Engine under tray and mpg effect

[eltax91]

Hi all

I'm not massively into driving for mpg or anything, but I do track the mpg on my daily driver using fuelly. It's my work hack and is an accord 2.4 auto that runs on LPG, I cover over 30k a year.

So, for the last two years my average mpg is 25.6 on gas. In the last 4 weeks and about 3k miles, it has increased to 26.9. Nothing has changed. The car gets serviced every 10k, but its not due for another couple of k.

The only thing I can think of that has happened about 4 weeks ago is that the engine under tray came loose. I drove it home about ten miles and then removed it and have not got around to jacking the car up and refitting.

So, is this a pure coincidence or do the trays really affect economy? Surely if anything they should marginally improve economy? Nothing else has changed though, not even the weather.

[Conscript]

eltax91 said:

Surely if anything they should marginally improve economy? Nothing else has changed though, not even the weather.

I would have thought so too. Is your undertray made of 5mm lead sheeting?

[anonymous-user]

I'd agree that if anything it should improve things (fractionally). It's only a guess but it could have been loose for a while, not noticeably so but enough for there to be a gap where air could get in and cause a fair bit of drag. That could have a noticeable effect at higher speeds. Just guesswork though

[trickywoo]

Is it possible you are subconsciously driving more smoothly knowing the tray is missing?

[eltax91]

charltjr said:

I'd agree that if anything it should improve things (fractionally). It's only a guess but it could have been loose for a while, not noticeably so but enough for there to be a gap where air could get in and cause a fair bit of drag. That could have a noticeable effect at higher speeds. Just guesswork though

Yes. One of my thoughts too. I do self service the car and often get a look at the tray. It has a separate small oil flap to drain the oil so doesn't get removed much.

The way in which it fits, it attaches with bolts at the front and slots above the bumper plastic, they were all still secure and it had failed on a plastic clip at the back. So I don't think air could have gotten in between tray and bumper at the front.

When it failed, it flapped like crazy and it was very noticeable noise wise.

[eltax91]

trickywoo said:

Is it possible you are subconsciously driving more smoothly knowing the tray is missing?

Maybe! However, outwardly at least I don't think about it. I generally regard them as a useless piece of plastic. I've had them go on old cars before and just jetissened them. The only reason I was going to refit it is because I believe Honda know a bit more about aero than I do.

[sfimirat]

Sorry to be picking this up after it's been dead for awhile. I have been searching for consensus on the effect of front splitters and then ran into the undertray effect. DO you guys think it would make sense to use both to increase MPG and to also protect from road debris? Thanks!

Mike

[996TT02]

sfimirat said:

Sorry to be picking this up after it's been dead for awhile. I have been searching for consensus on the effect of front splitters and then ran into the undertray effect. DO you guys think it would make sense to use both to increase MPG and to also protect from road debris? Thanks!

Mike

In all honesty what any casual PHer like me thinks is pretty meaningless. Such questions can only be answered accurately under proper test conditions. I was however under the impression that splitters were there to increase downforce, or, more accurately with most normal cars, reduce lift. Undertrays ought to provide a cleaner flow of air beneath the car. Beyond these basics - actual rather than supposed effects, and how such items interact, see previous.

[Rotor]

It is possible is may have an effect on radiator cooling ? high and low pressures /air flow through front grill ?, may also be wise to avoid/attempting going up side down in tunnels until refitted just in case...

[Haltamer]

sfimirat said:

Sorry to be picking this up after it's been dead for awhile. I have been searching for consensus on the effect of front splitters and then ran into the undertray effect. DO you guys think it would make sense to use both to increase MPG and to also protect from road debris? Thanks!

Mike

I recently picked this book up, and it may be of some intrest to you: https://www.amazon.co.uk/gp/product/1787112837/ref...

Adding an undertray should improve your underbody aero quite a lot; There are some case studies in the book. An improvement in underbody protection is pretty much guaranteed. If you'd like, I can probably send you over the relevant pages.

[samoht]

Haltamer said:

I recently picked this book up, and it may be of some intrest to you: https://www.amazon.co.uk/gp/product/1787112837/ref...

Adding an undertray should improve your underbody aero quite a lot; There are some case studies in the book. An improvement in underbody protection is pretty much guaranteed. If you'd like, I can probably send you over the relevant pages.

Interesting.. that's the same Julian Edgar who has written extensively on this topic at http://www.autospeed.com/ , which is a real treasure-trove of honest and scientific home experimentation on aero and other modifications.

[clarkey]

I used to have a mk1 Honda Insight and underbody aero had a big affect of MPG. On that, lots of small benefits added up - aero, weight, thin wheels, low rolling resistance tyres, etc all made very small differences but added up to an average of 70mpg and over 90mpg when trying.