13:1 c/r on pump gas?
I doubt hes thinking of taking his motor apart just to low compression. And if he were don't you guys think a head gasket spacer or just a thick head gasket would be a lot easier and cheaper way to lose some compression?
Dude spray the car!
Dude spray the car!
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From page 1:
Can someone explain how overlap effects dynamic compression?
As in, that's not accurate at all...
Erik would be correct in stating a poor breathing engine can benefit from
more compression to restore some of the 'punch', however you can run
all the compression you want until you're blue in the face... if you don't
have air and fuel combusting to provide the energy, the engine is a dog.
It is better to have a good flowing setup with less compression to arrive
at the same cylinder pressure.
Having a 13:1 SCR with "pump gas" means that IVC is going to happen later
to keep the dynamic compression within a window that is efficient for the
fuel you intend to run.
It also means the engine will be tuned for a higher RPM, and the torque and
HP will generally peak later in the band (high compression with late IVC).
Can someone explain how overlap effects dynamic compression?
As in, that's not accurate at all...
Erik would be correct in stating a poor breathing engine can benefit from
more compression to restore some of the 'punch', however you can run
all the compression you want until you're blue in the face... if you don't
have air and fuel combusting to provide the energy, the engine is a dog.
It is better to have a good flowing setup with less compression to arrive
at the same cylinder pressure.
Having a 13:1 SCR with "pump gas" means that IVC is going to happen later
to keep the dynamic compression within a window that is efficient for the
fuel you intend to run.
It also means the engine will be tuned for a higher RPM, and the torque and
HP will generally peak later in the band (high compression with late IVC).
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I was wondering about having the heads combustion chamber pushed out 5cc but wasnt sure of the effects on the afr heads, still looking for some solid info on this. as of now the plan still stands at cutting the pistons down, I already verified the pistons will be good for this route and its a short block now so Im not to far into it as to be a hassle to tear it down at this point.
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Your idea to reduce comrpession is much better than adding a thicker head gasket. Increasing quench height is not a very efficient method of reducing
SCR;it's fast and cheap, but hardly efficient. Raising quench height also promotes detonation.
SCR;it's fast and cheap, but hardly efficient. Raising quench height also promotes detonation.
Heres thing, theirs what works in theory and what actually happens.
418ci flat top pistons, AFR heads, .045" quench, 251/259 camshaft, TR6 plugs, timing @ 20º
SCR: 12.4:1
DCR: 8.5:1
pinging its nuts off at 4000 rpm in 3rd with a load on it
93 octane BP, in Chicago we're supposed to get best quality 93 octane, BP fuel is pipelined from BP's Whiting, IN refinery directly.
100 octane unleaded, fine, made 520rwhp though auto.
I know a few 13:1 LS builds even SBC street cars running 100 unleaded, in Chicago we have 100 octane available
in most suburbs on the pump. If you consider that pump fuel you're fine.
418ci flat top pistons, AFR heads, .045" quench, 251/259 camshaft, TR6 plugs, timing @ 20º
SCR: 12.4:1
DCR: 8.5:1
pinging its nuts off at 4000 rpm in 3rd with a load on it
93 octane BP, in Chicago we're supposed to get best quality 93 octane, BP fuel is pipelined from BP's Whiting, IN refinery directly.
100 octane unleaded, fine, made 520rwhp though auto.
I know a few 13:1 LS builds even SBC street cars running 100 unleaded, in Chicago we have 100 octane available
in most suburbs on the pump. If you consider that pump fuel you're fine.
i started at 27 and that sounded like someone was using an air hammer on my pistons!! Explain more please. Removing the dome off of a piston is exactly opposite of what its intended to do. Which is to go into the main combustion chamber of the head were spark nucleus, hatching out, and flame front all originate and make the air fuel ratio more vollitile thus allowing higher octane fuel to be more efficient and more easily ignited allowing for more ignition timing and ultimately more efficiency and power. Reducing the dome is fine but seems a bit extravagant. Not to mention exspense. Why not leave the short block alone and go with a custom ground camshaft?
