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OCB

Cool, but you have a big block. I am building a 400 SBC. I am trying to build this small block to deliver as close to a big block curve as I can. I know it will not be the same, but I also have none of the compromises a big block has. I discussed this in Post #1.

djquik1

https://www.extrudehoneafm.com/industries/automotive/

Using that and then gasket match it. Let them know what you are trying to accomplish and yes the Performer intake for the 1500-5500 rpm power band.

Ford used that on the OEM level with the SVT Contour to boost useable power across the rpm range and maximize torque of the 2.5.

leigh1322

Everyone kind of knows that cam sizes change according to cubic inches. Now we have some good data on how much.

OCBs and Corvette Passions power curves will actually be very similar, even tho one is a SB and one is a BB. The 400 has a 220* dur cam and the 496 has a 236* dur cam @ .050". But the 100 extra cubic inches requires more cam duration to let it rev and make peak HP at the same rpm. About 10-12* should do the trick. Since OCB has 16* extra it will rev just slightly higher, maybe 300 more rpm, than CPs.

They will both come on strong at 1500 rpm, and make peak power around 5500-5800 rpm. Both with very wide power bands and a lot of low end TQ. Both with near equal HP/TQ. OCBs was 550/570. HP/TQ. CPs should be down some with 100 less cubes, but probably make peak TQ & HP at similar rpms. OCBs peaks were 4400/5800. CPs should be similar, or just slightly lower. Actual #s are more of a guess. SB heads just have more velocity. I'd guess 510/510 or so. Maybe 480 on the low side. It will be milder than the 560HP 406 CP had before. But that IS the point. His HP/TQ peak RPMS will be down, maybe 300 or so from the prior 4500 rpm TQ peak. It will still be very strong, and a lot more streetable, than his old motor.

Just the way all THREE of us wanted ours. FYI I picked a 229* dur cam for my 454, for all the above reasons, and it should have a very similar power curve. My cubes are exactly 1/2 in between these two guys, and so is my duration.. My 1st cam was a smaller 224* cam, which dyno'd with a TQ peak at 3400, which I actually thought was lower than I wanted. So I re-cammed it to a 229* looking for more of a 4000-4200 TQ peak. My 224* cam made peak HP at 5300, and I wanted to move that up also a couple hundred.

So here are the intake durations @.050" the three of us came up with, on three different size engines, to get the same shaped ideal hi-perf streetable power curve with roughly a 4000-4200rpm TQ peak, a 5600-5800 rpm HP peak, and a 1500-5600 rpm cam power curve, and decent vac at idle. around 12-15" All HRs. And all around 500HP or more.

Anybody else see a pattern here?

400 cu in 220/224*
468 cu in 229/239*
496 cu in 236/245*

BTW it took EACH one of us TWO camshaft profiles to get there. So learn from our trials.
Dyno time is not cheap.

OldCarBum

Very good write up Leigh.
Thanks for taking the time to do the comparisons!

4-vettes

I find the Cam comparison very interesting as well.
Me, with a 350 and a Xtream Energy 268 on 1.6-1 rockers.

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Carb – Holley 750 Street HP Aluminum – PN - 0-82751SA - Summit Price - $714.95

I am a carb guy, on my Harley and on this car. There is no other option I am interested in, for all the same reasons I don't want an LS engine. This is the carb I am using. Its everything I need and nothing more. Jebbysan recommended this carb.

Holley is proud to introduce the all new Aluminum Street HP™. They utilize the best features from Holley's Race Bred 4150 HP™ carbs, but are tamed for street and light competition use. The Street HP features an aluminum contoured main body design taken from the race winning 4150 HP™ and has a tumble polished shiny finish and screw in air bleeds for precision tuning capabilities. The metering system includes a timed spark port for a vacuum advance distributor and is calibrated for a hot street/strip engine setup for outstanding throttle response, drivability and maximum performance. A street HP base plate is used on the 650 and 750 cfm models so there are plenty of vacuum ports for power brakes, PCV, etc. The Aluminum Street HP takes all the great features from the race bred HP and the Double Pumper and combines them into the perfect street/strip carburetor.

