Scott333

I met with my engine builder recently, after the engine was disassembled, to go over the goals again and decide how to proceed. It didn’t go at all how I expected. After thinking about it that evening, I called and asked him to stop everything until I figure this out. After giving it some more thought, almost all of the disagreement between what I want to do and what the engine builder says we need to do comes down to a disagreement over Compression Ratio and premium pump-gas.

What I read here (and on the NCRS forum) is that people are rebuilding these old 327’s at 10:1 CR or higher, with OE equivalent parts, with no problems running pump-gas. The engine builder says that won’t work, which leads to an entirely different kind of engine rebuild design and philosophy. I have never rebuilt an engine before, but I am responsible for deciding what parts to use, and deciding the general approach (factory spec & OE equivalent parts vs. everything else) to reach my goals.

Any advice or comments, either in a specific area or in general, would be very much appreciated.


The car:
1967 coupe, L79 (327/350), M21, 3.70 with A/C, trying to add “factory” power steering if I can figure out how to use deep-groove pulleys (seems to be a problem with L79-A/C-PS combo).


Overall objective:
A strong, reliable “daily driver” type engine that will run on premium pump-gas without any problems and last for many years and miles. I am hoping to accomplish this with an engine that looks, sounds and performs like a 1967 Corvette did when it was new in 1967. I don’t expect to race the car, but I do like winding through the gears under hard acceleration. Under casual driving conditions, I would like to get around 15-16 mpg in city driving. (Power Objective below)


External appearance:
Stock, except I’m leaning strongly toward 2.5” rams horn exhaust manifolds, to go with new Corvette Central N11 off-road exhaust pipes & mufflers. Stock Holley carburetor, L79 aluminum intake manifold, 462 heads, aluminum Corvette-script valve covers, stock alternator, stock starter motor, stock A6 A/C compressor, DeWitts restoration radiator, stock fan & fan clutch, etc.


Internal parts:
Original L79 cam grind, unless there’s a good reason not to. Forged pistons (.030 over, not sure on domed vs. flat, due to Compression Ratio concerns, comments below). OE spec valve springs - 3911068 or Sealed Power VS677. OE or equivalent for nearly everything, and I see ARP bolts and screw-in studs recommended for added strength/durability. If I understand correctly, the original rods should be fine if they Magnaflux okay, otherwise Crower Sportsman for strength and nearly identical weight. 3-angle valve seats, hardened valve seats for unleaded gas (if it doesn’t hurt anything).


Power objective:
If I understand correctly, in 1967 the L79 327/350 had around 250 actual rear wheel horsepower. If I can rebuild the engine to run safely on premium pump-gas, with OE equivalent parts and the L79 cam, and get the original rear-wheel horsepower the car had in 1967, that would be acceptable, but I would sure like to do better. If I can move significantly closer to 300 rear wheel horsepower using the original hydraulic L79 cam, while keeping a broad torque-bandwidth (90% peak-torque by 2,500rpms), I would like to do that.

After reading many Threads here in the archives, it seems the way to accomplish this is:

• build to OE design specifications, except lower CR from factory rated 11:1 (probably an actual 10.5:1 from the factory) down to about 10:1
• help the engine get more air by porting the heads / un-shroud the valves, 3-angle valve seats, matching gaskets properly, and port matching
• help the exhaust flow better with larger 2.5” manifolds, pipes and low restriction mufflers
• use the original L79 (327/350) cam grind
• advance the cam by 4° (to increase low-end torque, if I understand correctly)


Sounds fairly straight-forward, in theory.


Compression Ratio: The BIG problem?
I have struggled a lot with this, reading the archives both here and at NCRS. I want to build the engine as close to OE spec as I can, with as high of a compression ratio as I can safely run, because that is how the system was designed to work with the L79 cam.

If I understand correctly, the 11:1 CR rating for the L79 engine was probably closer to 10.5:1 when new. Besides reading the archives here on the Forum and Duke’s CR article in Corvette Restorer recently, my understanding of engine compression ratios is very limited. I don’t want to push the envelope too far and end up having to add race gas to boost octane. From what I have read on the Forums, from people who live and breathe ‘60s small-block Corvette engines everyday, a true, measured 10:1 SCR engine on premium pump-gas shouldn’t be a problem.

