Reversion
Thread Starter
TECH Veteran
iTrader: (21)
Joined: Apr 2003
Posts: 4,049
Likes: 3
From: Shreveport, LA
Reversion I'm understanding reversion as exaust gasses that were not evacuated from the combustion chamber in the amount of time that the exaust valve was open. Thus leaving a contaminated charge in the combustion chamber allowing it to contaminte the next incoming batch of fresh air. My question is, is this only bad with intakes that conect all the cylinders together, or would a individual TB intake help keep reversion under control? I'm trying to pick a rather large hydraulic roller for my 454 build, but from what I've gathered, going with the large duration (250+) should be coupled with a wider LSA to keep reversion amounts under control. This is the exact oppisite from the way I'm wanting to go. I'm wanting big duration,tight LSA, and 4 degree's of advance. Basically trying to keep the IVC early in order to keep DCR up in the 8.3-8.5 range, but I don't want to be going in the wrong direction. I will be using the Harrop intake with 8 ID TB's, so driveability isn't my main concern. I'm really wanting to see what I can get away with by going to the individual intake set-up over a solid manifold. 
My understanding is that reversion is reverse flow into the chamber or into the intake runner/plenum, and slightly different from simple contamination.
The IR intakes are supposed to significantly reduce the time available for reversion to happen since the pressure pulses are timed, but if the timing is wrong, the results could be worse.
IMO the biggest factor in reversion is matching the exhaust pulses with the cam timing. I think you just have to be flexible with the exhaust you run until you find something that works correctly.
There are formulas and all to help you set up pipe lengths, but there's too many variables involved for you to get it right using an equation.
I think it's one of those things that make good combinations appear as mysterious/lucky as they are scientific.
The IR intakes are supposed to significantly reduce the time available for reversion to happen since the pressure pulses are timed, but if the timing is wrong, the results could be worse.
IMO the biggest factor in reversion is matching the exhaust pulses with the cam timing. I think you just have to be flexible with the exhaust you run until you find something that works correctly.
There are formulas and all to help you set up pipe lengths, but there's too many variables involved for you to get it right using an equation.
I think it's one of those things that make good combinations appear as mysterious/lucky as they are scientific.
TECH Regular
Joined: Oct 2005
Posts: 430
Likes: 0
Originally Posted by Beast96Z
I'm understanding reversion as exaust gasses that were not evacuated from the combustion chamber in the amount of time that the exaust valve was open. Thus leaving a contaminated charge in the combustion chamber allowing it to contaminte the next incoming batch of fresh air. My question is, is this only bad with intakes that conect all the cylinders together, or would a individual TB intake help keep reversion under control? I'm trying to pick a rather large hydraulic roller for my 454 build, but from what I've gathered, going with the large duration (250+) should be coupled with a wider LSA to keep reversion amounts under control. This is the exact oppisite from the way I'm wanting to go. I'm wanting big duration,tight LSA, and 4 degree's of advance. Basically trying to keep the IVC early in order to keep DCR up in the 8.3-8.5 range, but I don't want to be going in the wrong direction. I will be using the Harrop intake with 8 ID TB's, so driveability isn't my main concern. I'm really wanting to see what I can get away with by going to the individual intake set-up over a solid manifold.
The first item to explore is: Why use 'big duration, tight LSA and 4 degrees advance'? Taking this approach to maintaining DCR means the intake valve opens probably 15 degrees too soon and the exhaust closes about the same too late for best power and you lose 30 or more HP vs. a correctly speced cam. It would give the engine a wicked rough idle (which is a plus for many) but an IR intake will tame 90% of that because the exhaust reversion won't make it past the almost closed throttle plates to contaminate an adjacent cylinder the way it would with a plenum.
The single most important valve event is the intake closing. This dictates the basic RPM range at which the engine will try to run its best. The other 3 events together have about the same overall importance. The reason LCAs need to get wider with long duration is that otherwise you get too much overlap to be useful at any RPM.
Narrow LCAs on long duration cams are mostly a method used by the cam companies to "save us from ourselves" when we pick a cam with too much duration for the application. As poorly as it runs at low/mid RPM, such a cam is not as bad for driveability and useable power as one that closes the intake too late for anything that won't be revving over 7,000 RPM.
Thread Starter
TECH Veteran
iTrader: (21)
Joined: Apr 2003
Posts: 4,049
Likes: 3
From: Shreveport, LA
So herin lies my question. I'm using the big duration to help promote cylinder filling with the larger cubes. The down side to this is that I get the late IVC, thus causing a major reduction in DCR and also causing my peak rpm to rise. My thinking behind the tight LSA and advance, is that I can use the big duration for cylinder filling, but still have the early IVC and a RPM band that peaks before 7k. The downside to this is obviouslly the large amount of overlap. I'm personally thouroughly confused as to which way I should be going with a cam for this application. I can't decide on which factors I can do less with like DCR,overlap, or duration. I was also unshure as to wether I should use the "driveability" advantage of the intake as a reason to use the tight LSA. Just one of those,"If I can get away with it, why not" type things. I'd really be interested in hearing what you guys would consider an appropriate size range that I should be looking in. Thanks
Thread Starter
TECH Veteran
iTrader: (21)
Joined: Apr 2003
Posts: 4,049
Likes: 3
From: Shreveport, LA
Originally Posted by ArcticZ28
Just wondering why something like a 244/250 on a 112 +4 (just making this up) wouldn't work for you.
Trending Topics
I dont think that that theory is all that true. I run with this sizing in mind.
stock cubes, 346 ranges, cams in the 220-228's seem to be the big cams.
400-408 cubes I find run great with 230-238' ranges.
420+ I find would not hurt going 240ish, but it still becomes big.
There is nothing wrong with a mild camshaft in a big cube motor. If you want RPM range, plan accordingly to your IVC, and make sure the runners on the cylinder head are big enough. ITs the cylinder head that willl cause you to fall on your face first. Especially with the cubes you have. Camshaft just dictates open and close. The flow happens from the heads. You can make great power with smaller cams as you can with bigger cams.
Rick
stock cubes, 346 ranges, cams in the 220-228's seem to be the big cams.
400-408 cubes I find run great with 230-238' ranges.
420+ I find would not hurt going 240ish, but it still becomes big.
There is nothing wrong with a mild camshaft in a big cube motor. If you want RPM range, plan accordingly to your IVC, and make sure the runners on the cylinder head are big enough. ITs the cylinder head that willl cause you to fall on your face first. Especially with the cubes you have. Camshaft just dictates open and close. The flow happens from the heads. You can make great power with smaller cams as you can with bigger cams.
Rick
LS1 Tech Stories
The Best V8 Stories One Small Block at Time
6 Common C5 Corvette Failures and What's Involved In Repairing Them
Pouria Savadkouei
Retro Modern Bandit Pontiac Trans AM Comes With Burt Reynolds' Autograph
Verdad Gallardo
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Pouria Savadkouei
Top 10 Most Powerful Chevy Trucks Ever Made!
Hennessey's New Supercharged Silverado ZR2 Has 700 HP
Verdad Gallardo
Coachbuilt N2A Anteros Is an LS2-Powered C6 Corvette In Italian Clothes
Verdad Gallardo
Awesome K5 Blazer Restomod Comes With C7 Corvette Power
Verdad Gallardo
10 Camaros You Should Never Buy
10 LS Engine Myths That Refuse to Die
Verdad Gallardo
TECH Resident
Joined: Dec 2004
Posts: 845
Likes: 0
From: K-W, Ontario
I wouldn't be too concerned about 'eliminating reversion' when selecting the
camshaft.
Select the camshaft for the range of RPM you want the engine to be most
efficient.
Reversion is inevitable and will occur above and below the tuned RPM and select
harmonics of the tuned RPM. The pressure changes within the intake port,
cylinder and exhaust port will be fighting against reflected pulses and opposing
pulse energy to keep the flow from intake > chamber > exhaust > atmosphere.
