Why do larger cube motors seem to tame big cams? - Page 2 - LS1TECH - Camaro and Firebird Forum Discussion

Notices
Generation III Internal Engine 1997-2006 LS1 | LS6
Sponsored by:
Sponsored by:

Why do larger cube motors seem to tame big cams?

Old 01-09-2019, 10:25 PM
  #21  
TECH Apprentice
iTrader: (2)
 
Ls7colorado's Avatar
 
Join Date: Jul 2005
Location: Jackson, TN
Posts: 374
Default

Originally Posted by 99 Black Bird T/A View Post
Why do larger cube motors seem to tame big cams?


You want to fill both buckets with water with your garden hose. The garden hose turned on flowing water is like the intake valve turned on by cam duration flowing the air and gas mixture. The time the water hose is turned on is basically like cam duration turning "on" the intake valve.

Filling the one gallon bucket takes a duration of 60 seconds of time with the garden hose "on" .

​​​​​Filling the two gallon bucket takes a duration of 120 seconds of time with the garden hose "on" .

The bigger bucket can absorb more garden hose "on" (up to 120 seconds) duration than the smaller bucket can (filled after only 60 seconds
exactly,
So what I’m saying is, what if you had a bigger hose to fill up the larger bucket that would allow the small and large bucket bucket to be filled in 60 seconds and then you changed the duration on each hose the same? Both would have the same percentage of increase in flow with the same increase in duration




Last edited by Ls7colorado; 01-10-2019 at 06:55 AM.
Ls7colorado is offline  
Old 01-10-2019, 12:01 AM
  #22  
Launching!
 
stockA4's Avatar
 
Join Date: May 2016
Location: St. Louis
Posts: 254
Default

I think some of that old David Vizard stuff is still worth a noobs reading. I think people discount his stuff now as it seems on the surface that he was caught up with cam lsa, when really I think he was ultimately showing that simply the amount of overlap used for a given displacement and valve diameter (not the actual displacement or valve size but the ratio of cubic inches per cylinder to inches of intake valve) for a given cam duration had the greatest effect on vacuum, power, and idle quality. We forget sometimes that the greatest restriction to airflow inside the running engine is the actual valves themselves until they are all the way opened.

Basically, the less cylinder volume per inch of valve you have the more sensitive the engine is to the overlap period and so less overlap is needed to get the job done.

For example, a 4.8 with 1.89" valves only has to feed 19.4ci per inch of valve, a 346 ls1 with 2" valves would be 21.6ci per inch of valve, and a 0.040 over 6.0 372 with some stock 2" valves on top would be 23.25ci per inch of valve.
This is why the big port, big Valve ls3 stuff works so well with more conservative intake durations.


​​​​​​

stockA4 is offline  
Old 01-10-2019, 12:11 AM
  #23  
TECH Addict
iTrader: (2)
 
Join Date: Apr 2006
Location: Pittsburgh
Posts: 2,072
Default

So theoretically, a small valve with higher lift would be be better than a larger valve with lower lift?
wannafbody is offline  
Old 01-10-2019, 12:37 AM
  #24  
TECH Apprentice
 
NSFW's Avatar
 
Join Date: Jan 2018
Posts: 344
Default

It's a little weird how many of these answers are just restating the premise of the question.
NSFW is online now  
Old 01-10-2019, 12:40 AM
  #25  
Launching!
 
stockA4's Avatar
 
Join Date: May 2016
Location: St. Louis
Posts: 254
Default

Originally Posted by wannafbody View Post
So theoretically, a small valve with higher lift would be be better than a larger valve with lower lift?
In a smaller engine yes, too big of a valve could hurt performance but most 2 valve v8's are under valved already. I think the smaller valve would need more lift and duration (time) to move the same volume of air. The larger valve is going to get the charge moving faster so long as the demand (read displacement) is great enough and the valve events compliment the rest of the combo.
stockA4 is offline  
Old 01-10-2019, 12:46 AM
  #26  
Launching!
 
stockA4's Avatar
 
Join Date: May 2016
Location: St. Louis
Posts: 254
Default

Originally Posted by NSFW View Post
It's a little weird how many of these answers are just restating the premise of the question.
A dead horse is an easy target pal, and you are in the land of unpaid experts
stockA4 is offline  
Old 01-10-2019, 06:21 AM
  #27  
10 Second Club
iTrader: (4)
 
Darth_V8r's Avatar
 
Join Date: Jul 2014
Location: My own internal universe
Posts: 6,536
Default

Originally Posted by 99 Black Bird T/A
Why do larger cube motors seem to tame big cams?

I think of it like this, it's sort of a hokey analogy. It's like filling two different sized buckets up with water.

Pretend we have the following...
One gallon (like a gallon of milk) is 4 quarts which is 231 cubic inches. Which is basically the size of a big V6 engine That is about a 3.8 liter V6.

You have two buckets to fill up, a one gallon or 231 cubic inches and a two gallon bucket or 462 cubic inches ( 454 V8 punched .030 over is ~460 cubes.)

You want to fill both buckets with water with your garden hose. The garden hose turned on flowing water is like the intake valve turned on by cam duration flowing the air and gas mixture. The time the water hose is turned on is basically like cam duration turning "on" the intake valve.

Filling the one gallon bucket takes a duration of 60 seconds of time with the garden hose "on" .

​​​​​Filling the two gallon bucket takes a duration of 120 seconds of time with the garden hose "on" .

The bigger bucket can absorb more garden hose "on" (up to 120 seconds) duration than the smaller bucket can (filled after only 60 seconds.)

Analogy wise - Cam duration in big and small engines works in a similar way to filling up the buckets.

BTW some of it is Perception. What's viewed as a "big cam" often comes from being more familiar with the smaller / medium cams typically used on smaller cube LS motors to achieve the desired result but not being as familiar with the cam size needed in a big motor to get the desired results.
Originally Posted by Ls7colorado
but wouldn’t the smaller engine also benefit from the same duration? Think of the smaller engine as having smaller runners, valves, and exhaust to match the VE of the larger engine.
Each engine takes 720 degrees to make a compression stroke.
Now change the assumption slightly ... fill the 2 gallon bucket from a hose that is 30% larger -- this is more like the engine scenario. And fill them in the exact same amount of time.

Water must flow faster -- higher port velocity -- to accomplish this.

At idle, which is what we are talking about, air moves slowly. During overlap, intake opens while piston is at or slightly before tdc, exhaust is still open, but is no longer really flowing any exhaust. Again, this is IDLE. small engine large cam, there is time for air to reverse direction and go back up the intake port -- hence the term reversion -- until the piston moves down, creating a depression, and causing air to turn around and move the right direction again.

Larger engine. Piston moves down, but creates a greater depression, causing air to move in the right direction sooner and faster. Creates increased manifold vacuum, which is a RESULT of less reversion, not a cause of less reversion.

Voila, tamer idle.

Stroke does it by increasing piston speed. Bore does it by increasing area. Either way (or both) the net effect is the same.
Darth_V8r is offline  
Old 01-10-2019, 07:04 AM
  #28  
TECH Apprentice
iTrader: (2)
 
Ls7colorado's Avatar
 
Join Date: Jul 2005
Location: Jackson, TN
Posts: 374
Default

Originally Posted by Darth_V8r View Post
Now change the assumption slightly ... fill the 2 gallon bucket from a hose that is 30% larger -- this is more like the engine scenario. And fill them in the exact same amount of time.

Water must flow faster -- higher port velocity -- to accomplish this.
.
I do understand how it would play out in a real engine with the same heads and intake,

BUT back to the bucket example if both got filled at the same time by increasing the diameter of the hose (not by speeding the flow up) then the duration will have the same PERCENTAGE of increase

Ls7colorado is offline  
Old 01-10-2019, 08:20 AM
  #29  
10 Second Club
iTrader: (4)
 
Darth_V8r's Avatar
 
Join Date: Jul 2014
Location: My own internal universe
Posts: 6,536
Default

Originally Posted by Ls7colorado
I do understand how it would play out in a real engine with the same heads and intake,

BUT back to the bucket example if both got filled at the same time by increasing the diameter of the hose (not by speeding the flow up) then the duration will have the same PERCENTAGE of increase
But that generally will not happen in a 2V engine. Port sizes, manifold size, valve size, shrouding not withstanding when you go to larger valves, it never keeps up.

About the only way you would get a larger engine to get same VE on the same cam as a smaller engine is if the displacement itself is the limiting factor. That means an intake that outflows the engine, ports that outflow the engine, exhaust the outflows the engine, etc.

It is desireable to gain air velocity in the ports and to be able to throttle the engine back. As long as that is the case, the hose will never quite keep up with the bucket. By design

If you want to see a bigger engine completely unrestricted, rotate the intake valve to dead center, canted to zero degrees, run four tiny exhaust valves, 350cc ports, ITB with really large throttles, on the order of 80mm.

In fact, ITB ALSO tames a larger cam in a smaller engine, so maybe that sort of makes the point...?
Darth_V8r is offline  
Old 01-10-2019, 09:13 AM
  #30  
TECH Senior Member
iTrader: (2)
 
99 Black Bird T/A's Avatar
 
Join Date: Nov 2001
Posts: 5,893
Default

Darth,

Playing with percent increase here's my example scaling about 20% and keeping the same cam. I really don't think any of this stuff scales in a linear fashion. Valve size, cross section area in the intake port etc, valve shroud in, etc.

What do you think of this "thought experiment" ? I think both set ups would suck but for different reasons

I think the airflow numbers are realistic based testing GMPP LS6 heads, various manifolds and what Land Speed Cylinder Head shared that the GMPP's can be optimized to flow 340+ cfm.

With cathedral LS6 heads we can sort of do a rough 20% linear increase by doing a comparison between cammed LS6 and 416 LS3 stroker with ported LS6 heads using the same cam. The cube inch increase, intake runner increase, and airflow increase are all fairly close to 20 percent.

The cam for both engines Hawk's Motorsports Super Sinister 231/236 .640"/615" It can be seen as big cam to turn a small intake port in some applications and see as a small to medium cam to turn a bigger intake port in other applications.

LS6 - 346 cid
WARR 92 TB, ported Fast92 intake
stock LS6 heads (210 cc intake runner, stock valves (peak flow 265 cfm, with intake in place 250 cfm )

In theory we have airflow to support ~5,900 rpm


416 LS3 stroker
Holley 105 TB, ported MSD intake
GMPP ported LS6 heads (250 cc intake runner, 2.08 valves, peak flow 312 cfm, flow with intake in place 300 cfm)
Hawk's Motorsports Sinister 231/236 cam

Again in theory we have airflow to support 5,900 rpm
99 Black Bird T/A is online now  
Old 01-10-2019, 10:04 AM
  #31  
KCS
Moderator
iTrader: (19)
 
KCS's Avatar
 
Join Date: Mar 2005
Location: Conroe, TX
Posts: 8,193
Default

So if you fill that 1 gallon bucket with the 30% larger hose, would you then decrease duration?
KCS is offline  
Old 01-10-2019, 10:21 AM
  #32  
Launching!
 
stockA4's Avatar
 
Join Date: May 2016
Location: St. Louis
Posts: 254
Default

Originally Posted by KCS View Post
So if you fill that 1 gallon bucket with the 30% larger hose, would you then decrease duration?
I read that its -2 degrees for every gallon your engine displaces. OP, if you are still reading go ahead and put your garden hose in the airbox and fire it up, just make sure you have your gallon buckets under the tail pipes first
stockA4 is offline  
Old 01-10-2019, 10:38 AM
  #33  
KCS
Moderator
iTrader: (19)
 
KCS's Avatar
 
Join Date: Mar 2005
Location: Conroe, TX
Posts: 8,193
Default

Originally Posted by stockA4 View Post
I read that its -2 degrees for every gallon your engine displaces.
I think I read that too, December edition of Home and Garden right?
KCS is offline  
Old 01-10-2019, 10:43 AM
  #34  
Launching!
 
