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Old 08-17-2012, 10:08 AM   #1 (permalink)
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sc1 cam timing

I was all ready to tear into the engine and advance the cam timing 5 to 7 degrees. The rational was since the engine in turning 10% slower than stock due to the large tires, that advancing the cam would allow allow me to lug the engine more. This would reduce frictional losses, since they are proportional to rpm squared, reduce pumping losses since I'd be lugging at open throttle and move the bsfc minimum point down, however after researching Google on the subject I am not sure if advancing is the right way to go.

TDI diesels and Cummings turbo diesels seem to respond to advancing cam timing with better low end power and better mpg, 2 mpg for the truck.

Ford 460 engines are reportedly peppier and have better mileage with the cam advanced 5 degrees. Many chevy 350 enthusiasts advance cam timing for street use.

AndyMan on the other hand has has retarded cam timing on 4 Hondas with favourable results.

Toyota Hybrid engine has a modified Atkinson cycle.

Analysis:

1) Diesels are not limited by pre-ignition.

2) Large low compression engines, 8.6:1 probably benefit from advanced cam timing.

3) When warm, my sc1 engine knock sensor is actively retarding the timing at low rpm and full throttle so the reduction in effective compression ratio due to late intake closing would not effect mean effective cylinder pressure.

4) Opening the exhaust valve later could increase efficiency or increase pumping losses.

5) On the highway, at 65 mph, the 2 liter engine is at 2040 rpm and on flat ground very little throttle is needed, due to the aerodynamic improvements (http://ecomodder.com/forum/showthrea...ods-15529.html) so retarding the cam timing should reduce pumping losses.

6) The right way to would be to mill the head to increased compression ratio and retard cam timing, but Saturn engines are a bitc_ to work on with the timing chain cover, not simple like old Honda engines. I may take a trip to the junk yard and scavenge a head.

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Last edited by arcosine; 08-17-2012 at 10:28 AM..
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Old 08-17-2012, 07:00 PM   #2 (permalink)
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ever do any more work on your modifying a thermostat to run hotter idea?

i thought it sounded easier to do, and/or reverse if need be.
i had shelved the idea myself because my car seemed to idle rough when it was up over 200deg, then recently the rough idle progressed and i figured out i had an intake leak. after repairing it idles fine at high temp. so i'm thinking it caused a noticeable difference in idle quality if i was at 200deg vs 185deg, because the engine uses enough less air at 200deg. that the leak would cause that cylinder to lean out enough to miss.
long story longer - i'm thinking about this idea again myself and it seems like it should be a sure thing, mpg improvement wise.
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Old 08-17-2012, 08:13 PM   #3 (permalink)
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I did on my previous saturn , a 97 sw1. I trimmed the plunger until it opened hotter, in a boiling pot on the stove with a thermometer. I don't remember the temperature. Warming up the temp would overshoot, then come back down when the thermostat opened.
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Old 08-18-2012, 12:09 PM   #4 (permalink)
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Well I change the oil to 0w20 today and I took off the valve cover. True to saturn form, the cam sprocket bolt is impossible to get to without taking the engine out. However it looks like it would be possible to deactivate one cylinder by removing the rocker arm shaft and sliding two rocker arms over. Something would need to be done to hold the lifters in place. I have an old injector that I could plug in as placebo. Doest anyone know if it will run this way without an error code?
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Old 08-18-2012, 01:29 PM   #5 (permalink)
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might not set an injector code doing that, but with a cylinder disabled your check engine light is going to be on because it will think it has a misfire
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Old 08-18-2012, 02:27 PM   #6 (permalink)
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I don't think so, the spark plug is still firing. I will run like it is misfiring, but there no sensor for that.
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Old 08-18-2012, 03:56 PM   #7 (permalink)
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crank sensor - i thought they were detected by the ecu seeing a dip in rpm
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Old 08-19-2012, 12:47 PM   #8 (permalink)
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sc1 - '98 saturn sc1
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rans - '97 rans tailwind

tractor - '66 International Cub cadet 129

2002 Space Odyssey - '02 Honda Odyssey EX-L
90 day: 28.25 mpg (US)

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That could be...

I went to the junk yard and looked at an engine. It is straight forward to remove 2 rocker arms. It needs something to keep the lifters from banging on the cam lobe, I thing an o-ring stretched around the lifter body would hold it up nicely. It look like the twin cam engine cam sprocket bolt is accessable unlike the single cam engine. I suppose it could be made to run with 1 intake valve opening.

I also got a thermostat to modify.
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Last edited by arcosine; 08-19-2012 at 01:36 PM..
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Old 08-20-2012, 06:06 AM   #9 (permalink)
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You can do just about anything with the engine in the car, but it would be easier with it out, for sure. On either engine, you'll probably have to get under the timing chain cover to adjust cam timing with the stock gear (moving it by one tooth). You'll probably have to release tension on the chain to do that, which means getting to the tensioner that sits about at the top of the crank pulley.

It can be done with the engine in, but it's a pretty huge pain. I rebuilt my DOHC engine in the car, and the timing cover is by far the worst part. You have to reseal it with RTV when you put it back on, and it's nearly impossible to slide it in without making a huge mess with that stuff. If someone made a premade rubber gasket for that cover, our cars would be the easiest things in the world to work on.
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Old 08-20-2012, 09:12 AM   #10 (permalink)
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sc1 - '98 saturn sc1
Team Saturn
90 day: 43.17 mpg (US)

Airplane Bike - '11 home built Carp line Tour

rans - '97 rans tailwind

tractor - '66 International Cub cadet 129

2002 Space Odyssey - '02 Honda Odyssey EX-L
90 day: 28.25 mpg (US)

red bug - '00 VW beetle TDI

big tractor - '66 ford 3400

red vw - '00 VW new beetle TDI
90 day: 58.42 mpg (US)
Thanks: 17
Thanked 297 Times in 201 Posts
Yes I have done the same dohc rebuild in the car, the cover is a pain, no room. The cover covers almost all of the cam sprocket in the sohc engine, the cam sits lower in the head.

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