Maybe I have unique data point here.

I built the C5R based 427 now in my '02 ZO6 with only 10.2:1 compression, so that later on I could SC it with a good amount of boost (thos C.r. seemed a good compreomise for what is purely a street car). Due to financial pressures, I had to drive it about a year with "only" the 507 RWHP it provided N.A.

I know I gave up about 30 HP by going to the 10.2 ratio, but it was a good street engine, but it faded past 6000 RPM -- peak power was at 5800, but throttle response was wonderful even with the low compression ratio: it just proved really difficult to tune well, but Jeff Creech at Carolina Auto Masters got it right.

I now have a D1SC on it and am going through a process of ever-smaller pulleys. Currently at 4.5 lbs boost and 564/595 RWHP with full cats& exhaust/open cutouts.

 

Look at a stock Z06 as an example. The stock timing table puts 22 degrees in. Its been dyno tested all the way to 32 degrees to not make any more power than the 22 that is in the car to begin with. The only power to be gained ont he car was leaning out the WOT a bit...

I'll have to go dig the thread up over on Z06vette, but they showed that timing on the stock motor made no gain in power. So, if it shows no gain, and you advance the timing, the car will actually slow down.

Compression on the other hand is worth ~4% per point.

As I said, its a matter of testing to find what CR you can get the most "squeeze" without detonation, along with enough spark lead to completely burn the cylinder charge.

People who are putting 32 degrees into a cylinder head like the LS series heads are stuck on in the world of 23^ SBC stuff where you need 32 - 36 degrees because of the design of the combustion chamber.

Its like everything else (cams, flow numbers, etc...), more isn't always better

EDIT: Found a couple of the threads.

http://www.z06vette.com/forums/showthread.php?t=20948
http://www.z06vette.com/forums/showthread.php?t=20818


Thats not to say when you modify a car and add overlap, etc... you might not need to add some timign. You may. My point is, if its not making power, don't keep thrwoing more lead at it. Its all wasted...

 

Nice examples, but if you added a cam and headers to the equation, would 22 degrees still be the ideal number? I'm curious.

 

Optimum timing is a "number" realative to cylinder pressure and temperature/fuel flash point. Who cares if that number is 22 or 42 or 38 or 2, as long as it is optimum. A 420+ cube motor isn't going to reach the flash point at 12.5 to 1 at 500/550 rwhp. Soooo..... who cares what that "number" is. 12.5 to 1 will make more power than 11 to 1 all else staying the same. Turbo buick for example: 231 cubes, 19.88 to 1 CR, 93 octane, 19 degrees total advance, 400 rwhp through a 200r4. Anything above that and I reach the flash point (auto ignition) and diminishing returns. Increase that 231 to 420+ and you have cut cylinder pressure roughly 81%. Ls1 and Lt1 combustion chambers a by far better in design. I think you'll be easily alright decreasing CR by 7.38 to 1/cylinder pressure by 81% and increasing hp by a 150 rwhp or so as long as your partial throttle timing curve is correct and EGR activation curve is correct.

 

The reason to raise compression ratio is to burn the fuel/air mixture in a smaller volume, thus creating higher cylinder pressures during the power stroke, particularly near the top of the power stroke.

The reason to retard timing is to burn the fuel air mixture later, so that the piston will be further down the bore, the volume inside the cylinder will be greater, and therefore maximum cylinder pressure will be lower to avoid detonation.

You see the problem here - retarding your timing just undoes the benefits you were trying to get by raising the compression. Actually, it is somewhat worse than just undoing the compression benefit - timing that is retarded from optimal is less efficient, since the combustion pressure is acting on the piston for less of the piston stroke.

So the textbook answer is: use the maximum compression possible with optimal timing, given the fuel octane you're using. With an LS1, depending on the cam you're using, that will tend to be in the 11 to 11.5 range for streetable engines, unless you start to do special optimizations like coatings.

 

Hi all!
Time for the newbie to wade in with a few observations:
1. Although 14 -19 degrees ATDC is a reasonable approximation of LPP (Location of Peak Pressure) for a number of engines under a lot of operating conditions, it is absolutely not any kind of universal constant that can be 'tuned to' by the engine controller in lieu of engine and emissions dyno development, etc. During my 35 years in GM Engineering (15 or so in engine calibration), I saw this concept surface twice, absorb millions of dollars in proof of concept and development phases and ultimately sink without a ripple, as the exceptions started rolling in. Far from adjusting to a single crank angle value, by the time the LPP concept was abandoned each time, it had been found necessary to completely 'map' the engine operating envelope to determine the correct angle for each condition, which pretty much defeated the purpose. If someone has a patent now on using the concept, A. They must have overlooked a couple of examples of "prior art" and B. I wouldn't hold my breath for it to appear in a production vehicle...

2. Regarding dynamic compression ratio and 'bleeding off" pressure to avoid low RPM detonation with long duration cams (specifically, late IVC), the reason it doesn't result (except in extreme cases) in detonation at high RPM when optimum cam events increase the V.E., is the time factor. We know that increased turbulence, higher pressures, etc. at high RPM combine to 'plateau' or even reduce spark advance requirements. If the burn is completed in the optimal time for maximum power with say 22 degrees advance at 3,000 RPM, and with 24 at 6,000, then the burn at 6,000 takes about half the time. As was explained in a previous post, detonation is dependent upon a number of factors, but one of the most important is time: After the rising pressure, radiant heat, etc. have created the right conditions for spontaneous ignition of the remaining charge, a measurable ignition delay or "cook time" must pass before the explosion occurs. With only half the time available for this at 6,000 vs. 3,000 RPM, the normal flame front usually arrives first, hence no detonation.
Also, rather than using a 'too long' cam to compensate for a 'too high' compression ratio, the cam should be selected first based on desired RPM range, engine airflow and durability factors, etc., then the correct mechanical compression ratio selected to optimize dynamic compression ratio for the fuel octane. Prolific automotive writer David Vizard, among others, has done extensive work determining DCR values for various applications, and there are formulas to relate it to cranking compression pressures.
3. I had another wonderful insight to share, but it must have wandered off while I was up on this here soapbox. That's what happens when you get old, I guess...

 

Best post by far in this thread, by a TON. Listen to this guy, fellas and try to absorb what he is saying. Note especially the order of events.

I know what you mean by getting old and forgetting. Memory is the second thing to go..or is it the first? Maybe you and I went to different schools together, MadBill. I like how you talk.

 

ding ding! exactly right, there is a graph somewhere where two lines intersect, that is the optimum compression/timing combination. think about the extremes.

4:1 compression and 45 degrees of advance (complete bullshit but bear with me)

18:1 compression and 10 degrees of advance (more to come)

both engines wouldn't make squat for power, but somewhere in the middle lies the 'perfect' compression vs. timing argument. only the dyno knows...

 

You're absolutely right, Steve! This tradeoff is addressed in painstaking detail by the OEMs. Ignoring for the moment the potential effects on emissions, the highest usable CR is vital for both power and often more important (to them at least) fuel economy. At part throttle, most engines could easily operate on regular fuel with 14:1 or more, so the limiting factor is always going to be at WOT. Historically, the OEM's optimum combination for a given octane fuel is a CR high enough that modest retard (say 5-6 degrees) from MBT (Minimum spark for Best Torque) is necessary to prevent detonation. In other words, the losses from a few degrees of spark retard are less than from a CR lowered enough to permit MBT spark.

 

Ok to put this into something most will understand, if we know our peak torque rpm and peak hp rpm what should we be doing for ignition timing below peak torque, at peak torque, approching peak hp and peak hp and beyond?

Seems to me that cruise should be significantly advanced, be how do you determine how advanced, where to begin and end.

I'd suppose most are not looking for exact numbers, as everyone's combination is at least a bit different. But an idea of, when one adds a point of compression and a bigger camshaft, where would the timing need to change.

 

Yes you design the cam and intake to trap as much fuel and air as possible and then add the compression you need. A good pump gas engine would be around 11 to 1 if the heads and cams and intake was working. You COULD run higher compression for a turd combination but it's a band aid of sorts then. If you CAN run 12.5 to 1 on a pump gas engine it basically has to be a turd usually. Most of the cars that are really going fast at this high a compression ratio are going that fast from weight reduction etc. Even if it works on a perfect tune and day and fuel it will detonate quite a bit on non-optimum days.

 

I always thought that if you have a smaller chamber, more compression etc. the burn would complete quicker therefor you didn't need as much timing because otherwise you would reach your peak cylinder pressure too early.

Isn't there like an optimum point for the burn to complete (sometime past TDC, I don't know much about this stuff) and for a larger/less efficient chamber you need to start earlier to achieve this desired point because it takes longer for the burn to complete across the larger, less compressed area? Does it even matter what timing you have as long as you achieve your peak cylinder pressure at that optimum point? If that is the case, then how could high compression be worse, given that peak pressure occured at the same point, because wouldn't the more compressed charge release more energy to push the piston down?

I'm not an engine builder, just curious because I always understood that more compression was always better as long as you can control the burn.

My experience with propelling things is with explosives and not gasoline, so I'm just asking questions not sharpshooting anyone.

 

Ben,

You are right for the most part. These new heads like the LS1 ARE a lot better than the older stuff and that's why people can run the compression as high as they do without problems. To me 11 to 1 is very high for a street car but with larger cams we never have problems. There are even some factory cars brushing the 11 to 1 territory.

 

Great info so far!

Your dynamic compression ratio combines the values used to obtain static compression ratio and then also factors in the timing of your intake valve in relation to to crankshaft degrees (specifically the number of degrees ABDC). Or the IVC (intake valve closing point). This is important because the a/f in the cylinder does not begin to compress until the intake valve is closed. Since 99.999% of all internal combustion motors will close the intake valve after BDC (ABDC) and thus effectively shortening the stroke in relation to the beginning of actual compression, the DCR will most always be lower than the SCR. Most performance motors will not close the intake valve until approximately 45-80 degrees ABDC so this basically takes alot of the stroke that is used to calculate the SCR out of the equation.

Motors that have a high duration/lift cam operating at low RPM will suffer a loss of low end torque torque due to the fact that the effective compression ratio is reduced by the late IVC. Ideally the intake valve will close just before the incoming air stops and reverses direction. This guarantees that the maximum amount of air/fuel mixture has been pulled into the cylinder prior to ignition. This is when people talk about the cam "coming on". In order to ensure that the a/f is still compressed sufficiently over the reduced effective compression stroke it is necessary to increase the static compression ratio in order to make up for it. This is why high performance engines with aggressive camshafts usually tend to need higher static compression ratios than normal.

Given this, it is much more beneficial to base the ability of your motor to effectively run on pump gas based on DCR rather than SCR.

 

I cant possibly see how running an excessivly high ion ratio and dead timing will be benificial. the benifits of compression taper off quickly after about 11.5:1 or so.

The benifits of advanced timing grow quickly when they are so far behind optimal.

JMO after seeing compression vs bhp and timing vs torque curves

 

I think I will jump in for us California guys. I seem to be the crazy one around here, but here is what I did.

91 octane is all we have. I still built a car that had 12.1 CR on a stock bottom end. Ran this sucker over 30 time or so on the dyno. And I use a loaded dyno also. No dynojet tricks here.

Also, but doing a compression test, the car has over 290PSI cranking. Basicly maxed out my gauge. Most people would freak, but I found the best out of this.

Found the car was not affraid of timing at all. AFR, lets just say you can be alot more efficient with it than most think. I am also taking the Wideband out of the colector, so no tail pipe lose.

The only thing I do is dump one or two gallons of race gas in when I hit the track. Other than that, the car is tuned in SD, and you want to go for a ride in a car that rips when you touch the gas, it does. Its scary responsive. Put down 456 and 433 TQ through an auto. Of course we are having trans problems now, good old 4L60E, so no new numbers with out new fast 90/90 and our velocity stack I built. But we will see. First time at the track the car ran consistant 11.6's with 3.23 gears.



Rick

 

Do good results imply optimal results?
Not to be a "hater" or anything, but i'm sure we can find stock ion cars putting down 430rwhp. I'm not sure if a few individual results mean much in this case since there are so many variables.

 

Wasnt trying to imply that, just giving a case. Sometimes when a setup is efficient, you can do alot of great things.

Rick

 

Probably in the wrong forum, but with water/meth injection you can have
the best of both worlds. I'm running 89 octane Hotcam &11+ compression and
29* timing.

 

i run my 13.0:1 LT1 motor on pump gas and it runs fine because of the same reasons stated above. The LT1 intake is vary restrictive and i also run a huge cam, 264/270 @.050 .631" 110 LSA, the motor does benifit slightly from race gas but why if it does not need it, especialy at almost $5 a gallon. the car is also setup vary safe to do this.
i run 110-112 leaded on spray,this is a nitrous car and that is why it is safe
the car also went 10.97 @125 on horse power