Horsepower difference ?

[onetinsoldier]

Heres a dumb question...........the first ZR1's had 375 hp , then it jumped to 400 hp, what change or addition made the difference in horsepower gain ?

[WARP TEN]

Here's a dumb question...........the first ZR1's had 375 hp , then it jumped to 400 hp, what change or addition made the difference in horsepower gain ?

The '93 to '95s were rated at 405 HP. I believe the gains were due to some better porting and exhaust also with some chip tweaking but Marc and others could say more definitively. --Bob

[onetinsoldier]

I believe you are correct, porting and some change in cam grinding , gave it the 30hp gain . I know chevrolet used to retard cam timing years ago for emissions and hp reduction to meet standards.

[rkreigh]

improved intake port alignment. Bit better heads and cams, improved exhaust and header/cats.

405 hp blocks also had 4 bolt mains and all of them were made in 1993

[Hib Halverson]

>Mild port/polish on intake and exhaust ports.
>better blending of the ports just above the valve seats
>Three-angle valve seats
>"Sleeve spacer" between the head and injector housings to improve alignment
>Revised cam timing but cam profiles remained the same
>Revised calibration
>New exhaust manifolds...which actually cost 5-hp but went to production to solve a problem with the engine ingesting debris if the cat converter substrate failed at high rpm.

[WARP TEN]

>Mild port/polish on intake and exhaust ports.
>better blending of the ports just above the valve seats
>Three-angle valve seats
>"Sleeve spacer" between the head and injector housings to improve alignment
>Revised cam timing but cam profiles remained the same
>Revised calibration
>New exhaust manifolds...which actually cost 5-hp but went to production to solve a problem with the engine ingesting debris if the cat converter substrate failed at high rpm.

Always glad to have your wisdom around, Hib. I had forgotten all the details and especially about the cat debris ingesting problem. --Bob

[Matt B]

>Mild port/polish on intake and exhaust ports.
>better blending of the ports just above the valve seats
>Three-angle valve seats
>"Sleeve spacer" between the head and injector housings to improve alignment
>Revised cam timing but cam profiles remained the same
>Revised calibration
>New exhaust manifolds...which actually cost 5-hp but went to production to solve a problem with the engine ingesting debris if the cat converter substrate failed at high rpm.That last point sounds really bad!!

[A26B]

That last point sounds really bad!!

That's because...... IF & When it does......... it Really is!
Referred to as catalyst reversion. Occurs during engine de-celleration, i.e. lifting throttle with clutch engaged at high rpm.

All that's left of the Oil Control Ring lands, is that short section on the left end.:eek:
https://cdn10.bigcommerce.com/s-nkg90o/product_images/uploaded_images/catalyst-reversion-damage-02.jpg

[Hib Halverson]

Always glad to have your wisdom around, Hib. I had forgotten all the details and especially about the cat debris ingesting problem. --Bob


Some of that info came from GM press info that was distributed in the 1993 long lead book.


The rest of it came from a conversation I had many years ago with Graham B.

[tiegsd]

That last point sounds really bad!!

Oh wow. Have to assume that happened to a few cars until someone figured out what was going on?!

[Hib Halverson]

That's because...... IF & When it does......... it Really is!
Referred to as catalyst reversion. (snip)
https://cdn10.bigcommerce.com/s-nkg90o/product_images/uploaded_images/catalyst-reversion-damage-02.jpg

"Catalyst reversion"...LOL
More like "catalyst regurgitation."

I don't remember what Graham called it but he did explain that because of lower pressure in the cylinder, the engine sucked pieces of the cat substrate past the exhaust valve. I'd imagine, if that happens, the damage sometimes had to be more severe than just a broken oil ring. GM wouldn't have spent the money to redesign the exhaust manifold if it hadn't spent too mucy money on warranty for that.

[A26B]

"Catalyst reversion"...LOL
More like "catalyst regurgitation."

I don't remember what Graham called it but he did explain that because of lower pressure in the cylinder, the engine sucked pieces of the cat substrate past the exhaust valve. I'd imagine, if that happens, the damage sometimes had to be more severe than just a broken oil ring. GM wouldn't have spent the money to redesign the exhaust manifold if it hadn't spent too mucy money on warranty for that.

For sure! This ring isn't broken. The lands have been completely eroded away. The cylinder liner was toast & piston badly scored. I just don't have photos of the other components. The damage wasn't confined to just a single cylinder.

In chats with Chris Allen & Graham, I don't recall this problem being associated with the few warranty returns. Maybe so, I just can't say.

My perspective on the manifold re-design and fabrication experience. Separating the converter from the manifold is a win-win. Simpler to manufacture & a lot less costly to replace a bad catalytic converter, in terms of both labor and components.

[Matt B]

So... Is that still a potential issue to all 375HP LT-5 that run with OEM cat?

E.g. when I bought mine the left cat was crumbled and had to be replaced.

Just out of curiosity. I don't think it's a common issue as up to that day I never heard about it.

[A26B]

So... Is that still a potential issue to all 375HP LT-5 that run with OEM cat?

E.g. when I bought mine the left cat was crumbled and had to be replaced.

Just out of curiosity. I don't think it's a common issue as up to that day I never heard about it.

Matt,
I don't think it's a common issue at all. As I understand catalyst reversion, it may occur when the engine is operating @ high rpm, throttle is closed and clutch is engaged. Think in terms of road racing and using the engine for braking, i.e. engine braking.
As Hib stated in his last post, "lower pressure in the cylinder." I think it's likely negative pressure, occurring on the downstroke. The intake air is shut off when the throttle closes, and exhaust reverses direction, flowing towards the negative pressure on the downstroke, picking up catalyst "dust" on the way.
I don't imagine it takes long, considering the high rpm. Likely not a good idea to be at very high rpm & just dump the accelerator. Engine braking during heel-toe is generally in the mid-range of rpm.

Not an expert by any stretch and just my 2 cents, so I will probably draw some flaming. ;)

[Matt B]

Very interesting consideration! Thanks Marc. Indeed your thoughts don't indicate a common issue. And it shows that cylinder head, camshaft and exhaust design is way more complex than an average brain would expect.