Flattop L28ET?

[Gollum]

Replacing the stock pistons with forged flat tops would be the ideal setup as the factory cast flat tops will easily lose parts of thier skirts on relatively mild detonation events.

 

That's all debatable relativity though. The overall piston specs from dish to NA are nearly identical and there's no reason for the flat tops to blow up before the turbo dish pistons. And I've run timing in all of my NA cars very aggressive and I've had times where they've pinged all day long and survived just fine.

 

It all comes down to the output of the engine when it comes to detonation. No piston should fall apart under detonation when pushing 80hp per liter. Now getting upwards of 120hp per liter things usually get temperamental in any production engine.

 

So this again, is something I feel we don't have concrete information on. The question I want answers to is:

 

"At what power level does mild detonation become damaging in the short term on both piston types."

 

We know that for dish pistons, once you're up to 400hp detonation will rapidly ruin your motor. What we don't know, is the similar limits of flat tops. I personally can't imagine it being a whole lot less.

[NewZed]

 

It all comes down to the output of the engine when it comes to detonation. No piston should fall apart under detonation when pushing 80hp per liter. Now getting upwards of 120hp per liter things usually get temperamental in any production engine.

 

 

You're saying that low HP detonation is different than high HP detonation?

[letitsnow]

I'd say that it's a reasonable assumption, the pressure in the cylinder of a high HP engine is much higher to begin with.

[Gollum]

You're saying that low HP detonation is different than high HP detonation?

 

Though the physics of what detonation is doesn't change, the effects of it change DRAMATICALLY is what I'm saying.

 

I'd say 90% of vehicles pre 90's were designed to exhibit mild detonation under heavy loads. Drive any truck from the 70's and you'll understand what I mean.

 

Yet most race cars that see any amount of audible detonation like the trucks mentioned above will probably be taking considerable damage.

[2eighTZ4me]

I think what Gollum implied was that - at the higher HP levels per cyl. - the effects of detonation become far more catastrophic with an inferior head design (from the 70's).

[Gollum]

I think what Gollum implied was that - at the higher HP levels per cyl. - the effects of detonation become far more catastrophic with an inferior head design (from the 70's).

 

Or ANY head design... It's a pretty blanket reality. Even the BEST head design engines can reach catastrophic engine failure. A 800hp supra engine WILL be damaged from detonation. A 500hp honda motor usually holds together if it's 100% detonation free.

 

Both those engines would NEVER fail from SEVERE detonation at stock HP levels. I don't speak from theory, I speak from experience.

[HowlerMonkey]

I've been running flat tops with P90 and turbo for 4 years myself and have fixed a few as far back as 1988.

 

I was relating from experience.

[NewZed]

Something has always bothered me about the flat top/ high CR/ detonation discussions. If someone can run a high CR and maximized timing, when other people can't, assuming no combustion chamber modification (quench factors), it seems reasonable that it might be because the setup has low volumetric efficiency and/or exhaust gas scavenging. The similar engine that detonates must be packing more active fuel molecules in to the combustion chamber than the other, without the dilution of inert exhaust gases.

 

For example, if I install a Maxima N47 head on a flat-top F54 block and run 38 degrees total timing on 92 octane, and don't get knocking, I should start looking for obstructions in the intake and exhaust systems, or poor cam design. It knocks for other people, why not me?

 

Detonation is just the result, chemistry and physics, of packing a certain quantity, of the right concentration of fuel and oxygen, in to a confined space at a certain temperature and pressure. When pressure, temperature and concentration are all correct, the mixture that that hasn't been burned from the spark initiation blows up, all at once. VE determines how much gets shoved in, which determines cylinder pressure, and the scavenging of exhaust gases determines how diluted the mixture will be.

 

You could say that if you build a high CR L engine and it doesn't detonate on pump gas before you get the timing advance maximized, you must have done something wrong.

 

Just an alternative perspective on an old topic. It should be a point of pride to get a relatively low CR engine to knock, it means your skill at maximizing the active charge in the combustion chamber is high.

[letitsnow]

IMO. that's a bad way to look at this.

 

The end goal of all this modification we do is to make more power. Power is the end goal, ignition timing is one small variable that we have to calibrate to make the power. Nobody should build and engine with the thought "I want to run xx degrees of timing", they should build an engine with the thought "I want xxx power".

 

Does ignition timing affect power? absolutely. Can you build and engine with too much compression? yes. How many of these claims of loosing power because of detonation at relatively modest(sub-11:1) compression ratio's have been substantiated with some sort of proof(dyno, trap speed, ET, etc)? Not many. It is possible to alter the timing value where max power takes place, just because you can or can't run 38 degrees doesn't mean you should. From research(and a small amount of experience) I've found many many people that claim that there is a point where timing increases don't yield more power, they also say that the 'tabletop' they experience is before detonation. Air fuel ratio also plays a BIG role in detonation control as well, a lean spot right around peak torque and you might have to pull 10 degrees to eliminate all detonation.

 

I've seen this in my own car(boosted L28, good fuel/timing control and intercooling), using a 'det cans' setup to listen for detonation. Even minor detonation can be easily heard. Taking the timing back 2-3 degrees from just over the threshold of detonation resulted in little or no perceptible loss in power, dropping another 3 degrees made a noticeable hit in power. No dyno proof yet, but when I do make it I'll post results, my trips to the track were plauged by collapsing lifters, so that data is invalid as well. I've also seen similar results on a SBC, advancing the timing past 33-34 degrees on this engine made no change in 1/4 mile performance, it didn't detonate(this time listening by naked ear) until 38-40 degrees.

