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Discussion Starter · #1 ·
Hi, sorry for my bad english.
Please I need your help.
I Have a 1972 Pontiac Lemans, my mechanic said that the cam is warn....... it has a Pontiac 400 engine, number 500557, i wish more power, nothing to race, only
for street but with some more power than original.
I think to change the camshaft, wich you recommend?? Comp Cams XE262h is a good option? , I was thinking of buying the kit
The engine has the original parts, valves, transmission (automatic), intake, only I change the carb, I installed Edelbrock 1406.
I can’t change cylinder heads, its too much money.
Or you recommend other cam?

Thanks a lot!

Best regards
 

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The XE series cams use a pretty fast lift rate which easily gets you back into lobe and lifter wear these days. Otherwise the XE262H should be decent for what your doing.

The Howard’s Street Force #2, part 410021-12 offers nearly the same duration but less lift at .450/450 inch which will be a little easier on the lobes and lifters.

Break-in is all important these days.

Bogie
 

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Discussion Starter · #3 ·
The XE series cams use a pretty fast lift rate which easily gets you back into lobe and lifter wear these days. Otherwise the XE262H should be decent for what your doing.

The Howard’s Street Force #2, part 410021-12 offers nearly the same duration but less lift at .450/450 inch which will be a little easier on the lobes and lifters.

Break-in is all important these days.

Bogie
BOGIE, Thanks for your answer.

i need to know, if i put the XE262H or howards, the car will be faster or more awake than with the original cam?

THANKS
 

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You’re a little bit stuck with this engine, it is a SMOG configured engine with only 8 to 1 compression. So without improving the compression ratio it is always short of the power it could if not should be making. The minimum means of improving that would be to mill some thickness off the bottom of the head’s which makes the combustion chamber volume smaller thus raising the compression ratio, generally .030 inch is good for .5 increase in the Compression ratio. Reducing Head gasket thickness also helps this but Pontiac head gaskets choices are getting pretty slim. Past a .030 cut on the head’s the intake also has to be narrowed bu milling on both sides. Correcting this situation beyond that requires different pistons and or head’s.

The cam choices outlined will bring the power up, actually by a lot but not as much as if would if the compression ratio was raised to around 9.5 to one. Also the bigger cam combined with the low compression will increase fuel consumption. This gets you into the differences between compression ratio calculations. The typical is the Static Compression Ratio (SCR) which is simply all volumes of the cylinder with the piston at Bottom Dead Center (BDC) plus those of the piston crown if any which reduce volume if a dome or add volume if a dish or valve clearance eyebrows, head gasket thickness and combustion chamber with valves in place divided by the volumes when the piston is at Top Dead Center (TDC) of any piston height above or below the head deck, and volumes of the piston crown, volume of the head gasket and that of the combustion chamber. OK, that’s the static or dimensional compression ratio.

However, there is also a thing variously called ‘Compensated’, ‘Dynamic’, ’Effective‘, or ‘Working‘ Compression Ratio. I’ll use Dynamic Compression Ratio or DCR just to minimize the jibberish of acronyms. What the DCR does is consider the working volume left after the point in crankshaft degrees where the cam closes the intake valve. Since this is some count of degrees past BDC the piston has risen some distance up the bore, thus there is a loss of displacement of the cylinder volume that lowers the compression ratio from the static SCR calculation. The result is the DCR is always less than the SCR. While seeming counter- productive holding the intake open on the compression stroke is the key to the high speed engine. Early engines only used a cam operated exhaust valve, for the intake that was a valve with a light spring that was opened by the pressure difference between the atmosphere and the decreasing cylinder pressure as the piston moved down its bore; these engines could not rev over about 900 RPM. By using a cam lobe to, also, control the intake valve it could be opened sooner thus the birth of overlap and late closing the intake using the inertia of the incoming mixture to offset the reverse pumping of the rising piston. For a mild production street type cam this calculation isn’t overly important because such cams close the intake valve pretty early after BDC. But for more performance from the cam the later in the compression cycle is the intake valve closed and there is more lift throughout the cycle so the valve is open more later into the closing cycle all of which increases reverse pumping also called reversion.

Reversion is active in the lower RPM ranges where the mixture velocity within the intake system is low, therefore, does not have much inertia. This displays as a lower end torque and horsepower loss and is a reason why big crammed engines need to idle fast and present a staggered sounding exhaust note. As the RPM comes up so does the mixture velocity induced inertia which at some point before the torque peak becomes a stronger force that the reversion pressure so it starts to force more mixture into the cylinders. This is what makes high RPM power possible without resorting to some form of compressor on the intake system. The downside with big to biggest cams is the loss of lower end torque and power as for a street geared vehicle this tends to be exactly where you want the cruise RPMs. A solution here is increasing the compression ratio.

