No Substitute For Cubes!

[mfg]

I happen to have an overhauled 400 CI Chevy 'small block' V8, which has been resting on an engine stand for over two years now.....I've continually toyed with the idea of dropping this engine into my '83 Avanti 'Everyday Driver', (currently 305 CI), and have been wrestling with the following two concerns...................

!) The 400 was originally backed up by the 'bulletproof' TH400 GM automatic tranny....I've been wondering if the power of the 400 engine would be too great for the GM 700R4 tranny presently in the car (I like the 700R4 very much, (overdrive!), and don't want to use the TH400)

2) How badly would installing a 95 CI larger engine in my Avanti decrease gas mileage?...remember, I drive this baby on a daily basis.

Thoughts/comments welcome!

Edited November 25, 2017 by mfg

[Gunslinger]

The tranny will likely take it...700R4s are very strong transmissions.  You would likely need to have yours rebuilt and modified to match the power and torque specs of the new engine.  

Gas mileage also depends on your driving style as much as additional cubes.  If you like to drive in a more "sporting" manner you likely will see a drop in fuel economy.  You might also see little difference...with the additional power and torque you might not need to feed it much pedal to accelerate to the same speeds.  Cam specs, induction, carburetor all are in the mix as well.

More power also means more heat.  Avantis are generally hot runners anyway...additional cooling ability might be required.  

Im not going to say do or don't do the swap...just be aware you might be opening up a can of worms that leads to just one more modification...then another and so on.  

[Avanti83]

Bruce has given good input but a couple more points. The 700R4 was standard issue in both large cars and trucks for many years. If it's in good nick, I'd just use it as is. In 83 GM and the other guys were beginning to pull themselves out of the MPG abyss that the clean air regs had created but were still a fair bit away from the mileage we enjoy today. I doubt the 400 would give better mileage but teamed with the 400 is might not be to bad depending on the interior configuration of the 400. The 400's era was the worst mileage part of the EPA regs. If your's was rebuilt with non-reg parts: decent compression, non-retarded improved cam and a decent distributor, it may not be terrible.

The thought on engine heat is real but a good system should handle it. a lot of mid-70's Avanti's did.

IAC, replacing one SBC with another SBC, as you know, is pretty straight forward as everything will bolt up. The only thing the 400 needs to run is power to the starter, gas to the carb and voltage to the coil simplistically speaking. 

Personally, if I had a rebuilt 400 with decent internals in my pole barn there'd be black streaks from the engine hoist traveling across the floor to install it. I'd also add a decent four barrel if you are still running the CCC carb.

On my 83 the computer control setup was an add-on, standalone system so if you still have that with the CCC, wiring the 400 would just require a different dissy and hot feed to it.

In summary, nothing ventured - nothing gained. Returning to the 305 isn't rocket science.

Good luck -Bob 

[mfg]

Thanks guys.I think we're all on the same page....and I really hate looking at that engine sitting there without a home!.....Thanks again!

(The engine heat consideration is real...as we know that Chevy small 400's have touching outer cylinder walls.....I do have a recored (brass) Avanti style radiator (as new) that would definitely be installed with the 400.)

[Joe Zeiger]

When i bought my 74, RQB2105,  a year and a half ago someone had already removed the original 400 and replaced it with higher horse power 350 that is coupled to a 700R4. (They also installed hood scope and 84 bumpers, and graphics ???)

GREAT daily driver, although you will burn more gas, YOU WILL ENJOY IT.  

I don't check exact mileage as spedo was never hooked up after eng conversion. I use the tack and with GPS spedo on my phone have speeds and rpms figured out to some degree. 2800rpm = 78mph on long trips.

Go for it, ENJOY YOUR NEW RIDE.

[mfg]

Very nice looking Avanti ll!....I have a set of those composite bumpers and am thinking of eventually replacing the steel bumpers on my '83 with composites.

Can't see the interior of your Avanti very well, but the pattern looks interesting too!..(More photos!)

[Avanti83]

I think you know this but you'll need the 400 flexplate and harmonic balancer unless the 400 was internally balanced during rebuild. Also check the flywheel teeth count to be sure they are the same or you will need a starter change. IIRC they were 168 and 153 or close.

Just being sure.

[mfg]

1 hour ago, Avanti83 said:

I think you know this but you'll need the 400 flexplate and harmonic balancer unless the 400 was internally balanced during rebuild. Also check the flywheel teeth count to be sure they are the same or you will need a starter change. IIRC they were 168 and 153 or close.

Just being sure.

Thank you!.............Regarding your first post on this thread;....are you saying that the 'factory 400' camshaft was machined with retarded valve timing?

(I re-used the original cam...but perhaps it should come back out before the engine goes in!!!)

