383 rotating assembly Q's
 

Hi all,
Hoping somebody has done this before and can help me out a little.

Building a 383 with 64cc vortec heads, had a few questions. I plan on starting with a 383 bare block from summit and a Scat rotating assembly.
1. Internal vs external balance. After getting the Bob weight, I plan on taking the crank to get balanced. I can buy the externally balanced assembly and have it internally balanced right? All my research says yes but figured i'd ask one more time.
2. What kind of compression ratio do I want to shoot for using 91 octane pump gas @ 4350 ft? (looking to stay a half point below to avoid any damage that way) I've been using this to calculate https://www.uempistons.com/index.php...5aa9049c025676
3. Two kit's im looking at, one lists 5.700" rod, one is 6" rod. Same stroke. Does that mean the 5.700 will be .300 deeper in the cylinder at TDC? How do I choose?

Sorry if these are dumb questions. This is my first short block build.
Thanks

Re: 383 rotating assembly Q's
 

You probably can balance the external crank for internal balance but i would have thought internal balanced cranks would have had some material taken off already and make the final balance job alittle easier

If you can go 6" rod. Tends to be easier to balance. Less piston weight.

Piston height changes with rod length in those kits so overal length is the same regardless of 5.7 or 6.0" rod length. Now a piston designed for a 5.7 rod will stick out .3" when placed on a 6" rod. Thats important to know when piecing parts together separately and not buying a kit.

If cam is decent sized 10:1 will work for compression at that altitude. Safe would be 9.5-9.8 ish

Re: 383 rotating assembly Q's
 

So the mystery I was trying to solve was piston dish size. As I understand it, this all depends on piston to deck clearance which I can only measure when the engine is assembled? Leaving the only way to decrease CR (should it be too high) to buying thick head gaskets?

Anyways, the block, rotating assembly & cam bearing installer tool comes to about $1760.
ATK makes a preassembled 383 for $1999. The $250 extra seems worth it to me right now, especially with summer pretty much here.
As i read it, the LT1 flywheel for my T56 is weighted to match the external rear balance on the 383 crank, right?

http://www.high-performance-engines....ain-p/sp15.htm

Re: 383 rotating assembly Q's
 

No not exactly. Piston will have a depression or dish measured in cc's. You can subtract this from the compression volume to get static compression

You know stroke, you know rod length and you know compression height on the piston. All this adds up to a set length that gets compared to the block deck height which standard unmachined is 9.025". Subtract rotating length from deck height to get how far in or out the hole the pistons are. Most rotating assemblies are 9.00" long from crank centerline to top of piston. So most machine shops cut block deck down to 9.00" so you can use a standard .039-.041" head gasket for optimal quench distance or piston to head deck.

Re: 383 rotating assembly Q's
 

You have all the info but 1 thing to figure out the compression.

The deck height.

Stock is 9.025" deck height (crankshaft center line or center of main journals to the top of deck) and stock 350 piston for stock 5.7" rod's compression height is 1.560" which leaves the piston 0.025" in the bore at TDC.

The summit block says "a machined deck surface" So we know it's not stock, and we know it's not 0 decked (9.000") or they would have said that.

You can buy the block and measure the deck height from center of bearing journal to top of deck.

Or you can go with a really good guess of the block being in the 9.010" to 9.020" range

Re: 383 rotating assembly Q's
 

Honestly, I am going to purchase the ATK engine.
$1760 is waaaayyyy too close to $1999 for an assembled short block.
Only thing I would change about the ATK build is the dish size in the piston. Something like 14 cc would be better.
Thanks for the input.

Re: 383 rotating assembly Q's
 

Talk to them about what pistons they can get for you that will get the compression you want.

Also could always take a grinder to the spark plug boss to shape it out, smooth it off and remove some cc in the process to help lower compression

Re: 383 rotating assembly Q's
 

Cam is 269/276 duration .501/.510" lift 112 LSA

is the 9.5-9.8 range a dynamic or static goal?
This is where I am sitting right now.


http://s7.postimg.org/87jlw7abf/compression.png

Re: 383 rotating assembly Q's
 

Static goal. Dont worry about the dynamic. That calc with the closing plus 15 deg figure wont be that accurate. The actual cam card numbers would be best

Re: 383 rotating assembly Q's
 

With your cam using static compression number should be good enough. For your vortec iron heads static compression would be 9.5 max but with your altitude u could test that and go a little higher if needed. And for aluminum heads 10.5 max is rule of thumb but again w/your altitude u could go higher.

