Need advice on a 68 L88 block rebuild....

[Terry F.]

Hi, need some advice on a 68 L88 block rebuild.

1) Would like to rebuild it and have it run on high octane pump gas. If someone could recommend a cam and piston profile (and where to get them) that would be great. Would like a radical engine but needs to run on pump gas and have the radicalness of the L88 sound. I expect there are some nice cams out there to do the job.

2) Cylinder wall clean up. If the cylinders are cut just 10 or 20 over to clean them up, what will piston availability be like. Would like to cut the cylinders to a minimum but it seems the pistons are not out there. Suggestions would be appreciated.

3) Other wise the engine will be stock rebuild but other suggestions would be appreciated. Just trying to be realistic.

Thanks, Terry

[Clem Z.]

there are engines out there with 427/435 HP

style pistons,11.25:1 CR, with a L88 cam using pump gas. not the best comb for max performance with closed exhaust but the L-88 sound is there. clean up the chambers and remove all sharp edges from the piston when doing this setup.JE can make you any O.S.piston size you want and their design allows for close piston to wall clearance.

[Tom B.]

Re: there are engines out there with 427/435 HP

As a rule of thumb you can run 1 point higher compression ratio with aluminum heads than iron and use the same fuel. Also it takes about 1 point higher compression ratio with aluminum heads to make the same power you would with iron due to aluminum's ability to disapate heat better. The high compression is part of the L88 sound along with the cam. Don't drop your CR too low with a radical cam or your engine will get sluggish.

Clem's suggestions above are good. It's smart to remove any sharp edges from the chamber and piston top (there is usually a sharp edge where the valve reliefs were cut) to help avoid hot spots that would cause pre-ignition. Just knock the edge off. Don't remove a lot of metal.

Tom

[Terry F.]

Re: there are engines out there with 427/435 HP

I am thinking 11:1 pistons max with a milder grind cam to be on the safe side of drivability and longevity.

I am not familiar with JE for piston source. If they blue print a set of pistons are they pretty much same design as original?? Would they be cast iron or aluminum or that other word I can't pronouce?

I would apprciate any other recommendations or ideas.

Thanks, Terry

[Terry F.]

Re: there are engines out there with 427/435 HP

Any recommendations for getting odd size pistons (10 or 20 over). Goal is to save the engine as much as possible from having to go 30 over. Terry

[Clem Z.]

Re: there are engines out there with 427/435 HP

most pistons come in .030 for the first oversize but don't worry because high performance BBC can be bored to .125 oversize with no problem. if you want smaller than .030 you need to go with custom made pistons.

[Clem Z.]

they would be forged aluminum and

if you you go with a milder cam you will not have the "L-88 sound"

[Terry F.]

Re: there are engines out there with 427/435 HP

Any recommendation on what the piston should be made out of? Cast iron vs that Hyper@@@@@@ word I cant spell or pronouce. PS I can't spell any way but....

Terry

[Terry F.]

Re: they would be forged aluminum and

Hmmm, so if a person goes with the aggressive cam and the aluminum heads, you will need the 11.25:1 pistons to make it work it sounds like. I suppose putting some Crane roler tip rockers on it might help the cam live longer. What do you think about using them in this set up? Terry

[Mike Baker]

I had some made 40 over.

Obviously more expensive than buying them off the shelf but, I would not hesitate to do it again.

http://www.jepistons.com/sportsman.asp

[Terry F.]

Thanks, Terry *NM*

[Mark #28455]

were you at the Orlando meet?

I brought my L89 which was dealer converted to L88 specs. Did you hear it?

The "sound" you are looking for comes from the cylinder pressure present when the exhaust valve opens. You would be better off with a slightly lower compression ratio and a shorter duration camshaft to keep the dynamic compression ratio up and more of the 427 cubes of air/fuel in the cylinder (so the engine isn't soggy). In addition, the L88 cam has WAY too much duration to run well through the stock exhaust system. Be careful with your real compression ratios! I rebuilt my engine as a 489 with an honest 10.3:1 compression using the JE closed chamber domed pistons and the 842 heads. I used a relatively agressive cam with only 244 degrees intake duration at .050" with .578 lift and 252 degrees exhaust duration at .050" with .600 lift, with 114 degree lobe separation (stock L88 was 264/268 degrees with 112 separation - so it's a LOT milder than stock). It gets me an honest 8:1 dynamic compression ratio which just makes it on 93 octane gas.

