Dyno'd the Cobra yesterday

[MalcolmV8]

Well since the engine rebuild I was curious as to where power was with the lower compression pistons etc. I plan on getting a twin screw next year and a few other goodies but for now I wanted to see where the car is at. Also my tuner asked me to log a few data logs while running it on they dyno so he could finish dialing in the tune. It made 487 hp and 495 ft/lbs to the wheels. Not to shabby for a baseline to start working with.



My only real issue with the car right now is gas mileage OUT of boost. While just cruising around town in normal out of boost mode I've suddenly lost about 5.5mpg since the engine rebuild. That's massive. I'm wondering if it's because I lowered the car and have not had a wheel alignment done yet?

[cgrey8]

My guess is lowering the CR is responsible for the loss in fuel economy. But that's just a guess.

[MalcolmV8]

That was my first thought too but I didn't think it would be that bad? From 19 mpg city/highway mixed driving out of boost to 13.5 mpg. That's a lot for sure.

[cgrey8]

By any chance, did you go back together with thicker head gaskets to get the CR lower? If so, you may have killed your quench too.

[MalcolmV8]

No stock head gaskets. I used deeper dished pistons to lower the compression ratio. I lowered it at the advice of various performance shops I called which said I needed to lower it to handle the extra boost from a twin screw blower. I've since spoken to many engine builders who tell me that was not needed as the 4.6 mod motors can handle a tremendous amount of boost/pressure compare to the 302 motors etc. I've also since found out a lot of people when rebuilding these 03 cobra motors go from stock 8.5:1 CR to 9.1:1 or there about where as I went lower.

[cgrey8]

Lowering CR does allow you higher headroom for more boost. Unfortunately pistons dished all the way around, not just under the combustion chamber area kills quench which kills cylinder swirl and can reduce fuel economy.

What I don't have a feel for is how much boost you can get with a given CR on mod engines. So they may be right that you didn't really need to lower the CR. But because you did, you won't have to worry nearly as much about detonation as you crank up the boost into the 20s....but it'll obviously be at the sacrifice of cruising fuel economy.

[MalcolmV8]

The factory pistons where dished all the way around too. Mine are just a deeper dish. What does it mean exactly if its killing quench?
My A/F, timing, fuel trims etc. all look really good in normal cruise. My tuner looked them over and says the car "should" be getting great gas mileage on what he's seeing in the logs.

Is there anything that I can do to help if that's what killed my gas mileage? I asked my tuner to add a couple degrees timing to normal driving to help compensate for the lower CR but he was hesitant because he said I may experience pinging on tip in. So if I'm just cruising along the highway and I suddenly stomp on it there may be pinging right as I step on it but I may not hear it because the blower and exhaust are so loud at that point. That was one of the reasons he asked me to data log a few dyno pulls too. He wanted to closely watch from cruising in 4th at 2k rpms to suddenly WOT and of course see the actual WOT run for improvements there too.

[cgrey8]

If stock pistons were dished too, then nevermind. Lost quench isn't to blame.

[Dave]

That looks good as a start! Makes me want to get mine in for a run this week. That new motor of mine is 8.8:1 running 14lbs boost. First gas check after 1500 miles showed 19.7 mpg doing the interstate at 70-72 mph. I would have thought you would have gotten some higher numbers..What kind of rear gear you running?
Dave

[MalcolmV8]

8.8:1 with 14 lbs will make some good numbers. I didn't get to see my boost at the dyno but on the street it varies anywhere from 13.5 to around 14.5 depending on weather. Yours will be more effieceint with the TVS. Mine's the stock eaton m110 ported and pullied to get that boost.

Yes I used to get a lot better gas mileage. It used to be 19 mpg city/highway combined running around town out of boost. 16 mpg city/highway combined when beating on it all the time. Now I'm at 13.5 tank after tank just cruising around out of boost?? I'm still running the stock 3.55 rear gears.
I'm wondering if it's because I lowered the car too and have not done a wheel alignment? I have 315's in the back and 275's up front all cross tracking out of alignment. I wonder if that's what's killing my mpg?

[cgrey8]

The alignment would make sense. Let us know how bad it is out and if having it corrected fixes your MPG issues. I'm as curious as you are to know why such a massive drop in MPG. I would've guessed 1-2 MPG, but not 5 MPG from a .5 CR drop.

[MalcolmV8]

Yeah I think it's actually around .2 ~ .4 CR drop. I won't be doing an alignment and new tires till the spring. I had to draw a line with mods to the car this year and I've stopped It's that time of year, kids birthday, thanks giving, christmas, personal property taxes etc. etc.

[MalcolmV8]

Got the car aligned yesterday. I was putting it off because a rubber boot is torn on a tie rod end in the rear and needs to be replaced and I need aftermarket caster/camber plates up front.
Well a local NTB gave me a 5 year alignment for $160 so once I get those other things in place they'll re-align in the spring at no extra charge.

The car drives totally different now. The alignment was very very bad lol. Hopefully this fixes my mpg issue. We'll know in a few days.

[Dave]

Good luck and like Roy and Dale always used to say "Happy Trails to You". You can really make this little motors into beast
Dave

[MalcolmV8]

Well that was it. Instead of 180 miles to a tank I just got 240 before the fuel light came on. What a massive difference. This last tank was a little over 18 mpg. That's 4.5 mpg increase just from getting a wheel alignment. That's huge. I would have never guessed but really glad that's all it was.

Later
Malcolm