How much overlap is wanted with a blower vs turbo?
07-06-2009, 02:37 AM
#1
TECH Fanatic
Thread Starter
iTrader: (1)
Join Date: Nov 2007
Location: NJ
Posts: 1,193
Likes: 0
Received 0 Likes
on
0 Posts
How much overlap is wanted with a blower vs turbo? How does overlap affect the engine with a turbo vs centi supercharger vs maggie/twin screw
Alot of people are say the you need zero overlap looking for you thoughs ?
Im saying with a supercharger a little overlap is good helps push out any unburnt exhaust gases. Also holds the valves open longer let more air and fuel in for more power ideas?
Looking for any Dyno testing or tracking testing to help back up theory's
Alot of people are say the you need zero overlap looking for you thoughs ?
Im saying with a supercharger a little overlap is good helps push out any unburnt exhaust gases. Also holds the valves open longer let more air and fuel in for more power ideas?
Looking for any Dyno testing or tracking testing to help back up theory's
07-06-2009, 06:52 AM
#2
9 Second Club
Isnt everyone !!! lol
Search, its been asked thousands of times. And unless you do the back to back testing yourself, you'll never get a definitive answer.
And everyones combo is different anyway. But mild cams will make power with FI, that is one sure thing.
07-06-2009, 07:19 AM
#3
TECH Fanatic
Thread Starter
iTrader: (1)
Join Date: Nov 2007
Location: NJ
Posts: 1,193
Likes: 0
Received 0 Likes
on
0 Posts
I know ive being seaching for the last year but was hoping i missed a few things or a few new things would come up. There be alot of talk about turbo cam But not one about why a blower cam might be different/how vs a turbo
07-06-2009, 08:07 AM
#4
I would think you could get away with more overlap on a turbo setup than a blower, because of the lower pressure ratio between the exhaust:intake, compared to the much higher of ratio of a blower
maybe not, seems logical though
maybe not, seems logical though
07-06-2009, 08:35 AM
#5
From what I have seen and read, a good performing NA cam will work even better with a blower. Because there is no backpressure on the exhaust side of a blower car (assuming proper exhaust) overlap is far more acceptable and yields more gains than with a turbo setup.
07-06-2009, 12:07 PM
#6
9 Second Club
07-06-2009, 05:43 PM
#7
Blowers and turbo's have the exact opposite problem during overlap.
Blowers produce much higher intake pressure than exhaust. The problem is pushing air/fuel through the cylinder during overlap and losing it out the exhaust pipe. Overlap should be limited, but no one really knows where the cutoff should be. I'd venture to say that 5-10 deg of overlap should be the limit.
Turbo's produce much higher exhaust pressure than intake. During overlap, exhaust reverts into the intake port. The alleged problem is that exhaust dilutes the intake charge, but I could argue this either way. Testing has revealed that adding overlap to turbo cams increases power, but the testing stopped at about 5 deg. I've venture to say that up to 10 deg of overlap should help power, maybe much more.
My experience was that I switched from a "blower cam" (224/236-114) to a "turbo cam" (226/226-114) with my turbo and lost power. That's the last time I base a decision on internet advice from the masses without first analyzing it myself to see if it at least makes sense.
Mike
Blowers produce much higher intake pressure than exhaust. The problem is pushing air/fuel through the cylinder during overlap and losing it out the exhaust pipe. Overlap should be limited, but no one really knows where the cutoff should be. I'd venture to say that 5-10 deg of overlap should be the limit.
Turbo's produce much higher exhaust pressure than intake. During overlap, exhaust reverts into the intake port. The alleged problem is that exhaust dilutes the intake charge, but I could argue this either way. Testing has revealed that adding overlap to turbo cams increases power, but the testing stopped at about 5 deg. I've venture to say that up to 10 deg of overlap should help power, maybe much more.
My experience was that I switched from a "blower cam" (224/236-114) to a "turbo cam" (226/226-114) with my turbo and lost power. That's the last time I base a decision on internet advice from the masses without first analyzing it myself to see if it at least makes sense.
Mike
Trending Topics
07-06-2009, 05:49 PM
#8
9 Second Club
point being, there is no right answer. Very few have done REAL back to back testing on their specific build with different profiles.
And even if they did, their testing would only be valid on their setup, running exactly as it is.
If you go for a mild cam with little overlap....it will produce good results, almost regardless of setup.
Aim for bigger cams, and proper testing is the only way to optimise. Which rarely happens due to costs involved.
So many just guess, or some try and use computer models. Although how good they actually are, is anyones guess.
And even if they did, their testing would only be valid on their setup, running exactly as it is.
If you go for a mild cam with little overlap....it will produce good results, almost regardless of setup.
Aim for bigger cams, and proper testing is the only way to optimise. Which rarely happens due to costs involved.
So many just guess, or some try and use computer models. Although how good they actually are, is anyones guess.
07-06-2009, 06:06 PM
#9
TECH Regular
Join Date: Mar 2007
Posts: 455
Likes: 0
Received 0 Likes
on
0 Posts
I agree...less overlap is better in a supercharged engine.A(supercharged) artificially aspirated engine can lose some boost pressure and a lot potential power due to too much overlap.This would be caused by the Piston forcing un-burned air/fuel out of the cylinder.In some high performance NA engines more overlap with more duration is beneficial as usually you are typically keeping the valves open longer also(duration).If you choke off the cylinder too soon you are cutting the engine way short of its potential VE.It is all about choosing the right camshaft for your intended use.Go too big on a NA lightly modified engine and the vehicle will be sluggish at low rpms.Cant go wrong with asking the experts such as Comp Cams what the recommended Camshaft for your application is.By the way Thunder Racing also has some good(CC)custom grinds from mild to wild you might want to look into.http://www.thunderracing.com/catalog...&vid=3&pcid=51I believe the negative effects caused by too much overlap would have the highest effect on centrifugal supercharged engines which generally dont produce as much low end boost as roots or twin screw.