stockA4's Avatar
 
Join Date: May 2016
Location: St. Louis
Posts: 254
Default

Originally Posted by KCS View Post
I think I read that too, December edition of Home and Garden right?
Thats right!
stockA4 is offline  
Old 01-10-2019, 10:55 AM
  #35  
Restricted User
 
Join Date: Mar 2014
Location: Ohio
Posts: 6,546
Default

Unless the plenum size is proportionate to the difference in displacement, the head flow won't matter.
You will still be trying to pull more cubic inches of air from the same available space. The larger engine will pull more idle vacuum.
In order for everything to be equal from an idle/cruise stand-point, every single piece of induction hardware will have to be proportionally larger. Idle speeds will have to be identical. Timing, compression, etc.
Even then, there are some more complicated factors that will help the larger engine run better.
JoeNova is online now  
Old 01-10-2019, 11:35 AM
  #36  
Launching!
 
stockA4's Avatar
 
Join Date: May 2016
Location: St. Louis
Posts: 254
Default

I'm in St Louis and the DA is -2300ft at 10am this morning, would that help?
stockA4 is offline  
Old 01-10-2019, 11:54 AM
  #37  
10 Second Club
iTrader: (4)
 
Darth_V8r's Avatar
 
Join Date: Jul 2014
Location: My own internal universe
Posts: 6,536
Default

Originally Posted by 99 Black Bird T/A
Darth,

Playing with percent increase here's my example scaling about 20% and keeping the same cam. I really don't think any of this stuff scales in a linear fashion. Valve size, cross section area in the intake port etc, valve shroud in, etc.

What do you think of this "thought experiment" ? I think both set ups would suck but for different reasons

I think the airflow numbers are realistic based testing GMPP LS6 heads, various manifolds and what Land Speed Cylinder Head shared that the GMPP's can be optimized to flow 340+ cfm.

With cathedral LS6 heads we can sort of do a rough 20% linear increase by doing a comparison between cammed LS6 and 416 LS3 stroker with ported LS6 heads using the same cam. The cube inch increase, intake runner increase, and airflow increase are all fairly close to 20 percent.

The cam for both engines Hawk's Motorsports Super Sinister 231/236 .640"/615" It can be seen as big cam to turn a small intake port in some applications and see as a small to medium cam to turn a bigger intake port in other applications.

LS6 - 346 cid
WARR 92 TB, ported Fast92 intake
stock LS6 heads (210 cc intake runner, stock valves (peak flow 265 cfm, with intake in place 250 cfm )

In theory we have airflow to support ~5,900 rpm


416 LS3 stroker
Holley 105 TB, ported MSD intake
GMPP ported LS6 heads (250 cc intake runner, 2.08 valves, peak flow 312 cfm, flow with intake in place 300 cfm)
Hawk's Motorsports Sinister 231/236 cam

Again in theory we have airflow to support 5,900 rpm
I think no matter what you do the bigger engine is going to idle better than the smaller engine with the same cam.

I do not know how to explain it other than swept volume keeps the air moving in the right direction.

No matter how big the plenum or anything else you throw at it, it is going to reach equilibrium, and the larger engine will eventually suck the plenum down to lower pressure and smooth it out.
Darth_V8r is offline  
Old 01-10-2019, 12:48 PM
  #38  
TECH Apprentice
iTrader: (2)
 
Ls7colorado's Avatar
 
Join Date: Jul 2005
Location: Jackson, TN
Posts: 374
Default

Originally Posted by JoeNova View Post
.
In order for everything to be equal from an idle/cruise stand-point, every single piece of induction hardware will have to be proportionally larger. Idle speeds will have to be identical. Timing, compression, etc
Exactly
Ls7colorado is offline  


Thread Tools
Search this Thread

About Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

© 2018 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.
 
  • Ask a Question
    Get answers from community experts
Question Title:
Description:
Your question will be posted in: