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Discussion Starter #1
I have a few questions that I am sure are stupid (not the first time and wont be the last time either;)) but I have been thinking about them for a while so here goes.

1. Piston skirt to bore clearance is that built into the piston or does the bore have to be adjusted to arrive at the correct clearance. What I mean is...if you have a set of pistons that are supposedly .030" oversize and require .003" in skirt clearance...is the bore actually .033 oversize or is the bore actually .030" oversize but the pistons themselves are only .027" oversize???? (Is that clear as mud?) :confused:

2. The old typical BIG cam problem is running a cam with too much duration and not enough compression to make the cam work correctly...what you normally end up with is a car that is sluggish at the lower RPM's but then "comes alive", so to speak, when the RPMs climb above a certain level...ie 3500rpm etc...anyway...is this still an issue if you actually run the required compression for the cam?

Thanks in advance:)
 

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Most rebuilders pistons are built with the clearance in the pistons. This meaning that if you buy a set of .030 pistons, they will usually be .028 in size or whatever the manufacure recommends for clearance. Not with standing whether you buy a set of cheap or expensive pistons, always measure the piston and make the piston to wall clearance to the manufactures recommended specs. I have seen pistons in the same set be a half of thousandth difference in sizing. As far as camshafts go, one of the biggest reason for sluggish performance is lack of convertor stall and gear ratio. A cam has to be matched to the engine around it. Putting in a large camshaft with stock commpression can be fine if you have a good induction system and good flowing cylinder heads and a high stall convertor with a low gear ratio. Bigger cams can hurt performance if you don't have everything to go with it.
 

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Pistons:
Piston to bore clearance is in the piston however, pistons are not as precise as you might assume. They vary so it's mandatory to have the pistons before you hone the block.
In the factory they hone the blocks to a nominal size and then select fit pistons to fit the actual bore size. There's more on the subject in Kev's budget 350 thread.

Cams:
In a normally aspirated 4 stroke engine, filling the cylinders with as much air as possible is critical to making power. If you watch the weather channel or paid attention in Physics class, Air only moves when there is a difference in pressure.

The camshaft timing can help create the conditions where the air pressure in the cylinder is less than the atmospheric air pressure. The very first 4 stroke engines opened the Intake valve at TDC and closed it a BDC. This seemed logical but they made very little power for their displacement. Later on it was discovered that changing the timing of when the intake valve opens relative to the piston, influences both manifold vacuum and cylinder pressure and performance improved. This is where the river gets wide, so to speak. Cam timing is often about compromises.

Racers found long ago that engines can overfill the cylinders beyond 100% at high rpm (inertial supercharging) with the right cam timing, however it wasn't without consequences.

Maximizing high rpm power often meant sacrificing low end torque. This was acceptable for a race car that spent most of it's time at high rpm anyway, but would be a problem for say a tow truck.

Fuel economy, emissions, driveability all are influence by cam timing and there isn't one perfect cam that get's it all right.

Honda's VTEC variable valve timing solves part of the delimma by varying cam timing to what's appropriate for the rpm. It's the closest thing to having your cake and eating it too.

On a small block Chevy you don't have that luxury so you have to select a cam that's appropriate for the useage and combination of parts. The biggest mistake people make is over camming. Basically comes from number envy and misjudging what they truly need. Too many people think that the bigger the cam you buy, the more power you'll make but in reality the wrong cam choice can make an engine a real dog.

One important cam choosing parameter is engine compression ratio. It is important to note that this not a number. It's the ratio of theoretical swept volume to fixed volume. Installing a cam shaft with later intake closing causes the effective swept volume to be less. This is often called the dynamic compression ratio or DCR and is all the buzz on the internet. By increasing the static compression ratio the effective compression is restored. Performance is increased with compression increases but octane must be increased as well.
This is by no means all there is to it. For the novice I recommend recording cranking compression before and after any cam swap. You don't want the pressure to get too low or too high. Remember while there are many, many wrong answers, there isn't one right answer to performance gains.

There's more on SCR, DCR and compression in Best of Tech.
 

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Discussion Starter #4
thanks guys for the response......anyway lets try it this way.

Lets say we have a car with a 355. Its running 9:1 CR, highrise duel plane intake, 750 holley, 3500 rpm stall, 4.10 gearing. The cam has the following specs lift .525/.549 with duration @ .050 241/249. The car seemingly works great above 3000rpm but below that driving around town its sluggish. BTW the cam card the calls for 10.5:1 + compression and the stall/gearing etc listed above is also called for on the cam card...


Ok now we have the same car except the compression is 11:1 with everything else the same.....would the car still be as sluggish in the lower rpms as it was when the compression was only 9:1???:confused:

Also lets assume that this is a measured CR not one just based on assumptions
 

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Probably cheaper to change the cam than it is to change the pistons.
 

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I don't think you can completely solve cam driveability problems just with a compression ratio increase. It's not that simple. To move the car smartly off idle requires sufficient torque. How much depends on the car weight and gearing and friction. Torque production is relative to volumetric effieciency (VE). This is a fancy term that is the measure of cylinder filling by percent. A cylinder that has 100% of it's capacity by weight has 100% VE. Maximum torque occurs at or near peak VE. A typical engine isn't 100% efficient even at peak torque.
Race engine can exceed 100% VE but sacrifice low RPM VE.
There is less air in the cylinder to compress in a race engine at low rpm than a similarly sized production engine. Depending on the severity of the cam difference, even with a compression increase they probably won't ever drive like a production car. Displacement and CR both improve low speed torque losses by adding more air and improving combustion efficiency.
There's also danger with bumping CR to compensate for poor lower rpm VE. Since SCR doesn't change, at peak VE the pressure may be too high and detonation can occur.
When the retrictor engines were allowed tons of compression the driver might have to be told to soft pedal the throttle until it got past peak torque. You don't want to have to remember to do that.


Performance can be had without sacrificing driveability or using crazy cams. Take a look at the new Z06. An amazing 505 SAE certified HP (530 by old standards) and you can drive it around town all day long. This is the future of hotrodding, but people want to be nostalgic about the sixties and still demand cams that actually have poor low rpm performance.

I think people spend too much time looking for magic cams when they should be looking at cylinder heads. Performance is all about getting air in. Heads are the restriction. Get good heads and you don't need cams with excessive timing.
 

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Discussion Starter #7
gstocker said:
Probably cheaper to change the cam than it is to change the pistons.

the above example isnt based on anything I actually have...its just me asking questions...since the motor isnt built I was just tossing around possiblities...:)
 

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Discussion Starter #8
Paul Wright said:
I don't think you can solve cam driveability problems just with a compression ratio increase.
thank you there is the answer I was looking for:)
 

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Actually I didn't answer your particular question example. I just rambled on.

Predicting how a cam will function in particular car really requires the dyno graph, vehicle weight and gearing. Real life experience with various tried and true cam/engine combinations really helps too.
In any event in this case I think the driveability would certainly be improved going from 9:1 to 11:1. Adding cubic inches helps too. If the CR stayed at 9:1 but the 355 was now a 383 then the driveablity would be better also. I'm just guessing that that particular cam would be dramatically better in an 11:1 383 over a 9:1 350.
 

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Discussion Starter #10
Paul Wright said:
Actually I didn't answer your particular question example. I just rambled on.

Predicting how a cam will function in particular car really requires the dyno graph, vehicle weight and gearing. Real life experience with various tried and true cam/engine combinations really helps too.
In any event in this case I think the driveability would certainly be improved going from 9:1 to 11:1. Adding cubic inches helps too. If the CR stayed at 9:1 but the 355 was now a 383 then the driveablity would be better also.
cubes seems to solve a lot of issues.:) ..not that I am thinking of doing this, this is just me thinking out loud, but if you build a 383 out of a 350 using 350 slugs and 400 rods and crank is there a down side to using the shorter rods???:)
 

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69NovaSS said:
cubes seems to solve a lot of issues.:) ..not that I am thinking of doing this, this is just me thinking out loud, but if you build a 383 out of a 350 using 350 slugs and 400 rods and crank is there a down side to using the shorter rods???:)
If you were using this 383 to compete with John Force you probably wouldn't want to use the stock 400 rods. If you were building this 383 to power a street car they will work just fine.
 

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Discussion Starter #12
Mike Goble said:
If you were using this 383 to compete with John Force you probably wouldn't want to use the stock 400 rods. If you were building this 383 to power a street car they will work just fine.

well shoot there goes my dream of running John Force down in the quarter with my Nova....heck I thought if Jungle Jim can do it why cant I...:rolleyes: ..lol..:D ..thanks for your reply...:)
 

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Rod length is certainly controversial. Of all the 383's I've seen, and IMO, the shorter rod versions don't perform as well as the longer 5.7 rod versions.
I've seen all the online mathematics saying the long rods don't reduce side thrust but I've also noticed more side thrust scoring and wear on short rod 400's than 350's. I've never seen an apple to apples comparison to prove my observations so it's just an anecdotal opinion.
I've learned to choose my battles so I won't argue the point without data to back me up.

I might also note that 5.0L Fords even shorter rods and they make decent power for their displacement. My Acura/Honda road race engine has short rods and revs to 9,200.

If I was given a challenge to make more power on a budget with either a long rod 350 or a short rod 400 I would take the 400 hands down. 50 cubic inches is much more desireable for power than a 135 thou of rod length.


It's certainly cheaper to go the short rod. For a quick and easy cube increase it's the least expensive way to go especially if you aren't buying new pistons.
In my view if you have to over bore and buy pistons, why bother with the short rods. There are plenty of 5.7 rod 383 pistons and good 5.7 rods readily available. Good short rods are not.

I was thinking about this discussion at lunch time. Even production cars have driveability problems. My SHO Taurus automatic has a 3.2L engine while the manual trans version is 3.0L.
The extra displacement was improve the driveablity when coupled to the automatic. The engine already has high compression and changing cam timing for low end torque would hurt high end power. They elected to do a time honored hot rodding trick. More stroke on the crank for more cubes. If the Yamaha engine had variable cam timing it would be awesome.

One other thing I forgot to mention was air velocity and driveability. Cam timing can reduce low speed flow velocity which impairs the vacuum "signal" to the carb. Lumpy cams often cause problems until the rpm and therefore air velocity get up to speed.
You'll get much better off idle performance and throttle feel with a higher vacuum camshaft. You can often get the same if not better high rpm performance with really good heads and a smaller cam. Rather than crappy heads and a big cam.
This is why improving air flow without hurting velocity is preferable to helping air flow at the expense of velocity. Translation: max flow numbers aren't everything.
 

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Paul Wright said:
One other thing I forgot to mention was air velocity and driveability. Cam timing can reduce low speed flow velocity which impairs the vacuum "signal" to the carb. Lumpy cams often cause problems until the rpm and therefore air velocity get up to speed.
This is why improving air flow without hurting velocity is preferable to helping air flow at the expense of velocity. Translation: max flow numbers aren't everything.
Paul, can this issue of reduced air volocity be somewhat compensated for by staying with the smaller 1.94"/1.5" valve size? I have always heard they help maintain the intake volocity speed when compaired to the larger 2.02"/1.6" valve size.:)

Maybe but that's a loaded question. A 220 cc head with 1.94 valves won't be a good combination.

Generally peak flow velocity is determined in part by the minimum cross section area of the intake path (from carb to chamber). A 2.02 valve has 3.2 square inches of area. A 1.94" valve has 2.954 sq ". Subtract the valve stem and you still have more area than the pushrod pinch area in the port. This is about 1.75 sq " or less on a stock head.
As soon as I can post pictures in Kev's Budget 350 monster thread, I'll be showing and telling more about that subject. Stay tuned.
 

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Paul Wright said:
Pistons:
Piston to bore clearance is in the piston however, pistons are not as precise as you might assume. They vary so it's mandatory to have the pistons before you hone the block.
In the factory they hone the blocks to a nominal size and then select fit pistons to fit the actual bore size. There's more on the subject in Kev's budget 350 thread.

Cams:
In a normally aspirated 4 stroke engine, filling the cylinders with as much air as possible is critical to making power. If you watch the weather channel or paid attention in Physics class, Air only moves when there is a difference in pressure.

The camshaft timing can help create the conditions where the air pressure in the cylinder is less than the atmospheric air pressure. The very first 4 stroke engines opened the Intake valve at TDC and closed it a BDC. This seemed logical but they made very little power for their displacement. Later on it was discovered that changing the timing of when the intake valve opens relative to the piston, influences both manifold vacuum and cylinder pressure and performance improved. This is where the river gets wide, so to speak. Cam timing is often about compromises.

Racers found long ago that engines can overfill the cylinders beyond 100% at high rpm (inertial supercharging) with the right cam timing, however it wasn't without consequences.

Maximizing high rpm power often meant sacrificing low end torque. This was acceptable for a race car that spent most of it's time at high rpm anyway, but would be a problem for say a tow truck.

Fuel economy, emissions, driveability all are influence by cam timing and there isn't one perfect cam that get's it all right.

Honda's VTEC variable valve timing solves part of the delimma by varying cam timing to what's appropriate for the rpm. It's the closest thing to having your cake and eating it too.

