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.
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.