# Compreesion ratio



## johnhalg (Oct 2, 2021)

Just pick up a 67 gto for restore, engine is a 4oo but not a 10.1 gto engine but a 400 8.6 compression, check compression and all cyl. Hot are 110 with carb open I though it should be 155 and info would be helpful


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## PontiacJim (Dec 29, 2012)

johnhalg said:


> Just pick up a 67 gto for restore, engine is a 4oo but not a 10.1 gto engine but a 400 8.6 compression, check compression and all cyl. Hot are 110 with carb open I though it should be 155 and info would be helpful


Depends on the cam. LSA and the closing times for the intake come into play. If they are all the same, then that in itself is good.


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## johnhalg (Oct 2, 2021)

TKY JIM, WHAT IS LSA


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## PontiacJim (Dec 29, 2012)

johnhalg said:


> TKY JIM, WHAT IS LSA


Lobe Separation Angle (LSA) the cam is ground on, ie 110 LSA, 112 LSA, 114 LSA. The 110LSA can build more cylinder pressure while the 114LSA would be used to lower cylinder pressure - along with duration and the closing number (degrees) of the Intake valve. With lower compression, you want to build more cylinder pressure - kinda like fooling the engine that it has more compression than advertised. On higher compression, you want the cylinder pressure to push out the exhaust a bit before closing the intake valve so it reduces compression - but you pick it back up as the RPM's get higher.

So cam choice can affect cylinder pressures, ie compression at your gauge. 
You want to do the compression test with the engine warm and all spark plugs removed so the engine will spin fast/freely. Battery should be fully charged and/or on a charger. Then take your readings.

Then with an oil squirt can, squirt a shot of oil into the cylinder trying to get it onto the back of the piston (intake side) so the oil will run down over the rings. Then crank the engine again and take your readings to see the difference.

If you have a big difference, then you could have bad rings.

Cylinders should be consistent in compression and no more than 10 percent between the best and lowest cylinder.


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## Baaad65 (Aug 29, 2019)

So PJ can you give me a crash course since you're taking about LSA and I don't mean to hijack this thread but I'm trying to understand, a bigger LSA number has less valve overlap thus increasing vacuum but then you say it reduces cylinder pressure so how can it do both? I have a Butler motor with about 9:36 compression according to there charts with a 114 LSA cam so why would they use that if it reduces cylinder pressure when it isn't that high to begin with? Then I remember you saying when I changed to 1.65 rockers that the intake valves slam closed faster and that those rockers should be used on the exhaust only and I had read that also but talking to Butler and Scorpion they said "well there's a reason we sell them in sets of 16". So to combat the the intake closing faster is it a good practice to flow as much air/fuel past those valves before it closes while taking advantage of the extra lift of the 1.65 rocker? And I know you said it before but why is the valve closing faster with the 1.65 ? isn't lift and duration increased with a bigger rocker ration? or are lobe profile and rocker ratio different species?


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## PontiacJim (Dec 29, 2012)

Baaad65 said:


> So PJ can you give me a crash course since you're taking about LSA and I don't mean to hijack this thread but I'm trying to understand, a bigger LSA number has less valve overlap thus increasing vacuum but then you say it reduces cylinder pressure so how can it do both? I have a Butler motor with about 9:36 compression according to there charts with a 114 LSA cam so why would they use that if it reduces cylinder pressure when it isn't that high to begin with? Then I remember you saying when I changed to 1.65 rockers that the intake valves slam closed faster and that those rockers should be used on the exhaust only and I had read that also but talking to Butler and Scorpion they said "well there's a reason we sell them in sets of 16". So to combat the the intake closing faster is it a good practice to flow as much air/fuel past those valves before it closes while taking advantage of the extra lift of the 1.65 rocker? And I know you said it before but why is the valve closing faster with the 1.65 ? isn't lift and duration increased with a bigger rocker ration? or are lobe profile and rocker ratio different species?


Keep in mind that a roller cam can lift the valve open faster and keep it open longer due to its design. So specs for a roller can be like apples and oranges when comparing to a flat tappet cam in that same manner a solid grind cam is different than a hydraulic cam grind.

Here is a real world example:

"With the first cam loaded in the engine (the 101 LSA) Westech's Steve Brul made three dyno pulls. The lumpy-idling 101 LSA cam churned out an average of 484.0 hp at 6,100 rpm with 493.9 lb-ft of torque at 4,400 rpm. It had a robust powerband with tons of usable torque. Idle quality was rough and the vacuum gauge showed only 9.8 in-Hg (inches of mercury). Brul spun the engine over with the dyno's starters and recorded 185 psi of cranking compression.