IR setups and exhaust runners can be designed to help direct more charge
into the cylinder, but the setup is only resonant at one frequency. At some
point there will be a pulse heading for the cylinder and that nasty intake valve
is going to shut and reflect it back up the runner.
camshaft.
Select the camshaft for the range of RPM you want the engine to be most
efficient.
Reversion is inevitable and will occur above and below the tuned RPM and select
harmonics of the tuned RPM. The pressure changes within the intake port,
cylinder and exhaust port will be fighting against reflected pulses and opposing
pulse energy to keep the flow from intake > chamber > exhaust > atmosphere.
IR setups and exhaust runners can be designed to help direct more charge
into the cylinder, but the setup is only resonant at one frequency. At some
point there will be a pulse heading for the cylinder and that nasty intake valve
is going to shut and reflect it back up the runner.
TECH Regular
Joined: Oct 2005
Posts: 430
Likes: 0
By far the best way to "promote cylinder filling with the larger cubes" is via big valve/port good flowing heads and a ton of intake lift. Opening the intake valve so early that even at 6,000 RPM the cylinder pressure is higher than the intake port's isn't going to 'fill' anything but the port. (with exhaust gas)
I just finished a Dynomation computer engine simulation series on a 396 LT1 with TFS heads, 1-3/4" headers and a Lunati 242/252 110 LCA, 0.560"/0.576" @ 1.6:1. The baseline results (480 CHP @ 5,800) were within 10 HP of what we dynoe'd with the old heads, and the flat power curve was very similar, so I have full confidence that the sim is right on for this engine. The simulation says the new heads (with some hand porting and better headers) are good for 554 HP @ 6,500.
BUT, even at that RPM and with 265 CFM @ 0.600" vs. the former 230, reducing the intake duration 8 degrees, bumping the lift to 0.640", together with increasing the exhaust by 6 and increasing the LCA to 112 (This is an open exhaust race engine, it would likely work better at 114 with mufflers) is good for another 22 HP!
I just finished a Dynomation computer engine simulation series on a 396 LT1 with TFS heads, 1-3/4" headers and a Lunati 242/252 110 LCA, 0.560"/0.576" @ 1.6:1. The baseline results (480 CHP @ 5,800) were within 10 HP of what we dynoe'd with the old heads, and the flat power curve was very similar, so I have full confidence that the sim is right on for this engine. The simulation says the new heads (with some hand porting and better headers) are good for 554 HP @ 6,500.
BUT, even at that RPM and with 265 CFM @ 0.600" vs. the former 230, reducing the intake duration 8 degrees, bumping the lift to 0.640", together with increasing the exhaust by 6 and increasing the LCA to 112 (This is an open exhaust race engine, it would likely work better at 114 with mufflers) is good for another 22 HP!
TECH Regular
Joined: Oct 2005
Posts: 430
Likes: 0
I'd say you couldn't go far wrong with about a 235-240 @ 0.050" intake duration with all the lift you can handle (preferably well over 0.600"), 250 degrees or so exhaust (depending on how free flowing a system you have) with not much more that 0.600" and an LCA of 112 to 114, the latter perhaps fiddled a bit in conjunction with cam advance to keep DCR in the ballpark.
Maybe not as glamorous as some grinds, but unless you want to trade off power for an intimidating idle, I think it's about the best you'll get without playing cut 'n try with a dozen or more candidates.
Maybe not as glamorous as some grinds, but unless you want to trade off power for an intimidating idle, I think it's about the best you'll get without playing cut 'n try with a dozen or more candidates.
Thread Starter
TECH Veteran
iTrader: (21)
Joined: Apr 2003
Posts: 4,049
Likes: 3
From: Shreveport, LA
Wow, 10-15 degree's of split? That's almost unheard of in LS-1 grinds. Also, when going with big lift on one side and smaller on the other, usaully indicates a change of lobe profile. In some cases, the 250* lobe could be the same, if not smaller at some lift points than the intake lobe. I'm not worried at all about the idle. I just want the most effective power I can get. The intake itself will tame down alot of the idle problems we have in these cars anyway. It's just so confusing seeing cars with stock motors running excellent with big 240+ cams. You just instantlly think that a larger motor would have to be that much larger, but I guess head/intake efficency plays a major role in the whole scheme.
