davebw31

Hello ALL:

Well took the drag car to a chassis dyno in Dothan, AL. this past Saturday.
Guy running the dyno seemed to know what he was doing. The dyno is a one year old Dyno-Jet 241XL (I think that is the model number).

Do not think the numbers for the three pulls really are correct for this car. He gave us a HP and Torque conversion at the flywheel from the rear wheel HP and torque readings. It just did not "add up" to be correct as I have had both SBO and BBO, both Stock and Super Stock engines dyno'd and they made more HP and torque than this car and this car has more performance items?????????? This is my first try at using a chassis dyno.

To start with I changed all fluids, rear end (80/90 Valvoline Semi-syn), trans. (TCI Maxi-shift fluid and high flow filter), and engine oil and filter(Brad Penn 10w-40 race oil and K&N oil filter). New NGK race spark plugs (#7 heat range, gapped @ .040")

Anyways here is the "numbers":

1st pull: Air filter in place, 35* of total advance, coolant temp. 160* reading made @ 6,000 rpm=

362 HP
399 Torque
13.1 Air/Fuel Ratio
93.5 *F
29.93 in-Hg
23 % humidity

2nd pull: removed air filter and base, 35* advance, coolant 160* reading made @ 6,000 rpm=

372 HP (+10 HP)
371 torque (loss 28 ft. lbs. of torque from 1st pull, huh ?)
12.5 Air/Fuel Ratio (better ratio)
93.82 * F
29.94 in-Hg
22 % humidity

3rd pull: advanced total timing to 37*, coolant 160* reading made @ 6,000 rpm

378 HP (+ 6 HP more from 2nd pull with total over 1st pull of + 16 HP)
379 torque (+ 8 more than 2nd pull)
12.1 Air/Fuel Ratio
93.80 * F
29.93 in-Hg
22 % humidity

So he used a multiplier of 1.25 to figure flywheel HP and torque so that equals 473 HP and 474 ft. lbs. torque. Christ I had stock engines that dyno'd at 430 HP and super stock engines at 495 hp?????

So what are your thoughts who have more experience. I am always open to learning, and "in the old days" it was what the "engine dyno said" during tuning and what the "time slip said" that told you if you made the right heat range plug, jetting, and timing changes at the track!

I was under the impression that chassis dynos were better and more accurate than those of 20 years ago!

Thoughts-opinions-agree/disagree?????????????????

BTW: Have not done any timed runs yet. Wanted to do this first !

380 Racer

Dave, I have never used a chassis dyno myself. After the talk we had the other night, I would have expected healthier numbers out that thing. My thoughts were very easy 500+ out of that BB. What were you using for fuel? Most usually from what I have read, BB like 32-35* of timing. Good luck.

Nick

davebw31

Nick I addded 3 gallons of VP 110 before the pulls. also Nick I am old school and I think the "et slip" tells the story!

jfb

Buddy usaully the difference is one fifth for fly wheel and rear wheel.

VORTECPRO

In my opinion thats good for one thing, changing a part, or making a tuning change and seeing if more power was made. To me, the numbers mean nothing when it comes to drag strip performance. I do think it will tell you if your change made more power. TQ converters affect the power alot.

VORTECPRO

My customer had his big block Vette on a chassis dyno, 3.36 gear, 10 inch tight converter, 440 RWHP, 10.59 @ 128 MPH @ 3720, hows that make sense.

SBORule

Chassis Dyno's are different from Engine Dyno's.

That HP/TQ at the tires and we all know it's one thing to make power but getting it to the ground is different.

I'm thinking tire slippage caused your lower than anticipated results.

Is this a BTR built engine ?

