Can of worms, gt40p related (p for pain)
- Thread starter DJCarbine
- Start date
Why would a reduced amount of overlap increase the dynamic compression ratio?
With a wider LSA (for a given duration), the intake valve opens and closes later, so the piston has traveled further up the bore at intake valve closing and therefore start of compression. Therefore, the cylinder volume at the start of compression is reduced while the volume at the end of compression (at TDC) remains constant, which means that the dynamic/effective compression ratio is lower.
with less valve overlap, less of the intake charge goes out the exhaust port. remember that when both valves are open the piston is still moving upwards. a later intake valve closing has less effect on losing intake charge due to inertia than having a large valve overlap.
At high rpm, you are effectively not losing any intake charge though. That is why the overlap is good at higher rpm. Also why they say tight LSA cams give more peak power and wider LSA gives a broader torque and power curve, but with less peak power.
I see what what you mean, but what you really talk about is the effect of the LSA on volumetric efficiency. Of course, the more charge is trapped in the cylinder, the higher the pressure at TDC will be.
However, that's not the same thing as a higher compression ratio. A wider LSA results in a later intake valve closing and a lower dynamic compression ratio. If, for some reason, the wider LSA also leads to a higher volumetric efficiency, the pressure at the start of compression will be higher. Whether or not the pressure at the end of compression is higher, depends on how much the volumetric efficiency is improved and how much the dynamic compression ratio is reduced.
Whether it's the overlap or the intake valve closing timing that has the largest effect on the volumetric efficiency depends on many things, including the engine speed, the scavenging effect of the headers and the ram effect of the intake.
I guess the cows got lost on their way home. You guys are hard to convince 
Maybe engines work differently on my side of the atlantic ocean:
http://www.sae.org/technical/papers/2005-01-0177
Maybe engines work differently on my side of the atlantic ocean:
http://www.sae.org/technical/papers/2005-01-0177
CraigMBA
New Member
I think we've all been leveled. I knew somebody knew what they were talking about in this thread, and I couldn't figure out who it was until now.
I saw a set of non P series GT40 heads with Ferrera valves and a fresh valve job the other day on a board for $450 for the pair.
I'm all about pinching pennies, but I'm not going to step over dollars to get them.
I saw a set of non P series GT40 heads with Ferrera valves and a fresh valve job the other day on a board for $450 for the pair.
I'm all about pinching pennies, but I'm not going to step over dollars to get them.
i don't know what an experimental diesel engine has to do with gt40-p heads or lobe seperation angles, but read this it may help you understand what D.Hearne is talking about.
http://www.wallaceracing.com/cambasics.htm
specifically this
Lobe Separation - This is the PHYSICAL configuration of the cam in relation to the actual spacing of the intake and exhaust lobes from each other. Lobe separation is ground into the camshaft. You CANNOT change it (Unless you reground the cam). You CAN change the Centerline by degreeing. These two terms are often confused with each other.
The lobe separation angle is basically the measurement in degrees of the angle between the intake and exhaust lobes, and is generally between 104 and 118 degrees. This measurement may also be denoted as the lobe displacement angle. Intake and exhaust duration along with the lobe separation angle and camshaft lift combine to determine the amount of overlap, which is the amount of time in which both valves are open. Excess overlap can hurt low rpm performance, and will cause an engine to idle poorly. However, in some engines large amounts of overlap can help high rpm performance by scavenging the cylinders and improving volumetric efficiency. Assuming the lobe separation angle remains the same, the more lift and duration a camshaft has the more overlap it will have. A greater lobe displacement will decrease the amount of overlap, lift and duration being the same. Generally, smaller lobe separation angles cause an engine to produce more midrange torque and high rpm power, and be more responsive, while larger lobe separation angles result in broader torque, improved idle characteristics, and more peak horsepower.
