Gear head question

[W1RKW]

My car has an 11:1 compression ratio.  Since I've had it I've noticed when the outside air temp is around 80 and above the performance of the car is less than optimal (poor throttle reponse mainly) but when the air temp is 75 and below all is good.  Is this typical of cars with big comp ratio's?  I run 93 octane and thought I'd try going lower just to see what happens but I'm not to enthusiastic about engine knock (even temporarily) and not sure if I'd gain anything anyway.  Would a lower octane improve performance at the risk of knock during hi temp days?  I don't really know what's causing this poor performance thing.  I'm thinking maybe an air density issue when it's hot out.

[WA1GFZ]

The air density goes down when it gets hot outside so your effective compression goes down. Changing A/F ratio of quality of the fuel will just make it worst.
My 400 +.030 firebird really likes the cold air. Only 9.6:1.
You need a huffer.

[Bill, KD0HG]

Try retarding the timing by a few degrees. That'll help the low end response and torque and help to prevent pinging, too.
At 11:1, there's no way I'd even consider using 87 octane regular. Don't do it!
Is your coolant temperature getting too hot when it's above 80 outside?
Is your EGR valve sticking?
Does your car have an intake air pre-heater, usually on the exhaust manifold? Make sure its thermostat isn't sticking.
Are you getting a partial vapor lock (bubbles in the fuel) when it's hot out? If so, re-route the fuel line or add a cool can.

Just some quick thoughts...What sort of car are we talking about?

[kc2ifr]

Any time u compress the air ...ie:  turbo charge or super charge.....the air temp goes up. Good practice in these cases is to use an intercooler to bring the air temp back down for a bigger charge in the cyclinders.

[W1RKW]

The car is a 2001  Honda Prelude FI DOHC. When it's hot out it just doesn't want to go the way I expect it to go.  I'd like to get around it somehow.  It's been this way every summer since the day I bought it.  The dealership said you have to expect that with a hi-comp ratio engine.  It's my first one so I don't know if I should believe them or not.  In any event, when I want to get on it, the throttle response is definitely different (sluggish) compared to when it's cooler out.  Once it's rolling it's seems OK. 

Everything appears normal. Coolant temp, EGR, is OK. It doesn't throw a code. 

My thinking has been I just need to cool the intake somehow and make the air more dense.

Thanks for the advice. I won't go lower octane. I'll try tinkering with the timing if it allows me too.

[WA1GFZ]

I bet you can't change the timing.
A lower temp thermostat may help.

[W1RKW]

Yep, can't change the timing.  Distributor doesn't allow timing adjustment.  Timing is controlled by the comptuer.

[W1ATR]

If you have a service manual for this thing, see if there's an "intake air temp" sensor on the intake manifold, or in the intake ducting somewhere. If there is one, maybe it's acting up, or just goobered up with grunge.

Most IAT's are just a simple thermister, and they do go bad after a while. IF your car uses one, and IF it's acting up, maybe it's just not out of spec far enough to make a code pop up.

What condition are the air and fuel filters in?

Just throwing some ideas. It's a lotta fun troubleshooting a computerized car when the sick light isn't lit.

[W2JBL]

     you are running way too lean a fuel mixture from the factory (lower emmissions and better mileage), which is common on computer controlled engines. the systems are SO good today they can run very well on a lean mixture, but everything has to be right on the money for this to happen. air density can be compnesated over for over a fairly wide range on these systems, but there is a limit. if you have trouble at only 80 degrees, the manufacturer screwed up.

    i had this problem, though it was not severe; only mildly annoying on my 1998 Crown Vic cop car. lacking power on hot humid days and big loss of torque. no codes when scanned, everything showroom perfect. back in April i got a Hypertech Power Programmer (435$) and reflashed the car's computer with a performance program even better than the Ford Police program. end of problem. car is way faster, and my combined city/highway mileage went from 10 (!!!) to 15 MPG. highway went from 17 to 25. faster and better economy? yes.  best of all now i can run wide open with the A/C on on 100 degree days and be just as fast as on a cold winter morning! the program gives more timing advance, a quicker advance curve, and way more fuel accross the entire RPM range (1500-6500 in my case). no more lean wheezing sound from the air intake, just a nice chainsaw like scream! it has allready payed for itself in savings on gas.

