Question about ideal carb'd a/f

[Alex]

Have a carb'd 408w.

I was studying my dyno sheet from last year and was wondering about the a/f (can't scan it unforunately, I will describe)

from 3000 to 3500, a/f is around 11.9
from 3600 to 6100 a/f is around 13
from 6100 to 6400 a/f is around 14.5

Does this sound like a good a/f for a carb'd 408w? By good, I mean decent power and safe. Not too rich, not too lean. I think it is, but wanted to get others ideas.

edit: Also, does the fact that from 6100 to 6400 the a/f really leans out means anything? (I have my rev limiter set to 6400, could theoretically rev to 6600 if I wanted but I dont')

 

[Andrew Robertson]

Hi Alex,

looks too fat 3000-3500, slightly on the lean side of safe but pretty good 3600-6100, too lean 6100-6400. I use 12.8 as a rule of thumb, or 12.5 if I'm being a bit lazy and want some safety margin and don't mind dropping a few HP.

Have you checked your high speed air bleeds (Holley) or air corrector (webber) are not too big, and checked fuel delivery at high RPM is adequate?

Cheers, Andrew Robertson

 

[Alex]

Havn't cheecked anything - too stupid to play with the carb settings since it seems to be working well now, haha.

I spent 1/2hr trying to get it to scan (too much corruption, need a new scanner, and camera sucks too much to take a picture) and I keep scanning it until the corruption didn't appear on the a/f section and just cropped it as best I could.

Based on the below, would you try to get it re-tuned, or say pretty good and as long as the car is acting normally, leave it as is? (I am a bit concenred about being lean above 6100, but it looks to be 14 and not 14.5 as i initially said, and I never rev that high)

The only reason I wonder is because I was reading about f/i ideal af before around 11.7 and i have no idea how f/i efi compares to n/a carb

 

[Bob Putnam]

That fast rise at 6K makes me think that either your fuel pressure is too low, your fuel pump flow is inadequate, or there's some restriction in the fuel line. In fact, the rise is so rapid, maybe there's a bad resonance that's disturbing fuel flow.

 

[Jack Houpe]

I have tuned EFI for for many years and I know your have a carb but the AFR is the same no matter what, I also have a Mustang rear wheel dyno in the floor of my garage and many many hours of testing on the road with graphic read out to study performance virus fuel economy. If you have a high compression engine with a other than street cam I find that 13.4-13.7 at idle will keep your engine running without dieing, any richer will make it run better but kill the guy behind you, under load (WOT) 12 to 12.6 all RPM range, anything leaner will loose HP and cause detonation, unless you retard the timing. From the looks of what you have posted you need to change the jets on the upper end and decrease them on the lower end, make sure your fuel line and filter is adequate for your needs, small lines means fuel flow drop no matter what the pressure. 14.5 is pushing it under load and if you have an EGT you will see that exhaust temps will be close to 1300 F or higher which means thermal melt down soon, I know this from experience with forced induction, and the low end rpm is way to rich, plug fowling rich. What O2 sensor are you using, an LS1 wide band?

I know that the book say stoichiometric is 14.7 AFR but thats under cruise and no load with a non performance engine, if we all have GT40s its for performance not economy, if it were for economy we would be driving a Prius so in my books 13.7 AFR is perfect under cruise and no load and 12-12.6 with your foot to the floor. I get 18 MPG out of my 427 on the hwy with these measurements.

 

[Alex]

That fast rise at 6K makes me think that either your fuel pressure is too low, your fuel pump flow is inadequate, or there's some restriction in the fuel line. In fact, the rise is so rapid, maybe there's a bad resonance that's disturbing fuel flow.