Overlap and Compression- A very common idea, although for the most part incorrect, is that overlap bleeds off compression. Overlap, by itself, does not bleed off compression. Overlap is the angle between the exhaust closing and intake opening and is used to tune the exhaust's ability draw in additional intake charge as well as tuning idle vacuum and controlling power band width. Cylinder pressure is generated during the compression cycle, after the intake valve has closed and before the exhaust opens. Within practical limits, an early intake closing and late exhaust opening will maintain the highest cylinder pressure. By narrowing the Lobe Seperation Angle 'LSA' for a given lobe duration, the overlap increases, but the cylinder pressure can be increased as well. Thus cylinder pressure/compression can actually increase in this scenario, by the earlier intake closing and later exhaust opening. By increasing duration for a given LSA, the overlap will increase, the intake closing will be delayed, and the exhaust opening will occur earlier. This will decrease cylinder pressure, but the decrease/bleed-off of compression is not due to the overlap, it is due to the intake closing and exhaust opening events.
http://www.wallaceracing.com/cambasics.htm
specifically this
Lobe Separation - This is the PHYSICAL configuration of the cam in relation to the actual spacing of the intake and exhaust lobes from each other. Lobe separation is ground into the camshaft. You CANNOT change it (Unless you reground the cam). You CAN change the Centerline by degreeing. These two terms are often confused with each other.
The lobe separation angle is basically the measurement in degrees of the angle between the intake and exhaust lobes, and is generally between 104 and 118 degrees. This measurement may also be denoted as the lobe displacement angle. Intake and exhaust duration along with the lobe separation angle and camshaft lift combine to determine the amount of overlap, which is the amount of time in which both valves are open. Excess overlap can hurt low rpm performance, and will cause an engine to idle poorly. However, in some engines large amounts of overlap can help high rpm performance by scavenging the cylinders and improving volumetric efficiency. Assuming the lobe separation angle remains the same, the more lift and duration a camshaft has the more overlap it will have. A greater lobe displacement will decrease the amount of overlap, lift and duration being the same. Generally, smaller lobe separation angles cause an engine to produce more midrange torque and high rpm power, and be more responsive, while larger lobe separation angles result in broader torque, improved idle characteristics, and more peak horsepower.
Overlap and Compression- A very common idea, although for the most part incorrect, is that overlap bleeds off compression. Overlap, by itself, does not bleed off compression. Overlap is the angle between the exhaust closing and intake opening and is used to tune the exhaust's ability draw in additional intake charge as well as tuning idle vacuum and controlling power band width. Cylinder pressure is generated during the compression cycle, after the intake valve has closed and before the exhaust opens. Within practical limits, an early intake closing and late exhaust opening will maintain the highest cylinder pressure. By narrowing the Lobe Seperation Angle 'LSA' for a given lobe duration, the overlap increases, but the cylinder pressure can be increased as well. Thus cylinder pressure/compression can actually increase in this scenario, by the earlier intake closing and later exhaust opening. By increasing duration for a given LSA, the overlap will increase, the intake closing will be delayed, and the exhaust opening will occur earlier. This will decrease cylinder pressure, but the decrease/bleed-off of compression is not due to the overlap, it is due to the intake closing and exhaust opening events.
This parts explains exactly what I was talking about.
If you still disagree with me, maybe we can at least agree that the original poster did indeed open a can of worms
In other news, I broke the timing cover getting it off
I also found a company that makes replica stealth manifolds in china, for about 120. All the specs are the same, +/- .100"
Went to a swap meet and picked up the stealth, picked up the knockoff... exactly the same. The ports were a bit messy, and the casting was a bit more rough, but I plan on gasket porting any ways so its not a big deal.
The timing gears were dead on, installed straight up. I was worried i might have gotten them a tooth off, but this was not the case.
Lifter preload. When installing the rocker arms, I find that they get tight on the pushrod about 3 turns away from bottoming out and fully seating the rocker arm. Non adjustable rocker arms BTW. Would too much preload result in dismal high RPM performance?
