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Exactly why when I rebuilt/upgraded my L-82 to a 355 in 2014 with consultation of a well respected SBC engine builder in my area, I opted for AFR 180 CC heads, not the AFR 195's, for the L-82 355, along with forged pistons yielding 10.2 Total Compression with a .015 head gasket, and a howards roller cam with 219/225 duration (right in line with your comments on max torque), LSA 110, and .525 lift. Tranny is a 4 speed Super T-10 with 3.70 rear gears. The engine is very snappy with strong low end torque along with BIG mid range power, pulling strong to 6,000 RPM.
And plenty of gear too.(2.64? & 3.70)
In your case that is like icing on the cake!
The first 60 feet you are running on TORQUE.
And any other time that you get caught below 3500 RPM.
Past 3500 it is all about the HP.
With my "big-cam'd" LT-1 I got caught below 3500 RPM a lot, on the street. (~240*)
And I got tired of being smoked by Hondas at stoplights, before it "woke up". Grrr...
But past 3500 it literally ran like I suddenly hit the "Warp Drive" button.
Then it just "sang", and it was in it's element, all the way to 7000 RPM (just like the movie! That scream is addictive too!)
Fortunately I raced it more than I street drove it.
Last edited by leigh1322; Oct 7, 2025 at 08:59 PM.
Great information on lower rpm torque. Thanks.
On the published GM dyno curves dated Feb of 1970 (great find) should I assume for the LT-1 that was the earlier #3849346 camshaft (thus the lower torque in lower rpm range)?
Is there similar published information for the later #3972178 camshaft with the lower intake (242) duration? If so I am curious to see the torque curve delta to lower in the rpm range to get a baseline.
My SB has a 218-224 HR. 300 rwhp @5600
It puts out 300 rwtq before 3K.
It is a little soft off idle, strong after 1500.
A .70 od should be 2200 at 70mph.
Very strong mid range, no down shifting required.
I took advice from people smarter than me! lol
The numbers say a smaller Comp 260H would run even stronger than a Comp 268H, until 20-25 mph or so. Very important on the street. The 268H responds very well to a little more gear. With 3.08 gears, the factory cam hits it power range at 7mph, and the 268 not until 17, so you would feel the bog. The 260 splits the difference.
Hello, I've been looking at this thread for a while now and just got the opportunity to finally put the cam in. Im thinking of picking up a weiand 8125 intake and running stock heads and Qjet until I have enough to do a full rebuild. I have a 4spd with 3.36 rears, stock tire, distrib advance, and I have headers and an X pipe.
Would the 260H or the 268H be better? I see that you're saying the 260H is better for autos and their 3.08 gearset but would I get more bang with the 268H since I have more rear gear?
How badly would a slightly thicker head gasket affect bottom line performance? Obviously the CR would be lower but I dont have the time or money to machine parts.
My biggest issue is the lack of power past 3000rpm, this should shift it slightly further up in range and add more power all around which is all I really want or need at the moment.
Im trying to squeeze the most out of this while in college. Apologize for any silly questions, hopefully I can revive this dead thread a little because its been EXTREMELY helpful for us low budget simple project people, not everyone's been ripping engines out for 25 years so any help is much much appreciated
I have a 4spd with 3.36 rears, stock tire, distrib advance, and I have headers and an X pipe.
Stock timing specs if I’m reading that right? Upping that will make a world of difference especially with the exhaust.
Personally I would leave it after that until you can do the engine modifications you actually want to do, but you can also check your block mating surface with a straight edge so see if it’s warped or not. I would try to avoid a thicker head gasket as it increases your quench area which increases chances of detonation.
Like the thread says, since you have more rear gear and the 4 speed go with the 268.
Get a thinner HG, adv timing, do a Lars tune up,
Which ex man do you have?
If you have the smog 2" rams horns?
Moving up to 2.5" RHs should be worth 10hp and tq.
JMHO
If you still have the stock timing curve setup, then I would add a good performance timing curve to the engine first.
It can really make a big difference in HP & TQ, especially below 3500 rpm.
It might feel 30-40 HP stronger, for just $7 & some time!
what is the link? I have a l48 1979 99% stock, no engine modifications. is it worth it.
Hello Eli, what link are you referring to? A 50%+ increase in horsepower is certainly nothing to scoff at, but with the prices of things constantly changing it’s up to you to decide if it’s worth it.
Hello Eli, what link are you referring to? A 50%+ increase in horsepower is certainly nothing to scoff at, but with the prices of things constantly changing it’s up to you to decide if it’s worth it.
I love the fact that I have it basically all original but do wish to get more power out of it. "The link for the part to get the 30 to 50 hp more". would it help much with the rest of it being stock? im new with this stuff so pleas be patient. also have school so I may not respond fast.
