This is very humbling; 30 years of working on cars had me convinced I was expert at something as seemingly simple as tuning a V8 Chevy. After making several modifications to our '72 LT-1 in the name of increasing performance (lighter advance weights, NAPA VC-1810 vacuum advance, larger Holley jets used on the '70 and '71 engines), the engine ran worse than ever, a situation that I couldn't remedy. Especially frustrating was the fact that those modifications have been discussed on this board and shown to be successful. So, what was I, an 'expert' (?!) to do. For the first time since we have owned the car (25 yrs), I brought it to a professional shop, a speed and performance shop, for some obviously qualified assistance, and basically said 'Help'. It turns out that they certainly could help, and the end result is that the car runs better than ever, and for what I consider the bargain price of only two hours of shop labor. The improvements were made with only timing and carburetor adjustments, but were beyond my scope of capabilities. I am literally in awe of what they did to the car, and only wish I had considered outside professional help earlier. My point is this: under the belief that I possessed a certain skill level that I in fact did not, I endured poor performance for too long. I learned that despite my misplaced faith in myself, the smart/right thing for me to do was to go to a place that does this type of thing for a living, and really knows what they are doing, and I got the results I was looking for. If my situation sounds like it might apply to you, look up and support your local performance shop; you might be very pleased. Larry, # 10731
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Re: Never Assume You Know Too Much
Larry,
I know what you mean. The more I learn about timing and carb adjustments, the more amazed I am that anyone's classic car runs well at all. As a PhD engineering professor that has rebuilt/restored cars for nearly 30 years, I thought I new a lot, but I encountered similar problems as you. I then met up with an amazing mechanic and he tought me a lot about adjustment options.
Since then, I have purchased an Innovate Fuel Ratio meter, a knock sensor, a Sun Distributor tester and a Sun MEA/LS2000 engine analyzer set. All of this was purchased via ebay for a total cost of less than around $800. For the past 2 years, I have been "tweaking" my timing curves and carb settings to achieve improved relationships on all of my cars.
I still have a lot left to learn, however I've accomplished more performance results in my last 2 years (with adjustments) than I did in my first 28 (from buying lots of speed parts).
Dave -
Re: Never Assume You Know Too Much
I assume you use the Fuel Ratio meter by attaching it to your exhaust to measure the amount of unburn exhaust emisions? The knock sensor is used to determine pre-detonation. The distributor testor to map your advance curve? What order do you make the adjustments in? Thanks, TerryComment
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Re: Never Assume You Know Too Much
I also know how you feel having a 65 365 hp i have learned to hate anything but gm parts, chevy knew what they where doing when they built an engine .the only part that i used aftermarket was a blueprint cam 30/30 duntov ,well i had plenty of problems tuning with this cam ,dont believe that these are the same cam, the lift and duration numbers are the same but the ramp lobes are what the company thinks they should be ,i would much rather have used what chevy offers in solid lifter pbobably the LT1 cam ,or just my opinion but the best cam chevy ever made for the smallblock the 327 350 hp 151 hyd cam that you can still buy over the counter from chevy and know that its the real deal it makes more hp and tourqe than the 30/30 duntov cam up to 5200 rpm .jus my opinion.good article!JRComment
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A "blueprint" cam used to be referred to
as an NHRA stock cheater cam. Same lift/duration but much faster ramps. Nothing like the original part.Comment
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Re: A "blueprint" cam used to be referred to
I hear ya mike ! thats a roger .JRComment
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Re: A "blueprint" cam used to be referred to
I used one of those for a short time in my '65 FI car, idled about 1500, woke up about 3000 and pulled like a freight train up to about 6500. About 3 tenths better in the 1/4 than my "real" 30-30 but essentially undrivable on the street.Bill Clupper #618Comment
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I became a true believer
in distributor timing after MANY years of hotrodding. Amazing how a slight bend in a couple of advance weight spring posts can turn a dog in to a tiger. Nothing like a Sun distributor machine to iron out a distributor. I have done many since then, and I have never seen one that was right when I started the job.Comment
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always "power time" the engine so you
that you have the correct "total" amount of timing in the engine.Comment
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Re: Never Assume You Know Too Much
Performance tuning of emission controlled engines can be problematic.
Most have relatively short and slow centrifugal advance curves, which combined with low initial timing usually results in the WOT maximum spark advance being below the optimum 36-38 degrees (for a SB). Attaining a proper WOT spark advance curve may require grinding the slot to increase centrifugal advance along with lighter springs to quicken the rate of advance combined with increased initial spark advance.
In the case of the low compression LT-1s you can use a very quick centrifugal advance if you're willing to use premium fuel. Use of regular unleaded grade fuel will place a limit on how quick the centrifugal advance can be before detonation sets in. The quicker you can achieve the full 36-38 degrees WOT total timing, the better the low end torque.
The VC1810 vacuum advance control is well matched to the LT-1's vacuum characteristics, but it MUST (REPEAT, MUST) be installed in conjunction with converting the OE ported vacuum advance to full time.
Carburetor idle mixtures must be set using pre-emission techniques, not emission control procedures such as "lean drop", and main jet changes may be required to get both cruise and WOT A/F ratios into proper ranges.
"Performance shops" offer a wide range of quality and knowledge. Many still disconnect vacuum advance controls because they don't understand their function or think that ported vacuum advance is best for a performance engine. Neither of the above philosophies is correct.
For early emission controlled engines, better performance, fuel economy, and less tendency to overheat can be had by setting up the spark advance and fuel maps similar to antecedent pre-emission control engines, but this involves more than just swapping the VAC and installing a different set of flyweight springs in the distributor.
The LT-1's antecedent pre-emission control engine is the '64-'65 L-76.
DukeComment
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Re: Never Assume You Know Too Much
Duke,
Thank you for the timing information you posted. I particulary noted your instruction that ported vacuum must be used. In the case of our car, the OE TCS equipment is no longer in place; the distibutor vacuum advance is plumbed directly to the carburetor, at it's original connection. At idle, that port is drawing vacuum. Is this the conversion you referred to?
Thanks again,
LarryComment
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Re: Never Assume You Know Too Much
No, full manifold vacuum must be used, not ported. I believe the vacuum source for most TCS equipped engines is ported, so there is no vacuum signal at idle.
On many emission controlled engine carbs you can tee into the choke vacuum break hose, which has full manifold vacuum including at idle, or there may be other full manifold vacuum sources.
Also, the ported source can often be changed to full time by judicious modification to the carb. The source must be below the throttle plate at all times including idle so full manifold vacuum is transferred to the VAC, including at idle.
DukeComment
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Re: Never Assume You Know Too Much
Duke,
I've heard also that the vacuum source may be ported, however, on ours, vacuum is present at idle; I can feel vacuum at the hose end.
I installed a new service replacement carburetor some years ago, and have made no internal modifications to provide full vacuum.
The vacuum hose routing on my engine has the choke vacuum break and the distributor vacuum advance both tee'd into the same carburetor port.
This is, I believe, the correct factory routing. Dobbins' 68-72 Fact Book has photos of '72 LT-1's with similar routing.
From what I've said, do you think my vacuum advance routing is for full vacuum?
Thanks again,
LarryComment
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Re: Never Assume You Know Too Much
Most OE emission controlled engines of that era have ported vacuum advance, and it was purely an emission control strategy, but there are many configurations, and I am not intimately familiar with all of them. With a few minor exceptions, all pre-emission engines have full time vacuum advance. The exceptions were short lived such as the '63 FI engine, which was changed from ported to full manifold vacuum in '64. Chevrolet learned that high overlap cams need a LOT of total idle timing because exhauast gas dilution from the overlap slows flame propagation speed.
A high overlap cam like the LT-1 cam needs about 25-32 degrees TOTAL idle spark advance to attain the highest idle quality, lowest idle fuel consumption, and the least heat rejection to the cooling system. This is usually provided by the sum of initial timing and full vacuum advance, plus maybe a few degrees of centrifugal if it has an aggresive centrifugal curve that starts below idle speed.
Choke vacuum breaks usually offer full manifold vacuum, and on C3s there should be full manifold vacuum sources that provides vacuum to the headlight and wiper door and power brake booster, and one should always verify that the VAC vacuum source is truely full manifold vacuum by cross checking it with known full manifold vacuum sources using an accurate vacuum gage.
The ideal spark advance map for a LT-1 would be 24-26 total centrifugal, all in as qwickly as possible with 10 to 12 degrees initial. This combined with a VC1810 VAC with full time manifold vacuum signal will provide the requisite 25-
32 degrees total idle spark advance and nominal 36 degrees total WOT spark advance.
Low compression LT-1s may respond well to slightly more total idle and total WOT spark advance because of higher exhaust gas dilution (due to the larger clearance volume) and a little more total WOT timing due to the larger chambers. This can usually be obtained by advancing the initial timing until you reach the high RPM detonation limit, and, of course, if you use premium fuel on a low compression LT-1 it will tolerate more total WOT spark advance
and a quicker centrifugal curve than if you use regular grade gasoline, however, in no case would I recommend exceeding 40 degree total WOT spark advance.
Idle speed should be "the lowest speed that yields acceptable idle quality", and this subjective measure will vary among individuals, but I suggest about 900 RPM as a starting point, and with 25-32 degrees total idle timing, manifold vacuum should be about 12" Hg.
DukeComment
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Re: Never Assume You Know Too Much
Duke,
Thank you very much for the time you have taken to respond to me, and for the amount of detailed technical information you have provided. I'm certain that you have forgotten more about this subject than I will ever know; and even if you haven't forgotten much, I believe that statement still holds true.
With appreciation,
LarryComment
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