Engine did 490RWhp

7psi of fuel pressure rock steady

Carb is holley 4150hp

Fuel setup is:

- -10pickup that converts to -8 line
- -8 line goes to 40 micron inlinefuel filter
- -8 from inline fuel filter to holley 150gph fuel pump
- -8 from holley fuel pump to aeromotive SS regulator (7psi here constant)
- 2 -6 lines from regulator to 2 Holley ports


So I ~think~ my fuel setup is adequate........as said above, I'm leaning towards it isn't jetted perfectly.

However, I spend 90% of my time between 2.5k and 4k rpm ..... I don't track the car or really even drive it hard (no top speed runs) so if it sees 5k+ RPM it's only for a second or two.

I just don't know if it's worth it getting it back on the dyno and re-tuned or not, simply because it drives well and feels okay.

What O2 sensor are you using, an LS1 wide band?

No O2 sensors; uncertain what the shop used to get the a/f, i wasn't there.

 

[Jack Houpe]

Engine did 490RWhp



No O2 sensors; uncertain what the shop used to get the a/f, i wasn't there.

Most important, the closer the O2 sensor to the exhaust outlet the better, and a wide band will give you a accurate reading. If they had the up the pipe type, it reads ANY exhaust leak you have and believe me the tiniest leak will show lean. I got to go eat, two cocktails and no food make a very poor typist.

 

[Alex]

Most important, the closer the O2 sensor to the exhaust outlet the better, and a wide band will give you a accurate reading. If they had the up the pipe type, it reads ANY exhaust leak you have and believe me the tiniest leak will show lean. I got to go eat, two cocktails and no food make a very poor typist.

Probably was up the pipe type. (65 cobra here, btw)

For what it's worth, I definately have an exhaust leak - my headers/side pipes are slip fit and arn't 100% sealed (e.g., I can clean the pipe, drive hard, and there's black residue on the underside that's kicked out from the seam). I assume this would also effect the dyno?

If so, are there any ways I can compensate for this?

 

[Randy V]

If so, are there any ways I can compensate for this?

Red or Copper High-Temp Permatex RTV...
It will be a bit tougher to remove the pipes when you want to, but it will seal it up.. Smear it on the sealing surfaces (cleaned) - assemble and let cure 8+ hours..

I'm in the 12.5:1 camp on AF ratio...

 

[Alex]

Red or Copper High-Temp Permatex RTV...
It will be a bit tougher to remove the pipes when you want to, but it will seal it up.. Smear it on the sealing surfaces (cleaned) - assemble and let cure 8+ hours..

I'm in the 12.5:1 camp on AF ratio...

Okay, stupid question time!

1)

I have circled where my pipes are slip fit

No side pipe attached

Side pipe

There's absolutely no question I have an exhaust leak at the slip fit joint. like I said, if I clean the area, then go for a drive, the sides and underside of the pipe are splattered with black crap leaking out from the pipe. When I join the pipes together, I usually gob anti-seize on them just because .... on first start, it SMOKES like crazy from that joint burning off the anti-seize.

SO I'm going to assume the first place just used a sniffer and (obviously) didn't stick it 4ft into the exhaust (past the slip fit joint)

So if I remove the side pipe and they just stick the sniffer at the slip fit joint, will this effect the a/f reading? FWIW, my sidepipe is just a straight through pipe; no muffling, no spirals, no nothing. Just a hollow piece of pipe.

Your idea of high temp rtv is an interesting (and messy, hehe) one, but if I could just have it dyno'd and remove the sidepipe, that'd be a lot easier (only takes 30seconds to pop him off)


2)

Does a/f differ on carb vs efi? or does a/f really only matter on n/a vs f/i?

For example, on my 04 cobra I'm told the ideal a/f is 11.7 .... any higher than about 12.2 and it can be a ticking time bomb...

 

[Randy V]

Okay, stupid question time!
So if I remove the side pipe and they just stick the sniffer at the slip fit joint, will this effect the a/f reading? FWIW, my sidepipe is just a straight through pipe; no muffling, no spirals, no nothing. Just a hollow piece of pipe.