Apparently .020-.060 is acceptable. I do believe this is around 2-4 threads on the allen bolts holding the rockers down, if so that is "correct".
I also found a company that makes replica stealth manifolds in china, for about 120. All the specs are the same, +/- .100"
Went to a swap meet and picked up the stealth, picked up the knockoff... exactly the same. The ports were a bit messy, and the casting was a bit more rough, but I plan on gasket porting any ways so its not a big deal.
The timing gears were dead on, installed straight up. I was worried i might have gotten them a tooth off, but this was not the case.
Lifter preload. When installing the rocker arms, I find that they get tight on the pushrod about 3 turns away from bottoming out and fully seating the rocker arm. Non adjustable rocker arms BTW. Would too much preload result in dismal high RPM performance?
Apparently .020-.060 is acceptable. I do believe this is around 2-4 threads on the allen bolts holding the rockers down, if so that is "correct".
Sometimes its hard to start up, I just attributed that to lack of a choke. I did have some really minor pop backs through the carb on occasion, I thought it was due to the carb flooding.
Perhaps I will do the marking method on the pushrods to see the preload. I believe I may have a few hairs too much.
Perhaps I will do the marking method on the pushrods to see the preload. I believe I may have a few hairs too much.
3 more turns after the push rods starts to get tight when spinning them between your fingertips (which I assume is what you meant) sounds like a lot. I never installed bolt down rockers, but on stud rockers, you usually give them 1/4 to 3/4 turn.
Three turns on a 5/16-24 bolt is about 1/8" and the push rod gets pushed down 1.6 times that (due to the leverage of the rocker).
Three turns on a 5/16-24 bolt is about 1/8" and the push rod gets pushed down 1.6 times that (due to the leverage of the rocker).
Maybe you should check the length of your push rods. Sounds like the ones you have are too long. You said they were for a truck engine with a roller engine. Does that mean you have a roller cam? IIRC, stock roller push rods are 6.250"
Right now, you seem to have about .20" preload (3 turns x 1.6). I think the correct length push rods would be a better solution than using an excessive amount of shims under the rockers. Changes are that you screw up the rocker geometry by doing so.
Right now, you seem to have about .20" preload (3 turns x 1.6). I think the correct length push rods would be a better solution than using an excessive amount of shims under the rockers. Changes are that you screw up the rocker geometry by doing so.
the P heads generally like around 28-32 degrees total. i have dizzy curved to 22 degrees mechanical and i run right at 16 degrees initial on my 351 with D0OE heads. most small fords like between 12 and 16 degrees initial timing, so i would shoot for that on initial and have the dizy curved to between 16 and 20 degrees mechanical to get in the 28-32 range total. of course every combo is different on what it likes though but this is about what i'd shoot for.
You are correct (about the push rod length, not about dynamic compression ratio
). The reason I mistakenly wrote 6.400" is that's the length that is used with reduced base circle (retrofit) Comp Cams roller cams. I thought maybe he got a set of those instead of the stock roller push rods (which would explain the problem) and ended up writing down the wrong length.
On second thought, I am not sure what pushrods I used. I think I went to autozone with my stock pushrods which were too short with 0 preload, but still "fit" and worked with barely any clearance/freeplay but enough to have no preload.
I then got the next bigger production pushrod in a ford engine that they had in stock. I am not sure if it was a 5.0, 302, carb'd/efi or anything. All I know is, compared to a stock 66 289 pushrod, it was barely noticable to be bigger. MAYBE .200 +/- .050
I'm going to measure the preload I have on the lifters today using the a straight edge on the head marking the pushrods before/after bolting down the rocker
I then got the next bigger production pushrod in a ford engine that they had in stock. I am not sure if it was a 5.0, 302, carb'd/efi or anything. All I know is, compared to a stock 66 289 pushrod, it was barely noticable to be bigger. MAYBE .200 +/- .050
I'm going to measure the preload I have on the lifters today using the a straight edge on the head marking the pushrods before/after bolting down the rocker
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