madkaw Posted April 16, 2015 Share Posted April 16, 2015 Are the L28 flatops thick enough to cut reliefs ? Like to do some combustion chamber matching and tried searching and couldn't find anything. Quote Link to comment Share on other sites More sharing options...
JohnH Posted April 19, 2015 Share Posted April 19, 2015 Depends on how much piston valve clearance you need. I have cut 1.5mm eyebrows into the stock l28 flat tops that was required for a 530 lift cam & a highly shaved N42 using a 1mm head gasket. Without the eyebrow reliefs I was getting slight valve marking on the piston crowns. I used the old hotrodders method. I brought the piston to be relieved up to TDC, taped off completely around the piston (oil & water ports also), JB welded 80 grit heavy pad backed sandpaper to an intake & exhaust valve, lubed the stem portion of the valves & popped them into the head, sat the head on the block, used depth stops on the valves for the required 1.5mm, attached an electric drill to valve stem top & created valve reliefs, lifted head off, vacuum up piston shavings & repeat. This is of course sans cam, cam towers & valve springs on the head. Engine is currently running & have had no problems in this regard. There are plenty of similar method valve relief cutters out there for US domestic V8's but I could not find any in a L28 size. I would say perhaps 2mm would be pushing it for valve pockets on stock flat tops. Quote Link to comment Share on other sites More sharing options...
madkaw Posted April 19, 2015 Author Share Posted April 19, 2015 Thanks for the response. I've seen some videos of your method. Not sure what the limit is as far as cam numbers that would need this kind of clearance. I don't think my 490/290 cam will be a problem, but I know it's a matter of measuring for sure. Quote Link to comment Share on other sites More sharing options...
Xnke Posted April 19, 2015 Share Posted April 19, 2015 Cam timing will be the biggest issue. As the cam retards, exhaust valve clearance gets looser, then quickly gets tighter again. The intake valve clearance, as the cam retards, will again get loser, then tighter. Advancing the cam can bring the Intake valve closer to striking the piston just as fast as retarding the cam brings the exhaust valve closer. Quote Link to comment Share on other sites More sharing options...
JohnH Posted April 20, 2015 Share Posted April 20, 2015 I believe the piston crown material starts around the bottom ring landing on the fire ring of the stock flat tops. I'm just going by memory as its been over a year. Should be easy enough to measure with a piston in hand. Quote Link to comment Share on other sites More sharing options...
DAW Posted April 24, 2015 Share Posted April 24, 2015 I had considered a project that I have not followed through on that is related to your post. I was looking at different piston head configurations that might maximize quench with a closed chamber head, and at the same time allow for enough total chamber (head + dish vol) volume to make reasonable, useful compression ratios in L4/L6 engines, and that have usable pin ht and pin dia. I selected pistons from a Toyota 3L V6 3VZ-E (87.5 mm bore) and would use 3 + 3 for the side that gave the correct side thrust/pin offset. (must buy 2 sets of 6; discard 6 for the other side of two V6's). I'd have to look up if I kept L or R; but it's easy enough to figure out from the L6 offset. The problem is that the valve reliefs are different sizes (to correspond to intake and exhaust valves) and half of the inline 6 cylinders will be mismatched re which valve is where on piston vs cyl head location. If you study the pistons you will see the conflict. So, I was planning on fly-cutting the piston tops for intake valve-sized reliefs to be symmetrical on all 6 pistons (because I couldn't go smaller on intake reliefs that were corresponding to exhaust valves in head; but I could make all piston tops match). I still have the pistons (new from Ebay) but haven't had the time to play with the project. It actually has more potential for L4 because there are no closed chamber/large vol chamber (like P79 and P90) heads to choose from. This limits even using flat top pistons if you want a larger displacement L4 and a round-bowl type dish would give up quench, so the configuration of the 3VZ-E piston top might be useful. This type of idea is for those on a strict budget. I would just order expensive custom pistons if that were within my budget. DAW Quote Link to comment Share on other sites More sharing options...
madkaw Posted April 24, 2015 Author Share Posted April 24, 2015 My thought on the reliefs was more of chamber matching then just clearing for pistons. Considering a mn47 head with oversized L28 pistons might be a engine to try this with. Lowering CR and clearing valves for what would be a necessary larger cam. Quote Link to comment Share on other sites More sharing options...
DAW Posted April 24, 2015 Share Posted April 24, 2015 I guess I'm not sure by what you mean as chamber matching. I think that is what I'm refering to re matching a flat portion of the piston top up with a flat portion of the cylinder head chamber to maximize squish. The dish portion of the piston top is needed to keep compression ratio down. This would apply in L6's to larger displacement than unstroked L28 where you want to use a closed chamber head but can't use full flat top (no partial dish), or the c.r. would be too high. Quote Link to comment Share on other sites More sharing options...
