Just had my engine dyno'd and air fuel ratio checked

Is the horsepower that you are getting at the wheels, or at the crank?

What is the difference in the heads to create more horsepower, or have you done some other things?

At what RPM was the highest horsepower created?

Just some questions because I'm looking into stroking my 4.0

Tim

The 4.2/4.0 mod should add about 40 horse power due to the porting on the

I'm not sure how the dyno test works but I think it's at the wheels.

Looks like they cut it off at about 3350 rpm.

Bill

I was told there is about 30% loss on 4 wheel drive vehicles. The guy that did my Cobra said, he had seen a 60% loss with some Audi all wheel drives. Auto or manual makes a difference too.

In addition, 60 miles is not enough to let everything seat properly. I had my Cobra tested at 2222 miles and was told that is still not enough miles for proper seating.

What was the A/F ratio in numbers? I would think it needs to stay around 12.

--James

I just talked to the engine rebuilder who told me about the same thing you said.

He said a true dyno would be an engine dyno, not the rear wheel dyno. Especially in my case since I have a drive shaft problem from the lift kit. Also, break it in at least a couple of thousand miles before making an evaluation.

The A/F ratio, they said they couldn't get a print out on their machine so they give it back verbally. They said a tad lean side if anything.

Bill

Bill,

They probably didn't get the engine to its peak power RPM. Remember, the 4.0 head makes peak power at about 4000 RPM. The head doesn't know what engine it is sitting on top of. If the engines are approximately the same size, the bore and stroke don't have much effect on the peak power RPM. A long stroke, small bore engine should make more low RPM torque than a short stroke, big bore engine. Also, porting a head will almost always move the power band up.

To explain this to some degree, a cylinder head port by design has a "sweet spot" where it best fills and empties the cylinder. This is basically where your power peak will be. This also works in conjustion with the intake, camshaft and exhaust. This is why it is important to match all of the components so that they all, by design, produce their peak power at about the same RPM. In your case, the head is probably helping a little in the lower RPM and will show more improvement as the RPM gets closer to 4000.

As far as the dyno test, if they used a chassis dyno and read the power at the rear wheels, then it is rear wheel horsepower. This number is generally about 20-30% lower than the flywheel rating due to friction losses in the trans, transfer case, and axle. An automatic trans will usually show more power loss than a standard because of the toque convertor and more friction loss internally.

Did they give you a copy of the dyno sheet? If so, look at the number in the BSFC (Brake specific fuel comsumption) column. This will give a good idea as to where the fuel mixture is. The number should be around 0.5 or a bit higher for the correct fuel mixture. This basically means that it is using .5 pounds of fuel per horsepower per hour. If the number is too high, it shows an inefficient engine. This would have also been a good time for you to play with your timing to optimize it. Normally, the timing that shows the best power on the dyno will be 2-4 degrees higher than what you should run because of temperature, barometric pressure, and load changes in real world driving.

Chris

Here you go I knew I could find it:

--James

(This is from the last page of the post)

--James

PS - I would take the torque, but I am not sure I would spend that kind of money on it.

Oh yeah, some place in the post he tried bypassing the snorkel, using a conical K&N. There was a HP loss, a pre-detination (ping/knock) gain and an overall loss in performance.

There is something to be said for the cool unrestricted air a snorkel on a TJ gives.

--James

so if we assume a 20% loss at the drivetrain...his HP comes out to be...

thanks for posting the results! i've been wondering about the power gains using a 4.0 head. i'm going to take mine in too when i'm finally done with it.

Well, let me just say this. At this time in my break in process, the my BUTT Dyno sees little difference between the previous 4.2 and the current setup.

After a couple of thousand miles I'll let you know how it feels then.

Bill

I would say so. And like I said in my previous post, I doubt he has reached the power peak at 3300 RPM. It should be more like 4000 or so. Given all that, Bill has a decent power output there considering he's apparently running a bit lean, the engine is nowhere near broke in yet, and in all honesty I would venture a guess that he may has more than a 20% loss through the drivetrain being a 4wd. The bottom line either way is that it runs in a way that makes him smile (especially given what he has gone through with this project), and that it has enough power when he needs it.

Chris

You are correct Chris. The Dyno tester said he didn't want to run it full throttle or as he would a already broken in engine.

The main purpose for this test was the Air/Fuel ratio test, not the Dyno. Why they needed to do the dyno I don't know. Especially since it's a new engine.

I have yet to get my smile on this one. Seems like the damn thing ran better with one cylinder sucking oil. I'm taking it to the Jeep Doctor (JeepsRUs) when I can get in which appears to be next weekend (after the holiday).

The engine seem has this kind of humming/buzzing vibration at idle which goes for a while, lets up, and goes again.

What would happen if the balanced Flywheel and Clutch plate were not aligned. Would that be enough cause a vibration? I've had the timing off before and it didn't feel like that.

Sheeezzzhh...

Bill

Or, could it be part of the break in process of the engine?

Bill

Nice Post Chris, just got to it.

They gave me a copy of the Dyno sheet but it didn't have the Air/Fuel Ratio number on it. They said that part of the machine has no print out capabilities so they give you their interpretation of the readings verbally.

Bill