Guest DaneL24 Posted March 14, 2003 Share Posted March 14, 2003 I'm planning on doing a .080" shave to P79/P90 head and putting it onto a stock flat-top L28 short block, with either a 1mm or stock head gasket. I wanted to know if anybody has done this and if there were any issues with pston interference, and if the pistons had to be notched with valve reliefs. I was also planning on getting a mild cam like 260 degrees for the motor. I'm also open to the possibility of having the cam reground to reduce the peak lift a little as long as I keep the duration. Would rather sacrifice high end flow than compression. Also, I have considered using an N42/N47 head instead of shaved P79/P90, but have decided that I would rather use the later head. Thanks everyone. Quote Link to comment Share on other sites More sharing options...
yo2001 Posted March 14, 2003 Share Posted March 14, 2003 No, I've senn P79 shaved .080" with big .480" lift cam on flat top engine and no problem. You can run the play-do test if you want to make sure. Quote Link to comment Share on other sites More sharing options...
DAW Posted March 15, 2003 Share Posted March 15, 2003 You should be OK and the same clearance as using an N42/47 head with flat tops because the P79/90 valves are 0.080" (int) and 0.090" (exh) shorter than the N42/47 valves and you are cutting 0.080" from the head which moves the valve edges to where they would be anyway with respect to the piston top using an N42/47. The reason that the longer N42/47 valves are transferred into the shaved P79/90 and thick valve spring shims placed, is to restore the cam geometry since the towers and cam had to be shimmed up 0.080" to take up timing chain slack. Now that I think about it, when I set up my son's go kart chain I used a 1/2 link to obtain the needed chain length. Maybe the P79/90 could be shaved 0.080" but instead of the accompanying cam tower shimming, valve swapping, etc. the chain could be shortened by 1/2 link...or an alternate diameter cam/crank sprocket could be used. This would greatly simplify building a flattop piston/closed-chamber-head engine. DAW Quote Link to comment Share on other sites More sharing options...
Guest DaneL24 Posted March 15, 2003 Share Posted March 15, 2003 Yeah, I've thought about shortening the timing chain too. Problem is you either shorten a whole link or none at all, so there isn't much room for error. I also thought about modifying the chain tensioners to take up the slack, but then that starts messing with the timing, just because the cam sprocket shifts slightly as tighten one side of the chain. Might as well just stick with the tried and true: Shim the cam towers and get the thicker lashpads. Good to know I won't have to worry about clearance issues. Thanks guys! Quote Link to comment Share on other sites More sharing options...
DAW Posted March 15, 2003 Share Posted March 15, 2003 There is a way to do a 1/2 link shorten, it involves removal of one roller only; not two, by using a special side plates that are wide at the end attaching to the outside of one end of the chain, and narrow at the end with the roller which attaches to the other end of the chain. You would remove a full link from the chain and install a 1/2 link. I'm not saying that this is sure to work, but I've not seen the idea discussed before. However, the 1/2 link approach may still be too much distance to remove from the chain...I haven't measured to see. The needed chain length change would not equal 0.080" of course, the easiest way would be to check a stock chain on a shaved head with the cam in place but without the tower shims in place. Tensioner modifications don't help because the stock chain is just too long if the towers aren't shimmed. BTW, you don't need to buy new lash pads if you do the 0.080" shave/shim technique as long as you swap in N42/47 valves, their added length is the same as the added thickness of lash pad you'd need if keeping the P79/90 valves. DAW Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted May 25, 2003 Share Posted May 25, 2003 The timing chain has plenty of tension, and that's not the issue, but rather timing retard. I can't imagine the timing being retarded that much though. I believe AEM makes an ajustable timing gear which can advance or retard the timing up to five degrees. Whouldn't that be enough to compansate? Quote Link to comment Share on other sites More sharing options...
DAW Posted May 27, 2003 Share Posted May 27, 2003 Datsun Dude, if the head is shaved 0.080" and the cam towers are not shimmed too, the chain doesn't have plenty of tension; it has plenty of slack...too much slack to be compensated for by tensioner, guides, etc. Either the towers have to be shimmed 0.080" (which means thicker lash pads or longer valves with shimmed valve springs), or the chain needs to be shortened a specific amount (this isn't a popular approach) and I'm saying that if that amount might equal 1/2 link that it might be possible to save a lot of work to accomplish the same goal. Adjusting/compensating for the incurred cam retard due to shaving the head is good until about 0.020" or so, any more than that and you've got to deal with the excess chain slapping around. DAW Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted May 28, 2003 Share Posted May 28, 2003 .488 lift, shaved 85 thou, no problems. Sorry but I have to disagree on the timing chain "slackness". You can take it up with the guides and tensioner, I have seen a milled 110 thou head put on a stock block with no shims, and it ran 8.40's in the 1/8th. You can slot the holes in the tensioner. NOT THAT I ENDORSE THAT IDEA. Shims are the proper way to do it. Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted May 28, 2003 Share Posted May 28, 2003 I know the way I wrote that last post made it sound, more or less, like I thought I knew what I was talking about, But it was intended to be a question. I've been condidering this mod for some time, and I'm trying to get as much info as I can. A few people have told me that timing chain tension issues can be corrected. Basicly, I'm looking to increase my compression, and I'm not interested in stroking the motor. So any tips on the head mod from someone who's done it would be sweet.. Quote Link to comment Share on other sites More sharing options...
