If a car generates 400 HP with an airfuel ratio of 15:1 at 4500 rpms, would the car loose HP if running at 12.5:1 afr? Or more accurately, how much power is lost by running at 12.5:1

I know that 15:1 is not safe but I'm curious how much power one picks up if running lean.

 

I just came from the dyno hoping to finally put down the big numbers, but once agian I was stumped by a hardware issue and I would like some suggestions.

You can see the car in the sig. Here is the story. The car pulls quite nicely to 4500 rpms and then the power dips, recovers and by about 5100 sinks quickly into the sunset. The strange thing is that the air / fuel ratio stays at about 12.6 - 12.7 throughout. The log on the FAST gives no hint as to the problem.

When I was street driving I would feel this stumble and attributed it to detonation and indeed one of the reasons to go to the dyno was to get my timing fixed up.

However, I'm assurred by those in attendance that it is not detonation. Currently it would seem that it is either coil, MSD or opti. The opti is three years old, the coil is 6 years old and the MSD 3 - 4 years old. To test the MSD we unplugged it but the same thing happened and abotu 600 rpms earlier which does not surprise me. Chances are the spark was being blown out.

Anyone have any experience with this syptom?

Thanks

 

From your description - I am leaning towards a valvetrain problem.

Hard to know for sure though(over the 'net) - but it sounds like your valves might be floating in the upper RPMs

But then you mention the MSD thing, and the problem happening 600 rpm sooner *shrug*

 

I considered valve float but I thought that 4500 rpm is a little low for that. Also I have AFR 190 heads with Crane springs etc etc. Shouldn't be a problem.

 

Mods: Feel free to move this to another board if you feel that would be appropriate.

 

What kind of timing are you running? I would presume valvesprings at first, but easiest to check the tune. Also, it seems to be running a bit lean for that combo. I would recommend it running around 11.8 on the dyno, as I'm sure the car weighs much more than the rollers. The car will run slightly leaner on the street than on the dyno, so you are approaching 13:1, which is NOT good for any FI car. What does your FAST info say with the Wideband on the street?

 

Well that is the other inconsistancy. The FAST has a target afr of 12.2 - 12.1. The log DOES indicated about that range but it is taking fuel OUT to get there. (I currently max itwith a 5.5% correction) BUT the dyno results say 12.6 - 12.7. I should point out that the FAST results are also from the dyno run. Thus the dyno results were about .6 higher then the FAST.

I should point out something though. I have a Borla exhaust and the "Y" was open on the right. However, I realised in hindsight that they had the WB sensor at the dyno put into the left side. I'm not sure if that would have anyway corrupted the dyno WB results.

Timing is base about 36 degrees and goes to a low of 22 at WOT. I get about 12.5 lbs of boost. However, driving home on the freeway, my buddy moved the base timing to 37-38 degrees at cruise and the car felt better.

I wonder about valve float! These are AFR 190 heads with Ferriara (sic) valves and Crane 13083 (from memory) springs. The heads and the FAST were installed over the winter and candidly, the dyno results UP TO 4500 rpms were not better then the last combo wich were modestly ported LT1 heads and LT1 Edit. Candidly I think I can take some timing out and get some more power.

Interesting to note that the two kind folks to reply did not express an opinion that it is an ignition problem. I really appreciate the help. i'm hoping someone says "Oh yeah I had exactly the same problem. It was my XXXXXXXXXX"

 

12.5:1 will probably have more power than 15:1, which is way too lean. 14.7 is "stoichiometric", theoretically the AF ratio where each atom of oxygen the car takes in has a fuel atom to react with. But combustion is not a perfect process, so to be able to utilize each air molecule there must be an excess of fuel molecules, iow the ideal AF ratio is <14.7:1. How much less depends on a number of factors, that's why no one will be able to give you some simple formula. Each combo has to be tested empirically to see what works best. A typical NA LT1 will make max hp in the 13.5:1 range though.

As a general rule, making a car a bit richer than optimum costs just a couple of hp, making it leaner cost proportionately more and increases the risk of engine damage. So assuming 13.5:1 is opitmum, dropping to 12.5:1 would cost fewer hp than increasing to 14.5:1, let alone 15:1. I have seen this many times on the dyno. Before you start dropping signficant hp you have be in the low 12:1 range. Start to get to 11.5:1 and lower and the decrease acclerates. Long before you go as far in the lean direction, you will have put a hole a piston and not been making power anyway.

Rich Krause

 

I am just curious if you are getting any knock running this lean? If I were to run this lean my engine would be knocking to hell, but I know all cars are different.