     11:1 is very high compression (for today) even with a good ECM system. 93 octane is 10% alcohol in this state, so it's just piss gas. it makes you go way lean. it takes about twice as much VOLUME of moonshine in your mixture to produce the same amount of energy as straight gasoline. the basic rule of thumb is if you are 10% alky, you need to go 20% richer to keep from leaning out and burning something expensive like valves or pistons. a well tuned/programmed ECM can do this. the BFC or LBS/HR numbers for gas vs alky are not good at all. nearly twice as many pounds per hour of alky to do the same job as gas. and the tree huggers say we need alky? i think not!!!

[W2VW]

Look around for an internet bulliten board and or interest group for the exact car in question. Switch the B.S. filter on maximum and maybe even post the question there. I did that with an older Mercedes and found a lot of useful info.

[Bill, KD0HG]

Good suggestion, Dave.

(Or buy all of the stuff in the JC Whitney catalogue) -)

[W1RKW]

With all the control systems on the engines these days one must guess where to start looking for troubles.  I did discover last night when talking about this with my  younger brother that he has a fancy scan tool that is capable of storing data (curves).  Might give that a try to see what's happening.  Probably the easiest thing though is just put ice in my air box.  I found a website that deals with this model.  We'll see what happens.

TNX for the input guys.

[W3SLK]

Bob said:

With all the control systems on the engines these days one must guess where to start looking for troubles.  I did discover last night when talking about this with my  younger brother that he has a fancy scan tool that is capable of storing data (curves).  Might give that a try to see what's happening.  Probably the easiest thing though is just put ice in my air box.  I found a website that deals with this model.  We'll see what happens.

I thought that is what those 'K&N' filters do. I've been told that if you modify the whole air intake system, you can get better gas milage/HP. However I've also been told that it only holds true until winter comes around when the air is too cold and your mixture becomes rich. Where's Frank the Slab Bacon? He's a pretty bigtime motorhead and probably has some good input on all of this.

[WA1GFZ]

Chris has a point if you can reprogram the car fuel control. When I lived in LA they were just starting to add shine to the gas. yup 40% BTU for shine so you have to go a lot richer. I also played with water injection and found that helps too. Remember you make it richer and get more horses you visit the man on Main St more often.
He is hanging tuff at $3.72.
the bigger the hole the greater the flow............................

[The Slab Bacon]

Speak of the devil and up he pops. I find it somewhat difficult to believe that in this day of concern for oxides of nitrogen emissions and now cat cons on diesels that anyone would still produce an engine with 11:1 compression. High compression engines are tough to tame what comes out of the pipe. However if it does have 11:1 compression that should make for very nice throttle response.

I find it interesting that only 80 deg is enough to cause poor running problems. That is not really that high of an ambient temp. As Chris said "gasahol" fuels really need recalibration of the fuel sustem to act properly although your computer should be able to sense that and compensate for it as the o2 sensor does that. down here they sell this crappy desextized gas in the summer that is "oxygenated to reduce exhaust emissions". That szht runs noticably more sluggish and gives noticaqbly lower milage than the "winter blend" gas.

I wonder if you have a temperature sensor somewhere (coolant or ambient air) that is working enough to not give trouble codes but out of calibration enough to make the ECM think that it is much hotter than it actually is and retard the timing to prevent detonation. 80 deg air is just not hot enough to cause an over rich situation due to its lack of density. And besides the ECM should be able to sense that and compensate for it. I would think something is retarding the spark lead. It is always much harder to diagnose a problem when you cant actually "lay hands" on it.

                                                                      the Slab Bacon

[Fred k2dx]

There's an add on piggy back unit - Apex- that allows you to correct the fuel curve. I have heard of good results with it on different forums. Beware of 'counterfeit' units that have been marketed, especially on ePay. The real one can be had discounted, I think around 250. Also c/o Unichip. I've been reasearching what's available should I need something for my Jeep stroker (almost ready to start it at long last).

http://www.apexi-usa.com/product_electronics_detail.asp?id=260&pageNum=1
<img src=http://www.apexi-usa.com/graphics/products/2601.jpg>

http://www.apexi-usa.com/product_electronics_detail.asp?id=202&pageNum=1
<img src=http://www.apexi-usa.com/graphics/products/2021.jpg>

Apex has another unit for VTEC engines, that may be variable valve timing?, your honda may have that. Their new unit does have that feature.