DAW Posted May 28, 2003 Share Posted May 28, 2003 Lockjaw, your friend who built the L6 with the head shaved 0.110" without shimming the cam towers at all, and ran 8.10 in the 1/8 mile...is that because he didn't make it the full 1/4 mile? just kidding. You can put it together however you want, but let's hear a 10,000 mile progress report (that's less than most people drive per year). If the tensioner is in its bore (it probably won't be unless a travel limiter has been added; big oil pressure loss too when this happens), the repositioned guides will likely be partially sawed through by the chain, the shavings into the oil, and the chain and sprockets excessively worn. Going fast on a budget is good but having to tear it down and rebuild it again early is a PITA. Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted May 28, 2003 Share Posted May 28, 2003 No he ran 8.40's but it was a long time ago, when there were not very many 1/4 mile tracks. He routinely turned it 7k plus, but got greedy and had this guy make an individual fogger set up for each runner of the sidedraft manifold, and squeezed it a little to hard. That car was wickedly fast for what it was, and as I said, I did not endorse the idea, but I have seen it done. That car had a huge crower cam in it, and it sounded some kind of wicked when you got on it. One of these days I am going to duplicate that engine, but not without shiming something. Quote Link to comment Share on other sites More sharing options...
Guest 2adz Posted May 28, 2003 Share Posted May 28, 2003 I'm currently running a .008" shaved P79 on an F54 flat top piston block. What I did was shim my cam towers an extra .008". Works and runs great. Quote Link to comment Share on other sites More sharing options...
yo2001 Posted May 29, 2003 Share Posted May 29, 2003 .080" not .008" right Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted May 29, 2003 Share Posted May 29, 2003 .008 is the alcohol legal limit... Maybe he got the two confused. -- -- Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted May 29, 2003 Share Posted May 29, 2003 I was going to say, for 8 thousandths, I wouldn't bother, but you know..... Quote Link to comment Share on other sites More sharing options...
Guest 2adz Posted June 3, 2003 Share Posted June 3, 2003 Sorry about that. I need to pay closer attention next time to what I am typing. Yes .080", you are correct sir. Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted June 3, 2003 Share Posted June 3, 2003 Just don't let it happen again... Kidding Hey, man--saw your site... Nice car! It should look pretty sweet when it's finnshed... I'm drifting off the topic so I'll stop typing now... Quote Link to comment Share on other sites More sharing options...
Guest hf240z Posted June 11, 2003 Share Posted June 11, 2003 hi, has anyone tried to re-route the path of the timing chain. like making it travel like an S on both side so that it will have enough tension on the same length of chain even after cutting the head and at the same time not altering the timing.? You should be OK and the same clearance as using an N42/47 head with flat tops because the P79/90 valves are 0.080" (int) and 0.090" (exh) shorter than the N42/47 valves and you are cutting 0.080" from the head which moves the valve edges to where they would be anyway with respect to the piston top using an N42/47. The reason that the longer N42/47 valves are transferred into the shaved P79/90 and thick valve spring shims placed' date=' is to restore the cam geometry since the towers and cam had to be shimmed up 0.080" to take up timing chain slack. Now that I think about it, when I set up my son's go kart chain I used a 1/2 link to obtain the needed chain length. Maybe the P79/90 could be shaved 0.080" but instead of the accompanying cam tower shimming, valve swapping, etc. the chain could be shortened by 1/2 link...or an alternate diameter cam/crank sprocket could be used. This would greatly simplify building a flattop piston/closed-chamber-head engine. DAW[/quote'] Quote Link to comment Share on other sites More sharing options...
Guest zrl Posted June 20, 2003 Share Posted June 20, 2003 Will flat top piston from 280ZX fit in 280Z? Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted June 20, 2003 Share Posted June 20, 2003 I suppose they would. I believe they have the same pin height. Ofcourse you'd have to concider your compession ratio... Quote Link to comment Share on other sites More sharing options...
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