Guest turbozed Posted May 5, 2008 Share Posted May 5, 2008 I have recently switched from a P90 to a N42 head. My cranking compression has gone from 150lbs. to roughly 170lbs. Using my P90 head I had no issues running 15lbs of boost on 94 octane fuel. With the N42 I am getting detonation at just 8lbs. of boost. The only thing that seems cure this is excessively rich air/fuel ratios (10-1) and little timing (in the neighborhood of 17 degrees with boost retard). Is this normal due to the higher compression and different combustion chamber design of the N42 vs. the P90 cylinder head? My setup details: F54 turbo block, T3/T4B turbo, Saab intercooler, 370CC injectors, SDS EM4F injection. Fuel pressure is set at 43 PSI, with a 1-1 rising rate regulator. Quote Link to comment Share on other sites More sharing options...
MJLamberson Posted May 5, 2008 Share Posted May 5, 2008 compression does play a big role in controlling detonation, why would you switch from the p90 in the first place? Quote Link to comment Share on other sites More sharing options...
Guest turbozed Posted May 5, 2008 Share Posted May 5, 2008 I changed to the N42 in hopes of gaining more streetability over the P90/54 combo's dismal 7.3-1 compression ratio. Was hoping that I could get away with running at least 12lbs. of boost with the new higher compression head. As a side note, I have already changed to cooler, non-projected tip spark plugs (NGK BR7ES), which did not seem to make any difference as far as suppressing detonation. Quote Link to comment Share on other sites More sharing options...
1 fast z Posted May 5, 2008 Share Posted May 5, 2008 Yea you just lost all quench benifits by going to a N42 head. P90 is a high quench design. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted May 5, 2008 Share Posted May 5, 2008 How much quench do you think he had with the dished pistons in there? I think the problem is the increase in compression. In Canada do they use the same RON+MON/2 octane rating we do in the states? If not, is there a higher octane gas you can use? Quote Link to comment Share on other sites More sharing options...
TimZ Posted May 5, 2008 Share Posted May 5, 2008 How much quench do you think he had with the dished pistons in there? I think the problem is the increase in compression. I agree. Honestly, I've never understood the obsession with trying to run slightly higher CRs for supposed off-boost gains at the expense of max boost. IMHO, the benefit of the higher CR is way overblown in these cases. If you do the math the power gain in going from 7.3 to 8.1 CR (which is what we are talking about here) is about 3.6%. On an engine that peaks at around 120rwhp without boost, we are talking about maybe 4 horsepower in the off-boost region. ...So you gave up say 60 hp worth of boost just to gain 4 hp when off boost. Yay. Quote Link to comment Share on other sites More sharing options...
jgkurz Posted May 5, 2008 Share Posted May 5, 2008 I agree. Honestly, I've never understood the obsession with trying to run slightly higher CRs for supposed off-boost gains at the expense of max boost. IMHO, the benefit of the higher CR is way overblown in these cases. If you do the math the power gain in going from 7.3 to 8.1 CR (which is what we are talking about here) is about 3.6%. On an engine that peaks at around 120rwhp without boost, we are talking about maybe 4 horsepower in the off-boost region. ...So you gave up say 60 hp worth of boost just to gain 4 hp when off boost. Yay. Exactly!!!!! Quote Link to comment Share on other sites More sharing options...
JMortensen Posted May 5, 2008 Share Posted May 5, 2008 ...So you gave up say 60 hp worth of boost just to gain 4 hp when off boost. Yay. I think you and Clifton (among others) have the right idea on this issue. If you're going to run the turbo, run a big one, turn the boost way up, and get more power. Deal with the lag, and forget about trying to run more compression with a lower boost turbo. If you want no lag and big power, get a V8. Maybe I'm oversimplifying, but that's my quick and simple advice... Quote Link to comment Share on other sites More sharing options...
fhptom Posted May 5, 2008 Share Posted May 5, 2008 You also need to watch how much surface material has been milled off the N42. Especially since that is a sought after head for N/A cars. Mine had been milled so much by past owners that my CR went to way over 10.5:1. I have to use 105 race gas to get rid off all the detonation. Tom Quote Link to comment Share on other sites More sharing options...
Administrators BRAAP Posted May 5, 2008 Administrators Share Posted May 5, 2008 I agree. ...So you gave up say 60 hp worth of boost just to gain 4 hp when off boost. Yay. Exactly!!!!! Ditto… Quote Link to comment Share on other sites More sharing options...
proxlamus© Posted May 5, 2008 Share Posted May 5, 2008 I learned something today! =) Quote Link to comment Share on other sites More sharing options...
jgkurz Posted May 5, 2008 Share Posted May 5, 2008 I have to use 105 race gas to get rid off all the detonation.Tom I have found that this approach puts a LARGE hole in the wallet. Better that then your pistons I guess... Quote Link to comment Share on other sites More sharing options...
Guest turbozed Posted May 6, 2008 Share Posted May 6, 2008 Is there anyone out there running a similar setup??? How much boost are you getting away with?, and what's your ignition timing like? Quote Link to comment Share on other sites More sharing options...