When I talked to an engine builder who has been building racing engines for 20+ years, practically every thing I had learned about rebuilding an old 327 Corvette engine was called into question, and then rejected. I need to understand why.



The engine builder has an excellent reputation. To the best of my knowledge he builds mostly racing engines. I don’t doubt his skill, knowledge or ability. This is what he does for a living, and he has been doing it successfully for many years. He builds racing engines for people who use them for that purpose, and they expect to be competitive. I don’t want to put words in his mouth, so I want to be clear that what follows is my recollection of our discussion, and I’m not meaning to come across negatively toward the engine builder at all, or make the engine builder look bad (he’s very good). I’m hoping to gain some understanding, so I can figure out what to do next, because I have some important decisions to make.

What I took from our meeting is that he doesn’t believe the engine can run safely on premium pump-gas with a 10:1 compression ratio, that 9.2:1 with the stock heads, or 9.7:1 with aluminum heads is the maximum. I said it was my understanding that the original L79 cam bleeds off a lot of compression, which is why it [i]should[i/] run fine on premium pump-gas at a measured 10:1 SCR, because the dynamic compression ratio (DCR) will actually be much lower, around 7.5:1.

I didn’t understand his reply because it was way over my knowledge level, but he didn’t seem to agree that there was anything special about an L79 cam compression “bleed-off” characteristic that would allow me to run an engine with 10 or 11:1 CR on premium pump-gas. He didn’t believe 2.5” exhaust manifolds or porting the heads will achieve more horsepower without killing the low-end torque, where most of my driving will take place, or that advancing the cam 4° will effectively compensate for any torque-loss from ported heads & 2.5” exhaust manifolds.

I was hoping to achieve 80% peak-torque by 2,000rpm, and 90% peak-torque by 2,500rpm, according to many rebuild Threads and dyno sheets in the archives. He said if we build the engine to do that, it will be all done by 4,500rpm. I thought that pocket-porting the heads, port-matching and gasket matching, with better exhaust flow through 2.5” manifolds should extend useable power by 500 - 1,000rpms, up to 6,000 to 6,500rpms. His response was basically, no way.

I explained that according to many people on the Corvette Forums, this is the approach (as best as my limited ability is to describe it) taken to achieve these objectives, and they post their dyno sheets. I have seen them, at least one guy here has his own chassis dyno. The engine builder said he reads the magazine articles too, but there’s a difference between magazine articles and the real world, and we can’t change physics. I realize some of the magazines appear to inflate or exaggerate on some things, and I understand we can’t change physics. But people here are using computer simulation programs, then testing various configurations, and most importantly, posting their real-world dyno results. Some people here have HIGHER than 11:1 compression and drive their cars regularly on premium pump-gas without problems.

I believe the accounts in the archives where people say they are successfully rebuilding their engines according to the general principles outlined above. And I believe the engine builder is giving me his honest opinion, based on his own years of experience. I just don’t know how to reconcile such diametrically opposed experience and results.

The engine builder does believe in 3-angle and hardened valve seats, an aftermarket cam that is specific to whatever the engine needs (I don’t know how to determine what the engine needs), and if I want more power and torque out of a 9.2:1 compression ratio engine that will run on premium pump-gas, he says one way to do it is with a full roller-rocker conversion to take advantage of the newer high-profile roller-cam technology, if I understood him correctly.

So I left the meeting much more confused than when I began. The original numbers-matching engine has been disassembled and hot-tanked (I think that’s the process used). I was very specific about not blocking the deck. No machine work has been done. It’s sitting at the engine shop while I figure out what to do. And that’s where things stand.

Do I need to find an engine builder who specializes in 60s Corvette SHP small-blocks, someone who will understand what I’m trying to do, why I’m trying to do it that way, and how to do it? Or am I missing something entirely? Is it just a communication problem, because I don't speak engine language very well? If the physics don’t change, how could a specialist Corvette small-block engine builder make this OE design approach with relatively high CR work on premium pump-gas when a racing engine builder says it won’t?

Thanks in advance to anyone who has read this far, and thanks to anyone who can help me figure this out and point me in the right direction,

Scott

NEVERL8

Scott,

You post was like reading my own L79 rebuild almost to the tee. I am not kidding you. I had almost exactly the same issues, questions and objectives. You will find an immense amount of opinion on compression (static and dynamic), octane and ignition issues. Mu engine was also built by a well reputed racing engine builder in NH. They look at things different (meaning much more precision and robustness) than the "normal" engine builders I talked to.

