8mm Dome piston success stories
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baltobernie
- Patron 2020
- Posts: 3466
- Joined: Sun Nov 25, 2007 6:00 pm
- Your car is a: 1973 Spider [sold]
- Location: Baltimore, MD
Re: 8mm Dome piston success stories
I have 8mm pistons with an 1800 head and motor. 40/80 cams. Blew head gaskets every 5-10k. Now running Mark's thicker multi-layer gasket and stud kit. So far, so good.
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evm1024
- Posts: 85
- Joined: Tue Feb 05, 2008 12:33 am
- Your car is a: Pininfarina Spidereuropa 1983
Re: 8mm Dome piston success stories
Thats good to know. I have a milled 2l head and a thicker gasket that Guy Croft recommended to me years ago. I ran all the numbers for the CR but will need to locate my notes.
I think I recall that Brad A. did a paper on CR with various pistons and heads. The 8mm pistons with an 1.8 l was giving 11.5:1 by my (oh so very bad memory).
I do toy with the idea to shave 1,2,3 or even 4 mm off the domes. I did measure the crown thickness and thought that there was plenty of material in the crown.
I think I recall that Brad A. did a paper on CR with various pistons and heads. The 8mm pistons with an 1.8 l was giving 11.5:1 by my (oh so very bad memory).
I do toy with the idea to shave 1,2,3 or even 4 mm off the domes. I did measure the crown thickness and thought that there was plenty of material in the crown.
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vandor
- Posts: 3996
- Joined: Sat May 23, 2009 1:23 pm
- Your car is a: 1971 124 Spider
- Location: Texas, USA
Re: 8mm Dome piston success stories
The ones I measure were 8mm thick at the center of the dome. The manufacturer said that no more than 2mm could be shaved off the top of the dome. I know a guy who shaved off 4mm and the car ran for years after.
Csaba
'71 124 Spider, much modified
'17 124 Abarth, silver
http://italiancarclub.com/csaba/
Co-owner of the best dang Fiat parts place in town
'71 124 Spider, much modified
'17 124 Abarth, silver
http://italiancarclub.com/csaba/
Co-owner of the best dang Fiat parts place in town
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klweimer
- Posts: 550
- Joined: Tue Mar 27, 2012 7:45 am
- Your car is a: 1982 Spider 2000
- Location: Arvada, Colorado
Re: 8mm Dome piston success stories
Another option to reduce the piston head volume is to turn the pistons on a lathe to remove some of the material off the flanks of the center pop-up section. This will eliminate volume without making the cross section of the piston appreciably thinner. The other thing to keep in mind when looking at compression numbers is that the measured compression can vary quite a bit depending on timing of the intake cam. Just 2 or 3 degrees can change the measured compression by 20 or more PSI.
Kirk
Kirk
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Kevin1
- Posts: 399
- Joined: Tue Feb 17, 2009 8:55 pm
- Your car is a: 1980 Spider 2000 FI
- Location: Maine, USA
Re: 8mm Dome piston success stories
Let me throw my 2 cents in here since last winter I rebuilt my engine with 8mm domed pistons and some head work. The engine had 95,000 miles on it when pulled from the car, is a 1980 2- liter with fuel injection. It was a fine running motor, burned about 1/2 a quart between changes, and had low-ish compression (118 avg.) but even across all of the cylinders.
Keep in mind I am not a mechanic, nor do I play one on TV. I am a pretty handy shade tree tinkerer though, and not afraid to dig into a project even if I get in over my head. That's when I call in the real mechanics.... All the machine work on the block was done locally. The head and intake were done by yours truly.
The block was decked just enough to clean it up and square it to the crank and cylinders, the cylinders bored to fit the new pistons (2nd oversize) and the crank journals line-bored. The head is from a 1.8l motor. It has blueprinted valves and seats, and was ported, and the runners were gasket-matched. The intake manifold was matched to the gasket as well. This head was on the motor before the rebuild, and though I don't have a dyno I can say that the head work made a big difference in power and smoothness with the old bottom end. All of these little things add up!
