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Opel Rallier since 1977
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Those DSS pistons are available in several compression heights and dish volumes so it would be easy to achieve pretty much any desirable CR from below 8 to as high as 13. This is based on a bunch of numbers I ran last night from the Summit website. So given a can such as the GM77 which Charles says runs about between the OR 66 and OR77 specs, what target CR do you think one should shoot for?
I'd have to run some numbers and it might take a couple of days to get the time.... have a project to build at Ft Knox this week.... not in the gold vault! LOL I need to dig back into those cam specs too.

But generally, in the upper 9's for SCR is where I would head just at first guess based on other motor work. Are you going to run that Sniper system?

BTW you can take your static CR and the cam specs and work through any good DCR calculator and look to get a DCR of high 7's to 8. The trick is to interpret the Opel cam specs, which give duration as seat-to-seat, but you need more like .006" lift duration numbers to get an accurate DCR. (The .006" number is for a hydraulic lifter cam; a solid would be different.)

Just be aware that your rod-stroke ratio is getting low, down close to 1.5, as opposed to the stock 1.9L rod-stroke ratio of 1.8. So it will be the typical low rod-stroke ratio stroker.... it'll not rev as high before starting to flatten out. (There is a reason that a 2.6L Misubishi kept a longer rod and deck height...to keep the rod-stroke ratio up.) So, you'll want to keep the low RPM torque good to keep a reasonable total RPM range. Of course, 2.6L in a light GT will no doubt move along.
 

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Opel Rallier since 1977
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OK, just managed to dig out those GM77 cam specs and took a stab (educated guess) at the .006" lift duration as being in the 285-290 range. If that is the case, your DCR ends up in the 7.7-7.9 range at first cut without any chamber opening, etc. So not bad.

But, the overlap is possibly an issue for EFI: 65-75 degrees which is very large for EFI.
 

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Opeler
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Discussion Starter #83
I'd have to run some numbers and it might take a couple of days to get the time.... have a project to build at Ft Knox this week.... not in the gold vault! LOL I need to dig back into those cam specs too.

But generally, in the upper 9's for SCR is where I would head just at first guess based on other motor work. Are you going to run that Sniper system?

BTW you can take your static CR and the cam specs and work through any good DCR calculator and look to get a DCR of high 7's to 8. The trick is to interpret the Opel cam specs, which give duration as seat-to-seat, but you need more like .006" lift duration numbers to get an accurate DCR. (The .006" number is for a hydraulic lifter cam; a solid would be different.)

Just be aware that your rod-stroke ratio is getting low, down close to 1.5, as opposed to the stock 1.9L rod-stroke ratio of 1.8. So it will be the typical low rod-stroke ratio stroker.... it'll not rev as high before starting to flatten out. (There is a reason that a 2.6L Misubishi kept a longer rod and deck height...to keep the rod-stroke ratio up.) So, you'll want to keep the low RPM torque good to keep a reasonable total RPM range. Of course, 2.6L in a light GT will no doubt move along.
Agree on the rod/stroke ratios. I did some research on these and the rule of thumb for engine builders is to hit 1.5 - 1.8 with 1.75 being the optimum ratios. Different pros and cons and depends what you want. Several Honda 4 cylinders are down in the 1.49 - 1.54 ratios and they are screamers. The Opel ratios are 1.83 for the 1.9L, 1.65 for the 2.2L and 1.576 for the 2.4L.

The biggest issue with the Opel engine I'm learning is the short block. With only 208 deck height to work with and piston compression heights no lower than 27.7 mm (1.09") there is little room for increasing the engine displacement without sacrificing the above ratio.

And yes I will be running the Sniper EFI system.
 

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Opel Tinkerer and Rescuer
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OK, just managed to dig out those GM77 cam specs and took a stab (educated guess) at the .006" lift duration as being in the 285-290 range. If that is the case, your DCR ends up in the 7.7-7.9 range at first cut without any chamber opening, etc. So not bad.

But, the overlap is possibly an issue for EFI: 65-75 degrees which is very large for EFI.
have a more aggressive OR77 on EFI and it didnt seem to have a problem at all ;)

Had some timing issues with the Motronic, but not the EFI itself.
 

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Opel Tinkerer and Rescuer
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The biggest issue with the Opel engine I'm learning is the short block. With only 208 deck height to work with and piston compression heights no lower than 27.7 mm (1.09") there is little room for increasing the engine displacement without sacrificing the above ratio.
You can twist yourself in knots, and go down a million rabbit holes trying to figure out every single hundredth of a CC. But, Math is math, parts are parts, the blocks all have their limitations, and the crank is what the crank is..

Anything more complex requires custom rods, custom pistons, etc.. A lot of work and cost for a street motor. $ years and 13 motors and I still don't know all of it.. but it still pretty much comes down to variations on what has already been done.

Everything is a compromise, Cost to Performance to Reliability all have to be considered.. then there is the time it takes the shop to do the work all factor in.

There is a reason I keep the compression in the 9:1 region, there is a reason for most everything I or the machine shop have done. See the above statement.

Teasing out those extra HP due to flow benching the head or porting the intake just right so you don't make it worse instead of better, etc.. etc.. is all theoretical to us..

I mean if you have Bobs skills, and all the tools or have a machine shop at your beck and call, or unlimited money laying around, then that's one thing.

But I and most everyone else have to pay a machine shop to do the work so that has to factor in as well. Where do you get the greatest improvement.. Where is that spot for diminishing returns on time and money..

These are all as big of factors as what to build as the smaller details.

Seriously though, what you do to the intake, what you do to the exhaust, what you do the ignition, all will have a far, far, greater affect on performance than a extra 40 ccs here or there.
 
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Opel Rallier since 1977
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It's just the matter that an EFI using manifold pressure can see some instability in MP at low-moderate throttle openings and RPM if there is a lot of overlap and thus reversion. (Once the rev's get up and the exhaust pull-through get going, that helps reduce the reversion.) I can only guess how a given system will handle any MP instability...so IMHO is is bit of a 'try it an see' thing.

And as to the Honda's being screamers with their ratios.... look at the bore and head chamber sizes more to understand them. Small chambers and smaller bores don't suffer as much. The physics of what is going on around TDC has to do with chamber width, shape and size, finite combustion speeds and how fast the piston is pulled down once past TDC, and you are dealing with a wider chamber with the CIH so it is apples to oranges to bring in the Honda's. The more compact the chamber, the more quickly the flame can burn through the whole charge, and so the piston can be jerked down from TDC faster (which is what happens with a low rod-stroke ratio) but still not compromise the peak pressures. (Hope that makes some sense...)

And, the Hondas are not all down around 1.5 by any means.... and we might as well throw in F1 engines with their high ratios....15K+ RPM. So the Honda's are really just low RPM engines after all LOL
 

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Opel Tinkerer and Rescuer
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It's just the matter that an EFI using manifold pressure can see some instability in MP at low-moderate throttle openings and RPM if there is a lot of overlap and thus reversion. (Once the rev's get up and the exhaust pull-through get going, that helps reduce the reversion.) I can only guess how a given system will handle any MP instability...so IMHO is is bit of a 'try it an see' thing.

And as to the Honda's being screamers with their ratios.... look at the bore and head chamber sizes more to understand them. Small chambers and smaller bores don't suffer as much. The physics of what is going on around TDC has to do with chamber width, shape and size, finite combustion speeds and how fast the piston is pulled down once past TDC, and you are dealing with a wider chamber with the CIH so it is apples to oranges to bring in the Honda's. The more compact the chamber, the more quickly the flame can burn through the whole charge, and so the piston can be jerked down from TDC faster (which is what happens with a low rod-stroke ratio) but still not compromise the peak pressures. (Hope that makes some sense...)

And, the Hondas are not all down around 1.5 by any means.... and we might as well throw in F1 engines with their high ratios....15K+ RPM. So the Honda's are really just low RPM engines after all LOL
Given the sniper is designed for bigger motors I think it will be more than fine, and the stupid Motornic with its ancient AFM was fine.

Its all about going up to the edge, and not crossing it. Thats why Isky designed the Cam for me. The OR-77 Hydraulic worked well but was a big rough around the edges, mainly at cold start and idle. We brought it down from the OR-77 specs to halfway between the tried and true OR-66 and the larger OR-77.

Everything I have tried to do in my designs was keep the cost within reason, the work the machine shop had to do repeatable.. and make it a streetable motor. The target was for it to perform as well as a 90s BMW motor. Never the intent to be a circle track or road racing motor.
 

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Opeler
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Discussion Starter #88 (Edited)
Hey folks, so after tinkering for a couple weeks and researching piston and rod options I've come up with a plan which would yield a solid 2.6L engine. But here are some rules of engagement (ROE) I've come up with by working with Charles.
- 2.4 block bored to 97 mm. There are reports of bigger bores but 97 mm seems to be the "safe" limit.
- 88 mm stroke achieved via offset grinding the 2.3 diesel or the 2.4 crank. Higher strokes are possible using the diesel 2.3 cranks but the rod length to stroke ratio is getting very low so long term durability may suffer. I'll explain below.
- You CAN"T use the Opel 2.4 connecting rods as the big end journal size is 52 mm and given the diesel crank is only 2 mm bigger, there is no room to offset grind any more than maybe 1 mm+.

Crank: By using a connecting rod that utilizes a 48 mm big end journal diameter, you now have room to grind the crank rod journals down and offset grind even the 2.4 crank. The best way to think about this is that you are increasing the stroke from stock 85 mm to 88 mm or 1.5 mm on the offset of the rod journal. The stock 2.4 crank journal is 52 mm diameter so reducing to 48 gives you up to 2 mm to offset. Assuming you might grind the journal down a little all the way around, it should be quite possible to use the stock 2.4 crank in this manner and achieve a 1.5 mm offset.

Pistons: DSS K1901-3820 pistons from Summit. These are 97.028 mm diameter flat top with two valve reliefs cut in the top, compression height of 1.115", 0.927" floating pin, 10 cc volume.

ConRods: BMW M10 connecting rods which are very easy to find brand new forged racing rods. 135 mm length, big end bore = 52 mm (journal diameter = 48 mm), small end bore = 22 mm, big end width = 23.88 big end width ( slightly thinner than opel rods). With these rods the small end bore would have to be hones from 22mm (0.8661") to 0.927" to accommodate the above pistons. That's only .0609" so hope that's doable.

With the above formula you would achieve 88mm stroke, 97 mm bore, 1.53 ratio on the rod length to stroke, and compression ratio of 9.51:1 which should be a very streetable combination using pump gas. This assumes 55 cc head volume, 0.8 mm head gasket and 0.679 mm deck clearance calculated from 208 (deck height) - 44 (half stroke) - 135 (rod length) - 28.321 (piston comp. height).

If you opt for the 2.3 Diesel crank then strokes bigger than 88 mm are very possible but a shorter conrod is required or a piston with a lower compression height.

Looking for thoughts or ideas. Did we miss anything crucial here? Is this stupid? LOL
 

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Opel Rallier since 1977
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1.53 rod/stroke ratio... won't be a high RPM revver.

But going from 2.4 to 2.6 is not much of a change so it's all basically solid. FWIW.... 8% is a really small displacement increase for the work. 15-20% is very common for stroker displacement increases.

The one thing to check closely IMHO is taking off .0305 from all around the pin bore in the rod's small end. The stock Opel 1.9L small end walls are not all that beefy.... but IDK know on those BMW rods; they may be fine with it.

And if the BMW rod small end walls can take it, I'd offset bore the new pin bores UP in the rod to move the hole up something like .020-.024" to get the piston tops within .005" to zero deck. Why? You'll get a nice tight quench/squish gap between piston and head of under .040" and that will help in fighting detonation and improve combustion efficiency, plus a bit more SCR. OR... just deck the block that much... that would be a lot easier now that I think of it! LOL

BTW: How much narrower is the BMW big end width vs the Opel BE width?

Stupid? EVERYTHING we do could be considered stupid LOL
 

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Hey folks, so after tinkering for a couple weeks and researching piston and rod options I've come up with a plan which would yield a solid 2.6L engine. But here are some rules of engagement (ROE) I've come up with by working with Charles.
- 2.4 block bored to 97 mm. There are reports of bigger bores but 97 mm seems to be the "safe" limit.
- 88 mm stroke achieved via offset grinding the 2.3 diesel or the 2.4 crank. Higher strokes are possible using the diesel 2.3 cranks but the rod length to stroke ratio is getting very low so long term durability may suffer. I'll explain below.
- You CAN"T use the Opel 2.4 connecting rods as the big end journal size is 52 mm and given the diesel crank is only 2 mm bigger, there is no room to offset grind any more than maybe 1 mm+.

Crank: By using a connecting rod that utilizes a 48 mm big end journal diameter, you now have room to grind the crank rod journals down and offset grind even the 2.4 crank. The best way to think about this is that you are increasing the stroke from stock 85 mm to 88 mm or 1.5 mm on the offset of the rod journal. The stock 2.4 crank journal is 52 mm diameter so reducing to 48 gives you up to 2 mm to offset. Assuming you might grind the journal down a little all the way around, it should be quite possible to use the stock 2.4 crank in this manner and achieve a 1.5 mm offset.

Pistons: DSS K1901-3820 pistons from Summit. These are 97.028 mm diameter flat top with two valve reliefs cut in the top, compression height of 1.115", 0.927" floating pin, 10 cc volume.

ConRods: BMW M10 connecting rods which are very easy to find brand new forged racing rods. 135 mm length, big end bore = 52 mm (journal diameter = 48 mm), small end bore = 22 mm, big end width = 23.88 big end width ( slightly thinner than opel rods). With these rods the small end bore would have to be hones from 22mm (0.8661") to 0.927" to accommodate the above pistons. That's only .0609" so hope that's doable.

With the above formula you would achieve 88mm stroke, 97 mm bore, 1.53 ratio on the rod length to stroke, and compression ratio of 9.51:1 which should be a very streetable combination using pump gas. This assumes 55 cc head volume, 0.8 mm head gasket and 0.679 mm deck clearance calculated from 208 (deck height) - 44 (half stroke) - 135 (rod length) - 28.321 (piston comp. height).

If you opt for the 2.3 Diesel crank then strokes bigger than 88 mm are very possible but a shorter conrod is required or a piston with a lower compression height.

Looking for thoughts or ideas. Did we miss anything crucial here? Is this stupid? LOL
Super cool engine build! I'll be following your development on this project!
 

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Opeler
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Discussion Starter #91
1.53 rod/stroke ratio... won't be a high RPM revver. Right but the stock 2.4 is only 1.576 ratio so probably not too much difference in power curve ....perhaps.

The one thing to check closely IMHO is taking off .0305 from all around the pin bore in the rod's small end. The stock Opel 1.9L small end walls are not all that beefy.... but IDK know on those BMW rods; they may be fine with it.

And if the BMW rod small end walls can take it, I'd offset bore the new pin bores UP in the rod to move the hole up something like .020-.024" to get the piston tops within .005" to zero deck. Why? You'll get a nice tight quench/squish gap between piston and head of under .040" and that will help in fighting detonation and improve combustion efficiency, plus a bit more SCR. OR... just deck the block that much... that would be a lot easier now that I think of it! LOL

From looking at the pictures of the BMW rods I would not offset hone/bore the small end as the bearing may not be conducive for that. I might be able to find a piston with a higher compression height and get the piston closer to the top of the block but then you would need more volume to keep the compression ration reasonable.

BTW: How much narrower is the BMW big end width vs the Opel BE width? BMW rods are 23.8 mm wide and the opel rods are 25 mm wide I believe.

Stupid? EVERYTHING we do could be considered stupid LOL
 

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So the con rod is bushed? (Not a bearing but a bronze bushing). Here is what to watch out for.....If the wall thickness of the bushing is .080" or thereabouts and you hone it out by about .030" all around, you'll have .050" left which is probably adequate. But if the bushing wall thickness is .050" or .060", and you hone the walls .030" then it wil be donw to .020" to .030" wall thickness and may not have enough integrity to hold up. The material will be so thin, it just may pound out of the small end. So you may have to hone out the small end of the rod itself to accept a larger OD bushing, and then you gotta look at the rod's wall thickness.

As for being end width. I am Ok with .025 to .035" inch side clearance and maybe a bit more... those numbers get run all the time these days. IDK the later cranks journal width. I'd also double check the rod bearing width on the BMW bearings, just to be sure.

BTW, I reworked your numbers for SCR and get 9.4 when I add in an extra cc for crevice volume down to the top ring. If the piston CH is raised .020" or the deck lowered that much, then the SCR goes to 9.8. I'd run that with a .039" quench/squish gap rather than at .060" gap and 9.4. I've run 10 and 10.3 on other street engines and with modest sized cams to boot. Not any issue if you make sure your DCR is low enough.....

Got a cam in mind so that DCR can be worked up?
 

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Opeler
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Discussion Starter #93
Manta, thanks for all the great advice and running numbers. The cam recommended by Charles is the GM77 cam. I'm told it lies someplace between the OR77 and OR66 specifications. Maybe Charles can jump in here and provide the specs.

So I'm still looking for combinations that can get the deck height higher as you suggest. Found some VW/Audi G60 rods that are around 135-136 mm in length. These may prove a good combination if I can find a suitable piston.
 

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Decking the block down .020" is pretty easy.....it'll retard the cam timing 1-2 degree per the data others have presented here... which may help the Dynamic CR anyway. If that timing is any issue, you ought to be timing the cam anyway and can adjust it.

OK, I dug around on the cam specs... I know Charles has mentioned them here in threads... and found that all again. So, I ran the DCR with a more conservative guess at the advertised duration based on the advertised duration numbers in post #2 here: Cams Cams and Cams

That guess would be 276* (halfway between the OR-66 and OR-77 cams), on an LSA of 110, and with an assumed ICL (intake center line) angle of 110.

With that, the DCR ends up at 7.7, which is fine for pump premium. You have to do the ignition timing right for that DCR and not be lazy and allow it to come in too soon. But I have run 8.3 DCR on the street before with iron heads and quench/squishh. That high DCR will really make the engine torquey and responsive at the low RPM's.

BTW, on your chamber volume, is that assumed or measured? That is getting to be the last item that is not 100% known for certain (at least to me).
 

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Opeler
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Discussion Starter #95
Not sure if the chamber volume is currently measured or previously known numbers from the 2.4 head. Charles?
 

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I "think" factor stock 2.4 head is 54 cc

What kinda valves, porting, head work, are you considering for this beast?
 

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Opel Tinkerer and Rescuer
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And the aforementioned piston steering..
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