Without the muffler the car will breath better and may show to be a bit leaner - but I seriously doubt it would be much different..

Your idea of high temp rtv is an interesting (and messy, hehe) one, but if I could just have it dyno'd and remove the sidepipe, that'd be a lot easier (only takes 30seconds to pop him off)

Nothing worth having comes without a price...

Does a/f differ on carb vs efi? or does a/f really only matter on n/a vs f/i?

For example, on my 04 cobra I'm told the ideal a/f is 11.7 .... any higher than about 12.2 and it can be a ticking time bomb...

The engine does not know or care how it gets its air and fuel from...

11.7 is actually a pretty rich mixture. The highest power readings on engine's I've built and dynoed came in at around 12.5:1. I've seen some guys run them much leaner than that with no ill effects.. This is where the thermocouples on the dyno come into play so you can read the EGT at the same time..

 

[Jack Houpe]

Randy hit it on the nose, 12.5 is perfect under load. I suggest to weld a O2 bung at one of the exhaust outlets closest to the head and use a LC1 for a proper reading. I know it will only read one cylinder but still if its sealed good at the head it will give you a good idea of what is really going on with AFR.

In my experience HP really drops off anything above 13.7 AFR (lean) and changes very little anything below 12 AFR (rich). The beauty of EFI is if you run in closed loop with a LC1 it will add or subtract fuel as load and rpm change. I have put a 5k ohm rheostat in series with the manifold air temp sensor and given 10% authority over the fuel curve to that sensor. While driving down the road I can tweak if necessary the fuel curve, this compensates for altitude change which most EFI systems do not do.

Alex thats a sweet ride you got.

 

[Alex]

I suggest to weld a O2 bung at one of the exhaust outlets closest to the head and use a LC1 for a proper reading.

That would probably be a good idea, but but but then I'd have to probably re-ceramic coat everything again, which I don't really want to do =(

 

[Alex]

Without the muffler the car will breath better and may show to be a bit leaner - but I seriously doubt it would be much different..




The engine does not know or care how it gets its air and fuel from...

11.7 is actually a pretty rich mixture. The highest power readings on engine's I've built and dynoed came in at around 12.5:1.

1 - muffler - but then I can't post anymore bannnnned videos and piss people off with my excessive engine

2 - in terms of the a/f comment, the 11.7 was for an 04 cobra (ported/pullied f/i cobra) .... I always though the ideal a/f was different for n/a vs f/i cars.

 

[Cliff Beer]

Good stuff. Here's a basic question: how does an accelerator pump affect things in a real world setting as far as tuning? Obviously, an accelerator pump enrichens the mixture concurrent with rapid butterfly opening - theoretically, I would guess that a steady state running AFR of 13-14 (which is probably OK for light/part load) becomes perhaps 12 or so depending on the size/duration of the supplemental squirt of fuel from the accel pump.

On a dyno running a WOT test with a carb (equipped with accel pump(s)) then the AFR is going to be momentarily enriched with a decent squirt, but lean out as the squirt is consumed.

So, I guess the question is, for carb'd setups with accel pump(s), does the solution for hitting the optimal AFR thoughout the rev range also include manipulating the accel pump volume, as well as monkeying with jets?

 

[Andrew Robertson]

So, I guess the question is, for carb'd setups with accel pump(s), does the solution for hitting the optimal AFR thoughout the rev range also include manipulating the accel pump volume, as well as monkeying with jets?

Cliff, basic answer - no. Accel enrichment (which is present in both carbs and EFI) is simply there to manage transition from a lower engine load to a higher load.

Accel enrichment compensates for the momentary lean condition that occurs in both carb and EFI set-ups when a sudden increase in airflow creates a temporary low velocity environment inside the intake runners and fuel falls out of suspension to a degree. In the carb scenario the pump shot also has to cover the time lag until velocity through the booster venturi activites the main circuit and fuel begins being metered in the required qty.

Once an engine is loaded and airflow well established accel enrichment is no longer in play and does not feature in AFR in either carb or EFI.

If you have a decent datalogger, you can tailor EFI accel enrichment to avoid a rich or lean spike in virtually any load transient condition. Unless your injectors are miles away from the intake valve, it's probably easier to over-fuel an EFI system in this area and end up with a momentary rich bog than it is to under fuel. Carbs take relatively more fuel to manage transient conditions because of the inherent time lag in the ability of the velocity-sensitive main circuit to respond, along with the usually much longer distance from the intake valve, especially if there is a large plenum sitting under the carb.

Cheers, Andrew

 

[Jack Houpe]

Cliff, basic answer - no. Accel enrichment (which is present in both carbs and EFI) is simply there to manage transition from a lower engine load to a higher load.

Accel enrichment compensates for the momentary lean condition that occurs in both carb and EFI set-ups when a sudden increase in airflow creates a temporary low velocity environment inside the intake runners and fuel falls out of suspension to a degree. In the carb scenario the pump shot also has to cover the time lag until velocity through the booster venturi activites the main circuit and fuel begins being metered in the required qty.

Once an engine is loaded and airflow well established accel enrichment is no longer in play and does not feature in AFR in either carb or EFI.

If you have a decent datalogger, you can tailor EFI accel enrichment to avoid a rich or lean spike in virtually any load transient condition. Unless your injectors are miles away from the intake valve, it's probably easier to over-fuel an EFI system in this area and end up with a momentary rich bog than it is to under fuel. Carbs take relatively more fuel to manage transient conditions because of the inherent time lag in the ability of the velocity-sensitive main circuit to respond, along with the usually much longer distance from the intake valve, especially if there is a large plenum sitting under the carb.

Cheers, Andrew

Andrew is Absolutely correct.

One of the major problems with EFI is people have a large range of options on acceleration, timing, volumetric and not to mention all the MAT CLT sensor inputs. And I have seen some clobbered up cars with tunes from guys who buy a dyno and now are experts in the area. Carbs are simple and useful, I still have a couple cars with carbs but given the same amount of CFM in two engines one with carb and the other with EFI, there is no comparison.

 

[Cliff Beer]

Thanks Gents, understood, makes sense.

 

[John]

...[/QUOTE]


Alex, sorry in advance for the thread hijack, but is that the Levy Racing exhaust? Looks really good. What is the advantage of those over normal headers?

Thanks,
John

 

[Alex]

Alex, sorry in advance for the thread hijack, but is that the Levy Racing exhaust? Looks really good. What is the advantage of those over normal headers?

Thanks,
John

Yes. I don't have any quantitative comparisons (back to back dyno), but

- definitely louder than stockers by a large margin

- equal length vs unequal (ffr) length

- seat of the pants feeling felt much better down-low and mid range power. Again, this is just qualitative, but they definately felt like they were flowing better than the stockers.

- flanges are true, meaning you can just bolt them right to the heads with some high temp rtv and not worry about sealing issues (with my ffr ones I had massive exhaust leaks everywhere...with these, none)

- ceramic coating option really works. I can drive for, say, 1hr, park, and touch the pipes with my hand and not burn them. Give it 10minutes and you can pull the pipe off because it's stone-cold then. Jet Hot coating does work wonders - anybody who chromes theirs or powdercoats is silly and deserves to get burnt. I had a bunch of tardbuckets arguing with me that 'ohhh, that's impossible, side pipes will always burn you in a cobra'.....driving in the middle of summer, oil temps at 200* (or so), water temp constant 180, the pipes are hot (but not singing hot) to the touch, and cool very, very quickly.

Only downside I've encountered with them was the cost - $4500 (but to be fair they're a limited production item, done by hand each time, so it will obviously be higher than somebody like bbk), and the fact several welds cracked and had to be re-welded (my friend said there was lack of penetration on several of them).

Overal I'm glad I did it.