Restoring '72 LT-1 Engine

[Patrick H.]

Re: Restoring '72 LT-1 Engine

If you're going to reinstall your original heads, seriously consider some mild headwork as Duke has described here extensively in the past.

Patrick

[Clem Z.]

throttle responce

feels better because you are pulling more air thru the stock size ports that were designed of less cubic inches and the velocity is up.

[Duke W.]

Re: Restoring '72 LT-1 Engine

Joe - you've mischaracterized the LT-1 cam. It idles at about 12"@900 on a 3.25" stroke configuration. On a 3.75" stroke configuration it will produce about the same idle vacuum at about 750-800. It's all about mean piston speed.

You cannot compare the .050" lifter rise duration of a mechanical lifter cam with a hydraulic lifter cam because part of that first .050" lifter rise on a mechanical lifter cam is clearance ramp and the valve is not moving. On a hydraulic lifter cam virtually all of the lifter rise is turned into valve lift.

Since the LT-1 has E/I clearance ramp heights of .012/.017", the .050" lifter rise ABOVE THE TOPS OF THE CLEARANCE RAMPS is what must be compared to a hydraulic lifter cam, so the comparable lifter rise is .062/.067" and these durations are 231/239, so the (inlet) duration is a little less than the Edelbrock hydraulic lifter cam. The BIG DIFFERENCE is overlap since the LT-1 cam LSA on about the same duration is 116, so it will be much more friendly to typical vehicle exhaust system back pressure. Also the Edelbrock cam is too far "advanced" for a long stroke configuration.

As I have said time and time again, most aftermarket cams have TOO MUCH OVERLAP for a street legal exhaust system - even one that provides modest back pressure.

Mark Johnson's '65 L-76 with massaged OE heads, 10.5:1 CR, and LT-1 cam - everything else OE equivalent - makes 80 percent of peak torque at 2100 and 90 percent at 2500 while making close to 300 SAE corrected RWHP at 6500 and useable power to 7200.

On a 3.75" stroke configuration, the torque curve will be shifted up in magnitude and down the rev scale so peak useable power would be at about the same 4000 FPM mean piston speed, which is about 6200. Retarding the LT-1 cam a few degrees from its 110 deg ATDC inlet POML will shift the useable rev range into the mid/high 6000 range at a slight loss of low end torque, but low end torque will still be substantially greater than the OE low compression LT-1 configuration and achieve the 80 percent of peak at 2000 criterion for a good, responsive high performance road engine.

Duke

[Clem Z.]

to compare hyd to solid lifter cams

cross reference is about 8 degrees. hyd lifter cam 222 degrees @.050 equals a solid lifter cam at 230 degrees @.050

[Joe R.]

Re: Restoring '72 LT-1 Engine

Hi Scott:

I would be inclined to leave the existing 383 short block as-is, with, of course, a new cam. If you are interested in making the engine look and sound more NCRS correct, you will need to change to a mechanical lifter cam. My sense is that a duplicate of the original LT1 cam, or something slightly less aggressive, would work fine for your stated goals. The larger 383 displacement will be more tolerant of the LT1 cam timing than a 350. In any event, it sounds like the LT1 cam is milder than the one you presently have, so it's probably a move in the right direction for your stated goals.

As long as you aren't trying to get the maximum possible power, now would be a good time to rebuild and reinstall your stock heads. With the intake already off, it's pretty tempting to just swap the heads while it is convenient. As Duke and others have pointed out, you could consider having the heads pocket ported to help close the gap between the stock heads and the AFRs, but if I take your stated goals at face value, you don't really need the marginal power benefit that you would gain from this expense.

As Duke noted, a key element that should be evaluated in the head swap is the combustion chamber volume and the effect on compression ratio. Be sure that you know what you presently have and what you will end up with if you swap the original heads back in. A knowledgeable engine builder can readily make the required measurements for you.

If you change back to the original heads and an original-type cam, the end result would be an engine that is very close to the original appearance and sound, and one that likely would judge reasonably well. The remaining differences (carb and ignition) are easy to change later if desired, and could be left as-is for NCRS judging if you don't mind some modest deductions.

[Mark #28455]

heads don't matter much with a stock exhaust

If you are going to run the stock exhaust manifolds and repro style exhaust/mufflers, you will be surprised how little difference a head swap alone will make. If you do various engine builds and just swap one component at a time, you will see there's not a whole lot of difference, that's why magazines like Hot Rod and Super Chevy and others ALWAYS swap heads and cam together so the new cam matches the heads intended power band. You quickly LOSE advertising $$$ if you show an aftermarket head is only good for 10 or 20 HP at a cost of over $1000!

