Horsepower in 302 vs 306????

[Blown331LX]

Well after driving a stock 100,000 mile 1994 bottom end mixed with a new upper end for the past 1,000 miles it has finally eaten up the oil rings and smokes like crazy at idle but runs great. Anyway...Im pulling the engine back out to replace the rings, and ditch those ****ty cast aluminum pistons and switched to forged. The cylinders have the perfect cross hatch pattern and no ridge and DONT NEED to be bored but I was thinking if Im going to pull it out I might go ahead an bore it .030 over. So now my debate is to use used stock 5.0 forged pistons standard bore with new rings / or / spend another $400 for the bore and bigger pistons and end up with a 306......How much more power will the 306 make over the 302 ??? I have no power adders but plan on adding juice or boost down the road. Also which rings are best for sealing and longevity????

 

[jhonda]

it might add about 3horsepower

 

[mrmustangman357]

the power gain would hardly justify the cost for that. paying the extra dollars would simply bring you peace of mind. If it was me, and I knew I was doing a power adder, I would add forged aftermarket pistons. that way you can tailor your compression ratio for boost if you are seriously considering it and are ready to commit. Either way, keep the stock bore. When you take it to a machine shop for a hone with torque plate (right??????? no bottle brush!!!), they will soon find if there is any taper or out-of-round issues. THEN, you might want to do an overbore.

 

[ninety15.0]

yup there is no advantage to boring the cylinders just for horsepower. The gain would be unnoticeable. I think you are better of just doing a cheap rebuilt with rings/bearings or just doing a stroker setup is you really want to tear into it. Thats the only way you will build actual power is with more cubes.

 

[jake2582]

it might add about 3horsepower

i was thinking 4

 

[Blown331LX]

LMAO, well 4 is better than 3, and I was only hoping for 2, so I geuss were on the up and up. Yeah I didnt think it would be much at all, Ive got friends running in the 10" with a stock bottom end with a little help from either a turbo or the bottle. Well thats what Im going to do. Stock forged pistons and new bearings and rings with a fresh ball hone run through the cylinders. I dont want a stroker, have had a few and they are a bitch for wear and tear on a street motor. But future plans do see a bored and maybe stroked 351W. Big money will go into that block in the future, for now my car runs amazing for what it is, just want to solve the burning oil with a cost effective method. What brand of piston rings should I look for ? Also what size do I need ? Thanks For the input fellas.

 

[cenok is family]

i have power seal moly rings...they're good. as far as size, if you bore your block out, you'll need a 4.030 ring, if not then a 4.00 ring

 

[mavmavv]

You overbore if you NEED to, not to add horsepower.

 

[Blown331LX]

I know about the bore size, when I mentioned ring size I was reffering to the ring groove size. I dont know the stock replacement size on a 1990 GT. Are they standard and if so what size? If they are not all the same Id assume I have to wait till the pistons come in and measure.

 

[blackened88lx]

You overbore if you NEED to, not to add horsepower.

Exactly. I doubt it would even be 2 horsepower. Probably .2 or something. The added power you get will be from a fresh bottom end, not the 4 cui. It's a tough decision to make, but I would say now is the time to decide what power adder you would like to run. Nitrous loves compression, and boost hates it. If you're going the nitrous route, whether you bore it or not, I would go with some nice forged high compression pistons, which would be a big power advantage both on motor and on the bottle. If you're thinking the boost route, stock forged pistons are plenty durable and will keep the compression ratio low.

 

[Foxfan88]

ASSUMING a said engine is making 1 horse per cubic inch the gain would be 4 HP going from 302 to 306

 

[Blown331LX]

Ive read the old article in MM&FF about running 10"s with a stock bottom end, and also because boost is free HP I think the stock bore forged replacement is the way to go. But FoxFan you are right about your comparo. Oh and how the hell do I post pics on here on my car??? Been on here a while but never found out how...Let me know and Ill post some current pics of what I have, its a clean bastard, one you guys see it youll be feeling as bad as I am about pulling it down.

 

[85_SS_302_Coupe]

With stockish pistons, there's no noticeable difference. If you get yourself some nice pistons that up the combustion to say 10:1 or so, then yeah the 306 will be worth a bit more than a 302.

 

[gungfusemaj]

i was thinking 4

Really.... man you must have crappy 306's I got 5.743689

 

[93mustank]

I would say less if its a fresh short block.....tight new clearances robbin power until it gets wore in

 

[mike50sa]

I'm doing the TFS upper kit and decided to get the TFS 10:1 pistons (made by Arias) that are for 0.30 overbored. These pistons aren't cheap but man, they're light as hell. I bought all this stuff before the stroker kits were affordable back in '97 and '98 (my 200k mile engine still won't quit, and I'm still collecting parts). My decals from the oversized pistons are worth lots of HP, so it'll add a bunch of power. Each TFS decal has to be 15hp increase at least.

Anyway, the factory pistons in the '87-'89 cars (and some others) are forged and are good pistons. I'd consider reusing them if they're good, and you're on a budget. Myself, I didn't want to find out that the cylinders needed to be bored during the machine process, then have to fork out the $$ for oversized pistons. Also, piece of mind. And, the lighter pistons will help with the reciprocating assembly, allowing to rev.