I now have 250 miles on my home built 381 CU IN LT1/4 engine...and I'm flat amazed it works! Ha.

I used:

4340 SCAT forged crank 3.75 stroke-stock type balance
4340 SCAT forged H-beams rods 5.7 length
4.020 SRP flat top forged pistons with 5 cc valve reliefs and floating pins
Speed Pro Plasma Moly rings (.017 and .021 filed gaps top and second, respectively)
GMPP Severe Duty Timing Chain set
GMPP 847 Hydraulic Roller Camshaft
CC Pro Magnum Hydraulic Lifters
Canton Oil Pan with OEM LT4 Oil Pump
CNCed LT4 heads (by CNC Cylinder Heads in Florida)
AFR Hydra Rev Kit
1995 Corvette LT1 block (4 bolt main-not splayed)

My block was rebuilt once before due to spun rod bearings...I had it checked and the main alignment required an align bore. More money. Even my stock LT4 engine had several different size bearing shells from the factory (like .0006 over/under).

I also asked for min deck cut as my compression was going to be atleast 11.7 with .025 piston in the hole clearance! I preferred to use Fel Pro gasket 1074 with .039 compressed thickness. After the align bore and the decking, the piston in the hole ended up being .016...giving me 11.9 CR! I know it has been done before and with a big camshaft I'm not freaking out, but that is some hefty squeeze! This results in a quench of .055 so not real close to the supposed ideal of .040. I figured if the stock LT4 runs 10.8 and pump gas with its .049 gasket and .025 deck height (.064 quench) then I should be ok.

I also ended up with an SRP piston that was .001 larger than the other seven and 10 grams heavier. These pistons also solidly contacted the crank counterweights. This also occurred on another 407 I put together recently...I spoke to SCAT about it and they take responsibility, but SRP is to blame as they are varying their piston skirt tab profile...this would be key if you balanced your engine before hanging the rotating assembly.

To dovetail with that, camshaft to rod clearance was a problem with my standard sized base circle GMPP camshaft-two rods hit the lobes...I ended up trimming 1 grams of weight from all 8 rods for clearance. This disassembly and grind was much more acceptable than new special camshaft (2 week wait), new pushrods, and possibly new lifters! By the way, the Pro Magnum Lifters from Competition Cams (also known as Comp R's) actually say both names on the box! In other words, I could never find the infamous Comp R's in their catalog, just the Pro Magnums...one and the same...:)

I also noticed on my OEM LT4 that the stub drive for the oil pump (gear drives the oil pump shaft) had a large crack in the plastic where it is bolted down. If this were to break, oil pressure would be lost-MORE Performance apparently has seen this as they offer a oil pressure saver for minimal cost.

Another point on the align bore, I was concerned about this as not many timing chain sets are available for the LT1/4 without massaging something. I bolted the brand new Severe Duty chain up and the slack was significant-.25 inch each side! Thanks to my trusty machine shop-they contacted Cloyes (the maker for GM) and they offer oversize crank sprockets-very slight oversize of course as chains have only so much tolerance. Called them up and a larger one was on the way...bolted it up and it is tighter than a banjo-ok not quite, but about perfect-my guess.

I did learn not to use the OEM oil pressure sensors as a handle as it came off quite easily in two pieces...that was ~$50...:(

Another thing not to do is remove a fuel injector from the rail when it is still pressurized with 50 psi of gas...otherwise a geyser of fuel will squirt you in the eye and 10 feet over your head! Ask me how I know...or use pressurized lithium grease and have it end up in your eye once again or install the wrong cylinder head on the wrong side of the block even after double checking or buy a $50 rod bolt stretch guage...either go with a quality ARP model (for $150) or just use a torque wrench like I did after getting fed up my Lunati stretch guage wouldn't fit my SCAT rods and 350 block and finally definitely check the oil pan crank clearance with the gasket and bolted down BEFORE you glue it into place for the final time...or you will end up removing it two more times grinding the contact points...:D

I prelubed the engine prior to installing the intake manifold. I took my time with that and made sure to turn the engine over .25 turn at a time and watch for oil to appear from each rocker. I then installed the engine 2 days later. I then pulled the fuel pump fuse and coil wire and spun the engine over for about 30 seconds. Reconnected and lit it off. The car started on the second try-large camshaft playing with the tune I'm sure, but idle right off-beauty of roller camshafts! Ran the lash in and seated the rings the first day.

Been tuning with LT1 Edit with major help from GS007 and the results are awesome. The car feels like stock-even pulls 6th from 1400 rpm or so on cruise (3.45 axle). I have yet to exceed 4000 rpm and 50% throttle, but I'm just blown away these cars are that flexible.

A ton of work to do yourself, but talk about a feeling of accomplishment...until the motor scatters on the dyno...that will be 16 April! :D

I hope to pull 6800 rpm reliably and road race, autocross, drag race, topend, and daily driver with the best of them...time will tell.


[Modified by No Go, 9:08 PM 4/4/2004]

 

:cool: :cheers: You and I may be talking quite a bit when I tackle the 396 stroker next year! :D

 

:thumbs:

 

:cool: :cheers:

 

first, i'm surprised a 3.75" stroke had clearance issues with the 847 cam. those scat rods must be larger in that area than the eagles in my 396.

second, can you tell me more about the cloyes gear? i did read on their website how they offer the timing set with the larger crank sprocket, but it doesn't say more or even show a picture. i have the "extreme" duty roller set from GMPP. is that same thing you have? and the cloyes gear took out a lot of the slack? if so, what's the part number?

thanks.

-michael

 

VERY interested in seeing how much power it makes......how well do your heads flow ?

Great job tackling it yourself, alot of work! Cant wait to see numbers. :thumbs:

 

My heads flow 282 and 192 at .600 lift on 'my' bench. Nothing spectacular, but I'm not expecting a stone either...:) An extra 40 WHP is about my hope...hopefully conservative???

I misnamed the timing chain set...it is Extreme Duty from Chevy...made by Cloyes. I called 1 479 963 2105 and extension 231 (Wayne) who apparently has a selection of different sized crank gears on his desk. He asked me about what slack I had and he sent on that he thought would work. I was slightly doubtful, but what do you know...nice and tight. I haven't received the invoice yet, so no cost info.

 

Wow, I just listened to an LT4 with an 847 cam and it sounded SICK! You are going to have fun with that MONSTER. :cheers:

 

:thumbs: