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Old 01-18-2009, 05:47 PM
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Brain Storming!! ? about valve sizes

Hey,I have been doing alot of reading,(books & Hotrodders). I have
looked at a Summit crate motor, the 350 with 290hp @ 5100,and
326ft# of torque @ 3750. Let's say this is very close to born-stock.
I would assume it has 1.94/1.50 valves...Would anyone say that this
is close to what was put in most half ton trucks? I called and got a
price on a long block from AutoZone.. Their best long block only had
260 HP........ ?? If I built a 350,bored .030 over, Put a better cam, say
a comp cam 256, 447/454 lift, Put Dart 165 runners and changed the
valve size from 1.94/1.5 to 2.02/1.60 with a 9.0 compression this is
what I get........ On 1.94/1.5 valves-318hp @ 4500,& 405ft# @ 3500,,
With 2.02/1.60 valves I get 337.1 hp @5000 & 411 ft# @3500.....
???,Why woud I be better off with small valves 1.94/1.5 when no matter
how you look at it I have a hell of a lot more power then stock form??
Most of the books and hotrodders say for a daily driver I'm better off
with small valves, Why is that?? Everything I read says that larger valves
only helps on higher RPM's (over 5000) All this under 5000, except for
the Summit crate motor!!! Can anyone gave me a better understanding
of this?????? THANK's
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Old 01-18-2009, 06:12 PM
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What are you trying to accomplish ?
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Old 01-18-2009, 06:22 PM
How fast is fast enough?
 
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smaller valves usually have slightly better fuel economy and low end power. You may make more over 5000 RPM, but how often does your car go over 5000 RPM?
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Old 01-18-2009, 06:58 PM
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Read this, cogitate on what they did versus your daily driver needs, then report back with what you think about what these guys did.

http://www.chevyhiperformance.com/te...320/index.html

http://www.chevyhiperformance.com/te...326/index.html

http://www.chevyhiperformance.com/te...359/index.html

http://www.chevyhiperformance.com/te...364/index.html

http://www.chevyhiperformance.com/te...370/index.html

http://www.chevyhiperformance.com/te...438/index.html

http://www.chevyhiperformance.com/te...461/index.html

http://www.chevyhiperformance.com/te...467/index.html


Bogie
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Old 01-20-2009, 12:33 AM
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An internal combustion engine is a pump. It draws fuel as fluid in through creating a vacuum, the fuel is drawn in the air stream, suspended and atomized and enters the combustion chamber. Depending on stroke and bore size, the intake stroke draws air & fuel in through a intake runner, port and through a valve opening controlled by a valve that opens and closes controlled by the the camshaft lift, (how far the valve is opened) and duration (how long the valve is held open). Now think of the port and valve like a soda straw. It is easy to suck soda through a regular straw, but what happens if you enlarge the size of the straw in diameter with the same vacuum signal, It becomes very hard to draw the fluid due to the increased volume. It slows down causing the liquid to creep on the side walls of the hose rather to remain suspended in the air flow. Although this is a very basic simple explanation, you can see how this is related to and its importance to match valve size to engine size. Camshaft design controls valve lift and duration to specific degrees of crankshaft rotation. This is where bigger valve size is not necessarily better. In most cases, bigger valves also mean more valve weight, needing stronger or bigger valve spring tension rates. Now imagine a small high revving engine like a motorcycle that has multi valve combustion chamber. Why multi valve? Small engines make power by increased RPM, using two small intake valves instead of one large valve means increased port volume while retaining good vacuum signal, (two small ports retain volume speed while actually having more flow volume), the valves are smaller, lighter with increased seating surface for heat dissipating. The valves are lighter, have same or more flow characteristics at lower lift and duration levels. Smaller and lighter valves allow lighter spring tension rates, decreased spring tension and valve weight means higher RPM reliability with less radical camshaft profiles. Although this is a simple explanation and actual engine design is obviously more involved, I hope this gives insight to bigger is not always better.

Last edited by Studebaker; 01-20-2009 at 02:18 PM.
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Old 01-20-2009, 03:54 PM
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Brain storming!,valves sizes

OK, I read all of Oldbogie's and Studebaker's reply and I apologize for
this on going issue.. I had finish buiding my 350 and I have been disappointed
with it. I really feel that I should have gotten alot more power. Athough I
injoyed building it. I am going to take it apart again and do it again. This
is what I have; 350 bored .030 over, KB193 12cc dish pistons, Dart s/s
165 int.runners 1.94/1.5 valves ( I found out that valves & springs were
stock). A Weiand Action Plus manifold good from ilde-6000, A Comp cam
EX256, 447/454 lift, Engine has a 8.5 compression., I changed the carb.
to a Holley 670.

I plan to change pistons to KB120 flattops which will raise compression
to 9.3 and rebuild heads with better spring and Manley Valves..
Can You'll help me pick a better Cam to go with these parts??, And
shoud I used straight stem valves or under-cut valves?? PS, I am going
to upgrade Stall to about a 2400 speed... Thank's for Everyones Reply!!
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Old 01-20-2009, 05:06 PM
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Quote:
Originally Posted by BourgD
OK, I read all of Oldbogie's and Studebaker's reply and I apologize for
this on going issue.. I had finish buiding my 350 and I have been disappointed
with it. I really feel that I should have gotten alot more power. Athough I
injoyed building it. I am going to take it apart again and do it again. This
is what I have; 350 bored .030 over, KB193 12cc dish pistons, Dart s/s
165 int.runners 1.94/1.5 valves ( I found out that valves & springs were
stock). A Weiand Action Plus manifold good from ilde-6000, A Comp cam
EX256, 447/454 lift, Engine has a 8.5 compression., I changed the carb.
to a Holley 670.

I plan to change pistons to KB120 flattops which will raise compression
to 9.3 and rebuild heads with better spring and Manley Valves..
Can You'll help me pick a better Cam to go with these parts??, And
shoud I used straight stem valves or under-cut valves?? PS, I am going
to upgrade Stall to about a 2400 speed... Thank's for Everyones Reply!!
This is a configuration that ought to dyno at the crank around 300 to 320 hp.

Obsevations:

- Dart makes a lot of heads, which ones are they?

- The 256 cam is a nice warm street stick, it shouldn't need anything but "stock" springs.

- 1.94/1.55 valves are stock 350 items and are appropiate to the cam and port volumes youre using.

- I think you compression is low by a ratio if these are current production Dart heads with fast burn chambers.

- Dart heads like most aftermarket heads really require some port cleaning. These guys expect you're going racing or at least in pursuit of more power and build the ports with that expectation. These heads really respond to pocket porting.

- The piston dish volume needs to be matched with the head volume to get the right compression ratio. Doing what it takes to get up to 9.2-9.5 if that's a KB120 with what your deck clearance and gasket thickness is, then they're the way to go.

- The Weiand Action Plus comes in a couple varieties, I think the 8120 (idle to 5500 RPM) is more appropriate for the cam in your engine.

- Bigger valves will further slow the low thru mid range mixture flow, this will drop off some power in this area and push the power peaks higher in the RPM range, while the intake you chose and the heads can support this, the cam can't get there. The net will be a softer responding engine with bigger valves.

- You have no need for a high stall converter with this cam. The 268 is just getting to that edge, your 256 cam is a couple/three grinds under that.

- Seeing you have an automatic, do you know if it's up to transferring the power? Either age or out of spec assembly leads to a situation where the automatic looses considerable power between the input and output shafts, so this needs to be checked out.

- There are many other technical issues that affect power especially when the engine is installed. There are also psychological ones i.e what your expectations are. A half ton truck weighs 3500 - 4500 pounds depending, that takes a pretty potent engine to stir the chassis up. Which is a thought that leads me to the condition of the chassis and suspension. All the power this side of the Space Shuttle is nearly useless if the chassis can't react it thru the tires into the ground. Flex in the frame and suspension components, dragging brakes, worn bushings (including those that attach the body to the frame, tired springs and shocks all take their toll on getting power to the ground.

- Exhaust, I don't think that's been discussed; this engine would be good with 1-5/8s long tube headers, into 3 inch collectors reduced to 2-1/2 inch head pipes which a crossover tube. Then to some low restriction mufflers that drop the outlet pipes to 2-1/4 inch to pick up some flow velocity as the gasses cool till you dump them.

You've got tons of tuning to do, power output is very sensitive to ignition timing and mixture ratios. Most distributors and carburetors don't come close to what's needed from the box, you gotta get in bed with these things for awhile till you figure out what the motor wants. I'm afraid that Desk Top Dyno doesn't solve these problems, it and other virtual power calculators, assume that all this tuning stuff is solved and the motor's world is a perfect place.

Anyway some food for though. But the motor you built, should be a nice, pretty warm street motor from the specs you sight. Not a screamer, but certainly no slouch either.

Bogie
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Old 01-21-2009, 10:51 AM
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Brain Stroming!! ? about valve sizes (Bogie)

Hey,Bogie, Thanks for all the information you gave me..I got the KB120's
in today and also I already got the comp-cam 1.254 valve springs last
week. ?, Do I need to rebalance the whole rotation assembly again or
can I just weigh the 193's on the rod's and remove the difference? The
KB120's are only 7 grams heavier than the 193's, I read somewhere that
there is a marginal amount of leeway that can be ok. Is that true? I have
the Weiand Action Plus 8024 which is a speader bore requiring a square
bore adapter ( Edelbrock 1" spacer) The Carb. is a square bore, Also the
engine is a 1993 which has the 4 middle intake bolts pointing straigt
down. Could this type of manifold with spacer be part of the problem??
If so should I go with another kind?
Thank's

I just went look at the manifold on Summit, It is a spreader bore flange,
but it says square-bore & spreader-bore. Does that mean I don't need
Edelbrock adapter on it? Could that be part of the problem?

Last edited by BourgD; 01-21-2009 at 12:30 PM. Reason: manifold
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Old 01-21-2009, 03:52 PM
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looks like you may have bit of a mismatch there, i don't think dart made any heads that had the later sbc intake bolt pattern. If your one inch spacer isn't of the 4 whole type and the adapter isn't either that may be leading to some of your weak lowend problems. Skip the whole square to spreadbore adapter and by either a square bore manifold or a spread&squarebore manifold this way no adapter is neccessary.

Oh yeah moving a truck isn't an easy task for any engine, but a low gearset should set you off right.
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Old 01-21-2009, 04:30 PM
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Brain Stroming,About valve sizes

Hey, Yes my Dart head are the late model bolt pattern, and I don't have
a one inch spacer.. The manifold is a weiand action plus for 87-95
model engines and it a spreader bore type flange which can be used
as a square bore or spreader bore. I didn't know that, so I put a Edelbrock
1" adapter ( spreader bore to square bore adapter) on it. Yes it has
the 4 holes..
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Old 01-22-2009, 01:52 PM
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Quote:
Originally Posted by BourgD
Hey,Bogie, Thanks for all the information you gave me..I got the KB120's
in today and also I already got the comp-cam 1.254 valve springs last
week. ?, Do I need to rebalance the whole rotation assembly again or
can I just weigh the 193's on the rod's and remove the difference? The
KB120's are only 7 grams heavier than the 193's, I read somewhere that
there is a marginal amount of leeway that can be ok. Is that true? I have
the Weiand Action Plus 8024 which is a speader bore requiring a square
bore adapter ( Edelbrock 1" spacer) The Carb. is a square bore, Also the
engine is a 1993 which has the 4 middle intake bolts pointing straigt
down. Could this type of manifold with spacer be part of the problem??
If so should I go with another kind?
Thank's

I just went look at the manifold on Summit, It is a spreader bore flange,
but it says square-bore & spreader-bore. Does that mean I don't need
Edelbrock adapter on it? Could that be part of the problem?

If you had the rotating assembly balanced with the original pistons then the the new pistons should be brought to the same weight as the originals if that's possible to do. Seven grams is a considerable difference, and is more than what one would consider "leeway" which is usually measured in fractions of a gram.

The 8024 intake is intended to provide peak flow at higher RPMS than the 8100 series. When configuring and engine, the camshaft is the principle characteristic for determining the engine's RPM range which in turn influences the selection of all other breathing components. These are then selected to support the cam's characteristics. If for example, you select a cam that peaks torque and horsepower at moderate RPMs, then you want heads, manifold, carb, and headers that emphasize the same characteristic. If you select components with too much flow capacity for the cam, the result will actually be a loss of power in the cam's best power range and most likely these components will not make up for those losses at higher RPMs because the cam can't support getting to those RPMs.

The things you learn about as time goes on like the spacer, are very important. Spacers are sold as a cure-all, when one is bolted on power "ipso-facto" goes up. Actually nothing is further from the truth, spacers are as good at taking power away as they are giving it, it just that you can't sell'em by advertising the negative potential, so it isn't mentioned. Spacers may or may not be effective, this is something you have to investigate on a case by case basis. Also spacer design plays into this between divided and open types. Divided passage spacers maintain the manifold's 180 degree function where open spacers allow the manifold to function like a 360 degree unit. The latter reduces the low and mid RPM response and will with a cam that's able to take advantage of it, enhance the upper end. But again the reference is to what the cam can do, configuring intake and exhaust parts around a top end response but using a mild cam leaves you with all the negative characteristics and none of the ability to take advantage of the positive ones.

Frankly, if I was building this engine around a carburetor, I would use either the Comp XE262 or 268 or the equivalent from another manufacturer. The 252 and 256 cams work well MAP sensed TBI and TPI, bringing up the power but keeping them SMOG test compliant. The 258 and 262 work well with MAF sensed TPI, keeping them emissions compliant without having to work hard at it. The 268 can come in on MAP or MAP TBI or TPI systems and be emissions compliant but you've got to work harder at the set up to where the average guy just bolting on aftermarket parts without any test or modification facility probably won't succeed at getting both the power and emissions compliance, unless blind luck just happens to dump him there.


Bogie
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Old 01-23-2009, 04:49 PM
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Brain Strom!! Valve Sizes

Hello Again, Bogie, I was able to remove the Edelbrock adapter spacer
and mount the carb. straight onto the weiand manifold and tested it
out. IT made a BIG difference!! I am thinking about the EX262 cam
and the compression alittle more. I might just shave the heads alittle
more to rise it some and put the new springs I got for it.

What is the ideal compression for the compcam EX262?
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Old 01-23-2009, 05:55 PM
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CR Vs Cam Chart>>http://www.chevytech.com/2c40o5.html
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