DAW Posted April 26, 2015 Share Posted April 26, 2015 (edited) Here is what I am referring to: ...the table on the piston top is juxtaposed to the table on the cyl head = squish/quench, etc = high c.r. resistant to knock. Edited April 26, 2015 by DAW Quote Link to comment Share on other sites More sharing options...
madkaw Posted April 26, 2015 Author Share Posted April 26, 2015 Dam, that's a hell of a shelf on that piston. Those aren't stock flatops right? Yes, we are talking about the same thing as far as cutting the shape of combustion chamber in the top of the piston. I was Suprised to see that you also needed room for valves with additional reliefs . What engine and head combo? Quote Link to comment Share on other sites More sharing options...
DAW Posted April 26, 2015 Share Posted April 26, 2015 they aren't custom pistons, they just happen to line-up flat-to-flat tables. Pins are 21mm; c.ht is 34 mm. After fly-cutting, the intake eyebrow would match symmetrically to what you machine in next to it...the identical eyebrow. It is an experiment but seems sound. Quote Link to comment Share on other sites More sharing options...
inline6 Posted April 26, 2015 Share Posted April 26, 2015 Here is what I am referring to: ...the table on the piston top is juxtaposed to the table on the cyl head = squish/quench, etc = high c.r. resistant to knock. Those look a lot like the JE pistons in my engine. Quote Link to comment Share on other sites More sharing options...
madkaw Posted April 26, 2015 Author Share Posted April 26, 2015 My thought on the reliefs was more of chamber matching then just clearing for pistons. Considering a mn47 head with oversized L28 pistons might be a engine to try this with. Lowering CR and clearing valves for what would be a necessary larger cam. I just realized that I wrote this wrong. I was considering chamber matching reliefs instead of just eye browsing for valves. A sunken pocket matched to the combustion chamber that would help clear valves and drop CR a little bit. I guess I could have asked the max depth you can cut on a flat top. Thanks for pics and examples. It's also nice e to know that 530 lift will work with a little help. Quote Link to comment Share on other sites More sharing options...
Leon Posted April 26, 2015 Share Posted April 26, 2015 I just realized that I wrote this wrong. I was considering chamber matching reliefs instead of just eye browsing for valves. A sunken pocket matched to the combustion chamber that would help clear valves and drop CR a little bit. I guess I could have asked the max depth you can cut on a flat top. Thanks for pics and examples. It's also nice e to know that 530 lift will work with a little help. I run a .540" lift cam on stock flat-tops, FWIW... Quote Link to comment Share on other sites More sharing options...
madkaw Posted April 26, 2015 Author Share Posted April 26, 2015 I run a .540" lift cam on stock flat-tops, FWIW... Reeeaaaaallllyyyyy! Reliefs? duration? Quote Link to comment Share on other sites More sharing options...
DAW Posted April 26, 2015 Share Posted April 26, 2015 Those JE pistons do look similar. See how three are the reverse configuration re exhaust/intake reliefs? The problem with using pistons from a V6 in an inline six engine is that three are going to be the wrong piston pin offset for the rotation direction. That's why I was going to cut the exhaust valve reliefs to be symmetrical to the intake valve reliefs on all six pistons. It is a compromise but not giving up much for the sake of attaining good squish and keeping c.r. useable on high displacement engines. I hadn't considered valve-piston clearance as even an issue in what I'm referring to because it looks like there is plenty there. (In that sense I hijacked the thread) Again, this is more of an issue on L4 engines because there are no raised roof/closed chamber heads available for L4 as there are for L6. Anecdote: I bought a rather hacked up road race 510 years ago, 1600 cc, and I had the head off for some reason and noticed that there were nicks in the piston tops on #2 and #3 cylinders from intake valve contact (not enough to bend the valves). That was because PO had the #2 and #3 pistons assembled into the wrong holes. They were nice forged pop-up pistons. I reversed them in their holes, sorted some other things out, and raced SCCA Solo I and II with the car. PO was a dyslexic engine builder I guess. Quote Link to comment Share on other sites More sharing options...
Leon Posted April 26, 2015 Share Posted April 26, 2015 Reeeaaaaallllyyyyy! Reliefs? duration? No reliefs, 270* duration. P90 shaved 2mm with 0.020" over flat-tops, no modifications to pistons. Torquey as hell. I checked valve-to-piston clearance and remember it being close but acceptable. Quote Link to comment Share on other sites More sharing options...
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