Stroker Advice 6.0/408 vs. LS3/416
 

A variety of threads touch on my questions here, but I haven't seen any that directly answer them.

In brief, I'm working on an LS conversion for my '67 Firebird. I've already purchased my TKO 600 and now I'm trying to decide on the short block--options are a stroked 6.0 (408) or a stroked LS3 (416). I have a set of L92/LS3 heads that are currently being ported. I'm shooting for ~600 hp normally aspirated. I'll be running a carb and an MSD 6AL ignition.

I'd appreciate any thoughts on whether the stroked LS3 is worth the extra $2,000 that it's going to cost. I've read about valve shrouding issues with the 6.0 and rectangular port heads, but I'm not sure whether the concerns are really going to mean that much on a car that will spend the bulk of its time driven on the street. It would be nice to shave some weight off the front end with the LS3, but I'm moving from a Pontiac 400 with cast iron heads to the LS motor, so I'm already shaving a fair amount of weight off the car.

Thoughts?

Thanks in advance!

Are you staying stock valve size on those LS3 heads?
I'm a big weight factor type of guy and would lean more towards the aluminum LS3 block vs the 408. With the right combo that goal can be done with either or. Much easier with the 416ci IMO.

Thanks. Yes. Stock valve size.

This is definitely what I would do...you said yourself, even with the 6.0 your going to be WAY lighter than before. And, that $2k will go along way towards all the little stuff your going to run into...

If the 67 Firebird was an automatic, I'd say go with the iron block for reasons already mentioned. However, your Firebird is a manual transmission car with a TKO600 which makes me think the build favors a driver's car.

In my experience and opinion getting weight off the nose of an older classic car pays off in better enjoyable driving more so than in a newer car. My 72 vette went from 3,450 with full tank of gas to 3,180 with a full tank of gas. You can feel the difference in normal driving. I kick myself every time I look at that iron block Dart, think that should have been an aluminum LS and 100 pounds less. :)


I think the aluminum vs iron cost difference is about $1,100. Thompson Motorsports lists an aluminum short block 416 for $5,300 and the short iron block 408 lists for $4200. For about $1,100 I'd go aluminum and take the extra 90-100 pounds out of the front of the Firebird.

67 Firebird weight AMA & option weight
See weight link for pdf with details.

I think 3,400 pounds is about what a 67 Firebird 400 with AC, power brakes & power steering weighed. About 1950-2000 pounds of that is on the front wheels, giving ~59% of the on the front.

Going aluminum LS would take about 200 pounds off the front of the car. ~3200 pounds, 1750-1800 up front, giving about 56% of the weight up front, similar to a 3rd/4th Gen.

The iron block LS ~3300 pounds, about 57.5% weight up front.

I think Pontiac 400's weight 600 to 625 pounds with an aluminum intake. The Pontiac engine isn't big block Chevy heavy ~665 pounds with aluminum intake.

Aluminum block LS about 430 pounds, maybe 445 with aluminum intake. Iron block LS about 520 pounds, with alum intake 535.

Thanks. Excellent thoughts. I'll probably be ordering from Thompson--their "motor of the month" right now is the 408 @ $3999. Looks like the 416 would end up around $5599. My gut keeps telling me to go with the 416 so I don't regret the decision later.

If your flush with cash, and money's no object, it's an easy decision, by all means get the 416...

I often give advice on my having to scrimp and scrounge for every penny I spend on a project...I guess that's why I lean toward the 408.

Both are great motors. We hope to have the opportunity to work with you either way!

Is 8 cubic inches worth about $1600 more?? Not to me.... @ $200/cubic inch...

I guess that's where my question about valve shrouding comes in. I'd normally think the exact same way, but there seem to be opinions that valve shrouding with the LS3 heads and less than a 4.065 bore will have a detrimental effect on performance. But I haven't seen any tests or evidence on this issue. Thus, if 8 cubes is only going to yield a negligible HP increase (10-12 hp), then it's not worth the money. But if those 8 cubes yield another 25+ hp, that could be taken into account.

Thanks for your thoughts.

The difference in power will be minimal. You are essentially looking at the cost difference being the in weight savings.

BOOM. There ya go! I capitalized the pivotal word.....

For just a basic street car I'd go with the iron block. You will have a ton of money left over to do whatever you want with. Besides, if you put the same exact top end on both blocks they will make almost exactly the same power. All else being equal of course

How about an aluminum LS2? They're cheaper than an LS3, right?

They fall right in the middle. The LS2 402 is $5199, I do have one in stock as well.

Yah but taking that weight off the nose of the car would offset the difference in cost. Think about what people spend to lighten there cars up now and days brakes clutches etc.. I mean every little pound counts.

You're talking a 100 lbs and 2k difference. With 2 grand you could easily offset the difference and then some. When I did my LQ9 I couldn't even tell the difference in weight

I would say two equally modded cars with the only difference being 100 pounds off the nose - the lighter car wins every time. But it all boils down to what the OP wants to use the car for, obviously if this is just a weekend cruiser than weight is not really a factor.

Definitely. And if the difference was more like $500 instead of $2000 i'd say aluminum for sure. But for 2k he could buy a whole lot lol