1/2 tooth off timing belt options

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miker
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1/2 tooth off timing belt options

Post by miker » Mon Sep 16, 2019 11:18 pm

I have a 1756cc engine with high compression pistons that ultimately required a thicker head gasket. As a result, my cam timing marks don't exactly line up when the crank is at its mark. It's 'a half tooth off'. The number of teeth between the cams would not have changed from stock.

The car runs fine as-is, with stock cams, later ignition, 36ADL carb, the nice flanged exhaust manifold, although I've never been impressed with the fuel economy.

My question is where should I be putting the 'extra' 1/2 tooth since I don't have adjustable cam wheels. And honestly, it seems like a big project to dial those in on the engine (vs. on a stand) anyway.

I'm due for a belt change this winter, so I thought I'd ask.
MikeR (mirafiori.com since 1995)


1977 Fiat 124 Spider
Previously owned:
2012 Fiat 500 Prima Edizione #236 (now owned by my son David)
'86 Bertone X1/9
'81 Fiat Spider 2000 #236
'78 Fiat 131 four door
'76 Fiat 128 4 door
'74 Fiat 128 4 door
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Re: 1/2 tooth off timing belt options

Post by fp55scca » Tue Sep 17, 2019 12:24 am

—Mike, advance the intake cam 1/2 tooth (counterclockwise) for now. Then order some adjustable pulleys.
—You can actually remedy the situation with one adjustable cam pulley, so you could split a set with someone if you like. You don’t really need two.
—Curious why you needed a thicker head gasket?
Jim Scurria
Norfolk, VA

1972 Fiat 124 Spider
1971 Spider - SCCA FP-24
1974 CSA Abarth Replica
1981 Fiat Spider Ratrod
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miker
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Re: 1/2 tooth off timing belt options

Post by miker » Tue Sep 17, 2019 1:23 am

When Ray Mortensen of Performance Apex in Renton, WA rebuilt the engine in the mid-90s, he built a head with Delta cams (I don't recall the details anymore) with extended valve stems so regular shims would work, and higher compression pistons.

Even with a single carb, there was so much valve overlap that the engine would not pass Oregon DEQ. It was just too dirty. So he made up a stock cam head and I sold the fancy head to a guy in Hawaii.

With normal overlap, the engine passed DEQ but the dynamic compression exceeded what pump gas could handle. I tried everything I could - less advance, no vac advance (I have the later distributor), recurved distributor, colder plugs, etc. But I would ping under load despite all of this. With a single carb or with IDFs.

After 20,000 miles, the head gasket blew between #2 and #3. I measured the static compression in #1 and #4 at about 205PSI.

I ordered a thick head gasket from Guy Croft and installed it, which knocked me down to 175PSI and my engine knock disappeared.

I'm assuming counter clockwise is as seen from the front of the engine.
MikeR (mirafiori.com since 1995)


1977 Fiat 124 Spider
Previously owned:
2012 Fiat 500 Prima Edizione #236 (now owned by my son David)
'86 Bertone X1/9
'81 Fiat Spider 2000 #236
'78 Fiat 131 four door
'76 Fiat 128 4 door
'74 Fiat 128 4 door
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Re: 1/2 tooth off timing belt options

Post by 18Fiatsandcounting » Tue Sep 17, 2019 11:32 am

miker wrote:
Tue Sep 17, 2019 1:23 am
I measured the static compression in #1 and #4 at about 205PSI.
Wow, with over 200 PSI, you've almost got a diesel engine there, my friend! :shock:

-Bryan
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Re: 1/2 tooth off timing belt options

Post by fp55scca » Tue Sep 17, 2019 12:20 pm

—Right Mike, that would be as viewed from the front.

—I see the head gasket issue. Typically, a 2L head will serve to reduce compression a bit as well.

—Just for clarity, there is no “setup” needed for adjustable cam wheels. Maybe you’re thinking of “timing the cams” which is another matter. With adjustable cam
wheels, you just rotate the cams to the timing marks, and then loosen the outer geared section from the center of the cam wheel,?and rotate as needed to accommodate the timing belt teeth.
Jim Scurria
Norfolk, VA

1972 Fiat 124 Spider
1971 Spider - SCCA FP-24
1974 CSA Abarth Replica
1981 Fiat Spider Ratrod
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Re: 1/2 tooth off timing belt options

Post by miker » Tue Sep 17, 2019 9:41 pm

That sounds pretty simple. Some have suggested I need to time the stock cams/crank if I'm using the adjustable wheels. You would seem to disagree.

In the meantime, rotating the intake cam counter-clockwise makes the intake valve open later which might tend to reduce power but improve fuel economy and reduce unburned HC. Right?
MikeR (mirafiori.com since 1995)


1977 Fiat 124 Spider
Previously owned:
2012 Fiat 500 Prima Edizione #236 (now owned by my son David)
'86 Bertone X1/9
'81 Fiat Spider 2000 #236
'78 Fiat 131 four door
'76 Fiat 128 4 door
'74 Fiat 128 4 door
fp55scca
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Re: 1/2 tooth off timing belt options

Post by fp55scca » Wed Sep 18, 2019 11:01 am

miker wrote:
Tue Sep 17, 2019 9:41 pm
--Some have suggested I need to time the stock cams/crank if I'm using the adjustable wheels. You would seem to disagree.

--rotating the intake cam counter-clockwise makes the intake valve open later which might tend to reduce power but improve fuel economy and reduce unburned HC.
---------------------------------------------------------------------
--Mike, if you are using stock Fiat Spider cams, then your lobe centers and lift profiles will be stock as well. All other things being equal, then "timing the cams" will result in stock timing marks. In your situation, you have only upset the relative position of the cams to the crank with a thicker head gasket. The same will occur if you should mill the head, or deck the block; and one remedy is to use an adjustable cam pulley.

--It is only when you change the orientation and lift profiles of the lobes (normally with higher performance cams) that you will need to use a cam wheel to account for new timing marks relative to crank rotation. Even after this is accomplished, then you get into "swinging" the cams, which is to vary the rotation relative to one another. This often results in chasing the torque curve from one range to another, and is best accomplished on a dyno for any reliable results. Mike, for a really good explanation of this, see GC's first book (Modifying and Tuning...) pages 67-71. I think it may still be available online?
Jim Scurria
Norfolk, VA

1972 Fiat 124 Spider
1971 Spider - SCCA FP-24
1974 CSA Abarth Replica
1981 Fiat Spider Ratrod
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