GTA twin plug: do it yourself? - Page 2 - Alfa Romeo Bulletin Board & Forums

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Old 11-14-2005, 07:55 PM
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This is great. How about a dedicated thread only for mods that were carried out by tuning companies in period? There must be a wealth of knowledge out there that is invisible until an engine is stripped down and the measuring calipers come out ...

Alex.
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  #17 (permalink)  
Old 11-15-2005, 09:41 AM
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Yes that could be a splendid idea. Three must be a welt of knowledge in what these old modifiers did with Tuning. Unfortunately three is not , to my knowledge, much material related to what they did. we know that Conrero produced a Formel 1 engine but what with the others, Abarth-Bosato-Delta, before they where called Auto delta-Carlo Facetti
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Old 11-19-2005, 07:35 AM
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I have 2 Litre that the plug hole was made smaller and centralized for better burn distribution and to install 46/41mm valves on 8mm stems. You could make it a Twin Plug and put 48mm intakes. Jose 404-374-1813
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  #19 (permalink)  
Old 11-20-2005, 03:27 AM
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Quote:
Originally Posted by alfa-sport
The question is: is it worth the trouble to modify a single plug head to get a twin plug version?
Depends on your reasons for wanting to do it. Nostalgia, performance, or just to have one?
I can appreciate the amount of time and skilled work to do this as I've repaired GTA heads before. Iv'e also run GTA and Twin Spark engines on engine dynos, pulled off one coil and found NO change in power or residual gas readings at higher rpms with wide throttle openings. This says that (with a good piston design), there is enough turbulance in the cylinder for complete combustion with only one plug ; - and it's not in the centre of the chamber either!
At idle and low rpm with small throttle openings, two plugs do help the combustion process; - especially with rich mixtures (hot cams) .
I suspect that much of the "twin plug" theory of years past was because of relatively crude mixture control and lack of understanding about flame propogation. Also the aircraft principle of two completely separate ignitions was a good back-up for variable mixtures and plug quality.
Oh, in the case of the Twin Spark they were chasing idle/low speed emission readings, again, when the cylinder turbulance is minimal.
Larger than 48mm inlet valve can be fitted with just one plug.......
Vince.
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Old 11-20-2005, 05:28 AM
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Thank you for the detailed post. Here in the US most people I have talked with say they get more power from a Twin Spark. A friend of mine owns a twin spark, stock with Jim Steck headders, stock injection in a 71 GTV and he blows the doors of 2.0 cars with 10.4 pistons, 45mm valves, 313 dgree cams, headders,Webbers. This is at Road Atlanta, both street cars, they run sports time runs. The Twin Plug has more torque, be the twin ignition, flat piston or whatever. I have a 2.0 head, not Twin spark and I had to make the plug hole smaller in order to install a 46mm valve since the seat should be 1mm larger than the valve. The easy way, just install 45mm valves on standard seats. Its not as god, flow wise, heat to seat wise, etc. When you install bigger valve seats/valves you will hit the plug, to avoid this, the choices are. Go to a smaller plug, leave in same spot, go to a smaller plug and center, or go to a twin plug and seperate them. There have been discussions on people disconnecting one plug wire from a new Twin spark. They claimed the engine did not run as smoothly. I have talked with a man with a nissan truch that had twin plugs, he said it ran better off idle with the twin plug. This is the reason why I centralized mine, made more sense. This is a hobby and we are trying to experiment and have some fun. If someone can do it, may they enjoy it in good health. Your statement larger than 48mm can be fitted with just one plug. Not the old 2.0??? Maybe one plug but of smaller diameter?
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Old 11-21-2005, 05:12 AM
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In selected "old" 2.0 heads you can fit a 48mm valve/48.25 seat. I've done six or more in the last six months. With a smaller plug you can go bigger.
From what you describe, the normal 2.0 with 313 cams etc, should if all setup and tuned properly, make 175-180 hp. The stock TS with headers certainly no more than 155-158. So something doesn't add up.....
It's hard to compare engines unless you can compare equal set-ups, ie. valve sizes , cams, induction system and measure them on an accurate dyno at the flywheel to test what you really have.
I can say that comparing the the two types as outlined above, the T/S idles more smoothly and up to maybe 3000 is slightly more responsive than a single plug, but over that there is very little in it. Maybe a percent or two, certainly not 20 or 30 hp as many people seem to expect.
I will have a genuine GTAm running next year, all my flow testing and other calculations tell me it will not be as good as my best "old" 2.0 on carbs. I'm hoping that the fuel injection will improve the result. I can't see it going anywhere near the old factory figures of 230hp.
Vince.
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  #22 (permalink)  
Old 11-21-2005, 08:58 AM
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The sole advantage of the modern twin spark engine is the valve angle which is 60 degrees compared to the older traditional 90 degree single and twinplug Alfa head. The modern twin spark engine simply has a better combustion chamber shape/valve port angle. Of course once you spin a 4 cyl Alfa fast enough to get near 200 HP you run up against the very severe life limiting factor of poor main bearing support. This is even worse with the Monoblock type blocks because of the extra machining that removes so much metal just above the main bearings. Monoblock blocks are known to be very flexible and to have very limited lifespan. The Dino Crescentini GT3 car, which had a 260+ HP twin spark engine, used a complex cradle bolted around the perimeter of the block base that pressed (and pinned with dowels as I recall) against the main bearing caps which had a flat spot machined on the bottom of each cap. The cradle was very large and thick and made of 2024 t-6 aluminum (very rigid material) and also served as part of the dry sump oil pan. The bottom end of these engines proved to be very reliable. The top end was a different story.