Brul slid the 113-LSA cam into the engine's heart. The 113 cam was way down on low-end torque. It made 472.4 lb-ft at peak, a 21.5lb-ft disparity from the 101 cam. Average power was also down, _*but the engine pulled ahead of the 101 cam starting at 5,400 rpm, picking up 6.5 hp at peak of 6,200 RPMs. *_On the cranking compression test, Brul recorded 175 psi, *a 10-psi drop* from the outgoing 101 cam. Idle vacuum showed *a considerable spike to 14.7 in-Hg*, likely the difference between being able to run power brakes and a mandatory leg workout."


*LSA**Horsepower**Torque**Cranking Compression**Idle Vacuum*101484.0 at 6,100 rpm493.9 at 4,400 rpm185 psi9.8107113490.5 at 6,200 rpm472.4 at 4,700 rpm175 psi14.7


Narrow LSA means:

Lower rpm torque range
Increases maximum torque
*Higher cylinder pressure*
Lower idle vacuum
Rough idle quality
Valve overlap increases
Wider LSA means:

Higher rpm torque range
Decreases max torque
*Lower cylinder pressure*
Higher idle vacuum
Smooth idle quality
Valve overlap decreases
“Lobe separation angles are influenced by the camshaft grind. If a street car has *smaller lift* (the amount the valve lifts off its seat) and *smaller duration numbers* (the degrees of crankshaft rotation for which the valve is held open) they might run 112 or 114. Widening their separation angle helps increase upper rpm power output. Alternatively, *if you are running a bigger camshaft* to gain maximum top-end power, cam makers often suggest *reducing the lobe separation angle* to recover power lost in the lower rev range.”

When engine builder Chuck Lawrence received the order to bestow a 520cu in big-block Ford with the sound of a Pro Stock engine, _he replaced the normal 112LSA hydraulic roller cam with one of 108LSA._ “The result *sounded wonderful*,” said Lawrence, “_*but it didn’t rev as enthusiastically and it made 30hp less than normal!”*_ “If you changed the lobe separation of a street engine from 112 degrees to 106 and didn’t do anything else,” says Jon Kaase, “the engine would idle a lot rougher and generate worse exhaust emissions largely because of unburned fuel.”


The *longer the intake duration*, the later the valve closes during the compression stroke, and* the less air it traps to be compressed.* To make up for this, *the longer the intake duration, the higher the compression ratio *needs to be *in order to maintain good cylinder pressure.

Longer Duration = Higher Compression Ratio Needed
Wider lobe separation = Higher Compression Ratio Needed *

Static compression is the mathematical calculation of the compression ratio or compressed volume. Dynamic is in reference to cylinder pressure based on the intake closing number. The two are different. The intake valve closing angle can be used to bleed off some of the air column (in a running engine), therefore 2 engines with the same compression ratio (static) will have different operating cylinder pressures at fixed rpm if they have different cam specs. Example, one with 106 LSA installed at 104 Intake Centerline (ICL) will have higher cylinder pressure than a 112 LSA installed at 110 ICL, presuming the same duration and static compression. This is why a tighter LSA does not need more compression because it builds more cylinder pressure (and work well on low compression engine builds). When duration is the same, if you widen the LSA you should raise the comrpession ratio accordingly to match the later intake closing angle and *subsequent intake bleed off*.

Dynamic Compression calculations can be helpful in predicting *the effective compression* an engine will see in an attempt to account for the bleed-off/increase in cylinder pressures that occurs from the Intake Valve Closing Angle. Think of it as "cam adjusted compression ratio" based on Intake Closing. And it can be adjusted by changing the installed ICL. *So DCR numbers just give us an idea of how changing the LSA and intake duration (intake closing point) affects how much air is trapped in the cylinder for the compression stroke.*

So a lot to swallow and it takes some reading to get a hold of all the various aspects of a cam to understand what affects what.

With regards to rocker ratio, 1.5 versus 1.65, it is the fulcrum point that makes the 1.5 a 1.65. The rocker arm opens the valve higher, but it also opens it quicker _*because*_ it opens the valve higher. Lets say we use the 1.5 rocker and the valve is opened .400". That same valve with the 1.65 rocker would be open .439". How did I figure this? .400" divided by 1.5 = .266" which would be what the cam lobe lift is (which is found on most cam cards). So if the cam's lobe lift is .266", multiply it by 1.65 = .439". So if the cam lobe lift is .266", with the 1.5 the lift at the valve is .400" while the 1.65 rocker lift at the valve is .439". The 1.65 rocker provides .039" more lift at the valve for the same given cam lobe lift, so the valve is opened higher sooner, thus faster.


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## Baaad65 (Aug 29, 2019)

Ok, wow...ya a lot there so I know you don't have a crystal ball but what's your thinking on why Butler would put a 114 LSA on a motor with a 9.36 compression other than wanting a higher vacuum for brakes? And did make a mistake adding 1.65 rockers? I'm not complaining the motor is a beast but I don't know if I really felt a seat of the pants difference just my in experience of wanting to add more horses and I sold the 1.5 rockers so I'm not going back just curious and always want to learn 👍


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## Baaad65 (Aug 29, 2019)

Sorry, 9.63 compression and did "I " make a mistake...time for bed.