Here is my post on the subject. It is in the C1/C2 FAQ section.

http://forums.corvetteforum.com/c1-a...and-specs.html

If I were to do it again I would have not gone quite so overboard on the costs but it still would have been a $6K engine if I already had the block.

MikeM

The 11-1 advertised compression was just that. Advertised. That would be the best you could probably get machining everything to the minimum side of the spec.

Reality- If your heads haven't been shaved and the block not decked and you use the thick (039?) head gasket, and the domed pistons on the rebuild, your compression will be just fine and your engine will run well on 93 octane fuel. Even if your heads have been surfaced (to clean up) and the block decked for a clean up, you still should be okay.

The only real way to find out what you have is to check the deck height of the pistons when you assemble the block and cc the heads after a valve job then calculate the compression.

I'd strongly suggest you buy the OEM replacement, forged, domed pistons and go from there. It's easier and cheaper to cut compression than it is to add it later. Even if your build resulted in a little spark knock, you can tune the ignition advance curve to cure that problem. The stock L-79 curve is pretty slow so you have a lot of room to work with.

Scott333

Hi Bruce, thanks for the reply. I have had your rebuild Thread bookmarked since around July of 2008, and I have read it many, many times!

I have gone back and forth on the engine subject, from a zz383 crate to original 327 engine to replacement 327 block (to save the numbers-matching block from any damage).

The zz383 would be a blast to drive and I am sure I would like it. I have read many of Steve's Threads (632C2) and how he was able to get great rwhp and drag-strip results out of a 383 crate with Dart platinum heads and rams horns, looking close to factory. That is one option that would be great, a little expensive though, especially if I need to beef up the rear axles and suspension.

The more I think it about it, I like the idea of driving a midyear Corvette the way it was back in '67, for the most part. I had a '69 350/350 M21 Corvette for 11 years, so I'm fine with that kind of power. There will always be faster cars, and I have spent a lot of time researching various other kinds of modifications, but there's just something I like about staying fairly close to the factory design.

Scott333

Back in a couple hours, thanks for the replies so far

MikeM

This is what often times happens to people that restore/rebuild their engines. Don't know what neverl8's engine cost but even $6k is way over priced for a driver/cruiser engine.

If you have a good, solid core engine to start with, you should be able to put an engine together that will last tens of thousands of miles for around $2k plus labor.

Now, if you're really **** and have to have everything to a knat's a.., the sky is the limit for what you'll spend with very little return on your extra investment.

Just my opionion.

AZDoug

Use the factory Speed Pro 2166 equivalent hi dome pistons for .30" over. I never had a problem with these on my 327 and 91 octane, WITH a .043" thick MLS head gasket. You can probably go skinnier on the head gaskets, that is one way to take compression out if needed, start skinny, go thicker as necessary Low 9:1 CR will kill your motor power unless you go to a mild flat tappet cam or roller hi lift short duration cam.

Doug

Bluestripe67

Scott, you may be getting into TMI terriority and I'm about to add to that, but what follows is my personal experience with my car over the last 37-38 years.
'67 convert. 327/300, 4-speed, 3:36 posi, and nothing else.
I went to a performance machine shop in '85 with a reasonable amount of knowledge and personal envolvement with cars and engines. I wanted more power just like you and here is what I did and got. I disassembled the motor, gave it too the shop, they checked everything; all OK. Bored .030, honed, and lined bored with a deck plate, rods magnafluxed, all components balanced, crank cut. .010. Heads done with 3 angle valve job, bronze guides, pressed in studs for roller tip rockers. I did all the assembly. Forged pistons, flat top, stock rings, L-79 cam, indexed straight up. I port matched the heads both intake and exhaust, and cleaned up the bowls. My deviations from stock include an Edelbrock Performer intake and matching 625 CFM carb, performance ignition system from top to bottom, headers and a fresh air intake system utilizing my BB hood. It runs very well, by turning 13.5 at the track and while crusing at 2800-3000rpm I get between 17.0-17.5 mpg, in town/city, 11-12mpg. This is a daily driver with 92-93K miles after the rebuild.

