Attn flat top 2.8 with E31/N42 head guys

[Derek]

In short, I do not endorse L28 builds with flat top pistons and E88, N42 N47 heads for ANY street application unless a thicker head gasket is used to drop the compression down to below 9.7:1 or so. If flat tops are is on your must have list, then I recommend the P79 or P90 head

 

Hi Paul.

Any thoughts on a L28 std bore with dished pistons and a shaved .080 P90 . With a felpro gasket that gets me (according to the L engine calculator) 8.3:1. This is with a 46cc combustion chamber.

 

My bottom end in in perfect shape and with all this talk going on about loosing timing points due to detonation I'm hesitant to change the pistons. Plus it's a lot of extra work! I also don't see pump gas getting any better as time goes on.

 

Thanks

Derek

[ozconnection]

 

Video of the dished piston motor just after installing the 44's (read carbs not tuned):

http://videos.streetfire.net/video/2000-autox-indisde-and-outside-I-think-my_8051.htm

 

Yeah, sounds like it too!!!!!

[z-ya]

Sorry to pick you on ya Pete, but your example is perfect.

 

Too common for the L-6 to have to back off ignition timing due to the compression ratio. I have been preaching for some time that the power lost due to the retarded ignition timing to keep it from rattling, for greater than the power gained by the bump in compression ratio. Gain of maybe 7-10 HP max for the compression ratio bump over the paltry 8.3:1, but over 20+ HP lost in retarded ignition timing! open chamber L-6 heads, (E88, N42, N47), have their optimum non octane limited spark advance in the 38-42 degree range. If you can't get 38 degrees total ignition advance above 3500 RPM at WOT, (N/A), with E-88, N42 or N47 head because it is rattling, you are laving HP on the table.

I'm not saying that 10.4:1 comp ration can 't be done and still have adequate ignition advance, it has been done but most of those combination rattle and and can't run full ideal ignition advance.

 

Hi Paul,

 

My setup is an example of how to get good power from a budget motor. No head work, no special cam, stock springs, etc. As we all know, a lot can be spent on head work. I spent $100 for a cam plus a regular valve job. This engine could easily be street driven as it runs fine on 93 octane. It IS very sensitive to detonation as you described. As long as you have some head room on timing (which I have 2-4 degrees backed out for safety), this is a good setup for the street. Or if you run closed loop with a knock sensor, you should be fine. I do well over 1000 track miles per year with this motor and it has held up well. I have blown a 2 head gaskets due to detonation, but this was due to fuel pump cavitation in the turns.

 

Do you think that if I swapped my short block for an 8.3:1 L28 it would make 170WHP? I have dyno tuned many stock L28s, or ones with stock CR and a mild cam like I have. If they make 155WHP they are lucky. Usually they are in the 140-150WHP range on the same dyno I tune my car on (sometimes on the same day). This is running over 35 degrees of advance. This is one of your theories I am not a believer in. You will need to show me some dyno measurements to convince me.

[skib]

next time I come out Ill be doing some bench racing with you about CR as well Paul

[BRAAP]

Hi Paul,

 

My setup is an example of how to get good power from a budget motor. No head work, no special cam, stock springs, etc. As we all know, a lot can be spent on head work. I spent $100 for a cam plus a regular valve job. This engine could easily be street driven as it runs fine on 93 octane. It IS very sensitive to detonation as you described. As long as you have some head room on timing (which I have 2-4 degrees backed out for safety), this is a good setup for the street. Or if you run closed loop with a knock sensor, you should be fine. I do well over 1000 track miles per year with this motor and it has held up well. I have blown a 2 head gaskets due to detonation, but this was due to fuel pump cavitation in the turns.

 

Do you think that if I swapped my short block for an 8.3:1 L28 it would make 170WHP? I have dyno tuned many stock L28s, or ones with stock CR and a mild cam like I have. If they make 155WHP they are lucky. Usually they are in the 140-150WHP range on the same dyno I tune my car on (sometimes on the same day). This is running over 35 degrees of advance. This is one of your theories I am not a believer in. You will need to show me some dyno measurements to convince me.

