HowlerMonkey Posted January 14, 2011 Share Posted January 14, 2011 Where's the P-79 vs P-90 discussion? Quote Link to comment Share on other sites More sharing options...
kce Posted January 16, 2011 Author Share Posted January 16, 2011 Where's the P-79 vs P-90 discussion? BRAAP pretty much wrapped it up on the first page: If using flat top pistons, the P79 is my first choice with the P90 being essentially equal. The only time I would prefer the P90 over the P79 is for very wild street an all out race applications that require crazy porting in the exhaust to match a cam and induction for an engine that only works wells above 4000-5000 RPM, idle at 1200+, etc, (.550â€+ lift) other wise for N/A street applications the nod goes to the P79. So as for the P90 vs. P79 discussion, it seems the determining factor in cylinder head performance is really the porting and any other modifications that are done. This of course, makes any sort of side-by-side meaningful comparison extremely difficult. As far as I can tell, the choice between what I call the more-quench combination (P79 or P90 with flat top pistons) and the less-quench combination (the N47 or N42 with dished pistons) really depends on your application and what kind of engine you want to build (I guess?). I have no educated reason to believe this, but the combustion chamber on the P79 and P90 look far superior to those of the N47/N42. The smaller combustion chamber and increased quench caused by the flat top piston, seems indicative to a more effective combustion chamber design. Good flame front propagation, swirl and mix and all those crazy fluid dynamics engineering-y things are important to making a performance engine. It's not just enough to have volumetric efficiency (moving fuel/air into the engine and moving exhaust out), thermal efficiency is also important (extracting the most mechanical energy possible from the stored chemical energy stored in the fuel). On Quench: The goal of performance engine builders should be to build their products with as much detonation resistance as possible. An important first step is to set the assembled quench distance to .035". The quench distance is the compressed thickness of the head gasket plus the deck height, (the distance your piston is down in the bore). If your piston height, (not dome height), is above the block deck, subtract the overage from the gasket thickness to get a true assembled quench distance. The quench area is the flat part of the piston that would contact a similar flat area on the cylinder head if you had .000" assembled quench height. In a running engine, the .035" quench decreases to a close collision between the piston and cylinder head. The shock wave from the close collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, average the chamber temperature, reduce detonation and increase power. Source Quote Link to comment Share on other sites More sharing options...
zpizzaman20 Posted January 16, 2011 Share Posted January 16, 2011 http://www.datsunspirit.com/engine.html Check out these heads. Quote Link to comment Share on other sites More sharing options...
HowlerMonkey Posted January 19, 2011 Share Posted January 19, 2011 (edited) The hardest part of comparing the P79 to a P90 with flattops is that most who swap the heads will swap the head complete with cam. I've just recently replaced a P90 with flattop long block assembly with a 1983 P79 long block assembly over a days time and notice no difference in performance between the two at stock boost. I'm sure the liners in the p79 don't affect performance until much higher power levels but I fear that extended running at high boost (standing mile....etc) could cause the liners to get hot enough to start to erode and possibly start losing chunks.....which is tough on turbos. I'm not sure how the comparison would go with a non-turbo engine. Edited January 19, 2011 by HowlerMonkey Quote Link to comment Share on other sites More sharing options...
kce Posted January 19, 2011 Author Share Posted January 19, 2011 (edited) The hardest part of comparing the P79 to a P90 with flattops is that most who swap the heads will swap the head complete with cam. I've just recently replaced a P90 with flattop long block assembly with a 1983 P79 long block assembly over a days time and notice no difference in performance between the two at stock boost. You don't have detonation issues running the P90 with flat top pistons and a turbo? I thought that combination would be get you pretty close to 9.1:1 static compression ratio (of course, your dynamic compression ratio is really the "compression ratio" that matters for determining your detonation threshold)? That seems pretty high to run with any boost but maybe I'm mistaken. EDIT: Also, excellent point about the cam swap. Edited January 19, 2011 by kce Quote Link to comment Share on other sites More sharing options...
