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I have heard much about the low and high compression 1.9 what was the ratio's of both and how much difference in hp? any pros and cons of each engine? why did they switch?
 

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Hi - Lo

Kent,
The High compression engines came first in 1969 and 1970 - They had flat top pistons and solid lifter cams and were rated at 9:1 CR ( but were actually nearer 8.5:1) The rate horsepower was 102HP but they had closer to 90HP.
Due to more stringent exhaust emissions laws and low octane unleaded petrol the compression ratio was lowered to 7.6:1 in about 1971 and at the same time a hydraulic camshaft was fitted. The engine was rated at 90HP but probably had nearer 70 HP. The CR was lowered by fitting dished top pistons - the cylinder heads all had the same combustion chamber size. This makes it easy to fit flat top pistons and gain an easy 20HP - nearly a third more power.
The hydraulic cam is slightly "hotter" than the solid lifter one so the later engines gain a bunch of power by just swapping in flat top pistons.
 

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Is there anyway to tell if you have flat top pistons vs the original dish without taking the head off...by maybe using a compression tester?? Reason I ask, is I know my engine was rebuilt by PO, but do not know if he upgraded the pistons or not.

Thanks.
 

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Just FYI guys, my experience is the jump from low compression to 'high' compression pistons is about a 6 hp gain, all other things being equal. Remember too the older engines had a solid lifter cam, and that cam had more duration than the later hydraulic cams. That is worth more hp than the compression increase.

HP ratings...

Early engines were rated at 102 gross (US) hp (this was before SAE standards for hp were used in the US). European versions of this same engine (no changes) were rated at 90 ps (metric hp), with SAE numbers being about 98% of PS numbers (88 hp then). In the real world, I've seen 82-85 hp on engine dynos from an early high compression engine with a Solex.

Later engines were rated at 78 SAE hp from '71-'72. This dropped to 75 SAE hp for '73-'74 engines, and went up to 80 SAE hp for the 1975 EFI engines. These engines rate a lot lower than advertised, usually around 62-67 hp at the flywheel, and 45-50 at the rear wheels! Pathetic at best....

The good news is it's fairly easy to get 100-110 real hp, and this makes a 40-50% improvement in 0-60 acceleration times. The real work is getting a streetable engine over 125 hp.
 
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Bob,

So you are saying, of the ~ 20 HP drop from 69/70 to 71/72, low compression pistons only accounted for ~6 HP and the solid vs hyd cam accounted for the other ~14 HP? Or, were there any other changes to note, jet sizes or spark curve?

What accounts for the drop from 71/72 to 73/74 models?
 

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"The good news is it's fairly easy to get 100-110 real hp, and this makes a 40-50% improvement in 0-60 acceleration times".

RallyBob, what might be helpful for us "newbies", is maybe a high level list of what we could do. Something like:

Replace with flat top pistons
Replace Cam with ????
Upgrade to Webber
etc

Then we could search the site for specifics of each one. I've been reading numerous posts on the engine mod forum, and there is alot of info that just makes my head spin. Not looking for an expensive tire smoker, drag racer but the easy economical upgrades to get 100-110 hp...which should add to the driving excitment of a daily driver.
 

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Paul said:
Bob,

So you are saying, of the ~ 20 HP drop from 69/70 to 71/72, low compression pistons only accounted for ~6 HP and the solid vs hyd cam accounted for the other ~14 HP? Or, were there any other changes to note, jet sizes or spark curve?

What accounts for the drop from 71/72 to 73/74 models?
For one thing, keep in mind that while one part alone may be worth 6 hp, and another part alone worth 5 hp, combined they might be worth 15 hp as they compliment each other better when used in unison (similar to the way a big valve head might be worth 8-12 hp on a stock engine, but on a racing engine that already has a big cam/big carbs/high compression it can be worth 25-30 hp).

There were other changes, later cars being leaner-running, having less aggressive timing curves, and having an EGR system. Truth is, the variation in hp from year to year can just mean having a good tune-up. A lean-running late model engine can often gain significant hp by being rejetted.

