I just measured the deck height of my 305 with the piston @ TDC with a feeler gauge. Is this an accurate way to measure deck height? I was told in a previous post that the piston should be .040 to .050 from the head for optimum quench-does this mean I'm screwed?

Thanks,

MatthewH

 

moving to general tech, you'll get more reply's

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, Jon (350 TBI!)
91 Red My website

 

How did you determine that the piston is actually at TDC? Eyeballing when the piston comes up and then starts going back down again is not all that accurate.

Either way .55 is acceptable... although it makes for a low compression ratio if stock.

If the engine is in pieces, have it milled. If not, don't worry about it.


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1986 Trans Am
carbed 350 - .30 over with 9.75:1 comp.
+- 315hp/390ft lbs.
700-R4 with 2500 stall
3.27 (ick!) gear
SLP 1 3/4" headers w/MAC catback and cat.
96 5 stars
SFC's, braces, arms, and a bunch more expensive stuff
no timeslips yet

 

If you're not certain the piston was at TDC, you can move a degree at a tiem and take another depth measurement. If you have a dial indicator and base, just watch the indicator.

0.055" would make for a pretty low static compression ratio. With .039" gasket and standard relieved piston tops and 72CC chambers on a 350 with that deck height you would yield about 8:1 (7.93:1 to be precise). That would be great if you're building for a blower or multiple turbos with a heavy wastegate, but will be a little disappointing with a naturally aspirated engine.

Are the pistons stock? Rods? Just askin'...

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Later,
Vader
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"Let the bodies hit the floor!"
Adobe Acrobat Reader

 

I think everyone missed the question. He wanted to know about the distance between the head and the piston. He was not giving us his piston to deck height. I think with a steel rod you have a min. of .035" and anything over .080" negates the quench effect. Closer is better. At a .035" Quench you have to keep your rpm below 6,500rpm.

Steve

 

Steve,

DOH! Good call - I guess I wasn't reading between the lines well enough. Never mind...

------------------
Later,
Vader
------------------
"Let the bodies hit the floor!"
Adobe Acrobat Reader

 

Actually I did mean deck height-sorry didn't mean to be confusing...So basically with a .015 gasket I'll have a total of .070 quench-is that pretty bad? The engine is already assembled (my machinist told me not to worry about decking because the factory job should be fine Also the rebuild was done with 12 cc dish Sterling pistons (p# 454NP) so with a 64 cc head my compression would be 8.1:1 I think...How much of a jump in power would I see if I could bump it up one full point?

Thanks,

MatthewH

P.S. It's a 305

 

Matt, if you don't mind me asking, why are you using 12cc dished pistons?

Steve

 

Well, I was kinda new at this at the time I had the block done and I didn't do as much homework as I should have...So now I'm stuck with a .050 (I remeasured it) deck, and 12 cc dished pistons. I calculated the compression ratio once again, but this time with a 55 cc head. I got 8.9:1 which isn't all that bad. The Vortecs can be taken to to 55 cc by milling them .045, but should I have them polished as well to help fight detonation?

Thanks,

Matthew C. Hall

 

Actually with a steel rod go 0. Zero decking... Even if the piston is slightly (I mean slightly like a few hundred thousanths) sticking out of the bore, this is excellent with composition head gaskets. Aluminum rods are a whole diff story (most alum rods are made undersize to allow for stretch)

But after all this you better know what your doing and check with clay by mocking up the head. Check piston to head and valve to piston clearence and meet the minimums.
At least zero deck is safe with steel and comp gaskets.

 

Wouldn't hurt to polish them and take off all sharp edges also. Is it really to late to deck the block or use some open chambered heads. I guess you could still use the .015" head gasket and be at .065" quence. Thats better than nothing at all.

Steve