Ok, so the block is done and didn't have to be line honed or decked. SO ... what should the "Piston Down for TDC" measurement be? And, no I don't have it here to measure. The 383 causes 67.73 more cc's of swept cyl volume.


[Modified by Glock'94, 2:59 AM 1/9/2002]

 

Well, darn... I guess you are just out off luck :jester If you are using a 4vr piston, it should be somewhere around 9.5 - 10.0. 4 valve relief is what the common piston type is for a rebuild, but since you don't have it, and you did not specify a deck or head clearance, this would be the best guess. Just run it on Premium gas and you should be OK.

 

oops.. sorry.. here's the piston: only 2 valve reliefs.



i had a post a while back about this and the closest we could come to was around 12.04:1 .. but we were'nt ever exactly sure.

I can tell you

volume chamber = 54.4cc's
headgasket = 8.65cc's (4.1 bore and .04 thickness)

the block wasn't decked, so it should be the same as the factory "down from piston TDC" ... which I do not know.

the pistons valve reliefs i think are 5cc's .. but not sure.




[Modified by Glock'94, 3:02 AM 1/9/2002]

 

I would guess that the piston will be down in the hole about .015.
The valve relief is probably 3.8cc. The quench is very important. Try to get it between .040-.050.,this will allow you to run higher compression without being as sensitive to detonation.

 

the usual block is 9.025 deck height. the stroke, rod and piston usually have a dimension of 9.00...therefore the usual distance from deck to piston is .025. At least that is what mine was and a few others i've measured. you can add 1/2 the stroke + rod length + piston compression height (dist from pin to top)....subtract that from 9.025....assuming the block has never been decked.

 

That usually is a measurement that is needed after the block is machined ( boored/honed/decked ). You will need to install the crank and 4 piston assemblies, one in each corner, then measure with a depth micrometer.

Typical machine shop decking will result in different measurements at each end of each bank, so to be really correct, you need to get four measurements and deck the block accordingly. I have seen a difference from .025 on one end to .015 on the other of tghe same bank. Normally, this will not be noticeable on an engines performance, especially a street engine.

Of course, the right way is to make sure they are all the same, which usually means you have to take all four holes to .020-.013 after all are machined even.


My last 383 had a deck height of .013 on all 8 holes, with a stock composition gasket and flat top pistons ( with 4 relief's ) and 72 CC heads, this worked out to about 10.4 compression. This high compression is due to the added swept volume of the 383's stroke ( 3.75" as opposed to 3.48" ).

If you are using flat tops and stock 58 CC heads...you will get a much higher compression.

 

Ok .. it's about .030 from the piston's TDC to the deck. Gasket is .040 , 54.4cc's in the head, and the valve reliefs are 5cc's.

The machine shop said that I should have around 11.5:1 compression. They said I should definitely be able to run on pump gas.

I'll calculate out the exact compression in a few minutes.

 

Just calculated it out. And I got 11.55:1 compression. That is pretty close to the 11.5:1 that the machine shop said.. hehe.

 

Thats a bunch, that oughta make her go.

 

Yup.. going to be loud too.... As if it wasn't already! :crazy: :crazy: :crazy:


[Modified by Glock'94, 1:11 AM 1/14/2002]

 

Glock, am I understanding you to say the distance from the block deck to the
piston quench area when measured at TDC is .030 PLUS the gasket thickness?

 

No .. it's about .03 from the deck to the piston at TDC ... add the .04 for the headgasket to this and the distance between the piston at TDC and the surface of the head is roughly .07"

 

Glock, apparently you didn't understand the content of my previous post.
Your answer should have been yes. BTW, there is a difference between
'squaring the deck' and the term 'decking'. Apparently your engine builder
doesn't know the difference either. Your block was/is definately a candidate
for decking. (easily .025") If production blocks were machined to the spec
given in the design specs there would be no need to go through the process
of 'blueprinting' would there? BTW, the .040"-.045" deck hgt. given in earlier
posts is a bit conservative, 'cuz minimum is considered .025". (actual running
dim.) More of course, if you are using aluminum rods.

 

Umm.. ok .. i'm a college student studying MIS, no I don't know everything about engines, and I guess I didn't understand what you asked. The engine builder uses terms that I don't know the meaning of. I thought decking was planing the surface of the block down a given amount. I thought this is what they always do when "decking" a block. I guess that squaring the deck is leveling it out to the lowest point or something. I do not know.. perhapse you could explain it. And what exactly is the "quench area" .. is that not the measurement from the top of the piston at TDC to the deck of the block?? :confused:

Why is my block a candidate for decking? The machine shop measured and said it didn't need to be decked, and it also didn't need to be line honed .... so what's the deal?

BTW, the .030 that I said was a guestimate. I don't have the paper work back from the machine shop yet. I have a feeling it is more than .03 after going back and looking at my spark plug gapper and seeing how small .03 is.



[Modified by Glock'94, 1:24 AM 1/17/2002]

 

Deck Height = Distance from the top of the piston, flat (quench) area, at TDC to top (deck) of the block.

Quench Distance (height) = Same portion of piston to the flat (quench) area of the HEAD. Quench distance (height) is "deck height" plus gasket compressed thickness.

The "Quench Area" is the area formed between the flat porton of the piston and the flat portion of the head.

"Decking to Square" is the decking of the block so that both decks are as close to 90 degrees to each other as possible and as parallel to the crank centerline as possible.

Simple "Decking" is usually done to either provide a flat, straight, deck and/or to reduce the deck height of the piston.

I interpert Glock94's machine shop's comment, to mean that the deck is flat and straight enough for a good gasket seal.


[Modified by CFI-EFI, 12:12 PM 1/17/2002]

 

Hey, shouldn't his quench be a little tighter? 11.5 sounds high to me for pump gas.

KM

 

Ahhh.. ok. No wonder he said i should have said yes.. hehe.

Ok...

Yes, the deck is flat enough that it will make a good seal. I didn't want to have to have it decked unless i needed it, because that would raise compression even more.

 

I don't know about the quench, but 11.5 should be ok, considering that I'm doing my own computer work and I can back the timing off a little bit if I get any detonation.

 

You might do better with a smaller quench and slightly higher compression - trading off quench area for compression seems to lead to more detonation problems - not less. A 0.035 to 0.045 quench is normally what we shoot for.

I don't know how much the piston is on the hole, but you may be able to use a thinner headgasket to get the quench back down.