 

I'm not sure how this thread relates to your "dyno" thread, and its not clear if the "dyno" thread belongs, but as a compromise, lets merge the two threads.

Are you saying you have seen your engine operate under load at 15:1?

 

Seems like a limited number of possibilities, and most of them have been mentioned...

1. Ignition breakdown. Getting worse without the "box" would indicate this may be the problem. You didn't mention the age/condition of the plug wires.

2. Running out of fuel... the wide-band results don't indicate a problem.... but where does the 15:1 come in, or was that a totally separate question.

3. Valve train problems

4. S/C belt slipping... does boost stay where it belongs?

5. Detontation... how do the plugs look immediately after the pull? What plugs are you using? Gap?

 

TY Fred: The 15:1 afr dyno run was from last year when running with LT1 Edit. BECAUSE I had been running lean and for other reasons, I had the motor refreashed over the winter and put on AFR 190 heads and the FAST. (Ironic in that the rings did not seat and this new motor burns oil)

Fuel: Injectors are 75# and duty cycle peaks at about 60%

Belt: That crossed my mind and I asked the dyno operator. He said in his experience when the belt slipped, the HP line would flat line.

Valve train problems: Would valve train lash adjustments be a candidate? I feel comfortable that the components themselves are up to snuff.

Detonation: Timing is VERY conservative. At WOT my timing is 22 degrees. Indeed I went to the dynp to tinker with timing to get more power. there was no audable detonation.

Plugs and wires have about 5,000 km on them. Basically new. TR6 gapped to 35 thou.

The other really strange thing is that the afr ratio was consistent at about 12:6 - 12:7 right through to 6300 rpms even though the power tumbles off. Also interesting to note that the FAST has a target AFR of 12:1 - 12:3 AND that the o2 corrections are having to take fuel OUT to reach that level (I only allow 5% correction). The FAST log indicates AFR are about 12:0 - 12.1. That has me a little confused.

I'll try and get a scanned graph and table on the web.

Thanks guys.

Fred: Thanks for your explanation. Bottom line is that 15:1 AFR does not geneerate more HP then 12:1 AFR. Thus I must conclude that either the heads and FAST have not improved performance or that the tune (probably timing) is not as good yet with the FAST as it was with EDIT from last year.

 

I was there for the dyno. The car is cutting out in the same area it has for the past year(before new heads). When it getts there is sounds like it hit a rev limiter at first but keeps pulling but breaking up with mis fires.

The MSD was totally by passed and the problem got worse and power fell of.

The shop where we were at suggested belt slip but they usually see power just level off when this happens and not dive.

There was NO detonation and the FAST was reading low to mid 12s AFR the wholw run(all three)

With the knowledge that this problem has transfered to the New FAST and heads i felt it my be the opti. May said Coil. One of the main reasons I felt it my be the Opti are just past experiance with them. Many times I have had them start to fail from carbon tracking and act this exact way. I just noticed it at higher RPMs in the earlier stages.

The nice part is if this is found to be the problem Shaun can go to the multi coil system fairly easy.

I have had problems with the Springs Shaun has listed but they did not float till 5,800 with a Comp Xtreme cam. I went to K-Motion springs and have never looked back.

Overe all Shauns car is showing brutal potential! 482RWHP breaking up at 4,500! Imagine when it runs and he can pull to 6,000

Good luck!

 

Another thought:

I once mis adjusted a couple valve about 1/2 a turn too tight. This was causing the hydraulic lifter plungers to collapse in the upper RPM range. Car would pull, but had a definete miss to it.


I still suspect something is not right in your valve train.

What kind of lifters are you running? How many miles are on your springs/lifters? How close to coil bind are you at max lift?

 

Curious, does the car have the same issue when accelerating on the street? Is the missfiring noticeable? I ask because a friend of mine had a VERY similar issue with a turbo honda. On the dyno the car would sound like it was hitting a rev limiter at 6000 rpms, start to missfire, and at 7000, power would nosedive. He has since sold the turbo kit, so we never pinpointed the problem. My guess was the cylinder head, since it had been ported, and the flow numbers were horrible, for a stock GSR head, much less a ported one. The thing was, he got the head from another friend that it ran fine for. I would then point to ignition, or weakend valvesprings. Granted it is an OHC design, but I'm sure 10K rpms takes its toll. I would still look at valvesprings on your car, but other than that, I would wonder about the heads themselves? Are they box stock? If so, you can be fairly sure that the flow is fine. If not, then I would question the porting on them. Also, about the a/f ratio, I have seen some super inflated torque numbers from high a/f ratios. Usually a turbo car, but once the a/f is put back to a reasonable level, it returns to normal.