Bob
 
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bozkm said:
RallyBob, what might be helpful for us "newbies", is maybe a high level list of what we could do.
This has been discussed heavily in the past on these forums. There are tons of tips and 'recipes' in the forums, particularly under the performance headings. The search engine can really help out here, but it's probably better to go into the performance forums and just scroll through the headings, as many of them are self-descriptive. I've probably personally posted 20-30 times on this very subject, not to mention everyone else's input. So there's a lot to absorb admittedly.

HTH,
Bob
 

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Bob,

I promise that I'll never ask again, but could you tell me if those roller rockers of Gregg's would work with a hydraulic lift cam, a higher compression (9:1) piston and FI? What would the max cam specs be for a street motor?

Thanks,

Dave
 

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Paul/Rallybob,
Thanks...thats pretty much what I was looking for. This checklist is good, but it is buried deep in another forum..thus the reason I haven't seen it. Maybe I'll put this into a nice checklist style word doc that could get posted in the download area, so its easily found...and of course giving credit to the original author...rallybob.
 

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Bozkm,

I know this is not the answer you want, but Its really tough to give someone a set of recommendations if we are not sure of your goals. I know I had to perform this soul searching process about two years ago in order to figure out which QUESTIONs to ask of the group (Bob).

You have to ask yourself and provide an answer as well, three big questions...

1. How much money do you WANT to spend?

2. How much HP do you want?

3 How much money CAN you spend?
3a How much over budget can you go?


From here, you can develop your shopping list, continue asking more detailed questions and refine your shopping list.

I've compiled reams of information (I'm sure others have as well) that I'm more than willing to share. I can help dig up the answers to questions that have been asked & answered before. However, I'm NO authority on the subject.


The first step for many is to make improvements without cracking open the engine. The next step is to put in flat tops, mill the head, valve job on stock valves and exhaust work.

Not knowing what you want for your efforts, Bob's recommendations for an ecconomical rebuild, is a great place to start with excellent bang for the buck.

If you need any info from me, best to contact me off list.

Good Luck
 

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This started as a discussion on the merits of flat top versus dished pistons, but it is heading into the "Opel Engine Performance Modifications" ether. So I moved the thread there to match.
 

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David McCollam said:
Bob,

I promise that I'll never ask again, but could you tell me if those roller rockers of Gregg's would work with a hydraulic lift cam, a higher compression (9:1) piston and FI? What would the max cam specs be for a street motor?

Thanks,

Dave
The roller rocker will work if you have custom inserts for the lifters that will work with the rocker arms. Spring rates will have to be stock or nearly stock however, if you collapse the lifters there will be problems. Max valve lift should be .430-.435" with stock springs, max duration depends on the lobe separation angle, and how much you can deal with a bad hesitation off-idle. Too much duration (overlap) will reduce idle vacuum and cause fluctuations to the air flow meter. After 220 degrees @ .050", the idle quality suffers enough to adversely affect the EFI.
 

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Discussion Starter · #15 ·
Very helpful info thanks all Bob how much difference do the intake mods make in overall performance? not the really agressive kind I'm not going through the bolt holes
 

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A Couple of RallyBob Recipes for Performance

KENT,
Here's something from a previous thread called "Looking For Performance Ideas" started last fall. Open this one up and you'll see lots of good ideas from a half dozen folks who really know what they're talking about.

One of the posts came from RallyBob, and I've copied it below. He does a fine job splitting it into "bolt-ons" and "Internals".
_______________________________
RallyBob wrote:

Here are a couple of 'recipes' for improving the performance of your engine without investing a ton of cash. I've broken it down into two categories, bolt-ons and internals.