Features:

· Aluminum construction for weight savings and long lasting good looks

· Optimized street/strip calibration for outstanding throttle response and drivability

· Contoured venturi inlet offers balanced airflow for increased horsepower

· Screw in air bleeds for precision tuning capabilities

· Four corner idle for precise idle control

· Mechanical secondaries for awesome tire turning performance

· Dual accelerator pumps for additional fuel under initial acceleration

· Dual feed fuel inlets for constant high volume fuel delivery

· Available in 650, 750, 850, and 950 CFM versions

· Intended for lightweight vehicles w/ manual trans or automatics with high stall & low gearing

· 100% wet-flow tested by Holley technicians to assure it arrives ready to run!

This is the carb used on the first engine builds, and one of the few parts I did not sell with the failed engine. It’s a fully mechanical double pumper, with no choke and a clean unobstructed air horn. I have added the secondary butterfly external adjuster that Holley sells, to make it easier to adjust the secondary butterflies at idle.

I will set it up to start with the butterflies set to Lars recommendations….and go from there. The car now has both an AFR gauge and vacuum gage mounted on the steering column, to provide the information needed to get the tune right, and I will make whatever adjustments needed to do so once up and running. This is the right carb for me. I like the lack of vacuum secondaries, no thanks, I will control the secondary butterflies with my foot,......and I have never once needed a choke. As for jetting, the dyno results showed it was calibrated well for high RPM. Of course, I will adjust it till it’s right on this new engine.

NEXT - I have discussed the heads and the carb…..now I need to talk about what is between it, the intake manifold. But, I am still researching this, and trying to make a decision. My writing these posts are forcing me to consider, document and “prove” my decisions to myself. I am not completely at that point yet with the intake manifold, but working on it.

gkull

Yes, I see an off whack pattern not following logic. It takes a little math to find an equivolent cam between various cubic inch levels. There are other things that come into the exhaust duration equasion. But generally the bigger the split between I and E like 9 or 10 degrees is for a very restrictive exhaust system. The additional E duration helps evacuate the cylinders.

I told the poster that it would be worth buying some engine simulation software. The real final say instead of dreaming about some hypothetical low duration cam is going to be DCR

The smaller number represents more intake duration per ci.

My 427 with 252/256 427ci/252 = 1.694444444444 for comparision

400 ci / 220 = 1.818181818181818.

468 ci / 229 = 2.043668122270742

496 ci / 236 = 2.101694915254237

gkull

since you are concerned with fuel atomization and low end TQ. You need to research Annular discharge boosters. I would send that low option carb back and buy something with sight glass bowl windows, pri sec adjustments, and from the factory annular disharge boosters. Or maybe buy and install them.

OldCarBum

Jeff,
I like your carb choice and it’s a lot like my Quick Fuel.
Look at Holley’s website and research the differences between their Holley brand and their Quick Fuel brand.
Quick Fuel was designed and built by several ex-Holley engineers who left and built their own carbs based on upgrading Holley’s carbs.
They are basically a premium grade Holley carb.
Same carb, but better components.
Holley bought Quick Fuel and now sell them under the Quick Fuel brand.
You’ll get billet aluminum parts and pieces over the cast pieces with the Holley.
They are a little pricey but better built.

60 SHARK

Intriguing information here,,, as I'm pondering a large displacement build in a 400 block. Thanks for sharing your thoughts.

leigh1322

Not sure what you are getting at GKull.
I have always heard this is not a strictly linear relationship, so simply dividing duration and cubic inches does not work to compare different cams.

