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Destroking a 350 NEW HERE PLEASE HELP!
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jtb0322
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PostPosted: Sun Jan 29, 2012 12:14 am    Post subject: Destroking a 350 NEW HERE PLEASE HELP! Reply with quote

I need to build a SBC 311 out of a 350 to run in limited stock class. I have a 262 (3.1" stroke) crank and I was considering putting it into a 350 4 bolt block with a 4" bore. If my calculations are right, I should be able to use a 6" rod with the 383 piston (1.425" pin height) and that should put me .050" below the deck height of 9.025". Here is what I am calculating:

.5 x 3.1= 1.55"

1.55" + 6" + 1.425"= 8.975"

Is this combination possible? Would it be a nice high rpm motor? And also how would it compare to a stock 305?

Thanks Alot!!

[/b]


Last edited by jtb0322 on Sun Jan 29, 2012 2:14 pm; edited 1 time in total
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10sec.et
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PostPosted: Sun Jan 29, 2012 3:25 am    Post subject: Reply with quote

i have no idea if your combination will work. i just thought i would say...

welcome to SMOKEmUP Cheers

someone should be along shortly to answer your questions Wink .

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clay
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1972 Chevrolet Nova

PostPosted: Sun Jan 29, 2012 11:26 am    Post subject: Reply with quote

Math looks good. You're very close to duplicating the factory 302 dimensions (4" bore - 3" stroke). I do have a few question for you though.
1. Will you deck the block to help get compression back where it needs to be and also helping quench?
2. What are the head rules?
3. What are the camshaft rules?
4. What are the carburetor rules?
5. Is there a max rpm rule?

I ask as your combination can work well with supporting parts that will let it turn some rpms. If you are really restricted with heads / camshaft or carburetor the longer stroke 305 may be better - but not very likely. Sounds like a fun project. Welcome to Smokemup! Clay

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jtb0322
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PostPosted: Sun Jan 29, 2012 12:31 pm    Post subject: Reply with quote

Thanks for the help! To answer your questions:

1. Yes I would have to deck the block to get the compression back to the correct ratio.
2. The head rules call for a factory head but says nothing about valves. I am running chevy 416 heads which are said to be the one of the best flowing heads.
3. The camshaft rules call for a hydraulic cam. No valve lift rule.
4. We must run the well-known rochester 2bbl carb
5. No max RPM rule

How far would I need to deck the block if my heads are already milled .025" ?

Thanks!
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clay
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PostPosted: Sun Jan 29, 2012 2:27 pm    Post subject: Reply with quote

Head milling will have nothing to do with how much you will have to remove from the deck. I usually mock up a piston / rod without rings at each corner and measure how far each piston is down in the hole. Write this down on the block. Some machine shops have fixtures to actually square the block the crank, some don't. If you write the amount down they'll set it up to take that amount and this will effectively square the block (not perfect but usually close enough). The next question is can you run the late model Vortec head? For a SBC factory head, they are the ones to beat. They do have a little bigger chamber than the 305 heads you have now but should make more power. Are you allowed to do any porting? If you can port, it looks like the 305 heads can be brought up to flow what the Vortecs flow from what I've found on the internet. Do you already have a camshaft picked out? Clay
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jtb0322
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PostPosted: Sun Jan 29, 2012 3:00 pm    Post subject: Reply with quote

In this class we are not permitted to run the Vortec heads, i forgot to mention that. I have heard that 416 casting heads are the best next to the Vortec heads. There is no rules about porting. They are gasket matched. Is there anything else I can do to improve the flow? At this time I do not have a camshaft picked out. Do you have any thoughts on what cam I should use? I also plan on using non-pump lifters... Any thoughts?

Can you also explain how to measure how far to deck the block again? I've never had to do it before...

Thanks for the help!!
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af2
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PostPosted: Sun Jan 29, 2012 3:55 pm    Post subject: Reply with quote

I would go with a 6.2 rod and a 1.250 compression height piston for weight savings and a good ring pack.

You could have the block (0) decked and run a .039" or .041" gasket and keep the squish good.

Or you could leave the block and run a .018" gasket if the compression is not too high.
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jtb0322
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PostPosted: Sun Jan 29, 2012 4:41 pm    Post subject: Reply with quote

I was using the 6" rod because I already have the rods. I want to build this motor to make more hp than my 305 but I would also like to keep the cost down a little bit by using what I already have available to me. I found a set of 383 pistons with a comp. height of 1.433. These are the pistons I would like to use in this build. With the 1.433 height pistons it would bring me to a total of 8.983" which is .042" below the deck height. How much would need to be milled off the block to use these pistons?

