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hat_man 04-03-2020 05:19 PM

What RPM range do YOU get the best mileage at?
 
Is there a general rule of thumb for better/best FE in certain RPM range?

For my Ranger it's around 2k rpm. That's 55 mph in 5th gear. At 1900 rpm the engine really gets smooth but I can't drive that slow on the highway without causing other drivers to have some "road rage".

I know there are a lot of factors that go into FE. Things like speed, gearing, tire size, etc. And that all vehicles are different. Things like weight, aero, drive train (FWD/RWD/AWD), etc. And where we drive them most often, either in town or on the highway.

So where do YOU get YOUR best mileage?

Frank Lee 04-03-2020 11:48 PM

I use 1000-1200 ft/mn piston speed as optimal efficiency zone. I have not personally tested it but several sources that seem legit to me point in that direction.

Ecky 04-04-2020 11:34 AM

As slow as possible. With both my current 2.4L engine and the previous 1.0L engine, it would peak around 30mph and remain nearly constant until ~50mph, where it would begin to drop pretty rapidly.

1.0L engine: 1075rpm - 1720rpm
2.4L engine: 975rpm - 1560rpm

Hersbird 04-04-2020 11:45 AM

You ask a question, then give your answer that doesn't fit the question. You are adding speed into the equation when you are looking for peak miles per gallon or liters per 100 km or however you measure fuel economy. If it happens at 35 mph, then it happens at 35 mph. If 35 mph isn't allowed on say an interstate, that doesn't mean that your peak efficiency now happens at 55 mph.

To answer the question for my TDI touareg I'd say right about 38 mph in 7th gear which puts me at about 1300 rpm and it goes over 40 mpg on a steady level road, and that's with winter fuel. The problem is, I can't force the shift to 7th gear until I'm goin over 40 mph so I have to get it up to 41, shift to 7th, and then let it drop back down to 38. If I go as low as 35 mph it will probably shift back into 6th with any addition of throttle. It still gets pretty good economy at 55 mph and will run in 8th gear there but it's more like high high 30's mpg vs into the 40's. At 65 it's dropped into low 30's

Ecky 04-04-2020 12:36 PM

Unfortunately most vehicles can't divorce RPM from speed, and it seems to me a rare case where going faster in top gear actually improves economy.

California98Civic 04-04-2020 01:30 PM

Quote:

Originally Posted by Frank Lee (Post 620822)
I use 1000-1200 ft/mn piston speed as optimal efficiency zone. I have not personally tested it but several sources that seem legit to me point in that direction.

How do you calculate/track that while driving? Is there a formula? Seems like you'd need a variety of piston and stroke data as well as RPM.

To the OP: my preferred shift ranges are 1500-2000 rpm or 1700-2200 rpm, and I kinda use the latter more often. My fav cruise option on the freeway is 60 MPH @ about 1900 rpm. At that speed I can get high 60s mpg on average. But I often do pulse and glide on the freeway, using 55-65 mph, 55-70 mph, or 60-70 mph speed ranges.

Tahoe_Hybrid 04-04-2020 03:20 PM

Quote:

Originally Posted by hat_man (Post 620794)
Is there a general rule of thumb for better/best FE in certain RPM range?

For my Ranger it's around 2k rpm. That's 55 mph in 5th gear. At 1900 rpm the engine really gets smooth but I can't drive that slow on the highway without causing other drivers to have some "road rage".

I know there are a lot of factors that go into FE. Things like speed, gearing, tire size, etc. And that all vehicles are different. Things like weight, aero, drive train (FWD/RWD/AWD), etc. And where we drive them most often, either in town or on the highway.