[W1RKW]

Speak of the devil and up he pops. I find it somewhat difficult to believe that in this day of concern for oxides of nitrogen emissions and now cat cons on diesels that anyone would still produce an engine with 11:1 compression. High compression engines are tough to tame what comes out of the pipe. However if it does have 11:1 compression that should make for very nice throttle response.

I find it interesting that only 80 deg is enough to cause poor running problems. That is not really that high of an ambient temp. As Chris said "gasahol" fuels really need recalibration of the fuel sustem to act properly although your computer should be able to sense that and compensate for it as the o2 sensor does that. down here they sell this crappy desextized gas in the summer that is "oxygenated to reduce exhaust emissions". That szht runs noticably more sluggish and gives noticaqbly lower milage than the "winter blend" gas.

I wonder if you have a temperature sensor somewhere (coolant or ambient air) that is working enough to not give trouble codes but out of calibration enough to make the ECM think that it is much hotter than it actually is and retard the timing to prevent detonation. 80 deg air is just not hot enough to cause an over rich situation due to its lack of density. And besides the ECM should be able to sense that and compensate for it. I would think something is retarding the spark lead. It is always much harder to diagnose a problem when you cant actually "lay hands" on it.

                                                                      the Slab Bacon

Hey Slab,

Looked up the compression ratio in the service manual.  Turns out to be 10.2 not 11.

In any case here's how it goes.  I can pull the car out of the nice cool garage drive around a while when it's in the 80's and all seems OK, throttle response is good, until I come to a stop, shut the car off and run an errand.  The sun can bake on the car for 30 minutes or so and that combined with under hood heat seems to make the response decrease. From that point on once it's warmed up the performance changes not radically or terrible but noticable. If that makes any sense.   

Never thought of the oxygenated fuel during this time of year and the behavior for this time of year.  It makes sense.

[WA1GFZ]

I wonder if you could relocate the temp sensor to a can of ice water and make it think the motor is cold so it runs richer. Better yet a pot mounted on the dash.
I never went anywhere without my full set of holley jets just in case.
Then a spring from a ball point pen and a couple of drill bits to add back fire protection to the power valve diaphrams. Oh and yes a spare set of needle valves and tap to add 4 corner idle. Oh yea then the full mechanical secondary parts with a 50 cc squirter in the back. tricked out 3310s 780 CFM stomp lightly or run a big gear.
Best of all drill out the idle circuit air passages so it doesn't stumble and works like a real carb.

Shine limits knock as it leans the motor and removes BAs.

[The Slab Bacon]

Bob

I still think that it may be more related to ign timing than mixture. Retarded timing causes very sluggish throttle response. The given symptoms are definately saying that it is temperature related. If it was more fuel mixture / fuel quality/ air density the O2 sensor in the exhaust pipe should be able to detect it and the ECM should be able to compensate for it.

However with a C/R of 10:1 or higher I'm sure that the software in the ECM is designed to retard the timing at higher temperatures to eliminate detonation. I still feel that it may well be the ambient air temp sensor. If it was the engine temp sensor it should not matter what the outside temp is as the engine's thermostat still keeps the engine at the same temperature regardless of the outside temp.

You may want to try to find the ambient air temp sensor and see where it is physically located. It may be picking up extra heat from the engine from a poor location in design. See if you can borrow a system scanner and hook it up while doing a test drive. See if its reading jives with the known outside temp. If not try like Frank said find the ambient temp sensor and drop it into an insulated container filled with ice and drive the car. If it runs better you know where to start. Either a defective sensor or a piss poor location for its location.
                                          the Slab Bacon

[W2JBL]

when i had these troubles with my Ford, and 9C1 Caprice they never showed any codes. very frustrating.  one often overlooked but critical detail: look for vacum leaks before you do anything else. cars are so lean these days a vacum leak will kick you where it hurts every time. not much in the way of vacum lines on modern engines but they are still there to cause trouble. after i put an intake manifold on my Crown Vic last winter it ran good, but not normal. a small vacum line feeding the HVAC
was leaking at the throttle body. fixed it and all was good.

     one last thing the tree humpers won't tell you about alky- it's CORROSIVE!!! yeah man- it eats many types of O rings, all kinds of aluminum, steel and zinc alloys. in short everything in your fuel system. roundy round racers who run pure alky often flush with gas after the race and keep gas in the system during storage.

