Highhat

I dialed in my A/F ratio and increased the timing several degrees to 34 on a chassis dyno. Gained 8hp/degree. Running 11.25 cr and want to stay with 93 gas.

A/F is now at 12.3. (84 degree air temp)

Question: keep increasing timing, or jet down at this point?

Gordonm

Is this at cruising or under acceleration. I try to run right around 12 at acc. and the best I can get is about 13.5 under cruise conditions. Any more and it starts to run like it is starving.

Highhat

That's at WOT

Matt Gruber

is this EFI?
carbs need track tuning to set up at max power.

Highhat

No. A Holley 950HP

dmaaero

11.25 cr is getting on up there. Make sure you listen for the valves ratteling. If it"s not ratteling now i"d suspect 36 will get you some.

Gordonm

WOT will require a jet change to really change the AF ratio. I would go a size or two larger and see what happens. I did this on mine on a chassis dyno and was able to get it tuned in pretty good. I was real close with my LM1 system just needed a few tweaks. I picked up 10 HP when tuning. I still feel maybe a few more are lurking inside.
11.25 is pretty high for the street but if you are running a cam with enough duration to bleed off some of the compression you should be OK. Are you going to take it to the track anytime soon?

gkull

The chamber type determines spark max advance. I have been told that 12.5 is the point of max power and cruise exceeding 14.7. As for compression ration my 383 was @ 11.2 and my 434 is 11.8. Both of then have done many thousands of miles on pump gas only

Ratio Requirements: A well-tuned engine used in normal road conditions has an air/fuel ratio that is constantly varying. At light loads, lean air/fuel ratios are used, while when the engine is required to develop substantial power, richer (ie lower number) air/fuel ratios are used.

Most spark ignition engines develop their maximum power at air/fuel ratios of 12.5:1 - 14:1, maximum fuel economy at 16.2:1 - 17.6:1, and good load transitions from about 11:1 - 12.5:1. However, in practical applications, engine air/fuel ratios at maximum power are often richer than the quoted 12.5:1, especially in forced induction engines where the excess fuel is used to cool combustion and so prevent detonation.

There is no one air/fuel ratio where all emissions are minimised. At an air/fuel ratio of 14.7:1 oxides of nitrogen peak, while hydrocarbons and carbon monoxide (CO) increase substantially as the air/fuel ratio richens.


The amount of fuel that needs to be added during cranking can best be determined by experimentation. This enrichment may be configured by just a one-dimensional variable based on engine coolant temperature, or it may be able to be controlled in a more sophisticated manner. Examples of the latter include post-start enrichment and enrichment decay time. Cold start is one of the dirtiest times in regard to emissions, and so if emissions requirements are to be met, a sophisticated ECU with multiple starting enrichment and decay maps should be used. Reducing the cold start enrichment but increasing cold acceleration enrichment will reduce the total amount of emissions. Some factory systems open the idle air bypass during cold deceleration, presumably to act as a form of exhaust air injection.

The air/fuel ratio required for a smooth idle will depend on the engine's combustion efficiency and the camshaft(s) used. Some engines with hot cams will require an air/fuel ratio as rich as 12-12.5:1 for a smooth idle, while others will run happily at 13-13.5:1. Engines with hot cams that are fitted with sequential injection management systems can run leaner idle mixtures than systems using bank or group fire. Those engines that can be configured to run in closed loop at idle will use an air/fuel ratio of about 14.7:1 when fully warmed, although they will still usually idle better at a slightly richer air/fuel ratio. However, keeping the engine air/fuel ratio as close to stoichiometric as possible will benefit emissions because the cat converter works most efficiently at this ratio.


