C3PO Vette

I know my stock 78 vette has 185 hp. I have a K&N air filter in it and free flow true dual exhausts with no catalytic converter. My question is what is my approx hp with these small upgrades? Also, what other simple upgrades can I do for better performance? Intake manifold, carb adjustments, headers, anything else?

scottyp99

Recurve the distributor. It costs, like 20-30 bucks, and you will notice the difference in power. Best bang for the buck you can do. And get rid of that K&N filter. Ya know why K&N filters flow so well? 'Cuz they're not very good filters. Think about it.


Scott

Shark Racer

Assuming the car is in good tune, I'd assume about 200 hp.

Headers will be the best bang for the buck. Intake manifold won't add much power but will lower the engine weight a bit.

Vette5.5

I agree on some distributor work, and in fact would pull it out for some clean up work. It's quite common for the mechanical advance shaft to gum up, and not properly advance. Since you opened up the exhaust, could be running a bit lean, so some carb re-jetting might be a good idea. The K&N filter is fine, if that's what you want to use.

C3PO Vette

I appreciate the advice, this forum is a really great tool to get our cars running the way we want them, thank you all!!!!

terry82

after all that time and money for improvments,about 185 hp.with about 20%less at the rear wheels.those improvements did not gain you any more than what gm gave you.a bad cat will restrict you .a bad flowing exhaust will restrict you ,but they dont make hp.a air filter can restrict you but it does not make hp.to improve you hp you need to look at cam and heads.for more power you need to look at pistons.when all that is done then you have to get all the bad gasses out .then you worry about headers and dual exhaust.i hope i answered your question with out disappointing you .decide how much hp you want and how much cash you have .with a little time and money you could have close to 300 hp.

jb78L-82

The only difference between a 74 L-82 rated at 250 Net HP and a 75 L-82 rated at 205 HP is the true duals on the 74 and the 2-1-2 exhaust on a 75 with the VERY restrictive cat which is a difference of 45 HP. There are a number of articles documenting that changing from a restrictive catted single exhaust to free flowing true duals will net between 30-45 HP depending on the restriction in the original exhaust. Headers will add about 15 HP to those numbers. There is zero wrong with a K&N airfilter-I have had one on my L-82 since 1985 with zero issues!

terry82

the debate about k&n filters has gone on for a long time.have your oil checked and you will see why fleets dont use them.

81pilot

Most fleets don't use them because of clauses in thier warranty that says non OEM items void warranty. Also The newer trucks have sensors calibrated for the exact air flow of the stock filter so it can show a warning when it needs changed, too much air and it thinks there is no filter. It will even shut down to a very low power output till it is changed. It has nothing to do at all with the filters being bad for the engines. Its another way for dealers to sell OEM parts.

Shark Racer

I did testing back to back of my car when it was still running the L48 and there were major performance improvements going from stock exhaust to true 2 1/4" duals, then stepping up to 2 1/2" duals, and finally putting headers on. The difference from start to finish was almost 3/4 of a second 0-60 time. To put that in perspective, it made my L48 run to 60mph as fast as the same year L-82, from exhaust alone.

jb78L-82

Exactly! Getting an L-48 to run the same time as an L-82 equates to about 40-50 hp as stated above! My L-82 with 2.5 true duals, free flowing mufflers and headers is MUCH quicker than with the stock exhaust.

81pilot

I agree. And for those that don't think air in and air out don't matter much, tell them to go outside and run around the block with a bathtowel wrapped tightly around thier mouth and nose, then take it off and do it again.....they will get the point.... efficiency matters...

jb78L-82

The usual dopey comments from some that are totally uniformed about exhaust efficiency not having much impact on HP as well totally wrong comments about k&N filters-Where do these comments come from?

TheSkunkWorks

FWIW, I'm a long satisfied K&N user and have never doubted the value of more freely flowing exhaust systems, but might it be worth pointing out that the OP already has the K&N and duals? He was merely asking for an estimate of current power and what simple upgrades to consider in addition, not for opinions against doing to the above.

To that end, I'd suggest an ignition and carb (in that order) super-tune, and 180* stat for beginners. If you don't already have headers, I'd do that first and then the tune. And, I'd consider a 3.55:1 or 3.70:1 ring & pinion if you've a taller (smaller ratio) FDR than that. Finally, weight savings add up in a hurry, so you might look at what you can do without.

C3PO Vette

Thank you all for posting, just what i was looking for skunkworks, im no expert mechanic but have a few bucks to throw into the car, just dont want shop to do work that wont make a difference

81pilot

Let me add another free mod that will help you. The timing is horribly inefficient when set to factory specs(factors sets them for emissions requirements). It will amaze you what a good performance timing adjustment will make. Lars(can I say his name?) has an awesome paper on setting a dist properly for optimal performance AND economy.

7t9l82

once upon a time, i put a set of headers on a car for a Guy the quarter mile times changed by an insignificant amount and i was a crook! i told him bring it back in rejetted the Carb set up the distributor and picked just under 4 tenths, again i was a crook because it was the Carb and distributor that made the change. the moral? it all works together and some don't know what they are talking about

terry82

better said than i did.

81pilot

I agree, however here is what is going on with a stock C3 engine. The Qjet is rated at 750 CFM...on an L-48 or even L-82 the engine will not need that amount. If you add headers the amount of decrease in exhaust restriction, it allows the engine to work less with what is available. There is no magical equation, but in general pipes including headers do increase HP. As for headers increasing 1/4 mile times, if that is the only missing equation then possibly, but if the carb had a restriction in the first place either in jetting, setup or CFM then that becomes the limiting factor, it is not a limiting factor on a stocker with a Qjet. So in your case the increase after the header install and carb adjustment was a factor of both things working in tandem, but the exhaust upgrade was not effective due to the limitations of the carb setup before the change.

7t9l82

i would bet a significant exhaust flow increase on any engine would benefit from Carb tuning.