roger3

76 Vette with 468bbc 60ft times 1.79 1.80 all day
Averaging 12.60 sec 1/4 mile

I was hoping for much better 60 ft times to get down into 11sec time for 1/4 mile

Here's my setup

468 bbc
540 hp at crank
990 heads
10:1 cr
Rpm air gap
850 Holley

350 auto trans with shift kit
3500 stall converter

3:36 gears
Mt ET street radials
340 composite rear spring

I don't spin when leaving the line

Tried launching at 1200rpm, 1500, 2000, and 2500 all give me similar 60ft times and 1/4 mile times

Any ideas?

I'll scan my last few slips and post later

Split327

Change that rear spring to a 420 and if you keep those 3.36 that's going to kill your launch. Sorry. Ran my 75 last year, 2003 LS6, 475 hp, 4,000 stall, glide (smaller first gear than yours) 4.11 rear, 420lb spring and ran low 11's with 60's in 1.55 range. Check your carb and timing for low rpm response, could be killing your launch. Hope this helps.

roger3

Split,

Thx for input. I was wrong about rear spring rate it's 360-lbs/in (not 320).

Glad to see your number s for comparison.

I know my carb setup is not perfect, I also have new shocks on front with sway bar that might be hurting my weight transfer.

I was running a 15 year old, (sat up for several years) Holley 850. Couldn't get it to idle below 1000rpm, would bog bad off the line, and when I had the engine dynoed it showed one side of engine richer than the other. Just for testing I borrowed a Holley HP 750 racing series with 4 corner idle screws. Now Engine doesn't bog off the line any more, feels pretty crisp, idles good at 800 rpm, but I lost 5 or 6 mph on top end when I cross the finish line and still have bad 60ft times.

Calculations show I need an 850. Thinking of putting a 950 HP Holley. Heard bad things about the 850's from time to time and mine certainly doesn't perform well.

Also want to get an air fuel ratio meter to tune with.

I was also told my 990 heads flow poorly at low RPM's and is the reason for my bad 60 ft times.

What do you think?

Roger

L79racer

What was your mph? MPH will tell HP. From there we see if its making the HP you think and then we see from HP what ET it should be running. Whats the weight also?

roger3

With the 850 carb mph were pretty inconsistent and ranged from 108 to 114mph.

With the 750 carb consistently run 109mph.

I don't know the weight. I need to find somewhere to get it weighed around Baton Rouge.

Roger

roger3

Using an online HP calculator with 112MPH and 3600lbs, RWHP is 416hp. 416 RWHP and 3600lbs equates to 11.96 sec so it looks like I am 0.8 seconds slow.

When I had the engine dynoed it had a made 540hp at the flywheel. If 416hp is correct, then I am losing 30% between engine and rear tires. Seems high to me.

Engine was dynoed at low water temp ( I think 140f if I remember correctly), with elec water pump and I have belt driven, without alternator, and with electric fuel pump and I have regular engine mounted fuel pump. Not sure if these differences and drivetrain could add up to 30% hp loss. Any thoughts.

Split327

Bad low rpm head flow plus the 3.36 will keep you from running good 60ft times. If you can live with that, just tune the car for peak mph in the quarter, you should be looking for a 12.5 to 12.9 AFR at WOT going down the track. If you set the carb up correct, you should get no bogs or hesitation off the line, the vette should dead hook with no wheel spin at all, and the result would be very consistent 60fts and low 12 second ET's. My car was lighter, 3050lbs with gas and driver. Get rid of the front sway bar and the weight transfer will work great. You can get away with a good gas shock in the rear but if you have any issues, the QA1's work great. Hope this helps

Split327

Also you can expect up to 25% loss for a stock rear vette because of all the u- joints. Set up the rear ride height so the half shafts are almost straight across. When the car squats you want them straight across for max strength. Best stock fitting axles and spindles are made by International Corvette (Axle). Don't let anyone tell you they have better.

Vettekid1992

only going 114, your not making any more then 450 horse, all LS1 cars go 110mph with stock motor making 350 horse in 3200-3500lb cars, but yes as stated corvettes loose a lot more in the drive train.

L79racer

Checked a corvette spec and says a 75 convert weighs in at 3660 plus 200 lb driver total weight 3860. Wallace racing says 445 hp for that weight and mph and that is flywheel hp. At that hp to weight an ideal run would be 11.96 with a car set up to race. gears, converter etc. Plus you have more hp loss for vette like split said. Seems you are not that far off. I'de be looking for a low12.20 et at that mph. You might be down on hp from carb, exhaust and maybe headers. Dyno headers usually flow better. Little more gear will help with 60'. What you pick up in 60' you double for 1/4 et

roger3

Thanks,

I'll remove front sway bar.

I do have the Dragvette Basic kit that has a 1" spacer to kinda flatten out the half shaft angles. I will measure the angle and see how much it squats and see about possibly using stiffer springs.

I'm gonna add an air fuel ratio gauge and check a/f ratio at WOT. If I can't jet the 750 enough I'll put a 950 Holley hp on it.

I'm gonna stick it out for a while with these heads, already spent more $$ then I planned. I want to switch to a hyd roller. Cost for heads, cam, intake, and lifters is about 3k. I might be able to sell my stuff and recoup some of it.

Thanks for the help so I can get the most out of what I have.

I'm learning as I go, but it ain't cheap.

Thanks

Roger

roger3

L79Racer,

I have run a 12.39 and 12.37, but most passes are 12.6 to 12.7. I dont know what I did different, or if it was a fluke to run the faster times. I was doing same routine during burnout, shifting at 6000rpms, and launching at 2000rpms.

With the 750hp carb I won the street class last weekend at our local drag strip. It's running pretty consistent. For the final round I dialed a 12.6 and ran 12.604 with a 0.023 light. Beat a 12.2 second rice burner.

L79racer

Different et's come with different air (DA) Nice cool air in fall gets nice ET's. Very nice package that run was.

roger3

My confusion is that I ran these 12.3x times on the same day as a bunch of 12.7x and 12.6x times. They were with the 850 Holley that was spiratic.

Glensgages

how-different were incrementals (60', 330', etc.) on the 12.3x to 12.7x runs that same day ~ with that much of a difference, you would expect to see the 60' times varying .2, maybe more

114 MPH shows that you have the power to run mid-11's (.60's?) if you can effectively apply it to the track off the line and the full 1320'

roger3

Here's my slips from April 15, 2012.

Let me know what ya'll think.

Vettekid1992

[QUOTE=roger3;1580813543]Here's my slips from April 15, 2012.

Let me know what ya'll think.

those r some inconsistent 60's, i'd try and focus on that more then anything, all my 60's are almost dead on with each other, (+ or - .01) u must be spinning or doing something different

ajrothm

Roger you are definitely inconsistant on all fronts.... I really think the lack of 60' and 330' is those 990 heads. Those combined with the 3.36 gear are probably not a good combo unless your converter works REAL well... You mention it is a "3500" converter.... Is that was your measured flash stall speed is? Or just the number on the box it came in?

Your suspension setup and tires are fine... Your car is not likely spinning...The lack of 60' is from low-midrange power and soft gear ratio. Your car is probably about 3700lbs raceweight (with you in it), if it made an honest 540 hp then it should be in the neighborhood of 118 mph ASSUMING the front half numbers are good...(also how much power are you losing in the car from parasitic losses and exhaust etc etc) You are showing a 25-26 mph back half gain so yours is making good power past the 1/8 but your 1/8 MPH is off...

I would look real strongly at your converter...My guess is its not giving you enough flash speed. You're 60's/330's are killing you... That is likely hurting your 1/8 mph and thus hurting your 1/4 mph by a couple.

The reality is, the combo is not matched well... Those heads are good from 5-7k+ rpms, if you dont have the cam/intake to get it there, you're not making the top end power you should, and if the converter/gears dont get you up there in that range quick enough, you're killing the clock.

Before making any changes though, you need to get it consistant., Try to race it on similar weather/tracks, keep track of the DA for the day. Write down everything you do on each slip...How you launched, where you shifted and what happened on that pass. Troubleshoot the inconsistancy and get it figured out, then start on the tuning changes.

Even running 114 mph, you should be able to get 11.70s or so once you get the 60's in the high 1.50s. It may not get there without a converter swap though. Personally I'd slap a nice set of ported ovals on it, decent SFT or HYD roller and I bet it picks up all over and much more responsive on the street... Keep the 3.36s in it for cruising. They'll work ok if you get the engine combo right.

ajrothm

Also you're definitely gonna need an 850....A regular 750HP won't fully feed 550hp IMOP.....That could be 2-3 mph loss right there. Also, you need to jet it for max MPH... What ever the AFR is, is what it is... My car runs its best MPH in 0' to -400' DA and the AFR is in the 11.6 range going down the track.... Any leaner it slows down.

A few other tips while my thinking cap is on...

*Make sure you are either running an air cleaner or preferbly the air cleaner base with a hold down. For some reason, in C3 engine compartments carbs dont like to be ran completely open.

*Make sure your air cleaner lid is not sitting too close to the carb throat... You need atleast 2" above the carb throat to the bottom of the lid.

*Make sure your hood is atleast 2" above the carb throat.

*If you have clearance, try to run a 1" open spacer under the carb, RPM air gap intakes need the volume. It will help power everywhere by about 10-15hp.

*Obviously dial in your timing and jetting at the strip

*Try to stage the car with the oil temps still warm but the water temp as cool as you can. I go in the burn out box at about 120*...I leave the line no hotter then 140*

*Usually shifting the 1-2 a little early will pick up some ET. Playing with shift points is a must. You never know what it REALLY wants.

Experiment and document everything... You'll start to widdle that time down for sure. I went from 11.22 with my new motor down to 10.50-.60s just from maximizing everything in my combo.

roger3

AJ,

I talked with Pro Systems about a new carburetor and they are recommending a Holley 1000 with Dominator metering plates. I think that's way too much for my setup. Calcs show I need about 830CFM. I have a real old holley 850 that I can't seem to get to work. I have went through it twice, soaking it overnight in carb cleaner each time and still think I have some partially blocked passage ways in it. I am pobably going with the Holley 950 HP.

My air cleaner is a 1" drop base with a 3" tall filter element so I have about 2" above the carb throat. I will try with the base only and see how that does. I have about 1/2" clearance between the air cleaner lid and the hood. I will try the 1" spacer and see how that affects me.

I haven't tried any jet changes at the track, that'll be next on my list. Work keeps getting in the way.

That's interesting to hear about the early shift point, because I did that accidentally and saw my time improve, but didn't attribute it to the early shift because I had too many other things going. I am a rookie at the drag strip, still trying to just be relaxed at the light.

My first few passes engine temp was between 180 and 200F. Then I added a toggle switch to my fans so I could make them run all the time. Then I was in the box at 160F, leaving the line at 180F. I'll let it cool off between passes to get it down to 120/140F.

Thanks for the help. I can't make it to the track this weekend but should be able to go next weekend. Hopefully i'll have my new carb and spacer by then. I'll try the carb by itself first to see one change at a time.

Roger