Tech Tips

Engine Rebuilding Basics:

No matter what kind of racing you’re into or what your goals are for the  season, when it comes to horsepower, all racers have the same thing on their mind. How can I get the most power for the least amount of money without sacrificing reliability? This is especially true with guys that are just getting started in hot rodding or running an entry-level class at the racetrack, so this article will be geared more toward them.

We’ll assume that you already have an engine to work with and that you’re going to do a good stock rebuild on it such as having the block bored and honed, the connecting rods resized, the crankshaft reground and a valve job on the cylinder heads. This is all stuff that must be done to ensure that you have a good sound engine to work with.

Now we can start to think about increasing the power output, so we’ll go over it in order of importance and what will give the most horsepower per dollar.

At the top of the list is cubic inches. Anything that can be done to increase the size of your engine such as boring more out of the cylinders or offset grinding the crank or even robbing the crank from a larger engine that can be made to fit into yours without a lot of hassle and expense is a common way to increase engine displacement. Of course if you’re running in a limited class you can’t go over the displacement limit, but you should be right on it.

Next on the list is compression. If you’re boring the block you’ll need pistons any way so shop for something that will increase your compression ratio. If you can’t find an economical piston to increase your compression ratio, check into decking the block or cutting the heads. Sometimes this is the most economical way to go. Again, compression ratio is limited by the fuel you’re going to run or by the class rules that you’re running in.

Now that our short block is as big as we can afford to make it and has as much compression as practical, let’s go to work on the heads. Many people will argue that the heads should be first in line for performance modifications and this is a good point, but I feel that you need a good sound short block with a reasonable amount of compression to take full advantage of cylinder head modifications. Before you spend any money on your cylinder heads make sure they are a good casting and have a large valve size for that model of engine. Many engine manufacturers made different versions of heads to fit the same engines and some are more suited for performance applications than others. So before spending any amount of money on heads, make sure you have something good to start with. Once you’re sure you have the right heads, we can start working on them. One of the most economical ways to improve performance is with a competition three-angle valve job. Another popular modification is to increase valve size, but this is more costly because it involves buying new valves and the machine work to have them installed. Also if the port is not opened up to the correct size and shape for the larger valve, the performance gain will be minimal.

Another economical way to increase performance is to port match and bowl blend the ports. In most cases a well-shaped valve bowl on a stock size valve will out perform an oversized valve that has not had the bowl bending done.

This should cover what machining operations the beginner needs to be concerned about. Things like the camshaft, intake manifold and carburetor need to be carefully chosen for the specific engine application. However, these subjects will have to wait for a future installment of tech tips. If you have any questions please feel free to call the shop, we will be happy to help you out. Until next time, good luck.

Well, I hope this info helps you to determine what’s best for your needs. If you have an idea for a future tech tip, just email them to

Spark Advance:

For this section of Tech Tips, I thought I would share some information that can help entry-level racers and performance enthusiasts who are involved in Drag, Oval, or Street performance. The info is on ignition timing and the cool thing about it is you can usually get a considerable performance increase for little or no money. But, some time and effort will be required.

Lets start with a simple explanation of why we need spark advance and how much of it we want.

First – When you hook up the timing light to the #1 spark plug wire and it flashes a light beam onto the timing tab, it’s showing you when the spark occurs, NOT when the ignition occurs and the explosion that starts to push the piston down the cylinder.

We want this to happen just as the pistons reach the top of the compression stroke, but it takes time for the spark plug to ignite the fuel mixture. This “time” is measured in crankshaft degrees and is the difference between when the plug fires and the explosion occurs to push the piston down the cylinder – producing the power stroke.

Next – How much “time” or how many degrees of advance is correct. Well, that changes with each application but in most cases it will be as much spark advance as we can get as soon as we can get it without having detonation or “ping”.

Now that we know why we want spark advance and that we want all we can get without detonation – How do we get it?

If the engine is already in the car and running, we will have to hope the timing mark is correct. If you are building an engine or having one built at a shop, make sure the timing mark is correct; use a dial indicator on the #1 piston head to find “top dead center.” Also, if the damper is easy to get to, put a 38_ mark on it to use as a reference on the stock timing tab. The formula we use is (Dia. X 3.1416 _ 360 x 38), this distance will be 38_ on whatever diameter damper you are using.

Now we know the timing tab is correct, and we have a 38_ mark on the damper, we can now start to work on our timing “curve.”

Most racing and street performance ignitions do not have vacuum advance and the stock type distributors that are sent to a shop to be recurved should have the vacuum advance removed and locked out. For this reason we will deal only with mechanical advance in this article. If you send your distributor to a shop to be recurved they can get you pretty close to the correct curve on a distributor machine. We usually install a 26_ mechanical curve that starts about 100 rpm higher than the engines idle rpm and have all 26_ in by 2800 rpm, this is a good general purpose timing curve when used with 12_ initial timing set at engine idle.

This, however, is not perfect or optimum for any one combination.

A street/strip car that runs on pump gas may “ping” with this much spark advance and will require less initial advance or heavier springs to slow down the “curve.” Where as a low compression, low stall speed converter car may respond better with more initial spark advance or lighter springs for a quicker “curve” – but watch it if you begin your mechanical advance curve at or below the idle rpm, the car will be a real pain in the ass to drive and tune.

For circle track applications, these motors almost always operate over the rpm where total advance is needed so the timing “curve” is not nearly as important as having the correct total spark advance. This is where the 38_ mark is very handy to have. We have had much success with instant advance curves in these engines. This is where we start engines on about 10_ of spark advance and the instant it fires, the timing goes to the total advance – as long as you use high enough octane fuel this gives a nice clean idle and very quick response.

As you can see, there’s a lot you can do with ignition timing, and every engine will require a specific curve and total spark advance, but if you take the time to sort out what’s best for your engine, you will gain performance without spending a lot of money, and isn’t that what we are all after.

Thanks for reading and checking out our website. I hope this info helps you get more performance from your machine. Remember, for all you performance needs, call Jensens Engine Tech at 570-379-3069

Connecting Rods:

Let’s talk connecting rods. What types are good for what applications, and I’ll touch on rod length, because this is a can of worms I’d rather not open since there are so many things that can change rod length characteristics.

The types of rods are broken into four families: stock, aluminum, 4340 forged and 4340 billet. I know there are some 4130 and 5140 after-market rods available, but I have no experience with these.

Stock rods vary from manufacturer to manufacturer, not only in size and shape, but in the amount of power and RPM’s they can withstand. For example, we’ve installed well prepared big block Chevy rods in some 800+HP/7500 RPM nitrous engines with no failures, but a big block Ford rod would never take that and it’s not that I’m pro Chevy, a small block Chrysler rod is a very strong piece. I just call them like I see them.

The preparation we do to a rod depends on the level of power or RPM’s it will have to withstand. The minimum, even for a stock rebuild is to shot peen, magniflux and re-size the bearing end. For more demanding use we will side polish the rod. This is to remove any stress risers on the beam area. A stress riser is a surface imperfection that will allow a crack to start. Once the rod is clean and side polished, we magniflux them. If they pass this test we get them shot peened. After peening, the rod is re-sized using S.P.S. bolts. We have had zero rod bolt failures with these and we feel that they are the best available. If a racer supplies a set of good cores, rods prepared in this fashion will cost way less than $200 and hold up pretty well depending on piston weight, RPM, horsepower, and as I stated earlier, the manufacturer.

Now we’ll get into the after market rods. First, we’ll look at aluminum. Aluminum rods are more suited for higher RPM drag engines. Some of the advantages are lighter weight and that they absorb some of the shock from the exploding intake charge and from the piston changing direction. They are also more economical than premium billet 4340 rods. The disadvantages are that they are bulky and hard to fit in an engine with a lot of stroke. Another disadvantage is their shorter life expectancy. Most manufacturers recommend changing them after 200 to 400 runs. I’m sure that there is more to the aluminum rod story and if you’re considering a set it’s best to call the people who make them to see if they are right for your application.

Now, on to 4340 forged after-market rods. These are a good compromise between cost and reliability. They usually sell for between $600 and $800 and are available from a bunch of different manufacturers. The ones we use the most are Oliver and Lunati. They are available in a wide range of lengths and the Oliver rods come in a standard weight and lite weight version.

The engines we most often use forged 4340 rods in are limited oval track and non-nitrous drag. For unlimited oval track or high RPM nitrous drag, we use billet 4340 rods. These are the most reliable rods on the market. They are also the most expensive (except titanium rods). They sell for between $1,000 and $1,200 per set. They are available in a bunch of different lengths and weight ranges. Since they are machined from a solid bar, it gives the manufacturer a lot of freedom to change the size and shape of the rod to suit a given application.

Billet rods are available from many different companies, the ones we use the most are Oliver and Crower. They both seem to be very reliable, but the Olivers are a little less expensive. We have never had a rod failure when using these rods. This should help in choosing what rods to use in your racing engine. I will shed a little light on selecting the length of your connecting rods, but this is a whole article in itself.

For limited induction engines the rod should be as long as possible. The engines usually don’t make a ton of horsepower so the piston can be pretty short and still hold up. On unlimited induction engines or engines with heads that have a lot of port volume, maximum rod length is not quite as critical so you can leave a little more piston height to get reliability and still not hurt horsepower. This is about as deep into rod length as I’m going to get at this time.

I hope this info is of use to the racer as well as the street enthusiast.  If you have an idea for a future tech tip, just email them to


In this section I’d like to cover the part of your racing engine that makes it all happen. It takes the reciprocating motion of the pistons moving up and down in the cylinder and transforms it into rotating motion that can be used to turn the wheels – The crankshaft! First we’ll cover what types of crankshaft materials are available and how to choose the right one for your application.

Beginning the list, because it’s the most common and economical, is cast iron. This is the material that is found in most stock engines. A lot of guys will say that a cast iron crankshaft is no good for performance use, but with the right preparation these crankshafts will endure a lot of abuse.

For mild street use or stock rebuilds, all we do at the shop is magniflux and regrind them. If a racer wants to use it for bracket racing or limited oval track use, we will remove all of the casting imperfections on the rod throws to eliminate stress risers (a place for cracks to start). We will also cross drill the oil holes to provide better rod bearing lubrication.

One of the most important things that we do to a cast iron crankshaft equipped engine is not done to the crankshaft. It is very important that we use rods and pistons that are as lightweight as possible, so they don’t tear the rod journal off of the crank when it tried to stop them and pull them back down the cylinder at top dead center. With the right prep work and a lightweight rod and piston combination, a cast iron crankshaft should handle 500+ HP with no trouble at all.

Next, on the crankshaft ladder is a factory forged steel crankshaft. If one was made for your engine, the most common steel used is 1038. This is pretty much just plain old clean steel with some carbon added for heat treating purposes. A forged steel factory crankshaft is usually a good compromise of cost and strength, especially when purchased used. These crankshafts will hold up in more demanding applications than cast crankshafts, but still need some prep work before being used in racing applications. Again, the first thing we do is magniflux it to make sure we don’t spend time and money on a cracked crankshaft. Also, we remove the stress risers from the rod throws and cross drill the oil holes. Another option, especially attractive to circle track racers, is to drill the rod throws parallel to the crank axis with a ¾ or 7/8 hole. This makes the rod journal hollow and much lighter, with only a minimal loss in strength and it can be done for under $100.

The last thing we do before having the crankshaft reground is to have it shot peened. This also stress relieves the steel and makes a more stable crankshaft. A steel crankshaft prepared in this fashion will survive in all but the most demanding racing environments. A racer can choose what degree of crankshaft preparation best suits their application and budget.

At the top of the ladder are the aftermarket forged and billet steel crankshafts. There are many different manufacturers and price ranges. So how does the racer determine which way to go? First, I’ll cover the most common materials available. There’s 4130 and 4340 – these numbers pop up all the time in crankshaft ads, but do we know what they mean?

I’ll try to give a quick explanation. The last two numbers tell us how many 100ths of a percent of carbon is in the material. Carbon adds to the harden ability of the steel. 4130 will have 30% of carbon and 4340 will have 40% and so on. Both 4130 and 4340 start with a #4, this tells us that the steel is alloyed with molybdenum, or moly. This adds toughness to the steel. More moly means a tougher crankshaft. 4130 has 20% moly, while 4340 has 25%. In addition to more carbon and more moly, the 4340 is also alloyed with nickel, this is noted by the second number in the alloy. A “3” nickel assures deep and uniform hardness in the crankshaft.

Clearly 4340 is the better material, but it does cost more. If you’ve reached the degree of performance that requires an aftermarket crankshaft, I can’t see buying one of mediocre quality. Save your money and buy the top quality stuff, you’ll be happier in the long run.

The next thing you should consider on a crankshaft purchase is how much it weighs. The lighter the crankshaft the more it will cost. Most company’s start with the same forging for their standard and lightweight crankshafts, so if you’re on a budget and can sacrifice the weight, the standard crankshaft is the way to go (as the strength and durability will be the same).

Well, that’s about all the time I have this month. I hope this will help you spend your crankshaft dollars more wisely. Until next time, keep your right foot down.  If you have an idea for a future tech tip, just email them to

Leave a Reply

Your email address will not be published. Required fields are marked *