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Discussion Starter · #1 ·
Which pistons or rings hold up to the high compression of 305 heads on a 350 best.

I've heard that the stock flats I have will burn through. True or false?

I took apart a 350 motor I bought and it had the 305 heads(untouched)
and 6 out of 8 compression rings were broke in two. I plan on working over the chambers which should relieve some pressure and cut the possibility of detonation.

What pistons would be best? I'm wanting to stick with flats, but I'm open minded. Thanks, Ryan.
 

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Hey there. First off my Friend you would need to work out what compression ratio your engine would equate to. Look up the head casting number at www.mortec.com to find out the combustion chamber volume and go from there. You then go to one of the many compression ratio calculating websites. By taking in factors such as bore size, stroke, head gasket thickness, piston flat tops - how many valve reliefs or dished pistons, deck height, etc your engine's compression ratio would be roughly calculated for you. If you're using 58cc heads, a rough guide would be that the comp' ratio is about 10.4:1.
Cylinder head Volume (CC) = 58.00
Piston Top Volume (CC) = 8.00 (flat tops with 4 valve cut outs)
Cylinder Bore (Inches) = 4.0
Cylinder Stroke (Inches) = 3.50
Deck Height (Inches) = 0.030
Head Gasket Thickness (Inches) = 0.020
Computation Results:
Computed Compression Ratio is 10.4 to 1
That would be too high for todays pump gas. However, if you were to work the heads by unshrouding the valves then that would lower the comp' ratio and help high RPM breathing. Combine that with a high duration and large overlap cam that can bleed off some of that compression at low RPM and you should be on your way to some streetable horsepower. Of course, you need to do specific research yourself with your specific data, ask as many people as possible for their opinions and don't rush the project.
 

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Do you WANT the high compression or is this basically a daily driver?

If this is a driver Sealed Power makes a hypereutectic 22cc dished piston that will bring compression down around 8.5 to 1 with 58cc heads and Fel Pro PermaTorque head gaskets. IIRC Summit had them for under $100. I have a set, I'm going to throw together a 355 with left overs and swap meet stuff and part of the left overs are a pair of 58cc 305 HO heads. They are a full dish though so you don't get any quench effect but if you you just want a driver and are going to rebuild it they'd work.
 

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That's a good question. 305 heads were an old-school way to raise compression, but they flow TERRIBLY. They barely breathe enough to let the 305 spin to 5000, so on a 350 they'd be downright pathetic.

The only reason I'd put 305 heads on a 350 is if I'm trying to build a 4500-rpm Ethanol grunt/torque motor.
 

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curtis73 said:
That's a good question. 305 heads were an old-school way to raise compression, but they flow TERRIBLY. They barely breathe enough to let the 305 spin to 5000, so on a 350 they'd be downright pathetic.

The only reason I'd put 305 heads on a 350 is if I'm trying to build a 4500-rpm Ethanol grunt/torque motor.
For the most part true but the 305 HO heads have the same port volume and flow as well the old 461 Double Hump heads that all us old timers hold so dear to our hearts. But of course I'd only use those on a 5500 RPM 350 "grunt" motor. ;)
 

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I'm pretty happy with my 305 equipped 350.
good torque (fries the tires) and gas milage, pulls real hard to 5000+

It's in the truck in my avatar. No it's not a race engine but it was cheap and reliable.
 

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Discussion Starter · #8 ·
Thanks for the input!

Thanks for the input everyone!

I tried to post a thread in this department explaining all of the details and after a couple attempts I decided to wait to see if it would come up later. I even PM'ed a MOD to see if there would be a delay.

After a few hours I threw this together, so I apologize for the vagueness, it was sort of a test thread to see if it would even post.

The only reason I'd put 305 heads on a 350 is if I'm trying to build a 4500-rpm Ethanol grunt/torque motor.
Exactly! Though after some homemade head work I think that they'll flow a bit better and I'll run octane boosted 93 Premium if need be after some unshrouding. I don't want it dependant on Alky yet. However, an Ethanol motor is on the drawing bored as well.

This motor is going into a full time off-road trail/mud rig. She's a '78 Chevy K20 Custom Deluxe, well....it’s 'custom' now, except for the stock 350.