Gollum Posted January 19, 2011 Share Posted January 19, 2011 (edited) You don't have detonation issues running the P90 with flat top pistons and a turbo? I thought that combination would be get you pretty close to 9.1:1 static compression ratio (of course, your dynamic compression ratio is really the "compression ratio" that matters for determining your detonation threshold)? That seems pretty high to run with any boost but maybe I'm mistaken. EDIT: Also, excellent point about the cam swap. It's actually closer to 8.5:1 I believe. The stock L28ET is in the mid 7's! BRAAP seems to be a lot more conservative on this debate than others, and has plenty of experience and knowledge to back him when he says that these engines are very detonation prone. Thus he doesn't readily recommend higher compression setups (that I've seen). There's actually a very wide spread of what people have seen from different setups, showing that there's just too many factors involved to make a conclusive statement that "X" setup will work perfectly for anyone. Now, if we want to say that X setup in X conditions in X circumstances then we can make an educated guess as a community and be in something of an agreement. All that being said, I think that many would agree to an extent if I were to say that a P79 or P90 with flat tops is perfectly fine at stock-ish power levels, even on the stock EFI and dizzy. Many might also agree if I were to say that the P90 + Flat tops are fine even up to the 300hp level with the stock cam if fuel and ignition tuning is capable. Obviously a more aggressive cam will help this maintain a level of possibility, but it's just a factor. There are guys pushing well over 400hp on the stock L28ET longblock. I personally thing there's almost no cause for concern for us guys with stock heads pushing a measly 7-12psi. If you're having detonation problems at those levels you have something else WRONG. I even run my P90/dish combo on california 87 octane which is like running on urine. If I switched to flat tops I might have to consider putting in premium, but even our premium isn't as great as some states regular. Gas is a big ping factor, so remember that when people start bickering about what combo will or won't work. Edited January 20, 2011 by Gollum Quote Link to comment Share on other sites More sharing options...
kce Posted January 19, 2011 Author Share Posted January 19, 2011 It's actually closer to 8.5:1 I believe. The stock L28ET is in the mid 7's! BRAAP seems to be a lot more conservative on this debate than others, and has plenty of experience and knowledge to back him when he says that these engines are very detonation prone. Thus he doesn't readily recommend higher compression setups (that I've seen). There's actually a very wide spread of what people have seen from different setups, showing that there's just too many factors involved to make a conclusive statement that "X" setup will work perfectly for anyone. Hmmm. I stand corrected then. I thought the static compression ratio was higher with the P90/P79 flat top combination. After doing a little impromptu research it does look like the P79/P90 have larger combustion chambers (53.5 CCs) compared to the smaller (~44 CCs) size of the N42/N47. Now, if we want to say that X setup in X conditions in X circumstances then we can make an educated guess as a community and be in something on an agreement. THIS! If I have learned anything from this thread (other than that I don't know anything) it is this. It is extremely hard to get an "accurate" side-by-side comparison of different cylinder heads because a lot of these setups are "one-off". Quote Link to comment Share on other sites More sharing options...
Dan Baldwin Posted January 20, 2011 Share Posted January 20, 2011 (edited) As far as I can tell, the choice between what I call the more-quench combination (P79 or P90 with flat top pistons) and the less-quench combination (the N47 or N42 with dished pistons) really depends on your application and what kind of engine you want to build (I guess?). I have no educated reason to believe this, but the combustion chamber on the P79 and P90 look far superior to those of the N47/N42. The key word being "look". Many have been able to run decent compression ratios with the N-heads, and it's easier to get higher c.r.'s with the N-heads (less shaving, no need to shim cam towers). Again, here's my N42-head 3.1's history: 3.1 liter, stock N42 head (shaved ~.010 from previous rebuild), stock N42 cam, unshaved KA24E pistons, 2mm gasket, 240Z carbs. 10.25:1 CR, timing had to be knocked down to 10* initial/35* total advance to eliminate ping (I had stupidly and wrongly been running +15* initial/+43* total) 290*/.490" Schneider cam, 2" Jag carbs 10.25:1 CR, timing bumped back up to 18* initial/43* total (again, a mistake) with no ping, 180rwhp 310*/.550" Sunbelt cam, Sunbelt porting and minor bowl work, 3x2 45mm carbs, thinner head gasket 10.8:1 CR, 217rwhp at 43* total advance, 235rwhp at 34* - 38* total advance (lesson learned, it's *stupid* to just run as much initial advance as you can while avoiding pinging) Rebuild w/ eyebrowed 89.5mm KA pistons, 11.3:1 CR, 255rwhp at 35* total advance Recent rebuild of middle 1/3, shaved head, 11.6:1 CR, not dyno'd, still running 35* advance on pump gas Edited January 20, 2011 by Dan Baldwin Quote Link to comment Share on other sites More sharing options...