Overall objective: I think your on the right track for a daily driver, but with your gearing, don't expect 15-16 mpg city.

External Appearance: Rams horns are excellant, and so is the intake and carb. The carb can be rebuilt by Lars (forum member) and will be perfect.

Internal: I don't think the quality of gas will get better, so I would stick with forged flat top pistons .030 over, factory rods, pressed in studs and magnaflux/balance everything that moves. If you can go the extra cost, get a roller cam kit (spec as close to orig. L-79 as possible) set up, it can provide a bit more power and less stress on the valve train. Get a three angle valve job, and bronze guides. Match all ports, both intake and exhaust. A high volume oil (not high pressure) pump with a all steel shaft will serve you well. The factory ignition can be vastly improved by sending your distributor to Lars (forum member) for a super tune and follow his recommendations for further settings.

A fresh rebuild and 3:70 gears will give you a real kick in the butt and a smile you may not be able to remove. My $.02 worth. Good luck! Dennis

Donny Brass

I run the 151 cam and 11.86:1 and do not have issues running on pump gas......

Kerrmudgeon

11 and a half to one on pump gas no problems. I would seriously consider switching engine shops. Nothing worse than getting something done at a shop after going against his advice. Like a selffufilling prophecy, also does he stand to make more $$$$ doing it his way?
=================== ===========

Scott333

Hi Mike,

That's what I have read, that off the assembly line they were often a half-point or more lower CR than advertised. So if it was a true, measured 10.5:1 or even 10.25:1 CR in 1967, lowering it to a true, measured 10.0 or 9.5 does not seem to be such a big step.


Nothing has been done to the heads at this point. It's a numbers-matching car that my Dad bought in 1976, and I was very specific about not decking the block, or doing anything else that would touch the number pad area. No machine work has been done at all, at this point. He's just waiting for me to decide what I want to do, and that's what I'm trying to do.


I'm confident this engine builder would measure each of these things carefully. The breakdown in communication/understanding between he and I (and I'm sure this lack of understanding is my fault, not his), is that he seems to want to purposely build the engine to not higher than a true, measured 9.2:1 CR. I am concerned that the L79 cam is the wrong cam for a true, measured CR in the 9.0:1 range, and the engine builder agrees with that, which is at least one reason why he wants to change the cam. But that seems to start a whole chain of events. If we're going to change the cam, then why not change to full roller-rockers to take advantage of a more aggressive roller cam (if I understand that correctly)? And if we're going to change to full roller-rocker, why not put on aluminum heads? And if we're going to put on aluminum heads, why not put on a better intake manifold, and if the intake manifold is too high for the stock hood, why not put on a different hood... and on and on.

That is one direction to go, and there's nothing at all wrong with it, unless one of your goals is to keep the car mostly original.

What I'm trying to understand, but can't figure out, is why the engine builder is trying to fix something that isn't broke, if the car will run fine at 10:1 CR on premium pump-gas with the L79 cam and OE equivalent parts? He says it won't, we can't go higher than 9.2:1. Is everybody measuring CR differently?


Either the engine can be rebuilt to a true, measured 10.0:1 CR and run fine on premium pump-gas, or it can't, right? Or is that not right? Is it not black & white like that?

Is it like carburetor tuning, where someone who knows how to tweak it just right can do it, but someone else might not be able to? Or is the engine builder using some kind of whole different CR measuring system? Because he's saying there's no way in this world that a 10.0CR engine is going to run right on premium pump-gas. And that's the opposite of most of the things I have read in the archives here on this Forum and on the NCRS forum. Donny is running his at 11.86CR on premium pump-gas! (but that's great, I'd be happy at just 10.0:1!)


Maybe I'm going about this the wrong way. If we were all sitting in an engine shop together, with a small-block Chevy on the engine stand, how would we determine the highest CR at which that specific engine can be run safely with premium gas? Is every engine unique, even with the same measured CR? Can we figure out the precise CR where the engine begins to knock, back it down a half-point on CR, and then I'd be good to go?


So lets say this engine builder agreed to build the engine to a true, measured 10.0:1 CR, using OEM forged domed pistons and the L79 cam with associated OE equivalent parts, despite his reservations.

When it's all done, we put it on the engine dyno. If it knocks and pings like crazy on premium pump-gas, can we take it down to 9.5:1 CR (after he finishes saying I told you so), just by changing some gaskets, or other relatively simple [inexpensive] methods, without redoing the whole engine?

If that's possible, then why not build it to a true, measured 11:1 CR with the plan to reduce CR by gradual, incremental steps on the engine dyno, until we find the "sweet spot" CR, where that particular engine runs great on premium pump-gas?


I'm not meaning to put you on the spot with all these questions, Mike. I'm just trying to work through this and figure it out, and everyones replies are very much appreciated.

Scott333

Hey Donny,

I know, I've read that many times in the archives, yours is one of the examples I cite, it gives me reason to think I could get somewhere in the ballpark compression ratio-wise, and be okay.

If the numbers on your engine pad matched the numbers on my frame, we could just strike a deal and I'd be all set

Scott333

Does this mean that we could build the engine with the OE equivalent pistons and gaskets you mentioned, carefully measure everything to determine the true static CR, and say (for example) it measures 11.25:1 static CR. Then we put it on an engine dyno, start it up, and if it has any problems, methodically drop the CR in graduated steps (like 0.25 each step) until we find the CR where that particular engine LIKES to run with premium pump-gas?

Or have I over-simplified it way too much?

Scott333

That’s okay, fire away. With too much information, I can take my time and work through it again later, and that’s better than not enough information.


I think I understood almost every word. I wouldn’t have understood it last year, but I’ve been reading the engine building Threads until I can hardly see straight, and it’s beginning to make a lot more sense than it did at first.

The machine shop magnafluxed the block and heads, everything's okay. The crank was off by .007 on the end of the shaft, the engine builder figured it got bent by someone trying to hammer the harmonic balancer on long ago. He said he can true it up, and the rest of the original forged crank is in good shape. I asked whether we could go just .020 over, I don’t mind paying a little extra to order special sized pistons if it will preserve more of the block, but apparently it needs .030 over. I have read that flat-top pistons offer a better burn (I probably didn’t say that right), but are lower compression (if that’s wrong, please correct me). If I understand correctly, forged pistons are stronger, and will flex under critical load, where hypereutectic pistons are less flexible under critical load, but either one should be more than strong enough for a street-driven car. The hypereutectic pistons can be fit to closer tolerances, and therefore the engine runs smoother, especially at start-up, when the forged pistons can be louder, or at least sound different. If there is no downside either way, then I would go with forged, just to keep with the OE engineered game-plan.

I have read a lot about the 2.0” vs. 2.5” rams horns, vs. headers, and Brzezinski Racing’s ported 2.5” rams horns. In a nutshell, according to what I have absorbed, the 2.5” is a worthwhile improvement over 2.0”, although hard data is unbelievably elusive, and Donny Brass actually picked up 7hp switching back to 2.0” rams horns if I remember correctly

Assuming for the moment that the conventional wisdom is correct (contrary to Donny’s actual results!), the theory goes that 2.5” is an improvement, and “hogging” out the 2.5” rams horn openings to match the exhaust ports on the heads improves it about as much as you can, without extrude honing. Still less exhaust flow than with headers, but maybe only by 15-20hp, instead of 30-40hp (which all may be out the window based on Donny’s results from switching back to 2.0” manifolds). I already bought the 2.5” Corvette Central N-11 Off-Road exhaust system, so I’m kind of committed at this point to locate a pair of 2.5” manifolds. At worst, hopefully I only lose 7hp, but gain in sound through the N-11’s with approximately 1.6 times the exhaust flow volume


I have his rebuild papers saved on the computer

I have thought about saving the M21 and using a wide-ratio M20. If I understand correctly, the combined ratio of the M20 with 3.70’s (2.52 x 3.70 = 9.324) would be roughly equivalent to the M21 close ratio with 4.11’s (2.20 x 4.11 = 9.042).


Thanks very much for all the information and for taking the time Dennis, I appreciate it



It’s way late, I’ll check back in tomorrow. Thanks to everyone for the replies so far,

Scott

AZDoug

I would CC your heads and know your exact piston dome volume and carefully calculate your compression ratio, using deck height etc,and adjust to about 10.5:1 true CR with about ~.016" head gaskets (or whatever the std steel single piece GM head gasket measures at), Using two steel head gaskets will double the thickness, you can get MLS gasket up to over .050" thick.

Use a compression ratio calculator, which you can find online and see what differences head gasket thicknesses make, I believe you will find you can change your CR from 10.5 down to about 9.8 by using available head gaskets.

I would NOT start at 11.25:1.

Doug

Doug

65 vette dude

I have a bone stock L79, with 52 degrees of advance, all in at 2800rpm, and have absolutely no problem running 91 octane gas.

Matt Gruber

when i build a motor i take the new pistons to the shop and tell him what i want done.
Scott
u r creating a liability nobody is going to accept.
When u r confident enough to accept responsibility for your parts selection, DO IT.
10.5 sounds good as i too fell for the 9:1 advice from books and car magazines in 1994 and as a result my 9.7 350's runs great, due to using modern cams, But on weenie 87 octane. i really wanted 93 motors but was talked out of it.(told i'd need 93 with 9.7)

MikeM

I think there is way too much being made of nothing here.

There are many, many SHP 327's running the streets that were rebuilt to strictly stock specs with no problems running on 93 octane fuel. The owners that report problems doing so either haven't investigated the real cause of the problem or just haven't fixed the problem.

In my own case, I successfully run my SHP engine(s) on 89 octane. I don't need 93.

I didn't do anything special. Didn't CC the heads, didn't shave the heads, didn't measure the deck before assembly. I did deck the block as I didn't like the gasket mating surface. I have .019 deck clearance. The rest of the engine was like it left Flint Engine and it ran fine for years so why not now?

It has the Speed Pro .125 domed forged pistons, 097 cam, stock '63 FI mechanical advance curve with the quick advance '64 vacuum cannister. I don't know what the compression ratio is but it's at least as high as what it was when it was new. I don't have any trouble at all running 89 octane. Same thing goes for an L-79 engine that is in another car. It's never been apart since built at Flint and it runs well on 89 octane.

You can do a "stock" rebuild on the cheap and have a good running, reliable engine. Once you start substituting hot rod parts for the stock components, you have started down the slippery slope of no return as far as cash investment goes. Your return will vary with the size of your wallet.

I don't know the OP and I don't know the engine builder. From what I've read here, the engine builder:

Doesn't know what he's talking about

Knows what he is talking about but is very happy to spend somebody else's money to prove out his superior knowledge of building engines. (I have observed some high performance shops tend to be like this)

Is trying to maximize his profit

Is being very careful to avoid/defend a comeback from some **** customer.

Whatever you wind up doing, I'd recommend you resist the tempatation to drop the compression ratio to a lesser amount than what was designed into the engine to start with. If this engine builder refuses to do this and guarantee his work, I'd find another shop.

Fordracer9

Using head gasket thickness to bandaid compression ratio is not the right way to do it. It changes the quench area/volume drastically. In a race motor, it may be OK, but for a street motor, once it's up to temp, you may not like the results.

Scott333

I'm hoping that won't be necessary, but it might be, and I understand what you're saying. It's also not fair to the engine builder to put him in a position where he's being asked to do something he doesn't believe is going to work.

I think this engine builder would do a great job from a work, skill and ability standpoint, and I don't believe he is doing anything to encourage me to spend any more money than necessary. It's really just an issue of engine build "philosophy", for lack of a better way to explain it.

Simply put, he does not believe the engine will run correctly unless we lower CR to 9.2:1. Problem is, if we lower CR to 9.2:1, we have to use a different cam than the OE L79. At that point, the engine isn't going to sound like a 1967 Corvette, and it isn't going to drive like a 1967 Corvette. Technically, it isn't even going to be an "L79" engine anymore, without an L79 cam.

What I'm trying to figure out is how so many people are clearly able to run at 10:1 CR or even higher on premium pump-gas, but the engine shop where my engine currently sits says that's not possible. Shouldn't this be one of those "it either is, or it isn't" situations? Shouldn't it be a matter of measurements and math, instead of "philosophy"?

According to some of the PM's that I have received, I am definitely not the first person to research all of the engine building Threads in the archives, then meet with an engine builder who says he understands everything I'm talking about, and it's almost all wrong.

I'm trying to understand how there can be two realities occupying the same space in time.