 

 

 

If the head was ported or bone stock on an L-28 that is “not” knock limited, it will make more peak power with 38-42 degrees total ignition advance vs 26, 30, or even 32 degrees! Flat top pistons with that open chamber head does not change the efficiency of the burn in the chamber, (still has no squish), as such retarding the timing below 36 degree IS removing available power that head could produce, (if not knock limited due to too much static compression ratio)!

 

If you dropped your compression ratio down to 8.5:1 or so, you could run full ignition advance of 38-42 degrees at WOT above 3500 RPM, you will gain more power than you have now at 26 degrees! If you could find a way to run 36+ degrees of total ignition advance on your current setup with NO rattle, you will see more power, probably in the neighborhood of 7% more than if it was 8.3:1 comp ratio at the same ignition advance.

 

Here is a simple test for anyone to perform on their own to see and feel this first hand, with an L-28 that has low enough static compression to take advantage of the heads ideal full ignition advance with premium pump fuel. The difference is so drastic, even the butt dyno will accurately show the results, back to back dyno runs would also be good.

 

Set total mechanical ignition at 38-42 degrees when above 3500 RPM, (with the vacuum advance disconnected, depending on the dizzy mech advance will be approx 15-20 degrees initial advance at idle), it will run quite well. Then retard the dizzy to a total timing to 26 degrees and drive it around! You will notice a HUGE decline in performance, a loss of far more power due to the ignition retard than that same engine would gain if you added 2 full points of static compression ratio to it alone, if it could still use full ignition advance!

 

 

Yes, I strongly believe with your 26 degrees of total ignition advance at WOT above 3500 RPM with a stock untouched N-47 head, you “are” leaving power on the table! If you can find a way to get more ignition advance without rattle, you will see more power.

Edited December 5, 2009 by BRAAP

[BRAAP]

Hi Paul.

Any thoughts on a L28 std bore with dished pistons and a shaved .080 P90 . With a felpro gasket that gets me (according to the L engine calculator) 8.3:1. This is with a 46cc combustion chamber.

 

My bottom end in in perfect shape and with all this talk going on about loosing timing points due to detonation I'm hesitant to change the pistons. Plus it's a lot of extra work! I also don't see pump gas getting any better as time goes on.

 

Thanks

Derek

 

 

Due to the low static compression ratio, you should able run full ignition advance allowing that head to make its peak power, not knock limited.

 

That head shaved .080" with flat tops offers some squish which in itself is a detonation deterrent. How much so, I don't really know. I would like to hear from more people that are running this that set up on premium pump gas and find out if they are still able to run full ignition advance.

 

One thing to point out, with a squish design, due to how the gasses are being "squeezed" out into the smaller open region of the chamber surrounding the spark plug, the gain in efficiency from that allows for less total ignition timing. How much, again, I don't know, but I would venture to guess it will make peak power, (when not knock limited), between 34-40 degrees total ignition advance. Again, just a guess, the gain in efficiency from the flat tops and squish might not be that much of gain.

 

Hope that helps.

[BluDestiny]

With more input/proof (dyno charts etc) this could be a good sticky. A lot of people do these kind of builds and I think it would be very helpfull. from my other post a while back I'm planning on doing this, but I do plan to get some headwork done and a bigger cam to compensate for the compression.

[z-ya]

Yes, I strongly believe with your 26 degrees of total ignition advance at WOT above 3500 RPM with a stock untouched N-47 head, you “are” leaving power on the table! If you can find a way to get more ignition advance without rattle, you will see more power.

 

I totally agree. The engine actually made closer to 195 WHP without detonation. Backing off the timimg 3-4 degrees (for safety) knocked close to 15WHP off.

 

Here are two plots. Both engines are L28s with an N series head. They both have similar intakes and exhaust systems (headers, 2.5" exhaust). The both have a stock camshaft. Both have EFI. The only difference is that one motor has flat tops, the other has dished pistons. WOT ignition advance was set just below the point of detonation.

 

 

 

Granted, the engine in the bottom plot is running too rich, and this can explain some of the HP differential. Anyway, it is a good comparison. Same day, same dyno.

 

Pete

[ozconnection]

I can't get the idea out of my head about running up to 42 degrees advance. Thats heaps!!!!....too much if you're going by the established 'rules of thumb' suggesting 38 degrees total advance (dta) is enough.