No real magic about the head, other than slightly enlarging the combustion chambers to get a measured 10.17:1 static compression ratio. All of the porting/blueprinting work followed Croft's book pretty much to the letter. The head was refreshed with new valve stem seals, a light touch-up of the seats, and checked for flatness. Valve springs, spring seats, spring caps and keepers are stock. The bottom end was assembled with new aux shaft, rod, and crank bearings, plus new seals and gaskets. The aux shaft was shortened and plugged to prevent interference with the rods, and the crank was polished. Oil pump is the original, it's clearances measured fine. The valves were shimmed and timed before installing it on the motor. All of the bearings were measured with plastiguage before final assembly - I swear I dry-assembled this thing five times before I convinced myself I had everything right.
The only other changes were using ARP studs instead of head bolts, and a multi-layer steel gasket. If anyone out there is rebuilding a twin cam I have to say I really like this stud/mls gasket combination. The cams were replaced with a set of Alquati A24/1 cams, which are, as far as I can tell, only slightly hotter than the stock cams in a 2 Liter. Cam timing was set at 110 degrees with adjustable cam wheels.
Tuning with the L-jet and stock ignition is limited, but thankfully within the range of this motor's needs. In the end my static timing is set at 7 degrees advanced with some mix and match springs (two afternoons of trial and error) for a total advance of 35 degrees which is arrived at by 3000 rpm. Running with 91 octane there is no pinging in normal driving. With 93 I can get away with 9 degrees advance, but 93 isn't available everywhere so I targeted the 91. The tuning, such as it is with the L-Jet consisted of using a digital meter connected to the oxygen sensor to set the idle mixture, plus adjusting the spring tension on the AFM so I wasn't running too lean with the throttle wide open.
Before and after results were very different. The original engine pulled well from 2000 to around 5000 rpm, with power dying off by then. After the first upgrade with the head work and stock bottom end it had pulled cleanly (and with more torque) to 6000 rpm. Now with higher compression ratio and cams added to the mix the motor pulls noticeably harder from 2500 to past 7500 - as far as I had the nerve to try it - but it still was accelerating hard. Bringing the compression up made a huge difference.
The engine now has 1,500 miles on it since the rebuild and I am very happy with it. It drives like a different car. Long-term reliability with the 8mm domes is yet to be seen but I see no indication anything bad is happening inside the engine risking a detonation problem, but time will tell. On a favorite "hillclimb" road I can now make use of 3rd gear whereas before I had to wring it out in 2nd because 3rd was too high of a gear to pull up the hill. Not any more. 3rd gear feels like 2nd used to, 4th feels like 3rd did...etc. I wish I had measured results to compare before and after that I could post for you all, but I would conservatively estimate a 20% improvement in power based on comparisons driving familiar roads, compared to the stock 2L.
On a side note, the cams were not chosen specifically for this combination, they were a set I had on hand so I put them to use. I am now curious if there might be a different set of cams which would work better with a high compression 2L. Anyone have some experience they'd like to share?
To sum it all up, the only down side is that with this particular combination of parts there is more power and a much wider power band to go with it. The only downside in 1,500 miles is the idle needed to be set at 1,000 rpm for smoothness. It will idle at 800 but it is not a s smooth. I can live with that.
Keep in mind I am not a mechanic, nor do I play one on TV. I am a pretty handy shade tree tinkerer though, and not afraid to dig into a project even if I get in over my head. That's when I call in the real mechanics.... All the machine work on the block was done locally. The head and intake were done by yours truly.
The block was decked just enough to clean it up and square it to the crank and cylinders, the cylinders bored to fit the new pistons (2nd oversize) and the crank journals line-bored. The head is from a 1.8l motor. It has blueprinted valves and seats, and was ported, and the runners were gasket-matched. The intake manifold was matched to the gasket as well. This head was on the motor before the rebuild, and though I don't have a dyno I can say that the head work made a big difference in power and smoothness with the old bottom end. All of these little things add up!