On a small block Chevy you don't have that luxury so you have to select a cam that's appropriate for the useage and combination of parts. The biggest mistake people make is over camming. Basically comes from number envy and misjudging what they truly need. Too many people think that the bigger the cam you buy, the more power you'll make but in reality the wrong cam choice can make an engine a real dog.

One important cam choosing parameter is engine compression ratio. It is important to note that this not a number. It's the ratio of theoretical swept volume to fixed volume. Installing a cam shaft with later intake closing causes the effective swept volume to be less. This is often called the dynamic compression ratio or DCR and is all the buzz on the internet. By increasing the static compression ratio the effective compression is restored. Performance is increased with compression increases but octane must be increased as well.
This is by no means all there is to it. For the novice I recommend recording cranking compression before and after any cam swap. You don't want the pressure to get too low or too high. Remember while there are many, many wrong answers, there isn't one right answer to performance gains.

There's more on SCR, DCR and compression in Best of Tech.
You are my sensei....I am your pupil!

I am a block of clay...mold me to your likeness!

Ha ha ha!
I think I could seriously sit in on Dr. Paul Wright PHD's school of engine building classes!!!

Very eloquent and makes me want to become an engine builder/engineer.
 

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Thanks for the endorsement and vote of confidence, however, in my old age I think I know less than than I thought I knew as a kid. Most of my apparent "wisdom" originates from making many, many mistakes. You live and learn. People that don't know should listen carefully to people that DO know.
If you tell a kid not to touch the stove he may still do it. He'll certainly know not to afterwards. It would be nice if everything the human race learns gets preloaded in the next generation, but we all start out as blank slates.

Not all advice from trusted sources is valid.
My dad once advised me that 8 tracks would outlast cassettes so I bought the 8 track. Turned out cassettes lasted all the way until the CD. He wasn't decieving me, he just believed it would be true.
Likewise, the internet is full of experts and opinions. The real trick is sorting them out. I've always felt if someone can't explain it then they don't really know it.

My Dad also gave me some good advice. Figure out what you want to do and then persue it. When I was still in High School he helped get me an after school and weekend job at an engineering company drilling precision holes. I learned about tolerances. measuring tools and many, many other things. Besides teaching me the essentials, my boss gave me some more good advice. Find a mentor, the best in the field, and learn from him, even if it means working for less money. However, I was young and that sounded dull and unrewarding. I went down several different paths in the pursuit of money but I wasn't happy. I eventually decided to take that good advice and have never regretted it since.

You have to like what you do and not do it for money. One question you should ask about your job is if they didn't pay you would you still show up for work?
If you say "yes", then you truly love what you do.
 

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Discussion Starter #17 (Edited)
Paul Wright said:
Maybe but that's a loaded question. A 220 cc head with 1.94 valves won't be a good combination..
Well I wont be running out looking for 220 cc heads for my motor anyday soon unless of course I added 60+ cubes to the motor or I decided to build an all out high RPM drag motor..neither will be happening anyday soon.....but I get your point...i was really refering to something more along the line of the vortec or etec kinda head.......looking forward to reading "The Rest of the Story"....when you get a chance to update Kev's thread....:)
 

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Here's the Reader Digest version on valve sizes and flow:


2.02" is an odd size when you think about it. If you look at a production GM head from the 60's a 1.94 head has about a nickles thickness between the intake and the exhaust. A 2.02" has about a dime thickness. I think they just put the biggest valves that would fit because that was the performance philosophy at the time.

What's been learned since then is at some point valve shrouding hurts flow more than valve size increases help. The Fast Burn head has 2.00" valves and the Vortec flows great with only 1.94" valves. Both flow more air than the old style 2.02" heads.

If you unshroud the chamber around the valve you gain volume which causes you to lose some compression.
Bottom line is use the minimum size valve that still yields best flow.
 

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i know this is off topic, but i would like to comment

Find a mentor, the best in the field, and learn from him, even if it means working for less money.
i have always strived for this point, i went to college for electronics engineering, got my degree but couldn't see myself doing that for life...so a year out of college i went to wyotech because i always loved cars, now lets make a living...i said to myself one day in class if i won a billion dollars, i would still do this...thats how i knew. my first day out of wyotech i went looking for jobs, i somehow walked into the shop where i'm at, and at that time he needed help....didn't offer much but working in an independent shop doing a solid mixture of custom/performance/and general repair with two guys twice my age and have been in the field longer than i have been on this planet, where could i learn more. I have become their "student" and i have now been there for just about a year, working in the best shop around, with the most respected fabricator and street rod builder in our area, i still make crap money....but i love it and would come into work tomorrow if i won a million today....
 
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