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-   -   All this work and can't even spin the tires??? (http://www.hotrodders.com/forum/all-work-cant-even-spin-tires-182491.html)

dry 08-10-2010 02:52 PM

All this work and can't even spin the tires???
 
Long time Lurker and now need your expertise!!

Ok here is the deal, My Corvette is a 74 L82 4 Speed car 73k original miles with the 3:72 rear end I have not been happy with the performance of the L82's Bottom End Torque so some changes were made.

Block was line bored .30 over and decked. the rotating assembly was balanced, forged flat tops, Crane Fire Ball II 290H Cam (1800-5200 rpm)-224/224 duration, 107 lobe centers,454/454 lift, Performer aluminum intake, Holley 4160-600 cfm single line, 882 stock heads professionally massaged and rebuilt with bronge guides etc, stock rockers, crane matched springs,Mallory unilite mechanical advance electronic ignition factory 20 degree mechanical advance "No Curve" timing is set at 18 btdc idling at 800 rpm, nice even compression across all cylinders, runnning 92 octane.......

Here lies the problem the car starts right up no timing drag hot or cold and runs great,no pinging, no stumbles or hesitations, temp stays about 190 drives great on the road 3500 rpm at 70 mph and runs well stop light to stop light when just driving it normal.........PROBLEM is when you try to stand on it from a stop light it doesn't have enough torque to even break the tires loose....this rebuild has about 5,000 miles on it so its broke in well and i am ready to find the right tune the car sounds awesome at idle but the torque performance didn't seem to change from what it was before the rebuild!!!

What am i missing here i just can't seem to figure it out, why have i not realized a torque increase etc...

All opinions and suggestion welcome!!

CanuckBird 08-10-2010 03:07 PM

One problem i see is not enough carb.670-725 would be more inline for your build.With that cam you should be coming out of the hole around 3000RPM.Try that and i'll bet the tires light up!

lmsport 08-10-2010 03:13 PM

What trans? What compression ratio? May need a stall, or alot more initial timing if compression is less than 9:1.

dry 08-10-2010 03:33 PM

Quote:

Originally Posted by lmsport
What trans? What compression ratio? May need a stall, or alot more initial timing if compression is less than 9:1.

Tranny is a 4 Speed Muncie t-10 i believe, comp ratio should be 10:1 it just seems flat down bottom when you roll off the clutch from about 2k and stab it. it just hooks and takes off even with only 60 series tires.......

lmsport 08-10-2010 03:43 PM

Double check compression. Flat top with zero deck and 76cc chamber is about 9:1, 10:1 requires 66cc chambers. I am a proponent of testing another known good carb before spending a bunch of time tuning on a carb to cover some other problem. Borrow a double pumper and test.

bigdog7373 08-10-2010 04:05 PM

Well, i noticed that a basic cam from jegs with 224/224 duration has an rpm range from 2200-52000 rpm. Maybe you just aren't launching at high enough rpm to break the tires loose. I also agree that you need more carb for that engine.

a1supersport 08-10-2010 04:43 PM

Like was suggested previously. Rev-up, pop-clutch, problem solved.

P.S. I'm actually witing for the Ford and Dodge guys to chime-in. I'm sure they're gonna be funny... :D

87monte385 08-10-2010 04:47 PM

that cam sounds kind of small for a big block honestly, and i would definatley get a bigger carb.! about a 700 would do fine, itd give you plenty of fuel and better throttle response. do you know off hand what gears you have in the rear-end??

ericnova72 08-10-2010 05:03 PM

Compression ratio too low and heads are terrible, everything else should work out okay.

Headers, or manifolds?? What exhaust system??

Has any tuning been done to the carb?? different secondary spring, pump nozzle, pump cam, etc?? Got a List # for the carb??

bigdog7373 08-10-2010 05:11 PM

While you're at it you should put some roller rockers in, they're supposed to add 15 hp acoording to the manufacturers. Also, an electric water pump, or an electric drive kit for the existing pump will free up some power also. :thumbup:

bigdog7373 08-10-2010 05:13 PM

Quote:

Originally Posted by 87monte385
that cam sounds kind of small for a big block honestly, and i would definatley get a bigger carb.! about a 700 would do fine, itd give you plenty of fuel and better throttle response. do you know off hand what gears you have in the rear-end??