Bolt-ons.
>38 DGAS Weber carb. It will work on a mild engine, even a bone-stock engine, but must be rejetted accordingly.
>Ported intake manifold, preferably '73-'74 smog-style. Easiest to port and get good results.
>1975 'Sprint' exhaust manifold from fuel injected Opel. You'll have to grind the triangular nub off the center pipes to clearance the intake manifold.
>Custom 2" exhaust pipe. GT's only have a 1 5/8" pipe, while Manta's come stock with a 1 7/8" pipe, yet they're rated at the same power (? I never understood this...). Even a stock engine will benefit from this mod, you will not lose torque, and it will help the entire power band. Use a perforated-core, straight-thru design muffler up front. For a GT, use resonated rear tips, and a custom 2.5" over-axle pipe splittting into two 1.75" pipes out back. Nice sound, not too loud, and looks good. A Manta can use a second straight-thru muffler (round body) and it will emulate a resonator nicely.
>Electronic ignition. At least a Pertronix unit, with an upgraded coil. Another upgrade would be a Crane XR-700, but for a few bucks more a Crane XR-3000 is a lot nicer, and you can eliminate the resistor wire in the ignition with this system. Use a Crane PS-91 coil with this, it works well. Lastly, another option for the ultimate in 'trick' bolt-on ignitions, the Compufire DIS-IX 'distributorless' system kicks butt.
>Electric fan.....much quieter, and adds a few hp in the upper rpm range.
>Tuning! Having the parts is one thing, but getting it setup right is another. Most cars are not getting nearly 100% of their part's capabilities, so setup is crucial. Consider having it dynoed on a chassis dyno. These days, $75-$100 will get you about 6 pulls on a dyno, and tell you more about your setup than a month's worth of driving on the street. If you can get 6-8 hp from tuning, then you just got more power than a 38 DGAS is worth, and for a lot less money. Not to mention, you will get better gas mileage, and save $$$ in the long run. Well worth it.

Internals.
>Bore block .020" over, install flat-top pistons. A small increase in power from the displacement, another small increase from the compression, and yet another increase from the 'newness' factor. It's no longer a 30-year old engine. Use Total Seal gapless rings on the second compression ring. Relatively modest investment but the engine holds compression nicer.
>Lighten flywheel. No power increase, but acceleration is improved. Stock flywheels weigh 22.5 to 23.25 lbs on average. You can very safely lighten to 17-18 lbs.
>Recondition the head, install hardened exhaust seats, larger 2.0 litre intake valves, and mill .050" off the head. This will increase compression a bit, to about 9.5:1 true compression. Still workable with pump gas. Don't worry about cam timing, I'll get to that in a moment. The head's ports can be cleaned up a bit. Don't enlarge the actual port area, just blend the bowl areas, and have the machinist mill about 1/4" off the intake valve guides and 1/8" off the exhaust guides where they protrude into the port. Blend the edges of the guides slightly.
There, you just improved airflow by about 10-12 cfm per intake runner, and 8-10 cfm per exhaust runner with minimal effort. If you do any more to the head, then you'd better know what you're doing, or you may do more bad than good! After the head is milled, make sure to deburr the edges of the combustion chamber to reduce the chances of hot-spots.
>Performance cam. Nothing crazy, if you are using stock valve springs (you should be), then you are limited to .425"-.430" max valve lift anyway. I usually keep it to .420"-.425" max, but that's just me. If you are not gonna rev it hard, use hydraulic cam/lifters. Less maintenance. Shoot for a custom profile, with split intake/exhaust duration. This helps the Opel's notoriously weak intake flow (note the trend? Most modifications are aimed at intake flow). For a truly mild driveable cam, I'd go with around 212-214 degrees of intake duration @ .050", and around 6-8 degrees less on the exhaust lobes. Grind at 110 degree lobe separation for a smoother idle and broad power band. Now, to correct for the milled head, have the cam guy grind the cam with 4-5 degrees of advance. This will correct the retarded cam timing, and give a touch of advance for better torque. Stock Opel cams are ground 1 degree retarded for comparison.

If you decide to put a bigger cam into the engine, expect to have to modify the distributor to get good throttle response. This mild cam I've recommended would run well with 3-4 degrees more initial ignition advance than stock, but can use a stock distributor. I'd use a later distributor too, a 72-74 model.

Anyway, just some ideas/feedback from someone who's built a LOT of Opels over the years, and has just about seen/tried it all.
 
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