The best example was when Dart first released their Merlin cast iron BB heads. We found that when swapped vs. GM rectangular port open chamber heads, they were worth about 8 to 10 HP even with their raised exhaust ports.

I'm willing to bet that if you did dyno runs and swapped the heads alone, up to about 4000 RPM the engines would be within about 15 HP (with stock exhaust) and you would start to see a divergence in the power bands only above 4000 RPM. Do you really run the car much above 4000 RPM?

Mark

[Duke W.]

Re: to compare hyd to solid lifter cams

It depends on the height and velocity of the clearance ramp. Here are the durations at .050" lifter rise above the base circle, clearance ramp heights, and .050" lifter rise durations above the tops of the clearance ramps for the production mechanical lifter cams.

Duntov: 228/231, .008/.012, 220/220
30-30: 254/254, .017/.017, 231/231
LT-1: 242/254, .012/.017, 231/239
L-72: 242/242, .012/.012. 231/231

The Duntov cam lobes are the same except the exhaust clearance ramp is .004" higher. At every point on the opening and closing flanks exhaust lift is exactly .004" more than inlet lift

The LT-1 cam uses the L-72 lobe with a slightly smaller base circle on the inlet side and the 30-30 lobe on the exhaust side advanced 4 degrees relative to 30-30 cam indexing.

The only why to accurately compute the .050" lifter rise above the base circle is to have the detailed lobe data such as what is on the GM cam drawings - lobe rise to five decimal places every cam degree. The Duntov inlet side difference is only 8 degrees given the relatively low .008" clearance ramp. The 30-30 clearance ramp is about double this height and the difference of 15 degrees is also about double.

For aftermarket cams it can only be estimated based on what can be inferred from clearance recommendations and using a typical clearance ramp velocity of .0002 in/crankshaft degree.

Duke

[Duke W.]

Re: heads don't matter much with a stock exhaust

Improving head flow by reworking the OE heads or going to higher flowing aftermarket heads is the best way to get "more power" without killing the low end torque or changing idle characteristics.

On OE SHP configurations the top end power limiting factor is always head flow, not the valve timing, but a ten percent improvement in inlet flow on medium performance engines will also yield about the same increase in top end power and an equal increase in useable revs without killing the low end torque or affecting idle quality.

Peak torque is basically a function of compression ratio and displacement, so head flow and valve timing are not significant factors. For example, the 327/250 and 327/375 FI engine both have the same OE peak torque ratings of 350 lb-ft, but it occurs much higher in the rev range on the 375 FI. Low end torque is highly affected by overlap and the inlet valve closing point. The high overlap and late closing inlet valve of SHP cams reduces low end torque and degrades idle quality. The only way to achieve more power at low and medium revs is to increase displacement assuming CR is limited by fuel octane.

Also, since reworking heads improves exhaust flow more than inlet flow, the E/I flow ratio is considersably increased, which means that optimum exhaust duration is LESS than inlet duration.

Clearly, the aftermarket and hot rod magazines do not understand this since I know of no off-the-shelf aftermarket camshafts that have less exhaust than inlet duration.

Valve timing is just a "tuning parameter" - like the spark advance map - and it should be tailored to head flow and the desired torque bandwidth and idle characteristics.

Duke

[Joe C.]

30 - 30 Correction

Duke:

Should be:

30-30: 254/254, .017/.017, 239/239

Joe

[Duke W.]

Re: 30 - 30 Correction

Correct, thanks!

Duke

[Scott S.]

Re: heads don't matter much with a stock exhaust

Well, thanks to you guys I've decided to stick with the 383 bottom end and probably re-install the old heads if funds permit. Good news is if I go with the old heads I'll have a pair of AFR's to sell which will help to offset the cost of cleaning up the old ones.

I must confess that a lot of what's been said with respect to camshafts has gone a bit over my head but I think I'm getting that either the Edelbrock or the LT-1 cam will work well with both the original and the AFR heads. So I can't go too far wrong with any combination so long as I ensure the swap to the old heads doesn't alter the compression ratio significantly. Funny thing, when I spoke to the previous owner about the cam a while back he told me that some knowledgeable guys told him after he built the engine that it would've been better with the LT-1 cam. Not sure if that means it'll make more power or just make it at more usable rpms.

Thanks again for the help! Cheers.

