TDC/DOHC pistons in RWD 3.4

[Naft]

Hey, sorry if this has been covered before, but I did a couple searches and didnt find anything on it. Basically I'm inquiring if anyone knows of this being done before, and come tomorrow im gonna spend some time researching it all myself, I just dont want to re-invent the wheel if i dont have to.

Basically im wondering if anyone has ever put 3.4 DOHC pistons into a 3.4 OHV block, and how that would clear the valves. im interested because of the identical bore & smaller piston dish, which would give a cheap higher cr piston . . . im just not sure how the valves will clear . . . i think with the dohc rods the piston sits .013-ish above the deck, with a .060 gasket, so if i use it with OHV rods(stock piston sits .040 below, right?) and a .040 gasket like the iron head OHV's use i should still have plenty of clearance, even without valve reliefs . . .

hmm i need to think about this after some sleep.

[tejohnson]

I'd say it would be based on the compression height of the piston. If it is taller, it would make me nervous... The only other thing to see about is if the piston pin is the same size (I would assume so...).

[RacerX11]

What heads are you using? DOHC pistons with iron heads has been done by several people. You will end up with compression somewhere around 9.5-9.7, and valve clearance will not be a problem unless you have a REALLY radical cam.

Marty

[Naft]

Iron heads. the biggest cam i was looking at was 228/238 @ .050, .490/.512 max with 1.6 rockers.

compression heights are the same, although the later DOHC is listed as .003 higher. the only question i have is supposedly some/all of the DOHC pistons came with .040 dome and then valve reliefs cut into that(for a total dish of ~2.7 cc still), so im wondering if i had my heads milled .010 and stayed with the iron head gasket (.040), would that only give me .030 quench(iron head 3.4 has pistons .040 below deck)?

anyway, thanks for the replies already, its good to know a few people have done it already, ill keep looking up the specifics on my case and ill put the results here for reference.

[RacerX11]

The DOHC has the same rod length as the 3.4 OHV, so the final piston position will end up being the same. My numbers show both pistons would be ~0.007" below the deck at TDC. The quench distance will be approximately 0.047" (0.040" gasket plus 0.007" from the piston being in the hole).

The dome on the DOHC pistons only covers part of the piston, and probbaly half of it is removed by the valve reliefs, so it really doesn't contribute any as far as quench is concerned. There isn't really much quench area on the iron heads (compared to the aluminum heads). Decking the heads will reduce combustion chamber volume, but not affect quench.

Here is a picture of the top of the Sealed Power H684CP DOHC piston.


Marty

[Naft]

Thanks for the picture!

I didnt run many calcs myself, but this calculator here: http://www.dunegoon.org/compression.html showed a deck height of .080 (with gasket) in order to get the 9.0 stock compression for an iron head 3.4

bore: 92 mm
stroke: 84 mm
head: 48.5 cc
dish: 8 cc
deck: ?

the other question would be the general size of the chamber on an iron head, so i can calculate roughly how many cc's i would be milling off. I'm thinking if i take .010 of the block and .010 off the head i should be able to get around 11 cr and .037 quench with DOHC pistons and still not have any problems with the intake manifold. Although the question then is how much more is a set of ross from ftpp.net without any of the machine work, lol.

never had anything decked/milled before . . . is it a $50 job or what?

edit: did some quick guestimation calculations, each .010 wont even take of a whole cc, so id probably have to do .020 off the heads to get above 10.5.

edit: wow my math was horrible . . . if the chamber opening size is ~3" in diameter, a .010 mill takes off ~4 cc., so .010 mill and .010 deck on a .030 bore with TDC pistons should net ~10.8:1 compression

[Naft]

found these:

http://www.kb-silvolite.com/spistons.php?action=details&S_id=613

http://www.kb-silvolite.com/spistons.php?action=details&S_id=203

second piston has .007" higher compression height, would give me .040 quench without any decking . . . but does it have the same displacement after the .040 dome? im assuming it would because it lists it as a stock replacement for the pre-96 DOHC engines, but ive found values ranging from 2.7 to 4 to 5 cc . . . meh.