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From: K-W, Ontario
Actually, reducing the dome is beneficial to flame travel. A dome impedes the
flame front from moving across the piston. If you can achieve the compression
ratio without using a dome (IE: reducing quench, or chamber cc), it's the
better scenario.
As for quench height, increasing the distance between the head and piston
reduces the quench effect. It also leaves air/fuel in an area furthest from
the spark plug.
http://racingarticles.com/article_racing-10.html
If you want more advanced discussion on these topics, I'd recommend
going to Speed Talk forum and register an account there. You will have
access to the advanced forum and discuss these tuning tips with seasoned
engine builders in the industry.
As for using a custom ground cam to dial in the compression I would agree
that's a better alternative than sticking in a thicker head gasket.
flame front from moving across the piston. If you can achieve the compression
ratio without using a dome (IE: reducing quench, or chamber cc), it's the
better scenario.
As for quench height, increasing the distance between the head and piston
reduces the quench effect. It also leaves air/fuel in an area furthest from
the spark plug.
http://racingarticles.com/article_racing-10.html
In a running engine, the .035" quench decreases to a close collision between the piston and cylinder head. The shock wave from the close collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, average the chamber temperature, reduce detonation and increase power. Take note, on the exhaust cycle, some cooling of the piston occurs due to the closeness to the water cooled head.
going to Speed Talk forum and register an account there. You will have
access to the advanced forum and discuss these tuning tips with seasoned
engine builders in the industry.
As for using a custom ground cam to dial in the compression I would agree
that's a better alternative than sticking in a thicker head gasket.
Last edited by Adrenaline_Z; Aug 12, 2010 at 12:28 AM.
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My thoughts on overlap;
For overlap to effect the ability for a high cr motor to tolerate pump gas, I would expect a delayed intake valve opening event to reduce the intake charge, and dilution of the intake charge with a delayed exhaust closing event to be the primary controllers
Theres enough cam gurus on this site that should be able to fill in some of the blanks
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From: K-W, Ontario
Yes you're on track with the theory. Compression cannot start until both valves
are closed, and in this case we're referring to the intake valve closing which is the
sole factor in determining the DCR value.
Overlap is tricky to understand and tune. You are correct that a late opening
intake and early opening exhaust will at times cause the exhaust gas to
remain in the chamber and displace the fresh air/fuel mixture.
Residual exhaust gas helps to cool the chamber and therefore acts like additional
quench. This is a variable effect and will happen at only certain RPM.
If setup properly overlap will help create better VE by using the exhaust
system to 'boost' the intake charge. By creating a depression (low pressure)
in the chamber just about the time the intake valve opens and the intake pulse is moving down the intake runner and head port, the engine can achieve
over 100% VE at certain times (Pro Stock tuners are masters at this).
So you can see that overlap can work with you, and it can work against you
but the Intake Valve Closing point (IVC) is what determines the DCR value.
are closed, and in this case we're referring to the intake valve closing which is the
sole factor in determining the DCR value.
Overlap is tricky to understand and tune. You are correct that a late opening
intake and early opening exhaust will at times cause the exhaust gas to
remain in the chamber and displace the fresh air/fuel mixture.
Residual exhaust gas helps to cool the chamber and therefore acts like additional
quench. This is a variable effect and will happen at only certain RPM.
If setup properly overlap will help create better VE by using the exhaust
system to 'boost' the intake charge. By creating a depression (low pressure)
in the chamber just about the time the intake valve opens and the intake pulse is moving down the intake runner and head port, the engine can achieve
over 100% VE at certain times (Pro Stock tuners are masters at this).
So you can see that overlap can work with you, and it can work against you
but the Intake Valve Closing point (IVC) is what determines the DCR value.
