spitfun

I just took my new car (1972 BB) in today for some very disappointing results. The car dyno'd 172 HP to the wheels! Even after tuning I only made 185HP and 299TQ,albeit it with a wider power curve to about 4800 rpm's.The tuner (very capable guy I've used before) said there is a mismatch somewhere. I went thru the records only to discover on the rebuild they used flat top pistons (maybe 8.0 to 1 comp) and a truck cam!! What do you guys think I will gain by just replacing the cam ( I'm thinking an XE268 ) to the horsepower and torque levels and should I go ported polished heads with larger valves... Thanks for the time Rich

DUB

I do not know what type of numbers you are looking to achieve. The 454 from the facotry was rated and 270 HP..and that was at the crankshaft...not rear wheels. I am sure you are aware that a HP/torque drop does occur within the driveline.

I am not 100% sure if GM lessened the quality of the internal parts due to the HP began to get dropped so no need in putting super good parts for and engine that was not going to need it.

AS for polishing and porting heads....from my understanding of it...although it greatly help flow...what I have been told is that due to the runners and such being now smoothed out....the lack of the rough surfaces does not help the air/fix mix bouncing off the inner walls of the runners and such to further break up the fuel droplets...thus....possibly causing the car to need to run at a higher RPM. A lot of this also depends on the intake and how those runners are also. SO...a straight runner intake with polished heads can cause for the air/fuel mix to be a bit more 'liquid' instead of being more of a vapor....so-to -speak...and thus can foul out plugs if the engine is not being used at a high rpm a majority of the time.

Many variables to possibly consider...but like I asked..I do not know what kind numbers are going to make you happy. I run into this with many customers that allow the results of a dyno to get inside their head. They were happy with the engine performance UNTIL they saw the results. Then it was the beginning of downward spiral from that point.

DUB

C3 Stroker

Better heads and cam will wake up the engine, but you are really limited by 8.0 to 1 compression..........(unless you consider a blower?)

cv67

Gm rated their fwhp differently you can only do so much with low compression
Most dont post thier dyno # as they are humbling
Some do....some are true most are inflated and dont know it

Ever see any shop or customer say "My dyno #s are lower than my competitors yay!"

Cam, some bowl/short turn work,, curved dist headers used intake you oughta be happy wiht the results. 72 stock 454 sucked on power stock but doesnt take much to make one run.

tuning on an 02 sensor helps a bunch can take a car that "feels good" to "runs hard" .

ddawson

I stroked my 427 to 496, Zero decked the block, Edelbrock heads, Straub roller cam, LS6 intake, QF 830 Carb

Heads got a little work, compression came in around 9.8:1

On a loaded dyno "not a dyno jet" I made 438RWHP and 509RWTQ. With a better intake it could be better but I like the stock hood.

I would check and see what your piston tops look like with either a USB camera or take a heads off and measure correctly. If your not going to rebuild it at least you can get a better recommendation with known parts and specs.

BTW my pistons are flat too.

Big Block Dave

I understand that the numbers are disappointing, but the dyno only says so much. Some guys dyno their stuff, and then take the car to the track, with unimpressive results. It can be a great tuning tool, but I must ask..

How does the car perform?

Even with the low compression, these cars can still run pretty well.
My 1974 LS4, with only modifications being Hooker side pipes and MSD distributor and ignition ran 13.44 @104 on radials.

What is your total timing? What carb? What intake? What exhaust.

Basically Im saying, don't lose hope, and focus on what really matters, and thats what the car does, not what a piece of paper says it might be able to do.

spitfun

The car has the the stock LS5 intake,Rochester carb and stock exhaust manifolds. I have had quite a few cars over the years and my "seat of the pants meter' has been pretty accurate. This car is EXTREMELY slow-Example 1: drop the hammer from 25 MPH and it MAY chirp the tires, example 2: in a CORNER in 2nd if I nail it I can't even get the tires to break loose! No I don't drive my car like this on a regular basis but come on guys this IS a big block with what should be about 350HP and 450TQ on the old rating system. I have a 2010 Buick LaCrosse that would beat this car 0-60 AND in a quarter mile,somethin' aint right. I knew it the first time I drove it-unfortunately that was AFTER I had bought it from BuyAVette and had it shipped to my house-I will say that so far they seem to be willing to meet me part way on resolving thse issues- I will update as the situation progresses. Rich

The13Bats

Seat o pants meter now i can relate,

15 18 years ago i spent my weekends with good old boys and their toys,

It was super common to take a smogger 454 and drop in in you name it and it was a low end / street terror with no upgrades besides carb and headers, because of the torque

Glenns 79 ta with a smogger 454 ran low 14s and would dip into high 13s but seat o pants he could scare ya, and smoke the meat off the wheels at any time,

Just seems something must be holding your engine back, forget dyno numbers it should be coming on low end better to me.

spitfun

I agree, I have a slow car that needs to be remedied. I did come up with s new license plate though "ITS2SLO". Rich