[Gunslinger]

What you might consider is checking with customer service techs at Summit Racing or a cam maker like Edelbrock, Trick Flow, Comp Cams or Lunati.  They'll ask you things like what cylinder heads you have, what you want out of the engine, intake and carburetor, etc., then they can provide solid advice about what combination of cam, flow, carburetor will give you what you want without downside.  I hear great things about the Lunati Voodoo asymmetric camshafts which have a reputation of giving good power without poor idle or low vacuum but they are more expensive.  You might find out what you have is optimum for your needs.  Summit Racing doesn't have a real stake in pushing any particular brand (though they do sell their own brand cams, intakes, carbs and heads).  I'd start with them.

The Chebbie 400 was used during the era of emissions tuning before the industry had a real handle on making power and low emissions.  The tune was not optimal and you can do better now with the right mix of parts.

[Avanti83]

3 hours ago, mfg said:

Thank you!.............Regarding your first post on this thread;....are you saying that the 'factory 400' camshaft was machined with retarded valve timing?

 

A fair question but I don't know the answer. The only 74 305 I rebuilt was that way buy not sure about the 400. Google may help if Jack or one of the other folks don't chime in.

Bruce's comments are dead on if you want to get the best performance though. With the 305 and my 383 I talked to several cam manufactures about my needs and came away with good answers. Weiand with the 305 and Voodoo for the 383. You are adding about $200-300 or so for the cam and lifter kit so the cost is building so only you can judge the cost vs performance.

A less expensive option would be to degree the OEM cam and bring it to spec with an adjustable timing set if necessary. Can't speak to the cam performance however.

One last question, you did say overhauled 400 IIRC. Are you sure the rebuilder didn't add upgrades to help the performance as that's very common around here as it usually requires a cam change anyway.

[mfg]

I'll definitely research the camshaft question.....There isn't an intake manifold nor lifters or harmonic balancer installed on  the engine right now, so changing the cam isn't a big deal labor wise!

A Hi-Po double roller steel timing set was used when this 400 went back together.

[plwindish]

Decent power can be attained from the SBC 400.  I had the 400 in my 76 rebuilt back in the Spring of 2011.  Motor was completely torn down and rebuilt with new pistons, hydraulic roller cam,headers,  Edelbrock aluminum intake, new carb and heads machined for better flow.  The motor dyno-ed at 360 hp and 460 torque before going back in the car, which was a substantial upgrade from the original 175 hp.The motor powered the Avanti to both coasts and many other road trips to SDC and Avanti Meets having a few hiccups along the way like not running worth a darn at altitude in Colorado and losing overdrive in the 200R4 trans in AZ during a Route 66 tour in 2015.

Fast forward to this past August, I decided to go with an MSD TBI Electronic Fuel Injection to cure altitude sickness.  The mechanic doing the install didn't like the way the motor was running  before starting to work on it.  I had thought it had been running more "cold blooded" than usual and also was noticing the motor needing about a pint of antifreeze monthly.  Tearing down the motor revealed a significant cracked head on #6 cylinder and a small crack on #2 with just under 36000 miles and 6 1/2 years since the rebuild.  The motor again was totally torn down, new crank bearings, crank rebalance, new rods and pistons, new roller cam with more EFI friendly lobe separation, new Dart aluminum heads were added in along with another .0010 over, making it a 408 was completed mid September.  The motor again was dyno-ed and this time is making 471.5 hp at 5400 rpms and 522 torque at 4400 rpms.  

I only had put around 600 miles since the rebuild and EFI switch so final EFI "learning" will need to be completed next Spring after the car comes back out of storage. The car starts very nicely, turn the key and listen for the electric fuel pump to stop pulsing, then start.  I had good fuel economy before the 2nd rebuild, getting up to 26+ a couple of years ago on a trip with the 200R4 AOD, but haven't seen those kind of numbers on the few tankfuls I have driven since the rebuild.  I'm sure some tinkering will be in order next Spring before everything is running the best it can.

I toyed with the idea of replacing the engine with an LS the first time it was rebuilt, but opted to keep it original. FWIW, it could have been for what the engine teardown, rebuild, and trans switch cost.  This time I wasn't expecting the whole teardown and engine rebuild together with doing the EFI, but I stayed with it.  Now the "original" 400 is comprised of the block, crankshaft and oil pan, everything else is new.

I'm looking forward to a trouble free driving season in 2018 and am currently contemplating driving the car from Chicago area to Spokane for the SDC/AOAI Meet.  

[Footer]

Plwindish

Before you make your hotel reservations in Spokane for the SDC meet you might want to check, it looks like it's in Tacoma, 300 miles to the west. I'm just a scosh north of Spokane in Deer Park and would love to toodle over in RQB3771.  Sounds like a great trip for you!

[jpepper]

Back in the day (1975) I built a 400 SBC for a 70 Nova. The short block was built stock but blueprinted. I used a factory 350 HP 327 cam and a set of small chamber double hump #461 head castings with 1.94/1.5 valves. Using a 4 speed and 3.08 rear end, this car regularly returned 24 MPG on the highway. Performance wise it ran out of steam at about 5000 RPM but it was a high 14 second street car. Today I would expect results plwindish describes.