Cruise: Light-load cruise conditions permit the use of lean air/fuel ratios. Ratios of 15-16:1 can be used in engines with standard cams, while engines with hot cams will require a richer 14:1 air/fuel. If a specific lean cruise function is available, air/fuel ratios of 17 or 17.5:1 can be used, normally at the standard light-load ignition advance. However, running too lean a cruise mixture will cause the cat converter to overheat. If a dyno and exhaust gas temperature probe is available, the cruise air/fuel ratio can be leaned out until exhaust gas temperature becomes excessive for these load conditions (eg 600 degrees C+), or torque starts to significantly decrease. Remember, an engine in a road car will spend more time at light-load cruise than in any other operating condition. The air/fuel ratio used in these conditions will therefore determine to a significant degree the average fuel economy gained, especially on the open road.
3. High Load

A naturally aspirated engine should run an air/fuel ratio of around 12 - 13:1 at peak torque. The exact air/fuel ratio can be determined by dyno testing, with the ratio selected on the basis of the one that gives best torque. Rich air/fuel ratios can be used to control detonation, and this is a strategy normally employed in forced induction engines. Thus, on a forced induction engine, the mixture should be substantially richer: 11.6 - 12.3:1 on a boosted turbo car and as rich as 11:1 on an engine converted to forced aspiration without being decompressed. As is also the case for ignition timing, the air/fuel ratio should vary with torque, rather than with power.

Highhat

At the very least, I hope to be at the Nostalgia Day again at ATCO on 10/14 and see you again, but I'd like to go to a few of the Forum's track rentals (there's one on 9/6).

Chamber - running Dart Pro 1 Platinum 215s, with flathead pistons. Gkull - so are you saying possibly increase timing and richen up the jetting?

Of course, this is all a baseline, and I will tweak at the track.

gkull

I've had two kinds of Dart heads. The pro1 215 with the large valve 2.08/1.625 option and now the 227CC Dart told me that max HP was in the 32-34 range at sub 2500 feet A.D.A. So even on a hot humid day at sea level you are not going to need more timing.

As for jetting. I would use 12.5 as max for WOT. When you start using fuel enrichment to control detonation or inlet temp and water temp you have gone overboard and are using bandaids

Matt Gruber

tell me what jets u have now
and i can calculate what to try.
12.3 now
my suggestion:
try12.0
and 12.9

or tell me what A/F u want and i'll give jet size.

Highhat

I am at 77/85 with 6.5 PV on a Holley 950HP.

I am not sure what to shoot for. I just figured 12.3 was too low. If you can calculate a range, I'll experiment within that range.

Many thanks Matt

Matt Gruber

Convert holley # to drill size:
#77=.086
#85=.100"
Holley doesnt make some of your sizes, so order max jets
to get 12.9-13:1 use .083 & .097

to get 12.0
.088 & .102"
i don't see big gains, but jets don't cost much either

Highhat

Thanks. I have the box of Holley jets to work with.

Matt Gruber

75=.082
76=.084
close. if u can run these u wlll save on gas!

but for .097 drill it yourself

Gordonm

[QUOTE=Matt Gruber;1561764785close. if u can run these u wlll save on gas!

[/QUOTE]


I don't think that is the really what he is concerned with.

Matt Gruber

true, but i've had faster cars pass me, then stop for gas more often. this is a fast car?

Gordonm

Faster to the gas pump!

ajrothm

If WOT AFR is at 12.3 to 1, I would leave it there, its not worth messing with it. Although you MAY gain a couple of HP leaning it out to 12.8-13.0 to 1, chances of detonation will be higher on an 11-1 comp engine. I would say you have it at a safe level that will still make good power... I wouldn't push it on the timing either since you are running it on 93 oct. Definetly no more then 36*. You may never "HEAR" it pinging(detonation) but I promise you its happening....by the time its so bad you can hear it, its already doing ring damage etc..

BTW, a little off topic but yesterday I went to a dyno day at a sportbike tuning center and about 95% of the bone stock bikes had WOT AFR in the 12.0-12.3 range...My stock GSXR1000 was 12.0-1 at peak.. Just goes to show you that even high performance manufacturers prefer the rich side of tuning. My turbo-hayabusa is tuned to run at 11.5-11.80 to 1 AFR...if I lean it out to 12.5 to 1, it MAY gain 3hp and it will push a head gasket at 13.0-1. FYI....keep it safe.