So where do YOU get YOUR best mileage?

at 45MPH = 40-45mpg reading on the instant gauge @1,080rpm
3.0L/6.0L V8 engine btw.. once it flips to v8 mode it drops to 25-29mpg but soon will shift back to v4 mode and 40-45mpg..

the best ever was 31.5mpg


I just reset my MPG average it's at 30MPG avg in 4 miles of travel..


the reason for resetting it as my suv have some issues with a dirty power cable causing issues with the engine. =poor mpg my fill up was only 18.9 but i did idle and short trips = lost mpg it probably would have been in the 22-25 mpg range otherwise

hayden55 04-04-2020 06:38 PM

I have found that for large v6 engines of the american style. (3.0-4.X L) anywhere from 1200-2700 RPM are within a couple % of peak BSFC.
I really tested this out on the Ranger when I only had 1,2, and 5th. I would drive anywhere from 1st to 2700 RPM at 90% load, 2nd to 3000 RPM at 90% load, then shift to 5th and hit about 1075 RPM at 30 mph and continue increasing speed at 90% load to 45 mph. (28" tire and 3.73s) Then I would just stay in 5th and cruise around at 45 mph since that is the speed limit here. A little pulse and glide with EOC and what not, DWB up to stop signs. I did note that you get charging issues doing that and wouldn't dare use the starter. I actually had a tank of cruising around town at 25 mpg for about 100 miles on a stock 1996 Ford Ranger 3.0 5 speed with EGR issues, vaccum leaks, and more sensor issues. Been working on getting the transmission back in it now that Quarantine has hit and I have more free time. It has been one hell of a pain in the ass!

*I think skip shift is the way to go on these bigger non 4 cylinder engines. Going gear to gear at 90% load on the bigger stuff is a little too brisk for me versus how slow the old 96 civic was at doing it.

Frank Lee 04-05-2020 01:11 AM

Quote:

Originally Posted by California98Civic (Post 620855)
How do you calculate/track that while driving? Is there a formula? Seems like you'd need a variety of piston and stroke data as well as RPM.

Get the stroke then online calculators can tell you the rpm range that 1000-1200 ft/mn piston speed is. If your car has a tach you are now golden. If not then you need gear ratios and tire size and another online calculator to spit out the ideal speed range in top gear. Incidentally most of my cars don't have tachs so that is how I did it, and they usually calc out to be in the 45 to 50 mph area.

Stroke differences are how a large engine can be putting along and a tiny engine can be screaming and they will both be at the same piston speed and optimal efficiency range.

serialk11r 04-05-2020 02:05 AM

Quote:

Originally Posted by Frank Lee (Post 620922)
Stroke differences are how a large engine can be putting along and a tiny engine can be screaming and they will both be at the same piston speed and optimal efficiency range.

This is a very good approximation at high load, but cars that are geared way too short or have really overpowered engines do better if you run them on the slower side.

Frank Lee 04-05-2020 06:17 AM

Right! It has me going too fast in the F150; there I pick a speed just above where the torque converter would unlock frequently due to slight grades and whatnot.

California98Civic 04-05-2020 08:10 AM

Quote:

Originally Posted by Frank Lee (Post 620922)
Get the stroke then online calculators can tell you the rpm range that 1000-1200 ft/mn piston speed is. ...

With my Honda Civic's 3.54 inch stroke, one of the online calculators suggests that my engine is in the ft/min range you talk about when the engine is turning 1700-2000 RPMs. Interestingly, as I pointed out in my earlier response in this thread, that's one of the ideal rev ranges that I use MUCH the time. Now I might begin to use it nearly ALL the time as an experiment.

Frank Lee 04-05-2020 08:31 AM

All signs point to that piston speed range as a good proxy for when you don't have a BSFC chart.

Years ago I inquired here about optimal piston speed and why wouldn't slower be better (because the calcs showed my F150 so different from the cars and bikes)? There is a sweet spot between too slow (excessive heat loss through head, piston, cylinder walls) and too fast (higher internal friction).

https://ecomodder.com/forum/showthre...peed-1477.html

2016 Versa 04-05-2020 09:57 AM

My Versa with CVT seems to get it's best mileage between 1400-1600 RPM and depending on terrain is usually 40-50 MPH.

Ecky 04-05-2020 11:44 AM

Using Frank's figure of 1100ft/min +/- 100, my engine would be most efficient at 1700rpm +/- 150, or 55mph +/- 5mph.


Something I wonder about, is how ignition timing plays into this.