In short, a traditional Alfa has weak points which are poor valve angles and weak main bearing caps and two spark plugs do not make much of a difference at upper (racing) RPM's.
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Currently: 70 Giulia Ti 2.0L

Formerly:
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Old 11-21-2005, 09:49 AM
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VSharp, one of the reasons for doing this extra stuff on the old heads is because alot of people run Vintage and are "not allowed" to run a Twin Spark head on a 2.0 motor. Vintage Prep, nice past auto's. I agree with the Twin spark. But some people have to play with an old head. The old head I have has 47mm and 42mm for exhaust. I used 46/41 valves so the next time you did not have to trash the head or have someone weld the seat area, machine to re-install 48.25mm seats. I will try and attach some PIC's
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Old 11-21-2005, 01:40 PM
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GAalfa you have obviously taken the old style 2.0 further than most. To go a little further without too much work, how about boring the intake ports at a steeper angle and epoxing pre-bent tubes in the bores and adding carb mounts similar to Horst Kweck's Trans Am car? Also you might want to bring the cooling water into the head from the top where those screw in plugs are if possible (similar to kweck but oposite flow direction) and exit water near the bottom of the block. Would an electric water pump be legal in the class of vintage racing that you do?
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Paul Van Der Linden

Currently: 70 Giulia Ti 2.0L

Formerly:
(1) 58 spider
(2) 67 duetto's
(4) 67 super's
(2) 67 gtv's
(1) 70 gtam
(2) 74 gtv's
(1) 78 spider
(1) 82 spider

Last edited by vintage prep; 11-21-2005 at 04:38 PM.
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Old 11-21-2005, 02:04 PM
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I can't think of any reason why you would want to centralise the spark plug myself.

The plug being so central restricts the valve size possible and I though the original reason why Alfa (Autodelta) went twin plug ... 2 enable larger valves (with the original 90 degree head).

In the end as vintage prep said the flatter combustion chamber of the latter GTAm heads and then the modern twin spark ones has to be the biggest advantage.

While we all spend a fortune worrying about valve sizes, etc. a simple Alfa Sud engine with only 2 valves in a dead flat, non central spark plug will flow enough air to make 200 hp out of 1600cc at 8500 to 9000 rpm ... and yeah I have not the faintest about emissions but she was very good right through the rev range (except maybe below 3000 rpm) and had bags of torque.

I have also seen (on the same dyno) a 1962cc Alfa Nord engine make 208 hp without moving the plug or fancy bottom end, etc. other than baffling/windage tray. Now as that engine was not mine I am not 100% on it being reliable or not. I do not even know if it had fancy rods, etc.

But it is all good fun , and reading all these sorts of threads makes me want to go racing again

I must say if I was going to race a Alfa Nord engine, the first thing I would investigate is increasing the bore so she's like 1999cc ... why give away capacity? and the bore is too small anyway, and has anybody tried to change the valve angle of a 90 degree head a little. With the larger bore you might be able to flatten the valve angle by as much as 5 degrees (assuming the cams might be able to run slightly closer together too). Now this would be massive work but we do know that flatter combustion chambers mean lighter and flatter pistons (thus more of the bang pushes down instead of sideways) and enable steeper inlet ports, etc.