Last edited by lovescamaros28; 07-06-2009 at 06:33 PM.
07-06-2009, 06:58 PM
#10
LS1Tech Co-Founder
iTrader: (34)
My experience was that I switched from a "blower cam" (224/236-114) to a "turbo cam" (226/226-114) with my turbo and lost power. That's the last time I base a decision on internet advice from the masses without first analyzing it myself to see if it at least makes sense.
Mike
Mike
More duration = more camshaft = peaks at higher rpms = how high can you, or do you want to buzz your combo.
07-06-2009, 09:25 PM
#11
Very true. I added 2 deg duration on the intake (negligible) and subtracted 10 from the exhaust. Everyone said a conventional split pattern is all wrong for a turbo, so I went with a single pattern. After losing power, I thought, "Doh! How did I ever expect to gain power by reducing exhaust duration?!" The reverse splits seem even more silly now than they did before. The point is to not blindly follow internet logic and generalizations on cam selection.
07-06-2009, 10:05 PM
#12
huh? i said the exact same thing as the post below.....
Blowers and turbo's have the exact opposite problem during overlap.
Blowers produce much higher intake pressure than exhaust. The problem is pushing air/fuel through the cylinder during overlap and losing it out the exhaust pipe. Overlap should be limited, but no one really knows where the cutoff should be. I'd venture to say that 5-10 deg of overlap should be the limit.
Turbo's produce much higher exhaust pressure than intake. During overlap, exhaust reverts into the intake port. The alleged problem is that exhaust dilutes the intake charge, but I could argue this either way. Testing has revealed that adding overlap to turbo cams increases power, but the testing stopped at about 5 deg. I've venture to say that up to 10 deg of overlap should help power, maybe much more.
My experience was that I switched from a "blower cam" (224/236-114) to a "turbo cam" (226/226-114) with my turbo and lost power. That's the last time I base a decision on internet advice from the masses without first analyzing it myself to see if it at least makes sense.
Mike
Blowers produce much higher intake pressure than exhaust. The problem is pushing air/fuel through the cylinder during overlap and losing it out the exhaust pipe. Overlap should be limited, but no one really knows where the cutoff should be. I'd venture to say that 5-10 deg of overlap should be the limit.
Turbo's produce much higher exhaust pressure than intake. During overlap, exhaust reverts into the intake port. The alleged problem is that exhaust dilutes the intake charge, but I could argue this either way. Testing has revealed that adding overlap to turbo cams increases power, but the testing stopped at about 5 deg. I've venture to say that up to 10 deg of overlap should help power, maybe much more.
My experience was that I switched from a "blower cam" (224/236-114) to a "turbo cam" (226/226-114) with my turbo and lost power. That's the last time I base a decision on internet advice from the masses without first analyzing it myself to see if it at least makes sense.
Mike
07-07-2009, 04:10 AM
#13
9 Second Club
07-07-2009, 08:17 AM
#14
I just don't see reversion being a problem on a well setup system when you can keep the EG drive pressure lower than the intake pressure or atleast have around a 1:1 ratio
but yes on a setup where you have high drive pressures and cant keep it around 1:1 then i can see reversion being a problem
that is why there is so much variance in what people think "works", because there is so much variance in what setup they are running
IMO
07-07-2009, 11:15 AM
#16
Teching In
Join Date: Apr 2005
Posts: 27
Likes: 0
Received 0 Likes
on
0 Posts
You'll never get there. You need to have enough turbine power to overcome efficiency losses and frictional losses.
07-07-2009, 12:06 PM
#18
9 Second Club
But for most...its a mere dream
07-07-2009, 02:38 PM
#19
LS1Tech Co-Founder
iTrader: (34)
To the OP, I'd think you need to define a bunch of parameters before asking generalized questions about camshafts. For example, let's say you had a 10:1 348 cid with better-than-stock heads, a cam, and a power adder. If you ran say a D1 Procharger, APS twin, custom single, or a MP112 Maggie could you run the same cam for all the above applications and it would work? For sure. I think we could agree that a single pattern 224/224//114 camshaft would make more power than say a stock 2002 F-body camshaft, and would work all around. How about a 224/228/112? Or 111? Or 224/232//112?
I remember a veteran cam guy at Comp suggesting a 240/244 or a 240/248 I can't quite remember, for my old 348 cid combo. I went with a 236/246. But Chris was suggesting the best hydraulic race cam, he wasn't focusing too much on driveability, idle quality, max rpms, etc. It all depends.
KevinP (kp) had a great post a few years ago where he talked about the 4-5 or so cams he ran in his black TA. One thing I remembered was that by the third cam or so I think he had to spin the engine a noticeable amount to get the gains he was looking for.
I remember a veteran cam guy at Comp suggesting a 240/244 or a 240/248 I can't quite remember, for my old 348 cid combo. I went with a 236/246. But Chris was suggesting the best hydraulic race cam, he wasn't focusing too much on driveability, idle quality, max rpms, etc. It all depends.
KevinP (kp) had a great post a few years ago where he talked about the 4-5 or so cams he ran in his black TA. One thing I remembered was that by the third cam or so I think he had to spin the engine a noticeable amount to get the gains he was looking for.
07-07-2009, 05:37 PM
#20
just saying that having a little bit of reversion from overlap with lets say a 1 : 1.3 (int/exh) pressure ratio with a turbo........ is going to be a lesser of evil when comparing the affects of overlap (bleed off) on a blower where you have a 5 :1 (int/exh) pressure ratio
again IMO