Thanks!!!
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squeeezer
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PostPosted: Sun Jan 29, 2012 5:49 pm    Post subject: Reply with quote

There are. 015 steel shim head gaskets you can use for quench and comp ratio
I use them alot its cheaper than decking assuming your decks are true
Still even then you quench is all wrong
Its hard to put a motor together with mismatched parts,in the longtime probably more expensive
How much will it cost to balance this stuff?
0 deck and a shim will get you what you need

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af2
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1933 Willys Coupe

PostPosted: Sun Jan 29, 2012 6:13 pm    Post subject: Reply with quote

jtb0322 wrote:
I was using the 6" rod because I already have the rods. I want to build this motor to make more hp than my 305 but I would also like to keep the cost down a little bit by using what I already have available to me. I found a set of 383 pistons with a comp. height of 1.433. These are the pistons I would like to use in this build. With the 1.433 height pistons it would bring me to a total of 8.983" which is .042" below the deck height. How much would need to be milled off the block to use these pistons?

Thanks!!!


To have a (0) deck it would have to measure 8.983".
You can not assume the deck is 9.025" because they can be anywhere. I have personally seen from corner to corner 9.013 and 9.021. That is why you have a block squared.

The block I am running has a 8.990" deck height so I would not worry about going to 8.983". I would have it at 8.985" though.
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af2
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1933 Willys Coupe

PostPosted: Sun Jan 29, 2012 6:22 pm    Post subject: Reply with quote

As far as heads look at this for some good flow info. You can find a few heads way better than the 416's and be legal.

http://users.erols.com/srweiss/tablehdc.htm
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jtb0322
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PostPosted: Sun Jan 29, 2012 11:55 pm    Post subject: Reply with quote

Thanks for all the help guys! I am going to have a machine shop assemble to bottom end of this motor for me but I need to know what needs to be done. I have a few questions about deck height and my combination... :

1. How do I measure my deck height?
2. What should the clearance be between the top of the deck and the top of the piston? For example if I have a total comp height of 8.983" and say the deck height is square at 9.025". Does the block have to be milled down to 8.983" or is there a certain range of clearance needed?
3.If it was milled to 8.983" is this considered a 0 deck and is that what I want for this build?
4. Would it be possible for me to get by using a .015" head gasket without decking the block?
5. My whole point of this build is to make more horsepower than a 305 with a rochester 2bbl carb feeding both motors. Will this 311ci combo accomplish that?

Thanks!!
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af2
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1933 Willys Coupe

PostPosted: Mon Jan 30, 2012 12:45 am    Post subject: Reply with quote

My whole point for the responses is to try and give good advice.

I would Zero deck the block get a better set of heads and have a cam ground to meet those #'s as the last thing.

Attention to the smallest details will make or break the deal.

I did say earlier what I would run for a piston rod combo but it's your deal.

Good luck and welcome to Smokemup!
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clay
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PostPosted: Mon Jan 30, 2012 1:24 am    Post subject: Reply with quote

To actually measure deck height will require something like a large set of calipers. You would measure from the main bearing housing bore to the deck surface. Then add half the main bearing housing bore and this will be your actual deck height. Since almost nobody had a set of calipers that large I usually put the crank in with a piston / rod assembly on each corner (1,2,7 and Cool. Then you can measure how far each piston is down in the hole at tdc for that cylinder. For simplicity sake and a safety margin I shoot for zero deck clearance or the piston flush. Add in a standard gasket (0.039" compressed) and you have the recommended clearance to allow for rod stretch and piston rock to keep from hitting the head. If you want to reuse the heads you have do everything allowed to them. Larger valves and any and all port / chamber work you can do. If your machine shop has a Serdi valve machine they can cut the throat and unshroud the valves for you. You still may have to do some minor clean up work if you can but they can take out the majority of the material. As far as camshaft I agree it should be last when you have all the other details sorted out. I would probably give someone like Mike Jones a try. His stuff may be slighty higher than maybe Comp but he seems to have a good reputation and I won't get into what I think of Comp's help line. I think what you are planning can make more power but keep in mind it isn't going to be a ton more power than your current combination based on the bore / stroke change. It's sort of like the classic 377 vs. 383 combinations. I'll look up an article tomorrow but I think the 383 made around 10 more ft. lbs. at the lower rpms. and the 377 made around 10 more h.p. at the higher rpm ranges. As well as I can remember they use the same cam, heads, intake and carburetor on both. I do realize in a limited class that 10 h.p. can rule though and I think you have several areas that you can make your proposed combination better than the 305 you are running now. Clay
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squeeezer
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PostPosted: Tue Jan 31, 2012 12:04 pm    Post subject: Reply with quote

i know you said you cannot use vortec ,rules is rules and all.
can you use 305 vortec????? im just currious

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