Power: It can't really be this simple? - Pontiac GTO Forum
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post #1 of 9 (permalink) Old 10-21-2017, 10:42 AM Thread Starter
 
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Power: It can't really be this simple?

In trying to get my 68 running "better" I've been doing homework and planning some changes. Some of that reading involved studying these articles: Building A Strong Street Machine | Dallas Area Pontiac Association

Something REALLY jumped out at me - this quote:

"The only hop-up tricks that work without some penalty are; increasing displacement, increasing compression within certain limits, improving the exhaust system, and of course, a good tune-up of the carb and distributor."
With displacement and compression (generally) bigger/harder/more expensive changes, it kinda hit me like a brick: Exhaust, carb and timing. That's it?

Is it really that simple?

And, more specifically.... While I don't know the engine internals on my car (no reason to pull it apart), I do know that I have a Holley 650 and Edlebrock Performer. This article is making me think that maybe this isn't quite the best setup.

If I assume the intake isn't "hurting" me too bad, I wonder if perhaps swapping out the Holley for a QJet is something to consider?

Also - this is big, I just ordered the RAR Ram Air Manifolds and a 2.5" Magnaflow exhaust - I'm hoping this is not too much pipe diameter? I wouldnt think so, with my 68 400 YC block and heads from the same year. Thoughts on that?

That statement quoted above seemed to clarify all the crap in my head about planning performance upgrades....

Love to hear thoughts on all this....
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post #2 of 9 (permalink) Old 10-21-2017, 12:49 PM
 
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I would say that is a very loose general statement that applies to most all engine types. Every change made is merely exchanging one problem for another. The value of each change is directly related to moving closer or further away from your goals. That being said.....then it stands to reason that starting with a good engine architecture is beneficial. In the world of iron head/block, push rod, carbureted engines....the Pontiac design is pretty good. It has a strong bore to stroke ratio and good flow on the intake side. The 400 ci engines have a lot of torque and can rev pretty high so it makes for a good street engine. The 1968 engine you have is pretty good to work with. In my opinion the 3 items that could be improved for maximum effect are the cam, exhaust and axle ratio/limited slip. A good split pattern cam can help with the exhaust flow issue. Headers would be best but ram air type manifolds are a good substitute since they will be less likely to have clearance/access issues, less noisy and less maintenance. There are mandrel bent down pipes that are available to work best with those manifolds....you might want to check into that if possible.

Very hard to beat a properly set up Q-jet carb. They have great street manners and excellent WOT performance. One issue that I have experienced with them in years past is the small volume of fuel in the float bowl.....something that will become apparent if you change the gear ratio. That issue is relatively easy to resolve so it shouldn't be a major factor in your decision making.
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post #3 of 9 (permalink) Old 10-21-2017, 05:53 PM
 
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My 2-cents worth. The improvements are those things you want to do on your stock engine to increase power - free horsepower in a sense. You are fine tuning what you have.

The factory Q-jet and cast iron intake are probably the best bang for your buck - the factory engineers knew what they were doing. The Q-jet can be fine tuned, just as any Holley, ABF, or other carb - you just have to know how and what to do. So fine tuning the carb will pick up some free HP and get the carb running "crisp" when you stomp on it.

Improving the exhaust on a stock Pontiac, ie the factory cast iron manifolds and pipes/mufflers will again provide free HP. Going to the RA exhaust manifolds and 2 1/2" pipes is the way to go and get better breathing on the exhaust. A low restriction muffler is another plus. Many on the market, but you probably want to get one that is somewhat quiet, yet flows well. Many good flowing mufflers are noisy. Personally, I don't have a problem with a loud muffler as I like the sound of my engine/cam roaring out the pipes. So your choice of RA manifolds and 2 1/2" pipes are fine.

BUT, now that you have the exhaust system flowing real well, it can lean out the cylinders a bit because you have decreased exhaust backpressure and the speed & flow of the exhaust gases can suck out some of the air/fuel mixture (not talking anything major when I say this, so don't panic). So here is where you might want to go with a little bigger jetting on the carb or richen it up by making some adjustments to the Q-jet "hangers" on the secondaries if you were running a Q-jet. So this slight increase in fuel of course equals a tad bit more HP that can be fine tuned from the engine.

Now that you have a good flowing exhaust and a little more fuel at the carb, you now want to tackle the ignition. You want to maximize you initial timing at the balancer and then play around with the "advance curve" in the distributor. This is where you will adjust your mechanical advance and your vacuum advance to suit your engine and maximize your engines timing to go with the changes in the carb & exhaust. This has been covered a number of times in other posts on the forum. You can do a search to pull up past articles.