Last edited by Eli hayes; Apr 27, 2026 at 03:37 PM.
I love the fact that I have it basically all original but do wish to get more power out of it. "The link for the part to get the 30 to 50 hp more". would it help much with the rest of it being stock? im new with this stuff so pleas be patient. also have school so I may not respond fast.
I see what you’re saying. Yes, absolutely having a proper ignition curve is good for the engine no matter what else has been done to it. I would email Lars at v8fastcars@msn.com and ask for a copy of his timing papers. It will have all the information you need as well as part numbers to buy.
The original style catalytic convertor (the can) was so restrictive that it added 1 full atm of backpressure (16psi).
Just fixing the exhaust is worth ~74 HP.
Check earlier in this thread. (post #43)
Someone dyno'd & documented this before and after.
The exhaust was so bad on these smog era engines it may as well have been a cork!
And you really must do that anyway, or any other change will be nullified also.
So you may as well do that first. Then the $7 distributor tune.
I did both on my 75.
It ran like a different engine.
And never touched the intake or valve covers.
But please read this entire thread.
Several engine builders weighed in here, with their own personal experiences, and they almost all agree.
There is like 100 years of combined experience in this thread!
Last edited by leigh1322; Apr 27, 2026 at 09:02 PM.
The original style catalytic convertor (the can) was so restrictive that it added 1 full atm of backpressure (16psi).
Just fixing the exhaust is worth ~74 HP.
Check earlier in this thread. (post #43)
Someone dyno'd & documented this before and after.
The exhaust was so bad on these smog era engines it may as well have been a cork!
And you really must do that anyway, or any other change will be nullified also.
So you may as well do that first. Then the $7 distributor tune.
I did both on my 75.
It ran like a different engine.
And never touched the intake or valve covers.
But please read this entire thread.
Several engine builders weighed in here, with their own personal experiences, and they almost all agree.
There is like 100 years of combined experience in this thread!
hello,
so, what is the way to un-smog-ify the exhaust, I don't have the money to just get a new exhaust and still need to register the vette as a classic. wont respond for like,
1 1/2-2 hrs, sorry. thanks for all the help
Last edited by Eli hayes; Apr 28, 2026 at 09:18 AM.
Look under and see if you have one of these: This is the original GM style "pancake" catalytic converter from the mid '70s.
It is (was) full of pellets and horribly restrictive.
Replacing just this one part with either a bypass pipe or a newer honeycomb convertor (much less restrictive) will get rid of the 16psi restriction and add 50+ HP.
That converter may as well have been a cork, it was so bad.
The dyno test in the above post also added better flowing mufflers, and that would add say 20HP more.
Magnaflow mufflers are highly used and liked by forum members.
Most replacement converters today will have this shape. They are the much better flowing honeycomb style.
If you already have one of these replacements, there is very little HP to be gained by replacing it, even with a straight pipe bypass.
Now whether you need a cat at all, on a 45 yr old classic car, totally depends on your local state rules for classic cars.
A by-pass pipe would be much less expensive, if allowed. Any muffler shop could do that.
Last edited by leigh1322; Apr 28, 2026 at 12:37 PM.
A muffler shop will never build or install a catalytic converter bypass pipe. Violation of federal law. It's just not worth it to them. "Test Pipes" are all gone.
Look under and see if you have one of these: This is the original GM style "pancake" catalytic converter from the mid '70s.
It is (was) full of pellets and horribly restrictive.
Replacing just this one part with either a bypass pipe or a newer honeycomb convertor (much less restrictive) will get rid of the 16psi restriction and add 50+ HP.
That converter may as well have been a cork, it was so bad.
The dyno test in the above post also added better flowing mufflers, and that would add say 20HP more.
Magnaflow mufflers are highly used and liked by forum members.
Most replacement converters today will have this shape. They are the much better flowing honeycomb style.
If you already have one of these replacements, there is very little HP to be gained by replacing it, even with a straight pipe bypass.
Now whether you need a cat at all, on a 45 yr old classic car, totally depends on your local state rules for classic cars.
A by-pass pipe would be much less expensive, if allowed. Any muffler shop could do that.
Originally Posted by stingr69
A muffler shop will never build or install a catalytic converter bypass pipe. Violation of federal law. It's just not worth it to them. "Test Pipes" are all gone.
I plan on having my uncle help me work on this with me, I was wondering if there are specific models that are bedder than others and for the muffler ppl say that a "pack glass" is way mor high pitched but how high pitched, also it looks like the only point on the exhaust that is 2.5in is after the exhaust manifold for not even 10in. is there a good way to also remove the (verry little) rust from the exhaust without possibly causing ma bigger problem. And is there something i should fix or check while I'm under there,
sorry about being slow and un-Knowledgeable because I'm 15 and drowning in schoolwork.
