LS1-450

iTrader: (33)

Why does a carb intake make best WOT power @ 36* timing when the same engine w/ an EFI intake makes best power @ 26* timing? Is the carb intake using 36* making more peak & under the curve power or is it a difference in VE?

The timing noted is a general for discussion example & is close to typical of what is seen on this site.

jimmyblue

iTrader: (11)

First I'd ask how "best power" values relate, between the
two.

My gut would say the carb style intake's short runners will
shine at high RPM, where high power sits, and the cylinder
filling ought to be better. But then you'd need less timing,
not more.

But the EFI manifolds seem like they have some issues
with back-cylinder airflow / fuel distribution, and maybe
the timing limit is foreshortened by ping rolling on?

I guess I'd be reluctant to pin the tail on the manifold
alone.

LS1-450

iTrader: (33)

Yah, it's the more timing that the carb set up likes that I'm trying to get a handle on. Generally, the carb makes better power, but, also likes more timing. Why?

I think that the central location of the intake opening is one advantage of the carb + the air makes a less restrictive turn into the runner, where as air entering the EFI intake has to make a sharp right hand turn before entering its' runner & the runners are not equally positioned around the intake opening. For these reasons, a larger a/f mix appears to be getting into the carb CC under otherwise equal conditions. However, how this relates to timing; IDK. If the carb is more efficient, how could it want more timing?

The highlander

all things equal (runner type etc) the carb might get more timing because fuel cools a lot better when shot through the intake. Thus can get more timing. Sort of like a methanol injection.

There could be better fuel mixture thus allowing for better spark advance... as it pertains to the bottom end engine combination and MBT. it will all be in the setup, not necessarily is a carb thing... but those are the things out of the top of my head.

jimmyblue

iTrader: (11)

I suppose there could be some more subtle difference like
atomization / stratification, from how the air tumbles and
so on, affecting the initial light-off speed?

stevieturbo

Who is making those claims about timing ? And how many test examples ?

Simply put, if the engine is needing more timing to make similar numbers it is less efficient, burn rate is much slower, perhaps less controlled combustion.

speedtigger

iTrader: (16)

There are countless examples of guys who have had best result with mid 30s timing on single plane, carbureted LS combos.

But, here is a curve ball for the OP: my car has a single plane, carbureted LQ9 with LS6/LS2 heads and it runs the fastest at 27 degrees timing.

stevieturbo

You're saying carburetted motors....whereas the OP mentioned carb intakes ?

I'm sure that would also make a difference. Are there any timing comparisons between carb and EFI, as well as carb intake and normal efi intakes ?

Are the builds of similar spec ? similar cams ? similar compression etc ?

10 degrees is a whopping difference on any engine.

speedtigger

iTrader: (16)

As far as the carbed, single plane combos go, there is a great diversity in the combos compression ratio and combustion chamber design. Guys could be running a 9.4:1 LQ4 right out of the junkyard, or an LQ9 with L92 heads milled .100" and every conceivable combination in between.

Perhaps my particular car likes 26 degree timing because I have 10.88:1 compression and a smallish cam that closes the intake valve at 41 degrees ABDC @ .050". And, perhaps some of the other combos like 36 degrees because they have less compression and a cam that closes the intake valve much later like 45 to 48 degrees ABDC @ .050". Or, perhaps they are slightly fatter on their A/F ratio, or they have a higher numerical final drive ratio and a bigger stall converter to where the engine wants to rev faster, or their car is lighter than mine. etc etc etc. Lots of interesting possibilities to consider.

LS1-450

iTrader: (33)

The question isn't regarding how one intake type requires more timing to produce the same amount of power. The question is why carb intakes make more power & accept more timing than do EFI intakes, "everything else being equal."
_______________________

In regards to how one carb'd intake engine runs best @ 27* & another @ 36* on otherwise similar engines, DCR is a major issue. Rarely, is the DCR noted when seeing it discussed in this section. Stating SCR doesn't clearly define the set up. When comparing best timing for max power, DCR needs to be part of the conversation. For example, an engine w/ 9.1 DCR running 93 Octane isn't gonna accept the same timing @ WOT as an engine w/8.4 DCR. The 9.1 DCR engine running on 93 Octane also isn't gonna achieve as much power as it would have @ say 8.6 DCR & more timing on the same fuel. The 9.1 DCR engine is limited by the 93 Octane fuel & will pre-detonate unless timing is reduced. Have gotten a little off topic, but is relevant to the conversation.

____________________________

I am claiming that all other things being equal except the intake & the timing that can be achieved with the intake, a carb intake will produce more power than an EFI intake. Spent several years tuning our GM EFI PCM w/ EFI Live before swapping to carb. Am here trying to learn why.

The comments that make sense to me are the cooling effect of the atomized fuel. Centralized location of the intake inlet. Smoother transition as the air enters the runner. Better fuel mixture. These things make sense as to why. Are there more answers?

speedtigger

iTrader: (16)

The DCR formula/calculation you are referring to is lacking too. It does not take into account the volumetric efficiency curve. So, the only thing that DCR can really predict accurately is cranking compression.