VORTECPRO

I wouldnt put to much stock in those numbers, just use it as a tuning device, when the car goes down the track he will know how much power is being made.

ah64pilot

My first thought is that the car wasn't in 3rd gear, but surely he knew better. I agree with the corrected numbers, if the chassis dyno is correct and you assume a 20% drivetrain loss then the 473 HP / 474 TQ makes sense. (473 X .80 = 378.4)

What is the timeslip goal for the car? I made 492 on the engine dyno, correct that for 20% drivetrain loss it comes out to 393.6 at the rear wheels. My last run (on a blown head gasket) was 12.11 @ 112. It had stock suspension when I made that pass, now it's set up for drag racing but I haven't run it yet. Either way, it's gonna be a high 11 street car. If you are planning on 10's or lower then you might have a problem.

I don't know the specifics of your build to know if that is low or not...can you post details?

chadman

I know loose torque converters and deep rear gears can give you low chassis dyno readings. I always just use a RWHP calculator that uses my vehicle weight and 1/4 mile trap speed to calculate RWHP. I do know one thing, you can dyno the same car with 2.56 gears and 4.56 gears and it will show more power with the 2.56's and we all know it will be faster with the 4.56's. I use this one:
http://www.wallaceracing.com/hpcalculatorquarter.php

According to it I make between 430-435 at the wheels and between 480-485 at the flywheel in my '65 using an 11.60 ET with a 116mph trap speed at 3825lbs.

380 Racer

Yes, a dyno is just a tool but can be a very good indicator of the performance. I guess I have been very lucky, both of the guys I have used have good reputations with good (calibrated correctly) dynos. And yes you have to get it to the ground.

VORTECPRO

A real dyno (SF901) is a very good indicator of performance, because when I come off it I already know what Im going to run at the track, or atleat come very close to knowing. The trick is how you put it to the ground.

davebw31

SBOrule:

No, not a BTR built engine. Machine work was done by a local machine shop in Tampa, FL and assembled by previous owner who also fully ported the stock 1970 "E" heads, 2.007 intake Ferra race valves, 1.650 exhaust, Harland Sharp Gold roller rockers, Crane springs 135# on the seat with 335# off, Ultradyne solid lifter cam with 257/264 @ .050 587/608 lift 108 LSA installed 3* adv., 2" Hooker Super Comp headers with 14 " collector extensions, Holly 950 HP carb., 1" parabolic spacer, ported and flowed EB Torquer intake.

VORTECPRO and chadman:

Has a JW 10" race converter that is stall rated @ 3,400 rpm (flash stalls @ 3,800 rpm). Rear is a 67' 12 bolt Ch**y with Richmond Pro 4:10 gear set, full steel spool, and Moser 33 spline axles. MT ET Drag 29.5X10.5X15 tires. Car weights 3350# without driver, 3550# with me in it.

380 Racer:

Got to make some easy passes to get use to this car on how it handles, top end stability, sounds to get use to, etc., etc. Got to get a "time slip in my hand" to really know. LOL. It will be another month or so before that happens!

ah64pilot:

Actually an automatic trans from what the dyno guy said and what I have read in the last few weeks, states that there is 25-30% loss in the driveline. Stick car is only 13-15%! According (Note: I say according) to the previous owner that he made 6 passes at the local 1/8 mile track with a best ET of 7.08 ET @ 99.5 mph only shifting at 5,700 rpm. The dyno graph showed torque falling at 6,000 rpm, but HP was still climbing! My partner and I agreed to "just be a little conservative this time." A time slip will tell ALL. LOL

chadman

Here is a good read about the difference a converter can make to your chassis dyno numbers:

http://www.popularhotrodding.com/tec...r/viewall.html

ah64pilot

I'm going to get mathy on you here. When you take a given number, say 372 and multiply it by a percentage increase, 1.25 you will get a number that is 25% more than 372.

EX: 378 X 1.25= 472.5

However, then number you want is that of 75% - 80% of the power you are making at the crank. The above formula won't work because you are only increasing your BASE number by 25%. You really need to find out what number 378 is 75% or 80% of.

For that, you would need a calculator and some algebra. By my calculation, 378 is 75% of 505, thus:

505 X .75 = 378.75

The latter example is more accurate because we are using the higher number as the base number and multiplying it by the percentage total power we will see at the rear wheel (less driveline loss).

The reason I said 20% above is as you can see:

472 X .80 = 377.60 (20% driveline loss)
505 X .75 = 378.75 (25% driveline loss)

So depending on how much you are losing to the wheels the engine makes anywhere from 472 - 505 at the flywheel. This is why I hate chassis dyno's, it still leaves open what the engine makes on it's own

Good luck at the track!

507OLDS

The chassis dyno that I use here is a Mustang dyno.It gives you the actual hp/tq at the rear wheels,as if you were driving down the road or track.We have made 1/4 mile passes on the dyno,and my car,as well as others,have always been right with the number,when taking them to the track.I know there is drag created on the drum,to simulate the weight of the car,wind resistence,etc.The rear wheel numbers always seem low,but they have always proved to be accurate.

davebw31

ah64pilot:

Wow, I checked out your link to your engine build/engine dyno tests and your build is very, very similar to my build, except mine is .030 over (461 C. I.) with professional lightened old TRW forged pistons, 10.67 comp. bigger cam (duration and lift), ported iron heads (yours alumn.), everything else appears to be the same! Owner claims he dyno'd this engine and it made 505 HP with 549 ft. lbs. torque. Again similar to your readings.

yankees

From what I've heard is that a stock eliminator 72 cutlass is putting out around 475 in his 350 sbo and runs low to mid 11's at 3700 lbs. His car suspension (alf setup) wise works on those 9 inch tires.

ah64pilot

In that case what are your expectations of the car? I can offer my times as a comparison, but as I said above my car is a full weight street car with stock suspension. It ran 12.11 @ 112.

After fixing the blown head gasket and front suspension I expect the car to dip into the 11's but I have yet to claim a time slip for it. Since the head gasket change my compression is up a little bit to 10.65. I'm interested to know what yours runs because I was contemplating a larger cam now that I have manual brakes.

davebw31

yankees:

You are correct. It runs J-K-L stick and runs 11:50 s @ K weight. There is a J-K-L auto (metric 200R) her in South Fla. that runs 11:60 s @ K weight. I know the guy here in Fla. and both cars engines are done by the same guy somewhere in south LA. The auto dyno'd last year @ 430 HP under old rules. Don't know what the stick dyno'd at, but would guess more than the auto motor. 2012 rules now allows for roller rockers and any duration camshaft as long as stock rated lift is maintained. The stick car in 72' is 440" lift max. and auto. is 400" max. lift. So the stick car would be faster and have a better mph. Would guess that under new rules I am sure they are making more than 430 hp!

ah64pilot:

When I make some all out passes I will let you know my ET s and MPH. If and I say "if" the guy wasn't BS'ing me it should run high 10 s in the 1/4 and high 6 s in the 1/8. I mean everything this car has says so. Need to wait and see! This car is not a street/strip car, it is a drag car only. Also, it is lighter by 500 #s than your car weight. Plus, this turbo 400 has the 275 low gear set!

VORTECPRO

When I use the Wallace calculator its shows 50 HP high from my actual dyno testing, something to consider, the Weiss calculator using MPH lines right up with my dyno testing.

VORTECPRO

What is K weight?

davebw31

K class weight is the actual shipping weight for the HP to weight ratio for that class per HP claimed by the manufactuer. NHRA allows you to add or lessen weight to move up or down one class from the class weight per HP. I would guess that "K" weight is in the middle @ 3550 #s.

joesw31

Glad to see that you are enjoying your new ride. I would not worry about the dyno numbers, and again wait for your time slips as you discussed in your post. The goal is to have fun with your car.

Run to Rund

I have always used a Dynojet 248 (meaning two four foot diameter rollers), 80 slugs of inertia. My Dodge diesel dyno'ed 800 hp at the wheels after a lot of work--single turbo, diesel fuel only, 359 cubic inches. It is common for people to expect more HP at the wheels than the Dynojet shows. Your results are not so much out of line.