autocross truck build

[PEZi720]

This is a bit of a read... if you're not into autocross it may not be worth it.

 

This will be an extremely long project but it began a while ago. I'm building my 720 to be an autocross only vehicle looking to in the end have a nationally competitive ride. I've been running in CO in G-Prepared and will be building the truck for GP pushing the limit on every rule in order to try and dominate. As I said... its already mid-build but the good stuff hasn't come yet.

 

The suspension as of right now is simply lowered with the torsion bars up front and 2" blocks in the rear with KYB monomax front shocks and gas-a-just rears. The front will eventually have 3" spindles, custom torsion bars, custom sway and adjustable shocks. The rear will also have adjustable shocks, another custom sway and the 2" block will stay with the rest of the lowering coming from a custom set of fiberglass leaf springs.

 

I've done a fair bit of lightening and relocating of things already as well. The battery for example is where the passenger seat used to be. I'll go ahead and list off some of the stuff I've deleted so far:

 

all of the interior except driver seat

AC system

power steering system

headlights

wipers and wiper system

rear tailgate and bumper

all emissions equipment

 

The list of stuff continues... with future deletions still being decided upon.

 

Now of course I can't forget power! I currently have no internal work done and I'm still running the stock carb. Of course this will not stay this way. My plan is to get a decently aggressive cam and do a port and polished head. Now my first question is: Should I attempt to put the KA head on the Z24 block? I know it has been done before and made decent power with the other mods it had done to it (and it was done by someone on this forum but i don't know who.) I will also be doing a full fuel system overhaul. I don't yet know if I'll be doing a Weber or Mikuni. Obviously the Mikuni would be ideal... but in GP I'm not sure that much power is needed and the Weber will cost less. As of right now I have a full exhaust done as well. I have the Pacesetter header, ceramic coasted, with the 2" piping it comes with back to where the cat was. In place of the cat is a custom 2.25" test pipe followed by a custom 2.25" 'catback' that leads up to a magnaflow resonator. The full system ends right before the rear axle.

 

Don't forget the wheels and tires!!! As of right now, I'm running 15" American Racing Outlaw II's.... and yes I'm on street tires! I currently run BFGoodrich G-Force Sports in 225/45/15. I'm not yet sure what the new wheels will be, but I do know that they will be 13". Thus my second question: What are some good 13" autocross wheels and do they make them for the 6 lug or will I need to convert? The new tires will be Hoosier A6's in 255/40/13.

 

For those of you who made it this far... some pics and video are now here! First of all... the backstory for video. This is me autocrossing the 720 with all the stuff listed above (street tires, stock carb, near stock suspension, etc.) It is my first run of the day and thus not my quickest but its the only one the guy got. A later run this day... for those of you who know about autocross indexed scores... was good for 874 points with the prepared class index (GP) even with the not so prepared parts I'm currently running. I'm hoping that with the mods to come listed above, a little more lightening and aero work and some good driving from me it will be nationals worthy....... On with the video:

 

 

and now a few pics... none of them are 100% up to date but I think you'll get a basic idea.

 

 

Any suggestions or help is welcome... hopefully some of you will get to see her nationally at autocross in the next year or two! :fu:

[Cuts metal like mad]

nice dash! I like it. :thumbup:

[bonvo]

i love the looks of this truck how much are the performance 13 inch tires costing you 14s have alot more tires to choose from. are you still running stock brakes?

[irsa76]

One thing about your rear suspension. Don't use lowering blocks. They cause axle windup and weird handeling. My mate's Toyota, the youtube link I posted in your other thread, had some serious issues when he ran blocks. He now runs proper lowered leaves. He's planning on running a 3 link when he goes 2JZ.

[PEZi720]

nice dash! I like it. :thumbup:

thanks man!

 

i love the looks of this truck how much are the performance 13 inch tires costing you 14s have alot more tires to choose from. are you still running stock brakes?

They're $207 a piece... no r-compound is cheap but with my 3.70 rear gear this will help the revs go up quicker without having to tear that apart as well

 

I have not upgraded to larger brakes... but i do have better ceramic pads that with the weight of the truck have plenty of stopping power. on my street tires they actually lock up pretty easy

 

One thing about your rear suspension. Don't use lowering blocks. They cause axle windup and weird handeling. My mate's Toyota, the youtube link I posted in your other thread, had some serious issues when he ran blocks. He now runs proper lowered leaves. He's planning on running a 3 link when he goes 2JZ.

Interesting... i actually love the handling with my blocks... i'm running the 2" right now and couldn't be happier with the rear... my only issue right now is the front because of how it gets positive camber under compression

[datzenmike]

If you are going total auto X get rid of that 3.70 rear! You need faster take offs and acceleration out of the turns. You have 5 speeds in that tranny and likely 3, 4, and 5 are just along for the ride and never used. You need a 4.375 from a 620 or any '80-'83 4X4 720. This will get the revs up into the power much faster. Relocating the battery is good but should be further in the back where it's weight will do the most good over the axle.

 

Oh yeah... Nice truck!!!

