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Upgrading Cylinder Heads On A L98 Chevrolet Corvette Motor - Heavy Breathing Deep Pockets

Cylinder Head Improvement Equals More Horsepower For The L98 Corvette

Richard F. Newton Aug 1, 2002
Vemp_0208_01_z Upgrading_cylinder_heads_l98_chevrolet_corvette_motor Salvage_yard_parts 2/12

These are the aluminum heads as they arrived from the salvage yard. Everything was removed and the bare castings were hot tanked. After this cleaning, they were shipped to Extrude Hone. I really paid $300 for a set of bare aluminum castings. You might think of them as a canvas for an oil painting.

This adventure started with a $300 set of used aluminum cylinder heads from a trashed '87 Corvette. Several months later, not to mention about $8,000, I found just over 80 hp, and about 90 lb-ft of torque. In an effort to pass a Porsche 944 Turbo-S, I learned a whole lot about cylinder heads-as well as spending money. Anytime you can spend a pile of money that is roughly equal to the value of your car, you learn a few lessons. You might want to call this Cylinder Head Consumerism 101.

The extent of misinformation about cylinder heads, not to mention power increase, is staggering. Every manufacturer has a set of claims that would lead you to believe that they're the only one that can produce a quality cylinder head. It's virtually impossible to sort the good from the bad, not to mention the ugly. Unable to deal with this situation, I fell back on my old performance rule: Go talk to the guys that go fast. Real racers have to have the best equipment-they get paid to win. Since most of us just play with cars, and don't depend on speed for our income, we're not totally informed. Trust me, people who go fast for a living have a pretty good idea about what really works. They also have a pretty good idea about what it costs.

The cylinder head is the key to improving horsepower and torque in your Corvette. I'm not going to tell you that my final selection is the best combination. I really don't know what the best combination is. It would take several hundred-thousand dollars and months to do the necessary testing. When the NASCAR BUSCH series went to a new set of motor rules last year, most teams spent about a $500,000 on research to find the best combination. We simply don't have that kind of money here at Team VETTE. What we can do is help you wade through a little bit of the cylinder head hype.

Let's talk about why this cylinder head swap was necessary. The old L98 Corvette motor ('85-91) is a great torque motor. It's not so good at producing horsepower at high rpm; and by high rpm we mean anything over 4,500 rpm. This motor was designed in the late-'70s to produce adequate torque and clean emissions. The emphasis was on clean emissions. The torque came as a side benefit.

Vemp_0208_02_z Upgrading_cylinder_heads_l98_chevrolet_corvette_motor Aluminum_cylinder_head 3/12

One reason these heads only cost $300 was the small problem where the head was eroded away at the edge of the chamber. This is supposedly very common on aluminum cylinder heads. We welded it and then resurfaced the head. In hindsight it was, just as everyone told me, no big deal, but I did have a few moments of anxiety about my bargain heads.

Chevrolet installed over 150,000 of these L98 engines in the C4 Corvette. Every single one of them runs out of power at around 4,300 rpm. These are great motors for driving around town. They have tremendous torque and can move away from a stoplight with the best of them. My '85 can run the first 60 feet of the drag strip in less than two seconds. That's what bottom-end torque is all about.

Over the past few years I had installed a TPIS ported plenum and big runners. I had also installed a set of TPIS headers. The result of all this was a great street motor that simply ran out of breath above 4,500 rpm. The peak horsepower was occurring at 4,300 rpm. This isn't all that bad for a street car, but on the back straight at Sebring I was relegated to a parade of Porsches going past me. That's embarrassing.

I run a half-dozen track events at Sebring every year. Sebring is a fairly fast track, and you can actually use higher rpm. If I could pick up 1,000 rpm, not to mention some real horsepower, I could get down the backstretch without shifting into overdrive. I would need horsepower to develop this speed. Anytime you're over 100 mph you should look at the horsepower, not the torque. A total lack of horsepower at 5,000 was the problem. A different cylinder head, a new camshaft, and a new manifold seemed to be the solution. Visa would make all this possible.

The earlier changes allowed me to get a fair amount of air into the plenum and down to the manifold. Then all this air was simply backing up in the manifold and cylinder heads. The manifold and the intake passages were one huge bottleneck. This was confirmed by looking at the fact I only moved the horsepower curve up about 300 rpm with all the improvements I had previously made to the car.

Vemp_0208_03_z Upgrading_cylinder_heads_l98_chevrolet_corvette_motor Cast_iron_cylinder_heads 4/12

This is a picture of the stock port. The stock aluminum cylinder heads don't have a lot going for them over the old cast-iron heads. The biggest plus might be the weight savings, which really isn't all that significant. The real advantage is that you really can't destroy an aluminum cylinder head. If you break anything the head can easily be rewelded. This is the major reason racers prefer aluminum cylinder heads to the cast-iron versions.

I needed to open up this bottleneck. This would be fairly simple except that I also drive the X-S Corvette back and forth to work every day. I knew I could get horsepower. The real problem was to get it without giving up too much torque. The old adage is that you "advertise horsepower, you drive torque." When you mash the throttle down on your Corvette, the torque puts you back in the seat. Horsepower is really not relevant for most of us.

Running the calculations through the computer, I found that every 500 rpm meant roughly another 10 mph. This meant that I would be happy with 500 more rpm, and if I got 1,000 rpm it would be just like Christmas in June. If I could pull 1,000 more revolutions, with real horsepower, it would mean roughly 134 mph before I would have to downshift and make the big right turn onto the front straight. It would also mean that fewer Porsches would pass me. Life is always better if you don't have Porsches passing you.

Cylinder Heads, Manifold, and Camshaft
It was pretty obvious that this wasn't going to happen unless I changed the manifold, cylinder heads, and the camshaft. You have to think of this as a complete package. Besides, once you have the cylinder heads and intake manifold off the engine, it's not much of a step to change the camshaft.

At this point I needed help. I needed the sort of help that you can only get from people who build motors for a living. If you build race motors for a living you have to be able to cut through the exaggerated advertising claims and find out what really makes power. If you're a professional engine builder, your customers actually race against real cars, and they get very unhappy when they lack power.

Vemp_0208_04_z Upgrading_cylinder_heads_l98_chevrolet_corvette_motor Accel_intake_manifold 5/12

This is the new ACCEL intake manifold. The passages are slightly different from the others on the market, which some folks feel gives you more power. There are actually three different castings of this manifold on the market. You can select from Edelbrock, TPIS, and ACCEL. I had this manifold opened up to match the cylinder heads. All the ports are very smooth. If this were a carbureted engine, the smoothness might be a problem, as the gas could drop out of the air/fuel mixture. In a fuel-injected engine this isn't much of a problem since you have dry air until the fuel injector adds the gasoline, which it does fairly close to the intake valve.

The Heads
I had already found a set of aluminum cylinder heads at my favorite salvage yard. These heads were from an '87 Corvette. After tying to make sense of all the claims, not to mention all the (mis)information on the Internet, I decided to stick with the GM program. General Motors knows a few things about Chevrolet engines. Every time I've used a GM part on my Corvette I've had a good experience.

Besides, these heads were cheap. I paid $300 for the pair. I have to admit I was a pretty uninformed consumer when I wrote the check. Looking at a set of cylinder heads on the floor of a salvage yard shop doesn't tell you a lot. There were no burned valves and no signs of a blown head gasket; so how could I go wrong?

Extrude Hone
I knew that I wanted to put more air through these cylinder heads than GM had originally planned, so the next question was how to make the ports bigger. There was a time when I would have gotten my trusty grinder out and done this all by myself, but that time has since passed.

I also knew that Ford and GM use Extrude Hone. This is a process where abrasive mud is pushed through the intake and exhaust ports. This not only smoothes the ports, but opens them up considerably. When I first started this project I sent an ACCEL intake manifold to Extrude Hone and was very pleased with the results. Why not send the heads?

Another reason I decided to send the heads to Extrude Hone was that I couldn't make any sense out of all the various claims for aftermarket heads. One person will tell you that port matching is very important, and another one will tell you it's a waste of time.

A friend of mine related how he purchased several very expensive heads from various high-tech race shops, and none of them could match the numbers he had been told he would get. That's when I realized that you could manipulate the flow bench numbers. It turns out that a flow bench is just like an alignment machine. Tell me what you want to see, and I can give that number on the screen.

Vemp_0208_05_z Upgrading_cylinder_heads_l98_chevrolet_corvette_motor Combustion_chamber 6/12

Here's the heart of this whole experiment. The goal was to get as much air into the combustion chambers as possible. I replaced everything from the old TPIS big runners to the combustion chamber. The best part was, it worked. The irony is that we've now reached the end of the road for the modified Tuned Port intake system. Now I'm back at the top of the circle with the runners and plenum being the limiting factor once again.

The other problem is that everyone gives cfm numbers at maximum lift. Maximum lift only lasts for a very short time in the cycle. In fact, most of the air is already in the combustion chamber by the time the valve reaches full lift. With all this in mind, it was back to "Run what the fast guys run."

After checking with some fairly serious racers, I made the drive to RPM Performance in Auberndale, Florida. RPM has been building motors for drag racers and circle track guys for a number of years now. I was quietly impressed with over 100 crankshafts hanging from the various racks. Even more impressive was several million dollars of machining equipment in the shop. This was a serious shop. I figured they could not only put my new heads together, but I could get some good advice, too.

Everyone talks about 2.02-inch intake valves. When I asked the guys at RPM about this they just smiled and looked at me as if the village idiot had just walked in the door. "We build motors that turn 7,000 rpm and still run stock valves. What exactly are you attempting to do here?"

This is the point where you sort of look at the floor and admit that you haven't got a clue. It's one thing to read all the magazines, and look at all the catalogs. It's another thing to actually build motors. It's also the major reason you need to pay attention to the professionals. Remember, you're going to spend some fairly serious money. Make sure that you get your advice from people who actually go fast-not just talk fast.

Vemp_0208_06_z Upgrading_cylinder_heads_l98_chevrolet_corvette_motor Fuel_injected_engine 7/12

Here is the intake side of the same head after I got it back from Extrude Hone. The one thing that you'll notice is a very smooth finish. With a fuel-injected engine this is no problem since you have dry air until the fuel injector. You'll also notice how much larger the port is. This means that you can push a larger volume of air down the port, but at a lower velocity. When you give up velocity for added volume, you could easily lose torque. We countered this by raising the compression ratio. In the end it all worked out fine.

My next question was why I should be using Ferrara valves-a company I had never heard of. Once again Allen, the owner of RPM, sort of smiled and said, "Because I've never had a problem with a Ferrara valve." This is from a guy who not only builds very serious race motors, but has also tried every other valve on the market. Allen said he's purchased virtually every aftermarket valve around, and that sooner or later he's always had a problem. "Ferrara has never let me down." Remember, Allen is a guy who builds 700-plus horsepower motors for offshore boat racers. If they work for him they should work for me.

Valve Springs
This was an easy one, actually. Allen said there was only one choice here. Competition Cams makes the strongest spring that fits on a stock Chevrolet head. There may be stronger springs, but they all require special machining of the cylinder head. I mentioned a few other brand names, and he just kept smiling. Hey, I'm not the fastest guy on the planet, but I finally got the point and called Comp Cams.

Rocker Arms
Once again there was really little choice here. (Is this starting to sound repetitive?) I wanted to run the stock Corvette rocker arm cover, and Competition Cams makes the only roller rocker arm that will fit under the stock cover. This is more important than you might first realize. There's a real shortage of aftermarket rocker arm covers for these aluminum heads. There are times when you start to think you might be the only person in the world using this GM aluminum cylinder head.

The Camshaft
This is where all the power really comes from. You can have the greatest valvetrain in the world, but if the valves open and close at the wrong time, your Corvette is going to feel like a Ford Pinto.

When I asked RPM for some advice about camshafts, he simply said, "Call Comp Cams and purchase whatever they suggest." He felt the same way about Comp Cams as he did about Ferrea valves. With the sound of 700-plus bhp ringing in my ears, I decided to take his advice. Who should I trust? A man who sells 502ci race motors in matched sets, or the guys in the Internet chat rooms?

Vemp_0208_09_z Upgrading_cylinder_heads_l98_chevrolet_corvette_motor Comp_cams_rocker_arms 8/12

Everyone is running 1.6:1 rocker arms these days. The best part of the Comp Cams rocker arm is that it will fit under a stock valve cover. I don't know a great deal about these rocker arms except that the fast guys use them. In my basic consumerism model, I've learned to respect what the fast guys tell me.

Putting It All Together
This is really the boring part. There was a time when I thought putting a motor together was pretty exciting. On the other hand, a lot of things are exciting the first time around. The real trick here is to find someone that's done the job before. A lot of folks can bolts cylinder heads to an engine block. The problem is all the little things.

There's a problem with Corvette modifications that no one ever really talks about. Once you make changes to your Corvette, you might as well marry the technician that did the work. No one else is ever going to want to work on your Corvette. They won't even be able to figure out what's going on.

Any time you change the computer, the chip, and a half dozen relays, you've just made troubleshooting very difficult for the next technician. If I were to take my X-S Corvette to my local Chevrolet dealer, I would probably be thrown out of the shop. The other alternative would be an incredible bill, since they would have to figure out all the modifications before they could troubleshoot the system.

That means not only do you need to locate someone who can do the required work, but you also need someone you can trust for the next few years. Performance work is a specialty. Once you contract for very specific performance modifications, you're entering into a long-term relationship.

Vemp_0208_10_z Upgrading_cylinder_heads_l98_chevrolet_corvette_motor Nascar_dyno_system 9/12

This is the true test. No matter how much horsepower you think you might have, the dyno test doesn't lie. Keep in mind that this is the same dyno that NASCAR uses to determine whether Ford, GM, or Dodge has the most horsepower. I'm no fan of the NASCAR system, but they do use the best equipment available. Good, bad, or indifferent, you should believe your dyno sheet. Real horsepower beats advertising.

A friend of mine once had Lingenfelter build him a monster Suburban. This was an incredible vehicle. The only problem was when he had a couple of problems, he had to ship the truck back to the Lingenfelter shop. Add up the cost of shipping your truck halfway around the country and you get the picture.

All of this is to simply point out that you're not only going to pay for the initial modifications, but you're also going to pay again at some point further down the road. You've always wondered why the Ferrari and Porsche guys had such incredible service bills? Well, modify your Corvette and you'll be allowed to join the same club. It isn't the type of car that makes repair bills expensive; it's the degree of specialized service that you're going to need when all the modifications are added.

When there are only a limited number of people available to perform this specialized service correctly, you can expect to pay a premium.

When you shop for someone to do this type of work on your Corvette, keep in mind that this is going to be a long-term relationship. A cylinder head and cam swap is not a one-night stand. Make sure you can live with this person. They're going to charge you for this relationship, but that's ok. Just make sure you get what you're paying for. Also, don't ever forget that age-old answer to the question, "How fast do you want to go?" is, "How much money do you have?"

The Great Dyno Numbers
The good part was that all of this actually worked. This combination is one very strong motor now. Prior to all this effort the car would flat run out of power at 4,500 rpm. Now it runs strong all the way to 5,500 rpm. The X-S Corvette feels more like the old LT4 engine than anything I've ever driven. Over the years I've absolutely loved two small-block engines. The first is the '68 Z-28 with the 302-cid engine. I'll never forget running through the gears with this high-rpm motor. The old LT4 motor was similar except it had a whole lot more torque. Now I have something very similar.

One of the interesting numbers we found on the dyno is that this new configuration gets from idle to 4,000 rpm almost 1/2 second quicker than the earlier version. This means things feel a lot more responsive than before. Engines aren't about any single number. A strong motor is really about a combination of things.

Vemp_0208_11_z Upgrading_cylinder_heads_l98_chevrolet_corvette_motor Exhaust_manifold 10/12

This is the next step. I've finally concluded that the plenum and runners are limiting my power. If you have an L98, this manifold is the answer for high rpm and horsepower. ACCEL has stopped manufacturing the old Lingenfelter L98 Intake system. We are now left with two choices: stock or TPIS. Stop back in a few months and we'll let you know how the TPIS manifold works.

The most interesting item is that for all this effort, and a decent pile of money, the torque curve peaks exactly where it did on the stock motor. It seems this motor wants to peak the torque at 3,400 rpm, regardless of what we do. The good part is that with the new heads and cam, the torque curve remains flat and just continues. In fact, the biggest torque gain was at 4,700 rpm. The torque at 4,800 is as much as the old stock motor used to produce at 3,400. This is a good thing.

The maximum horsepower now occurs at 4,700 rpm rather than the old stock number of 4,200. With a stock L98, you're lucky to get over 200 bhp at the rear wheels. With this new combination we're getting almost 266 bhp at the rear wheels. Better yet, it's happening at a higher rpm range. Remember, this whole deal was about being able to pull the long straights at Sebring.

I've reached the edge for the street. The car is faster than anything needs to be on the street. It is also right on the edge of being streetable. I managed to keep all the strong torque numbers of the basic L98, and the car idles right at 800 rpm.

The main reason for this, according to my friends at Vinci Performance, is that in the head swap we probably picked up around one point in compression. Roger Vinci feels a compression gain will allow you to maintain a strong bottom end when you swap camshafts. Indeed, they recommend that anytime you change to a more aggressive camshaft you need to raise the compression, too. We did this by using the '87 aluminum heads, which have a smaller combustion chamber than the stock cast-iron L98 heads.

Vemp_0208_12_z Upgrading_cylinder_heads_l98_chevrolet_corvette_motor Street_motor 11/12

Here's the whole thing all back together. It's truly a stealth motor, until I start it. The only real clue is the center bolt valve covers. Even that takes a real Corvette expert to notice. I've got all the horsepower and torque necessary to go fast on the track, and still have a reasonable street motor.

At the track I've had to learn to drive all over again. I can run the revs up a lot more than I could with the previous motor. I also have to be a little more careful coming out of a corner. When the power curve hits, the X-S Corvette goes. Road racing is all about exit speed in the corner. This Corvette now has tremendous power at the exit, without the hassles that you get from a high-rpm motor, where you need to keep things very high on the rpm scale.

After I looked at these dyno figures, I went back and looked at some old records from about ten years ago. When I wrote my book How to Restore and Rebuild Your Corvette: 1968 to 1982, I had an engine built by a company called Racing Head Services. They were located directly across the parking lot from Competition Cams. This motor was a 383-cid engine with 9.5:1 compression and an Edelbrock carburetor. This makes an interesting comparison with my 350-cid motor. Both motors were built as street engines. Since the 383 was run on an engine dyno and the 350 was run on a chassis dyno, I had to use a correction factor for drivetrain loss. I used a 1.17 factor for drivetrain loss.

Keep in mind that we're really comparing apples and bananas here. This is simply interesting material, and shouldn't be seen as a definitive comparison.383 -VS- 350

Some Interesting Numbers
Torque @Horsepower @4,000 rpm5,000 rpm350 cid FI 391 lb-ft298 bhp383 cid Carb 418 lb-ft360 bhp

There's no question that a 383 makes a whole lot more torque. Is anyone really surprised? What's interesting is that the 383-cid horsepower is substantially better than the 350-cid engine. The other interesting thing was the 383 peaked about 400 rpm higher than the 350 motor. That's probably a result of sticking with the tuned port runners on the 350: a decision I hope to rectify in the very near future.

As Long As We're Taking this Apart, Why Don't We...
This magic phrase will destroy your finances quicker than a NASDAQ stock. In order to make an improvement to your Corvette, you're going to take something apart. Once you get that part off the car there will be another part staring you right in the face. Hey, why not replace that at the same time?

Vemp_0208_13_z Upgrading_cylinder_heads_l98_chevrolet_corvette_motor Performance_valve_springs 12/12

The valves springs are very similar to what was used on the old Z28 motors. These springs don't require any machining on the cylinder head. Remember, you only need a spring that closes the valve. Too much spring tension will simply cause wear problems. There is truly an optimum spring tension, which is why you ask Comp Cams what's going to work best.

The problem is that this will continue until you've taken the whole Corvette apart, or until you've spent the kid's tuition money for the next semester. The easy part about any Corvette project is the starting point. The real trick comes in choosing a stopping point.

You have to make some decisions very early on about where you're going to stop. Deciding what to do is the easy part. Deciding what not to do is the hard part. Let's take the X-S Corvette as an example. The decision was made to swap both the camshaft and the cylinder heads. Since the best way to do this is to remove the motor, the radiator is going to have to come out. When a car has over 110,000 miles, it wouldn't hurt to at least send the radiator out to have it cleaned. No big deal; just a little more money.

A high-mileage C4 usually has worn throttle shaft bushings in the throttle body. Since the throttle body is coming off the car, why not have bronze bushings installed? This would cure the rough idle problem. Once again, no big deal; just a little more money.

Then Chris, at the Corvette Clinic, suggested that as long as we're doing all this work why not replace the slow '85 computer with something from an '89. He thinks we can find a used one for around $75. A little wiring modifications and everything will be fine. No big deal; just a little more money. One thing I've learned in this life is that it's easy to spend other peoples money.

Oh-and by the way, why don't we put a new chip from Fast Chips in the transplanted computer? There goes another couple of hundred dollars. Again, this was my money-not theirs. In the blink of an eye we had added $1,000 to this project.

Most of you have been through this process. If you haven't been here, then you're probably still happy simply waxing the paint on your Corvette. Actually, once you start a project like this one, you gain a whole lot more respect for the waxers. These guys have it all figured out. They simply buy a new Corvette then order a couple of gallons of Zaino Bros or Ibiz polish. They're good to go for the next few years. I know my wife thinks this is a good deal.

Those of us who have a need for speed have an addiction that financially is almost as bad as a crack habit. Just as the crack addict must someday quit the pipe, we have to be willing to tell the shop to STOP! When Chris called me and suggested that I might want to put new rings and bearings in the motor, I said "NO!" He was totally correct to suggest this additional work. Considering the mileage on the car, and the fact it gets a lot of track use, this wasn't a bad idea. The only bad part is that I might find myself sleeping in the garage alongside the car if I spent any more money on this Corvette.

The real trick of enhanced performance, and avoiding financial ruin, is to "know when to say no." Before you start the project, make sure you have some lines drawn. As the project progresses the lines may change, but at least you'll have some lines. Whatever you budget for your project, you'll exceed that number. How far you exceed that number will determine the state of your domestic tranquility for the next few years.

What It Costs To Go Faster
In one of my books I have a suggestion for calculating the cost of restoration work. You need to go through all the catalogs and make a complete list of all the items you'll need. When you reach a total, simply double that number and you'll be close. The time factor should be calculated by getting the best possible estimate for every job that's involved. Total the numbers-then quadruple it. You're going to be close.

Performance work turns out to be very similar to restoration work. The following list really isn't complete and accurate. I really don't want to know exactly how much this whole thing cost. What you have here is a rough idea about the cost. Remember, different things are going to be added, or subtracted, if you actually do this work on your Corvette. The interesting thing is that the cylinder heads, which was the purpose of this whole odyssey, represent very little of the actual cost. I actually paid more for the intake manifold than I did for the used cylinder heads. We won't even discuss what the completed cylinder heads cost. Also, look at the simple fact that half the total cost was labor.

When you're all done with this job, you've spent more than your Corvette is worth. The wholesale value on an '85 Corvette is $5000 if you're lucky. We spent that much, and a little more, by the time we finished. On the other hand, I lost a lot more than this over the past year in tech stocks, so why bother counting? The real problem is explaining to the family why you just spent over $8,000 on a $5,000 Corvette.

The real-world figure for performance, once you get past the bolt-on stuff, is going to be between $75 and $100 per horsepower. For instance, Vinci Performance sells a package for the C5 Corvette. It adds just over 100 bhp to your C5. It'll also cost you about $10,000.

You can look up just about any of the performance packages on the Internet and check out what horsepower costs. Keep in mind that the actual total bill, installed, will be more than the Internet price. You'll quickly see that about a $1,000 per horsepower is what serious power is going to cost.

All of these numbers actually make the bolt-on stuff look like a real bargain. Everyone laughs at the airfoils that fit into the throttle bodies, but they actually give you two or three horsepower at a cost of just around $60. Throttle bodies are another item that's a real bargain. Vinci Performance sells a polymer throttle body for the C5 Corvette that will give you about 8 hp for $129. This figures out to just over $16 per horsepower, assuming you install it yourself.

If you're looking for power per dollar, then you seriously need to look at some bolt-on items.

If you want real knock-you-back-in-the-seat power then you better start looking at the balance on your Visa card. When you put this whole deal on a cost basis, we see some real bargains.

For instance, a really strong ARCA or BUSCH motor will be around $55,000. This means you're only spending about $65 to $75 per horsepower. It's really cheaper to call Hendricks Motorsports and have a motor dropped at your doorstep than it is to have your local shop modify your current engine. Then again, the Hendricks motor would have a stupid carburetor on top.

I'm really not into retro power all that much.



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