It is cool to see some Jettas being built

[shwah]

I have considered it myself a few times.

On paper it seems the aero will be slightly better, with a nicer rear window angle, a trunk to fill the void behind the window and reduce the vacuum there and if a 4 door, smaller side windows open to the wind stream.

Theory is great and all, but has anyone ever run across empirical evidence to support the Jetta being more aero than the Golf?

How about effectiveness of the GLI rear lip spoiler? If there is still somewhat attached airflow there, the lip hanging of the back may actually reduce drag a teeny tiny bit (like .1% or something), but my bet is that this close to the rear window it is not really effective at all - but pretty cool looking. Anyone have or seen data on this?

Maybe I should start putting one together and run the hatch and sedan with the same drivetrain at RA to get some data...

[joeg]

If Mike Ogren says it is a better shell to work off, I would believe him.

[shwah]

I don't disagree, but I would love to see some supporting data.

The one big reason I never went that route before is because in a sprint race, I like having a shorter car, that will fit in tighter spots on track.

I am not convinced that the weight distribution is any advantage.

I think that the additional chassis stiffness could be a minor impact, though only 1 rear tire is on the ground at a time anyhow when the car is working in the corners - so maybe nothing more than a feel thing, sort of like an upper strut tie bar.

The aero stuff may add up enough to matter at triple digits, and the top of the straights are where I have issues with competition (Accords, 924s, 2.0 VWs).

I stumbled on a decent non-sunroof donor - seriously thinking about putting it together enough to swap components and do some chassis comparison between the two...

[pfcs]

I suspect the small front windows yield less drag than the Jetta tail does. At least it's a big add on Volvo 142/144s.

[D. Ellis-Brown]

There are two MK III Jetta's in Central Florida, Mike McCormick built one last year, and we built our's 3 years ago. It is a great car. As far as the Aero benefits, I can not say. We installed the rear factory spoiler on day one. But it was more for show than anything else, since we did not have any Aero data to say if it was beneficial or not. But since it was "Standard" on some models of the Jetta, we installed it. We see 6600 in 5th at Daytona with an 0.81 final drive. So it is fast. We built the Jetta for several reasons. 1. it was cheap, we found a pristine car in a junk yard with a seized motor for $200. 2. One of my friends, Randy Pobst, formally from Melbourne, Florida, and I had talked about "Jetta's, (Mk II's) in 1990, that a Jetta, with the extra weight behind the rear wheels, might rotate better to offset the normal understeer. 3. I wanted to build a new car but it needed to be a VW, 4. We figured that the car had alot of potential, because of the cross flow head, Motronic ECM, etc, and it was only 70lbs. heavier than the Mk II's. Well we were right. The hardest part of building the car was getting the car down to weight. We have done that. The engine is sweet, with lot's of torque. The next issue has been the ECM. We had two different companies burn chips for us but we could never get the A/FR down to were we wanted, so we had to give that up and installed a MegaSquirt. That increased the HP by 8%, and the Torque by 6%. David and Jeff at Atlantic Autoworks in Melbourne installed it and we saw the increase imeadiately on the Dyno. It is a neat car. Randy has driven it, Paul Ronie also has driven it, They had some very favorable things to say about it. So, we hope more people consider the Jetta's to. They are much more plentiful than the Golf's, and for me it has been a fun car.

David Ellis-Brown

[shwah]

Wow David. I honestly did not think the A3 Jetta could get to weight. You are definitely correct about the weight difference between the 2.0 and 1.8. It is a favorable classing, and if I had read closer when I started I would have gone that direction...

The AFR never seems right on the chips for that car. I would love to see that car in action. Any plans to take it North over the next year?

[Flyinglizard]

When dong the # per CC for the VW Cup, I get about 2400#(actually 2425) for the Mk 3. And that is not factoring the crossflow, usually considered about 9-10% at 6500rpm. If the G2, 1.8 is 125hp, the G3,2.0 should do a real 140ish. Should be faster than the 1.8.
The total airflow is about the same as the 1.8 16V at 6500rpm.

My thoughts; RE Jetta air. Small but definite variences; smaller windows,with better rear airflow off of the trunk. Better/less LF tire weight by maybe 20#. Slightly rearward and lower rear roll center, resulting in better inside front power delivery and better steady state corner power.
The negs; more sq in of surface area(drag), two more rear window /door edges, and trim strips. I plan on tuft testing both cars on the airport Jan 17. I know that the Golf 2 is not very good. It is very helpful to get the windshield angle as low as you can. IMHO. MM
FWIW, I have a sweet Mk 3 Jett for sale. No rust or drive line. 500$

[shwah]

Let us know your findings. It will take me longer to get any data since I don't have a Jetta to compare with yet.
What factor is impacting the roll center? Geometry is the same on the suspension.
I am sure someone will take you up on the clean shell some time - am too invested in cross flow/A2 stuff to change platforms now.

[Flyinglizard]

Right, Roll center is the same of course. CG is lower and rearward, due to lack of the heavy hatch, plus the trunk. There are not any huge differences, just some small ones that total a few percent. IMHO. ,
The tuft test will show flow off of the rear . I cant get the cars even enough for drag , nor measure the drag easily. I have 6000ft or runway that I can use sometimes,really late on Sun, ...

[shwah]

OK makes more sense now. Not sure on the CG moving though. The Jetta has more metal above the floor than the Golf, equal glass, and you can get the Golf further below weight and put more weight in the floor than the Jetta. LF (and RF for that matter) weight should go down though.

If I try to do this it will be done swapping the same drivetrain between the two, so the variables will be chassis and environmental variables. I am hoping that data aquisition on the RAm straights could pick up any differences in accelleration above 100mph as a useful parameter.

[D. Ellis-Brown]

Chris, we just would not go any further than RA or Roebling....after nearly 40 years I just don't have the desire to tow too far any more. We knew the weight would be a major issue. We wound up scrapeing every pound possible from the inside and underside. and removed everything the rules would allow. We also initially built the cage to a safety minimum, with the intention on adding the additional bars after the car was complete and we had an "actual" race configuration for the scales. After we knew the final weight, we added more cage. We also went to the effort to calculate mileage for our local tracksI, Daytona & Sebring, and add the minimum fuel plus 1 gal. Since our races a typically timed / laps races, we only add the minimum fuel. I even had to go on a diet to get to 185lbs. A car builder that I met a number of years ago had developed a theory, " that for every 20lbs of weight, you needed an additional HP to move it". So if we were 40 lbs over weight, we would need 2 additional horses to move it. In addition, we would need more brakes , or earlier braking to slow it down.

[shwah]

Sounds a lot like trying to get a 1.5 or 1.6 VW down to weight in FP.

Maybe I will see you at the ARRC some time.

[Flyinglizard]

20 # slows our stocker down about .1 sec on a 16.5 sec Lap.

[D. Ellis-Brown]

Chris, we considered going last year, but the economy got to our budget, maybe next year. I have not been to RA since 1976 in an IMSA RS race, and one of first races was there in 1970, in an FP Volvo P-1800. Thanks David Ellis-Brown

[shwah]

Here is the closest thing to data I have found. Compares Rabbit to A2 Jetta, but still illustrates much of what we have theorized here.

[jetta87]

so it sounds like i hasve a good car that i am building tht has better aero dynamics?

[Flyinglizard]

The published CD for the MK Golf was .032, the Mk Jetta was .033. (Per road test of the era.)
These values may also have been the best case.
IE: the Golf on the 5in x14 wheel and the Jetts on the 5.5 or6x14. Golf may have had one mirror, etc.
I dont hav e the frontal area to do the final /toatl drag value. I think that the Mk 3 has the CD of .030, but quite a bit more frontal area.
I still think that the Mk 3 Jett, is the best car, considering # per CC , and Cd. The usable rev range becomes pretty good when "Squirted" . Barring any cam changes.
TBS, If any Mk 3 runs by my MK 2 with a 4.2 gear. I will have a cam/valve spring discussion, over a beer. The stock valve springs wont go over 6100rpm...
No to worry too much ,as we are on the full time Spec Miata/ Chumpcar thing for 2010.
MM