The thumb rules I have always heard from 45 years in an engine building shop, with over 5000 engine builds, and many dyno results.
The trends I see are this:

For the same size engine, adding 10* duration moves up the HP & TQ peaks 700 RPM. (source cam catalogs)
To the peak HP at the same RPM, for different size engines, it takes about 7*-10* more duration if you add 50 cu in. (source dyno runs, including our 3 engines and 6 cams)
If you keep the cam the same, but change the cu in, the peak rpm varies. (source: Dyno runs)

3 engine size examples, with the same camshaft, CR., cylinder heads, similar intake & carb: Same valve jobs too. 100% factory correct builds. 12 dyno runs on 12 diff engines. On the same Dyno at Lorenzo's in Feasterville, Pa.
All 12 made within 20 HP of each other. All 460-480HP.
What changed was the rpm the peak HP was made at:
L78 396-375 peak HP at 6800 RPM = 460HP
L71/L72 427-425/427-435 @ 6300 RPM = 460-470HP
LS6 454-460 @ 5800 RPM = 480HP
Roughly a 600 RPM change per 25 cu in change.
50 cubes was a 1000 RPM change.
These are dyno verified.
And that RPM change is exactly linear to the cu in change.
It takes "X" RPM to move "X" airflow.
That 242* duration @ .050" solid flat t. cam measured at 231* after/above the lash, like a Hydr. (source: SWDuke)
That cam feels pretty wild in a 396, like a decent hot street cam in a 427, and almost mild in a LS6 454 with smoother low end TQ.

The SBC DZ302 or 30-30 254* cam is the same.
HP peak of 360 in a 302 at 6900 RPM
Peak in a 350 at 6200-6300 RPM. Also near 360-370HP
The 327 version would be in-between.
Same cam, -50 cubes = +700 RPM.

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Intake Manifold – Edelbrock RPM (Non-Air Gap) – PN 7101 – Summit Price - $281.95

Up to this point, many decisions are made, and parts have been purchased and delivered. That includes the short block, the heads, the cam and the carb. I have not fully decided on the intake, but here is what I got so for:

Here are my basics points to consider in selecting a manifold:

- With my discussion above on the heads, I have already talked about my thoughts on a closely matched port-to-port match. The AFR 195 Street head ports are 1.265” wide x 2.110” tall, w 5/16” radius corners. The AFR 6900 intake gasket fits them perfectly, and will be my likely choice. Felpro 1205 also fits well, but corners are more squared off, less round, which is not much of an issue.

- The manufacturer’s advertised RPM range, which indicates the airflow design parameter for the capability of the intake, and it makes sense to me you should try to match with the intended operation of the engine. For me, that is Idle-4500. It’s not a big demand any manifold.

- I will be using the Holley 750 Aluminum Street HP Double Pumper, No Choke Carb that I still have from the first build. So the intake must be a square bore.

- Will it fit? The 77 Corvette has the 73-79 higher rise hood, so it can handle a taller manifold height. With the first build, and the RPM Air-Gap, I used a one inch drop, 14 inch, 3 inch filter, and a ¼ inch carb base gasket, and the front edge of the air cleaner lid just barely clears the hood.

- Lastly, I have no interest in a single plane manifold, so it will be a dual plane manifold. Clearly dual plane manifolds were designed and best suited for street operations, especially like mine. It’s widely accepted that single planes are for high RPM, high speed racing. All it takes is to look at a torque curve on a single plane, and its an easy choice.

Edelbrock dominates the dual intake manifold world, but I know there are others. The books I have read, online comparison articles I have read, and numerous forums show Edelbrock outperforms most. They have performed well for me, as far as I can tell. But I would consider anything that works right for me and will be looking into options. And honestly, unlike everything I have written so far, I have not 100% decided yet on the intake manifold. All I have decided is that it must meet the above basic requirements.

So far, I have considered four different intake manifolds, including Performer, Performer EPS, Performer RPM, and Performer RPM Air Gap intakes.