Here is one of my ignition timing tables, which I built based on maintaining a roughly fixed peak cylinder pressure:

https://i.imgur.com/FSzzMrP.png


When spark ignited, a flame front spreads from the spark plug through the combustion chamber, and it's ideal to have peak cylinder pressure occur when the piston rod has the best mechanical angle to spin the crankshaft. Flame speed is pretty constant for a given fuel and air mixture, so as the piston moves faster (higher RPM), you need to start combustion a few degrees of rotation earlier to keep peak pressure at the same crank angle.

At higher engine speeds, there's a larger percent of of high pressure before the piston even reaches the top (trying to spin the engine backward) and also too late in combustion, where the angle isn't very good. At least, to a certain point. My understanding is that, although the piston moves faster as RPM increases, after a certain point one does not need to ignite it any earlier, and indeed my ignition timing tables suggest this is true. I don't have a firm grasp yet on why this is so.

Piotrsko 04-05-2020 12:51 PM

I recall 1100 fpm as a magic number, but it was related more to a structural limit, either you couldn't aspirate faster, or perhaps fatigue on the rod ends. Been 40 years too long since I was a hot rodder. You may be correct on the ignition limit as I recall there was no point going beyond a certain amount of advance.

MeteorGray 04-05-2020 01:15 PM

I have always figured that after achieving high gear in any car, the lowest RPM value will yield the highest fuel economy.

Ecky 04-05-2020 01:54 PM

Quote:

Originally Posted by MeteorGray (Post 620949)
I have always figured that after achieving high gear in any car, the lowest RPM value will yield the highest fuel economy.

That's probably true with fixed gearing, because load is *probably* more important than piston speed in the vast majority of cases.

But, let's say you have a hybrid vehicle, which can spin the engine at whatever RPM, and toggle it on and off whenever. Is it going to be more efficient to spin the engine at, say, 3000rpm 90% load for 30 seconds out of every minute, or to spin it at 1500rpm 90% load and leave it running all the time?

Hersbird 04-05-2020 07:02 PM

Quote:

Originally Posted by MeteorGray (Post 620949)
I have always figured that after achieving high gear in any car, the lowest RPM value will yield the highest fuel economy.

I used to think that as well but with direct injection, a turbos, and an 8 to 10 speed transmissions I'm not so sure about that anymore. They are building in lower, and lower highway cruise RPMs all the time looking for better economy at 65 mph. With the turbo and direct injection they can get a good BSFC over a big range, but with 2 or more overdrive ratios the spped can get pretty high before you reach "top gear". Then maybe one of the lower gears would have less aero drag while still being at peak efficiency.

serialk11r 04-06-2020 01:09 AM

Newer engines with high EGR dilution can run better at slow speeds since the diluted mix burns cooler and loses less heat to coolant.

redneck 04-06-2020 02:31 AM

Quote:

Originally Posted by hat_man (Post 620794)
Is there a general rule of thumb for better/best FE in certain RPM range?

For my Ranger it's around 2k rpm. That's 55 mph in 5th gear. At 1900 rpm the engine really gets smooth but I can't drive that slow on the highway without causing other drivers to have some "road rage".

I know there are a lot of factors that go into FE. Things like speed, gearing, tire size, etc. And that all vehicles are different. Things like weight, aero, drive train (FWD/RWD/AWD), etc. And where we drive them most often, either in town or on the highway.

So where do YOU get YOUR best mileage?





Yes.

At Zero r.p.m. and coasting... ;)



>

hayden55 04-07-2020 02:09 PM

Quote:

Originally Posted by Ecky (Post 620955)
That's probably true with fixed gearing, because load is *probably* more important than piston speed in the vast majority of cases.

But, let's say you have a hybrid vehicle, which can spin the engine at whatever RPM, and toggle it on and off whenever. Is it going to be more efficient to spin the engine at, say, 3000rpm 90% load for 30 seconds out of every minute, or to spin it at 1500rpm 90% load and leave it running all the time?

This is basically the difference between 40mpg in the Prius and 60mpg in the Prius. If... Its warm enough and the cars warm up protocol doesn't sabotage me.