TECH Regular
Joined: Oct 2005
Posts: 430
Likes: 0
"Wow, 10-15 degree's of split? That's almost unheard of in LS-1 grinds"
Not when you see the new LS7 HOT profiles!: https://ls1tech.com/forums/generation-iv-internal-engine/431521-anyone-seen-new-gm-hotcam-s-ls7.html
You can always use a lower rocker ratio on the exhaust if necessary.
Keep in mind that much of the time what seems like a really good combination because of the power it makes is actually still not reaching its potential. A 364 c.i. race small block (with spec carb, compression ratio and mufflers) I'm working on was predicted to make 660 FHP @ 7,600 RPM by the Dynomation computer simulation program, using the owner's last year's exhaust, new 2nd hand SB 2.2 heads and the cam suggested by the engine shop. The owner was thrilled, because the 2005 engine only made 598 on the dyno. With the computer-optimized cam (300/310, 112 LCA) and exhaust, the prediction is over 750 HP!
When I simulated an all-out race 427 LS engine with the Harrop intake, 14.0:1 ETP C5R heads and 2.0 to 2 -1/4" stepped headers with Burns merge/megaphone collectors, the solid roller cam Dynomation 'wanted' was advertised duration of 306/312 (~ 274/280 @ 0.050") with a 114 LCA installed straight up. I used a conservative 0.700" intake lift and a 'best power' 0.630" exhaust lift and it predicted well over 850 FHP @ 7,300 RPM.
Notice that despite the 63 c.i. difference in displacement, the two engines 'want' about the same duration. (Obviously my suggestion for your engine was a little more conservative, being a muffled street engine.)
Trivia fact: The original '67-'69 302 c.i. Z-28 'Duntov' cam made 30 HP more with open headers then the milder (to handle the optional automatic) cam used from '70 on, but the milder one made 15 HP more with the stock exhaust.
Not when you see the new LS7 HOT profiles!: https://ls1tech.com/forums/generation-iv-internal-engine/431521-anyone-seen-new-gm-hotcam-s-ls7.html
You can always use a lower rocker ratio on the exhaust if necessary.
Keep in mind that much of the time what seems like a really good combination because of the power it makes is actually still not reaching its potential. A 364 c.i. race small block (with spec carb, compression ratio and mufflers) I'm working on was predicted to make 660 FHP @ 7,600 RPM by the Dynomation computer simulation program, using the owner's last year's exhaust, new 2nd hand SB 2.2 heads and the cam suggested by the engine shop. The owner was thrilled, because the 2005 engine only made 598 on the dyno. With the computer-optimized cam (300/310, 112 LCA) and exhaust, the prediction is over 750 HP!
When I simulated an all-out race 427 LS engine with the Harrop intake, 14.0:1 ETP C5R heads and 2.0 to 2 -1/4" stepped headers with Burns merge/megaphone collectors, the solid roller cam Dynomation 'wanted' was advertised duration of 306/312 (~ 274/280 @ 0.050") with a 114 LCA installed straight up. I used a conservative 0.700" intake lift and a 'best power' 0.630" exhaust lift and it predicted well over 850 FHP @ 7,300 RPM.
Notice that despite the 63 c.i. difference in displacement, the two engines 'want' about the same duration. (Obviously my suggestion for your engine was a little more conservative, being a muffled street engine.)
Trivia fact: The original '67-'69 302 c.i. Z-28 'Duntov' cam made 30 HP more with open headers then the milder (to handle the optional automatic) cam used from '70 on, but the milder one made 15 HP more with the stock exhaust.
Thread Starter
TECH Veteran
iTrader: (21)
Joined: Apr 2003
Posts: 4,049
Likes: 3
From: Shreveport, LA
Very interesting. I wish I new what Dynomation said for mine, but unless you own a shop or spec alot of stuff, the price is hardly justifyable. Maybe I'll look into a LSK lobe for the intake so I can get the big lift, but use a XE-R lobe on the exaust for a milder lift??
