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Pedal Smasher
1973 Opel GT
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Discussion Starter · #1 ·
I know it may seem presumptuous of me to start planning what I intend to do with a GT when I have yet to acquire one, but I've always been the type of person to plan something out. Like till the point that the horse was dead a few years ago. I'll constantly refine my plan till I have such a clear image of what I want to do, that I can imagine every detail of the project, down to the nuts and bolts, in my head. My screen name is autoholic for a very good reason, I literally have to have daily car porn. So much so that I'm actually attracted to automobiles, LOL. No, I haven't stuck in the tail pipe...yet.

So, down to the point of this thread. I've done a fair amount of reading and the first thing that pops up with the GT is that it is down on power. The 2.0 L is a popular conversion simply because it only requires an increase in bore diameter. I've done the math and at maximum, you can only get just over 2 litres. I don't want to bore it out to the maximum of 97 mm, I'd rather shoot for 96 mm. This allows for a freshening up rebuild down the road. On a related topic, is the CIH a candidate for sleeves? This would allow the block to have a much longer life span.

I know that by increasing the stroke, you can extract a lot more from the 1.9 block. Doing some math, if you stick with the short pistons and short con rods, you can increase the stroke to 81.8 mm and with a 96 mm piston, you'll have a 2.3 L displacement. The only problem I have is that I don't know the precise deck height for the CIH if it hasn't been decked. I've looked but I can't find it. On a related note, I remember seeing the compressed thickness for the cylinder head gasket but I can't seem to fine it now. Can anyone help with this info? I know a stroke of 81.8 mm will require a custom made crankshaft. I can dig up all the math once I have these two figures to verify numbers. I believe my reverse engineering gave me a deck height of roughly 207.8 mm if the factory engines had zero deck clearance.
 

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Bikini Inspector
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5,771 Posts
It has been done with 1.9 block aND crank. I have one.

It sounds like you're more than capable. The directions are out there. I bought mine already done.

And yes, 2.0 is a nice improvement, but the 2.4 stroker is perfect for the gt.
 

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Über Genius
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9,401 Posts
It has been done with 1.9 block aND crank. I have one.

It sounds like you're more than capable. The directions are out there. I bought mine already done.

And yes, 2.0 is a nice improvement, but the 2.4 stroker is perfect for the gt.
Whoa there pardner...

Your 2.4 is made from a 1.9 block and crank?

How was it done with the crank?
 

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Pedal Smasher
1973 Opel GT
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2,574 Posts
Discussion Starter · #6 ·
From my understanding, there is no way you can even hit 2.2 L with the 1.9 L crank. You need the 2.2 L crank to go beyond 2.0 L.

I really need to know the deck height of the block and comrpessed gasket thickness fo the head gasket. Once I have this info, I'll put all my math on here. There is a method to my madness. :)
 

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Über Genius
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From my understanding, there is no way you can even hit 2.2 L with the 1.9 L crank. You need the 2.2 L crank to go beyond 2.0 L.

I really need to know the deck height of the block and comrpessed gasket thickness fo the head gasket. Once I have this info, I'll put all my math on here. There is a method to my madness. :)
I have the compressed gasket measurement. I measured mine at .030 pulled from a stock 1969 1.9L

And, if you want, I can send you an Excel file that will calc everything out for you.
 
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Pedal Smasher
1973 Opel GT
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2,574 Posts
Discussion Starter · #8 ·
Sure, I'd like to see it. I know all the calculations but there might be some info I don't have yet. I can't get a line on displacement yet as I don't know approximate volumes. I have an excel sheet I'll probably put up here, it's a gear calc that also gives you all the dimensions of a tire. You simply put in the tire size from the sidewall, your gear ratios and a few other bits of info and it spits out all the info you could ever want when selecting gear ratios and tire sizes. It gets addicting, as you can easily pick out the best theoretical tire size for your gear specs.
 

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Über Genius
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Let's see if this works.

I have some numbers in there already. They can be adjusted to suit your mood.
 

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Pedal Smasher
1973 Opel GT
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2,574 Posts
Discussion Starter · #10 · (Edited)
Ok so here are my calculations for just displacement. I'll start by listing the various options for component dimensions. Oh and the excel sheet worked, thanks! But I couldn't view the data behind your equations.

Stock 1.9 L Info:
Bore - 93 mm
Stroke - 69.8 mm

Stock 2.0 L Info:
Bore - 95 mm
Stroke - 69.8 mm

Stock 2.4 L Info:
Bore - 96 mm
Stroke - 85 mm

Con Rod Lengths
Short - 128 mm center to center (1.9 L / 2.0 L)
Long - 134 mm center to center (2.4 L)

Piston Compression Height Lengths
1.9 L Short - 39 mm
1.9 L Long - 45 mm
2.4 L - 31.5 mm

Knowing this information, I can figure out what would be roughly my deck height, +/- 1 mm, as long as the info is correct. The deck height is stroke/2 + rod length + compression height.

Using the 2.4 L info:
(85/2) + 134 + 31.5 = 208 mm

Using the 1.9 L info:
(69.8/2) + 128 + 45 = 207.9 mm
note: I believe this is the stock config.

Alternate 1.9 L config:
(69.8/2) + 134 + 39 = 207.9 mm

You can find con rods of other lengths, my research made me trust these lengths as the source was Wössner's parts catalog.

If you dive into engine math, one important thing to look at is mean piston speed.

MPS = 2 * Stroke * RPM / 60
It should be noted that you do not want to exceed ~25 mps and a good goal for performance is between 20 - 25 mps. Set RPM to whatever would be your maximum. I'm putting 7000 as my target (all comes down to valve springs and valve weights).

1.9 L MPS = 2 * 69.8 / 1000 * 7000 / 60 = 16.29 mps

2.4 L MPS = 2 * 85 / 1000 * 7000 / 60 = 19.83 mps

Due to this not including the stroke to bore ratio which can be very important, there is a corrected piston speed. It's argued that this is more accurate to determine stress on an engine, but one common metric for measuring engine performance is MPS.

CPS = MPS / square root of stroke to bore ratio

1.9 L MPS = 16.29 / sqrt(69.8/95) = 19.0 mps

2.4 L MPS = 19.83 / sqrt(85/96) = 21.08 mps

The benefit of a shorter stroke is that you'll make peak horsepower at a higher RPM, as long as you don't have valve float. Another metric worth looking at is stroke to bore ratio by itself. For more information, go here: Stroke-to-Bore Ratio: A Key to Engine Efficiency - Achates

1.9 L S/B: 69.8 / 95 = 0.735

2.4 L S/B: 85/96 = 0.885

This would indicate that for racing, a really well built 1.9 L would do better, as long as you could hit the MPS (~25 mps) redline at roughly 10750 - 11000 RPM. The 2.4 L would have a similar MPS redline at roughly 8800 - 9000 RPM. Will I be racing this GT? Chances are it might see some open track time but it won't be racing. However what car guy builds an engine just to cruise around town? Is this overkill math? Maybe, but I'm an engineer, overkill is what we do. All of this is just to try and help me figure out the right combo, and I haven't even dived into the math behind cam selection yet. In the end, I'm more likely to get close to peak power in a 2.4 L than a 1.9 L because I probably won't be able to build the engine to sing properly to 11,000 RPMs. The higher the RPMs go, the worse the street manors are for the engine near idle. F1 engines sing beautifully to around 18,000 RPM, but they also have an idle of around 3000 RPM. I think they'll even stall if you try to run them at 1000 RPM. This is all due to cam profile, even VVT can only do so much.

I was thinking of going with the shortest possible con rods and comp height, to minimize the moment of inertia while at the same time maximizing the displacement. You have rotational inertia and reciprocating inertia. Rotational is your crank, flywheel, etc. Stuff that rotates. Reciprocating is the small end of the con rod and the piston. This is where you want to decrease the amount of weight as much as possible. Imagine a car going 45 mph (20 mps), coming to a dead stop in a fraction of a second, and then going in reverse at 45 mph in just as much time as it took to slow down? The crankshaft's moment of inertia will come into play with how fast the engine responds, along with the inertia of the entire drivetrain, but the crankshaft doesn't come to a sudden halt like hitting a brick wall at 45 mph. It's your reciprocating mass and MPS that can really limit how fast an engine can spin before suffering from catastrophic part failure. So you have to find the right balance of con rod length, comp height, con rod weight, piston weight, stroke and bore to build a really phenomenal engine.
 

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Pedal Smasher
1973 Opel GT
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2,574 Posts
Discussion Starter · #11 ·
Building off of my previous post and the conclusion, I had a thought. I would stick with the 2.4 L crank / stroke, but decrease the con rod and comp height lengths as much as possible.

(85/2) + 128 + 31.5 = 202 mm

This would require a 6 mm decking of the block, which is just under a 1/4 of an inch. The timing chain length would have to be decreased a link or two. But at the same time, is taking 6 mm out of the con rod worth it, without increasing the stroke? That would require figuring out the moment of inertia differences. Then it occurred to me, can you increase the stroke to 91 mm? Would it result in interference with the block skirts or oil pan? A 91 mm stroke would give you a displacement of 2.6 L with a bore of 96 mm. Can this even be done on the 1.9 L CIH?
 

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Just another for the mix.

Stock 2.2:
95 mm bore
77.5 mm stroke

The 2.4 has a stock bore of 95 mm as well, not 96 mm.

Another consideration, while the 2.4 engine has an 8-counterweight crankshaft (forged like all the others), it's about 43 lbs. A 1.9/2.0 crankshaft weighs about 36-36.25 lbs., while a 2.2 crankshaft weighs about 34 lbs.

A 2.4 engine rules for the street, as it has very good torque and a useable powerband. Which means you can use a normal gearbox. If you built a 10,000 rpm 2.0 litre you'd have no useable torque below 5000 rpm and you'd need an $8000 gearbox to utilize the powerband.

I would never use a shorter rod in a 2.4, they already wear the bores like crazy thanks to the high rod angularity and increased thrust loads from the longer stroke and bad rod ratio.
 

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I would use a 2.3 diesel crank if I was going to build a stroker, oh wait I already did

(I actually paid someone else to do it but who's counting)
 

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Pedal Smasher
1973 Opel GT
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2,574 Posts
Discussion Starter · #14 ·
I would never use a shorter rod in a 2.4, they already wear the bores like crazy thanks to the high rod angularity and increased thrust loads from the longer stroke and bad rod ratio.
Thanks for mentioning this, it slipped my mind. With a shorter the con rod, lateral forces and ring flutter will be greater.
 

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Pedal Smasher
1973 Opel GT
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2,574 Posts
Discussion Starter · #16 ·
The problem with a short skirt is that it tends to have more play in it, rocking side to side as it goes up / down the bore.
 

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Your building or getting thoughts with building a stroker.
Billet block,cylinder head,roller cam...

BTW the short skirt piston(pictured) has over 175K on it.
 

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Whoa there pardner...

Your 2.4 is made from a 1.9 block and crank?

How was it done with the crank?
Umm, isn't it a modestly common thing to "Stroke" the 1.9 crank to get more displacement? I'll go looking for some threads, but a fellow Calgary Opeler (John Warga, aka "ftl") sent his 1.9 crank away to have it "Stroked" to a 2.4 throw. Cost some $ ($600 USD IIRC) as it entails welding up the throws and re-grinding to the larger stroke. One complexity was the piston and rod choice, but I don't recall the details.

Bob, isn't that your design?

Ps. I moved this thread to the "Opel Engine Performance Modifications" Forum, where there are threads on the topic of stroking the 1.9
 

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You can also offset grind the 2.75 crank and use smaller connecting rod journals.
The bearing speed is much lower BTW
 

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Umm, isn't it a modestly common thing to "Stroke" the 1.9 crank to get more displacement? I'll go looking for some threads, but a fellow Calgary Opeler (John Warga, aka "ftl") sent his 1.9 crank away to have it "Stroked" to a 2.4 throw. Cost some $ ($600 USD IIRC) as it entails welding up the throws and re-grinding to the larger stroke. One complexity was the piston and rod choice, but I don't recall the details.

Bob, isn't that your design?

Ps. I moved this thread to the "Opel Engine Performance Modifications" Forum, where there are threads on the topic of stroking the 1.9
Keith, I did design the 1.9-2.4 welded stroker engines, wrote an article in the OANA over 20 years ago about it.

In a nutshell:
*1,9 crank with welded rod journals, offset-ground to 3.25".
* stock 1.9 128 mm rods, pin end enlarged to .927" from 23 mm.
*off the shelf .030" oversized (3.766") 305 Chevy dished pistons with top edge milled .030", this represents an overbore of .105".

Net result is 2372 cc's, and about 9.23:1 compression ratio
 
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