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engine rebuild

 
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After_Thought  



Joined: 08 Oct 2019
Posts: 29
Location: Hudson ohio

PostPosted: Mon Jul 27, 2020 11:22 am    Post subject: engine rebuild Reply with quote

1977 Porsche 924

Pulled my engine today due to worn out piston rings and thinking about doing some performance improvements because i already have weber carburetors.

I am very overwhelmed right now and not sure what exactly i can do with this engine. (very first engine pull/rebuild).

I had the cylinder head resurfaced about a month ago and have heard after i fif it that it could mess up my compression pretty bad. (due to bad piston rings I never really figured out if it did)

(i also have heard that my year car has the worst compression out of the 924's)

I have found some pistons on eBay that ive been looking at but i'm not sure if that is what i need

https://www.ebay.com/itm/4x-Pistons-fits-Audi-100-WA-Porsche-924-XJ-XK-2-0-87-51mm-Bore-Oversize/184372416196?hash=item2aed73cec4:g:glwAAOSwmGde72OU

I have also found a camshaft that might be a little better for my Weber's aswell but again, im not sure if this is a good move.

https://www.ebay.com/itm/Newman-High-Performance-Camshaft-to-suit-Porsche-924-2-0-EA831/233600541328?hash=item3663ad6290:g:zqEAAOSwgLFejYFu

basically what im asking is for someone to help me in the right direction. im not sure exactly what to do and if these parts are even worth the money. Thank you so much for reading this, sorry if i sound like i completely don't know what im talking about.
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GregSC  



Joined: 19 Jul 2008
Posts: 93
Location: Lovely, KY

PostPosted: Mon Jul 27, 2020 11:08 pm    Post subject: Reply with quote

Pm sent your way!
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1981 931
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MikeJinCO  



Joined: 08 Jun 2010
Posts: 926
Location: Maysville, Colorado

PostPosted: Mon Jul 27, 2020 11:52 pm    Post subject: Reply with quote

Modifying these motors can get quite interesting, complicated and expensive, with not necessarily very cost effective results. If someone has a complete motor I'd seriously consider going that way.

The '77 motor is vastly different than later versions. It has 38mm intake valves that were changed to 40mm in the 77.5 version and stayed that way throughout. They also changed the cam and CR from 8.0 to 8.5 for the 77.5.

If you could get a complete newer motor then stick that Newman cam in it, it would make a real difference. I have a 264 degree Elgin cam in my track car and it helps quite a bit. The European cams are made with new blanks so are not undersize and you don't(shouldn't) have to fiddle too much with valve clearances.

I think the very poorly designed and made stock cast iron exhaust manifold contributes a lot to the motor breathing problems(ports in head awful also), grinding it out to match the exhaust and the head can help. A note on the exhaust manifold, check it for cracks as the later ones had more cooling fins. I have a '77 manifold that I improved, then found a small heat crack in it.
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Mike
Poco a Poco, #033 '78 Vintage racer, SCCA Dp-81
'77 924
cricketdesigns.com
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924RACR  



Joined: 29 Jul 2001
Posts: 8334
Location: Royal Oak, MI, USA

PostPosted: Tue Jul 28, 2020 12:35 am    Post subject: Reply with quote

So first of all, no worries about milling the head affecting your compression. That doesn't happen on these motors; none of the combustion chamber is in the head, so it won't get smaller.

Decking the block will raise your compression ratio. It will also change your cam timing slightly.

Yes, the '77 has basically very low compression. The more you can add, the better, more power. Hard to overshoot and ruin it if you run good gas, keep reasonable timing, don't run it lean or overheat. Make sure your cooling system is pristine and up to snuff. 9:1 would be a happy place, or even 9.5:1, though you may need to be looking at premium gas at that point, 91 or 93 or so. Really depends on how much timing you run, and how hot you run.

Cam will be a big step-up too, those early cams are weak, even the late cams aren't amazing.
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Vaughan Scott
Webmeister
'79 924 #77 ITB racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype
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anthonimartini  



Joined: 17 Feb 2020
Posts: 24
Location: portland oregon

PostPosted: Sat Aug 01, 2020 3:23 pm    Post subject: Reply with quote

i build engines for a living. mostly vintage porsche stuff, 911's and whatnot. i gotta say this last 924 engine i built for myself was kinda a nightmare. pistons are not available unless you get custom ones made. so if your bores are good you can just hone/re ring them. if you need overbore you may get lucky and find a set scouring the world or if you have around 1500 bucks you can get custom ones made to whatever specs you need. i got lucky and found a good shortblock that didnt need oversize pistons.
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safe  



Joined: 18 Mar 2017
Posts: 171
Location: Sweden

PostPosted: Thu Aug 06, 2020 12:25 am    Post subject: Reply with quote

anthonimartini wrote:
so if your bores are good you can just hone/re ring them.


Assuming the ring grooves are in spec, which they probably aren't unless its a low mileage car.... which dosen't need a rebuild to begin with...
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