 

There are also many differences in individual engines that effect the likelihood of detonation beyond calibration. Sharp edges in the chamber(this is a BIG one, my stock chambers had big burs in them, took less than 10 min to remove), casting flash in the cooling jacket, coolant temp, crud buildup in the cooling system, quench, camshaft specifications.

 

In short, there are WAY too many variables to simply assume that if one engine doesn't detonate that it has a lower VE or makes less power. You absolutely can not assume that people making claims have investigated the issue to the fullest extent before simply picking a reason and blaming that, ie: high compression.

[NewZed]

It's just a perspective that goes beyond "I run flat tops at 10:1 CR and don't have any problems". It's a starting point that gets past just the assembly of parts and gets in to what's happening in the engine. For example, cylinder pressure at TDC should mean more than the mathematically derived CR, calculated from stroke and chamber volume. CR is a vague, fairly meaningless number, as is the term "dynamic CR". The cam numbers and the mathematical CR determine cylinder pressure, along with VE factors. The discussions would be more enlightening if they went along those lines.

[Gollum]

Actually running cylinder pressure readings on dyno's would be wonderful. Unfortunately that's more rare than scientifically done testing on dynos...

 

I can appreciate HowlerMonkey's opinion as he has experience, but he also didn't specify very much information to go with his statement and of all the other times this has come up I still haven't heart hard data. I'm not saying he's wrong, I'm just saying we don't have the full picture of the scenario in which he gained his data regarding detonation and pistons falling apart. We don't know:

 

HP Achieved

Ignition Timing

Ignition Map Data

Ignition Logs to show signal scatter

Wideband Data to couple to ignition logs

Engine prep info before assembly

Turbo used

Intercooler setup used

ECU used

etc etc etc

 

It doesn't take very much research to find out that, again, IDEAL, timing (meaning if detonation doesn't happen, where the best power starts) has very little to do with VE at all. It has much more to do with chamber shape, mixture quality, mixture ratios, chamber temps, etc. Just because an engine has poor VE and only makes 60hp per liter, doesn't mean it's going to need 40 degrees of advance, or will detonate easily. Chevy's fast burn economy heads for their SBC engines can see similar total ignition timings (even with detonation free race gas) similar to very efficient honda engines that are making nearly double the HP/Liter.

 

By very nature, a higher compression L motor will have a LOWER max power timing because it has a smaller chamber. If you add quench (which flat tops on a P90 will) you'll have an even lower still max power timing setting. So again, you can't just use the same timing curve for both setups and go + or - on the dizzy and expect great results.

 

Just because some people haven't had as many detonation issues with high compression L motors doesn't mean they were making less power at all. It just means that most likely their timing setup was better matched to the fuel and engine combo they had. There's WAY too many factors that will effect timing to make any blanket statements about timing in my opinion. I've seen enough well seasoned racers with cars down as low as the 7's who tune their engines by reading plugs that will tell you that "don't look for max timing, look for max power timing" to know that the assumptions made by most car guys is a bit narrow minded. If you wanna play with HP, you gotta learn the dynamics if you expect your engine to hold together.

 

In my experience, if you're having detonation issues, I'd say you need to get out an IR temp sensor and figure out where your AFR's are during detonation events. Make sure you don't have hot spot issues, and make sure you're running plenty of fuel where you're detonating. Also take your injectors out and have them flow benched, you might have a weak injector causing issues. That's all as likely more likely to me than battling compression factors.

[rsicard]

To keep from detonation keep the heat in the cylinder head chamber and cylinder head as cool as possible. Use pistons that provide the greatest amount of squish possible with partial dish to achieve proper static compression ratio. Good squish makes for turbulence within the cylinder chamber which keeps fuel in vapor form and away from detonation.

[NewZed]

By "maximize timing" I was referring to advancing for maximum power. The typical dyno tune procedure, from my understanding. The knock on high CR has been that knocking occurs before you can advance your timing enough to get maximum power. But some say that they don't have the problem. It would be interesting to ask what their cylinder pressures are when checked with a standard compression tester. Even with the variability in gauges, a trend might show. A true measurement instead of a calculated guess.

 

Again, just looking for something new besides the "I did it, don't know why you can't" response that seems to pop up in every detonation/flat tops with N42/Maxima N47 head conversation. Doing something without understanding how it happened isn't very satisfying.

[duragg]

Very interesting thread.

With my F54/N42 head combo and total seal rings my compression tester shows 210+ psi typically.

Cam is the Isky L490 and I spent a lot of time researching "dynamic compression ratio" (DCR) before building.

 

I didn't CC the heads myself but using stock parts and the available calculators I got about a 10.3:1 Static CR.

When you figure the cam and valve timing into the formula the DCR is something just under 8.

 

Is this DCR the variability that is driving the differences in experience?

 

I honestly don't know if my motor knocks or not, way too loud to hear and the Knocksense picks up too much other noise.

If it blows up I'll make another one, not the end of the world.

 

Detonation in my relatively low power motor is less of a problem than a big boost turbo motor.

It could probably go on forever in my application, whereas a turbo may pop much quicker?