Stroking a 305

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87grandprix84

Not-quite-so-new-guy
Jan 30, 2017
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Hi all I'm still new to the forum but I'm looking for some help here. I have an 84 Grand Prix with a Chevy 305. The motor is getting old and tired (200,000 miles into its life) and it is time for a rebuild. The motor is starting to burn oil and I'd like to fix that before it gets too bad. The car is numbers matching and I'm intent on keeping it that way, I.E. don't want to drop in a 350 ect. I would However like to get a bit more power. I'm not looking for anything crazy as this car is my daily driver. I had started looking into stroking the motor to get a bit more out of it but can't really find much good info. Everything I find is people saying " just throw away the 305 and get a 350." I'd really like to get more input on what stroking a 305 entails. Questions like, will it have to be bored for con rod clearance or can I just get it bored say 0.010 for clean cylinder walls? Also reliability? This is my daily driver and I really don't want to have issues like overheating ect. I've never rebuilt a car engine before either and was curios how stroking it would effect compression ratio as I'd like to continue running regular 87 to 89 octane.

Thanks everyone.
 

565bbchevy

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Aug 8, 2011
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If your intention is to have power then a swap to a 350 or larger SBC would be the best / least expensive route and basically a direct swap.
The 305 is limited by bore size which also restricts your head selection that has to work with this small bore which also limits cam size etc. a 350 can easily make the power a 305 stroker will make and for less money but if you are content with your 305 then I would just do a stock rebuild on it and save the money for something else like maybe less restrictive exhaust etc.
IMO a 84' GP with a matching numbers 305 is not going to be worth any more than the same car with a 350.
 
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565bbchevy

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The 305 is a decent engine for what it was designed for (80's emissions) but spending the money just to stroke it to a whopping 334 cubic inches it's power potential will still be limited by it's bore size especially if you are trying to keep it factory stock with matching numbers.
 

87grandprix84

Not-quite-so-new-guy
Jan 30, 2017
6
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Thanks for the input but I'd like to keep the car numbers matching for my own personal reasons, not really to hold value. I'm not concerned with value because I don't ever plan on selling the car. That's a good idea about less restrictive exhaust though I already removed the muffler and cat, I also opted for larger tubing and a glasspack. When I do the rebuild I was planning on investing in some headers and a new crossmember for the trans so I could run dual exhaust. Regardless I really want to cover all my bases and understand my options before I make a decision as the time to stroke the motor would be during the rebuild if I do go that route.
 
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CaliWagon83

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Nov 12, 2017
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Cost-no-object, I'd get it bored .020 over, and put some good-breathing heads on it. If you're intent on keeping the stock heads, maybe you could get them ported. But if you're going for performance, I'd look at getting some performance aluminum heads. A lot of parts that work on the 350 will also work on the 305, with the exception of big-valve heads. Looks like you can fit up to 1.94" valves on a 305.

I know the standard pat answer is "get a 350." You can get decent power out of a 305, but when all is said and done, you'll probably end up spending more money than it would take to get an equivalent amount of power out of a 350. As long as you know that going into it, go for it.
 
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MrSony

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Nov 15, 2014
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Once you open up that motor, it won't be original.
Y'know, I was dead set on keeping my Regal bone stock. Then the V6 exploded.
Then I added this:
13268470_1077479819011193_7227129713376208511_o - Copy.jpg


Then this:
shinrybuick2.jpg


There is 100% no reason to keep the 305, and almost no one will know at a glance that there is a 350, or even a 383 hiding under there if you paint it the factory color, and put all the factory doo dads and what not on it.

About once every week or so a guy joins the forum, wanting to pep up his 305 or V6. Everyone of them, ends up at the conclusion that it would be best to just upgrade.

But in the end, it's your car. Do as you wish. Or, don't even bother opening up the engine, and boost it until it explodes. Then build a 383. :D
 
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TURNA

Rocket Powered Basset Hound
Jul 24, 2009
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Why would u waste $$ stroking a 305 when there are 327's and 350;s out there?

Then you are worried about drivability, overheating and using 87 octane?????





" just throw away the 305 and get a 350."
 
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565bbchevy

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My Monte is numbers matching also since the 565 on the dash match the cubic inches under the hood.
IMG_0897.JPG
 
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64nailhead

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Dec 1, 2014
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I understand your wish to keep it numbers matching. But cranking up the 305 is going to yield less HP/dollar spent than just about any other SBC. So I'll assume you're planning to bite that bullet.

Here's what to do, 1st - stroke to 330+ cubes. 2nd - send your heads to a professional, yes professional porter that should be able to get them to flow in the 230-245CFM on the intake side (this will be a $700-1200 deal), but do not hand them to some dude with a die grinder that thinks he can get it done. The job needs to be done by someone with a flow bench that knows what they're doing. Contact Chad Speier - extremely reputable and fair pricing, but there are others. 3rd, 1 5/8" long tube headers. 4th, cam selection will be critical and something along the lines of 220ish intake and exhaust to follow your head flow numbers - of course in a hydro roller.

Sounds like a good plan to you? Expect $4000-5500 to get less than 400HP and maybe as low as 350HP. You'll need all of the supporting parts to match, i.e. pistons to increase compression, roller rockers, custom length pushrods, intake manifold, 650 cfm carb, recurved distributor and a new converter.

Sound like a fun project, but keep in mind that you could spend the same money for an easy 425-475 HP 383.


As 565 mentioned, the heads are the real problem. Aftermarket head options for a 305 are few and far between due to the limitations of the valve sizing due to the small bore. Don't underestimate the value of a correctly ported stock head though.

Good luck - Jim
 
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