Race preparing any engine in a lot of cases means work against design restrictions/limitations made by the manufacturers ... unfortunately the Alfa Nord engine being such an old design has plenty of really big limitations. It is amazing and a credit to all who develop these motors that they do so well on the track ... maybe these design limitations are overstated?. Then I get back to my little simple Sud engine which goes against ideal combustion chamber theories ...
Pete
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Last edited by PSk; 11-21-2005 at 02:09 PM.
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Old 11-21-2005, 03:33 PM
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Thank you Paul and Pete for your interest and replies. I can tell you guys appreciate good work. The plug I use is smaller than the typical instaled by Alfa. Thats the reason they had to move it to one side so the valve would not hit the plug. Also there were not as many fancy electrode plugs as today. In later engines GTAM etc, since the combustion chamber is flatter in order to install bigger valves they had to move the plug to one side, but it was too far for the other and to get better combustion they went DUAL. I already have an Electromotive dual plu HPV1. "Paul do you have any pic or instructions on these mods"??? The electric water pump would not be allowed. am sure. I made the valves 46/41 with 47/42mm seats. If I were to go to larger valves I would make it a Twin Plug. weld and flatten the area where the plug is, some. Make the plugs go in deeper in the cylinder. Changing valve angle is way to much machining involved. I get enough complaints from the welder and machinist as it it!!! We have no AfaSud in USA.

You can get 86mm bore barrels made and gasket. I have both, but need a Twin Spark piston. Custom Made. You get more like 2046cc. I believe the 208HP on a well built engine. I like doing things a little different and wanted the same distance from one side to the other for the flame! I am selling that head, which has 8mm valves, titanium retainers, keepers and ouil seals with it. This was a project before I decided to go Twin Spark. VSHARP How much do you charge in US$ for a head? Twin Plug or like one in PIC above?
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  #27 (permalink)  
Old 11-21-2005, 04:36 PM
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here is a picture of a kweck engine. There are others in the forum if you search.
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__________________
I was wrong once before,
Paul Van Der Linden

Currently: 70 Giulia Ti 2.0L

Formerly:
(1) 58 spider
(2) 67 duetto's
(4) 67 super's
(2) 67 gtv's
(1) 70 gtam
(2) 74 gtv's
(1) 78 spider
(1) 82 spider
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Old 11-21-2005, 04:58 PM
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Very Nice, thanks. The manifold is similar to my Twin Spark one. I have one like on the cover of Talamakis book. If I were going to use the old engine, I would do this, or maybe if someone wanted to pay me to do it for them. Some people want 45,48. I would do 50DCO or bigger with a smaller chocke like a 42 or 44 on a 50 or 55DCO for the 2.0. You have to be carefull as not to destry the bottom of the cam follower bore. Its not very thick there. I fill the floor of the entrance to the head instead.
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  #29 (permalink)  
Old 11-22-2005, 04:57 AM
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PSk, a two valve 1600cc sud that makes a genuine 200hp? Hmm.....I'll have to think long and hard about that one. I used to work with Brian Hart Lotus twin cams, and I know what it takes to make 180 + out of them. I can't say I've seen anything about the sud that would suggest equalling that. Genuine GTAm "230hp" heads max out at about 190hp on my Superflow, (with the complete intake system fitted). I'll let you know next year what it actually makes at the flywheel, but I'll bet it's ultimately not much different to the flow prediction.
I've also done single plug (84.75mm bore, 1999cc!) and T/Spark engines with heads that flow 215HP with carbs fitted, and they return around 210 genuine hp , measured at the flywheel.
I find that with suitable port filling/re-profiling that an old style head will match a T/S at high lift and is better at mid lifts as the closeness of the cylinder bore to the valve opening path in a T/S creates flow issues on the short turn exit. Pro-rata, you get less gain per mm of valve increase in a T/s than the old head. However, the better combustion chamber shape of the T/S requires much less ignition advance, reducing negative work on the piston, so in the end the result measured at the flywheel is much the same.
Vince.
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  #30 (permalink)  
Old 11-22-2005, 05:04 AM
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VSHAP. How much do youcharge in US$ to do a head. Make a Twin Plug out of an old head or weld the plug hole in and install 46-48mm valves?
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