So, starting with the port dimensions and advertised RPM range:

- The low-rise Performer PN 2101 port dimensions are 1.14 x 1.88. Those are NOT a close match to the 1.265 x 2.110 head ports, with net difference of 0.165 x 0.230. But the RPM range is perfect at idle-5500. The carb pad is designed for both Quadrajets and Holleys, and the carb height of 4.05 inch would provide lots of clearance. For a Holley, the dual purpose pad just looks wrong and his manifold would be a poor choice over the next, the EPS manifold.

- The Performer EPS PN 2701 port dimensions are 1.15 x 1.87. Those are almost identical to the 2101 and again, NOT a close match to the 1.265 x 2.110 head ports. The RPM range is perfect at idle-5500. This intake is supposed to be an improvement over the 2101. The carb pad is higher at 4.127 inch and designed for Holley carbs. This would provide me some additional hood clearance. If I were to use a NON-RPM manifold, this would be the one. There would be no reason to choose the 2101 above over this one.

- The taller Performer RPM PN 7101 port dimensions are 1.16 x 1.92. These dimensions are closer to the AFR head port dimensions of 1.265 x 2.110, with a net difference 0.105 x .190. The carb height is 4.27 inch, same as the Air Gap version I had on first engine. There is no cutout in the plenum divider.

- The Performer RPM AIR-GAP PN 7501 port dimensions are 1.14 x 1.95. These dimensions are the closest to the AFR head port dimensions of 1.265 x 2.110, with a net difference 0.125 x .150. Carb height is identical to the Performer RPM. The Air Gap has a ¾ inch tall cutout in the plenum divider. And of course, it has the Air Gap feature. I have read that the intake tract is slightly improved over the Non-Air Gap RPM, others say its identical.

DECISIONS

The Performer EPS starts off to be the choice I want to make, the RPM range is perfect, the carb height slightly lower for more clearance, and it’s made for a Holley, but the intake outlets are also considerably smaller than the intake ports on the AFR 195 heads. And as previously stated, I don’t believe porting an intake an inch into the port is the right way to make this work. By my definition, if the ports don’t match the heads well, then you must change the intake or change the heads. I am not changing heads.

The RPM manifolds are clearly the leaders in the industry, but the “RPM” part of their name, as well as the higher 1500-6000 RPM range advertised, as well as the increased height, all are meant to provide the airflow needed at the higher RPM; however, the question… is the RPM manifold design a significant sacrifice in the idle to 4500 RPM range? Or is it more important to use a manifold, like the RPM, that the intake track is closer matched to the intake ports of the heads?

Next few posts will go add some additional factors I am considering. I am leaning towards the NON-Air Gap RPM manifold.

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AIR GAP VERSUS NON-AIR GAP DESIGN

The original engine builds used an RPM Air Gap Manifold……which seems to be the best rated dual plane manifold made. However, popular and what the "masses" use have been proven to be usually the wrong thing for me with my version of performance and operating range.

The “air gap” part of this has stated advantages in the concept that the air charge is “cooler”. Some argue that as the engine warms up in a hot street engine compartment, eventually it is no cooler and the “air gap” thing is a gimmick on the street. It works best on drag strip…possibly less significant on a street driven car. I don’t know and will not argue about it. It’s one simple consideration in making the decision. My conclusion on this is that the theoretical colder air charge of an Air Gap may be questionable, may be a gimmick, and therefore not a big requirement for me. I don’t put much weight on this. I think the real reason this manifold is more successful with the high RPM crowd is the next feature....and not the "Air Gap"..

The RPM Air Gap also differs from the NON-Air Gap Manifold because the center divider has been cut down ¾ inch….to provide some “single plane” benefits, again for high RPM HP numbers. Published dyno runs on BS TV engine dyno drama shows only show the improvement at the top end, with the rest of the curve is a direct overlay.

The purpose of the dual plane over a single plane is to allow the carb to see only half the engine at a time in terms of airflow demand. It’s the entire design benefit and purpose of the “dual” plane. The center divider cutout compromises that at some level, but benefits HP at top end. It seems this is a compromise that has made this very successful. I just have to decide if that is worth it to me.