Edited August 24, 2009 by datzenmike

[WagDatto]

I'll agree with what a few people have said so far. Lowering blocks cause axle wrap, which is lost torque to the rear wheels. You may "like the way it handles" right now, but you're losing out on a lot. Also, blocks cause the axle to shift side-to-side more, too, as there's more torque around turns pulling on the axle. Take the springs somewhere and have them stiffened and lowered 2" with an extra leaf for the pack.

 

I have the same issues with the wagon axle and lowering blocks with my KA24. You might not feel it much right now, but if you upgrade the motor, you'll definitely be feeling it. Better now than later, IMHO.

 

Also, the battery should be mounted directly over the rear axle on the passenger side. This will put more weight over the rear tires for added grip. Plus, it looks a bit unsafe with the wires literally hanging from the firewall to the battery. Extend the battery cables to a proper length when you relocate it again.

 

The other suggestion I have is to buy some dry ice, chip it up, put it in some plastic bags and let it sit on the sound deadening tar that's on your floor. In a 240sx, people are saving 20-30 pounds in the tar that's all over the floor and tranny tunnel. Especially in a race car, there's no need for a comfortable, quiet ride!!

 

I'll also agree about the diff gearing. I don't know much about axles, but I'm assuming from the way Datzenmike worded things that the diff from a 620/etc would be a direct swap into your diff housing.

 

I think the concept of a race truck is super cool, and I'm glad to see videos of you out there beating on it! Keep it up and keep us updated!

[PEZi720]

If you are going total auto X get rid of that 3.70 rear! You need faster take offs and acceleration out of the turns. You have 5 speeds in that tranny and likely 3, 4, and 5 are just along for the ride and never used. You need a 4.375 from a 620 or any '80-'83 4X4 720. This will get the revs up into the power much faster. Relocating the battery is good but should be further in the back where it's weight will do the most good over the axle.

 

Oh yeah... Nice truck!!!

 

i understand exactly what you're saying... but let me explain why it is i don't (at least currently) want to do that... feel free to criticize this as well because i'm open to as many opinions as i can get

 

yeah... as of right now 3,4 and 5 are typically along for the ride... but even the best guys in our region have that same issue. one guy in specific has an E-Prepared AE86 that is constantly winning the 1,000 points and is going to be a threat for the win at nationals and he only uses 1 and 2. he claims the right gearing is more important that having more gears to choose from and that having to shift around will cause you to lose a tenth or two (which means a lot sometimes when it comes to points.) he as well as many others ride out the rev limiter for a second or so at the end of long straights in order to avoid the extra shift... and i can't argue with their results for sure. one guy in an old corvette that runs XP doesn't even make it out of 1st on most tracks and he does extremely well too :blink:

 

now yes... i'd love to be able to take off a little quicker out of corners and get up to speed faster and all that good stuff... my worry is that with the lower redline that the z24 has compared to these other guys i'd be doing too much shifting. now don't get me wrong... i know how to rev match and do all the things that help the time lost due to shifting and have actually had to do it on a few of the tracks we run... but nothing beats not having to shift.

 

i guess what i'm saying is that with what i've seen at the track is that all you need is 1 and 2. now once i get cam and head work done... hopefully i can rev it higher and will thus need the 4.xx gearing... but for now i hope the 13's will get me where i want to be.

 

Take the springs somewhere and have them stiffened and lowered 2" with an extra leaf for the pack.

 

Also, the battery should be mounted directly over the rear axle on the passenger side. This will put more weight over the rear tires for added grip. Plus, it looks a bit unsafe with the wires literally hanging from the firewall to the battery. Extend the battery cables to a proper length when you relocate it again.

 

The other suggestion I have is to buy some dry ice, chip it up, put it in some plastic bags and let it sit on the sound deadening tar that's on your floor. In a 240sx, people are saving 20-30 pounds in the tar that's all over the floor and tranny tunnel. Especially in a race car, there's no need for a comfortable, quiet ride!!

 

 

I think the concept of a race truck is super cool, and I'm glad to see videos of you out there beating on it! Keep it up and keep us updated!

 

1. after hearing what you guys have been saying about axle wrap... i plan on just getting the leafs lowered to 3" on their own so i can get rid of the blocks... they'll probably still be fiberglass though unless i somehow find out that they aren't as stiff

 

2. to both of you guys... the battery is a personal preference. i'll try to explain that a little but in the end it really is just that... a personal preference. basically my logic behind it is keeping the truck balanced from left to right and then mess with the front to rear handling with the suspension. now yes, the battery obviously doesn't weigh as much as i do but its at least an attempt.

 

3. now, datto... the safety of the battery is no longer a concern. the picture up there that you are talking about is old and i just had it connected to see if everything ran right. as you can kinda see in the picture with the dash back in, the cables aren't just hanging anymore... i have them tied and out of the way, proper length with the terminals insulated and all. no longer an issue.

 

4. sound deadening is on my 'to do' list... i know it saves weight and it obviously isn't needed... i just haven't gotten around to it

 

thank you both for the comments as well... and of course i'll keep you updated but with money being tight as a college student i'm not sure how long this will take

Edited August 25, 2009 by PEZi720