Congrats on using the ATK build. Ive only heard good bout ATK engines. As far as pistons go u need to contact them and insure your getting compression in the 9ish range with your 64cc votec heads. They will have limited choices for an on the shelf short block - otherwise u have to pay for custom.

Ok looking at the ATK w/s is see their 383 sb w/18cc dish pistons. They list 9.8 static compression w/64cc heads which is a little high and again will test it for detonation at 4500ft. In this case i would check dynamic c.r. for your cam. U may need a little bigger/more duration cam to lower your dynamic compression. Did u already buy the cam? Can u return it? Nail your dynamic c.r. first - practice, ask questions and u will get it right.

Since this is an external balance block u should just be able to bolt your original flywheel and dampner w/o changes to this ATK block.

Re: 383 rotating assembly Q's
 

Well i ran the numbers for static c.r. using 18cc pistons and 64cc heads, 0.039 gasket, 0.025" deck height and i get only 9.21:1 static compression. If block was zero decked then i get 9.24:1. So the ATK numbers are way off (so are blueprint engines and summit racing).:doh:

It shows u need to measure and calculate your own compression ratios.

I would go with that ATK short block, your heads and cam. Should be a great combo.:imo:

Re: 383 rotating assembly Q's
 

If you zero deck block, then compression would be 9.68:1 not 9.24

Re: 383 rotating assembly Q's
 

Ok lets double check here:

Static c.r. = (static vol + swept vol)/static vol

Static vol = piston dish + crevase vol + deck height + head chamber vol + gasket vol
= [18cc + 5cc + 0 + 64cc]/16.387cc/inch + [(4.125"/2)squared x pie x 0.039"] = 5.830inches cubed

Swept vol = Cyl radius x pi x stroke
= (4.03"/2)squared x pi x 3.75" = 47.833inches cubed

C.R. = (5.830 + 47.833)/5.830 = 9.204:1

So yes i was a little bit off. Howd u get Whatch u got?



BTW GM chamber volumes are conservatively larger than published.

Re: 383 rotating assembly Q's
 

cardo, No valve reliefs need to be added to the 18cc dish volume; take the 5cc out of your calcs and you get the correct 9.6 CR for zero decked.

Re: 383 rotating assembly Q's
 

I am defiantly excited! Its a waiting game now with the "busy" season. ETA is 6 weeks. I went with the 18cc pistons, seems like they should be fine.

I ran these numbers in 3 different calculators and all come up with 9.83 static.

Engine Bore/Stroke: 4.040 x 3.750
Advertised head volume: 64cc
Using Mr Gasket 5716G = .026 compressed gasket (4.100 gasket bore)
They said they do not zero deck their engines so they have the possibility of being rebuilt down the road if wanted.
Average deck clearance is .008 for them, so I am told.

where are you guys getting these other numbers?

Re: 383 rotating assembly Q's
 

I used .039 gasket and .030 bore. It will change with .026 gasket and .040 bore

Re: 383 rotating assembly Q's
 

When the engine arrives, I plan on checking pushrod lengths, exact deck clearance, & camshaft accuracy w/ TDC mark.
I'll probably do a once over on the rod and cap bolts for peace of mind.

I am going to touch up the runners on the head a bit, gasket match w/ intake. Good idea to cc the heads?

Anything else I need to put on the list?

Re: 383 rotating assembly Q's
 

That's correct for .008" deck, .026" gasket and .040" over.
I agree with your shop's philosophy of decking only to ~.008" clearance and using a thinner gasket. I did the same on my LT1 in the Impala.

Re: 383 rotating assembly Q's
 

Hmmm, never heard of this "no vlv reliefs need to be added to the 18cc dish vol". Dish vol that aint correct? Curious where u find this? But if true thats good info to share on the forums. And once the OP gets the block home he can nail the above piston vol and know for sure.

Yes to the CC the head volumes, it doesnt take much effort at all. In fact thats the easiest part of measuring the c.r. and i wouldnt skip this step while heads are off. BTW if u need to lower the compression some then thats the place to do it w/heads off. Unshroud the vlvs is a good thing and can open the head chamber up a few more cc (and have them all matched at the same time). Shouldnt cost much either as the chambers are very easy to work on (compared to the runners).

Re: 383 rotating assembly Q's
 

I just assumed the manufacturer's cc rating on the piston was already inclusive of the valve reliefs. True flat tops are 0cc but the valve reliefs are commonly 4-5cc and labeled as such. The dish with valve relief would include the valve relief with the dish volume. No need to double down. That was always my understanding

Re: 383 rotating assembly Q's
 

:iagree: