modified clone questions

Bartlome27

New member
Building a modified clone. Need advise on what to run. must look stock, must run on gas. Its a 1/8 mile dirt track. most of everyone is turning 6400 rpm.
what would you recommend?
Cam?
Big valve head or small valve?
Springs?
rod?
Head cc? Ported?
max stroke crank i'm sure.
flat top piston.
Let me know what you think
 

alvin l nunley

Premium User
Compression is the Holy Grail. Of course the higher you go, the higher the octane gas you’ll need. There’s a limit.
Of course that’s just one thing, along with all the other things you mentioned.
Comments, compliments, criticisms and questions always welcome.
 

Wes Snow

New member
A good intake port will trump compression every time. A 9 to 1 motor with a great intake port will out power a high compression motor with a stock intake port easily. A great flowing head is the number one most important thing for making great power. Getting a great flowing head is also very hard. Getting compression? Not so much! I spend allot of time welding and redesigning ports. The results are always worth it.
 

Rickwilson6

New member
I agre with wes a good flowing head is the key isky has a pretty good deal on a ported head and cam package but be sure to use a billet rod
 

Devil-D-Dawg

New member
Its a good thing until it isn't! Just keep the flatside down and make more friends than enemie's - thats fundamental! Make sure you razor sharp intake pattern and leave the exhaust just the way nature intended it. Also make sure you really know your profiles - otherwise you will have to adjust either the timing or the carb constantly. Know your ratios - dont leave it to a book - know it!
 
You will want to turn more then that most motors or stockers are in the 6000 to 6500 range
Modified or sa will go a lot more.cam and carb have to match along with header.
big valve is not always better lots of things to think about.just tossing stuff out here.

cam in the 308 range with 26 springs and 1:3 rockers
or any thing over 330 lift stay 1:1 with rockers with 32-37 single springs
if you go dual you will need to cut the spring pockets.
head if a 14cc head mill .040 off don't deck the block but un shroud the valves
if a 22 head mill off .080 and still don't deck the block and on both port them well
then play with rod and piston and head gaskets.
now if you go big valve use the 14cc head don't mill it and it will come in around 16cc !
if big valve in a 22 head then mill it .080 again nothing set in stone just giving some thought
on what can be done.as for header go .0930 or bigger and for carb go with the dover
sa carb.other thoughts may vary on all this good luck.
 

Yakattack

Member
Now can you explain how a great flowing intake is the secret?
I can make a intake flow great, but if the exhaust port is not upto the task it is a dog, and vise versa.

For my builds I like the exhaust slightly out flowing the intake at various lifts. Not a lot but a little bit.

I did a head that I made the intake flow great, and left the exhaust alone, on the dyno not too bad, the track horrible...

When I made the exhaust flow the same if not a bit better, than intake, it did not have max flow or dyno numbers...but track time picked up quite a bit...
 

Wes Snow

New member
Kart 43 is correct. This helps with reversion. On my modified animal I run 1.35 intake and 1.05 exhaust. Do we make allot of hp. Yes! I try to get 80 percent of the intake flow for my exhaust. This is a common formula. You will see a little hp loss with exhaust flow loss. Remember you are pushing air out. Part of the hp loss is the residual drag of just pushing the air out. Just like you get drag from an intake port not filling the cylinder. However if you don't fill the cylinder you suffer a large power loss.
 

alvin l nunley

Premium User
I agree...Basic physics of thermo flow from a high pressure to a low pressure. Ya get an extra acceleration "Tug"...
Can you tell us about this “tug” or give us a reference to study?
Comments, compliments, criticisms and questions always welcome.
 

Kart43

Member
Al I don't know about the tug, but the step into the header prevents a back flow of exhaust. Gasses flow back toward the cylinder are problematic at low speed or long duration cams. Don't be scared of a large step either, within reason of course. Pipes should be built larger than the port and do not open the port up to the pipe size, make certain the gasket is not smaller than the port opening. It can also show improvement when the pipe is close to flush at the floor and the majority of the step is at the top. D shaped exhaust ports out flow most of course the D shape has to be laying on it's back.
 

alvin l nunley

Premium User
Al I don't know about the tug, but the step into the header prevents a back flow of exhaust.
I’m sure you are aware of the reason for the venture in the carb, but if you're not, it’s to create a low pressure area in the air flow. When moving air, flowing thru a carb, encounters the venture, the air speed increases and the pressure drops. Not my theory, you can read about in any text book on the subject.
Well, if there is a step in the flow of the exhaust flow, like the header pipe being bigger than the exhaust port, the exact opposite happens, as the air flow exits the exhaust port, and expands into the bigger diameter pipe, the air flow slows down and the pressure increases. Again, you can read about this in any text book.
The increase in pressure is going to be an impediment to the flow of the exhaust gases. I think during the overlap of the valves this is going to have its biggest effect.



Comments, compliments, criticisms and questions always welcome.
 
I believe the problem from a header being to small is at the flange. If the header is smaller, there will be a "wall" for the exhaust to hit. Now, if it was tapered down smaller (smoothly), it wouldn't be the same problem. Years ago, we took an engine (SBC) out of a Nova (ran great, turn 7000rpm, and just really ran good), installed it in a Vega with manifolds. The heads were ported to a header gasket (real close), well the manifolds were smaller..it wouldn't run very well at all, and wouldn't turn over 6500 at WOT.. We removed the manifolds and opened up the flange...it would turn 7000 again. That's the only thing we did..
 

Kart43

Member
Al I think you will find that because of the high pressure that the exhaust gasses have and they are traveling at a high velocity, when they reach the enlarged section both velocity and pressure will decrease, this assists in scavenging and after the initial flow is dissipating the resulting reversed pressure wave will be impeded by the step.

On the intake side we only need the venturi to cause the carburetor to function and assist in atomizing the fuel. The intentional venturi has been all but been eliminated with efficient fuel injection systems.
 

astrowrench

Member
^X2 on KART 43's- Answer; Some people may find reading expansion chamber design and tuning; informational It gives a lot of insight into exhaust flow characteristics you just have to "read" between the lines
 
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