Honestly Doug. My 850, and your 750 demons are too different for direct comparison of jets. The best move you can make is a wideband O2 right now. Trust me it is worth every penny. If I was in the US, id just send you mine to play with for a while.

 

Does your carb have a choke horn at all? If so... and that air cleaner is drop base...

 

Exactly what I was getting at

 

Looks just like this....

 

Yea, me too, but I wonder how many of the combos even had 26* thrown at them? It seems to be popular to start at 32-34* and go up from there. All that really tells me is the combo likes XX timing between 34 and 40*!! LOL.
It could also be big cams with big heads need the extra timing to wait for the slow air to make it to the combustion chamber. I do know from looking around that the EFI cat to rec head timing shows just the opposite of what we have seen on carb cat to rec timing and that the rec head CCs are suppose to be the next step in the efficiency evolution from GM. I guess its all in how you set the compression and cam timing. I still have to wait till fall to experiment with lower timing on my rec head setup, so I cant speak for myself yet,

 

I had a better MPH with the lower 32-34*
But a better ET with the 36-38*

Now, conventional wisdom says the heat mark on the sparkplug ground strap should be on the bend. AT 38* mine was halfway between bend and threads.
Also need to remember I need to jet the carb yet...but really don't think that will change the mark much if any....unless I'm WAYYYY off to the lean side.

 

FWIW, Dave Mizell indicated that he ran his best times at 32° total with that same cam and combo
- LQ4/L92 - 9.8:1 (heads milled .030)
- GM single plane
- Pro Systems 780, squirters:30/30 - jets pri/sec:74/84 - power valve:6.5
- .624/.624, 239/247, 110+4
- TH350/Ultimate prepped ATI 4500 8"
- 4.10/28"
- 7000 RPM shift points
- Nicely tuned to a best of 9.87/137 @~2900lbs.

Full details of his combo: https://ls1tech.com/forums/carburete...l#post13221158

http://www.chevelles.com/forums/showthread.php?t=367754
(Dave Mizell = chvl71402 )

 

Great info...and nice find

 

https://ls1tech.com/forums/carburete...atabase-2.html

Virtually same info as above in the database.

I've read the converter was loosened up by ultimate before the 9.87 was achieved as well.

Note that Dave ran less than 10:1 static, and I think that some people running high comp will be experiencing some high speed detonation, problem with detonation is in most cars you aren't going to hear it happening. Not to be mistaken for pre-ignition, which is often easily heard and happens usually at lower rpm, totally different phenomenon as detonation occurs post-ignition.

 

Dave had 10.0X:1 comp and I only have .4-.5 more compression (10.4-5:1), so I don't feel that it will be an issue.
I am off the week of June 23-27, so I hope to have the exhaust dropped. I hope to sneak out to a TNT

As for a looser converter....not this year.

 

If you read the chevelle page, Dave clearly states 9.8:1 and my previous calculations on his setup showed roughly that from memory as well, just under 10:1 static comp.

Not to worry, most miss or overlook the finer details in this game, and thankfully not many have the mind or eye for extreme detail, because that's what it takes to be faster than the next guy!

 

LQ4 is 9.4:1 with 72cc heads
He claimed a .030 cut on the LS3's (they're 70cc factory)

What did I miss ???

 

No sense arguing about a couple decimal points of compression, it is not the cause of your issues Doug.
Im looking forward to you dropping the exh, and running no air cleaner.
Did you get a wideband?

 

I have seen many say the factory square ports are 68cc. If Dave really cut his heads .030" my math says he should be between 10-10.25:1 depending on his final CC on his heads unless he is running some super thick gasket which I doubt. With the .050 you cut off, you should be around 10.5:1. Definitely NOT a problem. LS3s are 10.7:1 right from the factory. I would not give that another thought.

 

 

Even though I think that the piston is unlikely to be .008" out of the hole, and say it was .005" positive deck, and the heads were 64cc.. still only 10.11:1 but I think many may not realise that the piston volume is actually roughly 7cc or thereabouts.

It is actually pretty important to get this calculation as accurate as possible IMO, to be able to accurately work out dynamic comp with the camshaft being used. And to therefore assess whether power is being left on the table and that it falls within desired range for fuel being used, and to ball park tuning window etc.

