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Stock engine should have plenty of vacuum for that. To determine what you engine is capable of producing, remove and block off ALL vacuum lines coming from the intake manifold and carburetor except for the one to the vacuum advance can on the distributor, if you have one. Start the engine and measure vacuum level. When you have done that, now pull & block the hose on that vacuum advance can and measure again. If the amount of vacuum didn't change between those two measurements, reconnect the vacuum advance can as you have proved that it is not defective. Now, you need to place an index mark on the distributor housing to intake manifold, loosen the lock-down bolt on the distributor, and slowly turn the distributor one way and then the other to find the maximum vacuum level while the engine is still running. Finally, you can use spray brake cleaner or carb cleaner to shoot little puffs of cleaner at various areas on the carburetor and around the sealing joints of the intake manifold. In doing this, if the engine increases RPM with any puff of cleaner, that is where a vacuum leak can be found. Some opening/gap is allowing that cleaner to be ingested into the air/fuel charge.
This is the method used to determine maximum vacuum capability of your engine and/or identify areas of vacuum leakage. Let us know what you find.
Stock engine should have plenty of vacuum for that. To determine what you engine is capable of producing, remove and block off ALL vacuum lines (...) cleaner to be ingested into the air/fuel charge.
This is the method used to determine maximum vacuum capability of your engine and/or identify areas of vacuum leakage. Let us know what you find.
Thank you for the super educational write up . I am not quite there yet...
Originally Posted by dennis
my 70 L46 idles smooth with 15" vacuum
Thanks !! Super helpful.
I am contemplating buying over someone's used long block 400. Seller says there is a slight lope to the idle. Measured, it pull 13 inches. I would imagine that is close enough to run brakes and accessories. I am loathe to swap out the cam. Maybe rhodes lifters in a pinch ? Hmmmm The problem is I hate lifter ticks as well.
If you have an L46 engine, why not rebuild it? That is a very desirable engine for many reasons and has good resale value. If the block is damaged beyond repair, swapping it out is understandable. If the block can be salvaged and most of the other components are still usable, you should at least consider rebuilding it. Even if it would cost a little more to get it in good shape, the L46 is worth significantly more than having a 400 block in the car.
That being said, if you are going for a resto-mod vehicle, a well-built 400 block will still make more power than any 350.
With moderate duration cams like this raising the idle 100 rpm will raise the vacuum as well.
So when some tells you a vac reading what rpm is it at?
My 230/240 duration 454 made 17 inches vacuum at 800 rpm with 35 degrees timing. If I let it idle lower it would drop some.
Last I heard you were beginning a L31 build. Have you had a change in plans or setback?
If you have an L46 engine, why not rebuild it? That is a very desirable engine for many reasons and has good resale value. If the block is damaged beyond repair, swapping it out is understandable. If the block can be salvaged and most of the other components are still usable, you should at least consider rebuilding it. Even if it would cost a little more to get it in good shape, the L46 is worth significantly more than having a 400 block in the car.
That being said, if you are going for a resto-mod vehicle, a well-built 400 block will still make more power than any 350.
Super sadly, the only thing left of the L46 is the tach That would indeed have been my very first choice.
The engine in the car seems like it was built by Dr. Victor Frankenstein himself. It's an ugly collection of parts from multiple sources... and not working too happily together
ADD The 400 seems well built and it has a 100 miles on it. I will be going to see it on Friday.
With moderate duration cams like this raising the idle 100 rpm will raise the vacuum as well.
So when some tells you a vac reading what rpm is it at?
My 230/240 duration 454 made 17 inches vacuum at 800 rpm with 35 degrees timing. If I let it idle lower it would drop some.
Last I heard you were beginning a L31 build. Have you had a change in plans or setback?
Good catch ! I am not counting my chicks yet, but I am counting my nickels and dimes.
I did the math on what it would cost to rebuild the L31 and it comes out to a tick more than it would to buy this fellow over. To boot the 400 comes with a few new parts that I can flog on eBay: MSD 6AL box, MSD distributor and coil, Holley carb, polished intake, polished water pump, valve covers, etc. I'll keep the new starter
The seller, who seems very trustworthy, tells me the SBC400 only has 100 miles on it. In fact, he has locked the distributor out at 3,500 RPM.
I saw a video of the vacuum reading. The needle was stable. I like that. And agreed - I need to find out what RPM it was at.
One last and very important reason, other than the power potential of a 400... I sold a car that was very dear to me that had a 400 I built myself. I really miss having a 400.
ADD As for the L31, the heads I can reassemble and sell. No loss there. The block I am not sure what to do with it.
Seems like a win-win. Less $ - more HP. And I know you'll like the TQ.
As long as you like the cam he put in it. That's usually a very personal decision and gives the motor it's "personality".
With 13" of idle vacuum it must be pretty healthy.bigger than a stocker anyway. .
Know anything else about it? Do you like what the idle sounds like?
Translated from Dutch as best as I can. 4-bolt 400 ci block 1972,fully machined, .030 overbore. Brass freeze plugs. Eagle steel 400 crank 3.75 stroke. Eagle forged i-beam rods. (5.7 inch). Speed Pro hypereutectic pistons (coated) 10:1 compression with moly rings.
Aluminium cylinder heads, equivalent to Edelbrock Performer RPM (64cc)
Stainless steel valves (2.02-1.60) Competition Cams springs. King main en rod bearings, Clevite cam bearings. Hydraulic flat tappet Howards cam, custom ground,
488-522 lift, 239-248 duration, advertised at .050 288. Cloyes Tue Roller timing gears and chain. Competition Cams pushrods. (7.8)
Modified Melling M-55 oil pump, ARP bolts.
7.5 quart oil pan (baffled).
Pro Products dual plane polished intake.
ARP rocker studs 7/16, new cylinder head bolts. Stainless Roller rockers 7/16 1.55 ratio.
Fel pro en Mr. Gasket gaskets with stell reinforced rubber oil pan gasket.
New sfi flexplate. New balancer. ARP bolts. Aluminium polished high volume water pump. New polished Holley type 4160, 750 CFM. MSD 8361 Pro billet ignition with vacuum advance. MSD 6AL-2 ignitionbox met two step rpm limiter. MSD 8207 SS blaster coil. MSD High performance plug wires.
Sounds like you are going to see it on friday. Hope it is still in the car and you get a chance to drive it.
The motor specs look really good Steel crank, good rods, heads, etc.
Most aluminum heads these days are way better than the 50 yr old original steel ones.
Lost to me like a solid quarter mile bracket racer street-strip setup. Should be way fast. Light years more so than your tired L48.
The cam, being custom ground, is not in Howards catalog. It is a little big for what you are trying to do. 239 & 248 duration is pretty serious for the street. The power curve will not kick in hard til 2800 & you should have a convertor like that if auto. But I believe you have a manual? Cams like this are usually recommended for Pro Street or Competition. If the lobe center is wide enough it could still be streetable. If it is a tight lobe center it will idle and stutter like the race car cam that it is..The idle in the Monte Carlo sounds good, but to me sounds like the idle is high, like 1000-1100 rpm +. (It's an automatic so I would expect that.). It will definately get choppier if you lower the idle. Slow the idle speed down and you'll get a better feel for it.
If it is running, and you are going to see it, get him to lower the idle to 800-900 to see what it sounds like and if you like it. This is not a cam that will like a stock idle speed of 600-700. Dropping the idle speed will also drop the vacuum, which could easily go lower than you would like. I would expect around 10-11" vac at a lower idle. 850-900 is an OK street idle with a stick, did it for years. And it may not idle well much lower than that.. If you can get 11 or 12 inches of vac at that idle speed it should run your vac stuff. So do a lowered idle speed sound check, and a vac check. Also see if he has vac advance on the distributor or not. It will idle better and make more idle vac with it. Unplug it and you'll see what it does. And how much more wild the cam will sound.
You may like the idle rumble though. And you will probably love the power curve. Should pull like gangbusters at 2600+ and go til 6500. Don't fall so much in love with the crazy power that you forget the low rpm stuff, where you really drive it. Whether you live with the cam or not will depend if you will like it's less than 2600 rpm performance.
Have fun! I'd guess 400-425 HP even with the stock manifolds on it! Headers would give it an easy 30+ more? Call it 450!
The dual plane manifold should fit in the vette if it is not too tall, and you run a drop base air cleaner ala the LT-1. But I doubt the 7.6 qt oil pan will fit. Vette pans are pretty specific.
Last edited by leigh1322; Aug 5, 2020 at 05:53 PM.
Unless you install a very aggressive cam, a SBC engine should make 17-21" Hg vacuum. An aggressive cam might drop that to 15-17" Hg. Wild-*** cams can be down around 10" Hg.
From the aspect of vacuum level maintained by the engine, elimination of all vacuum leakage and proper set up of the carburetor are the prime contributors.
@leigh1322 Yes, the plan is to see the motor tomorrow afternoon. I don't expect to drive it, but we will be going for a ride for sure.
I will do as many tests as the guy is willing to let me do; I will bring along my stethoscope, timing light, compression gauge and vacuum gauge. (And let's not forget wrenches and screw drivers )
Agreed, his idle does look high, like 1000~1100. He has manifold vacuum on his distributor.
@7T1vette As it is I am getting about 21 inches on my current powerplant. So, I know I am working with a good, leak free system.
I did think about that oil pan and I am worried that it will hang. Where would the interference be ? How is a corvette oil pan different from a standard oil pan ?
The question is indeed, can I live with the cam ? Will it be too wild for me ? It would be a shame to swap in a cam considering this one has barely been broken in - that or Rhoads lifters.
