[ChallengeThis340]

Hey, I was driving around in my 70 challenger 340 the other day when I started hearing loud snaps in the rear on turns and even while giving it gas in a straight line. I added posi additive concentrate with no luck, im pretty sure that the posi is shot or possibly one of the gears. Anyway, while I was back there I figured i'd give the car some much needed gear. It runs 3.91's now and its just flat out not enough gear for this little 340. Im just wondering what you guys would recommend. I dont drive the car that often, maybe once ever few weekends and hardly any of my driving is highway. If I do get on the highway, it is for no more than 10 minutes and its very rare. I just want the car to be fast. They closed the only track here on Long island, NY so I have no way to tell how fast the car is, but I think it was or should have been in the low 13's or high 12's. Thats just not enough anymore, can't stand the idea of getting smoked by stock 06 vettes and slighly modded STI's and Evo's....


here are my motor specs

69 340
balanced rotating assembly
bored .030 over
speed pro pistons (I want to say 9.5:1 compression but im not exactly sure)
Eagle forged rods
comp exhaust valves and valve springs
3 angle valve job
edelbrock performer duel plane manifold
holly 670cfm street avenger carb
hooker super comp long tube headers
MSD 6al
Freddy Brown built 727 tranny with manual valve body/shift kit

Here are the comp cam specs: intake lift: .480/ Exhaust: .480
230/230 duration at .050
Grind: CRS 280h-10

My boss at the speed shop I work for, who is also the guy who built the motor thinks 4.88's would be good for my application. When we can finally get it to the track in jersey, 4.88's and a higher stall (right now it only has a 2300) might be enough to get it into the low 12's (?) which I would be very happy with. My only problem is that I plan to run nitrous in the future and I don't want to run out of gear on the spray. On the other hand, I also want the car to run as best as it could N/A, as that is how i'd be driving it 90% of the time. Im using M/T L60 I tread sportsmans right now. I think they are about 28" in diamter. I guess I would go with a similar sized slick.


thanks!

[dave-r]

Few! Where to start?

First point would be that 3000rpm stall sounds reasonable to me for that cam.

Horsepower and weight will determin how fast you go in the quarter mile. Only the best rear gear will make you go the fastest in the quarter. Too low a gear and you run out of rpms. Too high a gear and you don't use the full potential of your power.

Idealy you want to cross the line at peak HP rpm. For you that will be around 6000rpm. Your ideal shift point will be around 7000rpm but 6500 will do if you don't want to rev that high.

I would guess your car will run very low 13s in the quarter mile. At maybe 104mph. Depending on how heavy you and the car are.

I think a 4.88 gear is way too much. You would only be doing 95mph at 6000rpm and 103mph at 6500.

A 4.56 will give you 101mph @ 6000 and 110mph @ 6500.

A 4.30 will give you 108 @ 6000 and 117 at 6500.

Of course a taller tire will give you more mph for your rpm.

So a 4.56 gear is looking like the best choice to me at the moment. Especially if the car or yourself is on the heavier side.

An alternative is to use a much lower gear and give yourself a 4th gear by fitting a Gear Vendors overdrive unit.

Check out the Excel spreadsheets I have placed at the top of this section of the board. You will be able to see exactly how tyre size and gears will effect your speed.

[dave-r]

Just thinking about the NOS.

A 75hp shot will get you low 12s @ 110mph so in that case the 4.30:1 gears might be better as they will suit you all the way into the high 11s if you decide to shoot even more laughing gas in.

My 440 runs high 11s on 4.30 gears with 28" tall slicks. I find the extra gear in the GV unit very handy as it gives me a 145mph top speed on the faster roads.

[ChallengeThis340]

Thanks, we are going with 4.56's. I talked to my boss from the speed shop about the whole thing, and he said he was reccomending 4.88's because it would work better after a cam swap I told him I had no intention of going back into the motor for a third time, and at this point, with the brand new motor and everything running great i'd rather just spray it. Shes got a forged crank from a 69 340, Eagle I beam 4340 rods and SRP forged pistons so I think a 125 shot would be safe. The ultimate goal was actually deep into the 11's, but my boss told me to use a 29" or 30" slick for the track. With a 29" slick we're looking at a top speed of about 123@6700 rpm's. That should be enough, but I also know the converter has a lot to do with it as well. If we raced it on N20, it wouldn't be often at all, and I trust that the engine was built and machined very well (machinist actually worked for Sox and Martin) so I wouldn't mind revving it to 6700 once in a blue moon.

So I think the 4.56s are my best choice right now, expecially because as you said, they would work the best with my current, N/A setup and thats how I plan to drive the car most of the time.

How much improvement in 1/4 mile time should I expect with the 4.56's vs the 3.91's? The car hooks VERY well right now with the current suspension setup so tractions shouldn't be an issue. I was hoping maybe a half of a second but maybe I am expecting too much.

thanks!

[dave-r]

I am not sure I am clever enough to give you an idea of how much faster it will go.

But it will have around 14% more torque at the rear wheels.

[ChallengeThis340]

We got em in today. Wow, the car feels MUCH better...I think this is thing the car has been missing all along. Theres still a little lag off the launch from the converter, but it blows through that lag a lot quicker, and once it gets moving it GOES. I didn't get a chance to drive it too much as I am working on my other car at the garage, but from what I felt so far, I am very happy. From a 3k rpm roll, the car just squatted and ran. I've never had the front feel so light before. Feels like I cut the length of that long first and second gear in half.

Thanks for you assistance Dave r

[dave-r]

I like a happy Challenger ending.