jaybird81

Ok, I am really sorry of this is a dumb question (I'm sure for some of you it is). There are a lot of post here with late model C3's with the stock L48 or L82 or L81 engines and the owners wanting to make more horse power. It seems that with all the knowledgeable replies the majority of people say change the heads. I know these cars had low compression and maybe the heads were like 76 cc's or higher. Now when you change the heads you would go with bigger valves and smaller chamber volume size. I know decreasing the volume size would increase the compression ratio. So Is that where the power you are looking for is coming from the CR? I guess my question would be if you rebuilt a motor that had 76cc heads and 8.25 to 1 CR, left the heads but put in pistons that gave you 10.25 to 1 CR are the heads still holding you back. So can you get the same results, rather than leaving the pistons and changing the heads to a smaller cc, say 64, if the overall result is a high CR? Again, sorry if this is a dumb question, I am not sure if the over goal is to just increase the CR and the easiest way is swap the heads rather than take the motor apart to replace the pistons? I may just be missing the point.

Revi

Yes increasing compression will give you more HP, but that isn't the best answer. Realistically you need a combination of compression, head flow, and cam selection. It's a "matched" set of all three that will get you the best results.
The orginal stock intake manifold/heads just don't flow all that well. If you replace the heads/manifold, then a new cam is in order to compliment them. Just replacing any one of these items or installing "unmatched" parts cobbled together isn't the best way to go.

I remember reading an article years ago where they engine dynoed a 1970 LT-1 (64cc, 11:1) vs a 1971 LT-1 (76cc, 9:1). The 71 actually made more power do to the larger cc heads flowing better.

billla

Ultimately, the cylinder heads determine the power level the engine is capable of delivering. Intake flow, and the intake flow/exhaust flow ratio are the major determinants, but as with everything else the devil is in the details.

The right cam allows the heads to deliver what they're capable of, and the rest of the engine is built to support that power level. Mismatching components (wrong cam, no headers, wrong intake, etc.) all significantly impact the power level.

Chasing CR for HP is a fool's game to a certain extent, especially at the power levels most folks are looking for here. Anything less than about 9:1 or so isn't very efficient, and more is better at higher power levels, but anything over about 10.2 on pump gas starts getting a little wonky. Someone will come along and talk about DCR, encouraging very high CR and bleeding it off at low RPM with overlap...but that's not really a street approach IMHO.

Note that quench - the distance between the top of the piston and the head deck - is a critical measurement and is typically terrible on a stock engine. Usually the piston will be .020 or better down in the hole, and the gasket is .048 or thicker...an .068 quench vs. an ideal of around .040. A thinner gasket (FelPro 1094 is the most common) can increase CR AND improve quench - both good for power.

There's my $.02. There will be a whole bunch of people along shortly to tell you how wrong I am

langg

Not a dumb question at all. The main reason for replacing the heads is air (and fuel) flow capacity. The more air then engine flows, the more power it makes. That is why good heads coupled with the right cam are the best means to increase power output. That is not to say that compression ratio is not important but as you pointed out, that can be changed by changing pistons. The key to being happy with the results of your changes is to realize the engine is a system and that to perform best, the whole airflow piece of the system must be coordinated. Generally, a street engine can greatly benefit from upgrading the heads and cam without changing the stock pistons. However, a 9 -10ish compression ratio should limit which cam duration/timing is chosen. There are a number of very knowledgeable people on this forum who would be happy to help with specific questions.

Hope this helped

Langg

v2racing

It's the overall combination of parts matched to a specific outcome along with good tuning that makes a successful build. That goes for anything from a mild regular gas burning engine to an all out competition engine.

Billa, I have to call you wrong on one thing. It is the intake valve closing point that determines DCR, not overlap.

Peoples idea of what is a street engine can be vastly different. I like my Corvette to be just shy of a race build, but to me it is very streetable. I have a 64 SS with a mild 350 if I just want to cruise too. Some people driving 2000 HP twin turbocharged beasts would look at my 406 as a small low powered street engine.

I have dealt with customers for decades though and try to evaluate their needs and guide them towards a build that will make them happy. I would never push someone into an engine that they wouldn't know how or want to handle the special needs it takes to drive it on the street.

billla

My reference was to what's happening at low RPM with the cams recommended by DCR advocates...and the intake closing is of course just one aspect of overlap.

REELAV8R

Yes and no.
CR by itself will not produce a significant increase in power. However it may allow a cam with more lift and duration and later intake valve closing, which will change where the power happens. That can mean more power where you want it to acommodate your gearing and transmission as well as your application, ie street, strip, racing.
A different head has many more benefits other than just smaller chambers. modern heads have much better burn characteristics, better flow and more lift available at the valves, bigger valves, as well as better designed valve seat areas, bowls runners. better heat exchange or heat retention depending on the need, etc. The list can go on.
So it's more about the technology and advancements that have been made in head designs since the 70's vs the chamber size.
But CR is and important part of the total equation. It would serve little purpose to bolt on nice modern heads and have a CR of 7.5:1 unless your talking super or turbo charging.

dochorsepower

To echo REELAV8R, it's the combined package, and the knowledge of what will work best together. Choice D, all of the above. Port design, chamber design and burn characteristics, cam, compression ratio as well as intake and exhaust manifolding. It is all interdependent. That's why the top engine builders make big horsepower and big bucks.

v2racing

Exhaust contamination of the intake charge lowers detonation sensitivity, IE: EGR valves. So yes, overlap does help in that large amounts of it allow exhaust contamination of the intake charge therefor lowering detonation problems at low rpm with high compression. The lumpy sound heard from large cams is a result of this affect.

Intake closing point still dictates calculated DCR.

billla

Thanks - I understand how overlap works and the impact of it as well as how to calculate DCR. I'll stand by my previous statement - let's leave it there and not take this off into yet another off-topic C3 Tech pizzing match.

v2racing

Sounds good!