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Discussion Starter · #1 · (Edited)
My current 2L is almost as optimized as can be. Excellent power & revability, 34 mpg, starts good hot and or cold. Pulls clean from 2000 rpm in fourth gear etc..... Great motor!

The last thing left to "tweak" is the compression ratio. From all my current information and old measurements I'm 9.35:1 +/- , my end goal is near 10:1

The block to head O ring has been weeping a bit of antifreeze for two seasons so I'll be taking this opertunity to inspect and rework the head a bit.

At the moment I "believe" 0.030 cut would do it, but I won't know for sure till after the Opel season ends and I pull the head and take more measurements.
 
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Sound quite good right now! Got any actual measurements and things like piston specs, piston to deck clerance, etc for double-checking? (Assuming you would care for others to check...) Cam info?

Your actual computed goal has to be dynamic compression ratio (DCR), not just static CR (SCR). DCR is derived from SCR with the cam specs and cam timing. DCR is what really limits how far you can go with a specific fuel type, not SCR alone. If you are running pump fuel, then a resonable-to-run limit on the street is 8.0 + or - with iron heads. (Well, there IS another step beyond DCR that accounts for the lower effective compression ratio cased by altitude ... but I don't think you are at a very high altitude....maybe 1000' AMSL?)

The cam specs play as big a part in DCR as the SCR. If you increase the SCR with too small of a cam, then your DCR can get too high, and you will have to work around that to avoid detonation problems; typical solutions used: change cams, change cam timing, reduce the SCR back down with a thicker head gasket, change the cooling or coolant, or limit ignition timing.

Part of the deal with the work I've been doing on profiling Opel cams is that the specs available for these engines is mostly inadequate for computing DCR. The general Euro seat-to-seat duration specs that we have are not at all correct for DCR computations. Some of the modern Euro cams (like from Enem) have duration numbers that may be usable; Isky numbers are a mixed bag since many of them are over 50 years old and were taken to different standards through the years.

And sincerely...This is not pie-in-the-sky theory... this is how you do it to avoid problems. Once one learns and 'catches on' to this, it is a step up in one's engine design knowledge.
 

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Discussion Starter · #3 · (Edited)
95 x 69.8 mm (not measured from my engine just generic numbers)

0.032 crushed head gasket

3cc valve reliefs

Combustion chambers volumes
#1 - 49cc.
#3 - 50.5cc.

Piston height vs deck.
#1 - 0.06 below deck
#3 - 0.04 below deck

Isky OR-66 hydraulic.420" lift, 268 deg, advertised 228 duration @ .050"

Am I missing anything?
 
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Discussion Starter · #4 ·
Sound quite good right now! Got any actual measurements and things like piston specs, piston to deck clerance, etc for double-checking? (Assuming you would care for others to check...) Cam info?

Your actual computed goal has to be dynamic compression ratio (DCR), not just static CR (SCR). DCR is derived from SCR with the cam specs and cam timing. DCR is what really limits how far you can go with a specific fuel type, not SCR alone. If you are running pump fuel, then a resonable-to-run limit on the street is 8.0 + or - with iron heads. (Well, there IS another step beyond DCR that accounts for the lower effective compression ratio cased by altitude ... but I don't think you are at a very high altitude....maybe 1000' AMSL?)

The cam specs play as big a part in DCR as the SCR. If you increase the SCR with too small of a cam, then your DCR can get too high, and you will have to work around that to avoid detonation problems; typical solutions used: change cams, change cam timing, reduce the SCR back down with a thicker head gasket, change the cooling or coolant, or limit ignition timing.

Part of the deal with the work I've been doing on profiling Opel cams is that the specs available for these engines is mostly inadequate for computing DCR. The general Euro seat-to-seat duration specs that we have are not at all correct for DCR computations. Some of the modern Euro cams (like from Enem) have duration numbers that may be usable; Isky numbers are a mixed bag since many of them are over 50 years old and were taken to different standards through the years.

And sincerely...This is not pie-in-the-sky theory... this is how you do it to avoid problems. Once one learns and 'catches on' to this, it is a step up in one's engine design knowledge.
All super interesting stuff... Hence why I started this thread. Thanks for sharing such good info

Started screwing around with some online calculators for dynamic C/R, it's fun stuff.
 

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An increase from 9,35:1 to 10:1 won't make a big difference unless you have a cam with too much duration and overlap. If the engine runs fine as it is, why not just skim the head some 0,1-0,2mm to straighten it and have a little cushion against pinking/detonation ?
 

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That is pretty good stuff, Vincent, and very close to what I plugged in yesterday to get a start, at least on SCR. Using the following:
  • Your bore, stroke, and head gasket thickness
  • 3.82" head gasket bore
  • 50 cc chamber volume
  • 4 cc for valve reliefs.... your 3 cc + 1 cc gets added for the volume in the crevice down to the top ring
That comes out to 9.1 SCR.

Is that the OR-66 solid or the OR-66H hydraulic cam? That, plus cam timing is part of the DCR computation; at this point, IDK if Isky grinds some advance or retard into the cam, so we can start by assuming there is neither and assuming a 108 LSA. That would put the ICL at 108 as a start.

With that, a 268 advertised duration, and a stoke rod length, the DCR comes out to 7.4.

If you up the SCR to 10.0, then the DCR is going to rise point-for-point. So that would put you around 8.3. That is tunable for most engine with some real care but you have to be paying good attention and 'creep up on it' in the ignition timing department. I have run 8.3 on the street in another engine with iron heads, and had to be careful to limit ignition timing a bit. And when I went from 1000' elevation down to sea level, it was a bit more prone to 'ping' (detonate).

Some other observations:
  • I know a couple of guys who run DCR in the high 8's; one of them even did it on 87 octane LOL. But they have spent days or weeks tweaking and know the whole deal inside and out. Most of us (including me) don't want that bother. That high of a DCR is not for the faint of heart or the inexperienced.
  • This is a computation and we put numbers in and get a number out. The actual detonation process depends on peak temperatures and pressures (which are interelated). So we get a DCR number to make sure we have a good idea where we are, but the actual detonation margin can vary from there.
  • Other factors enter into the detonaiton margin like:
    • slopes on the cam profiles near valve closing
    • keeping localized hotspots in the head from developing (coolant selection, air purging for the colling system, and system pressure)
    • quench/squish effects that help to swirl the mixture around and cool the surfaces (ever little bit helps; these Opel engines have this)
    • spark plug placement which effects the burn time and about which we can do little
    • higher energy spark, which gets the burn process started faster and more completely; anything you can do the speed the burn process tends to improve detonation resistance
    • the temp/humidity conditions on a particular day; cool, dry air lowers the margin to detonation, and hot, humid air increases the margin. (FWIW, I read a study from the 80's that found that high humidity was like adding 3 or 4 points of octane.)
So DCR computations help you get close to optimum and avoid gross mistakes in compression & cam combinations..

If you want to do this yourself (and you seem to be pretty hands-on), I use the Pat Kelley calculator. It is a Windows macro that you can download from here:
That page has a pretty good discussion of all this.

If I want to get an idea of cranking compression and the effects of altitude, then I take the Pat Kelly SCR and ICA (intake closing angle) outputs, and plug those into the Wallace Dynamic Compression Ratio calculator, along with altitude.
 

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FWIW, I had a street engine that used a .020” overbored 1.9 block, pistons set flush with the deck, with a 1.9H head (smaller chamber) that had been fitted with 2.0 valves and milled .030”. True compression was measured to be 9.8:1. the chambers were fully polished and the chamber edges were deburred to reduce the chance for hot spots.

It used a stock hydraulic Opel 1.9 cam, ported intake, 38 DGAS, Sprint exhaust manifold and 2” exhaust system.

With a stock 1973 distributor set to 36 degrees total timing it pinged slightly on 87 octane, but ran great on 89 octane.
 

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An increase from 9,35:1 to 10:1 won't make a big difference unless you have a cam with too much duration and overlap. If the engine runs fine as it is, why not just skim the head some 0,1-0,2mm to straighten it and have a little cushion against pinking/detonation ?
I hope everyone will forgive my rambling on below, but this is a favorite topic of mine. I somehow got educated on this matter early on, and have been working these numbers for over 4 decades years now.

Here is my direct expereience:
  • Increase SCR/DCR by .25 points will be noticeable to any car enthusiast. It will show up in low to mid RPM throttle response. (And FWIW, you can get the same effect on DCR by advancing your cam timing by about 4 degrees.)
  • Increase SCR/DCR by 0.5 points and the engine will feel very much different
  • Increase SCR/DCR by 1.0 points and it will be a an entirely different engine
  • The effects are more pronounced the lower the SCR/DCR.
  • Most of these differences will be felt in the low to mid RPM's (below approximately 3000 RPM in these engines and cams). Once you get 'up on the torque curve', above 90% or so of maximum torque, then the benefits become less and less (because the valve overlap makes cylinder filling very high regardless of DCR).
  • You are not only increasing pressures with a higher SCR/DCR, but significantly improving cylinder clearing at low/mid RPM's (i.e., getting more of the exhaust gases out of the cylinder) with the higher SCR/DCR. Most folks don't realize that clearing the exhaust gases out after a combustion cycle is not at all a 100% thing when you are operating at RPM's below the peak of the torque curve. Low SCR/DCR works exactly like having an EGR device in the system, and leaves more residual exhaust gases in the fresh mix.
  • The increased timing advance that many folks put in their distributors is aimed at the same thing: increasing peak cylinder pressures at the proper crank angle to improve net torque. Higher SCR/DCR has a higher level of benefit. And if you have an agressive distributor curve, you may have to take some of it out if you increase DCR. A 8:1 DCR engine will need significanlty less initial advance (5 to 10 degrees less is typical) than an engine with a 6.5-7 DCR.
I once increased the SCR in my rally Starion from 7.3 to 8.2. That was the only change; the same cam and all else was kept. I promise that I am not exaggerating.... the difference was SUPER significant in the lower RPM rangebelow 2500 RPM at least (before the turbo kicked in). For rally use, torque across a very wide RPM range is very important. If you only have 4 or 5 gears, you need all the torque you can get at the lower RPM's so it was a BIG deal in that car's rally performance. Now I don't expect Vincent see as dramatic a change: that Starion engine changed from a DCR of around 6-ish to maybe 7 at most. He is making less of a change.

Those hobby racers who operate their engines all the time at mid to high RPM's (like drag racers) tend to put less value on this. But it is because they intentionally never operate their engines at low to mid RPM's and don't experience the changes. If you are on the Autobahn a lot, at 3500-4500 RPM all the time, it is of limited value there too. The higher echelon and professional racers don't leave this 'stone unturned'. And street driving and local cruising involves a lot of lower RPM operation, and the benefits will be felt there.
 

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FWIW, I had a street engine that used a .020” overbored 1.9 block, pistons set flush with the deck, with a 1.9H head (smaller chamber) that had been fitted with 2.0 valves and milled .030”. True compression was measured to be 9.8:1. the chambers were fully polished and the chamber edges were deburred to reduce the chance for hot spots.

It used a stock hydraulic Opel 1.9 cam, ported intake, 38 DGAS, Sprint exhaust manifold and 2” exhaust system.

With a stock 1973 distributor set to 36 degrees total timing it pinged slightly on 87 octane, but ran great on 89 octane.
OOPS EDIT ALERT! I used the exhaust duration rather than the intake duration (248 vs 260) in the original post. I have corrected that and the numbers and observations below.

Bob, I put your 9.8 SCR into the same calculator and the advertised intake duration (260 at .006" lift) for a stock 1.9L cam (which I profiled a couple of months ago) The DCR came out to 8.05.

So your experience seems to me to fit well with mine, and fits with the general advisory numbers. My 8.3 DCR engine had polished chambers too, for the same reason. I stuck with 93 premium; 89 was marginal. Your DCR looks to have been a quarter point lower, so perhaps that explains why you could use 89. I don't have maximum advance numbers (that was over 40 years ago LOL)

That was on a Ford 351 Cleveland that I used for all sort of uses: drag racing to towing (which is a detonation-prone use). This all tends to work the same form engine to engine in that that era of design; the same principles apply across the board.
 

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I have never raced so I have no experience of race engines, but the difference in feel between a 115hp DIN 9,2:1 C20NE and a 122hp DIN 10:1 20SE isn't particularly big(both use the same cam and Motronic, only differences are the static compression and the lack of a catalytic converter on the 20SE). On the other hand, if we compare an early 52hp DIN 7,8:1 12N engine to a 60hp DIN 9,2:1 12S the difference is very notable. Although the engines differ more, a single outlet exhaust manifold and smaller Solex 30PDSI carb on the 12N vs dual outlet exhaust manifold and Solex 35PDSI on the 12S. And a larger diameter exhaust.
 

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Discussion Starter · #11 ·
I'm enjoying everyone's point of view on this! Thanks

If I were to pop on a degree wheel on my crank pulley, take rocker for number l#1 intake off, and put my magnetic dial indicator on the lifter. Measure at what degrees of crank rotation the intake actually closes. Then when the head is off I can measure how much stroke volume is left at that moment. I "think" this would give me good ball park numbers, aspecially after I verify all other critical measurements

What do you guys think of this idea?
 

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I have never raced so I have no experience of race engines, but the difference in feel between a 115hp DIN 9,2:1 C20NE and a 122hp DIN 10:1 20SE isn't particularly big(both use the same cam and Motronic, only differences are the static compression and the lack of a catalytic converter on the 20SE). On the other hand, if we compare an early 52hp DIN 7,8:1 12N engine to a 60hp DIN 9,2:1 12S the difference is very notable. Although the engines differ more, a single outlet exhaust manifold and smaller Solex 30PDSI carb on the 12N vs dual outlet exhaust manifold and Solex 35PDSI on the 12S. And a larger diameter exhaust.
That seems like a good observation, C.The SCR/DCR changes will be felt most by far in the low-mid RPM ranges. It won't make much difference on the Autobahn.

And having fixed my Jetronic as much as it can be and noting the limitations on performance due to that FI system, and hearing Sci-Fi Guy's comments on his Motronic performance, I have to question how much the FI system is limiting the engine feel versus the SCR/DCR.
 

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I'm enjoying everyone's point of view on this! Thanks

If I were to pop on a degree wheel on my crank pulley, take rocker for number l#1 intake off, and put my magnetic dial indicator on the lifter. Measure at what degrees of crank rotation the intake actually closes. Then when the head is off I can measure how much stroke volume is left at that moment. I "think" this would give me good ball park numbers, aspecially after I verify all other critical measurements

What do you guys think of this idea?
I think you are getting there. I see a couple of issues:
  • Pull all the plugs. You going to have to find the exact TDC angle with a piston stop. The BB on the flywheel is not set at TDC, at least based on the way that the TM documentation reads.
  • If hydraulic lifters are used, then find the angle at which the intake lifter is .006" from closing. What you'll have to do is take a number of degree versus the lifter's vertical position readings from the dial indicator as you slowly close the intake lifter from, say, .040" lift down to close, and then find where the .006" data is in that series of numbers. The intake closing angle at .006" lifter lift is what you want to use as it reasonably reflects the 'effective' intake closing angle. The compression starts when the valve is still a tad off of the seat, plus a hydrualic lifter will compress typically something like .004-.005" as it takes the valve spring pressure. (Finding the angle for actual close is an exercise in frustation as the cam ramps can have very long, slow tails on them.)
  • If you have solid lifters, then I would find the crank angle for .001" lifter lift with the same procedure.
  • You don't want to go backwards (CCW) as the chain slack will get on the wrong side and mess up the angles. If you only rotate slowly in the CW direction, then there should be no chain slack on the drive side. Make sure the cam does not 'jump' at any point (due the pressure from an open valve spring traying to rotate it in the forward direction) or the cam vs crank timing may get changed.
  • The idea of getting the piston position at that intake closing angle is intriquing. Just be aware that the calculators figure this out for you based on stroke and rod length inputs. So, it is not necessary. (It could be used to confirm the calculator, but I would trust the geometry computations more.)
If you want, send the cam to me and I'll profile it. I have done an Isky OR-4H, a stock 1.9L, a stock 2.4L and 2 more cams so far. I'd like to get data on more Opel cams.
 

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FWIW... EDIT ALERT! In my follow-up post above on RallyBob's 9.8:1 compression engine, I used the stock 1.9L exhaust advertised duration rather than the intake duration (248 vs 260) in the original post. I have corrected that and the DCR number, and added to my observations.
 

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Discussion Starter · #15 · (Edited)
I think you are getting there. I see a couple of issues:
  • Pull all the plugs. You going to have to find the exact TDC angle with a piston stop. The BB on the flywheel is not set at TDC, at least based on the way that the TM documentation reads.
  • If hydraulic lifters are used, then find the angle at which the intake lifter is .006" from closing. What you'll have to do is take a number of degree versus the lifter's vertical position readings from the dial indicator as you slowly close the intake lifter from, say, .040" lift down to close, and then find where the .006" data is in that series of numbers. The intake closing angle at .006" lifter lift is what you want to use as it reasonably reflects the 'effective' intake closing angle. The compression starts when the valve is still a tad off of the seat, plus a hydrualic lifter will compress typically something like .004-.005" as it takes the valve spring pressure. (Finding the angle for actual close is an exercise in frustation as the cam ramps can have very long, slow tails on them.)
  • If you have solid lifters, then I would find the crank angle for .001" lifter lift with the same procedure.
  • You don't want to go backwards (CCW) as the chain slack will get on the wrong side and mess up the angles. If you only rotate slowly in the CW direction, then there should be no chain slack on the drive side. Make sure the cam does not 'jump' at any point (due the pressure from an open valve spring traying to rotate it in the forward direction) or the cam vs crank timing may get changed.
  • The idea of getting the piston position at that intake closing angle is intriquing. Just be aware that the calculators figure this out for you based on stroke and rod length inputs. So, it is not necessary. (It could be used to confirm the calculator, but I would trust the geometry computations more.)
If you want, send the cam to me and I'll profile it. I have done an Isky OR-4H, a stock 1.9L, a stock 2.4L and 2 more cams so far. I'd like to get data on more Opel cams.
Cool. I'm definitely doing this.

These are indeed hydraulic lifters. I have the cam set with 3 degrees advance. (Chevy offset bushing.)

Also when I assemble this engine I had established true top dead center with my dial indicator and bolted my flywheel so my ball and pointer show true TDC
 

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FWIW... EDIT ALERT! In my follow-up post above on RallyBob's 9.8:1 compression engine, I used the stock 1.9L exhaust advertised duration rather than the intake duration (248 vs 260) in the original post. I have corrected that and the DCR number, and added to my observations.
Another consideration is this. I’ve found that bore diameter, chamber size, spark plug length, and piston design all affect this too.

Big bore engines are more sensitive to spark advance issues. Longer flame travel...

I’ve also noted I can run a large displacement engine (such as 2.4) with a dished piston and small chamber head (1.5, 1.6, 1.7, or milled 1.9) and get away with a lot more compression for a given fuel octane compared to a large chamber head and a domed piston.

Specifically, the 1.5 heads with longer plugs place the electrode closer to center of the bore as well as lower in the chamber. I’ve seen better detonation resistance, better combustion, better fuel economy.

Lots of potential variables!
 

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Discussion Starter · #17 · (Edited)
Think all I'm going to do is go for a cruise, then while the engine is hot and presumably the hydraulic fully pumped up, I'll set my dial indicator on #1 intake lifter. I'll fallow your advice in the way the measurement is done. After I'll mark the crank pulley at the position when the intake closes. Then once the heads off, reposition the crank at that Mark and measure cylinder volume.

I think my new goal will be 8:1 C/R after cam timing factored in....
 

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Great thread! SCR/DCR isn’t easy to wrap your head around but this thread has been very educational so far.
I like MR’s suggestions, I only wish I’d taken the duration readings @ .006” off base along with the .050” I don’t believe Isky goes by the .050” numbers.

I did & do agree with Vincent on degreeing the cam before and after you have made your mods. It’ll give you a good idea as to where you’re at now (a pretty good place from what I’ve been reading), assuming that you have the modified sprocket, preferably with the linked timing chain, then upon assembly dial in the cam timing right where you want it 🙂

I used the dial caliper to verify #1 piston TDC (head off) I was surprised the bead on the flywheel, after what I’ve read, was right on the money. Setting the dial caliper up on the deck I found was by far the best way to verify true TDC.

If you do take a little off the head, this is the best I’ve seen on cam degreeing I like it maybe because it’s simple.
 

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Discussion Starter · #19 · (Edited)
I had posted some calculation based on some cam degree measurements I had written down in the past. But realized I had to rethink my formulas some more
 

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Discussion Starter · #20 · (Edited)
......
 
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