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Discussion Starter #1
I recently purchased a 74 GTV with a rebuild done some time ago. The compression is strong, 190 to 205, and from searching here I believe it has Alguati 306 cams as the 306 is on the ends of both cams. It has been converted to Webers, 40 DCOE 72/73's. I trying to figure out what I have before striking out on any further changes. I cleaned up one of the pistons a bit and think I can see an AE marking plus 84 but nothing else. So I'm wondering AE?, AE Borgo?, or what? Again I did a bit of searching but found nothing conclusive by looking at other profiles. Thanks for your help, Ken
 

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It looks like a standard piston - nominally 9 to 1.
 

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84 is the diameter in mm. Agree, looks like regular ol' pre-Motronic piston.
Curious if it needed a rebuild, or if you're upgrading? That's good compression, yes.
Andrew
 

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Discussion Starter #5
Thanks guys. I was told it had a "performance rebuild" and was once used at the track some. I agree about the similarities to a stock piston but the valve cut outs look deeper to me and would you pair those Alquati cams w/ stock pistons? Those supposedly have like 11mm lift? I'll try to do a little more borescope work. I'm just trying to get a baseline before I tinker w/ the carbs or ignition(which is still original) Hint: More questions to follow! Ken
 

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would you pair those Alquati cams w/ stock pistons?
The head could have been machined to give a higher CR. I don't know anything about those cams but raising the CR is usually part of the recipe for a higher output engine. My motor is happy with 10.6 to 1 on South Carolina premium gas.

Also, you have emissions Webers. I like them but I seem to be in the minority. I would be interested to know which emulsion tubes are in them, probably F41 or F34.
 

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Looks like the same piston. The above pic includes the liner, at first glance it looks like a small done with a wide squish area. But I see it has the small scalloped chamfer.

Motronic have a flat-topped dome, and 10.4 HC pistons have a noticeably higher rounded dome. Motronic is a good upgrade if you can find them, in general allows more advance to be run that the 10.4.

Andrew
 

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Discussion Starter #11
Ok, educate me on a machined head. Pluses and minuses? Any easy way to measure?

Ed - I think the carbs are kinda standard meaning I think a kit was used for the conversion. I checked the idle and mains and they match to the charts I've seen. I'll be looking at the carbs closer soon. There is presently a cube pump on which I can't find a number. So I just got a regulator and gauge I want to install before tinkering with the carbs. The ignition is still stock marelli. I don't know much but I know all these things work in concert so trying to avoid a piecemeal upgrade approach. Thanks, Ken
 

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As I understand it, the HC 10.4 give you more compression, obviously, but the high dome gets in the way of the flame front, and so the burn isn't as good/effective, and you can't run as much ignition advance. The Motronic piston gives slightly less compression, but the flat piston top allows better burn, you can run more advance without pinging, more power. I got most of this from Norman Racing. I don't imagine it's a big difference, but it's what their dyno testing shows.

Modern pistons, after Honda and Cosworth revolutionized combustion chamber shape in the 60s, try for a most flat surface with "squish" at the eges to force the mixture inward. A narrow valve angle is preferable to a wide valve angle, in part to allow a flatter piston with high HC. The old designs like Alfa and Jag, with 90* valve displacement, require a huge dome to make high compression, and that dome interferes with good combustion.

Here's a Motronic piston when I put them in my GTV 10ish years ago.

Andrew
 

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Better Motronic picture, when I rebuild the 2000 in my race Super, since traded out for a hotter 1600.

I don't have a pic of the 10.4 pistons, but it's a big rounded dome, not flat topped.

Andrew
 

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Discussion Starter #14
Thanks Andrew. I borrowed my stock pic above from this IAP site.

Identifying 2-Liter Alfa Romeo Pistons

I now see AE did make stock pistons so maybe that is what I have with a machined head. Those cutouts did deceive me into thinking I had some older oddball piston I just hadn't been able to discover. You guys have been a big help. Ken
 

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Regarding compression ratio, ignition timing and combustion chamber shape. AFAIK most of the fast race motors are running about 12 to 1 and there is little to be gained by going higher due to the resulting poor shape of the combustion chamber, Andrew thinks that this effect occurs at lower CR's, The pistons that are used in these high compression motors have higher domes than Borgo 10.4's that he does not like.
Higher compression ratios require less advance but more advance does not necessarily mean more power so there may be no disadvantage to running with a bit less advance. I run my 10.6 motor with 34 degrees advance. I tried it at 36 when it was on the dyno. It did not ping but it made no more power than at 34. It lost power when we retarded it to 32.
IMO, for a hot street motor it probably makes little difference what kind of pistons you use provided that you machine then pistons and/or the heads to get the compression ratio that is good for your application. We get 93 octane in SC (currently $2.74/gallon) so I run 10.6. I could run a higher CR if I were running cams with more overlap.
 

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Discussion Starter #16 (Edited)
So I'm back. I've got the engine out and doing more investigation. With Rich's help we determined the head is stock and these are 10.4 pistons. We used some pics to compare the dome width and did the depth test from the top of the spark plug hole. 1.32". This is probably consistent with the Alquati cams and what I suspect is a 15 year old rebuild. We found the AE mark and 3498 on the underside of the piston and if anyone has one handy they could double check that number.

So assuming I'm not crazy to keep these pistons and cams(they don't have many miles on them due to some storage years), I'm making some changes I've discussed with several of you. The valve lash is tight at approx. .008" in and .009" out and I'll increase to .010" and .012". I have changed the cam LC's to 106* and 106* from earlier questionable settings and will now move the exhaust to 108*.

Are these suitable settings now I know the Pistons are 10.4?

As a refresher, running Weber 40's and the stock marelli ignition at the moment. Engine runs fine but a bit rich. Engine out for trans syncros and RMS leak.

Thanks again guys, Ken
 
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