Look at all these Jeeps that I have to keep up with!



Actually, the only place I really had a problem was this hill climb in the soft pea gravel, but so did half of the Jeeps. I'll make the top this year!!



I'm wanting to run flathead pistons, any suggestions which ones are best suited?

I have two sets of the "601" castings to hone my porting skills on, I'd love a set of HOs, but maybe I'll find some for the 383 alky that I'd like to have for next year. :D I've heard the 601s are 53 cc from some sources, 58cc from others. :confused:

HEY 6426yy, what 'set up' are you running in that outfit? What fuel?

Thanks again all!
 

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What cam are you running? That will affect your dynamic compression ratio, if you're running a cam with a fair amount of duration and overlap it will bleed off some cranking compression and allow your engine to run lower octane than if you have a short duration short overlap stump puller cam. Regardless I'd run forged pistons for what you're doing. Looks like a blast BTW! :cool:
 

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Discussion Starter · #10 ·
Hippie said:
What cam are you running? That will affect your dynamic compression ratio, if you're running a cam with a fair amount of duration and overlap it will bleed off some cranking compression and allow your engine to run lower octane than if you have a short duration short overlap stump puller cam. Regardless I'd run forged pistons for what you're doing. Looks like a blast BTW! :cool:
Hey Hippie,

Once I figure my CR I'll be able to select a cam on that basis. I've been studying David Vizard and some online for weeks now.

I've heard now that these 601 heads are 53, 55, and 58 cc chambers. Once I deshroud and take away the sharp edges, the chamber volume will increase, bringing down the CR and reducing detonation. I need to cc the heads soon to answer the question of stock chamber volume for myself and then go from there. Thanks for the cam suggestion, I could do that, and I might for a build up for a DD.

Mr. Vizard did mention that, but recommends that I go with a shorter cam that fits the CR minimum. I don't want to over cam it, so I will be conservative. On the trail, there is a need for it to be driver friendly, and there is a lot of idling, so I'm not wanting too much loopy lope.

I'll look into some forged pistons once I get the block work done, so I know what to get. Thanks for the advice. Any particular name brand to shoot for or avoid?

Thanks again! :thumbup:

It was a blast btw! The stock 350 and the Qjet did better than I thought, but the power to weight ratio must increase for future endeavors! :mwink:
 

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Old School Baja said:
. On the trail, there is a need for it to be driver friendly, and there is a lot of idling, so I'm not wanting too much loopy lope.

I'll look into some forged pistons once I get the block work done, so I know what to get. Thanks for the advice. Any particular name brand to shoot for or avoid?

Thanks again! :thumbup:

It was a blast btw! The stock 350 and the Qjet did better than I thought, but the power to weight ratio must increase for future endeavors! :mwink:
My money was on the "stump puller". ;) As far as forged pistons I think they're all pretty good where a "low" rpm application like yours is concerned, let your wallet be your guide. I'd actually use a small "Reverse Dome" or D-cup rather than a flat top, especially since it sounds like you're going for the stump puller cam. A 10 or 12cc cup isn't going to drop your compression that much over a flat top with 8cc valve reliefs and will improve flame propagation especially if you run a tight quench distance. It can more than offset the slight compression drop due to improved mixture burning.
 

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Looks like fun.

I'm running stock dished four valve relief pistons and '83 62cc heads with nothing other than a fresh valve job.
GMPP 330 HP cam, edelbrock streetmaster intake (single plane) not my first choice but it's what I had. 600 holley with 4 hole spacer, 1-5/8 headers and 2-1/4 duals with flowmasters. And a pete jackson gear drive for a little attitude!!

She drinks 87 octane all day!! And @ 15.3 MPG with 3.08's
 

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Don't live or die by what Mr. Lizard has to say.

I have measured the 601 heads at 54cc.

The Sealed Power H620CP (hypereutectic) will give you around 9:1.

I have built several 4X4 truck engines with those heads and piston with a Edelbrock Performer (clone) grind. They perform very well in the truck if care is taken with the timing curve.

tom
 

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I suggest that every tool box have a 100 cc veterinary syringe. $ 3. You can do a pretty good job of measuring chamber volume to +- 1/4 cc to see what you actually have. Works well for port volume also.
 
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