Monkeywrench

I'm going to look at a BB to stick in my 71 and the guys says it was out of a '74 so that would make it a LS4. My car originally had the LS5. Everything I see says they are the same

so why did '74 have no where near the HP as '71? Was it all due to emissions. Are the engines the same? Basically should I hold out for a 70-72 or is it possible to make this LS4 just as powerful...??? :confused:


[Modified by Monkeywrench, 8:41 PM 10/18/2002]

SuperFast80

1971 was rated at 365 GROSS and 285 NET. The LS5s and the LS4s are essentially the same hp output from 1971-1974. In '70 the compression was higher.

A C

I am not a big block guy, but weren't the LS-5 internals forged vs. the LS-4's cast?

AC

73 LS-4

That's been put to some debate as well. I've read some vintage articles on the LS-4 and some say the manual equipped cars did have forged cranks while the autos had cast. I think all the LS-5's had forged cranks but some were two bolt mains and some were four. Both engines did have forged rods as far as I know.

:cheers:
Pat Kunz

73 LS-4

Basically should I hold out for a 70-72 or is it possible to make this LS4 just as powerful...??? :confused:


[Modified by Monkeywrench, 8:41 PM 10/18/2002][/QUOTE]

Well if you want to get technical the 73 LS-4 was rated 5hp more then the 72 LS-5. Probably due to the 73's nice 049 heads, went back to 270hp for 74. In any case if your looking to build an engine up, it really doesn't matter much between the two, the 70 with its high compression is the only large difference between any of them. All the LS-4 and 5's make excellent low end torque, and this is what you will notice most when you drive one.

:cheers:
Pat Kunz

Yellow73BB

My LS4 has a cast (original) crank, and is a manual. So that theory isn't the case in all cars. Course my crank now has a crack too, and will soon be replaced by another cast one.


[Modified by Yellow73BB, 5:09 AM 10/19/2002]

73 LS-4

I read about the cranks somewhere on tom 73's site I believe. It might be something like the 4 bolt mains on the LS-5's, some do and some don't. By the way, how did you crack the crank?

:cheers:
Pat Kunz

Tom73

I had the same question and post it over on the NCRS board as to what was the differences in the 454 from 70 to 74. You can check the thread here: http://www.ncrs.org/forum/archive5.c...ames;read=8175

One of the responses was from Joe Lucia and he advised:

1) All 70-74 Corvette 454 cid engines were two bolt main configuration EXCEPT 1971 LS-6;

2) Most, if not all, 70-72 Corvette 454 cid engines used a forged steel crankshaft. There may have been some exceptions, though, with those using a cast nodular iron crankshaft;

3) All 73-74 Corvette 454 cid engines used a cast nodular iron crankshaft;

4) All 70-74 Corvettes with 454 cid engines, except 71 LS-6, used the same size intake and exhaust valves which, as I recall, were 2.06" intake and 1.72" exhaust.

5) 1970 LS-5 cylinder heads were unique to that year. For 1971-72 LS-5 engines, although different castings were used for each year, the cylinder heads were, essentially, the same. 1973 and 1974 LS-4 engines used cylinder heads specific to each year which were different for each year. As I recall, all 1970-72 LS-5 engines used cylinder heads which had about the same intake and exhaust port volumes. 73-74 LS-4 cylinder heads had smaller intake and exhaust ports, although I believe that 73 and 74 were about the same. All cylinder heads, except 71 LS-6, were cast iron material. 70-72 cylinder heads did not use induction hardened exhaust valve seats, including the valve seat inserts used on 71 LS-6 aluminum heads. 73-74 LS-4 cylinder heads used induction hardened exhaust valve seats;

6) All 70-72 LS-5 engines used the same camshaft, GM #3883986, which was also used on 66-69 L-36 427 engines. 1973 and 1974 LS-4 engines used a different camshaft, GM #353040, which was the same for both years. This camshaft was an "emissions profile" camshaft;

7) All 70-74 454 LS-4 and LS-5 engines used the same connecting rods which were forged steel with 3/8", knurled shank rod bolts;

All 70-74 LS-4 and LS-5 engines used cast aluminum pistons. The piston dome configuration was unique to each specific year, however, even though compression ratios may have been the same (72-74). 1970 LS-5 used 10.25:1 compression ratio; 1971 LS-5 used 8.5:1 compression ratio; 1972 LS-5 used 8.25:1 compression ratio; 1973-74 LS-4 used 8.25:1 compression ratio.

9) All 1970-74 Corvettes with 454 cid engine used the same oil pan, including LS-6;

10) 70-72 LS-5 engines used unique-to-each-year cast iron intake manifolds. However, all of the manifolds were similar. 73-74 LS-4 intake manifolds were also cast iron but had smaller port size to match the 73-74 cylinder heads. 73-74 manifolds also incorporated provisions for EGR.

11) All 70-74 Corvettes with 454 cid engines used the same side-specific exhaust manifold castings. 1971 with LS-6 and all 73-74 LS-4 manifolds were drilled and tapped for A.I.R. fittings.

12) All 70-74 Corvettes with 454 used the same valve springs except the 73-74 exhaust valve springs which were shorter to accomodate the rotators used during those years.

