BlackRocket

When did the LT5 go to a 4-bolt main + what other differences were made through the 5 year production run?

The following #s indicate '92 but if my memory serves me correct it was '93.

LT5:
- HP: 375@5800 ('90-'91); 405@5800 ('92-'95)
- Torque: 375@4800 ('90-'91); 385@4800 ('92-'95)

Is there a big deal with the 2-bolt block having stress issues?

Also, what was the big deal with the Al Dunn heads?


Thanks!
B-R

wfot

The 90 to 91 were 2 bolt, the 93 to 95 had the 4 bolt. There are other changes that had to be made to go with the new bottom girdle, the oil pan. The intake has a few differences as well. From what i have read, the 2 bolt main blocks can withs5and 200 hours of the saw tooth test and the 4 bolt mains can take 400 hours. This means full throttle from peak torgue to peak hp and back to peak torgue for 200 straight hours with now significant wear. Pretty impressive. ..

Goldcylon

1993

No

They Flowed better and sometimes you did not get a pair on the engine but a mismatch

GC

Paul Workman

Marc Haibeck documented the differences in the 2 vs. 4 bolt (90-92 vs. 93-95) LT5 blocks. (See excerpt from a paper of his [below]).

But, to get ahead of the question, it is an error to compare any reference or concerns with the 2-bolt SBC mains to the lower end of the LT5...if that is where this is going.

The 93-95 4-bolt configuration increased the bolt cross-section area by only 12+% compared to the larger 12mm bolts for the 90-92 LT5 blocks. So, the use of 4 bolts, in this case, was more to increase the stability of the main's steel to aluminum bonding within the ladder, more so than the bolstering strength of the 2-bolt caps, per se'. (Kevin Costello's 10.3 second, closed deck 441 cubic inch LT5 and several others too have amply proved the integrity of the 90-92 2-bolt blocks).

The point is, there is no burning justification to hold out for a 93-95 LT5 based on the minor 2-bolt vs. 4-bolt difference. Both blocks are good to more HP than practically anybody would every require anyway.

As for the Dunn vs. Birmal heads: The Birmal plant closed down before the end of the LT5 run was completed. According to "HEART OF THE BEAST" by Young, the core specs were transferred to the new supplier - intact. Tho there were improvements in some materials used and casting accuracy was improved, the performance otherwise was essential identical to the point that some LT5s had one of each of the two manufacture's heads on them.

In the context of someone having concern(?) for whether it is better to have a 4-bolt LT5 block AND the Dunn heads combination, experience has shown no practical differences in strength or reliability in the first place. And, in the 2nd place, porting the LT5 in preparation for 500-800+ HP (for example) completely negates ANY factory comparisons: Dunn vs. Birmal is entirely moot. Building a big power LT5? Have at it and don't give bolt or Dunn heads a second thought! They ain't yo daddy's SBC!

bb62

Paul,

The 10HP or so that the Dunn Heads adds is certainly nothing significant - and it's one reason that GM declined to recertify them (and agree it's a moot point if mods are planned), but one improvement that the extra Dunn head offered was an elimination of the chain slap issue on start-up.

FASTAZU

Excuse me if I'm wrong here but didn't the 93-94 LT5 have the same HP increase without Dunn heads.

bb62

The Birmal heads are what were used in the calibration process and, yes, did deliver 405 HP. They were machined versions of the 375 heads. When the Birmal facility was going bankrupt and Dunn was selected to produce the final sets of heads, Dunn, instead of copying the Birmal casting and machining, had the design changes incorporated into the tooling for the casting. Upon testing, they found that the cast Dunn heads flowed better than the machined Birmal heads, to a point where about 10 extra HP became available. But with the Chevrolet discontinuing the LT5 and the power difference not significantly different from a sales perspective would not have wanted to certify a new HP rating (i.e. - it's very expensive), so the vehicle was sold with the new heads without any fanfare.

FASTAZU

So the Dunn Head 95's are actually 425hp?

KGV

It would be infesting to see if this was the case, and the last batch of cars are 425HP. If GM were to allow the release of the dyno sheets, then we would know for sure, but this may be a long time coming.

Paul Workman

405+10 = 415

A pretty good argument can be made that the weight of the ZR-1s grew from the 1990 model to the 95 MY; enough to mitigate most (all?) of the increase in HP

It is the 10 hp gain for some minor runner shaping was only the tip of the iceberg! Porting and headers and a tune brings the motor to 510+ HP on stock cams.

Drivability is practically unchanged, including gas mileage! Is it any wonder why the "500" package is so popular among ZR-1 owners?

