engine rebuild decision time

[Steve L.]

I'm doing a body off resto for my 73. I've owned it since new and all parts (although some are rusted) are original.

The engine is in for reboring. It's the original L48 attached to a 4 speed. The engine rebuilder suggested I put in flat top pistons and a better cam to increase HP. (the pistons are toast since they were pounded out).

What do you guys think? I'm inclined to replace with originals piston and cam since it'll never be raced or anything like that.

What increase in HP would I get with another cam and flat top pistons?

Steve L.
73 coupe since new

[Joe L.]

Re: engine rebuild decision time

Steve----

Whether the pistons had to be "pounded out", or not, you would still have to replace the pistons, anyway, if the engine is rebored (i.e to a larger cylinder diameter).

Flat top pistons with 4 "eyebrow" valve reliefs (by far, the most common type for small block Chevrolet engines) used in conjunction with your 75 cc cylinder heads and a .030" overbore will produce a compression ratio only marginally higher than your stock compression ratio of 8.5:1. You will end up at about 8.7:1 or 8.8:1. You should still be able to use regular gas with no detonation problems. I would definitely recommend using the flat top pistons. Use the hypereutectic cast variety and NOT the cheap "rebuilder type" cast pistons that many machine shops ordinarily use. Excellent hypereutectics are manufactured by Keith Black/Silvolite (my favorite) or Federal Mogul/Speed-Pro.

You can gain significant performance advantage by changing the cam. You definitely don't want to go too wild, though, and "over-cam" this engine. My recommendation for your application would be a Speed-Pro retrofit hydraulic roller of their grind # CS-1070R. This is available as a kit with all the parts you need under KS-1070R. It's not cheap, but I think it's worth the price.

[Steve L.]

Re: engine rebuild decision time

thanks Joe,
What about originality and points deduction?
Also, what HP increase would I expect?
Are there any links to Speed-pro, I can't seem to find the web sites.
Which KB pistons would you recommend.

Thanks
Steve L.
73 coupe since new

[Duke W.]

Re: engine rebuild decision time

A good upgrade would be to install the L-82 cam and there is essentially no cost since on most rebuilds the cam and lifters are replaced anyway. If you want to trade a little top end power for more low end torque, advance it four degrees, which effectively yields the L-79 cam with a few degrees more duration.

Increasing compression ratio to the range of about 10:1 is a good idea, but you will have to use premium fuel. In current configuration your engine only requires unleaded regular. In not sure what pistons would be required to achieve this with your 76 cc heads, but they will probably be domed.

Finally the 2.5" manifolds would be a good idea if they can be easily integrated on your engine with not much more than replacing the head pipes, but I'm not sure what is involved on '73s. This would be the only visible change.

The L-82 cam would be detectable on PV, but would probably not loose any points on ops check.

Duke

[Joe L.]

Re: engine rebuild decision time

Steve-----

The change to the flat top pistons will produce no discernable difference so I don't think that any "originality points deductions" would occur. I don't think that the change to the Speed-Pro cam that I suggested would generate enough difference in idle quality to be detectable. But, I suppose that depends upon how well "tuned" that the judge's ears are.

The pistons, alone, will produce minimal HP increase. With the cam I suggested, I'd expect a minimum of a 25 hp increase and, likely, more.

Speed-Pro is part of Federal-Mogul; try their web-site. Last time I checked, though, the F-M web-site wasn't too useful, but it's been awhile so maybe they've improved now.

KB 198 are the flat top pistons with 4 valve reliefs for your application.

[Steve L.]

Re: engine rebuild decision time

I thought you guys would have said to go completely stock for the L48 and keep it completely original??

Steve L
73 coupe since new

[Clem Z.]

Re: engine rebuild decision time

if you are not into having a NCRS legal car i would look into a GM crate engine. they have all rating of HP and you get a 3 year,36,000 mile warranty which your rebuilder will not have and sometimes they are even cheaper than a rebuild. JMHO

[Patrick H.]

Re: engine rebuild decision time

It's not going to change things much to upgrade your pistons, at least. When I did my 71, I wanted to keep it close to stock, but have some minor upgrades internally.

I used Speed-Pro hypereutectic pistons like the STL-H631P30 shown on Summit's web site. Two valve reliefs, no dish. It raised the compression from 8.0:1 to 8.8:1 "for free" while allowing me to upgrade to the more durable hypereutectic material. If you're going to replace the pistons anyway, you might as well pay the small amount extra for hypereutectics, even if you stay with the slight dished style piston of stock design.

I also changed the cam to a CompCams 265-DEH. It does have a slight lope at idle and does give me slightly better performance in the upper rpms, but if you never drive there you will likely be happy with a stock replacement cam. My 72 is still bone stock, and while I can tell a difference from my 71, now that the 72 is tuned up they aren't worlds apart.

If I had the money, a roller cam would be nice, but I'm not sure you'll gain "that" much from the conventional camshaft. I've put one in another car, but that motor has yet to fire up so I can't give you any information on it with regard to real world usage.

Patrick