Mark IV, V & VI BBC question [Archive] - Team Camaro Tech

: Mark IV, V & VI BBC question

South Side Goons & Hitmen
Mar 27th, 07, 07:06 PM
Calling all the engine experts out there. Does anyone know what year Chevy went to the Mark V & Mark VI big blocks. Was it 1991 for the Mark V? The reason why I am asking---I'm thinking of buying a used 454 and rebuilding it.

Two questions:

1) Which is the better 454, the Mark IV, V or VI and why?
2) When did the Mark IV become a Mark V and a Mark V a Mark VI

Thanks!! All comments and replies are welcomed!!


Mar 27th, 07, 07:43 PM

Mar 28th, 07, 04:48 AM
Excellent link! But my 88 Dually 454 has non-adjustable rocker arms. Not a big concern.

Larger Dave
Mar 28th, 07, 09:07 AM
Biggest difference is the heads. Mark IV heads are either rectangular (Unsuitable for anything smaller than 500 cubic inches, or all out racing with an under 500 cid engine) and the oval port heads. The new "Peanut round port heads are fine for your tow vehicle: as they generate lots of bottom end and mid range torque, but they are all finished by the time you hit the built in rev limiter on a TBI equipped BBC at 4700 RPM. The Oval port heads flow much better in stock form and reworked with 2.19" or 2.30" intake and 1.88" exhaust valves are excellent for a race engine (with a little pocket porting).

The cooling system is very different on the Gen V engine and if you use a Mark IV head on a Gen V block it will leak coolant into the lifter valley. Gen VI blocks fixed the incompatibility issue with the revised coolant system, so that it will no longer leak coolant, but if you use a Mark IV head on a Gen VI block it will overheat unless you revise the water passages in the head to match the block (use Gen VI head gasket to bore new coolant holes). Aftermarket aluminum heads are designed to run on all blocks and have holes and coolant passages to fit any street driven configuration.

Gen V and Gen VI blocks have no provision for a mechanical fuel pump as they were designed to be used with EFI. They were also designed to be used with an automatic transmission, and many (not all) Gen V and Gen VI blocks have no mounting hole drilled and tapped for a bell crank (Z-bar) ball stud, nor a pilot bushing in the crank to accept use with a manual transmission. You can modify the casting by boring and tapping a hole, or in the case of a 502 block mount the ball stud on a bracket that bolts to the transmission bell housing.

The Gen V and Gen VI all use roller hydraulic cams, which is a much better choice than the old Mark IV block. But there are also more choices of aftermarket cranks available to buy for the old two piece rear main seal than the newer one piece Gen V and Gen VI blocks if you want a cheap stroker crank. You are not limited in the stroke you can use (the ZZ502 block claims to accept up to a maximum of a 4.5" stroke, just the price; as there are many more two piece cranks in the world than there are one piece cranks (though you can buy a two piece to one piece adapter for $70 bucks).

The best 454 block to buy is an aftermarket block (my favorites are by Dart or Merlin). They are stronger and can be bored out to at least a 4.6" bore. They come in a variety of heights and even come in aluminum if you dont like the color of cast iron blocks. All aftermarket blocks are HiPerf blocks with priority oiling and four bolt mains, made with a high nickel content. The cost to remachine a stock block added to your find will subtract about a thousand bucks off the price of a quality aftermarket block, cutting it almost in half. If your going to build a high horsepower thumpper start with a good foundation.

Larger Dave

Fred Ficarra
Mar 28th, 07, 09:41 AM
Great info Dave. What year were the cut-overs?

Vintage 68
Mar 28th, 07, 10:33 AM
Great info Dave. What year were the cut-overs?

As my Grandpa used to say - "I asked what time it was and you discribed how to build a watch" :D

The Gen V was first installed in the 1991 year models.
The earliest casting I've decoded was a very late (Nov/Dec?) 1989 date.

The Gen VI was first installed in the 1996 year models.