The combination of these 3 things, exhaust/carb/ignition timing are "free" horsepower on a stock engine without changing any major mechanical items like the cam/lifters, better heads, intake/carb combo's (assuming that the stock intake and Q-jet were in place), etc..
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post #4 of 9 (permalink) Old 10-21-2017, 08:39 PM
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Nice, that web site you went to? I'm the lucky guy that maintains it Those articles came from Jim Hand, the same Jim Hand who wrote this:

https://www.amazon.com/How-Build-Per...words=jim+hand

And yes, it really is that simple. An engine is nothing more than a big air pump. Make it capable of pumping more air, and you can make more power. All a "big" cam-shaft does is alter the rpm at which the "air pump" is working at its maximum efficiency, getting as much air into the cylinders as possible. Turbochargers and superchargers? All they do is force feed more air into the engine. The more air you can get in, the more fuel you can burn efficiently - and the more fuel you can burn efficiently, the more cylinder pressure you're going to get -- which in an engine translates into more torque. When you use a "big cam" to make the air pump more efficient at a higher rpm, all you're doing is taking advantage of the mathematical formula for horsepower. HP is a calculated value, and that formula is (torque x RPM) / 5250. So, if you have an engine that makes say 500 lb ft of peak torque at 3000 rpm, it's making 285.71 horsepower (at 3000 rpm). Put in a 'bigger' cam such that peak torque of 500 lb ft now occurs at 4500 rpm instead of 3000. Now through the magic of math, the engine is now making 428.57 horsepower! What changed? Peak cylinder pressure is the same, the engine still has a torque peak of 500 lb ft. All we did was move that peak to a higher rpm. Viola. That's all a cam change does - moves the torque peak around by changing the point where the engine is its most efficient.

Go look at any dyno sheet. You will 100% of the time find that the torque and HP curves cross each other at exactly 5250 rpm. That's because of the formula, (torque X rpm) / 5250 = horsepower. When rpm is 5250, it 'cancels out' that 5250 constant in the denominator. At 5250 rpm, every engine's torque and HP numbers will be the same. Always.

Back to your specific question. I'd bet that you'd pick up some torque by getting rid of that 650 Holley and replacing it with a QJet. Why? 650 is borderline too small for a good running Pontiac 400. Even standard model QJets were capable of 750 cfm, and some like the 455-SD version were capable of 800. However, there definitely is such a thing as having a carb that is too big. In order for all of its systems to work properly, a carb depends on a certain minimum air flow velocity going through it. Put on a carb that's "too big" for the engine its on, and the engine won't be able to generate sufficient air flow to make the carb work right. Same with headers - there's such a thing as 'too big' there too. Remember that air (and exhaust gas) has some mass, and therefore has some inertia (Newton's laws of motion). Get your header primary tubes too big, and it's possible for the exhaust gas velocity to get so slow that it will actually stop, and reverse back into the engine the next time the exhaust valve opens. Not only does this take up space in the cylinder that could otherwise be used for fresh air and fuel, now the engine has to expend energy to push this stuff back out. That's energy that won't be going to the rear wheels. It's a balancing act. You want header tubes that will help keep flow velocity up so that when that exhaust valve opens, the spent gas that's already in the header primary tube is still moving outward and will actually help 'suck' things out of the cylinder (this is what is called 'scavenging'), but you don't want tubes SO small that they start causing back pressure - now the engine has to do work again to push stuff out, and we already know we don't want that.

If you really want to get into the details of air flow in an engine, a fellow by the name of David Vizard has some great books out on the subject. Some of them can get pretty 'deep' in places, but I found them fascinating.

Bear
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post #5 of 9 (permalink) Old 10-22-2017, 06:37 PM Thread Starter
 
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That is a TON of great insight. Thank you!

After today I have some practical observations and (of course) a bunch of new questions to find answers to....what a journey.

I read in a sticky over at the Tuning forum by Lars: "First trick to any tuning, and I emphasize this constantly during the Seminars, is to tackle the ignition system first. Never, ever, try tuning a carb without first having tuned the timing curve. 90% of all carb problems are ignition problems."

With that in mind, I went out to Pep Boys this morning and bought a digital timing light. Then the fun began.

First, I checked initial timing to see where I was starting from. With my old fashioned light, I had measured what I thought was about 9 degrees. The new light showed it was a a bit over 10. Then I went to get high RPM. I set the light to 36 degrees and took the RPMS to about 27/2800. With the light at 36 and the RPMs at speed, I turned the distributor counter clockwise to bring the lines to "0."

Jeez - I hope that's the right process!

So, at that point, I have 36 degrees of timing. I turned off the engine and tightened up the distributor. By the way - That lock down nut is a total pain in the a** to get to.

Then, I turned it over again. Uh Oh.

It cranked and kicked and I hit the gas a couple times and it reluctantly fired, but didn't sound quite right. I turned it off.

I realized that I still had the vacuum off and plugged - put that back on and started again and it fired right up. With that start okay, I realized that the idle was noticeably higher - almost 1000. I adjusted the idle down to about 750 (light read 769). When I put into drive with the brake on the idle is about 650.

So....then I took it out for a test drive.

WOW.

Before this, if I went wide open, the car would bog out, then catch up. If I feathered the throttle just so, I could get the wheels to squeal and do a little burn out. Now, I can't get it to bog at all...and I tried. Now, when I give it about 50-75% of pedal throttle, it does a CRAZY balls to the wall burn out. It NEVER did that before.

After that test drive, I got everything buttoned back up. I did some errands to get some regular driving in to really get a feel for the new timing. Virtually every aspect of the driving is better. The aforementioned throttle response, regular driving at speeds from 20-50 mph seem smoother, car is actually quieter - it feels and sounds "smoother." Subjective, I know, but I can't deny it.

So - a really good day of tuning.

BUT.... I see now that I have a carb issue to tackle. Look at the attached pics. There is definitely gas dripping out the back of the carb. Not a lot, but it's there. It looks like the gasket is just spent, but it's tough too say. My plan was to replace the Holley with a Q Jet - I'm thinking this is the time to do it.

So - questions that come from this experiment:

1) I started with initial timing 10ish and total 32. When I made total 36 and retested, initial timing now 15! That seems really high - is it? My Dad (chevy small block guy) thinks we went too far (of course then I think of the crazy ass burn out I did). So... is 15 degrees of initial and 36 total sound like it could be right for my 400?

2) How concerned should I be about what is clearly a little seepage out of the carb? See pics.

What a fun day - learned a TON by doing, and had a blast doing it.

Thanks as always to you guys on here...
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post #6 of 9 (permalink) Old 10-22-2017, 09:12 PM
 
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"...is 15 degrees of initial and 36 total sound like it could be right for my 400?..."

It seems that most iron head Pontiac engines run better anywhere from 34-36 total mechanical advance.

The best initial advance is wherever it idles best. Some dist require a positive advance stop in order to get both the correct initial & the correct total advance. I think that most agree that the total is more important than the initial. Therefore they recommend finding the best total for your engine, then doing what needs to be done to your dist, in order to get the initial you want, without changing the total.

I ran about 13 initial in most of our drag cars. I've read of some who like as much as 16.

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post #7 of 9 (permalink) Old 10-22-2017, 09:35 PM
 
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Bredfan; " So... is 15 degrees of initial and 36 total sound like it could be right for my 400?"

PJ: Keep in mind that the outer ring an original harmonic balancer can slip IF the rubber that bonds it to the center ring has deteriorated or degraded. So if you did not (which I am sure you did not) build your engine and set the No1 piston at TDC and ensure the timing mark on the balancer was at "0", then you cannot be 100% that your timing mark is correct. But, we will assume it is correct.

If the engine fires up easily at 15 degrees, then it should be OK. Hard starting is usually and indication that the timing may be too far advanced.

36 degrees is good as long as you do not experience "pinging" or "detonation" which means you have too much advance and it will eventually damage the engine if it is not corrected. If you did not experience any "pinging" under wide open throttle, then you are good.

With regards to the carb, I am not a Holley man so just throwing out some guesses, but it does look like you have some sort of leak. It could be the gasket or gas coming out a warn throttle shaft (?). You may also be experiencing a flooding condition or even gas boiling over due to the alcohol used in the gas which can do this when the engine is hot. So you might want to check the float level and even the needle and seat to make sure all is well. Could just be time for a rebuild to freshen things up.
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post #8 of 9 (permalink) Old 10-22-2017, 09:40 PM
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That's the way I always set my timing. I set the total mechanical to where the engine is happiest, and just let the initial land wherever it lands. As long as it runs good and isn't hard to start, doesn't overheat, etc, it's fine. I'm glad you're having fun and success!

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post #9 of 9 (permalink) Old 11-01-2017, 09:04 PM
 
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Bredfan,

Welcome to the power zone... I just had mine rebuilt and have had a great time tuning and playing with mine.

Have a ball and as someone told me, I hope you get to find out what the difference between a tire on fire and a burnout smells like. LOL

*67K miles... Under Construction* :)
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