Dyno tests have proven that if you center cut the divider in the non-Air Gap……the numbers are exactly the same as the Air Gap…..on an engine dyno. Might be different running in a hot engine compartment in a street car, we don’t know because the BS TV drama dyno shows conveniently don’t mention it, nor can they test that.....which is WHY I label this show and these tests useless BS for making decisions for a regularly driven street car.

FUEL ATOMIZATION

What is often not talked about, but I have read in numerous forums, is that there is also an often forgotten “compromise” with a cooler intake, like the Air Gap, and that is that fuel atomization is improved with heat. It’s one reason stock intake manifolds have an exhaust heat crossover to intentionally heat the intake manifold…designed especially for winter weather in a daily driver. This is NOT irrelevant to me, and is a strong consideration.

If heat improves atomization, then its logical a cooler charge can reduce fuel atomization, ending up with fuel mixtures entering the cylinder with excess fuel droplets…. rather than an ideal atomized air charge going into the combustion chamber, which is the primary purpose of the intake. The engine is then “tuned” around this richer, droplet included mixture, because it takes that to simply RUN, and now this richer / wetter mixture going into the combustion chamber creates the undesirable effect of washing cylinders bores with excess fuel, removing oil, increasing wear, and decreasing fuel mileage. I intend to block the exhaust crossover on the non-Air Gap Manifold with the gaskets, but the bottom of the intake manifold will still be exposed to the block valley heat, providing some smaller level of heat into the intake manifold……providing quicker warm up, better fuel atomization, and less cylinder fuel wash than an Air Gap manifold.

CARB BASE GASKET

I will also be using a thin gasket between the carb and intake, rather than the ¼ inch thick open center gasket I previously used, which only exaggerates the single plane crossover compromise. I want to maximize the features of the dual plane by keeping the two sides completely isolated, without any crossover. This should provide the carb with the full benefits of a dual plane manifold and sacrifice nothing that matters to me.

400 CUBIC INCH

Any given engine displacement demands an intake charge that does not choke it. The carb, and the intake, should not become the limiting factor. So the question becomes, does a Performer EPS intake, with its small intake port dimensions, and by design intake runners, become a choke point for my 400 SBC engine, in the operating range I drive? Or, is it far better to use the larger intake ports and runners of the RPM manifolds, so that the engine receives all or more of the intake charge it needs. I know the answer for a 350, and I know it for a 454, but where does a 400 fall......where is the line? This is one area of research that I have not become very well informed on. I think it has someting to do with min cross sectional areas, and some measurement of airflow. I got more work to do here.

And so, based on all the above:

- Performer 2101 is out.......no reaon or benefit to use this over the EPS manifold
- RPM Air Gap 7501 is out......the NON-AIR Gap RPM manifold the better choice.

So the decision is between RPM NON- Air Gap versus EPS. The next post, which may take more time to finish, is further discussion on port-to-port match.....which is why its still not decided.

Or I will just consider this all overthinking and just use the RPM Non-Air Gap manifold that I am leaning towards. If I had to buy a manifold TODAY, without any further thought, it would be the Edelbrock RPM NON-Airgap PN 7101


.

4-vettes

I ran that non Air gap RPM manifold for many years on my 350. It seemed to work very well. On a very streetable low RPM 350 CID engine. In recent years I have done a number of changes. And continued to run that manifold for awhile. And still found it worked just fine. Put on a single plane thinking it would help my EFI. this is of course no consideration to you.
I'm thinking you would be well served by this time proven dual plane.

leigh1322

One thing to remember is that these "RPM" recommendations are for the ubiquitous 350 cu in size engine.

Your 400 moves 14.2% more air at any rpm vs a 350.