13) All 70-74 Corvettes with LS-5/LS-4 engines and manual transmissions used a 14" cast nodular iron flywheel with an 11" clutch. LS-6 used a 14" flywheel with a special dual disc clutch;

14) All 70-74 Corvettes with LS-5/LS-4 used the same oil pump

15) 1970-72 with LS-5 used the CCS (controlled combustion system) exhaust emissions control system. All 1971 LS-6 and all 73-74 LS-4 used the AIR (air injection reactor system) exhaust emissions control system.

Many of the horsepower/performance differences, particularly during the 72-74 period, were the result of emissions control system differences.


tom...

Jughead

WOW! Thanks for sharing that info Tom. I miss Joe Lucia. He helped me many, many times. Wealth of information. :yesnod:

73 LS-4

I have to deffinitly disagree with #5. The 049 heads have the largest exhaust ports of any of the large oval port heads. If I quote the Chevrolet big block parts interchange manual it says " The largest, as cast, large oval port exhaust runner seems to be found on the #-353049 head. It measures a maximum of 119cc." According to Chevy high performance the 049 casting has the same 253cc intake port as the 70 LS-5's 290 casting, but will actually outflow it marginally on the intake side but the 049 has a deffinite improvement on the exhaust side over the 290. The 049 head is one of the better production large oval port heads as far as flow, if you read a lot of the performance magazines they often use this casting for comparison purposes when comparing large oval port heads flow numbers. If you go to Chevy high performance's site they do some stock and aftermarket head comparisons with flow numbers.

:cheers:
Pat Kunz

Monkeywrench

So it seems like the only real difference was head design andthe intake/ehaust valve sizes. So stick a pair of '71 heads on a 74 block and you have LS5???

73 LS-4

The heads varied year to year, but all the LS-4's and 5's had the same valve sizes 2.06/1.72. See my post above for the heads, I don't know where that guy got the info on the LS-4 heads but it's wrong, especially in the case of the 049 heads . Do some research on this and you'll see. Now most of the LS-5 heads did have smaller cumbustion chambers then the LS-4 heads so most ran a slightly higher compression ratio of 8.5 vs 8.25:1, the 70 being the exception to this, I think it had a 105cc chamber.

Tom73

"That Guy" is probably the most knowledgeable guy in the Corvette world when it comes to stock/factory Corvette parts.

tom...

Yellow73BB

quote:
--------------------------------------------------------------------------------

I read about the cranks somewhere on tom 73's site I believe. It might be something like the 4 bolt mains on the LS-5's, some do and some don't. By the way, how did you crack the crank?

--------------------------------------------------------------------------------


I think the crank got too hot. My cam disintegrated last winter (apparently the shop that assembled it didn't lube it properly and it died pretty quickly). I redid the top end, but didn't do anything with the bottom (bad mistake). It only ran a few hours after putting all back together before the main and rod bearings were toast, and apparently it go so hot it cracked the crank at the #2 piston journal. Also scored the pinstons and bores, wore the rings substantially (.040+ end gap), and probably wrecked the oil pump although I haven't taken it apart yet. Bummer. Fortunately, the new cam and lifters are OK.


[Modified by Yellow73BB, 5:19 AM 10/21/2002]

73 LS-4

Tom, I'm not trying to say the guy isn't knowledgeable, I'm sure he knows a lot about the factory parts. But I can pull flow numbers and cc's that completely contradict what he said there about the 049 heads. If there's any evidence to the contrary to what I have said about these heads, I'd love to see it, and I don't mean that in a mean way, more like please educate me if I'm wrong, but I need to see evidence to back it. If you want to see my evidence I'd be more then happy to show it. Or simply go to chevyhighperformance.com and look at their head comparisons there.

:cheers:
Pat Kunz

szayerg

It's a very old post, but I red it many times as it summarizes the best of LS4, LS5 and LS6 specs and differences. I also believed that all 73-74 454 CID engines have cast crankshaft.

Today, I have found in an old specification docs, that '73 454 with manual transmission has forged crankshaft:



I have attached the whole specification file about 1973 Corvette. I don't know where I have found it before, I have it on my drive since years. The crankshaft is on page 46.

Kind Regards, Géza

Greg

Yes, ancient thread but that is good additional info you posted, Geza.

67:72

That document, as well as GM docs for hundreds of other Chevrolets, is available through the GM Heritage site: https://www.gm.com/heritage/archive/...formation-kits

leigh1322

And since no one has mentioned it yet, the distributor timing specs were changed every year. And not for the best for performance. GM kept retarding the timing more and more to try to meet the new emission standards, until the catalytic convertors came out in 1975. Retarding the timing and lowering the compression helped NOX tremendously, but did it by making the engine run hotter on purpose. These years engines are helped tremendously by re-curving the distributor, both for running cooler and for performance.

The 73 & 74 LS4 had EGR in the intake, I am surprised no one mentioned that. To me that would be the biggest difference vs an LS5. IIRC the cam changes were miniscule.

Rescue Rogers

you need compression, intake / head flow and a bigger cam. Unless you plan over 600hp the 2 bolt and cast crank should be fine. Depending on how much you want to spend I would look at some flat or domed pistons over the dished ones, some brodix race rite heads which can go pretty low on the cylinder head volume which would help increase the compression if you want to leave the pistons alone, Last you need a cam in the.500 lift range with a duration over 220 at .050 lift. That should get you above 400hp.