chopstic Posted April 11, 2006 Share Posted April 11, 2006 I just came across a good deal on a complete l28 n42 FI engine out of a 75 datsun. I have decided to go n/a with megasquirt. How much compression can i SAFELY get away with assuming i get megasquirt tuned decently Quote Link to comment Share on other sites More sharing options...
JMortensen Posted April 11, 2006 Share Posted April 11, 2006 I would guess 10 or maybe 10.5:1 Quote Link to comment Share on other sites More sharing options...
Mack Posted April 11, 2006 Share Posted April 11, 2006 on that head, probably about 10.5. Its not exactly a "high quench" or a "fast burn" design. I just got megasquirt and am goingto be putting it on my 11.7:1 flat top motor with an MN47 head on it. Ill let you know how it goes. I think if I can tune it decently (as in a 13:1 AFR across the board under WOT) I dont think it will ping much if at all. Quote Link to comment Share on other sites More sharing options...
chopstic Posted April 11, 2006 Author Share Posted April 11, 2006 mack... are you going to run that on 92 octane? Quote Link to comment Share on other sites More sharing options...
Guest Cronic Posted April 11, 2006 Share Posted April 11, 2006 93 octane under nominal circumstances is good for around 12.5:1. I would imagine you would be good up to 11.8-12:1 compression under normal pressure and humidity. Quote Link to comment Share on other sites More sharing options...
olie05 Posted April 11, 2006 Share Posted April 11, 2006 i hope you guys are talking about aftermarket cams on these heads as well. The stock cam will not cut it. Just thought I'd throw that out there Quote Link to comment Share on other sites More sharing options...
z-ya Posted April 11, 2006 Share Posted April 11, 2006 It all depends how much timing advance and what camshaft/valvetrain you will be using. With flat top pistons, N42 head, stock cam, header, you can probably run around 26 deg of advance. Your CR will be close to 10:1 with a stock head gasket. You can expect to get from 150 to 160HP at the wheels from a setup like this, mine did. Quote Link to comment Share on other sites More sharing options...
Guest norm[T12SDSUD] Posted April 12, 2006 Share Posted April 12, 2006 If the N42 head is unshaved keep it below 10:1 or it will ping.You can shave the head 70 thousandths and it will change the shape of the combustion chamber. Years ago I was running 13.4 @ 101 mph on a 9.6:1 CR N/A 70 thousandths shaved N42 equipped L28 motor in my 240Z. I have seen an N/A Lseries motor lose 16 HP on a dyno simply by retarding the timing 6 degrees from 37 to 31 degrees, so don't think you can go high on compression and then eliminate pinging by retarding timing and not expect to lose HP. Later,Norm Quote Link to comment Share on other sites More sharing options...
JMortensen Posted April 12, 2006 Share Posted April 12, 2006 '']I have seen an N/A Lseries motor lose 16 HP on a dyno simply by retarding the timing 6 degrees from 37 to 31 degrees, so don't think you can go high on compression and then eliminate pinging by retarding timing and not expect to lose HP. Thank you for saying that Norm. I've known it to be true for a long time, but I've never had access to a dyno to prove it. There is a lot of hp in those last few degrees. Quote Link to comment Share on other sites More sharing options...
Guest norm[T12SDSUD] Posted April 12, 2006 Share Posted April 12, 2006 Hey there is a guy in town that picked up 90 HP on a Supercharged Mustang with just THREE degrees of extra timing!! My buddy was at the Dyno and saw the pull or I would have never believed it to be possible!! Quote Link to comment Share on other sites More sharing options...
Mack Posted April 12, 2006 Share Posted April 12, 2006 there is the flip side of the coin to take into account as well guys... a smaller chamber with a proper shape (as in a heart shape cough *MN47* cough) would be considered a fast burn head. with a fast burn head you dont need as much spark advance, and you can actually lose HP by too much advance. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted April 12, 2006 Share Posted April 12, 2006 For every different engine combo there is a "best" amount of timing to run, and if you can't get there because you're pinging, you're losing hp. It might be 37 degrees, it might be 32 degrees. One well respected racer I've talked to is running over 40 degrees on his 510 with an SSS head (look it up Mack, it's a heart shaped chamber too). The bottom line is you need to figure out where that best amount of timing is, then you need to provide the octane to run at that timing to get max power. Quote Link to comment Share on other sites More sharing options...
Mack Posted April 12, 2006 Share Posted April 12, 2006 true. there is no way of telling what optimum ignition timing for any set up is unless you have access to a dyno. 1 question tho, I always thought the SSS heads were peanut/kidney bean shaped? Quote Link to comment Share on other sites More sharing options...
JMortensen Posted April 12, 2006 Share Posted April 12, 2006 Same thing really: P79: http://www.geocities.com/zgarage2001/head.html 219: http://www.worldaccessnet.com/~dcmurphy/620/tech/head_ID_2.htm Quote Link to comment Share on other sites More sharing options...
Dan Baldwin Posted April 13, 2006 Share Posted April 13, 2006 FWIW, I've seen an L-series motor (mine) gain 20hp by retarding the advance from 45deg to 35deg. I bet there are as many Z's out there that are losing power due to too much advance as there are losing out due to too little! Like an idiot, I had just set mine to ~20 at idle and figgered I was done because I couldn't detect any ping. Little did I know I was robbing myself of hp and running the risk of high-rpm (harder to detect) detonation. I've also been to the dyno and LOST 18 hp by running VP11 fuel, vs. 93-octane pump. I got 8hp back by advancing the timing from the 34deg (lowest advance that didn't hurt hp on 93-pump) up to 39deg with the VP11, but still didn't make as much as I did at 34deg on 93 pump. Go figure... Pete's 164rwhp stock-cam/stock-N47 head ~9.8:1 CR build indeed has to be retarded to 26deg to run pump fuel without pinging. But my then-10.35:1, then-stock-cam/stock-N42-head 3.1 liter, ran at 10static/35total without pinging. When I put a cam in, I was able to advance it to 20/45 (FOOLISHLY going by the "advance it til it pings, then back off a bit" method, see above) without detectable ping, though I think it must have been detonating at higher-rpm as it was costing me quite a bit of hp. ANY motor will lose hp with too much advance, regardless of fuel, CR, quench, etc. Juggling any or all of these variables around will no doubt change the optimum ignition advance. Like Mack says, it's worth it to determine what's best for YOUR engine at the dyno. IMO, it's best to run the least advance you can without hurting torque/hp production. Last time at the dyno I made the same hp/torque with from 34 to 38deg advance, so I keep it at 34-35. My .02... Quote Link to comment Share on other sites More sharing options...
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