9.5:1 and so I thought that is what stock was.

 

[LxVert]

I spoke more with the machine shop today and asked about rasing my compression and they told me that I could bring in my heads and have them shaved down and I might see about 15 hp getting it to about 10:1. The only concern they had was piston to valve clearance issues. My heads are standard E7's flow bench ported with gt40 valve package, E303 cam with 1.7 rockers.
Before I bring the heads in would anyone have any input for this? Would I be able to shave the heads down to boost up the compression or would it be putting the valves to close to the pistons.

 

[NIKwoaC]

Stock compression ratio is about 9.0:1, with some variation depending on head gasket thickness. You can increase compression ratio somewhat by installing a thinner head gasket, shaving the heads, or buying pistons with less head volume (dome pistons). While you should always check piston-to-valve clearance, I'd think that dome pistons with valve notches would be less of a concern with clearance, because they are designed to work with normal valve travel to begin with. Shaved heads lose that function.


On the boost note, in my experience/understanding, 8.5-9.0:1 is more ideal for boost. Seems to me that starting with 10.0:1 static compression ratio, hypereutectic pistons and some amount of forced induction is recipe for disaster. Nitrous is especially finicky, and using large amounts of that with hypereutectic pistons is risky business, too.

 

[NIKwoaC]

Also, keep in mind that if you go 9.5:1 compression ratio or higher, you are likely going to have to use higher octane gas (i.e. 91-93) to avoid compression-ignition (knock/detonation).

Just a thought.

 

[LxVert]

Alright, well boost is something that is a long long long ways away, for now it is only goin to be N/A. The octane note, I have run 93 in that since the day I bought the car so that doesn't bother me much.
I think i'm goin to drop the heads off there and have them see how much they can safely get the compression up to.
With goin to 10:1 would people agree with the machine shop that its about a 15hp difference?

 

[Stopsign32v]

Alright, well boost is something that is a long long long ways away, for now it is only goin to be N/A. The octane note, I have run 93 in that since the day I bought the car so that doesn't bother me much.
I think i'm goin to drop the heads off there and have them see how much they can safely get the compression up to.
With goin to 10:1 would people agree with the machine shop that its about a 15hp difference?

Go with 10.1:1 compression. Even if you plan to run boost you will be FINE unless you plan to make 700rwhp on pump gas. The absolute only reason to go with low compression is the ability to run very high horsepower numbers on pump gas. But then you are talking a $2,000 block, $1,000 worth of machining the block, internals, fuel system, and power adder. For now put the compression around 10.1:1 and enjoy your car.

Also I would like to throw in the fact that you could get a cast Scat crank for $200 and a set of FORGED Scat I beam 5.4 rods for around $250. So add $450 to your build and you can have a 331 or 347 stroker instead of a 306. A 331 or 347 at 10.1:1 compression would be a nasty little street car. Might change your mind about boost....

 

[LxVert]

Ok, Saturday i'm goin to the shop with my heads and they are goin to take some measurements to make sure that I will be able to shave down my heads to get that 10:1 ratio.

I have a long ways to go with this, I'm great with standard repairs, but with trying to make more power I dont know my a$$ from my elbow.