"283" Short Block

[jack alexander]

Is there any difference in a 58 and a 61 short block, besides the obvious external ID dates and numbers. I was looking at a 61 vette 275 HP "feulie" short block for a friends project 61 and I was told on the QT that it was really a 58 "non feulie" restamped. can I tell by outward appearance?

[William C.]

Re: "283" Short Block

the 275hp SHORTBLOCK was the same as a 230hp single 4-barrel engine. The difference was in the haeds and the FI. The only 283's that used a different shortblock was the solid-lifter engines and the only change there was the camshaft (270hp version) except for the solid lifter FI engines which changed to pop-up pistons.

[Duke W.]

Re: "283" Short Block

The NCRS spec guide list the 275 HP FI as having 11:1 CR - same as 315HP FI versus 9.5 for the 230/245/270 HP, so wouldn't that indicate that the 275 HP also used the same pistons as the 315 HP?

Duke

[William C.]

Re: "283" Short Block

There is some contraversy over the 275hp engine on that count Duke, Some references indicate the use of the popup pistons in the 275 hp, some, including the GM parts book, do not. The 275hp FI is a rare enough bird that I don't know how to nail it down 100% but I wouldn't discount the possibility of that engine using the higher compression, although with the short hydraulic cam I would think it would be detonation prone.

[Clem Z.]

Re: "283" Short Block

the NHRA tech sheet list the 275 hp corvette engine with .122 high 5.7 cc domes. these specs are supplied to NHRA by GM. i was the national tech director for a east coast drag racing assoc back then so we also had these specs.

[jack alexander]

Re: "283" Short Block

Thanks Guys

As usual I get a lot more info that I asked for, and am glad to have it. In my original question, I think I failed to mention that what we are looking at is a bare block. No guts whatsoever. Because it may be stamped, I was looking for something outside the block that would indicate a solid lifter motor,a 275-315 "feulie"? I know they had a different harmonic balancer, but that is not there either.
Thanks
Jack

[William C.]

Re: "283" Short Block

Nothing different in the blocks except the variations in casting numbers thru the years and the stamped ID numbers on the front pad.

[jack alexander]

Re: "283" Short Block

Bill

Thanks for the help! Regarding the correct cam, popup pistons, etc. Where would be a good place to find these parts.
Jack

[Duke W.]

Re: "283" Short Block

The original cam is no longer available, but the 3896929 cam that replaced it in '67 is almost identical, and will provide the same operational characteristics.

The forged domed pistons should still be available from Federal Mogul (They bought the TRW piston operation) L2275F-30 or 60, but Bill made a very good point, 11:1 compression with a short, low overlap cam is going to be prone to detonation on today's best premiums, so you should consider an aftermarket hypereutectic piston with no more than 10:1 compression.

If you want to use the domed TRW pistons I would recommend having the dome machined off.

If you want to turn it into a (upgraded) 315 HP fuelie install the LT-1 cam, and if you want the eqivalent of a 315 HP fuelie with hydraulic lifters, use the L-79 cam.

Assuming the 275 HP did have the 11:1 pistons the only basic internal difference between it and the 315 HP was the cam, as both used the 461X heads. If you go with the L-79 or LT-1 cam you can go with the 11:1 pistons as the additional overlap of these cams and later closing inlet valve will kill enough low speed dynamic compression to keep them out of detonation.

Duke

[Jerry C.]

Re: "283" Short Block

On the rear of the block the last three digits should read 519 and the other side should be the casting date IE c 12 1 for March 12th 1961.If the c 12 61 is seen it is not a block cast in Flint,Mi and is not correct for a corvette. 1958 block also had a unique rope rear main seal. If you could get the numbers from the rear of the block and post we could tell what year it is.

[jack alexander]

Re: "283" Short Block

Thanks Duke, Jerry and all you guys.
We went ahead and got the block, even if it is a stamped block, at least they did a great job. I really appreciated the info on the cam and pistons, but I have to wonder, will I be giving up a lot of performance in downgrading to a 10:1 from a 11:1 piston? And on the subject of gas, what is best Premium? I've used mostly American and Sunoco.

[Duke W.]

Re: "283" Short Block

Realize that advertised 11:1 compression ratio and actual compression ratio as built are two different numbers. Typical as built CRs are probably at least a quarter to a half point less than advertised, and in late '62 and '63 the 11.25:1 SHP/FI engines were assembled with two head gaskets to lower compression ratio even further to address customer detonation complaints.

When assembling the engine you need to select a suitable candidate piston, then carfully measure the deck clearance and know your the precise head gasket thickness and nominal (or preferrably measured) head chamber volume to compute the actual compression ratio, and I would recommend no more than 10:1. The difference in peak power and torque between a true 10:1 and a true 11:1 is probably on the order of three to five percent - not a big number, and you can make this up by pocket porting/port matching the heads and doing a three angle job.

If you built an 11:1 engine and had to retard the ignition timing to keep it out of detonation with the best pump premiums, you might actually end up with less torque and power than a 10:1 engine that can be run with optimium spark timing.

As far as fuels go, in most parts of the country gasoline blends are regulated by the EPA, and there is essentially no difference between brands, so you want to use whatever available octane rating that will allow the engine to run detonation free with an optimum ignition advance map. A key variable in this equation is compression ratio, and it's best to err on the low side.

If you want more power and torque the best way is to sneak a 327 crank into the block with a suitable set of pistons, but don't try to stretch the CR to the ragged edge.

Duke

[William C.]

Re: "283" Short Block

I believe that engine becomes a 307 at that point. If anyone needs a set of domed TRW's for a 307, I have some.

[jack alexander]

Re: "283" Short Block

Thanks Bill and Duke
Bill when you say not to push it ,are you saying that the 327 cam and porting job might be too much for and older 283 block? I don,t want to build a rod,but it is a Corvette! Also would you know what is the part # for the 61 315 HP cam. and is the crank the same in the HiPo 283,s as in the lower HP? Duke if I was interested in those pistons, how do I get ahold of you?
Thanks !
Jack

[William C.]

Re: "283" Short Block

The 315 hp cam was the "duntov" cam, same as used in the '57-60 and '62-62 mechanical lifter engines. Reproductions of these cams are available, and there are several threads in the archives regarding this issue. I have some recollection from the period when the 327 was introduced that the crank is a "drop in" unit but there mey be interference between the connecting rod bolts and the block in some areas. This is taking my memeory back a long ways, but it is worth checking before you plan that adventure. Both 283 cranks and 327 cranks at least thru '65 were forged parts, but I believe the SHP versions may have been subjected to a process referred to as "nitriding" or "Tuftriding" that was a very thin surface hardening treatment. Any good crank shop has similar treatments available for high performance applications. With respect to compression, there were a couple of very good discussions about this recently, but the significant thing to remember is that the factory never shipped engines much over 10.0 to 1 ACTUAL compression, but the SHP engines could be "blueprinted" to 11 to 1 or so for racing applications. Be it a 315hp 283 or a 360hp 327, an actual 10 to 1 compression, combined with carefull assembly and attention to the valve job will run very well.