Big block???

oldbogie 08-10-2010 06:08 PM

Quote:

Originally Posted by dry
Long time Lurker and now need your expertise!!

Ok here is the deal, My Corvette is a 74 L82 4 Speed car 73k original miles with the 3:72 rear end I have not been happy with the performance of the L82's Bottom End Torque so some changes were made.

Block was line bored .30 over and decked. the rotating assembly was balanced, forged flat tops, Crane Fire Ball II 290H Cam (1800-5200 rpm)-224/224 duration, 107 lobe centers,454/454 lift, Performer aluminum intake, Holley 4160-600 cfm single line, 882 stock heads professionally massaged and rebuilt with bronge guides etc, stock rockers, crane matched springs,Mallory unilite mechanical advance electronic ignition factory 20 degree mechanical advance "No Curve" timing is set at 18 btdc idling at 800 rpm, nice even compression across all cylinders, runnning 92 octane.......

Here lies the problem the car starts right up no timing drag hot or cold and runs great,no pinging, no stumbles or hesitations, temp stays about 190 drives great on the road 3500 rpm at 70 mph and runs well stop light to stop light when just driving it normal.........PROBLEM is when you try to stand on it from a stop light it doesn't have enough torque to even break the tires loose....this rebuild has about 5,000 miles on it so its broke in well and i am ready to find the right tune the car sounds awesome at idle but the torque performance didn't seem to change from what it was before the rebuild!!!

What am i missing here i just can't seem to figure it out, why have i not realized a torque increase etc...

All opinions and suggestion welcome!!

The heads are early SMOGers certainly not worth the effort you put in to them. My SCR calculator (static compression ratio) gives you 8.6 to 1 with flat tops and these heads with a zero decked block and .019 gasket. My conversion to DCR dynamic compression ratio gives you a useful ratio based on intake closing point and what that does to mixture density is 6.4 to 1 which is way low, the DCR needs to be around 8 to 8.5 to one. The Fireball 290 is an old design cam with too much overlap, a very late closing intake combined with relatively low lift. I'm afraid this engine is DOA on arrival. You really have to be careful of pistons as has been mentioned, there are short deck pistons designed to maintain stock compression with decked blocks, if you have a set of these the static compression is in the sevens or less.

It needs a modern fast lift cam with less duration, more lift, less overlap, and a sooner closing intake against these heads to run the DCR up. A quick and dirty improvement could be to advance the cam you have from 4 to 8 degrees. You need to check for cam to valve interference before firing the engine but with the low lift that's probably not going to be an issue. This cam has a nasty sounding street idle but in the big picture is no great shakes as a performance item, it has all the bad features (rough idle, weak torque curve) and few of the good ones (top end power output). Actually you intake and carb selection are pretty good considering this bump stick.

The carb's small for this combo and the Performer leaves a lot of power on the table, the Performer RPM is a much better unit. Actually the two of those probably are helping a lot right now by keeping the mixture velocity high, what with the late closing intake a slower mixture velocity that comes with bigger carbs and intakes would allow the piston to blow the mixture back out the carb further reducing bottom end and mid range torque. The carb and intake you have really aren't a problem till you wind this up and this doesn't sound like it was built to wind high so changing them right now is a moot point. If you change any of this start with the intake not the carb.

I don't understand what you're saying about the ignition timing, my first impression is that's it's well F'ed Up, but that comes from my intreperation of your description. Some questions to help me;

What is the static setting in degrees?

Does it have a vacuum advance? If so how much advance for how much vacuum?

Sounds like it has a centrifugal advance, that brings the question as to how much? When does it start in RPMs? What is the rate of increase in degrees per 1000 RPM?

Do you know how much total advance there is when it's all in, that is measured degrees against RPM not guesses and not some brochure data, but real measures.


Recommendations:

I'd start with advancing the cam from 4 to 8 degrees. This results in ending the intake cycle sooner against crank degrees which will help the bottom end but hurt the top. Then! Them heads gotta go, you spent a lot of money on those things making a silk purse out of a sows ear. They're now just a shiny expensive sow's ear that's as useless as before. And give me some real ignition timing data. This cam makes for a weak cylinder charge on the bottom end it takes a lot of compression and ignition lead to overcome at least some of that. Cut back on the octane to see if you can get this thing to ping, the fact it doesn't isn't telling me what I want to know. Don't worry for a street engine it takes a lot of pinging to do any harm.

Make one change at a time and test, don't rip it apart and do a bunch of stuff then hope it all works, then you never know what did and didn't work for it.

Bogie

john-dickjr 08-11-2010 12:46 AM

its a chevy , so did you really think it was going to make any power at all, i loved to kill vettes with my little 347 motor, but serious, is the motor a 350 i guess, and a smog motor at that, with a stock rebuild, it make like 180 hp, so no, its not going to spin the tires, unless you clutch dump it

adantessr 08-11-2010 01:59 AM

Quote:

Originally Posted by oldbogie
The heads are early SMOGers certainly not worth the effort you put in to them. My SCR calculator (static compression ratio) gives you 8.6 to 1 with flat tops and these heads with a zero decked block and .019 gasket. My conversion to DCR dynamic compression ratio gives you a useful ratio based on intake closing point and what that does to mixture density is 6.4 to 1 which is way low, the DCR needs to be around 8 to 8.5 to one. The Fireball 290 is an old design cam with too much overlap, a very late closing intake combined with relatively low lift. I'm afraid this engine is DOA on arrival. You really have to be careful of pistons as has been mentioned, there are short deck pistons designed to maintain stock compression with decked blocks, if you have a set of these the static compression is in the sevens or less.

It needs a modern fast lift cam with less duration, more lift, less overlap, and a sooner closing intake against these heads to run the DCR up. A quick and dirty improvement could be to advance the cam you have from 4 to 8 degrees. You need to check for cam to valve interference before firing the engine but with the low lift that's probably not going to be an issue. This cam has a nasty sounding street idle but in the big picture is no great shakes as a performance item, it has all the bad features (rough idle, weak torque curve) and few of the good ones (top end power output). Actually you intake and carb selection are pretty good considering this bump stick.

The carb's small for this combo and the Performer leaves a lot of power on the table, the Performer RPM is a much better unit. Actually the two of those probably are helping a lot right now by keeping the mixture velocity high, what with the late closing intake a slower mixture velocity that comes with bigger carbs and intakes would allow the piston to blow the mixture back out the carb further reducing bottom end and mid range torque. The carb and intake you have really aren't a problem till you wind this up and this doesn't sound like it was built to wind high so changing them right now is a moot point. If you change any of this start with the intake not the carb.

I don't understand what you're saying about the ignition timing, my first impression is that's it's well F'ed Up, but that comes from my intreperation of your description. Some questions to help me;

What is the static setting in degrees?

Does it have a vacuum advance? If so how much advance for how much vacuum?

Sounds like it has a centrifugal advance, that brings the question as to how much? When does it start in RPMs? What is the rate of increase in degrees per 1000 RPM?

Do you know how much total advance there is when it's all in, that is measured degrees against RPM not guesses and not some brochure data, but real measures.


Recommendations:

I'd start with advancing the cam from 4 to 8 degrees. This results in ending the intake cycle sooner against crank degrees which will help the bottom end but hurt the top. Then! Them heads gotta go, you spent a lot of money on those things making a silk purse out of a sows ear. They're now just a shiny expensive sow's ear that's as useless as before. And give me some real ignition timing data. This cam makes for a weak cylinder charge on the bottom end it takes a lot of compression and ignition lead to overcome at least some of that. Cut back on the octane to see if you can get this thing to ping, the fact it doesn't isn't telling me what I want to know. Don't worry for a street engine it takes a lot of pinging to do any harm.

Make one change at a time and test, don't rip it apart and do a bunch of stuff then hope it all works, then you never know what did and didn't work for it.

Bogie

I LOVE reading replies from oldbogie , techinspector and cobalt327 . They REALLY have their poop together when it comes to chevy engines .

Irelands child 08-11-2010 05:02 AM

Well looks like another brand new guy, 2 posts new, proud as heck with what he has done to his car (right or wrong) ripped to shreds by the usual suspects. Hope he returns!!


Dave W


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