 

It is not about me being correct or incorrect. It is what people are reporting. I am just sharing what I am seeing carbureted LS people report. If you truly care about carb combos, you can read the carb combo sticky or browse through the carb section threads and see what people are saying.

All of the carb guys who report their combos timing could all be wrong. I guess that is possible. I think it is more likely that carbed engines behave in a way that is different than your EFI engines do. And if you read enough threads on carb combos, you will see what I am talking about.

This is not the dynamic of the conversation. Sounds like you are not familiar with Doug's other threads. Doug told us in a previous thread that his car ran the fastest with 36 degrees timing. In this thread, he said he moved it up to 38 and it slowed down and was going to try 37. I suggested he put it back to 36 because he said that setting worked best for him in the past. I never told Doug 36 degrees was optimal, he told us that it worked the best.

When I first read all of these mid 30s timing settings from others, I was surprised too because mine likes 27 degrees. But, I am not going to type on here that they are all wrong with their timing settings. I merely point out what mine likes and make note of what others are reporting they have the best results with and take into consideration the difference in our combos.

With everything that I have seen in here, it is more than obvious that most of the carb guys are running more timing than the EFI guys. And, typically, the guys with the 317s and 823s are using more timing than the guys with the 243s.

 

I'm hoping this thread stays informative and we can all take something good from it. Please no pissing matches to get this thread locked.

I understand that a big cam and low compression (low DCR) is bad and will require more initial timing due to the mismatched parts....been there and done that

I understand that these LS motors have good-great heads and are very efficient.... and add a little compression (good DCR) they react really good
So that will lower the timing since it doesn't need the "head start".
I also feel that the EFI fuel "mist" burns better/faster that a carbs "fuel droplets" and needs a little more "head start" just due to the way it burns. So this alone will require a little more timing only due to a change in the way the motor gets its fuel.

Again....I could be wrong. I'm not a "Pro" tuner or engine builder with access to a dyno and unlimited "tinker time" to dial a combo in.
I want my motor to run good, and not looking for every .001. I know where it "should be" and am asking for a little help from the guys who are running these combos for their input.

Thanks to everyone posting

 

Agree with this 99% but the higher the DCR the less forgiving it will be with tuning, more sensitive to lean condition, more susceptible to detonation etc. Which I wouldn't doubt you are well aware of anyway.

 

This discussion kind of reminds me of my old buddy Fred. Fred is fond of saying that no matter how much he tried to explain theory to an engine, sometimes they just won't listen to reason.

Seriously though, I totally understand your logic. And, I agree with the idea that more cylinder pressure under a heavier load will generally want less timing. Pretty straight forward. With that being said, the fact is that these engines with a carburetor are continually reported by members to want more timing than their EFI counterparts.

So, what would be the control examples that we could use to test the validity? The best example I can think of is the 5.3 and 6.0 truck motor swaps into the Mustangs that retain the EFI. There has got to be plenty of those around. You being a tuner would know what kind of timing they like in pickup trucks both stock and cammed heavily right? Now all we would need is to know what the guys in the Mustang swaps found their ideal timing to be. If they are running mid 30s timing like the carb guys, then you could say that it is not the carburetor and intake that was the difference.

Another control would be comparing and EFI car using a Victor Jr. compared to similar build with a carburetor and Victor Jr. That would really let us hone in on the carburetor vs EFI factors. Those two example ought to be out there. So, I will keep that in my thoughts as I see those builds in the future, I will ask them about their camshaft specs and ignition timing.

I will say that two things ought to be added to the foundation you have built with your SCR and DCR factors. I say you must also consider volumetric efficiency and fuel atomization behavior into the mix. Looking at what is different about our engines in this section is that the great majority of them are equipped with single plane, very short runner intake manifolds and they use a carburetor that mixes the fuel earlier in the process than the factory style EFI setups. So, the question should be asked: What is it about this arrangement has these engines wanting more timing? Different fuel behavior or perhaps the difference in volumetric efficiency curve of the short runner intake? A combo of both? Food for thought.