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Compression suggestions anybody?

2416 Views 17 Replies 6 Participants Last post by  nobody
To start at the beginning my 1.9 motor is apart to fix an oiling problem. It is pretty much still brand new. I have 9 to 1 pistons, dual sidedrafts, 290 435 cam and 2.0 valves. The head was completely done when it was built the first time too. Last week I got this wild idea that I wanted those high compression pistons that were on Ebay, the 2 Arias. I talked to Arias and they will make 2 more for around 250. The idea and question is this. Do I put these in this motor now while I have it apart? I'm not tied to this block or crank I could do another for the same cost, roughly 25 a hole. The Arias's are 12 to 1 I know I can feed them with the setup I have. This car will never be a daily driver, but can I power it on pump gas? Have I gone completely nuts and need the dinner jacket that goes with that? Or should I go for it, and what kind of bump in power? 7.6 to 1 was 78 horse 9 to 1 was 102 what would 12 to 1 be?
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I'm pretty sure the curve is different but for the sake of just using ratios, the hp would be around 152 if an increase of 1.4 on the pistons provided 24 hp then an increase of 3.0 would translate to roughly 50 more hp.
Figure a 4% increase in power for every point of compression increased up to 10:1. After 10 to 1, the returns begin to diminish. By 12:1 you're only about 2% more than 11:1. However, you will have to retard the timing significantly on pump gas, and will lose more than that in power. I'd cap it at about 11:1 max. Unless you put a bigger cam in, with somewhere in the neighborhood of 255 @ .050" duration. Otherwise, too much cylinder pressure.....

I've run 11.2:1 with 93 octane pump gas, but it did NOT like hot days, and just adding 2 degrees more ignition timing made it ping like hell. That was with a milled 1.5 head and flat-tops, while domed pistons and a larger combustion chamber are not as detonation resistant, and will kill flame travel as well (less combustion efficiency and hp). You balance a fine line here.

BTW, European 1.9's were rated at 90 ps (about 88 sae hp), the 102 hp rating was a gross US rating, and very overly exaggerated on Opel's part! So the difference in power between the early and late engines was not that great, with the cam making more of the difference (solid lifter cams had 9 degrees more duration).

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Ok so say I cap it at 11 to 1 and 40 over, where do you suppose it might put me as far as power? Your right with this cam I only have 230 at .050 so that will have to be it unless I decide to do more work with this head. Doing the math puts me around 140, is this correct? I guess the other question is spring bind if I do change the cam. How far can you go before that becomes an issue with stock springs? It does sound like I need to be fitted for the dinner jacket with the fancy ties in the back.
I'm running the 11:1 Venolia on pump gas with octane boost. I did remove the vacume advance and limit the total advance to about 32 deg, if I remember right.

I still have an extra set of Venolia's that I never sold....if you're interested.
If you're running 40 mm sidedrafts with 30 or 32 mm chokes, you should be at around 135-140 hp with a small valve head. With bigger valves and some serious porting you might see another 15 hp, but after that you'd need bigger carbs and cam.

Stock valve springs (without shims) will bind at somewhere between .450"-.460" valve lift. So with .430" lift, minus valve lash, you'd have about .415" true lift at the valve, which is safe. If you're running the late-model valve springs (hydraulic type), they're actually the stiffest valve springs Opel used on the US 1.9's, at around 180 lbs of open pressure @ .400" lift. A fresh set of these springs will float the valves at about 7800 with a stock Opel hydraulic cam (yes, I've stupidly actually tried this on numerous occasions...)

If you need stiffer valve springs and higher rpms safely, I've found that you can use 2.0 litre valve spring retainers (stronger than 1.9's by a LOT), and the exhaust spring rotators from a 2.2 or 2.4 head, then use an off-the shelf Pioneer valve spring that fits intake and exhaust seats, and will have nearly stock seat pressure (easy on the valvetrain), but around 210 lbs of open pressure. Used to run these on 'stock' class Opel circle track engines, and revved them to 8200-8400 regularly.

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i would give James a call
i have an Arias piston in my hand that came out of my Race GT

it says Alcoa and Arias underneath also has the number 375

they where supposed to be 12 to 1 popups but when i measured it cam out to 10.4 to 1

Have a real close look at those pistons before you buy its been since last week since I looked at them but my spark plug notch was on the other side of the piston on my Arias pistons.

I did look at them real close and also compared them to a known 11 to 1 by Arias. The outer radius on the pop up is steeper and the plug cut is deeper, other than that they are the same. I also talked to Arias to verify a few things, price of the other 2 and so on. The more I look at it, now is the time to do something. I'll probably leave the flat tops alone and set that block aside for now. Start with a fresh one and fit pistons to it. I can go the Arias route for 350 or 400 or do something else like that. Doing the math on it says I can do horses around 8 to 10 bucks each with the way the motor is now. I just have to buy 3 or 4 dozen, for 4 cylinder power thats cheap. James I PMed you, I am interested.
no problem just trying to be helpful

when i talked to Arias they acted like they never herd of Opel

they wanted me to send them a piston

do you happened have a phone number maybe I talked to the wrong person.

