Performance Tips

[captras]

I have a 1975 convertible with 32,000 original miles. My goal is to keep it as original as possible, but I would like to boost the performance a little without any obvious mods. I am thinking about replacing my L48 cam with an L82 cam and replacing the existing Cat Conv exhaust with true dual chambered exhausts. How much HP increase can I expect, and would a set of headers make any more significant difference?

Thanks in advance!

 

[Tom73]

... but I would like to boost the performance a little without any obvious mods. I am thinking about replacing my L48 cam with an L82 cam and replacing the existing Cat Conv exhaust with true dual chambered exhausts.

Don't know about you but, to me, replacing a single exhaust with duals and adding headers would be very obvious mods. But if that is what you are thinking, then...

No matter what mods you make to the engine you must be able to get the exhaust out or the power will not be there. My theory is to start at the back and work forward. Trash the stock exhaust and replace it with true duals off of the '74. Bolt on operation. Next, if you want is headers. They will require some cutting and welding to hook up. Then a good cam. The cam is the traffic cop of the engine. I would not go with the L82 cam. It is good but is Very Old Tech, been around since the late 60's. Call one of the cam companies and they will match the cam to your engine.

Then if you want more power, try new heads, intake and carb. But I would do those last. Start by getting the exhaust out of the engine, then decide where you want to go from there.

Also be sure to look at a crate engine. May be much simpler to just pull the stock engine and drop in the crate engine. It comes with a warranty. Just buy what HP you want

tom...

 

[CQRT]

First of all, congrats on such a great looking, well preserved roadster....it looks awesome!

I don't recall Texas regulations on smog testing, or whether there's an exemption for cars over 25 years - - but I agree with Tom that exhaust gets my vote as the place to start and may provide the most benefit for relatively low cost and modest effort. We went the same route several years ago on our 75 L-48 roadster as well.

Rather than block off the EGR valve, we replaced our intake with one off a `72 to maintain the stock appearance and rejetted the carb to get more fuel into the better-breathing engine after running the larger LT-1 dual exhausts.

None of it gave us head-snapping performance, but I figured we picked up some budget HP in the process and although unscientific, felt we were stronger off the line with much better mid-range power.

Regards-

 

[captras]

Thanks to all for the tips. And thanks for the kind words about my "baby" There are no inspection requirements in texas for a car registered as an antique, so I can, and will, put duals on. Just trying to decide on mufflers, or chambered exhausts. I am not looking for a head snap acceleration, but something a little stronger and something a little better sounding. Thanks again...as usual, I get answers here, and get NOTHING on Corvette Forum

 

[vetteboy86]

True dual exhaust would be great with headers. It is nice to work with these cars because you can do anything without a computer to get mad at what you are doing.


Craig

 

[Vettefan87]

Headers and a free flowing exhaust is probably the best bang for the buck considering how easily it is to work on a carbed car with hardly any electrical. Did you have to run any cats in 75, I know it was somewhere around this time that they started really cracking down on emissions. Also headers for older cars like yours I don't think are as pricey as the newer ones, no matter what you do very nice Corvette.

 

[Tom73]

Did you have to run any cats in 75, I know it was somewhere around this time that they started really cracking down on emissions.

Cats were introduced with the '75 model year

 

[captras]

Cats were introduced with the '75 model year

Yes, it does have a cat, but Texas does not require them on cars prior to 78, and, since mine is registered as an antique, there are no annual inspection requirements anyway. So I am good to go on true duals!!! VIVA LA TEXAS

 

[MMM]

I had a '75 Convertible, but mine was an L82 (M20 four speed). When I bought the car I was badly disappointed with the performance. I also owned a '73 LS-4 (Auto, full option) coupe at the same time and figured the L82 would have some kick, but it was dead.
Did frame off restoration to the '75 which included engine and trans removal. It went back in looking dead stock but:
- recurved the distributor to '72 LT-1 specs
- repiped vacuum advance system to use manifold vacuum
- Gutted AIR pump and blocked AIR port in the exhaust pipe to prevent burning of the AIR system.
- Removed the L 46/L82 cam and replaced it with a L-79 cam set at 2 degrees advance.
- Removed the CAT and Y pipe and installed a '74 L-82 exhaust system.
- pocket ported the heads, cleaned up the exhaust ports, and did a three angle valve job to the heads.
- blocked the EGR.
- Completely revised the internals of the Q-Jet, changed main jets, metering rods, power piston spring, secondary rods and rod hanger, increased accelerator pump travel.
The car came alive. Ran consistant 14.2's in the quarter
(3:55 rear).
I still think the GM cams are the best. And for intuitive reasons I like the 327/350hp cam (L-79) better than the 350 / 350hp cam. I have been around along time and have seen many chevrolets with lots of supposedly great aftermarket speed parts that ran worse than they did with the stock parts.
I had to sell my '75 in 1990 to build a house. I would like to find it. So if anybody knows of a 75 convert with:
both tops (vinyl covered hardtop), dark red exterior, black interior, L82/ 4-spd, standard interior and no air conditioning, let me know. Thanks!
Mike