Cam belt drive

Maybe you're squeezing all you can out of a 998, or going large-bore with a 1380 (or even larger!) - either way, let us know all about it here!

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Re: Cam belt drive

Postby ACDodd » Sun Nov 11, 2012 8:05 pm

Best I managed on a Mk2 Midas was 58.5 mpg @65 to 70mph Over a 200mile run, Satnav figures) from a 1380 1.5 rockered Twin SU economy engine. It also used to return a regular 45 to 47 on the trip to work. This engine used a custom curved 65Dm4 dizzy and custom needles.

The bottom line is you need some sort of mapped ignition to get the best fuel economy and you need proper custom needle, none of this filed rubbish, a proper machined item that produced the fueling where you need it. It is perfectly possible to get fueling at around 1 to 1.5 % CO for cruise then richening to 5.5 to 6% when you floor the pedal. How do I know? This is the aim when I setup a car, on a rolling road. In 95% of cases, it won't be done with an off the shelf needle though. Having a vacuum gauge and using it while cruising is a must.

If any one is interested, an economy engine build can be supplied by MLmotorsport 01474 825123 for more details, and tell them I told you to call. The bottom line is though fuel economy is only as good as the driver.

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Re: Cam belt drive

Postby ACDodd » Sun Nov 11, 2012 8:13 pm

For the record I don't rate the cambelt drives the seals go all to often. I prefer the Duplex setup. Budget versions are perfect until you get to a competition motor.

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Re: Cam belt drive

Postby Rich » Sun Nov 11, 2012 10:56 pm

Rich wrote:I'm running a ported 1330 with a 266 and a HIF 44 carb, on a run I see 55mpg and that's with an electronic speedo/odometer that is dead on. The only thing that's any different is I'm also running Megajolt.


Oh, and ACDodd did the needle together with the rest of the engine... ;)
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Re: Cam belt drive

Postby lankyjames » Thu Nov 15, 2012 2:15 pm

I don't understand the need for high-lift rockers if you are already changing the cam for something with high lift. I was always under the assumption - standard cam + high lift rockers, as an easier alternative to changing the cam to something sporty.

With a sportier cam it provides greater lift etc and high lift rockes arent required - will change the profile of the cam further. I've also read various and varied accounts of such rockers creating greater wear on followers and lobes.
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Re: Cam belt drive

Postby Hans Efde » Thu Nov 15, 2012 3:50 pm

"Sporty" camshafts have different model lobes. They change the timing of the valves. When they open and when they close. This is most noticable in the way the torque curve runs in relation to the rpm. If you go from a 1.25 to a 1.5 rocker shaft, it increases the valve opening which will give more bhp. At least this is the theory what I learned. My engine gets a medium sport cam, the Swiftune SW5 and has 1.5 rockers. Combined with a stage 2 head and a 1st oversize I hope to see 90bhp.
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Re: Cam belt drive

Postby ACDodd » Sat Nov 17, 2012 11:02 am

1.5 rockers increase the valve duration that the engine actually see's, ie at any given crank degree the valve is open much further, or to put it another way the valve reaches the same lift point earlier in terms of crank position. There is also an increase in the valve lift during the overlap which is the aspect that increases the camminess and lets the engine rev harder for a give cam type. Also the total lift is increased in relation to the the overlap lift. This causes a small widening of the total available power band. The secret to make 1.5 rockers work on a road engine is to use them with a road cam with minimal overlap lift, funnily enough, just like the SW5. Older designed cam are all predominantly quite high in overlap lift, this fitting 1.5 rockers together with these cam produces lopey idles high emissions an d poor low speed running.
For my own road engines use my own cam profiles and recommend the use of a 1.3 rocker. This reduced the surface loading on the cam and follower, which I have found to give a better service life and maximises torque between 2 & 5Krpm. For me 1.5 rockers are used for competition spec units where maximum engine performance is needed and reliability is not an issue.

To sum up the 1.5 rockers work very well on the 5 port A-series as long as they are matched to the cam and the engines/users requirements.

For more info about my cams please see below;

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For a better image see the link;

http://img.photobucket.com/albums/v311/ACDodd/Camsheet.jpg




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Cam belt drive

Postby b1zbaz » Sat Nov 17, 2012 12:23 pm

So what's wrong with a standard 1275 a+
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Re: Cam belt drive

Postby ACDodd » Sat Nov 17, 2012 2:06 pm

Nothing wrong with a standard 1275 A+ engine.

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Re: Cam belt drive

Postby Hans Efde » Mon Nov 19, 2012 11:40 am

max midas wrote::shock: WARNING!! :o
If you're going to put a timing wheel on your crankshaft pulley so you can install megajolt/squirt/Emerald/Canems/etc,
DON'T fix the trigger wheel before you've decided the most sensible place for the VR pick up.
I did :roll: and it's taken ages trying to make a bracket that has the VR sensor in the right place, but doesn't foul the water pump/alternator belt.
Perhaps the pictures show better than my words:
http://i1273.photobucket.com/albums/y40 ... ELARCH.jpg

I've made this rather complicated bracket to hold the sensor in the right place (pensil hanging over the side gives correct diameter of trigger wheel)
http://i1273.photobucket.com/albums/y40 ... 012229.jpg

hopefully the belt won't catch, and the VR sensor is supported in two dimensions so it shouldn't vibrate and break.
http://i1273.photobucket.com/albums/y40 ... 012232.jpg

Any comments or suggestions?


I had a discussion with my engine builder about how and where to fasten the sensor and fastening the timing wheel in what position. I asked Emerald and this is their advise:

QTE
Hi
The wheel position depends on where you fit the sensor. The correct position is when the first tooth after the gap aligns with the sensor at 90 degrees BTDC. Therefore, fit the sensor, set the engine to 90 degrees BTDC then align the wheel with the first tooth after the gap opposite the sensor. It does not have to be exact as you can move the reference in the software. The idea is that crankshaft speed is most stable at 90 degrees BTDC.
Regards,
Dave.
UNQTE
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