How do cylinder heads work?

[tscott9330]

Ok i get the basic concepts of the cylinder head, but what i am a little fuzzy on is how things like intake port and exhaust port sizes affect torque and horse power? There has to be a differnce in all these race heads i see for sale and the ones designed for street, what is it?

Could you put a set of race heads on a 383 stroker and expect i to drive around town or will it drive like crap?

Tom

 

[cntrhub]

I'll just give you a little basic physics about heads and why valves differ in size. Cold air travels slower than hot air. that's why you need a large intake valve. It takes more air to pack the volume inside the combustion chamber. Small exhaust valves need not be big to move hot air out the exhaust. That's why some racing intake valves are just a few mm's bigger than stock in size to suck in more cold air into the chamber. Now as far as port design, the hot curve is the "D" configuration. Do a little research in the D theory for more information. For the ports, the story goes that port polishing is trying to move more air with less friction. But you will sacrifice atomization if the port walls are too smooth. As for the exhaust ports, the air travels so fast (remember your heat/cold theory) it really isn't that important (polish wise) as much as the intake ports. A cam is what puts it all together. The more the valves stay open (overlap) to suck in more air, the bigger bang.
Are racing heads good for the street? The heads would work fine. You would have to play with the fuel mixture. It would be a little touchy when the seasons change though. It wouldn't take much to find the right jetting to make the heads streetable.
As you can see, this was just a very short and quick answer. Hope it helped a little.

 

[TomOB]

cntrhub , i think when you said "how long the valves stay open (overlap)" you meant duration.
I just read half of Lingenfelter's SBC book....to save tom the read, although you should read.....overlap is when the intake valve opens before the exhaust has closed. This itself i don't think is an issue, but what is an issue is when the pressure in the cylinder is still greater than that of the intake port, then you get the exhaust going through the intake, and a bad intake charge once it comes around again.
I blieve the point of the book is to decide what you want to use the engine for. If the car is just a street car with some oomph, look for torque down low and drivability. If its a race car where you'll spend alot of time in the upper ranges, the drivability isnt an issue. If you're doing 1/4 runs, lingenfelter put alot on the transmission, but then you want to really focus on the powerband you'll spend the most time in.
hope i helped rather than confused.....i'm looking for similar information....One path coudl be to determine the application and call up a main cam manufacturer and let them help you figure out what you should be using.
tom

 

[AKRAY4PLAY]

Tom,
generaly speaking, there are several things that stay true with head designs
: the bigger the intake port, the higher the operating rpm for the head. small port = low rpm and good torque, big port = high rpm and poor idle do to less signal to the carb.
: chamber design dictates compression ratio and burn pattern for maximizing the eff. of the motor.
: straight plug vs. angle plug, angle places the spark closer to the intake valve and improves the burn pattern and scavenging.
: inake and exhaust port design adjust flow chacteristics for different set-ups, ie. cam profiles.
: polished ports flow higher numbers. intakes can be mirror polished if you are running a very good carb, Barry Grant or other racing type carb, that has very good atomizing characteristics or EFI.

then you get into very technical stuff about making small amounts of power. most head manufacturers won't disclose more info than what is listed above. Lingenfelter's book and other engine building books are very good sources of information about heads. heads are the most important part of the engine for maqking power. to put it in perspective, my brother-in-law's nephew just graduated from a performance engine building school in Houston, Texas. the school puts engines together and the graduates are snapped up by NASCAR and USAC teams. the course was 18 months and ten months were spent on heads alone!!! this school built a street legal Camaro with operating AC, stereo and power windows, it runs the 1/4 in the tens with a small block!! Brian

 

[tscott9330]

What school was this? I am currently getting my engineering degree at the University of Central Florida. When I graduate I will be attending a High performance automotive tech. school. I was planning on going to UTI. Is UTI Where Your brother in laws nephew went?

Tom Scott

 

[Twin_Turbo]

Ok i get the basic concepts of the cylinder head, but what i am a little fuzzy on is how things like intake port and exhaust port sizes affect torque and horse power? There has to be a differnce in all these race heads i see for sale and the ones designed for street, what is it?

Could you put a set of race heads on a 383 stroker and expect i to drive around town or will it drive like crap?

Tom

It has to do with charge velocity. The higher speed the intake charge has, the more effective it can "cram" into the cylinder. The effetiveness of this spec determines max. torque and where it happens. Max torque is that rpm where the cylinder filling is at it's most effective, generating the most force from each stroke. Max HP is the rpm where the sum of powerstrokes is highest. This means that with higher rpm the power per stroke can decline but the Hp can increase solely because of the rpm factor in it.

As for port design, it all has to do with flow dynamics. The majority of the flow always occurs at the long side radius. It's because of intertia, when the flow has to take a turn the molecules are flung to the outside by centrifugal force. There are some things that help improve a port and the combination of those can make a port effective or not. Making things too large slows down the charge speed, too small and it becomes restrictive.
The selection of the cylinder head is VERY closely related to the cam selection. If you choose a large port head you will need a longer duration cam to take advantage of this, the head chamber should be relativly small to bump CR so you won't loose too much torque down low.

Selecting a short cam and a small port head with small chambers (something like an L98 head) can give very high DCR (cranking pressures), the L98 head is aroun 170cc pors with a good design and good swirl. The chamber is small at 58cc.

 

[AKRAY4PLAY]

Tom,
sorry for my tardy reply, but things have been a little crazy. the school was not UTI, it was Specialized Automotive Machine or something like that. i called him to get more info, but he is on vacation. hopefully he will answer my email soon and i will relay it to you. i think the teacher's name is Jed Messingale. sorry i can't be more helpfull right now, it was a couple years ago since i spoke with him about the name of the school and instructor. i should have done a better job of filing it away in my head. i'll keep you posted, Brian

 

[AKRAY4PLAY]

Tom,
i just heard back about the school in Houston. sounds like it has really gone down in the last six months. all the good instructors are gone and the job service / public assistence departments are sending unemployed gangbangers there to learn a trade. so your original choice of UTI may be your best choice. good luck finding a good school. if i hear anything positive about other schools i'll drop you a line, Brian

 

[AKRAY4PLAY]

Tom,
more bad news. i just got another reply from the brother in law's nephew about UTI. he said several guys in his class attended UTI just before getting to SAM (school of automotive machinists) and they all said it was a great place to go to learn how to turn brake rotors and change batteries. he included the link to SAM so you could give it a look see. sorry for getting things so confusing for you, just trying to help, Brian
http://www.samracing.com/
ps. good luck in the hunt for a school!

 

[tscott9330]

Hey I do appreciate all the help. You have given me alot to think about. If you here anything else just drop me a line here. Thanks again

Tom