 

I'm going to give it a try though, bumping up from my current 35 dta. I'm going to disconnect my vacuum advance mechanism during these tests, just to be safe. Should be very interesting!

 

Pete, your dyno graphs are what we're after, but that pig rich mixture on the second graph/engine isn't really close enough to the other AFR's to say that there's that much of a difference in engine output, based solely on quench setups. Sure, the AFR's from both engines will never be 'exactly' the same, but I feel they're too different to sell the whole story in this case.

 

Don't get me wrong, I'm a subscriber to this arguement though and interested in hearing from more people on the topic.

[z-ya]

 

Pete, your dyno graphs are what we're after, but that pig rich mixture on the second graph/engine isn't really close enough to the other AFR's to say that there's that much of a difference in engine output, based solely on quench setups. Sure, the AFR's from both engines will never be 'exactly' the same, but I feel they're too different to sell the whole story in this case.

 

Don't get me wrong, I'm a subscriber to this arguement though and interested in hearing from more people on the topic.

 

I agree, AFRs make a huge difference in power output. In this case the 8.3:1 motor is about 1 point richer than the 10:1 motor. Is this enough to cause such a difference? Although I don't like to guess, I would have to say no. Mainly because I have tuned other L28s with the same mods that were running a median 14:1 AFR a never came close to even 150WHP.

 

As far as seat of the pants tests, my 10:1 motor, even with a stock cam, pulled way better than an L28 with a stock CR.

 

I would like to see a dyno plot for an L28 with stock cam and CR make much over 140WHP. I've never seen one.

[ozconnection]

 

I would like to see a dyno plot for an L28 with stock cam and CR make much over 140WHP. I've never seen one.

 

I've attended and participated in many Z car club dyno nights here in Sydney. I too cannot recall any L2X's make over 140 rwhp. The most I think I ever saw was one guy with a 280zx and an aftermarket management make 95 rwkw's which is about 127 rwhp.

 

Even my own efforts with a stock cam/head and CR grossed out to 90 to 92 rwkw's (through a L4N71B auto) which I thought was pretty bloody good, since I started with a bone stock N42/N42 combo with a 2 barrel downdraught Hitatchi which dyno'ed at 60 rwkw's

 

Can anyone prove us wrong?

[SHADY280]

im with brapp on this one, i had my engine at 11.8:1 cr (ported polished mildly worked over n47). it was a mistake, i couldnt run full timing, it had good throttle snap, but you could feel it was missing its full potential. i installed a p79 with less cr (10.8) and ran full timing and it actually felt better, still had good snap and better top end.

[johnc]

E85 should solve all of these compression problems.

[BluDestiny]

So this only applies to heads that are just stock and directly bolt on?

 

Should I begin to rethink my idea of an E88 on a L28 flattop?

 

I'm getting a .495 lift/ 290 duration cam and port/polish done

[z-ya]

im with brapp on this one, i had my engine at 11.8:1 cr (ported polished mildly worked over n47). it was a mistake, i couldnt run full timing, it had good throttle snap, but you could feel it was missing its full potential. i installed a p79 with less cr (10.8) and ran full timing and it actually felt better, still had good snap and better top end.

 

This is not really me against Paul. I jsut would like to see some actual measurements. I have made measurements, and that is why I think it is worth it. "Snap", "feel", etc. are not a very accurate way to measure power output .

[BRAAP]

I don’t recall the actual dyno figures from the example I quoted above where we dropped the compression ratio and swapped out the aggressive cam for stock one, maybe the owner will chime in? Mikey, you out there?

 

My particular L-28 280 Z, I don’t have any dyno numbers for, but I did weigh it and run it at the strip. 2800 lbs with driver and half tank of gas, ran the ¼ mile in 14.4 @ 97 MPH all day long! Engine was bone stock ’75 dished piston short block. ’78 N-47 head that was my very first attempt at porting heads, included extensive valve unshrouding, dropped the compression ratio down to between 8.0:1-8.1:1. Cam was a stock ’78 cam. Stock ’75 EFI. Lightened flywheel, stock ’75 ignition system and module with Jacobs coil, Jacobs wires and recurved ignition advance that brought full mechanical timing in by 2500 RPM and full mechanical advance was over 44 degrees! Was my daily driver for several years in that configuration. Power to weight ratio, possibly puts that set up in the 160+ hp range?

