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Old 04-12-2012, 20:16   #1 (permalink)
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302 rodded 351 or high comp 351?

Hi guys! Im new to the forums and I'd appreciate some opinions about my next move engine wise. I'm saving introductions and going to sound simplistic to save everyone some reading, but I assure you I've done my measurements!

I've got a 71 Fairlane that I just blew an engine in, Was a 351 with hypereutectic flat tops at 0 deck height, open chamber (67cc) heads, torque and economy cam, extractors and carter thermoquad sitting over the top with a non-restrictve gas collar in the air cleaner to save $ on highways. I run a 292 diff and c10 auto as the car is build predominantly as a cruiser, but I love to flick it to petrol and open her up on the odd occasion. I really want to build something with a bit more grunt this time!
This engine build I'm not sure what cam I'm going to use yet, but it will probably be something more in the 1500-4000 RPM range rather than just a torque and economy type. I figure it's all very free flowing as a low RPM engine, so I guess I'm chasing power increases from increasing compression.
I have HEAPS of clevo spares and I'm hearing lots of talk about 302 rodding a 351 to increase torque, so do I-

build a 351 on 302 rods in a standard bore block I have laying around, using flat tops (so it'll be identical to my old engine with the only difference being 302 rods, same rough 9.48:1 comp). Will using different rods make much difference?
OR
build a 351 in a 40 thou over block I have using Keith Black KB149-040 pistons, which have a 1.5mm high dome which matches the head shape, which at the same deck height (zero) as the flat tops increases my comp to 10.73? I know this will be fantastic running LPG as it doesn't mind the extra comp, and realistically this won't be the last engine for this car so I don't mind hanging onto the standard bore block for a later build. Is the 1.5mm high dome big enough to affect flame travel? Will this type of build try to run hotter than usual?

Any input would be greatly appreciated!
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Old 04-13-2012, 07:14   #2 (permalink)
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Re: 302 rodded 351 or high comp 351?

Quote:
Originally Posted by cuttlefish_22 View Post
Hi guys! Im new to the forums and I'd appreciate some opinions about my next move engine wise. I'm saving introductions and going to sound simplistic to save everyone some reading, but I assure you I've done my measurements!

I've got a 71 Fairlane that I just blew an engine in, Was a 351 with hypereutectic flat tops at 0 deck height, open chamber (67cc) heads, torque and economy cam, extractors and carter thermoquad sitting over the top with a non-restrictve gas collar in the air cleaner to save $ on highways. I run a 292 diff and c10 auto as the car is build predominantly as a cruiser, but I love to flick it to petrol and open her up on the odd occasion. I really want to build something with a bit more grunt this time!
This engine build I'm not sure what cam I'm going to use yet, but it will probably be something more in the 1500-4000 RPM range rather than just a torque and economy type. I figure it's all very free flowing as a low RPM engine, so I guess I'm chasing power increases from increasing compression.
I have HEAPS of clevo spares and I'm hearing lots of talk about 302 rodding a 351 to increase torque, so do I-

build a 351 on 302 rods in a standard bore block I have laying around, using flat tops (so it'll be identical to my old engine with the only difference being 302 rods, same rough 9.48:1 comp). Will using different rods make much difference?
OR
build a 351 in a 40 thou over block I have using Keith Black KB149-040 pistons, which have a 1.5mm high dome which matches the head shape, which at the same deck height (zero) as the flat tops increases my comp to 10.73? I know this will be fantastic running LPG as it doesn't mind the extra comp, and realistically this won't be the last engine for this car so I don't mind hanging onto the standard bore block for a later build. Is the 1.5mm high dome big enough to affect flame travel? Will this type of build try to run hotter than usual?

Any input would be greatly appreciated!
Was thinking gas liked 11 or 12 better, but lower would be better for when you want to use petrol. In theory the long 302 rods gives you an increase in performance from increasing the time the piston is @ TDC. Haven't seen any proven results of a real world improvement though. In the States the '71 M code was 10.7 & called for 98 US octane, timing had to be backed off with todays octane levels for petrol, but where your using gas...
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Old 04-13-2012, 09:55   #3 (permalink)
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Re: 302 rodded 351 or high comp 351?

Hey George! I appreciate the reply. I'm in Victoria Australia. I know our quality of diesel isn't as good as in the U.S but I'm not sure about petrol. I only ever put 98 in it, I figure i run it on petrol for fun so I may as well make the most of it!
So do you lean more towards putting the standard bore on the bench for later and going the higher comp engine?
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Old 04-13-2012, 18:03   #4 (permalink)
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Re: 302 rodded 351 or high comp 351?

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Originally Posted by cuttlefish_22 View Post
Hey George! I appreciate the reply. I'm in Victoria Australia. I know our quality of diesel isn't as good as in the U.S but I'm not sure about petrol. I only ever put 98 in it, I figure i run it on petrol for fun so I may as well make the most of it!
So do you lean more towards putting the standard bore on the bench for later and going the higher comp engine?
US & Australia figures octane diffrently, think I saw that 98 in Au is 89 or so in the US. Where you already have the KB pistons, I'd go that way, w/o those in hand I'd go for the std block.
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Old 04-16-2012, 08:03   #5 (permalink)
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Re: 302 rodded 351 or high comp 351?

Thanks once again mate, that's a fair difference in fuel quality. I'm going to take my time slowly with this decision! I can't identify one of the blocks I intended on using, so I'm going to try work that out first. It just has odd numbers and no serial.
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Old 04-16-2012, 08:39   #6 (permalink)
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Re: 302 rodded 351 or high comp 351?

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Thanks once again mate, that's a fair difference in fuel quality. I'm going to take my time slowly with this decision! I can't identify one of the blocks I intended on using, so I'm going to try work that out first. It just has odd numbers and no serial.
Not quality, but how it's figured, like the different tempeture scales, 0 & 32 are both freezing point of water.
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Old 04-29-2012, 15:27   #7 (permalink)
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Re: 302 rodded 351 or high comp 351?

the octane diffeence is the way it's being measured, many different methods
Octane rating - Wikipedia, the free encyclopedia

i'm a little confused when you state Open chambers @ 67cc's, are they milled quite a lot?
Open chambers start life in the 76cc range and have a full round shape to the chamber

long rods in a 351C isn't gonna make a hill of beans worth of difference in the real world, and we're only talking about a 1/4" or 6mm longer at that. unless you can find a great deal on the right pistons and those are the only rods you have... just build with stock dimensions.

why Hypereutectic pistons and not Forged? spend what you were gonna on the lower compression height hypers on better metal!

i don't see a 1.5mm dome affecting flame travel much at all... ?
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