TOM B1

If you are a original engine type person then you may not want to grind and port #s matching heads BUT if not then check if the flat top pistons have 2 valve relieves in them. if so then you can get a thinner head gasket or mill the heads to raise the compression.
Then get a mild performance cam with good mid range HP and Torque or the stock cam if you wish. but get that truck cam out of there. as for porting and polishing the heads, dino testing proves that opening the ports to the gasket size on the head and manifold makes HP and polishing only the exhaust port and leaving the intake port ruff makes HP and mileage work on the street.
Just doing this will make the car fun again.
This is the budget way to do this, but as said above if you want real HP then get aluminum heads and intake ect.
An old saying is still correct, HOW MUCH MONEY DO YOU HAVE AND HOW FAST DO YOU WANT TO GO

ajrothm

Have you verified the secondaries are opening on the Q-Jet (the air valve)? Have you verified the timing is set correctly?

172 rwhp is very low, even for 8-1 compression and a truck cam.....

Porting the heads and changing the cam will help, but honestly that is band aiding something else...

I think a compression test and leak down test is in order...then go from there.

FWIW, my stock 71' L48 (350/270hp) with 8.5-1, L79 cam and headers made 195 rwhp on a Super Flow dyno, through a T400. Pulling the air cleaner got it up to 205 rwhp.. Car was dog slow, ran 15.0@91 mph. I got beat on the street by an old fella in a Toyota Tundra crew cab, and I ordered an engine a few weeks later...

TOM B1

Thats funny, I hope you meet up again

7T1vette

You can do 10.5:1 compression with iron heads and use today's premium pump gas. You can do 11.0:1 with aluminum heads on pump premium. You'll never get good power out of 8:1 compression engine.

You need to decide how badly you want a bunch more power. To do it with the engine you have, it will require a major overhaul. Or, if you just want a "kick in the pants" every now and then, you can get that with nitrous for much less money. But that will only work for short bursts...unless you want to melt your pistons.

spitfun

I have a compression test of when the engine was built (2800 miles ago) and it shows "140" approximately acrossed all 8 cylinders. The timing was changed when they dyno'd the car along with some carb adjustments. I have attached the dyno sheet-he did find a few horsepower but he also got it to at leasdt have power up to about 5K,before the tune it made less and died at about 2900!! The engine is not original to the car,so I don't have a problem having work done to it. I think I will have the headwork done,change the cam and take a look at the pistons to see if I can use a thinner gasket to up the compression along with maybe milling the head and see what results that gives me. I'm not expecting 500RWHP,but I would like to see 300 plus to the wheels,which I feel is attainable with the mods. Rich

Big Block Dave

I recently borrowed a Q jet from a friend, and haven't gotten the secondaries to open. Its pretty tame and uninspiring.

Don't be fooled into thinking that sub 10:1 compression can't make power. 8:1 may not be the best foundation to build on but don't believe for a second you can't make any power. There are guys on chevelles. com that will disprove that.

spitfun

Sorry,I forgot to answer the secondaries question. When I got the car,no they didn't open hardly at all-but I did get them to open completely after adjusting the linkage. The sad part is it didn't make much of a difference...Rich

Revi

Is your car an automatic or manual? Does it have A/C, P/S, A.I.R. installed?

When I bought my 1970 L-46/M-21 the engine had been detuned by the PO. I'm guessing so it would run on 87 oct. fuel.
I ran the car on a dyno jet to get the detuned RWHP, I then pulled the engine and had it run on an engine(lab) dyno at the rebuilders. Once rebuilt it was put back on the same engine(lab) dyno and after being driven a couple of hundred miles, ran once again on the same dyno jet. Same car, same dyno's, same exhaust/drive train.

Detuned 1970 L-46 with flat top pistons and RV/Boat cam, 8.7 c/r.
285hp/334tq (engine dyno)
217hp/282tq (dyno jet)

Rebuilt 1970 L-46 with domed pistons, valve job, head porting, cam xe262-H10, 10.4 c/r.
341hp/387tq (engine dyno)
245hp/305tq (dyno jet)

Comparing your engine results with those of my detuned L-46, it seems like you either have the perfect storm of less than 8.0 c/r and a lazy cam or you have a tuning issue.

Are you sure the timing is advancing as required (36* all in by 3000rpm) and that the secondaries (mechanical) are opening along with the air valve (the flapper on top)?

rklessdriver

It won't with that horrid Air/Fuel curve.

That Q-Jet needs some work bad.....

Also only 140lbs on the compression test tells a lot about the cam/valve timing.

Will

StraubTech

With OEM heads the duration split needs to be at least 12 degrees. The OEM exhaust port needs more time (duration) to get the exhaust out. A camshaft change will help quite a bit. Don't let the compression worry you that much. Cylinder pressure can be manipulated with camshaft. Just had a 475CID BBC with 9.86 to 1 make 680HP and 600#/ft.

StraubTech

Here is the dyno sheet.