[Naft]

ok after a lot of hunting, i think ive sorted out some discrepancies i found between the DOHC pistons.

1. There are 2 distinct designs, one from 91-95, and one from 96/97.
2. For the 91-95 pistons at least, there are also 2 other distinct differences, between auto and manual cars.
3. None of the piston manufacturers bother specifying most/any of these differences

For 91-95, auto engines had a CR of 9.25, manual had a CR of 9.5
96/97 had a CR of 9.7

I have found:
Silvolite P/N 3409
91-95 3.4 DOHC 207
Compression Height: 1.457"
Flat Head w/Valve Reliefs
Compression Ratio(s): 9.3
-----This must be the piston for auto 91-95, note: flat head, no .040 dome.

Silvolite P/N 3483
91-97 3.4 DOHC 207
Compression Height: 1.464"
.040" dome w/Valve Reliefs
Compression Ratio(s): none listed
-----This is definitly the piston for 96/97, it has the 1.464" compression height, but is dissimilar to the stock 91-95 manual

TRW P/N H684
96/97 3.4 DOHC 207
Compression Height: 1.464"
.040 dome w/Valve Reliefs
----96/97 piston, as above. Note Compression Height difference with next piston

TRW P/N H
91-95 3.4 DOHC 207
Compression Height: 1.457" + .040" dome
domed head w/Valve Reliefs
----91-95 manual piston. here TRW lists the compression height plus dome under "compression height", but in the 96/97 listing they dont.


Fierobsessed measured 2.7 cc net dish in his DOHC pistons, but i havent been able to find out what style piston that was.

assuming it was 91-95 manual, with standard bore and no machine work, those would yeild 10.44:1 compression on a stock iron head 3.4 with stock .040" gaskets. A .040 bore and decking the block .010 to get .037 quench gives 10.91:1, which is excellent. The only issue is if the 91-95 manual pistons actually have a 2.7cc dish, or if that was a different piston. i think im just gonna cc 'em when i get a set and go from there.

[RacerX11]

What specs are you using to come up with these numbers? Reverse-calculating a deck height based on published compression numbers is not right. For hypothetical compression calcs you need to use the blueprint values for crank stroke (84mm), block deck height (224mm), rod length (5.7"), etc. to come up with close-to-real numbers. The most accurate method is to cc all the components to account for machining tolerences. 10.4 is too high for stock DOHC pistons with stock iron heads.

Marty

[Naft]

nah i redid everything using piston compression heights, block deck height, stroke, etc.

i came up with the same thing you did, .007 below deck.
everything worked out fine with all the specs for the DOHC stuff, i think some of the head values were off. (manufacturer specs for the iron head pistons match the numbers i had been using, 8cc dish)

i just ran the iron head 3.4 pistons and the head chamber size must be too small, i was using 48.5, but 48.5 cc with stock iron head pistons creates 9.66 cr, if you use 54cc like the numbers i had for early DOHC heads then it works out to 8.98 . . . maybe those numbers got messed up somewhere

so yeah, if i use 54 cc for the chamber size it only comes up with like 9.7 cr, which makes more sense. Hmm i guess im just gonna have to cc a set of heads first anyway, i was hoping to do some rough guestimation first but maybe not.

[Naft]

found a random jpg on which someone listed a measured cc value for the iron heads: 50.3 cc.

http://users.spec.net/home/emxjc/JE_piston_form.jpg

running it all through a calculator i think 3.4's run a .060 head gasket, because that adds up to 9.02 compression, whereas a .040 gasket raises it to like 9.4.

I dont feel that safe running the DOHC pistons with a .020 shorter gasket and maybe a .010 deck though, cause the valve reliefs really wont be any relief, so i think im gonna grab a set of forget flat-tops from ross at ftpp.net and get some valve reliefs cut into them by a machine shop. the flats are only like $500, and i think i can scrape up a few extra $100 to insure the safety of my valves & pistons.

[RacerX11]

3.4 iron heads run 0.040 head gaskets.