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## Baaad65 (Aug 29, 2019)

PontiacJim said:


> Keep in mind that a roller cam can lift the valve open faster and keep it open longer due to its design. So specs for a roller can be like apples and oranges when comparing to a flat tappet cam in that same manner a solid grind cam is different than a hydraulic cam grind.
> 
> Here is a real world example:
> 
> ...


PJ I don't know if you got to read my questions in my response about the LSA choice and my rocker choice and also you mentioned you don't like double pumpers as well as vacuum secondary's, so what's the thinking on that because everyone and everything you read says if you have a manual trans which I have, that you don't want vacuum secondary's.


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## PontiacJim (Dec 29, 2012)

Baaad65 said:


> PJ I don't know if you got to read my questions in my response about the LSA choice and my rocker choice and also you mentioned you don't like double pumpers as well as vacuum secondary's, so what's the thinking on that because everyone and everything you read says if you have a manual trans which I have, that you don't want vacuum secondary's.


Did not say I don't like vacuum secondaries, if I did, then it was in error. I like vacuum secondaries carbs over a double pumper - which in my book is mechanical secondaries.

As you found out, double pumpers/mechanical secondaries will work, but can require various adjustments and trial & error testing to get the carb dialed in to your engine/drivetrain. Double pumpers have 2 accelerator pumps and use mechanical opening secondaries. I am not including the 660 center squirters used for drag racing which do not have an accelerator pump on the mechanical secondaries.


Double pumpers typically pop the secondaries open without the benefit of building up air/fuel velocity prior to the secondaries popping wide open. The vacuum secondaries create a transition in that you have some means of opening the secondaries "slower" or building up some velocity in the air/fuel mixture before the throttle blades are completely open or/if the secondary throttle blades are completely open there is an air flap above the throttle blades that can act as a way to create some air/fuel velocity as the air flap becomes fully open - such as a Q-jet or AFB.

With a double pumper/mechanical secondaries, they are better suited to a car that has a driveline that will allow the engine to spin up quickly or you could experience a bog, flat spot, or even back fire, as the secondaries pop open too quickly and there is not enough fuel being introduced into the secondaries to compensate when the engine takes a gulp of air when the throttle blades go wide open - which is what you experienced. Can this be adjusted out? Sure, but look at all that it can take in adjusting, buying parts, testing, etc... This may be fine for someone who has the knowledge/understanding/skill to do this.

On a street cruiser over 3,300 pounds with a mild cam (218/220 degrees or less duration at 0.050) that won't see more than 5,500 rpm, has a stock or near-stock converter, uses 3.55:1 or milder gears, and is fed by a dual-plane intake, a vacuum-secondary carb is likely the better choice. And, a dual-plane intake versus a single plane intake is more forgiving with an overly large carburetor.

On a near-stock engine in a heavy vehicle up to 3,500 pounds (remember, most of out GTO's are closer to 3,800 pounds) with mild rear gears a mechanical-secondary carb can still be made to work by increasing converter stall speed about 800 to 1,000 rpm over stock. If the engine has high compression, a serious cam, a single-plane intake, 3.70:1-or-steeper gears, and a manual trans or an automatic with a high-stall converter, then a double-pumper may be best.* Double-pumpers require proper size selection and careful tuning to perform at their best. * 

For most the vacuum secondary type carb can be a bolt-on-and-go carb because the vacuum secondaries can compensate for a carb that may not be perfectly tuned for that engine. Slap an 800 CFM carb with vacuum secondaries onto an engine that needs no more than 600CFM's and the vacuum secondaries are only going to open what the engine needs. Slap on an 850 CFM double pumper and you are probably not going to get it to run good.

The vacuum secondaries can also be tuned using the air flap. You can have the air flap open sooner or later, thus tuning the engine's need for more fuel and at what point and you can adjust much easier if the engine has a bog/stumble because there is too much/too little fuel being dumped in by the secondary operation. You cannot do that with mechanical secondaries, you have to make more adjustment in fuel enrichment or leaning it out to dial it in - again, more work/testing. *With a double-pumper, a heavy car and/or a slow-revving engine may have a hard-to-overcome bog.* And, when you do get that mechanical carb adjusted up, you could experience a change in your settings based on gasoline quality, engine temps, timing, altitude, weather, and the season. I could have these same things affect a vacuum secondary carb, but could also/possibly make an adjustment to the opening/closing rate of the secondary air flap by adjusting spring tension - which would be much easier.

