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Pedal Smasher
1973 Opel GT
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Discussion Starter · #1 · (Edited)
Howdy Opelers,

I could use some advice, especially from Rally Bob and others who have built 2.4s. I need help choosing some of the parts. I know, dangerous idea to ask others what parts should I buy. :cool: The engine will be mostly for the street, but I don't mind it if I need to run premium gas. My GT is meant to be a classic sports car, not a daily, so I'll prioritize performance over fuel costs. I want my GT to be able to go from ~11,000 ft down to sea level, provided some tuning adjustments. So, it's best to target sea level and make adjustments for where I live.

I don't know exactly what cam to install. I've had some recommendations to get a custom cam grind done by Isky. So, cam recommendations to go with best piston choice are appreciated and the head will be ported / blended & smoothed. I'm looking at Wiseco and Wössner for pistons, I just don't know which would be the best choice for my application. I'm pretty sure I want to go with 96mm, but there are a few options between these two companies. I picked 96mm for two reasons. 1, I don't want to push this engine to the very limit and 2, leave room for it to be rebuilt down the line if needed. Hopefully my GT winds up being a cherished car for a long time (I want her to hit 100+ years old) and that goes into my plans.

The first option is Wiseco KE167M96 but I don't believe the 11.2:1 SCR. It's a flat top piston with a valve relief. My math put this closer to 10:1. I'm wondering if I'm leaving too much potential even for a street engine on the table with a flat top piston.

The second option is Wössner 2.0L pistons that might work in the 2.4? These pistons were made for the 134mm rod length of the 2.4L.

The third option is Wössner 2.4L pistons but I'm worried that the SCR will be too high and not work with pump gas.

I know the cam has to be considered and help picking the right cam would be great. I also need help picking valves and valve springs. I have a set of Harland Sharp roller rockers but I will need to get some solid lifters. I have an Isky OR-77 hydraulic cam that I'll likely sell because RB pointed out that this came is probably too mild for a built 2.4 when looking at valve lift and duration.

I'm ready to pull the trigger on the remaining parts I need to buy (pistons, rods, valves, valve springs, cam, retainers, lifters, and all the bearings). Down and down I go, deeper into the rabbit hole. :) I've been looking forward to sourcing the remaining parts for this engine.
 

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The Wössner 2,0 liter piston can't be used in a 2,4 liter, because of its higher compression height. And don't mix stroke and rod length. The 2,4 stroke is 85mm vs the 2,0 liters 69.8mm.
Are you going to use carbs or FI ?
 

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Just Some Dude in Jersey
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Charles Goin has been the most active builder of 2.4/2.5 engines recently, so I suggest giving him a call to talk about mm's, rod lengths, OR77's, SCR's, etc.

Personally, I'm sick to the teeth of loping engines due to cams. The stock 2.4 cam has a slight lope and has a unique sort-of split profile grind: It opens the valves fast, but closes them at a normal rate. Charlie really loves the power of OR77's in 2.4's, but a recent engine he made, a 2.5 with race gas worthy approximately 210-220 compression and flat top pistons, has been giving him and Harold Collins nothing but trouble. Broken starters, blown head gaskets, etc. He is currently pulling the engine from that car, The Doctor Wagon, and is going to switch out the flat top pistons in favor of dished ones to reduce that final compression. Send me a message if you need his number.
 

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Opel Rallier since 1977
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If this has to go to sea level, then you have to maximize the DCR (dynamic compression ratio) to sea level conditions, and then live with the results at 11,000 feet. It is definitely going to be doggie at those high altitudes; you're going to just have to live with that if you insist on the sea level operation, unless you elect to add a turbo- or super-charger. Push the DCR to 8.0 at sea level and use premium fuel. That will be safely tunable at sea level, while avoiding detonation. (And, if you are willing to detune some ignition timing before driving to sea level, then you can even go past that 8.0 number.)

Your driving preferences are very important in this. Please tell us in more detail how you will use the car and engine. I see too many cases of over-camming an engine and then getting an engine that has a peaky torque curve, which indeed has a higher peak HP, but has a narrow torque band and is not as pleasurable to drive in everyday use on the street. The duration of the OR77 is not the issue if the static compression ratio is high enough; the matter is for the valve lift to take any advantage of the breathing capacity of the head at lower and middle range RPM's for the 2.4L. But be ready to upgrade valve springs with a higher lift cam; the valvetrain accelerations get higher and you need the stiffer springs to maintain control. (That is the one good thing about the old, slow-ramp Isky designs: they are easy on the valvetrain. They had to be to work with the more limited valve springs of the 60's.)

The Wiseco pistons can be set up to over 11:1 SCR (static CR) with the right combination of deck height, combustion chamber size, and head gasket. You can adjust down from that with head gasket thickness. Where you may be getting misled is thinking that a flat top cannot go that high. But recall that the stock 2.4L piston is a dished piston. The use of a flat top takes away that dish volume and so drives the static CR up. The 2.4L is also a 'stroker' design right out of the factory, due to its very low rod-to-stroke ratio, and that rod-to-stroke ratio tends to push up static CR. The Wiseco is most probably going to be the piston you use here IF you are going to be at or around sea level.

Back up a bit and:
  • Give more detail on how you plan to drive the car and engine. I would assume 98% street driving and cruising but we should not make assumptions. That will determine the general cam characteristics. The cam and target DCR at what altitude then drives the piston selection, along with the associated parts and dimensions that set DCR.
  • Has the head been milled or do you plan to do so? This is tied in with the matter of combustion chamber volume. Eventually, you need to measure what chamber volumes you have ('cc the head') so as to work out the SCR. That is then used with the cam data to get to your DCR.
  • The measurements of things like the chamber volume are going to have to be done with reasonable precision. Similarly, installed deck heights are critical, as will be cam timing and adjustment. So prepare to do this slowly and with lots of measurements and some partial assembly and disassembly, and maybe even a trip back to the machine shop. The reason for all this care is that you need to push up the DCR at sea level to that 8.0 number if you want to have anything left at 11,000 feet.
  • What is the trannie, manual or auto?
  • Once built, spend some time to optimize and push the ignition timing at altitude. You might eke a bit more torque that way. It won't make up for an engine design optimized for the higher altitudes but it oughta help.
  • And you may be aware, but carbs tend to go rich as you go to high altitudes. So with carbs set leaner than normal for high altitude use, you are gonna have to adjust richer as you go to sea level. I assume that most newer FI system will do a fair job of compensating for that.
 

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The first option is Wiseco KE167M96 but I don't believe the 11.2:1 SCR. It's a flat top piston with a valve relief. My math put this closer to 10:1. I'm wondering if I'm leaving too much potential even for a street engine on the table with a flat top piston.
I looked up the Wiseco’s. They claim a zero deck height and 4.9 cc valve relief.

Based on that, I get 10.406:1 compression with a .032” head gasket and a stock, modified 2.4 combustion chamber (54.6 cc’s). Of course, different valves, receded valve seats, head milling or chamber unshrouding can change that very quickly.

I seldom go over 10:1 on a street 2.4/2.5 because they build cylinder pressures very easily. The only way to compensate is with a much more radical cam.
 
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As far as I know, the rod to stroke ratio has no effect on the static compression ratio. The static compression ratio is just the cylinder and combustion chamber volumes at BDC divided with the combustin chamber volume at TDC.
 

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Pedal Smasher
1973 Opel GT
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Discussion Starter · #7 · (Edited)
The Wössner 2,0 liter piston can't be used in a 2,4 liter, because of its higher compression height. And don't mix stroke and rod length. The 2,4 stroke is 85mm vs the 2,0 liters 69.8mm.
Are you going to use carbs or FI ?
Yep, I meant rod length. Not sure why I put stroke, maybe it was too late to be writing anything technical. Are you sure the compression height is a problem still if the piston was made for a rod length of 134mm? Deck height is still the same between the two engines. I'll be running a carb.

And, if you are willing to detune some ignition timing before driving to sea level, then you can even go past that 8.0 number.
My math had me around 8.3 at sea level with advancing the cam a little (ICA of 66) to help at higher elevation. I don't mind having to retune a little to drive at sea level. The reason for this wide range, is road trips. It would be nice to go to Carlisle and maybe try to get the GT into a show out in Cali. Some of my personal goals for the car is to get the car in Petrolicious quarterly publication (which should start up again after the pandemic) and I'd love to have Jay Leno see the car, always wanted to meet the guy. Ya, I know. Lofty aspirations.

Your driving preferences are very important in this. Please tell us in more detail how you will use the car and engine.
Spirited cruising, maybe some HPDE track time just to enjoy the car with a tad bit hard cornering. Sometimes simple runs around town. Some road trips. Mid range power would be needed. I'm fine with sacrificing a little peak power for a broader power band.

  • Give more detail on how you plan to drive the car and engine. I would assume 98% street driving and cruising but we should not make assumptions. That will determine the general cam characteristics. The cam and target DCR at what altitude then drives the piston selection, along with the associated parts and dimensions that set DCR.
Already covered I think. But, to add... look at Sandia Crest and the road that goes to it. Ton of fun. We also have a small track here that I'd probably go to for HPDE days.

  • Has the head been milled or do you plan to do so? This is tied in with the matter of combustion chamber volume. Eventually, you need to measure what chamber volumes you have ('cc the head') so as to work out the SCR. That is then used with the cam data to get to your DCR.
Not yet but it will be ported a little, more of an emphasis on blending and smoothing, unshrouding. All details of the engine will eventually be recorded, so I would at some point check the chamber volumes. That will decrease the SCR over a stock head but not too much is my thought.

  • The measurements of things like the chamber volume are going to have to be done with reasonable precision. Similarly, installed deck heights are critical, as will be cam timing and adjustment. So prepare to do this slowly and with lots of measurements and some partial assembly and disassembly, and maybe even a trip back to the machine shop. The reason for all this care is that you need to push up the DCR at sea level to that 8.0 number if you want to have anything left at 11,000 feet.
Precision I can do. I'd create a Lexan plate to bolt down to the head, with a small hole for a burette to fill the chamber. I believe in the scientific process of taking a measurement more than once whenever possible, to account for error in measurement.

  • What is the trannie, manual or auto?
Getrag 240, either a 3.44 or a 3.67 in the rear. I plan on getting a Quaife LSD.

  • Once built, spend some time to optimize and push the ignition timing at altitude. You might eke a bit more torque that way. It won't make up for an engine design optimized for the higher altitudes but it oughta help.
  • And you may be aware, but carbs tend to go rich as you go to high altitudes. So with carbs set leaner than normal for high altitude use, you are gonna have to adjust richer as you go to sea level. I assume that most newer FI system will do a fair job of compensating for that.
I want the engine optimized as much as possible for where I live, but it needs to be possible to drive the car at sea level if needed. I wouldn't mind that much if I had to rejet the carb at some point for a trip like that. I know there is a rule of thumb for jets to adjust for elevation once you have the carb set for a particular elevation. A sea level road trip wouldn't be common but I don't want an engine so dialed in for a mile high that I can't go to sea level without running race gas automatically.

I looked up the Wiseco’s. They claim a zero deck height and 4.9 cc valve relief.

Based on that, I get 10.406:1 compression with a .032” head gasket and a stock, modified 2.4 combustion chamber (54.6 cc’s). Of course, different valves, receded valve seats, head milling or chamber unshrouding can change that very quickly.

I seldom go over 10:1 on a street 2.4/2.5 because they build cylinder pressures very easily. The only way to compensate is with a much more radical cam.
I know I've asked you about cam recommendations before. You said to pretty much get a 1.9 race cam and increase the LSA. So, I'll get whichever piston is recommended with any cam specs that are deemed the best to go with.
 

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Well, if the deck height and rod lengths are the same, but the pistons move in a wider arc, the 2 liter pistons would pop out 7,6mm out of the bore at TDC. The compression heights are 31,5mm for the 2,4 vs 39,1mm for the 2,0.
 

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Opeler
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Official Wiseco catalogue, claims that those pistons may protrude above the deck. When I assembled my engine using their 96 mm pistons, they were very slightly protruding above the deck indeed. Rather than using thick gasket, I opted to machine the pistons to bring them to deck level. I also dished them 0.5 mm. Combined with Enem X1 camshaft, compression was around 165 psi and the engine worked fine with 91 (premium) gas.If I would build such engine again, I would probably not bother to dish pistons.
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Pedal Smasher
1973 Opel GT
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2,533 Posts
Discussion Starter · #10 ·
PJ, thanks for the info! What power output did you get?
 

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Opel Rallier since 1977
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As far as I know, the rod to stroke ratio has no effect on the static compression ratio. The static compression ratio is just the cylinder and combustion chamber volumes at BDC divided with the combustin chamber volume at TDC.
The rerference to rod-to-stroke ratio was as an indicator of whether this is a 'stroker' type setup or not. At around 1.5 rod-to-stroke ratio, the 2.4L is definitely such an animal. These typically do have a lot of static CR with flat top pistons, compared to higher rod-to-stoke ratios in the same engine family. So it is just something for the OP to know as he tries to grasp the higher-than-he-expects SCR of a flat top in this use.

The real effect of rod-to-stroke ratio is how well the engine will rev. Low rod-to-stroke ratio hurts high RPM breathing and high RPM peak cylinder pressures, hence why they are low RPM engines. It 'is what it is' with a particular crank and rod and block and you account for it in the other parts of the design.
 

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Opel Rallier since 1977
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I guess that settles which pistons to get. Now to pick a cam profile, valves, and spring rates.
Once you select the cam, then it is important to recheck DCR at sea level. DCR is the end goal to know and combines the cam and SCR characteristics to see how close the engine wll be to detonation-land at sea level. It is an essential computation IMHO if you want to push the engine to its best-at-altitude performance with the constraint of pump gas at sea level. Skipping this step brings in risk; see the post above about the problems of a (very possibly) too-high compression with too small-of-a-cam engine above running over 200 psi cranking pressures. That can all be avoided; there is no need to guess.
If you need help running those numbers, I'll be happy to assist. It can be discussed in this thread or via PM or email if you prefer. Things like combustion chamber volume measurements are part of all this.
 

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Pedal Smasher
1973 Opel GT
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Discussion Starter · #15 ·
Thanks. I've done the math before. My math gave roughly 145 psi at sea level. The problem is, those calculations don't really help me pick a cam. I can determine an intake closing angle and that's it. As long as the cam winds up at that ICA, then the rest doesn't show up in the DCR and DCP numbers. That's not really giving me much of how the cam will perform, which can change those values A LOT depending on how it impacts the efficiency of the engine which does show up in the calculation. So, I need help knowing what cam specs to aim for. I do kinda dig a lopy idle. It's rather sexy, as long as the engine still idles fine and has good low end power.
 

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Pedal Smasher
1973 Opel GT
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Discussion Starter · #16 · (Edited)
Just purchased the Wiseco pistons (KE167M96) and corresponding set of K1 rods (012EI17134). Soon I'll get all the bearings for everything. :cool: Now to figure out the valvetrain... I could have tried a high compression piston, but if I went to sea level I bet I'd be in trouble. A good cam could split the difference.
 

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Pedal Smasher
1973 Opel GT
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Discussion Starter · #17 ·
So, the cam I have right now is an Isky OR-77 Hydraulic. It has more or less the same specs as the OR-77. Looking at the OR-99, duration is 300, the LSA is 110, and lift is 0.48". If I get a custom ground cam, should I increase the lift? If yes to that, do I increase the duration? What should the LSA be? If I widen the LSA, that decreases the overlap which decreases scavenging.

When I did the math for the SCR, I had pretty close to 10:1. RB had roughly 10.4:1. My goal, with PJ's post, is to machine the pistons to zero deck.
 

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Pedal Smasher
1973 Opel GT
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Discussion Starter · #18 · (Edited)
Found an interesting article to help me figure out how I wanted to move forwards with the cam...


Going back to the OR-77 and OR-99, the only significant difference I see is the amount of lift. They both are ±2° from 250° of duration at 0.05" of lift and have a LSA of 110°, I doubt 4° more duration makes a huge difference. But the OR-99 has 0.05" (1.27mm) more of lift, which translates into 0.08" (2mm) more of valve lift with the Harland Sharp roller rockers (ratio of 1.6).

I'm thinking, maybe a little less LSA (107?) on the OR-99 cam??? This would increase the overlap a little, from 80° to 86°, which would be 7.5% increase. That would help a little with low end power and a broader power band. Of course, the only way to really know how much impact changing the LSA will have on the engine, would be to compare it to the stock OR-99 on an engine dyno.
 

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Pedal Smasher
1973 Opel GT
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2,533 Posts
Discussion Starter · #19 · (Edited)
Does anyone know what the valve spring dimensions are? It might be overkill, but I want to get a set of dual conical springs made.

Little worried I'm starting to just talk to myself now. LOL
 

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Opel Rallier since 1977
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Thanks. I've done the math before. My math gave roughly 145 psi at sea level. The problem is, those calculations don't really help me pick a cam. I can determine an intake closing angle and that's it. As long as the cam winds up at that ICA, then the rest doesn't show up in the DCR and DCP numbers. That's not really giving me much of how the cam will perform, which can change those values A LOT depending on how it impacts the efficiency of the engine which does show up in the calculation. So, I need help knowing what cam specs to aim for. I do kinda dig a lopy idle. It's rather sexy, as long as the engine still idles fine and has good low end power.
Right... you pick the cam for the overall use... THAT's the cam knowledge part. Longer durations natrually raise the RPM range, but when you do that, you also drop the low RPM torque and reduce the ratio of top to bottom RPM range for good usable torque. That latter matter is important with a manual trannie, where you don't have a torque converter with which you can play with the stall speed. If you pick too big of a cam duration, then you drop off the low RPM end of the good torque range in the 2-3 shift of the standard Opel 4 speed, and life really sucks for a while LOL. Street driving with a manual trannie is all about good low-mid range RPM torque, not peak HP, because the good low-mid-range RPM torque widens the good usable torque range. Street driving is not racing.

That is why you optimally pick the cam first, and THEN pick the pistons to give you the desired DCR. The DCR limit is based on fuel type, normally aspirated or turbo/super-charged, and the lowest altitude.

BTW, if you are getting 145 psi crankpressure, you CAN go up in either SCR/DCR or go smaller on the cam. A 160-165 psi cranking pressure is a good limit for pump gas. Your 145 psi corresponds to a low 7's DCR, and you're leaving a significant amount of low-mid RPM torque on the table and that is going to show up all over the altitude range, especially at the high altitudes. Trust me, you're gonna really feel that 0.75 point difference.

Since you have the pistons, then my approach for street use would be to work the cam duration and timing numbers with the piston choice to get that 8.0 DCR (or 160-165 psi cranking pressure)..... at sea level for your case.

One you know that duration etc., then you look at what you can do with the lift. Bob knows best what lift works better with what port type so use that as a starting point. Then you go to a cam mfrs and see what profiles can give you what lift with that duration. Then you ask them what spring rates go with that cam lobe profile. Then you gotta find the springs. If nothing is available or reasonable, you back up, try to compromise some paramter, and re-work with a different profile and spring rate. It can take some iterations, and this simplifies this a lot just to get it down into the basic steps.
 
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