Increasing the compression ratio gets more work from any trapped volume of mixture. So boosting the compression ratio with a big cam adds more lower RPM torque and power. It also carries the top end power while increasing mid RPM torque and plateauing its curve so more torque is generated over a longer period of RPM. Of course more torque yields more horsepower. Torque being a force and horsepower being a measure of work accomplished by that torque across a timed distance (RPM).

By test we know that a DCR of 8 to 1 is the effective minimum for a throttled engine. Throttling being a limiter on air consumption so the molecular density of the mixture changes with throttle position thus the internal operating pressure is independent from compression ratio to where WOT (Wide Open Throttle) is the only condition where compression ratio represents compression pressure. This compared to a diesel where the fuel fraction is controlled not the intake air throttled. So a diesel less the variable effect of the fuel fraction always operates with a compression pressure nearly equal to its compression ratio.

Where your at is an engine with an 8 to 1 SCR and we know that the DCR is always lower than the SCR, so even with the factory mild cam you engine is operating below the established minimum effective DCR so adding more cam timing only makes this efficiency condition worse. Back in the 1970’s the key technical driver was to get the NOx levels down to reduce SMOG and get acid rain production down as at that time the technology of converting this to N2 and O2 did not exist in a cost and space effective manor. Since the heat of combustion is what produces NOx the answer was to drop the combustion temperatures, that was done by lowering the compression ratio, and reducing ignition timing advance and adding EGR.

So the DCR is going to be what you are fighting as with the bigger cam the DCR will fall deeper into the inefficient mode of the efficiency curve. In the end yes you will reap more power that you currently have, but not nearly as much as you would have if the SCR was corrected to get the DCR up around 8:1 or 8.3:1. Going higher is possible but this gets into combustion chamber shape and head material. If this was a Chevy engine the quick answer for combustion chamber shape is the L31 Vortec, you can look on the web to see what that chamber looks like compared to your Pontiac chamber. The differences are very significant in their effect on the burn in terms of efficiency and detonation resistance. The other is head material in that aluminum’s faster rate of heat removal allows much higher compression ratios without getting into detonation.

I know this is a long and complicated read but for searchers of torque and power the devil is in these details. If you can harness these your fastest you can be under this you’re faster than it was originally but leaves a lot on the table unused.

You can run SCR/DCR calculations on WEB based applications like Wallace Racing calculator see the link. This is an iterative exercise as the SCR and DCR have a interdependent relationship. Behind the calculator is a lot of trigonometry as piston movement in the bore is based on opening and closing triangles whose corner angles are constantly changing, so the effective length of the piston movement in the bore is not linear to crankshaft degrees. It also means the piston speed varies with crank degrees.

This is the whole pile you will need both the static and dynamic calculators, lot of other math toys to play with at this sight and Wallace has a lot of Pontiac stuff to browse as well.


Cams that time and lift like the XE262H are close to as far as you can go with an automatic transmission and the usual high ratio rear axle that Detroit loves so much without going to a higher stall converter. The XE268H is right on that cusp it is better with a bit more stall and or final gear ratio but it will run decently with what the factory typically provides for stall and gearing. Manual gear box is more tolerant in that you can select a gear that better matches the engine’s power curve to the vehicle’s power needs. Automatics unless beefed for manual dont last long as how the power is moved through the transmission is different for manual than automatic controls as in manual a roller element sprag clutch is engaged that provides engine braking on a dropped throttle. These things are not designed for constant use and will fail if your used thusly. So stock automatic will not last when used like a stick transmission.

Do not confuse the terms overlap with the late closing intake. Overlap is another complicated discussion by itself. David Vizard refers to this as the 5th cycle of a 4 cycle engine.


Bogie
 

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Might I add one concern I have.
If indeed you have a flat lobe, then it’s likely all those fine little metal shavings are now dispersed throughout your motor. Everywhere including in the bearings.
any new cam you install is a waste of time and money. A complete motor tear down with complete cleaning, plus new bearings etc. Is what needs to be done.
I speak from experience. I put a new cam in a motor that the cam went flat in and 3 months later I had to replace everything. That was over 40 years ago. I learned my lesson early.
Best to do it right the first time.
 
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