To sum this up, a vacuum secondary carb, along with a dual plane intake, it a better choice for most. If you have the knowledge/understanding/skills to make needed adjustments on a double pumper to dial it in, then it can be an option for some. Vacuum secondary carbs are better suited for heavy car and stock drivelines along with stock to mild build engines. Double pumpers are better suited for high compression, big cam, fast revving engines with high stall converters if using an automatic, and 3.70 gearing or more that favor the faster revving of the engine.

BUT, those who tinker with carbs is becoming smaller and smaller so the EFI systems are for some the better choice in avoiding carbs and adjustments altogether.


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## PontiacJim (Dec 29, 2012)

"Why Butler would put a 114 LSA on a motor with a 9.63 compression other than wanting a higher vacuum for brakes? And did make a mistake adding 1.65 rockers?"

PJ: 114 LSA is a good street engine choice. It is good for engine vacuum for power brakes, wider power band over a larger RPM range, drops cylinder pressures at the lower RPM's to help with cranking the engine easier and can keep the engine away from the "detonation zone."

1.65 rockers are fine if the springs are not going into coil bind and strong enough to keep the valve shut at higher RPM's and not allow them to bounce off the valve seats. You get more lift and a tad bit more duration out of the cam. If you are not having any issues and it runs good/smooth, then leave well enough alone.


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## Baaad65 (Aug 29, 2019)

PontiacJim said:


> Did not say I don't like vacuum secondaries, if I did, then it was in error. I like vacuum secondaries carbs over a double pumper - which in my book is mechanical secondaries.
> 
> As you found out, double pumpers/mechanical secondaries will work, but can require various adjustments and trial & error testing to get the carb dialed in to your engine/drivetrain. Double pumpers have 2 accelerator pumps and use mechanical opening secondaries. I am not including the 660 center squirters used for drag racing which do not have an accelerator pump on the mechanical secondaries.
> 
> ...


Got it and good information as always, and that's exactly the problem I had to overcome, the thing I hear all the time is like I mentioned if you have a manual trans vacuum secondaries don't work as well so that's what I'm trying to understand.


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## PontiacJim (Dec 29, 2012)

Baaad65 said:


> Got it and good information as always, and that's exactly the problem I had to overcome, the thing I hear all the time is like I mentioned if you have a manual trans vacuum secondaries don't work as well so that's what I'm trying to understand.


They work fine. You might have a slower opening of the secondaries with some vacuum secondaries based on how the secondary air flap is operated. The AFB has a weighted air flap, so it may not open as quick unless you modify the weights by adding or subtracting. The Holley uses a vacuum diaphragm/spring tension.

So the double pumper can snap open the secondaries faster due to the mechanical linkage compared to waiting for the vacuum diaphragm to open the secondaries. For racing, the double pumper/mechanical secondaries may have the edge - all things matched.

Read this from Summit:






Carburetor Secondary Types







help.summitracing.com


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## PontiacJim (Dec 29, 2012)

And just a few more follow-up comments on double pumpers from one of my car magazines, 1972.

"If you want to run softer gears in the range of 2.75 to 3.50, I don't recommend any of the Holley double-pumpers. There are definite areas in the general range of 1200 to 2500 rpm where you can get bogging if you punch hard at very low speeds and pull slowly with long gears. The big accelerator pump shot dissipates before the engine really begins to pick up. Using better rear axle gearing, 3.90 and up, can help this or use a smaller CFM carb so venturi velocities are higher at lower RPM/speeds. Holley recommends the 650CFM for engines under 400CI. Carb CFM/venturi size is more critical with mechanical secondaries than vacuum secondaries. You can get into trouble real fast if too big. A vacuum secondary or air valve carb can be used nicely on a broader range of engine sizes. These types of carbs will pull a little stronger from 1000 to 2000 rpm with the restricted venturi area, but generally don't give full air flow until around 3500 to 4000 rpm whereas the double-pumper provides full air flow as soon as you punch it and can show a HP advantage in the 2000 to 3000 rpm range when all components of the engine and driveline are matched. You don't have to be quite as careful about choosing the right sized CFM carb when selecting vacuum operated secondaries or an air flap carb for your engine, or its softer driveline."


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## Baaad65 (Aug 29, 2019)

Good stuff, I had a 770 street avenger vac secondary on this motor and just wasn't as quick as I thought it should be so that's when I went looking and I saw multiple places and the guy who built it all said you don't want a vacuum secondary carb on a manual transmission car, so this one seems to put me back in the seat more but maybe it's my timing adjustments or the 1.65 rockers idk. I did have to overcome that wot hesitation and just for comparison I stuck the #31 nozzles back in today went a drive and noticed a little hesitation so I'm sticking with the #35 nozzles and we'll see how it goes. I do get the air speed problem from idle but I have a 2:99 first gear with a 3:42 rear so it gets going quickly but it's that split second after hammering the throttle that I had to overcome, maybe I could bump my base timing 2° for 36° total instead of 34°, don't hear any detonation even lugging up an incline.


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