just back from the dyno. running lean

Hi everyone,

Went to the dyno this morning with my '76 gtb. We started doing a pull and stopped at 5200 rpm due to how lean the car was running. From 2500-5000 it was running at 15.4:1 and we stopped right at 5200 when it hit 16:1. Who knows how lean it would have been if we continued. It did hit 156bhp at 5200 and 157 torque prior to stopping.

I know I have stock main (135) and idle jets (55), so should I get a set of 140 mains and 60 idles to clear this up? Or should I look for another possible issue. BTW, I just replaced my plugs and air filter.

thanks in advance- john


one more thing. I had the car weighed right afterwards. With a quarter tank of fuel and me not in the car, it weighs just a few pounds over 3000.

 

Change your air correctors to 200's and try again or even 190's. Im running 140 mains and 55 idles as well as the 200AC.

 

Should I go a size larger with my mains as well as the 200ac?

 

Can you expand on your as-weighed configuration please:

Originally a US version?

Presently with or without cats?

Presently with or without air injection?

Presently with or without the fuel evaporation control system?

With spare and jack and tool kits?

 

As-weighed configuration is as follows:

-original 1976 steel US car with US bumpers

-all emission and evaporation equipement still installed

-tool kit in the car, but not the jack kit

-1/4 tank of fuel.

Car was weighed at a truck weighing station and is advertised to be accurate within 25 pounds. The printout they gave me shows 3000 pounds flat, but when I asked for a more correct number, they said jsut a few pounds over 3000 came up on their screen.

 

Thanks for that clarification.

Does that mean that you measured the A/F ratio with the air injeciton system disabled?

 

air injection stuff was disabled during the run.

 

Just got off the phone with Mike at Pierce manifolds. He advised I run a 190ac with 145 mains and 60 idle. Pretty big main.

 

Give it a go, and let us know your results. I will advise that sometimes you may need to go up one heat range on your spark plugs or they may "load up" with the richer jetting. What plug brand and number are you using?

 

OK. From a different perspective: You said you replaced the plugs, but didn't mention anything about your distributor(s). If you haven't already done so, be sure and check both the static/initial timing AND the advance curve. Make sure they are spot on before chasing the mixture...and pulling any harder on the dyno! (We've heard some horror stories about dizzys in the 206/246 forum.)
'just a thought.
Cheers,
Coop

 

Sorry to ask that question, but as a german guy i do not understand this ratios...
Is it the air-to-fuel ratio meant expressed within these nubers?
Would that mean 14,7:1 is ok?!
In Europe we have the "Lambda" value, which expresses the air-to-fuel ratio based on Lambda =1 for 14,7kg air to 1 kg fuel based to stoichiometric ratio for burning 1kg fuel!

Would that mean, that 16:1 is about 10 percent lean???
Or what do you guys form overseas mean by it??
(just to know it in later discussions ;-))

THX,
Alex.

 

You get all yes'es
Yes, 14.7:1=1 lambda, or stoichiometric
yes 16:1 is about 10% lean.
yes it's the ratio of mass of air/mass fuel

about 13.0-13.2:1 is what he'd want to see. 11 is about as low as a naturally aspirated engine should be, but it will makes the best hp at 13.2

 

the timing is spot on. I just changed back to points (call me old fashioned), recurved both distributors to correct spec, and running NGK BP6ES. the engine has 170-175 compression on each cylinder

 

Thats lighter than I would have expected. I was thinking it would be more like 3200lbs. I will weigh my 77 to compare.

 

Thanx alot!!!

Cheers,
Al.

 

I was highly surprised about the weight. I know that the scale I used is pretty accurate due to how many cars I have weighed there, and then confirmed the weight on professional auto scales. For instance, my '86 Porsche carrera weighed in at 2750. I later weighed it on pro auto scales and it came in at 2746, so I assume the Ferrari wiehgt is very close.

Regarding the jets, I ordered a TON of jets. 8 140 mains. 8 145 mains. 8 190 air correctors. 8 200 air correctors. 8 57 idle jets. 8 60 idle jets. 8 34 mm chokes. I am going to hit the dyno for a full day and post the results of what several setups run so people will finally have a reference point with hard numbers regarding bhp, and mixture. I understand each car is going to be a little different, but this may be a good baseline.

 

That is a big investment in time and money, thanks for your dedication!

 

Actually, I ordered all my jets from a colleague in Europe for $1.90 each, and the dyno time is done at a friend's shop.

 

Im not sure you'll be happy with the choke results, the jets yes, chokes not really on a stock engine. If anything you'll get a drop in throttle response.

 

sorry to be a little dense, but I'm looking for clarification. Does the A/F ratio use A/F mass ( lbs. or Kg.) or A/F volume (cu. inches/ liters)?

It seems like volume is the measurement even though the word "mass" is used.

 

Good to know about the chokes. I was advised by Carobu to try 34mm chokes. Sounds a bit big to me too, but this is more academic than anything else.

 

The air pumps alone would be 30lbs. I don't have the heart to replace the US bumpers with Euro style though. I would be too afraid if someone accidentally bumped me.

One key fact I did not even think about that I should have mentioned. My exhaust is a light ansa sport, and my rear valance has been modified to the Euro look./ I bet that saved a ton of weight.

 

No it's mass!

Otherwise you have to burn 1 liter of fuel within 4 cycles of a 3l engine ;-)

 

From my jetting experience, I'd recommend:
32mm chokes
140 mains
190 A/C
55 idles (60 is way too rich)
F24 ETs