[Scott S.]

Re: heads don't matter much with a stock exhaust

Is it possible to find a solid roller cam with the same spec's or very close to the LT-1 cam? Also, is this an easy retrofit/upgrade? Someone told be that the block would have to be modified.

Thanks again.

[Joe R.]

Re: heads don't matter much with a stock exhaust

Hi Scott:

I have not checked, but I'm sure you can find a solid lifter roller cam that has specs similar to the LT1 cam. Hopefully, others on this board will chime in with some suggested parts. Based on what you have told us, all you really need is a mechanical cam (for the correct sound of mechanical lifters) with timing that is reasonably similar to the LT1 cam timing (for similar idle characteristics). Unless the timing of the cam is WAY off from the LT1 cam, it is unlikely to attract attention during NCRS judging.

For your stated purposes, the only benefit of a roller cam is that it will be less likely to wipe cam lobes with today's modern passenger car oils. Roller cams can have some modest performance benefits too, but that is not your focus.

The main disadvantage of a roller cam is that the cam and lifters cost a lot more, and it takes a little more work to install one. You will not have to machine the block, but the end of the cam behind the timing cover has to be fitted with a "cam button" to restrict fore and aft movement. It's not a big deal.

The subject of wiped cam lobes and low-zinc oils has been discussed extensively on this board, and the discussions can be found in the archives. Just search on Duke Williams and oil.

Duke will probably comment here, but I think you will find that his bottom line is that it is FINE to use a flat tappet cam IF you do ALL of the following:

1) Use a duplicate of an original GM cam that has GM's original lobe shapes, since these typically have less aggressive ramps than aftermarket cams.

2) Use valve springs that match GM's original specs (aftermarket springs are typically much stiffer).

3) Carefully follow the installation and break-in procedure for the new cam.

4) Use a high zinc oil such as those specified for diesel engines.

So, if you use a duplicate of the LT1 cam and you follow the other steps listed above, you are very unlikely to have wiped lobes and you will save a lot of money compared to a roller cam.

On the other hand, the only down side of the roller cam is the cost. Roller cams are less sensitive to break-in and to zinc content in the oil, but the cost is considerably higher.

I hope the above has answered your questions and provided a bit more insight into the tradeoffs. I'm sure that others will comment as well.

[Scott S.]

Re: heads don't matter much with a stock exhaust

Thanks Joe. You're absolutely right, I don't need the performance benefits of a roller, just looking for durability and less vulnerability to the downside of modern oil. To some a wiped cam might mean a day (or two) in the garage, for me it's been a much bigger deal finding someone to fix it, working to his schedule etc, not to mention the added cost of having someone else do the work. So if a roller is $500 more and twice as durable it may cheaper in the long run to go for broke now.

Then again, I may be over-thinking this and maybe I should just order a case of racing oil and be done with it. :-)

[Duke W.]

Re: heads don't matter much with a stock exhaust

There is no such thing as a "roller cam that has similar specs to the LT-1 cam" or any other flat tappet cam. By design a roller cam has much more aggressive flanks, which results is very different valve dynamics so a roller cam with the same .050" specs will produce very different engine characteristics. Mechanical roller cams are racing cams that are totallly unsuited to a road engine.

It's possible, but I doubt it, that a hydraulic roller cam exists that may produce a torque curve and idle characteristics similar to the LT-1, but it will have very different "specs". If you have the time and budget to test a couple of dozen cams, let us know what you find.

I repeatedly fail to understand why guys want to "upgrade" vintage engines to "modern" hardware. The cost is high and the benefits are marginal. If you want a "modern" engine, just throw away the numbers matching vintage engine, install a LS crate engine, be done with it, and enjoy your hotrod.

Most vintage engines are very well engineered - even by modern standards. Considering the design and development tools that were available decades ago compared to today, the results they achieved back then are truly amazing.

Some vintage engines have weaknesses and some can benefit from slightly later OE parts in addition to head work, which is all important, and careful attention to assembly detail.

A OE spec cam built to OE specs with correct OE replacement springs WILL NOT WIPE LOBES even if you use the wrong oil, and if you use CJ-4 oil the probability goes down even further, even if you don't do a "cam breakin". It's the aftermarket cams that require gorilla valve springs that wipe lobes as is very likely the case with your situation.

Unless you're a qualified and experienced engine system engineer, stick with quality OE replacement parts and you will not have any problems as long as assembly is correct.

Much discussion in the archives on these subjects.

Duke