So my "revised" manifold RPM ranges for your 400 would be:
2101 or 2701 = idle - 5500 rpm @ 350 turns into idle - 4800 rpm on a 400
7101 or 7501 = 1500 - 6500 rpm @ 350 turns into 1300 - 5700 on a 400

It has been my experience that when a intake manifold hits the top of it's rpm range, the engine runs out of air, and it chops power pretty severely. My choice for you would probably be the 7101. Like you said the ports "fit" better.

Height:
Edelbrock says the 7101/7501 RPM series does not fit a "Corvette" However it is only .25" taller than an LT-1 intake and we know those fit.
They must be referring to the std cars "flat" hood. A LT-1/BB hood clears an LT-1 intake with around 1/4" clearance, and the 73+ up hood is even taller, by a little, and you got it to fit before.

OldCarBum

When I purchased my Edelbrock RPM Air Gap I called Edelbrock about the “Will not fit Corvette” statements for their intakes.
The customer service rep told me they legally have to base their fitment measurements using the oem air cleaners and all oem equipment.

One reason I purchased my Air Gap was the raised air gap.
It makes sense that there would be some airflow through the intake, while driving but how much, who knows.
My thought was it also might keep the carburetor cooler when the engine is shut down and might aid in keeping fuel from boiling in the carburetor bowls preventing vapor lock.

Another thing you might consider is going with a four hole carburetor spacer over an open type.
That way you can raise the carb up to keep the fuel cooler and retain the dual plane function of the intake.


The number one reason I went with the air gap was that the intake and head ports were an exact match with my Edelbrock Performer RPM heads

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Yes, the Edelbrock statement that it does not fit a Corvette has to regarding the early 69-72 hood, the 73-39 hood is considerably higher, and as I already wrote in my post above, I have been running the RPM Air Gap since the first 406. It fits, with a one inch drop base aircleaner. The Non Air Gap RPM is the same exact pad height as the AIr Gap.

GordonR

Hey Jeff there is another Gorilla in the room to address and that's wheels. What are your plans? Your past engines had your power curves higher in the rpm ranges. On the street going through a turn with the new power plant with 500 ftlbs of instant torque will have some pucker moments for sure. Possibly Installing a cammed throttle lever also?

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Post #2 shows that I have 17 inch wheels, with modern sticky Nitto 555GT tires, with 10 inches each on the ground. And my foot on the pedal will regulate whatever power the engine has. It already was supposed to have 565 lbs of torque with Mark Jones cam and rebuild, but at a higher RPM. So I understand it, but lots of mild big blocks have that kind of torque at that lower RPM. Point it, it will be managemeable, and I will have to be careful.

gkull

scientifically a cubic inch is a cubic inch. We always have to remember to compare apples and apples. So you need to have everything the same and then change the cubic inches. To compare BBC to SBC you would have to have equal cfm flowing head ports and induction system. Then you throw in the smaller sbc valves with cylinder shrowding. So my above math will get you in the ball park.

An example. I had Comp Cams make a billet cam with endurance racing solid roller lobes really long duration at .200 lift points. From nice simulation software I played with all kinds of duration numbers and splits. I ended up with 236/242 which comp cams sells today in the XE cam line up. Like all hot rodders 383 ci wasn't enough jam after a couple of years and I had a couple of nice 3.875 stroker cranks laying around. So I took another 4 bolt block and other than the differrent crankshaft/rotating assembly used all my parts from the previous 383 like cam 227cc race ported heads,single plane, and 830 sfm carb. The 396 just seemed to run so much better than little 383's.

So then I decided to build a 427 and I used that math to get a cam in the ball park because I was going to be using those same 227cc heads and induction system.
The 383 and 396 actually have more duration slightly more duration per ci, but they actual run very much the same with peak hp and tq at the same rpm although you can feel the exta ci power

My 427 with 252/256 427ci/252 = 1.694444444444 for comparision
my 383 and 396 236/242 396/236 =1.677966101694915

You just have to think outside of the box and not buy some 40 year old cam off the self that was disigned for cast iron manifolds.