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343 advice needed |
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myrambler 
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Joined: Oct/10/2012 Location: Cedar Rapids,IA Status: Offline Points: 298 |
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 Topic: 343 advice neededPosted: Mar/18/2018 at 12:36pm |
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I have parts from several engines to use to build a 343 stroker.
One of the 343 engines has the original crank, bored .030 over with forged 360 27.5 cc pistons. Currently the pistons are .55 down from top of deck. This combination I'm told was a low compression engine, somewhere around 7.7-8.3. so I'm considering replacing the crank with a 304 that I have.
So I'm thinking I would use the 343 short block bored .030 over, 304 crank, stock rods, forged 360 27.5 dish pistons that the specs say are 1.6 comp ht, same as factory, 1.6 roller tip rockers and stock 8558 heads. Not sure if the heads are 67-68 53ccc or 69 51cc version. The heads that came with the engine have had some work done, larger screw in studs for the adjustable roller rockers and dual valve springs. Plan to use a 66-69 R4B intake and 1968 Carter 343 carb. Machine shop will have to check rods to see if they have already been conditioned and are shorter than stock specs and check pistons for deck height. I am shooting for pistons flush with the deck or slightly proud if valve clearance will allow. So I'm trying to determine compression ratio cam for street car should I use the balancer flex plate/flywheel from a 304 or 360? (I have the 304 balancer and flex plate original to the 304 crank) With the pistons being forged I assume they are lighter than cast but still the rotating assembly is likely heavier than the smaller bore 304 so I was leaning towards 360 balancer and flex/flywheel? Haven't decided if I stay automatic or switch to manual. I have a 727 that came with the 304 that could be used and access to the smaller 904 or 998. Either way the rotating assembly will all be balanced. Looking to build a torquey quick revving engine with decent HP. Budget build since I have all these parts. I will use new bearings, timing set and distributer gear. Not sure if I will use factory logs, modify free flow dog legs to fit, or headers. I have 2 1/4 dual exhaust. |
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Lyle
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Joined: Jul/17/2014 Location: None Status: Offline Points: 772 |
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Posted: Mar/18/2018 at 1:41pm |
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Your going to have to determine the compression ratio before selecting the cam.
Unless you have all the parts from the 343, weigh everything and match balance the new components then re-balance is required. If you change the crankshaft you will have to re-balance.
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myrambler
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Joined: Oct/10/2012 Location: Cedar Rapids,IA Status: Offline Points: 298 |
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Posted: Mar/18/2018 at 2:29pm |
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I was hoping with the information provided someone could tell me the approximate compression ratio. Then suggest a cam, and yes I mentioned no matter what was done it will be properly balanced. Just a question as to what balancer and flex plate/flywheel to use, 304 or 360.
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WesternRed
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Joined: Aug/03/2010 Location: Australia Status: Offline Points: 5799 |
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Posted: Mar/18/2018 at 3:02pm |
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You have pretty much all of the information you need, just go here and put the numbers in:
Use the 360 flex plate/flywheel and balancer, you are basically building a short deck 360. I'd suggest some better aftermarket rods if you are going to give it some stick and are already considering forged pistons.
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wantajav
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Posted: Mar/18/2018 at 3:24pm |
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Hello,
I'm working on a 343 myself. I didn't see what gasket you are using, I assume FelPro type - 0.048 and compressed around 0.040. I also assumed you are 0.055 down in the block not 0.55...plugging in gives you about 8.0:1 with a 3.28 stroke, and 8.4:1 with the 304 crank/360 stroke (3.44). I'm using dogleg headers on mine and 8558 heads too. Using a summit 8600 cam and AMC "390" intake. Want me to weigh a stock 343 rod/piston/end cap assembly for you? I'm using cast 362NP's this time around, but not having it rebalanced. Main thing is: what front damper are you using and what flexplate...if the answer to both are 360 stock items, and your rods 707 type are the same as a 360, maybe you are good ? The rotating mass does not know what block it is in. Other question do you have a decked block or stock deck height?
Edited by wantajav - Mar/18/2018 at 6:12pm |
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myrambler
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Joined: Oct/10/2012 Location: Cedar Rapids,IA Status: Offline Points: 298 |
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Posted: Mar/20/2018 at 9:09am |
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I stand corrected, the piston is .055 down, so I don't think the deck is blocked other than to flatten it out. stock is typically ,08-.06 down, plan to use felpro gasket. I've been told by the machine shop when ever you change pistons or rods or crank the rotating assembly should be balanced. you should weigh the new rods and piston vs what was in the engine before to make sure they are the same.
There is an old post from PHAT69AMX that did a similar compression check and there are slight differences used 51 cc 69 8558 heads decked to 49 cc, his pistons were 1.58 vs 1.6 , rods conditioned so shorter, and his estimated compression was 9.53. Ill look at the sites suggested to play with the numbers. I would prefer to be closer to his 9.5 number vs 8.4
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Boris Badanov
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Posted: Mar/20/2018 at 10:50am |
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Set the pistons to be flush or slightly
above the deck with a .045 gasket and the thing will really wake up. I like a .035-.045 quench. Tightening up the quench will increase the net compression ratio but will also reduce detonation. Sir Harry Ricardo discovered this a century ago and it is still true today. https://en.wikipedia.org/wiki/Harry_Ricardo |
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Gremlin Dreams
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myrambler
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Joined: Oct/10/2012 Location: Cedar Rapids,IA Status: Offline Points: 298 |
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Posted: Mar/20/2018 at 6:42pm |
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Sounds good Boris,
I have some mathematical formulas from a Hot Rod Magazine article to determine deck clearance. See if you agree Deck Clearance = (Deck Height - stroke/2) - (piston comp height + rod length) 304/360 crank(stroke 3.44/2=1.72) 343 block with 304/360 crank. 9.175-1.72=7.455 360 dished piston with stock rods. 1.6+5.875=7.475 7.455-7.475=.02 above the deck So piston would be proud/above deck by .02 so is that ok with the .045 compressed height of the head gasket? FYI: pistons are .030 Forged 27.5 dish TRW 1858P same as L2358F Johnson speed indicated they are .190 deep recess and 1.6 comp distance vs the newer replacement pistons 1.58 undersized. In a 360 block 9 to1 with two barrel heads (1970-1978)? perhaps two barrel intake and heads from 70-78? even that is confusing as the heads have different cc's depending on year. Thoughts on compression and cam?
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myrambler
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Joined: Oct/10/2012 Location: Cedar Rapids,IA Status: Offline Points: 298 |
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Posted: Mar/20/2018 at 7:02pm |
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I checked the summit racing compression calculator and came up with the following results
9.82/1 with piston .02 above deck with a .040 compressed gasket thickness 9.70/1 with piston .02 above deck with a .045 compressed gasket 9.39/1 with piston flush with the deck and .040 compressed gasket 9.29/1 with piston flush with the deck and .045 compressed gasket Other posts have indicated less closer to 8.0 - 8.4 so should this be calculated another way? I'm certainly happier with the compression in the +9s
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billd
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Posted: Mar/20/2018 at 8:00pm |
You introduce turbulence - you don't let unburned gasses get trapped and compressed to the point of self-ignition by the approaching flame front. We've heavily covered that here - the source of the discovery is cool info.
Detonation is the colliding flame fronts of the gasses ignited by the plug and those compressed and trapped and - which self-ignite. It blows away the barrier layer of unburned gasses which normally protect the piston. |
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Boris Badanov wrote: