Did you ever wonder how 2 people with the seemingly exact same car could run such different ¼ mile times? Here’s a peek into why that happens and what you can do to turn your GP into a factory freak.
There are many reasons that some people with similar mods run much faster than others in the ¼ mile. Some you can control, while others you can’t.
This takes practice. Running a 2.0 60 instead of a 2.3 60 will drop you from a 14.9 to a 14.5 in the ¼ mile. This is because time dropped from your 60 equals a larger gain that that amount off of your ET. For tips on improving your launch click here.
Driver’s weight, believe it or not, but some members are over 200 lbs heavier than others. This means that the greatest improvement in this category would come from them going on a diet!
While some feel that you can’t do anything to your car and call it stock, others do things like remove the spare tire and headlight. Others will race with only a small amount of fuel in the car. For more tips on track prepping your car, click here.
Some tracks spray their launching section with VHT Track Bite, which greatly contributes to a car launching ability. VHT Track Bite is a custom formulated resin used to increase the traction of a car’s tires.
The temperature of a track attributes to traction as well. Here is where warm weather can actually help you. A hot track sticks like glue.
The temperature of the outside is very critical. Members who live in the Southern states know that it is very difficult if not impossible to run the types of time that us Northern guys do. Cold air is denser, holds more oxygen, resists detonation and usually is less humid than warm air.
The humidity of air is another factor. Humid air does not hold as much oxygen and isn’t great for going fast.
The barometer can make more of a difference than a pulley swap in some cases. Trying to race in altitude is all but futile if you are trying to break some records. Even at the same track, the air pressure can vary from day to day.
Now on to the interesting stuff. Why is it that some cars just make more hp than others when they have the exact same mods and are in the exact same atmosphere conditions?
When Eaton made the M90 they decided for one reason or another to machine the first ½” of the inlet of the SC housing. Unfortunately because of mass production and the fact that this machining process isn’t perfect, some of the housings would come out better than others.
Eaton blower housings seem to prefer certain rotors to others. This means that when you take 2 brand new M90s one will inevitably produce slightly more boost than the other. Even when you switch housings from one to another there is no way to predict whether you will gain some output or lose a little.
The bypass plate varies from blower to blower as well. On some SCs, the bypass seals much better than others producing more efficient operation and decreased outlet temps with higher boost.
The front exhaust manifold does not have large holes in it that just cover the head exhaust outlets. Instead they are shaped in a way that makes their placement affect their performance. There is a little play in the mounting holes causing performance to vary from vehicle to vehicle. The front manifold is made of cast iron meaning that the flow will vary just a little from car to car.
The rear manifold is made of tubes kind of like a header. These are put together quickly and while doing the work on our ported manifolds we noticed that some cars were setup much better than others. This includes how far the #6 tube extends into the main piece and how far the O2 sensor mount extends into the factory header.
The intake manifold gaskets seem to vary wildly from car to car, making a big difference in how your ports will flow.
The TB gasket is sometimes misaligned with how your blower has been machined.
Checking your copper exhaust manifold gaskets for carbon deposits can reveal a misalignment.
The HEAD GASKET is very critical on a GTP. In pulling a few heads off we noticed a few things about the L67. One is that the gaskets are not perfectly round. They have a strange shape to them. One can only speculate that this is to contour to the shape of the head. The other thing we notice is that each car’s gaskets look a little different and sometimes on certain cylinders the gaskets come right up to the edge of the cylinder wall. This can cause detonation in a big way. The edge of the head gasket is very thin metal and doesn’t have a sink to dissipate the heat like the head does. A head gasket can glow red hot if it overhangs into the combustion chamber even a few thousandths. We believe that this is the biggest difference between GPs and as to why some have so much more KR than others.
GM states that the rocker arm ratio on a Series II 3800 is 1.60. Now that we have modified rockers available we know how much difference in hp a different ratio can make. From the factory the rockers do not always come exactly 1.60, after measuring many we have found that each stock rocker’s ratio seems to be slightly different.
A stock GTP weighs about 3500 lbs. If you do some calculations for weight effect on GP performance you will find that every 8 pounds of weight on a stock GP is equal to around .01 in the ¼ mile. With the variances in driver weight, different levels of fuel or other fluids, and some people removing all the extra stuff in there cars times can vary as much as .3 or more. With a large stereo these differences can be even greater.
After measuring the lobes on a cam we found that they can vary from lobe to lobe. This means that some cams are going to inevitably be better than others.
Worn tires can sometimes be a good thing. They are slightly shorter and if they are less than a year old the rubber is still soft enough to give good traction. This added gearing as well as the tire being lighter will help with launching and trap speeds. Using different brands of tires can amplify the above differences.
For reasons unknown, certain PCM’s seem to reset and favor higher timing than others. Why this is we have yet to figure out but without a doubt it could cause one car to have more hp than another.
Every Series II 3800 has two knock sensors, one on the front of the engine and one on the rear. Thanks to the wonders of mass production every knock sensor is going to have sensitivity variances. Because the knock sensors are used in determining timing this alone can cause vast differences in total advance.
Most people have a brand of gas they like to buy. For me, it’s Shell but depending on where you buy your gas you may end up with a little higher or a little lower octane of fuel.
Believe it or not but oil can make a difference. Using synthetic oil will reduce friction in your engine. This has many benefits including increased power output. The Viscosity of your oil will also determine how fast your lifters bleed down.