Jump to content

Experiences with 420 fuel economy


Recommended Posts

Not that it's really important, but I've notice that my 420R (with aeroscreen, 5 spd trans, roller barrel intake, stock CC tune) gets terrible gas mileage. It seems I get about 16-17mph on the highway cruising 70-75mph. I guess I expected something on the order of 20-25mpg? Sure, the Cd is bad, but there's still not a ton of frontal area, and it's still just a 2L engine. My supercharged exige got 30mph on the highway. Interestingly though, I get better mileage on track in the 7 by a significant margin over the Exige.

 

Is this typical, or is my car running unusually rich at low RPMs?

Link to comment
Share on other sites

I'd say not typical, as my r500 (stock) gets close to 30mpg highway.  Major difference could be that I'm running a 6-speed transmission.  What is your cruising RPM?  

 

 

Link to comment
Share on other sites

1 hour ago, jeffs said:

I'd say not typical, as my r500 (stock) gets close to 30mpg highway.  Major difference could be that I'm running a 6-speed transmission.  What is your cruising RPM?  

 

 

 

I believe your 6spd is 1:1 in 6th. His 5speed should have overdrive and actually be better off.

 

With that said, I barely average in the teens even just cruising, but I have all sorts of issues going on. 

Link to comment
Share on other sites

When my Duratec was in 420-ish guise (same cams but with Jenvey's), it would generally return low 30's on high-speed sections of tours.  The car has a windscreen and very large wind deflectors, both of which negatively impact aero, but that is partially offset by the 3.62 diff vs. 3.92 in your car, which translates to 3079rpm at 70mph.  I typically average 25-26mpg in normal use; no freeways, running errands in town, back road blasts, and bursts to redline.  

 

-John

Link to comment
Share on other sites

Hmm definitely something going on. It starts instantly and idles smoothly. Pulls like a raped ape to redline. Not sure if I should chase the issue or not. Freeway cruising is c. 3000rpm. I always note the mileage after getting to the track and filling up at the pumps at the track. However that after a cold morning drive - my oil runs extremely cold because of the dry sump setup. I see temps < 110F on cold AMs while on the freeway. Keep in mind sender is in the tank, after the cooler. I wonder if the cold/thick oil could be affecting economy as well. Another variable is that I let the car idle for a while in my driveway before leaving, trying to get a little heat in the coolant/oil. Typically I will let it idle for 5-10mins. This may be using a not-insignificant amount of fuel? I will go fill up and see if the drive home from the track (warmer ambient temps, no idling period) used significantly less fuel.

 

Thanks

Link to comment
Share on other sites

I know this is a very unlikely scenario, but if your odometer is wildly pessimistic, it could result in a lower mpg calculation than the car is actually returning.  

 

-John

Link to comment
Share on other sites

What % of the time is your idle warm up?  If you let it idle for 10 min for a 15min drive (though I think that's a mythical thing in the bay area) I could see it having an impact.  A 10 min idle for an hour of highway driving probably not more than a 1-2 mpg impact.  

 

Probably not an issue since your car is running well, but I recently had my mileage drop horribly.  Turns out that one of the two coolant temp sensors has gone bad (one feeding the ECU not the dash) causing it to run really rich.  I'm getting under 20mpg city now. 

 

Highway for me is typically a little over 4k rpm, also running dry sump (a little warmer but not much over yours), aero screen, roller-barrels, stock tune.  Seems really weird your mileage would be so much worse than mine.  The 500 came with a few extra aero bits but I can't imagine they make that much of a difference.  

Link to comment
Share on other sites

What are the coolant temps? I suggest installing an oil thermostat to bypass the cooler when the oil is too cold or blocking off the cooler airflow unless you are at the track. The oil works with the coolant to cool the engine so you might consider a 195f coolant stat also. I would want to see around 175f in the tank where the circulating oil temp is at least 212f on every drive to cook off moisture, acids/deposits, and reduce potential sludging.

The oil cooler must be thermostatically controlled to maintain oil temp, just like the coolant stat. It bypasses the cooler during warm up. If the oil is running that cold, the coolant must be running too cold also, which makes it run rich/less mpg. Cold oil is more critical than cold coolant.

If I wanted to drive it while waiting for the parts, I'd block the airflow through the oil cooler and run a 195 coolant stat.

I'd like to see 2500rpm at 70mph but it will effectively widen the gear spread a bit.

Link to comment
Share on other sites

10 hours ago, jeffs said:

What % of the time is your idle warm up?  If you let it idle for 10 min for a 15min drive (though I think that's a mythical thing in the bay area) I could see it having an impact.  A 10 min idle for an hour of highway driving probably not more than a 1-2 mpg impact.  

 

Probably not an issue since your car is running well, but I recently had my mileage drop horribly.  Turns out that one of the two coolant temp sensors has gone bad (one feeding the ECU not the dash) causing it to run really rich.  I'm getting under 20mpg city now. 

 

Highway for me is typically a little over 4k rpm, also running dry sump (a little warmer but not much over yours), aero screen, roller-barrels, stock tune.  Seems really weird your mileage would be so much worse than mine.  The 500 came with a few extra aero bits but I can't imagine they make that much of a difference.  

The last two AM drives, where I noted the mileage were 1 hr drives, so a 10min idle is not insignificant. I will eliminate that variable and calculate again.

 

3 hours ago, MV8 said:

What are the coolant temps? I suggest installing an oil thermostat to bypass the cooler when the oil is too cold or blocking off the cooler airflow unless you are at the track. The oil works with the coolant to cool the engine so you might consider a 195f coolant stat also. I would want to see around 175f in the tank where the circulating oil temp is at least 212f on every drive to cook off moisture, acids/deposits, and reduce potential sludging.

 

The oil cooler must be thermostatically controlled to maintain oil temp, just like the coolant stat. It bypasses the cooler during warm up. If the oil is running that cold, the coolant must be running too cold also, which makes it run rich/less mpg. Cold oil is more critical than cold coolant.

 

If I wanted to drive it while waiting for the parts, I'd block the airflow through the oil cooler and run a 195 coolant stat.

 

I'd like to see 2500rpm at 70mph but it will effectively widen the gear spread a bit.

 

I have a dry sump, so there is no way to have thermostatic control that I am aware of. My coolant runs 197F rock solid no matter the ambient temp (while on freeway). On AM drives to the track I do block off the nosecone opening in front of the oil cooler (I have the 420R race nosecone). Because I have a pusher fan, I can't block directly on the cooler itself. Even with the blocker in place, I was running 106F while on my way to Laguna in Feb. I assure you, I was much colder...

Link to comment
Share on other sites

The sump type doesn't matter with an inline oil stat. There are many examples like this one: https://www.merlinmotorsport.co.uk/p/susa-inline-oil-thermostat-with-m22-ports-40-aot180-22

 

I'd consider capping the cooler and bypassing all together. There is no benefit and only down sides if the oil never gets over 230f on the track without it.

Link to comment
Share on other sites

Ok interesting - I have never seen a thermostat on a dry sump'd engine so I assume it just wasn't a thing for some reason. I will do some more research. I haven't done a real heat stress test on the car yet, as it hasn't been very warm on the 7 trackdays I've done in the car, but I assume it will need the cooler for warmer ambient temps.

 

 

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...