Piston to Head Spec? Options

[Jetsetsurfshop]

I'm in the process of ordering some custom pistons and my Dad and I were thinking there must be a preferred distance from piston to head for the best combustion. Any thoughts?
(Its a 4, not a 6)
Thanks
(IMG:style_emoticons/default/driving.gif)

[Bills914-4]

I'm running .032 , I've heard as little as .028 (Jake) , but I believe .035- .040 is
good (optimal) something to with quench, maybe some others will chime in , Bill D.

[stugray]

Do some googling of "deck height".
It should provide many hours of reading enjoyment (or not).

I spent weeks getting a handle on how to measure & set mine (including a trip to the machine shop in between iterations).

[r_towle]

.040 is safe.

deck height will set your compression ratio.
8.5:1 to 9.5:1 is a safe range.
A higher lift camshaft will require more of a deck height or valve pockets in the Pistons to avoid having the valves hit the piston.

Too little deck height, and one good over rev will float the valves and they will hit the Pistons....not a good thing.

As Stu said, go google this topic.
Learn about what you are doing to make a good decision based upon what you are building....

Street motor, race motor, what type of power you want, in what part of the rpm range do you want that power.....all of this will help you decide on the camshaft and deck height.

Rich

[Jetsetsurfshop]

I googled it. Found some info here and on Pelican.
Are concern started because the PO shaved the heads. We are going to a bigger cam and need to add valve pockets. We didn't want to lose our compression in the process. Thinking we can get some back with little dome on the top.

[sean_v8_914]

head chamber volume in CCs?
what is bore and stroke?

[Jetsetsurfshop]

head chamber volume in CCs?
what is bore and stroke?

[r_towle]

You can order any pin height you require to get that compression ration in check.

Rich

[ChrisFoley]

That's a small combustion chamber for such a displacement (unless you're building a high compression race engine).
Dished pistons may be needed to maintain a tight deck, depending on the camshaft and desired CR.

[Bleyseng]

What heads are those with a 49cc chamber?

[Bills914-4]

Are concern started because the PO shaved the heads. We are going to a bigger cam and need to add valve pockets.

My experience ,valve pockets only add 2cc-4cc max. , your cam choice
is going to determine what C.R. you should setup ,

10cc dish pistons would give you 9.5cr , valve pockets & a little chamber work, might
get you 10.2 maybe 10.0cr ,

tighter deck = snappier throttle response (IMG:style_emoticons/default/smile.gif) ,

CB calculator
http://www.cbperformance.com/v/enginecalc.html

[Montreal914]

On my 2056 D-Jet build, I used .040" deck and 8.4 compression. Both of which seems to be within the recommended range.
Fun engine (IMG:style_emoticons/default/driving.gif)

[stugray]

If they shaved the heads down to get the comb chambers to 49 ccs, they may have shaved them too far.

I tried to do the very same thing, but the Porsche mechanic/machinst advised against it.
He said that machining the heads that far will make the metal too thin where the exhaust studs thread in risking blowing them out.

So he shaved them down for me but only to the point where I had 54 ccs comb chamber volume.

The deck height requirement of > ~0.028 (call it .030 for some margin) is to keep the piston tops from hitting the flat portion of the head when the pistons reach max temp.

I have .035 and ~.5 valve lift (webcam 86b) and have plenty of clearance from the valves hitting the piston (stock valves).

Once you get everything dialed in, do the clay method of checking valve clearance.
Put a lump of clay on the piston , reassemble, and crank once around.
Then disassemble and measure the indentations in the clay.

http://www.914world.com/bbs2/index.php?showtopic=200247

[Dave_Darling]

For illustrations of how fly-cutting the heads can result in exposing the exhaust studs, see the cross-sections on this thread:

http://shoptalkforums.com/viewtopic.php?f=1&t=71523

--DD

[Jetsetsurfshop]

Ok, let me see if I can address everyone here.

First off, its for a track car and we only run 98 octane fuel. So the having a compression of 10 to 1 wasn't a issue. It was what we wanted. I think I said it wasn't me that flycut the heads it was the PO.

Pin hight is already into the oil ring already and I want the compression ratio high.

I dont know who did the heads but they have monster valves and there's R.J. initialed on them.

last thing to address is that i will never cut your heads down this low. What Sturay and Dave said is correct. Check out the picture. The hole was there when we rebuilt it last year. We stuck some JB weld over the hole and ran it in dozens of HPDE's and one 14 hour Chump Car race. When we pulled the head the JB weld was gone and the exhaust stud worked its way out a little bit. We could of just pulled the head and did a quick repair but as a race engine we wanted to see the bearing and camshaft wear. (the web cam was pretty beat, fyi)
The plan is to run it for another year and order up some new heads then. Need to save a few bucks for those!

[Jetsetsurfshop]

Cant find a picture right now, I'll take one tonight. (IMG:style_emoticons/default/confused24.gif)

[Jetsetsurfshop]

(IMG:style_emoticons/default/driving.gif)

[r_towle]

Ok,
So what is the question again?

[ChrisFoley]

Find someone who can TIG weld that hole shut, grind smooth and move forward.
(open it up with a carbide burr first so the hole is well exposed)

[Jetsetsurfshop]

Find someone who can TIG weld that hole shut, grind smooth and move forward.
(open it up with a carbide burr first so the hole is well exposed)

Thanks for the advise. (IMG:style_emoticons/default/beerchug.gif) On it.