 

In short, my personal preference is do NOT run any more compression ratio that would force you to retard ignition timing due to knock sensitivity, because you WILL give up more power in retarding the ignition timing, (from the heads ideal), than you gain in compression ration increase.

[JMortensen]

So this only applies to heads that are just stock and directly bolt on?

 

Should I begin to rethink my idea of an E88 on a L28 flattop?

 

I'm getting a .495 lift/ 290 duration cam and port/polish done

My L28/E31 with a similar but slightly smaller cam required 95 octane to prevent pinging. I would go bigger on the cam or lower on the compression especially if it's one of the open chambered E88 heads with no quench.

[letitsnow]

In short, my personal preference is do NOT run any more compression ratio that would force you to retard ignition timing due to knock sensitivity, because you WILL give up more power in retarding the ignition timing, (from the heads ideal), than you gain in compression ration increase.

 

I think this might be better put in it's own thread, but, why is it that the L series engines are so sensitive to detonation? An iron headed(wedge chamber, not vortec) sbc can run 10:1 without much issue and my TFS headed sbc can run on 89 with 10.5:1 without retarding the timing. It's rather obvious that the much better CC of the TFS heads combined with the thermal conductivity of AL are the reason it is so resistant to detonating.

 

The combustion chamber on the P90/79 isn't that awful with a flat top piston, it's still not the best though, a welded and profiled chamber would probably be the best, willing to comment on your experiences with them?

[BRAAP]

I think this might be better put in it's own thread, but, why is it that the L series engines are so sensitive to detonation? An iron headed(wedge chamber, not vortec) sbc can run 10:1 without much issue and my TFS headed sbc can run on 89 with 10.5:1 without retarding the timing. It's rather obvious that the much better CC of the TFS heads combined with the thermal conductivity of AL are the reason it is so resistant to detonating.

 

The combustion chamber on the P90/79 isn't that awful with a flat top piston, it's still not the best though, a welded and profiled chamber would probably be the best, willing to comment on your experiences with them?

 

I agree entirely! As I covered in my first post of this thread, the L-6 head, a 2 valve design who’s chamber design isn't that terrible, but for some reason is hyper sensitive to knock!

As you mentioned, an iron head small block Chevy, Ford, or Mopar V-8 can safely run 9.5:1, 9.8:1 and even 10:1 on premium pump gas, aluminum head can run on more point of static compression, so 11:1, (10:1 for Iron heads and 11:1 for aluminum heads really is about the max safe static comp ratio for most production 2 valve heads on premium pump gas). Many other manufactures, Toyota, etc, are also able to realize greater than 10:1 static compression ratios with their aluminum heads without knock! The L-6, for some reason, is very knock sensitive! The Turbo guys are regularly popping head gaskets at 10-12 PSI boost on conservative tunes, with only 7.5:1 compression! That low of static C/R should be able to safely support more boost than that!

 

As I mentioned in my first post in this thread, my theory as to why has it roots in this thread linked below;

 

I as many other have noticed with the L-6, it is highly sensitive to detonation. With an aluminum head, it should be able to handle 10.5:1 compression ratio N/A on premium pump with no rattling, (many Chevy, Toyota, and other valve designs do just fine), but far more often than not, the Datsun L-6 will rattle at anything above 9.5:1 comp ratio on prmeium pump gas! Why? I have my theories and I firmly believe it is rooted it eh coolant through the head which this thread address. Again, just my theory...

http://forums.hybridz.org/showthread.php?t=125186

 

 

Again, that is just my theory as to why the L-6 is so sensitive to knock, and the findings of those guys testing in that thread seem to be supporting it. To some degree at least.

[z-ya]

No head mods, hot cam, just bump up the CR from 8.3:1 to 10:1. That is all I'm talking about. Set the max advance on both motors just before detonation. You will find that the high CR motor produces at least 20WHP more than the motor with a stock CR. I'm not guessing here, I've made the measurements that prove this. Has anyone else?