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Low Compression Street Engine's & Tuning Techniques Idea's...How Too's on low compression Engines.

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Old 09-30-12, 04:44PM   #1
boomers T/A
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Default L-78 with a XE 274

2200 stall 3.42 gear and a 3600lb car. If tuned right will it still be a dog?
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Old 10-01-12, 11:06AM   #2
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Can't say. Is it a "dog"?

We've used XE274H in such applications. Most customers are VERY happy with the results. Your converter might be a little "tight", but probably not.

Jim
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Old 10-01-12, 09:31PM   #3
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Can't say. Is it a "dog"?

We've used XE274H in such applications. Most customers are VERY happy with the results. Your converter might be a little "tight", but probably not.

Jim
2400? don't want a huge convertor. Just figured this cheap combo would ge me going
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Old 10-02-12, 12:35AM   #4
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Did you already buy it? I would like to see a smaller cam in that combination but with the right tune you can make it work with your low compression. I would prefer something along the lines of a single pattern cam between 214-218 intake duration @ .050". Build that low rpm cylinder pressure and let your gearing take care of the top end!

Good Luck,
Paul
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Old 10-02-12, 11:21AM   #5
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"2,400" is not "huge". 3,500 for a streeter could be...

Pontiac has always used a dual-pattern cam (even the 2-bbl. engines). Only in circumstances where the exhaust has been upgraded MORE than the intake, should a single parttern be considered. We've significantly improved performance in engines with the old "Magnum" grinds (all single-pattern) by using 1.65 rockers on the exhaust side. Pontiac heads don't enjoy the same level of exhaust port their main GM rival does (but we have a better intake port!).

FWIW

Jim
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Old 10-03-12, 12:49AM   #6
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Default Is This L-78 Stock?

Perhaps I have assumed too much. I assumed he had a STOCK 8.2:1 400 with a restricted secondary intake, stock cylinder heads, and exhaust manifolds.

If this is indeed a low compression engine saddled with mostly stock components, street friendly torque convertor, and mild rear gearing, I prefer to utilize relatively short cam timing to build low rpm cylinder pressure, maintain good driveability with a stock or mild torque convertor, and utilize a conventional ignition curve.

To the O.P., if you want to use a longer cam such as the 274 with a LOW compression ratio such as 8.2:1 (if your engine is indeed stock), have your distributor recurved to give you somewhere in the range of 20-26 degrees initial timing with a total of 36 all in by approximately 2,500. Utilize vacuum advance, ported or manifold (10-15 degrees), whichever the engine likes best. You won't need the vacuum advance as a crutch if you have enough initial. Talk to a good carb guy, I hear Cliff is good, because you will have less vacuum so you WILL need to recalibrate your carb's part throttle calibration. Check your plugs to see if they are loading up at idle and at cruise. If after all that, you find that you need just a little more low end, advance the cam ANOTHER 4 degrees from the stock 106 intake C/L.

OR.... you can go with a milder cam and set your timing at 12 degrees initial and drive away.

I agree, Pontiac DID indeed utilize dual pattern camshafts in MOST of their engines. It was found back in the early sixties when experimenting with camshafts for NASCAR that more top end was achieved. There is no doubt in applications where the end user wants some level of rpm capability AFTER peak power, a typical dual pattern camshaft with a exhaust lobe 6-10 (or more) degrees than the intake will be of benefit. If one is trying to maximize torque at a lower rpm, then a single pattern or possible a reverse split can be utilized. I haven't done a reverse split on any of my engines but I have heard some good things.

I will admit that our intake ports are bigger than a stock small block chevy intake port, but that's it. There are just too many variables to consider such as the size of the engine being fed by those ports, the deck height, stroke (piston speed), location of the choke within the port, combustion chamber shape, shape of the short turn radius. If our ports were that good, then we wouldn't need High Ports, Pro-Ports, Wide Ports, and dare I say, CV-1's.... all radical departures from our stock intake port.

IF YOUR L-78 ALREADY HAS 9.5:1 COMPRESSION, AFTERMARKET INTAKE, AND HEADERS...... nevermind.

Good luck with your upgrade!
Paul
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Old 10-04-12, 12:17AM   #7
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Quote:
Originally Posted by paul View Post
Perhaps I have assumed too much. I assumed he had a STOCK 8.2:1 400 with a restricted secondary intake, stock cylinder heads, and exhaust manifolds.

If this is indeed a low compression engine saddled with mostly stock components, street friendly torque convertor, and mild rear gearing, I prefer to utilize relatively short cam timing to build low rpm cylinder pressure, maintain good driveability with a stock or mild torque convertor, and utilize a conventional ignition curve.

To the O.P., if you want to use a longer cam such as the 274 with a LOW compression ratio such as 8.2:1 (if your engine is indeed stock), have your distributor recurved to give you somewhere in the range of 20-26 degrees initial timing with a total of 36 all in by approximately 2,500. Utilize vacuum advance, ported or manifold (10-15 degrees), whichever the engine likes best. You won't need the vacuum advance as a crutch if you have enough initial. Talk to a good carb guy, I hear Cliff is good, because you will have less vacuum so you WILL need to recalibrate your carb's part throttle calibration. Check your plugs to see if they are loading up at idle and at cruise. If after all that, you find that you need just a little more low end, advance the cam ANOTHER 4 degrees from the stock 106 intake C/L.

OR.... you can go with a milder cam and set your timing at 12 degrees initial and drive away.

I agree, Pontiac DID indeed utilize dual pattern camshafts in MOST of their engines. It was found back in the early sixties when experimenting with camshafts for NASCAR that more top end was achieved. There is no doubt in applications where the end user wants some level of rpm capability AFTER peak power, a typical dual pattern camshaft with a exhaust lobe 6-10 (or more) degrees than the intake will be of benefit. If one is trying to maximize torque at a lower rpm, then a single pattern or possible a reverse split can be utilized. I haven't done a reverse split on any of my engines but I have heard some good things.

I will admit that our intake ports are bigger than a stock small block chevy intake port, but that's it. There are just too many variables to consider such as the size of the engine being fed by those ports, the deck height, stroke (piston speed), location of the choke within the port, combustion chamber shape, shape of the short turn radius. If our ports were that good, then we wouldn't need High Ports, Pro-Ports, Wide Ports, and dare I say, CV-1's.... all radical departures from our stock intake port.

IF YOUR L-78 ALREADY HAS 9.5:1 COMPRESSION, AFTERMARKET INTAKE, AND HEADERS...... nevermind.

Good luck with your upgrade!
Paul
theortical cheap build.......stock compression. That's the reason I'm considering A XE 274 I understand they "boost" compression with fast ramp technolgy and shouldn't be used in anything over 9:0 to 1 compression. I would like to splurge and get ram air exhaust manifolds and true 2 1/2' dual exhaust. Yes I was thinking a Cliff's carb......
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Old 10-04-12, 01:00PM   #8
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True enough, the X/E cams have aggressive ramps but to split hairs, they don't "boost" cylinder pressure. For a given .050" duration, they allow a combination to "not lose as much" cylinder pressure by virtue of their shorter advertised duration. Ex.: Let's say you have a Magnum camshaft with 280 degrees duration @ .006" and 230 degrees @ .050", an X/E will have a shorter 274 degrees duration @ .006" whereas the .050" numbers will still be 230. Durations @ .200" lift are 140 (Magnum) and 143 for the X/E lobe that most people run. See the trend? As the lift goes up, the valve is staying open longer AT THAT LIFT, allowing more air/fuel into the cylinder.... ALL WHILE USING A SHORTER ADVERTISED DURATION. That shorter advertised duration is what the engine sees when it comes to cylinder pressure so that is why it can be run with a lower compression ratio. Imagine attempting to run a RA IV cam with its 230 duration @ .050" in a 8:1 combo.... IT HAS 308 DEGREES ADVERTISED DURATION!

As far as ONLY using the X/E cams with low compression, just like any other camshaft out there, you must match the duration with the engine configuration and intended usage: A very short cam will work in a low compression engine (closing the intake valve early for low rpm cylinder pressure) just as a high compression engine can use/will need a later closing intake for a given octane. The by-product of the later closing intake will be an increase in top end power with a slight reduction in low end torque. IF the engine is built right, you won't lose much at all down low and you will gain MEASURABLE top end power. Otherwise, if you were to just install the cam retarded in a given combination, you will lose measurable low end, but only gain marginally up top. It all goes back to the "combination", change one thing, like going to too long of a camshaft for the compression, and other necessary changes must be made to get it to work.

If I want a rough sounding idle but still retain low compression street manners, I will just order a custom cam with (SLIGHTLY) tighter lobe centers.

I realize I spoke mostly of duration, and while there is a lot more to camshaft science than just duration and lobe separation, I feel it is too easy to get caught up in all the jargon and overthink everything when it comes to street applications. But to be fair, it's also VERY easy to UNDERTHINK things and just throw duration at a combo when it comes to STREET/STRIP combos. Ex; 250+ duration @ .050" with ported heads yet a guy can't get his engine to make power past 5,800 rpm..... SO HE GOES BIGGER! Oh well.

I hope this helps,
Paul

P.S. That example does not pertain to any certain person out there. If you know someone out there like that.... sorry.
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Old 10-04-12, 11:57PM   #9
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all right throw a recommedation out there.........would like to have power to 5000 rpm's, decent vaccum, healthy sound, not a race car by any strech of the imagination.
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Old 10-05-12, 12:41PM   #10
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PM sent.

For anybody else following the thread, it's a 218@ .050". Boomer has the lobe numbers, LSA, I/C so he can order it from anyone.
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Old 10-05-12, 02:50PM   #11
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XE262H... 218/224 @ .050. We use it as a "replacement" cam in T/As and GTO engines with compression under 9:1.Workls great with a stock converter, pulls hard to 5,400 in a 400. One of our customers with an 8.5:1 406 has chassis-dynoed his car. He has Hooker "Comps" and Performer, with a "Cliff's Q-Jet". Put 276 to the rear wheels. Car ('68 Firebird convertible) went 13.80s and dipped into the'.70s in hot weather, 100% street (rear tires, 2.25-70/R15 BFGs). Happy man. This cam has more of a "gurgle" than a "lope". Sounds a LOT like an 068. Much better vacuum and low-end than 068.

Jim
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