EF enthusiast
 

I bought a 1990 Honda DX (EF)hatchback with an SI swap(D16a6) in 2003 cause I needed a car to drive back from PA to CO. I fell in love with that car.
I did some basic susp. upgrades and was competitive in the local autocross club with it.
That car has great power and still gets 40mpg+ on the highway. One time I drove 80mph+ all the way from Denver, Co. to K.C. and I got 44mpg. There was a huge tailwind the whole way but I often was driving 85 to 90. This is back when gas was cheaper. Just shows the potential of these cars.
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Right now I am driving a '89 hatch
with a d15/5spd that gets between 38 and 44 on the highway,
and it gets about 36 to 38 around town if I drive carefully.
Interestingly this car was regularly getting 46 to 47 overall mpg before I changed the timing belt, which was retarded one tooth on the sprockets. I also had a '90 hatch with a D15 engine that got that kind of mileage for me. I sold it to a friend without ever changing the timing belt, and I am curious to see , when they change it, if it had a retarded timing setting too.
Both my cars
are over 200K and need rebuilds as they both use oil. I really abused the '90, racing it and driving it really hard on the street, and it is the worst(blows smoke). The '89 uses about a quart in 3K, unless I am pushing it hard over Vail or Loveland Pass, then it uses more. But if I just go 50 over the passes it does fine.
I would love to do something with the D15 motor to squeeze a few more mpg out of it.
I'm wondering if I can put a VX (D15z1) head on a D15b, with a VX ecu of course.
Are there any forum threads on this?
And I have another question about rebuilding the D15 engine that I would like to get opinions on. What is the effect of high compression ratios on fuel economy?
I remember reading about Smokey Yunick building a direct injection, straight six, gas motor in the 60's, with high comp. pistons (like 14 to 1), that he put in a Mustang and ran like low 13's in the quarter mile and yet still got 30+ mpg.
I figure Honda put the best compromise, compression wise, in the VX engine, between fuel economy and emissions, but maybe left something on the table when it came to absolute efficiency, as I have always felt that higher compression ratios would give better economy, that is if a driver keeps a light foot on the peddle. Not the easiest task with a sweet sounding high Comp. motor.
Anyway, any info. on that?
I know I'm throwing quite a lot out there for everyones consideration, but I have been thinking on these issues for a while and am really curious about your opinions.
And no, I don't drive peddle to the metal all the time anymore. I try save it for the track mostly. I find myself coasting a lot here in the mountains. It really does help with fuel expenses.
Larry

Re: EF enthusiast
 

Welcome to the site, Larry.

Higher compression ratios increase efficiency and power, the down side is that it also increases the tendency for spark knock. But- since you live at a high altitude, running higher than stock shouldn't be as much of a problem.

I am very interested in your experience with good fuel economy while the cam timing was retarded one tooth. Many ppl say that advancing cam timing will yield better economy.

Everyone does agree that advancing cam timing will move the power band down to lower rpms, while retarding cam timing will give the engine more power at higher rpms. Did you experience this? Could you tell a big difference in the amount of power at various rpms after the cam timing was changed?

I just installed an adjustable cam gear on my CRX D13 engine (10:1 compression ratio) and will be experimenting with it soon.

Re: EF enthusiast
 

Seems like I gained some driveability down low but not a lot of difference.
It seemed to me that if the exhaust is opening late that the power wouldn't be affected too much. But when the intake valve opens late then some power would be lost, especially at high rpms I would think , and maybe a little less fuel used.
Do you have any links to a discussion of cam timing??
I have some friends who build V8s for drag racing. I will ask them.

Re: EF enthusiast
 

Sorry for the delay, but here are some links on the effects of changing cam timing:

End of page 2 start of page 3 for this article https://www.circletrack.com/enginetec...ing/index.html

https://www.aa1car.com/library/camshafts.htm

Here's a graph from https://www.hottr6.com/triumph/CamBasics.html

You can see the different curves for advancing cam 5 degrees over stock (no dots) and retarding cam 5 degrees (lines have dots)

https://www.hottr6.com/triumph/TR6cams/Advance_graph.jpg

Re: EF enthusiast
 

Thanks, interesting articles