Guest turbozed Posted May 6, 2008 Share Posted May 6, 2008 also.... could one point of compression really effect max. boost capability that much??? (especially when going from an exceptionally low 7.3-1 to a seemingly boost friendly 8.5-1).. Or is the P90 combustion chamber just that much better than the N42? Quote Link to comment Share on other sites More sharing options...
m4xwellmurd3r Posted May 6, 2008 Share Posted May 6, 2008 8.5:1 isn't really boost friendly imo. the highest I would ever go would be 8:1, but ONLY if everything was 100% perfect. otherwise the stock 7.3:1 is perfect Quote Link to comment Share on other sites More sharing options...
X64v Posted May 6, 2008 Share Posted May 6, 2008 Your comp ratio should not be causing that pinging. On my L24 (8.9:1 SCR with open chambers like the N42) I ran 10psi, non-intercooled, on 91 octane with ~21 degrees of advance and 10.7:1 AFRs. No ping, no problem. Quote Link to comment Share on other sites More sharing options...
racerx260 Posted May 13, 2008 Share Posted May 13, 2008 I am running an n42 head on a 54 block bored .40 over with dished pistons. Stock turbo at about 8 lbs. timing at 20btdc. on 93 octain gas . I have no pings or knocks.I love this setup because of the off boost tourque. I get full boost at 2300 rpm. from the higher compession. I have great on boost power and get a surge at 5000 to 6500.I am using the stock n/a cam from a stock 76 n42 head.I recently drove through the mountains fron nc. to ky. and got 26.5 mpg at 85 mph.Using the engine calculator I have about 8.4 to 8.5 compression. i Quote Link to comment Share on other sites More sharing options...
510six Posted May 13, 2008 Share Posted May 13, 2008 First I would run at least a NGK BR8ES with a 8.5 compression ratio.My 3 liter turbo motor has a 8.5 compression and 24* of total advance using 35% methanol as my "total' fuel, detonation is non existant( confirmed by datalog from the AEM EMS) and off boost response is great. I would imagine even with a very basic methanol/water injection kit that it would do wonders to eliminate detonation. On my previous setup an old school Spearco water/methanol injection kit was used with 8.3 compression ratio running 12 psi on a CT26 turbo wasn`t a problem even without an intercooler. Quote Link to comment Share on other sites More sharing options...
racerx260 Posted May 14, 2008 Share Posted May 14, 2008 What Plug Gap Would You Recomend On That Heat Range Plug? Quote Link to comment Share on other sites More sharing options...
Tony D Posted May 15, 2008 Share Posted May 15, 2008 If you're going to run the turbo, run a big one, turn the boost way up, and get more power. Deal with the lag, and forget about trying to run more compression with a lower boost turbo. If you want no lag and big power, get a V8. Maybe I'm oversimplifying, but that's my quick and simple advice... This is my 'Freeze Plug'... Using a larger turbo generally results in a higher Boost Threshold, meaning full boost will not be realized until a higher rpm. This is not the same thing as 'lag'! Lag is the time it takes for the compressor, once given a WOT situation, to come onto full boost. This time is so quick in modern turbos as to be almost immeasurable (if that is a word). People get conditioned to drive 'on the cam'---and this is supposed to be acceptable in an N/A engine. Yet when you go WOT on a cammed engine at idle you get soggy response (V8's included). Run it up so you are 'on the cam' and go WOT and you get instantaneous response. It's well known you have to drive a Cammed N/A engine 'on the cam' for proper response. This is no different than a TURBO engine. Yet, apparently slapping a hairblower onto a car is supposed to magically transform the engine into an 'all rpm performer' and ignore the basic laws of physics. Drive a turbo car properly, and there will never be an instant where you will notice 'lag'---that went away well before the 70's were done and the 80's came along. Now, having been in JeffP's car, I can say his engine runs like a much larger CID simply because his engine breathes so well. Sure, you only get 3psi at WOT from 1000rpms, but the dyno will show it pulls linearly from there on up. Sure, you feel the torque peak pull harder at 4500, just like any cammed engine, but you really don't feel a surge at 3400 rpm when the boost goes like a switch from 3 to 20psi. You wouldn't think the curve would stay as flat as it does with boost jumping so quickly, but it does. The current generation of turbos are not what terms like 'Lag' were meant for... Maybe I'm missing something, or misinterpreting it, but "lag" from the above quote looks like a mis-driven old-school turbo setup. Drive even an old school turbo setup properly, and 'lag' really isn't an issure either. And driving it properly means keeping the r's above boost threshold, same as when you drive a heavily cammed engine. As for the detonation issue, I agree. I don't know what 'drivability' issues you were having with the stock P90, but raising the compression probably didn't accomplish much other than giving you another headache. I run a Non-US N42 and have run that to 17psi, but that's not with dished pistons. I think you messed with a working combination and learned the hard way theory sometimes does not work in the real world. Did your 'drivability' issues get any better after the head swap. Did you feel the difference you thought you would? If not, that's a good indicator to go back the way you were. It's only a couple of hours work and another couple of gaskets. Sucks when experiments don't work out. Quote Link to comment Share on other sites More sharing options...
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