Thread Starter
TECH Veteran
iTrader: (21)
Joined: Apr 2003
Posts: 4,049
Likes: 3
From: Shreveport, LA
Only problem is that the XE-R's stop at 248*. I wouldn't be able to get 10 or more degree's of split.This is why I wished all cam vendors listed there lobes like comp does.
If I'm correct, I believe we can use some BBC lobes and some Ford lobes in the LS-1. If this is the case, how important is it that I get the .200" numbers split apart? If I use the LSK intake, no other lobes are even close to being similar to them. Therefore, the even bigger exaust lobe has close to the same .200". Then, if I get the .200" up to 10 degrees apart, I have a 20* split at .006", and.050". Here's one grind I was looking at heavily:
Intake: 2131--.006=293 ---.050=243----.200=168---lift=.653"
Exaust: 3661--.006=307 ---.050=254----.200=169---lift=.595"
It has a nice split, but is only 1* apart at .200".
If I'm correct, I believe we can use some BBC lobes and some Ford lobes in the LS-1. If this is the case, how important is it that I get the .200" numbers split apart? If I use the LSK intake, no other lobes are even close to being similar to them. Therefore, the even bigger exaust lobe has close to the same .200". Then, if I get the .200" up to 10 degrees apart, I have a 20* split at .006", and.050". Here's one grind I was looking at heavily:
Intake: 2131--.006=293 ---.050=243----.200=168---lift=.653"
Exaust: 3661--.006=307 ---.050=254----.200=169---lift=.595"
It has a nice split, but is only 1* apart at .200".
Last edited by Beast96Z; Feb 3, 2006 at 11:04 PM.
TECH Regular
Joined: Oct 2005
Posts: 430
Likes: 0
It does get a little tricky, comparing durations and split at different lifts between lobe families, but I feel they relate to flow, reversion and triggering intake and exhaust pressure wave action, which are basically low-lift activities and so the 0.050" lifts, or better yet, the 'advertised' or seat to seat duration, is what counts. More duration at 0.200" means more flow, but doesn't directly affect the natural RPM range of the cam.
Thread Starter
TECH Veteran
iTrader: (21)
Joined: Apr 2003
Posts: 4,049
Likes: 3
From: Shreveport, LA
OK, so even a car like mine, that will have virtually minimal exaust, will still benifit from the long duration split? My exaust will consist of 1 7/8 to 2" headers, 3 1/2" collectors into 3 1/2" pipe w/X, into 4" Borla or Magnaflow mufflers, then dumped at the axle. It should scavenge quite well with the stepped headers and the X. I just want to make sure I don't hang the exaust out there to bad. I would look into the diffrent ratio rockers, but the Jesels for these heads don't come in any other sizes. BTW, I just noticed in another thread that one of the cam grinders spec's a 256* hydraulic intake lobe for a 408. He had the high intake lift, low exaust lift thing going, but are cams like this really making the most power? Seems like he would get into the problems that I was looking at with my initial cam choice at the begining of this thread???
TECH Resident
Joined: Oct 2005
Posts: 787
Likes: 0
[QUOTE=MadBillTrivia fact: The original '67-'69 302 c.i. Z-28 'Duntov' cam made 30 HP more with open headers then the milder (to handle the optional automatic) cam used from '70 on, but the milder one made 15 HP more with the stock exhaust.[/QUOTE]
Huh? The 302 was never available with an automatic and only had 1 cam from the factory, and 2 "off-road" offerings and that wasn't the "Duntov" cam. Maybe you're confusing the 302 Z-28 cam with the 350 LT-1 cam.
302 Z-28 254 / 254 .485 .485 114 lobe sep
350 LT-1 242 / 254 .435 .455 116 lobe sep
Huh? The 302 was never available with an automatic and only had 1 cam from the factory, and 2 "off-road" offerings and that wasn't the "Duntov" cam. Maybe you're confusing the 302 Z-28 cam with the 350 LT-1 cam.
302 Z-28 254 / 254 .485 .485 114 lobe sep
350 LT-1 242 / 254 .435 .455 116 lobe sep