California98Civic 04-08-2020 02:46 PM

Quote:

Originally Posted by 2016 Versa (Post 620939)
My Versa with CVT seems to get it's best mileage between 1400-1600 RPM and depending on terrain is usually 40-50 MPH.

I think it is important to remember that mileage (fuel Economy) and volumetric efficiency are not the same. You may get your best mileage in specific freeway conditions at 40-50 mph and 1400-1600 rpm. However, Frank's formula applied to your 2016 Versa with its 3.29 inch piston stroke should be moving at a rate of about 1100 ft/min at 2000 RPM. That might be your greatest volumetric efficiency. But if you drove 65 mph or more to cruise at 2000 rpm your aero drag would be MUCH higher and you fuel economy would suffer.

MeteorGray 04-09-2020 06:24 AM

Yep, actual results trump theory every time.

We seem to be seeing this in action as the predictive models are embarrassed by actual results during the current pandemic.

Could Global Warming modeling be far behind? :-)

GreenTDI 04-15-2020 05:25 AM

The best mileage so far, for a long distance (150 miles) was 71 MPG
The second best mileage I achieved for a very long distance (400 miles) was 68 MPG. Those MPG's are real MPG's, measured at the fuel pump.

Both distances were traveled at speeds between 55 and 60 MPH at 1750 - 1900 RPM in 5th gear. The car is not yet modified.

I recon that lower speeds will increase the mileage a little. However, the highest gear is quite long for the tiny three cilinder diesel engine, and lower speeds will make the engine hum with greater load.

The very very best mileage could probably be achieved in 4th gear at 35 - 40 MPH? Less drag at those speeds.

Ecky 04-15-2020 07:39 AM

Quote:

Originally Posted by GreenTDI (Post 621699)
The best mileage so far, for a long distance (150 miles) was 71 MPG
The second best mileage I achieved for a very long distance (400 miles) was 68 MPG. Those MPG's are real MPG's, measured at the fuel pump.

Both distances were traveled at speeds between 55 and 60 MPH at 1750 - 1900 RPM in 5th gear. The car is not yet modified.

I recon that lower speeds will increase the mileage a little. However, the highest gear is quite long for the tiny three cilinder diesel engine, and lower speeds will make the engine hum with greater load.

The very very best mileage could probably be achieved in 4th gear at 35 - 40 MPH? Less drag at those speeds.

US or UK gallons? Care to start a fuel log? Which vehicle exactly?

Wish we had 3 cylinder diesels here.

RedDevil 04-15-2020 08:02 AM

For me it's 1100 RPM.
My Insight refuses to drop RPM below 1100 RPM and the CVT maxes out ('top gear') at 50 km/h per 1000 RPM, so I get best economy driving 55 km/h @1100 RPM.
In nice weather that can be 35 to 40 km per liter (80 - 92 MPG)...!

60 km/h @1200 RPM is hardly any worse though, but it climbs ever faster with more speed. Anything above 80 km/h (50 mph) @1600 RPM hurts a lot.

GreenTDI 04-15-2020 08:51 AM

Quote:

Originally Posted by Ecky (Post 621705)
US or UK gallons? Care to start a fuel log? Which vehicle exactly?

Wish we had 3 cylinder diesels here.


US gallons, I'm converting the km/l that we use into MPG (US) The car is a Skoda Fabia Estate with the 1.2 CRTDI engine, which is a VAG product and similar to the Volkswagen Polo.
I'm certainly going to start a fuel log here. Fact is that I don't drive a lot right now because of the virus lockdown …
I have some fueling data from a couple years ago on spritmonitor.de
https://www.spritmonitor.de/en/detail/465392.html
The car is definitely more economically now than those first 5 years.

Edit: link of my early fuel log in my signature

DGXR 05-05-2020 10:28 AM

In nearly all the road vehicles I've owned, best MPG is around 1500-1600 RPM. That's where the torque curve gets fat, but low enough in RPM to keep the fuel consumption down.

Enki 05-05-2020 02:00 PM

This probably depends alot on overall engine design. While done for reasons other than MPG, my engine is destroked and has large custom cams and much higher than stock static and dynamic compression; yet it seems to run best/most efficient between 2700 and 3000 RPMs (compared to about 2500 RPM stock). My piston design is kind of terrible for DI, though, so I'm sure that's costing me quite a lot of efficiency; I have yet to implement piggyback PI and/or WMI so it remains to be seen if going to a full time hybrid fueling setup can reclaim any of that.

MN Driver 05-05-2020 05:26 PM

The lowest RPM in the highest gear with the engine in lean-burn gets me the best mileage.


If I'm on a long stretch of road I'd rather be going 50+ mph
1000rpm is 30mph and it gets over 100mpg but you lose lean-burn if it drops below 1000rpm and there isn't much reserve torque for anything but a completely flat road.

On a warm or hot day with windows closed and air conditioning compressor off
50mph, 1700rpm, 90mpg
58mph, 2000rpm, 85mpg
64mph, 2200rpm, 80mpg
70mph, 2400rpm, 75mpg
70-78mph can be acheived under lean-burn which has a 65-70mpg(lower at lower altitudes and if you are near 10,000 feet you might see 80mpg as the limit but with less air drag I've cruised in the 75-78mph range at 80mpg on a high density altitude(hot and high altitude, no AC) day) can be achieved on perfectly flat roads and after 78mph the car typically will drop out of lean burn when VTEC kicks in when you are above 2700rpm.

The brake specific fuel consumption is a smaller factor than air drag in a car and for the engine, you want the throttle plate as far open as possible without triggering fuel enrichment, which usually means the lowest engine speed that can accomplish the task. The Honda Insight does this with lean-burn to reduce the pumping losses and keeping the engine at roughly 80-90% load at a low RPM relative to its smaller engine displacement and power output capability. Cars with CVTs can target, although they might not necessarily do this or do it well depending on the CVT, for a specific engine load or throttle position to maximize throttle opening while keeping the engine at the lowest speed possible. If you can achieve a high enough engine load to where the engine couldn't produce more power through throttle opening and there isn't much excess power, you are now in a position where it's better to hold that load than to do pulse and glide(engine-off).

Most engines are overpowered or their transmissions won't bring the engines to a low enough RPM for this to be the result and we need to use engine-off coasting to drive with enough load to add efficiency.

In my Insight, if I use a speed higher than 40mph, I might as well use lean-burn, anything below that and pulse and glide makes sense, but I don't care anymore because gas is not above $3/gallon and my driving below those speeds without traffic behind me is far more uncommon with the driving I do today(pre-COVID) than it used to be. ..now I'm just barely driving at this point

cerec 05-06-2020 12:08 PM

Grand Cherokee Diesel Jeep 2016 mileage
 
Quote:

Originally Posted by hat_man (Post 620794)
Is there a general rule of thumb for better/best FE in certain RPM range?

So where do YOU get YOUR best mileage?

This Jeep, got the best mileage at 57MPH in combined 80 city, 20 Hwy (28MPG)

At 100% Hwy, I achieved 40MPG!

hotshoetom 05-06-2020 04:13 PM

Hi all, I have an 07 Legacy wagon automatic w/lockup torque converter and live in a hilly part of VA and for me with that car 55-60 mph seems to be about optimum, any slower than that and i come out of tc lockup and then mileage really suffers. At 60 on level ground I am at about 2350 rpm. Best mileage under real world mixed driving conditions is 26-27 mpg in mixed mostly highway driving. Iím running 5-30 synthetic oil and factory transmission fluid (synthetic). Any suggestions on how to get better mileage with this boat?

Ecky 05-06-2020 08:00 PM

Quote:

Originally Posted by cerec (Post 623328)
This Jeep, got the best mileage at 57MPH in combined 80 city, 20 Hwy (28MPG)

At 100% Hwy, I achieved 40MPG!

Which Jeep? Those are rather extraordinarily claims.

ksa8907 05-06-2020 08:08 PM

Quote:

Originally Posted by Ecky (Post 623371)
Which Jeep? Those are rather extraordinarily claims.

I also wonder what city has 57mph speed limits...lol

EcoCivic 05-06-2020 08:46 PM

I have not done a specific test but my 05 Civic's engine doesn't seem to run efficiently at lower RPMs. Whenever I try to get more MPG by keeping RPM down such as by locking the torque converter and cruising at 1500 RPM in 4th gear going 35 (the soonest I can without the engine lugging) I have always gotten lower MPG than when I leave it in D3 around town up to a sustained speed of 40+ MPH.

I also seem to get my best MPG at higher speeds. When I was on my way to North Carolina to visit a friend it was raining most of the way there so I was going 60-65 most of the way and I averaged about 33 MPG. On my way back the weather was good so I was going 70-80 MPH at 3000-3400 RPM and I averaged 38 MPG! Sure there are other variables, but these results seem to reflect my experiences around town. I don't have any significant aero mods either, about my only aero mod is I lowered my car by 1.5 inches.

I have a Crower stage 1 cam and other engine mods though, so my results may be different with the stock cam, I started manually controlling torque converter lockup after I installed this Crower cam. I'm thinking that my Crower cam might just not be efficient at lower RPMs, but I'm not sure.

Ecky 05-06-2020 09:38 PM

Quote:

Originally Posted by EcoCivic (Post 623376)
I have not done a specific test but my 05 Civic's engine doesn't seem to run efficiently at lower RPMs. Whenever I try to get more MPG by keeping RPM down such as by locking the torque converter and cruising at 1500 RPM in 4th gear going 35 (the soonest I can without the engine lugging) I have always gotten lower MPG than when I leave it in D3 around town up to a sustained speed of 40+ MPH.

I also seem to get my best MPG at higher speeds. When I was on my way to North Carolina to visit a friend it was raining most of the way there so I was going 60-65 most of the way and I averaged about 33 MPG. On my way back the weather was good so I was going 70-80 MPH at 3000-3400 RPM and I averaged 38 MPG! Sure there are other variables, but these results seem to reflect my experiences around town. I don't have any significant aero mods either, about my only aero mod is I lowered my car by 1.5 inches.

I have a Crower stage 1 cam and other engine mods though, so my results may be different with the stock cam, I started manually controlling torque converter lockup after I installed this Crower cam. I'm thinking that my Crower cam might just not be efficient at lower RPMs, but I'm not sure.

What engine management are you using with your cam and mods?

If you haven't remapped your fuel tables (and to a lesser extent your ignition timing values, as they're fuel dependent), they're probably a mess.

EcoCivic 05-06-2020 09:50 PM

Quote:

Originally Posted by Ecky (Post 623384)
What engine management are you using with your cam and mods?

If you haven't remapped your fuel tables (and to a lesser extent your ignition timing values, as they're fuel dependent), they're probably a mess.

I'm unfortunately using the ECU that came with my JDM Honda Stream VTEC engine and it's not programmable. It's just a stock Honda Stream ECU. There are 2 aftermarket options available for these engines: Kpro and AEM EMS. Kpro isn't an option because I have an auto and AEM EMS isn't an option since I can't afford $1300. Other than that it would be a nice option though, I could use it with my D17 now and use it with a K24 in the future when I swap.

I don't think the mods I did messed anything up too badly though because my MPG is no worse than stock and performance has greatly improved, so it is still an improvement even though I'm probably not getting the most performance or efficiency possible out of my mods.

Ecky 05-06-2020 10:18 PM

Makes sense. I don't want to get the thread too off topic but the issues you can expect to see from changing how the engine breathes are thus:

Big cams (broadly speaking) move the torque curve up. You lose torque at low RPM, and gain it up high. There's no such thing as a cam profile that does both. I haven't looked into the exact functioning of the D17's VTEC system but my guess is that it's either 1) a low and a high lift lobe, and each lobe drives both valves depending on VTEC, or 2) a low and a high lobe, where the low lobe actuates one valve at low RPM and the high lobe the other, and a locking pin allows the high lobe to operate both valves after VTEC engagement. This arrangement is more likely.