Hmm, what would an L46's torque/rpm distribution look like ? Is it as peaky as this one ?
My guess is the only way to do this is to throw it in the Corvette* (with the pan on to see if it fits) and go from there. We'll also immediately know if the manifold will fit. If I can't stand the cam, I guess I'll have to swap it in the car...
* well, I will need to swap on an externally balanced flywheel first
Actually, that is a good question... I have no doubt the L46 wouldn't be making as much power, but would it be more streetable ? Would the 400's torque distribution be similar ?
Regarding vacuum potential, the 'overlap' between intake and exhaust lobes is what causes the drop for performance cams. The greater the overlap, the lower the vacuum potential. The lift on that cam doesn't seem terribly aggressive. I'm guessing that the engine would make adequate vacuum. For proper operation of vacuum systems in a C3, the car NEEDS a suitable vacuum reservoir tank.
Actually, that is a good question... I have no doubt the L46 wouldn't be making as much power, but would it be more streetable ? Would the 400's torque distribution be similar ?
Very good question. Here is a Desktop Dyno model to answer that question.
Graph 1 is a stock L46 with big tube headers.
Graph 2 is a 400 with Eddy heads and same L46 cam..
Graph 3 is a 400 as above with 239/249 Howards cam.
Absolute power numbers are often off by 5-10-20, and the rpm is sometimes low, but the trends and curves we can easily see.
Please do not take these numbers as Gospel, but the trends should be correct.
L46: 379 HP at 5000 and 455 ftlbs at 3500 Lowest HP & TQ
400 / Eddy / L46: Best TQ. Exact same curve shape, just higher curves. Up to 416 HP & 494 ftlbs. Up by 40 everywhere. Highest TQ Middle HP
400 / Eddy / 239/249 cam: Best Power. 435HP @ 5500 and 478 ftlbs @ 4000 Less TQ and more HP. Curves move upscale . HP moves up 500rpm. TQ is softer early and peaks later by 500 Highest HP & Middle TQ TQ drops to 350-L46 levels and does not recover/pass it til 4500rpm. Even the HP is less til 4500 rpm.
Your self-question is: (Howards cam vs GM L46 cam)
Is the 19 HP gain at 5500 worth the 40 ftlb TQ loss from idle to 4000 rpm????
But it is already installed, so is it worth changing it?
And your current L48 is down around 270HP, so all three of these choices are going to be 40-50% stronger!!
Very good question. Here is a Desktop Dyno model to answer that question.
Graph 1 is a stock L46 with big tube headers.
Graph 2 is a 400 with Eddy heads and same L46 cam..
Graph 3 is a 400 as above with 239/249 Howards cam.
Absolute power numbers are often off by 5-10-20, and the rpm is sometimes low, but the trends and curves we can easily see.
Please do not take these numbers as Gospel, but the trends should be correct.
L46: 379 HP at 5000 and 455 ftlbs at 3500 Lowest HP & TQ
400 / Eddy / L46: Best TQ. Exact same curve shape, just higher curves. Up to 416 HP & 494 ftlbs. Up by 40 everywhere. Highest TQ Middle HP
400 / Eddy / 239/249 cam: Best Power. 435HP @ 5500 and 478 ftlbs @ 4000 Less TQ and more HP. Curves move upscale . HP moves up 500rpm. TQ is softer early and peaks later by 500 Highest HP & Middle TQ TQ drops to 350-L46 levels and does not recover/pass it til 4500rpm. Even the HP is less til 4500 rpm.
Your self-question is: (Howards cam vs GM L46 cam)
Is the 19 HP gain at 5500 worth the 40 ftlb TQ loss from idle to 4000 rpm????
But it is already installed, so is it worth changing it?
And your current L48 is down around 270HP, so all three of these choices are going to be 40-50% stronger!!
WOW !!!! Thanks so much for this write up ! Super stuff
For the record, I’m nowhere near 270 ponies. More like 170 ! I presume I’ll certainly be making more off idle torque than I am now.
I guess it does grind down to that: it’s already installed. Is it worth swapping ?
The seller seemed super meticulous using a primer every time he started the engine. Yes, yanking the distributor and all.
He took me for a drive and it was a bit underwhelming. But... that’s probably because he was running:
- unmodified TH350
- 10-inch converter
- no kick down cable
- granny gears
- peg leg, and a
- barge on tires.
The vacuum was 13.5 inches at 850 rpm. It sounded like 1100-1100 but I measure it, it’s 850. The idle was smoother than I was expecting.
Also was going through no-performance cast iron exhaust manifold and had only tuned the idle of the carburetor.
IMHO, it may be worth a shot to drop it in as is.
ADD. and he didn’t really tune the carb in any way.
L46 vacuum at idle.
. I am not quite there yet...
The problem is I hate lifter ticks as well.
That would indeed have been my very first choice.
)
