Why won't it run hard on the backside?
 

Getting things ready to go back to the track but want to find everything I can to try for better times. My car just doesn't seem to want to run on the back half like I think it should.

Best 1/8: 7.52@91.5mph but has ran some 92mph passes, this I'm happy with
Best 1/4: 11.92@111.03 mph with best MPH of 111.81


I've since swapped from a 28" tall 325/50r15 to a 25.5" 275/50r15 to better suite my 3.42 rear gear. Car feels better on the street and spins a little more on the street. I also added a 85mm MAF to replace the stock one.

Car weighs approx 3750# with me in it

98 ls1
799 heads (shaved .010", rocker boss removed, worked around the intake valve boss, fully sanded intake/exhaust ports)
EPS 230/238 113+2 (eps/lsl lobes)
Fast 90mm with wedges removed from runners, 90mm tb
K&N cone filter with 3" outlet
GM 85mm maf
1 3/4" longtubes, 3" duals (collectors do neck to 2.25" for 2", will be fixing this soon)
4l60e with Yank SS3600, no signs of slip or anything abnormal
12 bolt 3.42 gears

Tune: 12.6 a/f (tried between 12.4-13.0 with no gains), 26* timing, shifts at 6400rpm (tried higher up to 6650rpm with no gain), I need to confirm no vacuum at WOT



Is there anything else at all I can do or check? Is it really asking too much for closer to 115mph?

Since no one else has answered yet, I'm gonna say picking up 20 out the back with your combo is about right. Trying to get in the 25 mph range is gonna need some gas or FI with your engine combo and weight.

Seems pretty normal sounding to me with your mod list.

At 3750, you won't see the mph you're looking for with the power you are likely making..... Get so
E weight out of it, and I bet you start seeing the car pick up a bunch

It takes alot of power to make a 3700 lb car do it on the back half. The power/converter/gear might not be perfect either. I bet the shorter tire will be faster as long as it hooks. Get some gear back in it.

What did it 60'?

I think you should do more on the backed as well my car seems really close to yours. My 60ft is worse than you but I did 7.76@91.2 and 12.02@115.6 with a 1500 something density altitude

When the Formy had the stock engine in it with LS1 intake and LT Headers at 3650 lbs it ran 7.50 and 11.94 with a mid 1.50 something 60 foot. The MPH was about the same as what you posted except I had 3.70 gears and a 26 inch tire. It looks like to me the car is doing correctly. I do believe the smaller tire will help ET but MPH may go down!

I did not see where you posted your 60 foot times what are they?

Good luck on your next outing!

N2

Whats a formy?

Looks about right to me...

I was 11.8X @ 114-115 with 7.5X @ 91-92 660's

Just to compare....

33-3400# approx. w/ driver.

That 28" tire was just too tall for your engine power and gearing. The 26" will work way better but lose a little mph...

Cutie name for Formula.:nod:

Thats in the norm

My car is very similar to yours, so I went through my old tickets to find one that was similar in ET. The slowest ticket I could find was from the first trip to the track. Here it is:

1.671 60'
4.802 330'
7.445 1/8 Mile
92.42 MPH 1/8 Mile MPH
9.774 1000'
11.76 1/4 Mile
113.36 1/4 Mile MPH

I was picking up 21 MPH on the back end then. I would imagine my aerodynamic drag coefficient is even worse than yours.

My car:
3725#
6 liter 10.9:1
Ported LS6 heads
2800 converter TH200-4R trans
3.90 gear
28" tire

At the time I ran those numbers, my cam was 222/224 on 112 LSA with 110 ICL and a dual plane carb intake.

Now I have a 226/234 on 110 LSA cam and a single plane carb intake. The last trip to the 1/8th, the car went 7.25 @ 96.5 MPH.

This is the Formy or at least that is what the wife calls it when I am not spending money on it! :nod:

With my 325rwhp lt1 setup, I was doing low 7.50's@ 91-91.5mph ran out the back to a low 11.8x@115.5-115.8 on a 1.67-1.69 60' 1200 DA, 3500lb race weight.

I was running a 275/50 m/t drag radial, and a 3.73 in the 10bolt, th350 and an edge Racing 3400rpm converter.

Seems to me like you're 3.42 and 28's were putting you in a bad spot in your power curve past the 1/8th mile mark,
The tire change should help your overall gearing out, and help you in the big end IMO.

I really think your intake setup is killing you. You should make a track only 4" ram air setup that goes to the head light area? The GTO guys and even the RX7 swap guys that run a 90" degree bend CAI usually pick up 15-30 rwhp going to an over the radiator setup, or similar.

Thanks for all the responses guys. I guess I'm not as far off as I thought.

My 60ft are in the 1.69-1.74 range when it hooks well.

I spent some time over the weekend cutting out the exhaust neck down right behind the collector. It was a 3" slip on made to exit to 2.25" pipe crammed into my 3" exhaust, but the exhaust was at a downward angle so the 2.25" was almost against the 3" inside created a wall where the exhaust could only take a sharp turn down between the gap. (Think of it like sliding a pipe into another bigger pipe at a 45* angle but pushed in past where the smaller pipe slips in.)

The car is noticeably more responsive and smooth during normal driving, WOT there isn't a seat of the pants change but I bet it picked up at least 5hp from this. The exhaust note is more open sounding and the high load low rpm gurgle it had is gone, just sounds smooth and clean.



I will keep toying with ideas for an intake pipe setup without 90* but it won't be easy. It would need a 90* to even get to the headlight area but maybe I could do it with 2 45* spaced apart.

I did talk to a 6.0 nova who went to the 1.8th mile track with me last time. His car is a 6.0 with carb/intake, MSD box, TSP 233/239 cam, 1 3/4" headers to 3" exhaust, TH350 with 3500 converter, 3.73 gears, 26" tire and likely weighs around 3400 or less with him in it. He ran a 7.4@93mph to my 7.6@91mph in the humidity. His car dynoed 380rwhp on a land and sea dyno. He was out launching me but it seemed like I was just barely pulling on the top end. Stupid 1/8th mile tracks haha...

Also to mention when comparing the car to a Fbody, it has the aerodynamic capabilities of a Brick, so those things are working against you, add more power, take away weight, and/or converter fine tuning.

A friend has a 4" inlet cone which is about 3" longer than mine and a 4" to 4" elbow. I ordered some 1/4" reducers to make them fit my MAF. Going to try it out versus my current 3" setup.

I know with the car running at idle (especially cold) there is a very loud hissing sound of air coming from the filter.