FASTAZU

My bad was way to late for me to be up

So that would make the split head LT5's 410hp LOL

By the way I love my 500hp 2 bolt main LT5 with 3:73 gears..

Paul Workman

QUOTE=FASTAZU: By the way I love my 500hp 2 bolt main LT5 with 3:73 gears..

I didn't dyno my car after I ported the top end, so I don't know what the rwhp actually was. But, ZOWIE! What a difference it made!

The lion's share of the improvement came from porting the top end and adding headers/exhaust. So much so that after I finished porting the heads, the added 25 or so hp felt a little...anticlimactic? (Even tho it dyno'ed 432 HP at the wheels.)

FASTAZU

So true Marc did mine many years back, had all the porting and gears done at the same time gained about 80hp plus gears at once.

bb62

Glass,

With all due respect - please don't tell me how my business is run. I spent the majority of my career as a senior powertrain planning manager for that other large OEM so I know how things operate from the inside. If you change the HP of an engine, with the rules that were in place at the time, it required a rerun of the emissions testing certification (since the EPA equated HP with a change in C02 emissions at the very least). This EPA testing is one of the reasons why there are so few engine options compared with the vehicles offered in the 1960s. It wasn't until the debacle with the Mustang (at least for Ford) advertised HP numbers (well after the ZR-1 program was complete) that the HP was certified, but that is NOT the only certification required. Sometimes, if the changes were minor, you could have gotten away with some minor emissions testing - but testing of one sort or another WOULD have been required. Ford has their WC requirements that all suppliers needed to adhere to - GM has the same set (although slightly different) of design/development/calibration testing requirements.

As for proof of the extra HP from the Dunn head cars, that comes directly from conversations with Gary Cline, the Mercruiser Project manager for the LT-5 program. Please tell me what other statements that I make where I have a "tendency to post things that aren't correct at times"......

ZR-71

ZEN357

So basically if you are looking for one buy a 93 and up?

Tom400CFI

No. As was posted above, in this very thread, there isn't a meaningful difference in strength from 2 bolt to 4 bolt. Any diff in power from early to late heads is offset by weight increases (and car to car variations blow any "10hp diff" out of the water anyway), and lastly, if you're planning to mod, the mods will eliminate any perceived difference that the various years may or may not provide.

bb62, I'm not sure you want to go down the road of "where I have a tendency to post things that aren't correct at times"...that probably isn't going to be constructive...but tell us this: How do the trap speeds typically compare for a STOCK "birmal" head car vs. a STOCK "dunn" head car? My guess is that you couldn't accurately categorize birmal and dunn cars based on 1/4 mile trap speeds. I'd bet that LT5's have a greater variation in traps due to assembly tolerances, than any variation in heads.


.

wfot

I heard that the 2 bolt block passed the sawtooth test for 200 hours and to strengthen the block they went to 4 bolt mains and the 4 bolts mains can take 400 hours of the saw tooth test.

The 2 bolts mains are not just 2 bolts, they have 10 mm bolts on mains 2 and 4 and 12mm on 1, 3 & 5. The 4 bolt mains have 10mm on all mains with 4 on each main.

​​​​The 2 bolt main blocks will show some crank walk at BIG cubes and high rpm and NO2. Most owners will never come close to even hurting the 2 bolt main blocks feelings, let alone damaging it.

Rkreigh

clam shell ladder style bottom end makes the difference between 2 and 4 bolt engines not worth talking about

the duns heads had some slight QC improvements and a better tensioner and indeed many of the engines had one dunns and one birmal head.

I think the lore is more than the HP. the 93 engines (all of em built then), had slight better alignment of the intake and better intake that eliminate the port bump for the injectors, a bit different cam timing, rods, pistons, and a few nice improvements to hit the 405 target. EGRs for better emissions, ect....

I have had a 90, 93, 2 95s and I can tell you the 93-95 c4 has a bit better QC overall , but not so much difference in the lt5, they are all animals. early cars are lighter, and just as quick, later cars had a bit better top end.

all ZR1s 93-95s had engines made in the final production run in 93

Paul Workman

Much to the dismay of many steeped in SBC lore, no valid comparisons can be projected from the iron SBC to the LT5, far as 2 vs. 4 bolt mains [for just one] go: apples vs. oranges.

If I might add to your list of changes between the 90-92 and 93+ injector housing (runner ID), the ID of the 93+ IHs primary runner was 34mm vs. the 33mm for the 90-92s. But, of course when the top end is ported, that as well as Birmal vs. Dunn head nuances go out the window!