[W1RKW]

Hi Slab, Chris, Frank, etc.,
Ambient air temp sounds logical to me.  Would that be located off the engine in the engine compartment or in some other location on the car. Where are they typically located is probably the better question to ask.  Is it possible that this temp sensor is located on the intake? 

I'm looking at my factory manual and it shows two temp sensors on the engine.  One is a engine coolant temp sensor and the other is an intake air temp (IAT) sensor. 

Here's what else it says about the (IAT):  The IAT sensor is a temperature dependant resistor (thermistor).  The resistance of the thermistor decreases as the intake air temperature increases...

Then there's a graph which I'll describe: At  -4 degrees F the approximate resistance is 18kohms.  At approximately 248 degrees F the resistance is 200ohms.  It's almost linear (a very slight curve) with a negative slope.  Does the temp in an intake manifold get up that high?

I wonder if I can fool this thing into thinking it's sucking in cooler air by varying the resistance a little. Would this cause other problems.

As far as being lean I'm not so sure.  When I compare the soot at the end of the tail pipes in all my cars  this car has the sootiest tailpipe.  Also when I get on the throttle hard it leaves a pretty good dark cloud especially when the valve timing changes. Top end is fine it's just low end (1000 to 2500rpm) when it's 80 and above it's behavior is different.

I found a Honda Prelude bulletin board and I posted a note regarding this and apparently everyone who has this particular model complains about the same thing.  So it must be a poor engineering thing or maybe designed that way to limit emmissions that coupled with the crappy fuel we get.

Anyway, that's what I have so far. Thanks for the input guys.

Bob

I still think that it may be more related to ign timing than mixture. Retarded timing causes very sluggish throttle response. The given symptoms are definately saying that it is temperature related. If it was more fuel mixture / fuel quality/ air density the O2 sensor in the exhaust pipe should be able to detect it and the ECM should be able to compensate for it.

However with a C/R of 10:1 or higher I'm sure that the software in the ECM is designed to retard the timing at higher temperatures to eliminate detonation. I still feel that it may well be the ambient air temp sensor. If it was the engine temp sensor it should not matter what the outside temp is as the engine's thermostat still keeps the engine at the same temperature regardless of the outside temp.

You may want to try to find the ambient air temp sensor and see where it is physically located. It may be picking up extra heat from the engine from a poor location in design. See if you can borrow a system scanner and hook it up while doing a test drive. See if its reading jives with the known outside temp. If not try like Frank said find the ambient temp sensor and drop it into an insulated container filled with ice and drive the car. If it runs better you know where to start. Either a defective sensor or a piss poor location for its location.
                                          the Slab Bacon

[The Slab Bacon]

the hot rod set often refer to these cars as "tuner cars", sounds like you have found a good tuning opportunity. If the incoming air temp was 250 deg, with 10:1 C/R that engine would probably run without an ignition system.

See where the IAT is located, and see if it really gets a good accurate indication of what the air temp really is (not influenced by the engine temp). That could be a design flaw. Putting a variable resistor in series with it may just the ability to Fine tune the problem out. If it works, post it on the Prelude reflector and you will become a big hero!! QTF!!
                                       The Slab Bacon

[W1RKW]

I wonder if I just pull the sensor out and plug up the hole.  It's worth a shot to see how it behaves.

[Bill, KD0HG]

Good idea, Bacon!
A trick I used on my former Ford 460 truck was to put a resistor in series with the intake air temp sensor thermistor, which was screwed inside the air filter box. The higher resistance fooled the engine into thinking it was 40F when it was 90 outside. Ran *much*better when it was hot outside. I had added 4-tube headers so a bit of mixture richening is what it needed.

The resistor value is easy to guesstimate, unplug the sensor and stick it in the refrigerator and/or freezer for a while, then measure its resistance. Measure its resistance at room temperature, too. It'll go up when it's cold, go down when it's hot. Pick a value to add in series so it thinks it's much colder outside than it is.
If you go too far with it. it'll set a code.

[N9NEO]

I would try some low oxygen battery cables.