More lift and duration degrades torque and combustion quality at lower RPM. This can be seen in extreme cases such as in classic large domestic V8s which have loping, rough idles from their high lift and duration cams. VTEC can minimize this, depending on how it operates, but you'd be trading mid-range for high end torque in a best case.

At low load the engine runs in closed loop. The O2 sensor can compensate for differences in airflow, but the ECU has to relearn the fuel trims on every startup, and you may find it runs rich for a very brief time when changing throttle, as it expects more air. There will likely be some changes to cylinder scavenging if the low cam has changed, which would have a minor effect on what ignition timing is needed.

At high load, most likely the ECU goes open-loop and relies on its fuel tables without O2 feedback. If you're seeing a 15% difference in torque, for example, you'll be running 15% more rich in the mid-range and 15% leaner up high, since the tables won't be corrected and that air would have been moved from one spot to another. If the base target AFR was 13.5:1, that could mean at (for example) 4000rpm it could be as rich as 11.7:1 AFR, and up around 7000 it might be as lean as 15.5:1. I doubt from a cam alone you'd get 15% more high-end, but the principle applies.

My 2 cents, and my word of caution about changing anything past the butterfly valve without adjustment of ECU maps. :thumbup:

EcoCivic 05-06-2020 10:42 PM

Quote:

Originally Posted by Ecky (Post 623389)
Makes sense. I don't want to get the thread too off topic but the issues you can expect to see from changing how the engine breathes are thus:

Big cams (broadly speaking) move the torque curve up. You lose torque at low RPM, and gain it up high. There's no such thing as a cam profile that does both. I haven't looked into the exact functioning of the D17's VTEC system but my guess is that it's either 1) a low and a high lift lobe, and each lobe drives both valves depending on VTEC, or 2) a low and a high lobe, where the low lobe actuates one valve at low RPM and the high lobe the other, and a locking pin allows the high lobe to operate both valves after VTEC engagement. This arrangement is more likely.

More lift and duration degrades torque and combustion quality at lower RPM. This can be seen in extreme cases such as in classic large domestic V8s which have loping, rough idles from their high lift and duration cams. VTEC can minimize this, depending on how it operates, but you'd be trading mid-range for high end torque in a best case.

At low load the engine runs in closed loop. The O2 sensor can compensate for differences in airflow, but the ECU has to relearn the fuel trims on every startup, and you may find it runs rich for a very brief time when changing throttle, as it expects more air. There will likely be some changes to cylinder scavenging if the low cam has changed, which would have a minor effect on what ignition timing is needed.

At high load, most likely the ECU goes open-loop and relies on its fuel tables without O2 feedback. If you're seeing a 15% difference in torque, for example, you'll be running 15% more rich in the mid-range and 15% leaner up high, since the tables won't be corrected and that air would have been moved from one spot to another. If the base target AFR was 13.5:1, that could mean at (for example) 4000rpm it could be as rich as 11.7:1 AFR, and up around 7000 it might be as lean as 15.5:1. I doubt from a cam alone you'd get 15% more high-end, but the principle applies.

My 2 cents, and my word of caution about changing anything past the butterfly valve without adjustment of ECU maps. :thumbup:

You are totally correct, great information! This is a great cam, the idle quality is just as good as stock and I don't feel a loss in low end torque, if anything it feels a bit better. The mid range and top end improved noticeably though. The main difference I noticed is that the car now pulls strong all the way to redline, the power doesn't drop off as it approaches redline anymore. It now feels like it would keep pulling strong past redline if it wasn't for the rev limiter.

IIRC what Crower did is they kept the non VTEC cam lobes the same and they increased the lift and duration of the VTEC lobes so low end torque and idle quality are retained, but the engine breathes better at higher RPMs when VTEC comes on. Or something like that, it's been a while.

Great point about air fuel ratios. My Scanguage shows AFRs, but I don't know how it calculates it or if its reading can be trusted. For what its worth it shows low 12s at full throttle though, so that should be good. I told myself I had enough gauges when I added a dual temp gauge for my oil and trans temp and an oil pressure gauge, but you are making me want to get an AFR gauge too lol.


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