Gollum Posted January 20, 2011 Share Posted January 20, 2011 And that right there is good information that I'd consider reliable. So what can we learn from his setup? That it takes a "whole package" mindset. He's running a fairly high duration cam and he's certainly done his research on timing curves, not just initial or total advance. I think some people also need a bit of a wake up call when it comes to compression ratios. Even 10:1 can be considered "low" in certain circumstances. There are Honda B engines running over 13:1 on pump gas out there, making 300+hp naturally aspirated out of 2 liters, again, on pump gas. Many people just assume a detonation issue is compression related, but it's a much more complex equation than that. Example: Say you're running stock SU carbs, that haven't been rebuilt or touched for a long time. So your engine starts to ping every now and then, depending on gas quality. You check your mixtures and you seem to be running plenty of fuel, maybe even a bit rich. You can chase your tail over and over again but your carbs might be the biggest issue, not the head or block needing to be pulled apart and cleaned. It could very well that you have dirty needles in the carb that aren't allowing as good and even of flow of fuel as they should. This results in poor atomization thus poor fuel distribution of fuel in the chamber, thus ping! Detonation isn't just heat related, and very rarely will you see ping in an engine across every cylinder. Tracking down what caused the ping is the biggest battle, and many times we tend to over look things. Maybe you have a cooling issue, or just simply your radiator cap is going bad so you're not at the psi you should be and running a tad hot. All that being said, just throwing together a high compression setup without doing your research can lead to a setup that just doesn't work. The problem might not be the compression ratio! Just some food for thought. I think for NA application any head will work. For force induction I personally prefer the P heads for the added quench while keeping compression ratios lower. Ping issues will come down to what pistons you run, your cooling system, your head prep work, and your fuel and ignition system and control. To Dan~ What fuel do you run? And what state are you in? Quote Link to comment Share on other sites More sharing options...
kce Posted January 20, 2011 Author Share Posted January 20, 2011 The key word being "look". Many have been able to run decent compression ratios with the N-heads, and it's easier to get higher c.r.'s with the N-heads (less shaving, no need to shim cam towers). I didn't mean to say that I think the N-heads are bad in any way, nor that I actually have a good reason to support my earlier statement. I can't argue with your results. Maybe the whole more-quench vs. less-quench argument is really just a red herring. Like you said, it's easier to get higher compression ratios with the N-heads and with a proper well designed, cammed and managed engine build you won't see any more detonation problems than you would with a P79/P90. I suppose comparing the two cylinder heads is really just a specious comparison anyway because of the amount of variability in each build (as mentioned earlier). Like they say, "Numbers talk and bullshit walks!"... and I can't argue with your numbers. Quote Link to comment Share on other sites More sharing options...
HowlerMonkey Posted July 14, 2011 Share Posted July 14, 2011 I mentioned earlier I swapped a stock p79/f54 longblock onto a turbo car running stock boost. Last week, I swapped the p90 head onto that block out of fear that the p79 liners would come apart under boost and it runs identical as far as I can see. The liners were undamaged. Quote Link to comment Share on other sites More sharing options...
Gollum Posted July 14, 2011 Share Posted July 14, 2011 I mentioned earlier I swapped a stock p79/f54 longblock onto a turbo car running stock boost. Last week, I swapped the p90 head onto that block out of fear that the p79 liners would come apart under boost and it runs identical as far as I can see. The liners were undamaged. Mind sharing a view details for the record? I'd like to reference this post in the future when this topic comes up. 1. What boost levels were you running? 2. What EFI system is the engine on? 3. Intercooler yes/no, size? 4. What fuel where you running? 5. What was your timing configuration and settings? 6. Any difference between max timing? Quote Link to comment Share on other sites More sharing options...
HowlerMonkey Posted July 15, 2011 Share Posted July 15, 2011 (edited) No intercooler Stock M30 ecu with stock 280zx turbo injectors and dropping resistors. 19 degrees of initial timing 7 pounds of boost no intercooler 93 octane position #1 on the cam sprocket victor reinz head gasket stock m30 exhaust system with catalytic convertor in place. Edited July 15, 2011 by HowlerMonkey Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.