Arias talks to everybody that way, especially the foreign car crowd. I've always had great luck with them. My first motor I had done through them out in California. When I bought the tubo GT in Denver I had new pistons and rings for it before I got the car home. When I saw the ones on Ebay I couldn't help but have fond memories. I paid 31 including shipping for them, 2 more could be 125 each from Arias. On the ones you have what is the piston top to top of dome height? Also another good dimension to know would be pin bore top to to piston top. Thanks. Oh and I just got thier 800 number off the web, it helps when they start acting stupid, if you talk to them as if they were. They get smart real quick.
i just went up on there web site

Think I will spend my money elsewhere.

the best i can tell my piston dome is 0.314

the compression ht looks to be 1.7835 that's eye balling the stuff

intake valve relive is .241
ex is .183

not sure if i am measuring the valve relief's properly

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This is special, after seeing those numbers I got out the calipers and did some checking. Top of pin to top of piston is 1.352 and the dome is .234 over that. Just to satisfy my curiosity I checked a stock piston, they are 1.3 from top of pin to top of piston. If memory serves me stock piston are flush with the top of the block. I checked a new head gasket and it is .042 uncompressed. That gives me a .010 or better press fit for the piston into the head. Anybody want a really cool ash tray? Oh I checked and the piston won't fit into a head either without serious grinding around the spark plug hole.
Deck Height

The only accurate way to check deck height is with the piston in the bore and on the rod, all tightened up. If it is above the deck only a wee bit - 1mm or so (.040") then it is far easier to recitfy than if it is down the bore as it is much easier to remove material from a piston than not have enough! Once the two you have are checked out then the revised specs can be communicated to Arias for the other two. Just don't hack at the piston tops with a Dremel or some such. They need to be held in a piston vice and accurately milled, both the same.

This looks to be a serious redesign of the dome and total heights. Comparing it to an 11 to 1 it appears to be more like 10 or 10.5 rather than 12. It was an interesting idea but if I'm going to go to that length for the first 2 then the price for the second 2 will be higher as well. It would be more cost effective to go with something else that will fit or redesign on paper and have them built. I already contacted the seller about them not being Opel as advertised. I'll see what happens or they do make a good looking paperweight too. I don't have enough money in them to get mad, just dissappointed maybe.
even if you go the route you originally planed i don't think 10.5 to 1 is to bad for a street motor

There is a scan of a Dyno run from my race GT in the PIC section (if you can figure out how to get to it) with Arias pistons and they worked great.

For street applications I think one would be very happy with 10 to 10.5 to 1.

i did not make much power when i took my car to the dyno but i still had a lot of bugs to get out like jetting and timing

after the Arias pistons i bought the last set of some HI Dollar HI PO pistons from OGTS

supposedly spec out by TGIS well they needed a lot of work at the machinist

they did not fit the combustion chamber either

the next set i will spec out myself if i can

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I did get ahold of James and possibly got that ball rolling. The set he has are also 2.0 litre so that is another point to think about. The Arias pair I got probably for sentimental reasons, one of my favorite motors of all time had 11 to 1 60 over Arias pistons in it. These just look to need a complete redesign of the combustion area, then get two more to match that. I think it would wind up being cheaper to get a set made by Arias than to use these two. I agree that 10.5 or so would be fine, that's another reason not to use these. Without cc ing everything there is no telling what they are. To do that would mean alot of road time as I don't know or trust the machine shops where I am now. Laniers in the Springs did the last one and will do another if I decide to go that route, There's a good machining speed shop up by you Davegt27, Doug's Balancing I can recommend for quality work. I'm kind of glad these pistons showed up when they did. It put the bug in my head to bring more power to the table at a time when it's easiest for me to do it. Motor already being pulled for other reasons and more blocks than I know what to do with.
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These just look to need a complete redesign of the combustion area, then get two more to match that. I think it would wind up being cheaper to get a set made by Arias than to use these two.

i see what your saying
thanks for the tip on the machine shop i will check them out

right now i have two heads being flow tested. the one was taking so long i dug another head out and took it to a second shop

i was thinking Paul, at Auto Know How for my next Opel Motor
he was on the 2.5 liter Trans AM Opel team back in the 70's

The idea about a 200HP Opel motor peeked my interest
I cant give these Opel parts away so I might as well use them.

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I just want to thank you guys for not openly telling me I'm an idiot and pointing me in a good direction. The numbers Rallybob set out were extremely helpful. After making sure the application was right the 2.0 set from James shipped today and will arrive on Friday. They go to the shop the first part of next week, I'll probably be putting it together the following weekend. I still need a 2.0 head gasket and maybe a set of rod bearings if I haven't packratted some away. Then again do I need the 2.0 head gasket? I've got 3 1.9s with no place to go. I've got some time to think about that one so either way it will be fun.
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