No real magic about the head, other than slightly enlarging the combustion chambers to get a measured 10.17:1 static compression ratio. All of the porting/blueprinting work followed Croft's book pretty much to the letter. The head was refreshed with new valve stem seals, a light touch-up of the seats, and checked for flatness. Valve springs, spring seats, spring caps and keepers are stock. The bottom end was assembled with new aux shaft, rod, and crank bearings, plus new seals and gaskets. The aux shaft was shortened and plugged to prevent interference with the rods, and the crank was polished. Oil pump is the original, it's clearances measured fine. The valves were shimmed and timed before installing it on the motor. All of the bearings were measured with plastiguage before final assembly - I swear I dry-assembled this thing five times before I convinced myself I had everything right.
The only other changes were using ARP studs instead of head bolts, and a multi-layer steel gasket. If anyone out there is rebuilding a twin cam I have to say I really like this stud/mls gasket combination. The cams were replaced with a set of Alquati A24/1 cams, which are, as far as I can tell, only slightly hotter than the stock cams in a 2 Liter. Cam timing was set at 110 degrees with adjustable cam wheels.
Tuning with the L-jet and stock ignition is limited, but thankfully within the range of this motor's needs. In the end my static timing is set at 7 degrees advanced with some mix and match springs (two afternoons of trial and error) for a total advance of 35 degrees which is arrived at by 3000 rpm. Running with 91 octane there is no pinging in normal driving. With 93 I can get away with 9 degrees advance, but 93 isn't available everywhere so I targeted the 91. The tuning, such as it is with the L-Jet consisted of using a digital meter connected to the oxygen sensor to set the idle mixture, plus adjusting the spring tension on the AFM so I wasn't running too lean with the throttle wide open.
Before and after results were very different. The original engine pulled well from 2000 to around 5000 rpm, with power dying off by then. After the first upgrade with the head work and stock bottom end it had pulled cleanly (and with more torque) to 6000 rpm. Now with higher compression ratio and cams added to the mix the motor pulls noticeably harder from 2500 to past 7500 - as far as I had the nerve to try it - but it still was accelerating hard. Bringing the compression up made a huge difference.
The engine now has 1,500 miles on it since the rebuild and I am very happy with it. It drives like a different car. Long-term reliability with the 8mm domes is yet to be seen but I see no indication anything bad is happening inside the engine risking a detonation problem, but time will tell. On a favorite "hillclimb" road I can now make use of 3rd gear whereas before I had to wring it out in 2nd because 3rd was too high of a gear to pull up the hill. Not any more. 3rd gear feels like 2nd used to, 4th feels like 3rd did...etc. I wish I had measured results to compare before and after that I could post for you all, but I would conservatively estimate a 20% improvement in power based on comparisons driving familiar roads, compared to the stock 2L.
On a side note, the cams were not chosen specifically for this combination, they were a set I had on hand so I put them to use. I am now curious if there might be a different set of cams which would work better with a high compression 2L. Anyone have some experience they'd like to share?
To sum it all up, the only down side is that with this particular combination of parts there is more power and a much wider power band to go with it. The only downside in 1,500 miles is the idle needed to be set at 1,000 rpm for smoothness. It will idle at 800 but it is not a s smooth. I can live with that.
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klweimer
- Posts: 550
- Joined: Tue Mar 27, 2012 7:45 am
- Your car is a: 1982 Spider 2000
- Location: Arvada, Colorado
Re: 8mm Dome piston success stories
Nice write up on your work. I would agree with your description of improved performance with higher CR and hotter cams. I have Mark's 274 cams for FI and raised the compression to about 10:1. I live at 5000 feet in the Denver area, so everything slows down a little with the altitude, but you get less pinging. I find the car more drivable and much stronger in typical local driving. I think that's a practical performance goal for these cars.
Kirk
Kirk