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Streetracing.co.za • View topic - HIgh Lift cam - valve clearance.
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 Post subject: HIgh Lift cam - valve clearance.
PostPosted: Mon Feb 11, 2013 9:13 am 
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Hi Gents.

I am looking at ordering a race spec cam for the GX and I know that this spec cam from Lambco has a 2mm higher lift than the standard cam I am unsure of the duration of the cam at this stage.

Now the big question:

I have flat top pistons in that block with no valve pockets as it is Mazda pistons and I know at TDC the pistons are level with the top of the block (they might protrude 0.25mm or 0.5mm)

I also know that the valve can not possibly be fully open when the piston is TDC (or am I wrong) so how does one measure or determine that the valves will not smack the pistons with this new high lift cam?

This is a push rod motor so no use taking the head off, an putting the straight edge across the combustion chamber to see if the valve will touch the ruler as the cam sits in the block......

Please help a boer that is trying to play Mechanic in the Garage :oops:

mossie.

I know you guys love pics so here is a old pic of when we build the motor please ignore the 4 valve pockets as they are for the 16V mazda head.

IMG_4748s.JPG
IMG_4748s.JPG [ 189.3 KiB | Viewed 13307 times ]

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 Post subject: Re: HIgh Lift cam - valve clearance.
PostPosted: Mon Feb 11, 2013 9:23 am 
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u have 2 putty fit the motor together and turn it over by hand 2 see how much clearance u have!

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 Post subject: Re: HIgh Lift cam - valve clearance.
PostPosted: Mon Feb 11, 2013 9:46 am 
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hmmmmm,,,,, so put window putty ontop of the piston, fit the head, turn the motor, take the head off and see how deep the indents are....

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 Post subject: Re: HIgh Lift cam - valve clearance.
PostPosted: Mon Feb 11, 2013 9:49 am 
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 Post subject: Re: HIgh Lift cam - valve clearance.
PostPosted: Mon Feb 11, 2013 1:59 pm 
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 Post subject: Re: HIgh Lift cam - valve clearance.
PostPosted: Mon Feb 11, 2013 5:40 pm 
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 Post subject: Re: HIgh Lift cam - valve clearance.
PostPosted: Mon Feb 11, 2013 5:40 pm 
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What engine btw ?

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 Post subject: Re: HIgh Lift cam - valve clearance.
PostPosted: Mon Feb 11, 2013 9:45 pm 
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 Post subject: Re: HIgh Lift cam - valve clearance.
PostPosted: Mon Feb 11, 2013 9:47 pm 
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A15 crank? Same as the E15 langley crank?

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 Post subject: Re: HIgh Lift cam - valve clearance.
PostPosted: Tue Feb 12, 2013 1:06 am 
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Just make sure the cam timing is 100% right or you can bend the valves while turning it.
We bend my brother's BMW valves like that and had to order new valves.
That valve just have to nip the piston to bend it.

Can't you pocket the piston for the 8v setup to make sure it will not touch?

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 Post subject: Re: HIgh Lift cam - valve clearance.
PostPosted: Tue Feb 12, 2013 6:28 am 
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We can pocket the pistons but that means pulling the motor apart and I dont want to do that.....

the A15 crank has 5mm longer stroke compared to the A14.....

We never got the A15 Motors in SA but they are plenty overseas....

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 Post subject: Re: HIgh Lift cam - valve clearance.
PostPosted: Wed Feb 13, 2013 1:50 pm 
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Hi Guys

Another Question.

I have always set the valve clearance on the GX to 0.35 When I put in the race cam does the clearance stay the same or do I have to make it bigger or smaller?

mossie

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 Post subject: Re: HIgh Lift cam - valve clearance.
PostPosted: Wed Feb 13, 2013 2:12 pm 
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The people who cut the cam should have given you the clearances with the cam. Maybe contact and ask them.

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 Post subject: Re: HIgh Lift cam - valve clearance.
PostPosted: Wed Feb 13, 2013 2:28 pm 
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 Post subject: Re: HIgh Lift cam - valve clearance.
PostPosted: Wed Feb 13, 2013 2:36 pm 
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It shouldn't change by much if it does, I'm just used to them giving you specs with the cam. the 0.35 should be ok.

I find it weird that a valve that doesn't seal properly would cause a flat spot, I'm more thinking dizzy and/or carbs.

Also, how do you check the CR? remember some of the compression will leak past the rings. You won't get the actual CR measuring unless you calculate cylinder and combustion chamber volume and use that to determine what you CR is. Dynamic CR is always lower than calculated CR, because of air resistance to flow and stuff like that plus the leaking of gasses past rings.

EDIT: If they all measure the same with the compression tester then they will be ok, no way all 4 cylinders' valves won't seal properly unless the guy who did the machining is a complete poephol.

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 Post subject: Re: HIgh Lift cam - valve clearance.
PostPosted: Wed Feb 13, 2013 2:50 pm 
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it seems like when the flat spot happen it only runs on the the front 2 cylinders and that prompted us to do the compression test. but all 4 cylinders have different readings and are way to low they should be between 10 and 11 to 1.

I have flat top pistons in there so the compression should be higher than STD (9.5:1) or atleast standard.....

I know the engine builder put new "vw Golf" valve guides in to the head but he never had the valve seats cut so I think those valves are not seating and throwing away the compression......

well I am hoping it is that as I dont want to rebuild the bottom end again and she is not smoking via the exhaust and is also not breathing at all.... ( I dont have any breavers connected to the tapped cover or the block at this stage so would see any thing immediately) and the plugs came out perfect.....

so that points the flatspot to maybe ignition... but the low compression can only be valves in my mind.......

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 Post subject: Re: HIgh Lift cam - valve clearance.
PostPosted: Wed Feb 13, 2013 3:00 pm 
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But dynamic CR will be lower than calculated CR...

How do you distribute the fuel to the two carbs? is the rear one not maybe starving of fuel?

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 Post subject: Re: HIgh Lift cam - valve clearance.
PostPosted: Wed Feb 13, 2013 3:01 pm 
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 Post subject: Re: HIgh Lift cam - valve clearance.
PostPosted: Wed Feb 13, 2013 3:04 pm 
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 Post subject: Re: HIgh Lift cam - valve clearance.
PostPosted: Wed Feb 13, 2013 3:07 pm 
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I have a facit pump that is split at the carbs with a T-pice....

I get the flat spot even when we rev up the motor in neutral......

I dont know anything about engines but I thought if you screw in the compression gauge in to the plug hole and push the throttle to WOT then crank the motor that the needle on the gauge should go all the way up to the green on the gauge after a couple of seconds of cranking.... bit it sits on the edge between red and green (low)......

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