Wanted to get a baseline even though I have already modified the exhaust, but before I do the pulley change.

This was done today on a Dynojet, peak HP before hitting the speed limiter at the rear wheels was around 415 in 5th gear, 373 in 4th gear, torque was about the same, 76F & 32% humidity. Seems high to me for a 4.2. How much more should I add to get power at the crank? They said 18%.

Maybe my exhaust mod is better then thought! Will be interesting to see what happens after the pulley and before the tune!

 

Odd that it would make more power in 5th than 4th. What was the order of the runs? 15-18% is generally reasonable but the number you come out with is not really all that meaningful. Dynos are great for comparison, but generally not that useful in locking down actually hp numbers. For what it is worth, dyno jets are generally considered to read on the optimistic side. How much so can vary from shop to shop. Nonetheless, it is great to have a baseline number to see how much of a difference the changes will make. Many around here like to see that information.

 

4th gear pulls are what you look for, and the 373 looks realistic for a dyno number, but not reliable for use with an even less reliable driveline loss guestimate factor to predict crank hp. Just use the dyno measurements as a way to measure your change in engine power, and that is the real increase in power.

But as far as Dynojet numbers go, I think 373 whp sounds great for an engine rated at 420 hp with just an exhaust mod, and more what I'd expect with exhaust that had a high flow cat or not cat at all.

Congrats so far, and good luck with your tune!

 

Thanks for posting your experience. I think, it's always best to baseline before the mods. Especially, when you're posting results and others are contemplating a similar build.
BTW, did your exhaust mod include removing the mid-resonators and replace with the "X"- over pipe?

 

Yes, I replicated the performance exhaust option (replaced both resonators with an X pipe) & also disabled the active exhaust valves. If I had found the dyno before the change I would have done it then. At least I have one now.

 

If we hadn't hit the speed limiter, wouldn't 5th have given us a more accurate HP number?

 

You want to dyno in the gear that provides a 1:1 gear ratio, or closest to that. For most cars that is 4th, including the XKR.

 

See Bruce H's post above. Generally when you get into a higher gear below 1:1 you will see a decrease in horsepower. That is why I was surprised you made more power in 5th than 4th. Was there a wheel spin issue in 4th?

Great to see some numbers, regardless of what they are. They will really help you assess the effectiveness of whatever changes you make. If you can, post plots at all stages (preferably including A/F). Best of luck.

 

I actually thought 5th was closest, as least from what I have read elsewhere. Unfortunately their wideband sensor was faulty. I will make certain it is working when I go back. They also gave me the drf file but there is something wrong with it as it does not have the 4 pulls I saw when they did the dyno. Somewhere I saw max HP at 403 but the graph showed 415 (5 HP increments). Hopefully they can send me the full files.

 

Yeah, I'd like to see the A/F curve, as well. All of my performance tuning has been with modifications made to my 289/331 mustang engine and it's normally aspirated WEBER carbs. I'm "green" to EFI systems.

 

New pulley has been fitted and I do notice an immediate undeniable improvement in acceleration. Will be interesting to see what the dyno says when I go back.

 

You may have told us what size pulley you were going to install but, I missed it. So, when you get the dyno results, please let us know about the pulley size you installed.

 

It is a 1.5lb pulley from Eurocharged, but they have advised me not to dyno the car and that I should not drive it until I tune it as the increased boost will cause the ECM to run the motor rich.

Not certain when I am going to do the tune, but is it really true that I can no longer drive it until then?

 

I have been lurking on here extensively for a long time. I have seen time and time again that even the 3.5lb pulley requires no real tuning as there is so much left in the ECU. Jaguar really crippled the 4.2L XKR because of warranty concerns during a time they were dipping into parts bins to build cars and bleeding cash. I have heard nothing but good about Eurocharged and intend to buy from them in the near future but I would take what they said with a bit of skepticism as they also are the top "tuner" for 4.2L Jags, so take that for what its worth since its obviously in their interest to sell you a tune also.

Also jahummer I have followed you the most as your car is the closest to mine (2007 XKR) and one of the only people I see actively modding their car.

 

I'd say "should avoid" rather than "cannot" - you won't do any real damage running rich, but you will soot the engine up a bit. Might contaminate the cats a little too (I think I recall that unburnt fuel is bad for them, though I may be wrong). I know other people have driven around with the pulley & no tune. Anyway, as you have an alternative ride, probably best to take the Eurocharge advice and keep the XKR parked unless you have to use it.

 

I drove my '13 5.0 XKR for 3 months with just a pulley before Eurocharged tuned it. I had no problems at all but I can vouch for a 4.2.

You absolutely need a tune to get everything out of the Pulley as the ECU will just blow off all the boost over 9LBS. The reason you are feeling a performance boost is that you are getting to 9LBS quicker with the smaller pulley but it will fall off over 4k RPM..

I know I've posted this before but look at my dyno with only pulley, then pulley with tune. Its over 70HP difference, your not getting that without a tune.