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Old 01-26-2013, 01:04 AM   #1
beatmaniac1
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Is it possible ? Bad compression on SVT

Soooo, I had a compression test on my svt and found that my #3 cylinder isnt building pressure (100-125-85-125 ), I 'll proceed to make a leak down test soon to confirm but I think that there is blow-by around the rings (I saw smoke coming out of the tailpipe when I reved and engine sips some oil ). Seems like a need a rebuild ( my engine as now 160 k miles )

Many reasons lead my to think about a solution that I had never heard of on this forum : Could I use a stock zetec bottom end to replace my svt ? I mean is it physically doable ? I know the crank and block are identical. But I was wondering if the pistons would have enough valve clearance for when the vct would ret/adv the camshaft ? I've seen that the zetec pistons have deeper dishes so, given most of the CR raise is provided by the svt head, they should clear the high lifting valves of the svt head since the svt pistons are basically flat top pistons... Another way to look at this is : as anyone ever used high compression zetec pistons in a build svt motor and still use the vct ? I know I would lose the safety of the forged rods doing this but normal zetec rods have been proven for over 200 hp and I do not plan to boost/race/over-rev this car so I'm not worried about this weakness ( I'd still use arp conrod bolt just for the peace of mind )

Also, is there any gain/loss in the combustion efficiency because of going from a flat top piston to a dome/dishes piston ? Like flame traveling speed or whatever ...

I know i've asked a lot of questions... I've searched for these informations all over the forum but couldnt find it.

Thanks for the replies


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Old 01-26-2013, 08:14 PM   #2
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Moved to "General Technical Chat".
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Old 01-26-2013, 09:44 PM   #3
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Thanks , let's hope i'll receive more inputs **fingers crossed* *
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Old 01-27-2013, 05:48 PM   #4
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You can use the Zetec bottom end, but I fail to see how that would be easier than simply swapping out rings.

I don't know if using a Zetec bottom end with hi-comp pistons would work. Surely if you have one built already, the person who built it with hc pistons would have put forged rods in. Now if you were wanting to build a Zetec while keeping your engine in the car, then taking yours out, swapping the head and intake over, then installing it in the car- I can understand. You'd probably want to used forged rods too, and this method would be more expensive than using your SVT block since it probably doesn't run with only 85 psi on one cylinder. Besides, you don't know if it's pistons that are the problem right now or valves, so you'd have to had the head worked on also.

The only reason to go to a dished piston is for future forced induction plans, so the engine can handle more boost. Forged rods and pistons required- obviously. To go to a domed piston, I'm not sure. You need more of an expert than me. It's possible there is an 11.5:1 set out there that will work with the VCT, or cams can be used that are designed to work with those pistons and without the VCT.

I think this is a question best answered in the SVT forum, not in Gen Tech Chat.
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Old 01-27-2013, 09:01 PM   #5
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Thanks whynotthinkwhynot !! Yeah my point was to build a motor while I still use my car. I'm currently studying in university, 200 miles away from my father's garage so it's not an option for me to remove the motor, order parts needed for rebuild, wait to receive them, rebuild and then reinstall the motor. I need the car running so a week-end swap seems to be the only option for me. Furthermore, if I needed to send the block to a machine shop it would add time to this ( I personnally wouldnt rebuild an engine with a block that has out-of-round bores or non-flat mating surfaces ). Also there are no svt engines for sale in my area. So buying a zetec bottom-end seems for me to be a good option... But I wasnt sure if stock zetec rods/pistons would clear the svt head valves ??

You're right about not knowing whether the problem is my valves or the rings, I'm going to run a leak down test for sure before I'd buy another engine or bottom end (maybe my problem is only a head gasket ). I was only asking for some infos before I'd run into the process of swapping an engine if I ever find the problem to be the rings ( that and a little bit of curiosity)

So basically you were telling me that it was possible if I am not mistaken ?

Thanks for your reply !!
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