The LT5 is not forgotten

[ZRapid-1]

Good to see the LT5 recognized as one of the best engines. Made it into the super Vette ZR-1!

LT5; 5.7-liter V-8, 375 hp
The LT5 went under the hood of the first Corvette ZR-1 in 1989 but wasn't made in-house. GM contracted development of the engine to Lotus Engineering (which was actually owned by GM at that time). Originally, Lotus designed a clean-sheet design for the LT5 that was intended to make 400 hp. However, GM wanted to use the engine block from the existing small-block V-8, and the Lotus modified its design to meet GM's requirements. The consequence was that the power output for the engine dropped to 375 hp. The new engine also ended up with both a longer stroke and a smaller bore than the old V-8. Production of the LT5 was contracted out to Mercury Marine.
Why is it one of the best? The LT5 was all-new engine that veered from the small-block norm, it was truly a global collaboration that helped to produce this super Vette.

http://www.automobilemag.com/features/news/1110_10_best_chevrolet_engines/index.html

[VetteMed]

Uhh, too bad it didn't use a SBC block... poor research right there.

[Bob Eyres]

Uhh, too bad it didn't use a SBC block... poor research right there.

The Brits at Lotus did want to use the 90deg. block, but GM mandated the 60deg. block so that the massive heads would fit through the frame rails from the bottom during assembly.

for your "research", read the book.
http://www.amazon.com/Heart-Beast-Anthony-Young/dp/tags-on-product/0971146845

[Aurora40]

The Brits at Lotus did want to use the 90deg. block, but GM mandated the 60deg. block so that the massive heads would fit through the frame rails from the bottom during assembly.

for your "research", read the book.
http://www.amazon.com/Heart-Beast-Anthony-Young/dp/tags-on-product/0971146845

The LT5 is most certainly a 90-degree V8.

Lotus blamed the bore spacing on the lower output, but my guess is that was just because it was easy. The made 400+ on the '93 engine with the same spacing, and were shooting for 475+ on the "1995" engine with the same spacing. If the engine didn't fit in the car, it wouldn't matter how much power it made.

As an aside, that article has lots of "issues" or odd phrasing. Like the '94 LT1. There was an LT1 in '94, but it debuted in '92 on the Y-body, and '93 on the F-body. Why mention '94? And I'd always heard the '62 Olds Jetfire was the first turbo production car (the article mentions the '64 Corvair, though like the LT1, the turbo Corvair engine had been around before that).

[Kevin]

gm mandated the bore spacing so they could still say it was SBC

[Bob Eyres]

Thanks Bob, it was the bore spacing that was the issue. But wasn't there a packaging problem that the engine was too wide to load from the bottom?

BTW, Both the Olds Jetfire and the Corvair came out with Turbos the same year, 62'. But the Jetfire was released first.

[VetteMed]

for your "research", read the book.
http://www.amazon.com/Heart-Beast-Anthony-Young/dp/tags-on-product/0971146845

Maybe you should take your own advice, champ.

[Kevin]

The Brits at Lotus did want to use the 90deg. block, but GM mandated the 60deg. block so that the massive heads would fit through the frame rails from the bottom during assembly.

for your "research", read the book.
http://www.amazon.com/Heart-Beast-Anthony-Young/dp/tags-on-product/0971146845

gm wanted to use a stock 350 block and bolt 4v heads onto it, lotus told them it wouldn't work but they could build them a new block and heads, gm said fine, lotus designed it with a different bore spacing and then gm said no.

go reread your book

[Paul Workman]

Thanks Bob, it was the bore spacing that was the issue. But wasn't there a packaging problem that the engine was too wide to load from the bottom?

BTW, Both the Olds Jetfire and the Corvair came out with Turbos the same year, 62'. But the Jetfire was released first.

If I recall from my reading correctly, the size of the cam sprockets had to be reduced or there would be issues with the engine being too wide to fit between the rails on assembly. Initially an single chain was considered, but later proved to be insufficient at high sustained rpm, in conjunction with the small sprockets, and the dual chain was the final solution to the issue of fitting between the rails.

for what it's worth...

P.

[XfireZ51]

If I recall from my reading correctly, the size of the cam sprockets had to be reduced or there would be issues with the engine being too wide to fit between the rails on assembly. Initially an single chain was considered, but later proved to be insufficient at high sustained rpm, in conjunction with the small sprockets, and the dual chain was the final solution to the issue of fitting between the rails.

for what it's worth...

P.

I think Lotus wanted to do a "flatter" motor than the SBC. Lower CG that way, but production prevailed and I don't think it had to do w just the Vette. For the number they built they could have done that offline. If the LT-5 was going to be used in any higher production vehicles, it would need to adhere to the same production constraints. Alas, never happened.

[Bob Eyres]

Well, I went back to the book, Pgs.22-25, and from my reading many of the engineering decisions were dictated by the fact that this engine had to fit through those frame rails and could be "no more than 675mm wide". This was not possible with the connecting rod length in the Chevy 350 small block. So they sold GM on designing a whole new block, "As soon as we shorten the connecting rod, the cylinder head joint face moves inwards, and we can get it all in".
GM wanted rubber belt cam drives too, but that idea had to be ditched because the pulleys on the cam sprockets would be too large for the narrow packaging requirements.
Some of this isn't too clear a read for a non-engineer like me, but it's obvious a lot of design compromises were made to slip that LT5 in from the bottom.