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dynoIf you've been following the progress of our 302 engine buildup and Project 11.99, then know that we've been promising dyno numbers for the past couple months. We'd been furiously trying to arrange some dyno time with a shop we has used in the past. Unfortunately we never got a hold of them, so we ended up having to locate another dyno. Well two days before this issue we finally found a shop at Sears Point Raceway that had a chassis dyno. However let us tell you that we had some apprehensions about getting the car tested at this shop. Not because we didn't think the engine would meet our claims, but because this particular dyno operates on a different principle than the popular Dynojet 248. When we talked to Tony, the owner of The Racers Group, the first thing he told us was that their dyno is a 'load bearing dyno', and unlike 'inertia' dynos this type of chassis dyno offers more capabilities for tuning, at the expense of slightly less rear wheel numbers.Well this immediately raised a flag. Sure tuning capabilities are nice, but we want numbers. Just how much less were we going to see? On the order of are 10% less than what you'd see on an inertia dyno. We had quite a bit of experience with Dynojets, and frankly, the Dynojet is sort of an industry standard. But since we didn't have much of a choice, we decided to see what the Bosch/Dytech dyno would give us. The guys at Racers Group also reminded us that any dyno is simply a tuning tool, and what is important is how much you've gained at the end of the session.

dynoWe loaded the car onto the rollers, strapped it down, and let Tony go to work. Tony, the South African native, builds and tunes Porsche race cars for a living. Having squeezed out 250 rearwheel horsepower out of 3.6L Porsche engines, naturally aspirated, we had no doubts in his ability to tune our simple V8.

 

 


dynoIf you've ever had your car on a dyno, you probably ran it in third or fourth gear from idle to redline. Then the computer plots out a horsepower and torque curve. The Dytech is a little different. It has the capability to sustain a load on the engine so you can hold the rpms at a specific point. It is similar to being on a treadmill that increases intensity as you try to run faster. It is truly a strange feeling to have you foot to the floor, the engine screaming, and the tach needle stuck at 5000 rpm. What this allows is for dead-on precise measurements at a fixed point every time.

So with the car loaded and ready to go, Tony made a base line run. The initial run made us shake our heads in disbelief. The machine spit out a peak horsepower of 296 at 6500. However Tony detected wheelspin, and also felt the car was running lean. He also hooked up a wide-band 02 sensor in the header collector, lowered the tire pressure, and made another pull. This time he immediately noticed the air-fuel ratio was on the lean side, (around 17.0:1) so we worked on isolating if it was the primaries or secondaries that were lean. Holding the pedal so that only the primary throttle blade were open, we could see the AF meter read lean. We stopped the pull, and jetted the primary side up from 75 to 79, and repeated the test. Oddly enough the meter showed no change in the A/F ratio, leading Tony to believe it was not reading correctly. Upon further investigation he determined that the A/F meter was not exclusively reading exhaust gases, but was being contaminated with ambient air. The reason for this is pretty simple. An O2 sensor is made to work upstream in an exhaust system, where there is only exhaust flow before and after the sensor, however since we were running open headers, the meter was picking up outside air at the collector.

Tony then decided to go with a carbon monoxide (CO) meter, similar to the ones used in tailpipe smog checks, which can be inserted higher up in to the header tube. This time the numbers showed just the opposite, that in fact we were too rich (6.8% CO) and were passing raw fuel through the exhaust. We removed the ram air box, to let the carb breath on its own. We tore in to the primary metering block a few more times, bringing the jetting down to 70. This got the CO to 1.5%, which is great for passing emissions, but a bit to lean for making power. We bumped the primary jets to 73 and obtained a 2.5% reading. This would equate to an air fuel ratio of around 15:1, which is perfect for part throttle driving.
Next we focussed our attention on the secondaries, under wide open throttle conditions. We had pulled into the shop with 76 jets in the secondaries, which turned out to be lean. We ended up jetting up to 80's to yield a a wide-open throttle AF ratio of 14:1.

dynoWith the jetting dialed-in, Tony suggested we check the timing. In past dyno sessions, we'd proceed by adjusting the timing up or down a couple degrees, followed by a pull. Then we'd analyze the data to see if we gained or lost power. Tony said he had a better method; with the dyno loading the engine at 5000 rpm, we got in and put the pedal to the floor. With the engine stuck at 5000 rpm, Tony rotated the distributor until the LED horsepower display showed maximum power at 5000 rpm. We witnessed the power go from as low as 270 to as high as 300. That is a 30 horsepower difference by simply getting the timing adjusted properly. When then checked the setting with a timing light, and found total to be at 41 degrees, we previously had it set at 38.

With all the tuning out of the way we geared up for a full run to see what we'd gained. The pull was carried out to 7000 rpm to see where power drops off. This time the numbers were more palatable, as peak was now 326 horsepower at 6500 rpm. We had gained 20-30 horsepower across the board from our first run, not bad for a two hour tuning session. Torque, which is calculated using the horsepower data, showed a peak of 287 at 5500 rpm.


horsepower
click here for torque curve


Analyzing the Results
The Bosch manual for this dyno indicates that power loss for a manual transmission car, in fourth gear, is between 25% to 30% from flywheel to rearwheel. This seems high to us (inertia dynamometers like the Dynojet claim around 15-18% loss through the drivetrain for a manual car), but Tony ensured us that his experience with testing on both engine and chassis dynos, proves this to be accurate. Keep in mind the Bosch/Dytech shows 10% less at the rearwheels. What we'd like to do in the near future is make a couple pulls on a Dynojet, and compare the numbers with what we got here.

Did we meet our goal?
Definately. If we take the conservative approach and add 20% to the 326 HP obtained on this dyno, that puts flywheel horsepower at 391. Keep in mind this is without the effects of ram air, which we cannot measure in a dyno room. We figure the ram air is worth at least 10 horsepower at the rear wheels, in reality it's near double that. That would put us at over 400 horsepower.

We'd like to thank Tony and the guys at The Racers Group for taking the time to tune our car and answer our questions. We had only planned on spending an hour and making a couple baseline pulls, but we left with 30 more horsepower, and a greater understanding of using the dyno as a tuning tool. We thought the car was running great before we came in, so it will be very exciting to see what we turn the next time out at the track! F/M


Going to the Dyno
Scheduling a dyno tuning session takes some preparation. Most shops charge $60 to $100 an hour, and that usually does not include any tuning assistance, or the use of air/fuel meters. In an hour you will be able to make a couple baseline runs, and two or three timing or jet changes. If you really want to 'dial-in'an engine, plan on spending two hours.

Be sure to list out the changes you are interested in making, and meticulously record all the changes you make while tuning. We like to bring a camcorder so we can review the session afterwards.

Make sure you take an assortment of tools, a timing light, carb jets and parts assortment. Don't forget extra bowl gaskets, and a cup or rag to catch the fuel from the carb. Other things you may want to play with on the dyno are different size air cleaners, even different size carbs.
If you have a lot of time, and money, you can shoot for things like headers, manifolds, dropping the exhaust, etc.


The Racers Group
Located at Sears Point International Raceway Sonoma, CA

(707) 935-3999

400 Horsepower 302 Menu
Part I Short Block
Part II Induction
Part III Installation
Part IV Dyno Testing
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