The cams for the Mark IV and Gen V are interchangable for flat tappet lifters.
The Gen VI where the first equiped with Roller Lifters, but the main difference is the machined flats on the lifter bores of this block - you can still install an earlier cam without rollers.
The cam retainer plate holes are verticle on a Gen V~VI, rotated 90deg from the Mark IV's horizontal orientation.
The '91 on trucks with Gen V's had manual transmissions and use a bracket for the pivot, the later medium duty trucks have a hydraulic clutch.
The Gen blocks use longer main cap bolts than the Mark blocks.
The crank on the Gen engines uses one long key in the keyway slot for the cam drive gear and the damper - the Mark cranks have two short keyway slots and two seperate keys, one for the cam gear one for the damper.
As he stated, Gen cranks are one-piece seal, Mark are two-piece.
The Flex plates are interchangeable - but Flywheels, for truck and marine applications, are not interchangeable on Mark and Gen engines.

Hope this gives you a better watch;

South Side Goons & Hitmen
Mar 28th, 07, 12:46 PM
Thanks for the info guys. What about this so called new Gen VII block? I just read and saw a 496 listed for sale somewhere. The engine was the 8100 Vortec (8.1L) It was a GM warranty engine complete with oil pan, valve covers, flex plate, intake, etc. The engine has been sitting since December of 2000 and has never been fired. Since it sat so long, I know it would need to be rebuilt. I read the Gen 7 debuted on the 2001 models. According to the article I read people did not like the Gen 5 engines and called Gen 6 the Gen FIX.

Two Questions:

1) Can a Gen 7 be rebuilt to go into a 1969 Camaro without the crank sensor and all the other electronic cr@p?

2) Are the Gen VII 496's worth building up? Or just go with a 454 block from a Gen 6 or Mark IV?

Thanks!! All replies are welcomed!!

Larger Dave
Mar 28th, 07, 03:40 PM
The 8.1L (496) is a great choice for your dually but it will need the wiring harness and the matching computer to work. It shouldn t need to be rebuilt, just change fluids and prime it well, with some WD40 in all cylinders. Turn motor over a few revolutions by hand to verify it turns freely and then fire it up.

It will not go well into a Camaro because it is a tall deck block (truck block in Mark IV parlance) and will aggravate clearance issues people have with the big block (they don t call them big blocks for nothing). The heads are pushed out a further half inch which hits power brake boosters and keeps one from removing their valve covers until the three bolts holding the wiper motor are removed and set aside as far as the actuator arm of the motor allows, and it sits nearly three quarters of a inch taller so you can forget that cool Air Gap high rise manifold even with a two inch cowl scoop.

Otherwise the 8.1 liter motor is awesome (it is essentially a factory production version of the popular Mark IV 454 upgrade of a quarter inch longer stroke stuffed into a standard block height motor). It can be made to fit in a Camaro, but it will not "drop-in" without some concessions such as custom made headers, different than factory wiper motors and reduced circle dual vacuum diaphragm boosters or a Hydrostatic brake conversion. Additionally as I have said on other posts anyone seeing a tall deck block assumes you are packing some serious displacement of well over 570 cubic inches (since the 572 crate motor is the smallest displacement truck block sold), so few will believe a 396 decal if you try and run T/O claiming a slow time offered by a mild cammed EFI truck engine.

Larger Dave

Fred Ficarra
Mar 28th, 07, 04:13 PM
Hey Dave? Do you write for a living? Great composition.

Larger Dave
Mar 28th, 07, 04:23 PM
No but thanks. Old school engineer (slide rule and log tables), been retired close to seven years now.

Mar 28th, 07, 05:05 PM
My 454HO Gen 6 has a fuel pump boss and a threaded ball for the z bar - from the crate!

Mar 29th, 07, 10:35 AM
I've used a Gen VI roller cam and timing set in a Mk IV block with Crane retrofit HR lifters.

Gaskets and seals are better on the newer blocks, but I like the solid decks of the older blocks. Plus, that's what I have (older blocks & heads).

Good Luck!