Last edited by Adrenaline_Z; Aug 12, 2010 at 05:24 PM. Reason: Misread first part. Removed paragraph.
That's great stuff man I appreciate your willingness to share. Ill probably run super thin head gaskets from here on out. LOL. Ive always been one to build my own engines from start to finish out of parts I picked and hardly every run into problems with trying to lose compression.
For the record I knew that the piston dome actually hinders flame travel but it also makes the mixture more volitile and creates a give and take scenerio as far as making power goes. All my dome pistons have flame notch's cut into them. Most pistons do now days. Thing is with a wedge style engine that's really the only way to get 13.5 or better compression without some really custom stuff.
For the record I knew that the piston dome actually hinders flame travel but it also makes the mixture more volitile and creates a give and take scenerio as far as making power goes. All my dome pistons have flame notch's cut into them. Most pistons do now days. Thing is with a wedge style engine that's really the only way to get 13.5 or better compression without some really custom stuff.
Another thing to remember is the fact that heat also is a pre-detonation factor , so You run an over-sized rad with the same thermostat , even when the gauge says a given temp , You're engine is cooler . Also rod ratio makes a huge difference . An article in a Hot Rod mag from about 12 years ago has an article about 2 guys that made a chevy 350 using a 400sm bl .030 (4.155") bore and a 307/327 crank (3.25") stroke , Ford 300 6cyl. rods (6.209") machined narrower to fit chevy crank , custom JE pistons I'm guessing domed with pin location higher up on piston , "stock" dur and lift roller cam and aluminum heads with 58cc chamb . 11/1 scr . I think 2.02/ 1.6 valves a Quadrajet and headers . HEI . With pump 87oct were able to total advance of 36deg . 412 hp 435tq . So I estimated a gen 1 350 with the same heads ,comp, carb, cam, headers to put out about 245hp 285lb. ft. A diff of 167hp 150 lb. ft.
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We have a 355ci 650 hp on pump gas. From 11.7 to 1 to 12.7 to 1 picked up 30hp still on 33 deg timing, It is all in dynamic compression, If it still knocks on low loads it needs a tighter quench, say 30 thou on a steel rod, and a lot of chamber work no sharp edges blended nicely and polished, Remember tighter lsa gives higher dynamic compression, all we normally do is retard the cam timing 2 to 4 degrees OR run a wider LSA so if you are 112+4 go to 114+1 or 0. Run colder ects aswell.
One mans 408 for example may take more lead, some say it is because VE is reduced I know for a fact many engine builders would argue that, it is true to an extent but all motors are different and if it takes low timing generally if you look at the chamber it can be improved upon and also altering quench and wider LSA. I have seen reports of 102 lsas in hemis running 240psi cranking comp with no issue, then a Ford motor running 220 psi and struggling on pump gas in which scenario a 114 lobe sep cam is installed on a later ICL and problem solved. Use cranking compressin as a guide, Get your ballpark figure based upon experience and then alter cam timing slightly is the best way at the end of the day excellent results are achievable.
cheers
One mans 408 for example may take more lead, some say it is because VE is reduced I know for a fact many engine builders would argue that, it is true to an extent but all motors are different and if it takes low timing generally if you look at the chamber it can be improved upon and also altering quench and wider LSA. I have seen reports of 102 lsas in hemis running 240psi cranking comp with no issue, then a Ford motor running 220 psi and struggling on pump gas in which scenario a 114 lobe sep cam is installed on a later ICL and problem solved. Use cranking compressin as a guide, Get your ballpark figure based upon experience and then alter cam timing slightly is the best way at the end of the day excellent results are achievable.
cheers
Last edited by hymey; Sep 21, 2010 at 07:40 AM.
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Because a wider lsa has a later intake centreline and so the compression starts later. If you look at 2 stroke specs you will see they advertise compression by a dynamic value not static, because it does not start to build until the piston closes the intake port much like when the 4 stroke closes the inlet valve.
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