LS1-450

iTrader: (33)

IDK, I've never had an issue when ensuring that DCR is suitable for what is generally safe on 93 Octane. Can say that when playing around w/DCR a little bit by advancing the cam, pre-detonation occurs w/ timing set where it did not occur when DCR was lower (@ WOT). Pre-detonation also occurred near peak torque @ around half throttle (for reference), when it did not occur w/ lower DCR. This test was done when we had EFI Live & PCM tuning capability. We did not modify the VE table or any other related areas of the tune. So, am not claiming that the pre-detonation couldn't have been tuned out. It was just a simple test to see what would happen on 93 Octane if we increased DCR above 8.6:1 w/SCR @ 12:1. Paying attention to the results hasn't led us astray.

Once tuned, the carb set up is pretty much fixed w/ limited VE adjustment. There is only how the engine was set up & then tuned; (AFR, timing, RAM air or not, ability to cool intake, plugs...). VE curve is more a factor in the highly adjustable PCM or stand alone EFI set up. Realizing that I had asked the question in relation to the intake & not whether or not it was carb or EFI, when carb'd, I think that DCR can be an issue.

It is also possible that we have interpreted something incorrectly. If so, please let us know.

speedtigger

iTrader: (16)

I am not saying that there is not a correlation between the DCR formula and ideal ignition timing. I am saying it is an incomplete formula.

Consider this: DCR has inputs for SCR, intake valve close timing, rod to stroke ratio and bore to stroke ratio. So, if you change the SCR or the intake valve close timing, you will affect the DCR.

In your example, you took an engine where all of these elements are the same and changed just one item in the formula. So, of course you can measure the results and note a correlation. Right?

Now, take the same engine and change something that is not in the formula that will affect the engine's volumetric efficiency. For example, take off the FAST intake and put on a Victor Junior intake manifold. Now you have notably changed the engine volumetric efficiency curve, with the DCR staying exactly the same, yet the engine might want notably more timing. Why? Because the short runner intake manifold is not as efficient at lower RPM and now the torque peak is higher allowing for more timing due to lower cylinder pressures where the engine is most sensitive to timing.

A perfect example of this is how an engine likes much more timing at part throttle. At part throttle, you might run timing well into the 40+ degree range for maximum efficiency while at WOT it might like 26 degrees. This is all volumetric efficiency.

This is the reason why I say DCR is an imcomplete formula for predicting ideal timing or maximum compression or cam timing etc. While I have never tested it, I bet one could run a higher DCR with a single plane than a FAST or LS6 intake equipped engine might like.

A few more things not included in the DCR formula are combustion chamber design and quench. Both of those can affect burn rate and timing. The combustion chamber design can affect burn rate and timing dramatically. Neither of these are represented by the DCR formula.

LS1-450

iTrader: (33)

Agreed & this is an example of a set up change to adjust VE. I was trying to convey that once the engine is built (money spent) there are many posts in the carb section where members are discussing power gained or lost related to adding or reducing timing. In those discussions, there is little consideration of how DCR can be effecting the results. In many cases, the poster doesn't know the DCR of the engine.


You're probably right & another benefit of a carb style intake.


Yes, I see your point. Even though it is an incomplete formula in that it's only part of the answer, noting the DCR would be helpful in optimal timing discussions.

speedtigger

iTrader: (16)

I think the reason you don't see a lot of guys in the carb section get into DCR is that they really don't care. They are just being pragmatic. Many of them will just say: "give he engine what it wants". This works for them because they just test at the track to tune, so the don't have to spend much time philosophizing.

Take a street strip car with a 4500+ converter and 4.30 gears that only has 9.4:1 compression. This is actually a good example of where tuning for an " ideal" DCR is not only a waste of time, but wll likely even slow the car down. This car will make its best times with a cam that will cerainly result in a low DCR. But, since the RPMs are kept high, good volumetric effiiency will fill the cylinders nicely and the car will run hard.

jimmyblue

iTrader: (11)

Another aspect occurred to me the other day, which is
intake runner resonance. Like the T in Tuned Port Injection
(laugh, if you must).

Now, runner length even on the factory manifolds is on
the short side for normal RPMs, but could contribute a
skosh to cylinder filling at higher RPM. The carb intakes
are as stubby a runner as can be. Straight flow, OK. But
unlikely to see any resonant enhancement. Better filling
needs less spark.

But this would all be visible in the relative HP @ high
RPM, which was an initial question and has no 1:1 comparo
based answer so far, only generalities.

LS1-450

iTrader: (33)

IDK about this; I guess it's possible. Although, how much of the fuel injected @ the top of the head runner (EFI application) is sticking to the smoothed walls of most heads? I realize that the carb fuel passes through the same head runner, but only after it has been mixed in w/the air. Carb walls are good & rough. Whereas the cylinder head walls are usually smoothed in the name of increasing airflow & without consideration that fuel is to be mixed w/ air as it passes through (EFI applications). How much sprayed fuel sticks to the head runner walls & how much mixes w/ air in an EFI application? If resonance were a benefit on EFI, wouldn't the less efficient atomization of fuel negate a resonance benefit?

...Generalities, yes, but, are most likely the case. I could make a run down the strip & check MPH @ the risk of breaking yet another half shaft, but, probably won't because it's a street/road course car, not a dragster. To me, it's not worth braking another 1/2 shaft just to verify the better power seen in the carb manifold over EFI. Road course suspensions suck @ the drag strip. Best 60ft (manual transmission) was 2.1s. Went twice; broke half shaft each time, even M5 alloy shaft that was guaranteed not to brake. Anyway, the discussion contimues.