I made a recent trip to Western Oklahoma and they are having 106-108 F days. The old STR was a real dog. Felt like I lost 100HP or more. Just never had it effect the car so strongly before!

I really like cool/cold weather. The car just flies then!
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Wow a 100hp is quite a bit! Was this in stop-and-go driving or highway or two lane roads, or a mix? No other issues other than the feeling of lost power?

That is really friggin hot - how was the humidity?

Putting other factors aside, the high ambient temps shouldn't have made that much difference. Power decreases by 1% for each 11*F increase in inlet air temps so if the OP is used to 80*F temps, he'd be down about 3-4% or about 12-16 HP on his type R.

Possibly altitude was a factor.

Math says one thing, experience another. I too have noticed on the hot days the STR feels like its down 100hp from the heat. I did an extensive country tour/scenic blast this weekend in 90f+ weather, car was a real dog.

Got home, popped the hood, car cooled for a couple hours, departed later that (cold) evening, and car felt great again.

There again, there's nothing less reliable that a butt dyno.

A little higher altitude but lower humidity than in Houston.
I really noticed it when passing. It's usually just a quick jump around but she struggled in the high heat.

It might be showing up a weak fuel pump? Sometime during that trip I picked up a code. P0191 which is the fuel rail pressure sensor. It can indicate high or low fuel pressure.

I did change the fuel filter when I got home but the old one only had 5K miles on it.
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Do these car run a return-less fuel system?

Fuel itself can heatsoak as well.


Indeed, the "100hp loss" we speak of is entirely anecdotal, and a butt-dyno best guess can be taken with a grain of salt. A 10-15hp drop off you would barely notice though. A severely heat-soaked STR is quite noticeably slower, I really don't think a 50hp+ loss is stretch to consider.

My 2005 STR has a fuel return line but I know Jaguar went to a returnless system but not when.
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It does?

All the cars before then were returnless I believe and I thought ALL years were.

The only time I've heard of having a return line was those who've added LPG.

The problem is that heat has an effect on each sub system of the motor ect.

The intake temps start higher, which means the intercoolers are under more load, which means the engine bay it's self is hotter, which means the temps after the supercharger are even hotter beacuse it's not 100 efficient.

Tempature management is something these cars need plenty of. Start with a larger pump in place of stock ic pump, larger heat exchanger and a separate tank for the intercooler.

The engine bay should hardly change if at all, because the main heat source is the engine and it's cooled within limits or the car will defend itself (limp mode or whatever).

You know, when I first got my '05 STR and looked under the hood, my instant reaction was "looks great, but how does the heat escape from this engine compartment?". Those black plastic covers don't help. When I found out about the belly-pan, I realized this was a real heat-trap.

Fortunately, the STR is sooo beautiful and ambient temps are not too high in my neck of the woods. 108F, good grief!

Remember that engines aim to run (overall, i.e. coolant etc) at around 200F for efficiency so regardless of plastic covers that's the temp.

The intercooler is quite a bit lower temp and there's a sensor after it (IAT2) since the air charge temp really matters.

A "Killer Chiller" (Kincaid Cooling, $650, uncomplicated install, same system as MB E55 Kompressor) would definitely put the smile back on your face. It lowers the i/c coolant by more than 55 degrees. With the KC, one may either eliminate the stock i/c heat exchanger or run the KC in tandem (this'll allow the a/c to be turned off in cooler weather). Pop on the a/c and good bye heat soak during hot weather or hard charging driving. I spoke with Kincaid Cooling yesterday. This's how the Mustang Cobra and Shelby's do it.

So there may be something other than just temp and altitude going on here. Clear the code after fuel filter replacement? Has it gone away and stayed away?

I kinda missed the P0191. The codes PDF says the PCM substitutes a default pressure. That could mean change of power I guess.

Butt dyno or no butt dyno, when tbird6 and GT42R both report what feels like a 100 HP loss, there's no way they could be so wrong and have actually experienced a loss of only 12-14 hp.

But let's assume those 2 guys independently made a 100% error of judgement, and the actual power loss was more like 50 HP (about 13%). If Mikey's formula of 1% HP loss from an inlet temp increase of 11 deg F is correct, that would mean an inlet temp increase of (11 x 13) = 143 deg F over and above the base of 80 deg F. Since the ambient temp in Oklahoma was 108 deg F, that explains the first 28 deg F increase in IAT.

So, "heatsoak" must have been responsible for the additional increase of 115 deg F in IAT. And with an engine compartment unable to shed heat efficiently, the IAT will creep closer and closer to up to 200 deg F maintained by the coolant as the ambient temp increases past 108 F.

So, the moral - if you visit Oklahoma in July in your STR, remove all plastic covers and the belly-pan to help maintain performance.

Doesn't sound right. The incoming air first cools the SC radiator so providing the SC coolant pump is working you're sending fairly cool water round the separate SC coolant circuit. Heat soak isn't going to help but that SC radiator and separate coolant circuit are there for good reasons. The engine will be at about 200F no matter what you do, that's the design and the PCM etc will work to make sure that pretty much happens.

tbird's power loss was probably the P0191 and that substitute value causing maxing out of what the PCM was prepared to do.

It would be very useful to have actual temp readings at various parts of the SC coolant circuit to see what actually occurs, and to have these before and after doing any mods (i.e. a scientific approach).

I'm taking that last comment as tongue-in-cheek eh...anything short of removing the entire hood I don't see as making much of a difference in heat shedding capability in this scenario.

And as Jackie Gleason used to say, and awaaaaaaay we go.

The confusion again is presuming that engine power is moving in lock step with coolant temp which in turn moves in lock step with air inlet temps. It/they don't.

Without rehashing why engine efficiency/power increases with higher coolant temps, suffice to repeat what's already said that underhood temps may vary as ambient temps change but engine COOLANT temps remain stable at the factory setting. The engine doesn't know the difference IOW. Removing covers, belly pans, hood/bonnet etc. would simply increase the air cooling of the engine exterior, and lessen the load on the liquid cooling system. No net gain.

If the engine is ingesting engine compartment air at 200*F as suggested, then someone should write a stern letter to Jaguar HQ and demand that they engineer in some cold air inlets on our cars that take air from in front of the radiators. Oh wait a minute..........