The stock 435 cam had 242/242 duration with 114 separation and .520 lift - so mine is similar to the 435 on the intake and with 10 degrees more exhaust duration to get a little sharper exhaust sound. Does it work? I brought it to the Orlando meet in my crusty L89. I think it sounds about the same as the L88 parts that the dealer installed 36 years ago - same CRAPPY idle, and it pulls a LOT harder at low RPM (but then again, the extra 1/2" stroke doesn't hurt ).

All in all, when you start playing around with things, there is no guarantee of your outcome! If you're going to make changes, some of them WON'T work out and you'll have to start all over again. No matter what you do, always keep track of the dynamic compression ratio and keep in mind that the stock exhaust is restrictive and does not work well on a real L88 - the street mechanical cam was a really good cam matched to the stock exhaust.

Good luck,
Mark

[Terry F.]

Re: were you at the Orlando meet?

How far can you push the dynamic compression ration in your opinion and still run on pump gas? I assume the dynamic compression ratio is more important at determining the engines ability to run on pump gas??

I am far from being an expert on engines. I get by because I am conservative and never venture to far off the path of a stock configuration. I have never had to put together an L88. It sounds like you would either need off road pipes or open side pipes to let it breath on the exhaust side.

Need to know how far to push the dynamic compression ratio.

Thanks, Terry

[Terry F.]

Re: were you at the Orlando meet?

So let me see. Maybe start with a compression ration that is reasonable like 10.25 to 1 and then pick some fairly known cam profiles and run them through a dynamic comprssion ratio program?? Am I just full of it or what??? Pick the cam that produces the greatest dynamic compression ratio?? I expect then that after all this you would probably have to rejet the carb to get that right also?? Thanks for the help, Terry

[Mark #28455]

Like Duke says, it's a whole system

Any engine must work as a coordinated system. The BB Chevy with early heads has several limitations.
1) Any dynamic compression ratio over about 8:1 tends not to tolerate pump 93 octane fuel.
2) The rectangular intake port on the 1960's heads is WAY too big for street RPM on a 427 - lousy port velocity - that's why all my recent motors have been strokers (to boost the lower RPM torque within the limitations of the heads).
3) The stock exhaust on a BB Corvette is too restrictive for anything more than the 435 HP street mechanical cam - exhaust under pressure dilutes the intake charge and drops the torque production (your 427 isn't effectively pumping 427 cubes of air/fuel with each cycle due to the exhaust dilution) so you get the "nasty" L88 idle quality, but you actually do trade off by LOSING power (with the stock exhaust).
4) The L88 cam ONLY works well with an OPEN exhaust - forget dual 2 1/2" pipes, we're talking hooker sidemounts with NO mufflers.
5) The intake lobe of your camshaft works hand in hand with the compression ratio, so you have to look at them together. That's where dynamic compression ratio comes in - go to the KB-silvolite web site to read about it and use the calculators. You have to be honest about your intended RPM range and you pick a cam lobe to match it, then you look at the available pistons and input the piston and cam info to check the dynamic compression ratio. For street with the old BB heads, don't go over 8:1.
6) Be careful about lobe separation angle - all the aftermarket cams seem to go with an angle narrower than the stock 114 degrees. If you take the same camshaft lobes and narrow the angle (to 110 or 108) the engine is more sensitive to the exhaust backpressure so you get more mixture dilution with a full exhaust and mufflers, on the other hand, if you had open headers, you might see a slight improvement in power as the intake valve would close a little sooner and boost the dynamic compression, and you might get a little better cylinder filling if the headers help pull the intake charge into the cylinder (won't work with a stock exhaust).
7) Rougher idle WILL affect your vacuum accessories like the opening of the headlights. In my engine, my cam idles at 1000 RPM with 12 mmHg vacuum so all the accessories work OK - but no guarantee given!

What was my design goal? I wanted the L88 raspy idle, but still wanted to maximize lower RPM torque. It was a trade-off! My engine would be a better all around performer with a STOCK 435 HP cam, but then would lack the idle quality. As I said before, I did have a stock type open chamber L88 in it before, and this engine definitely pulls harder through 6500 RPM with a full exhaust.

P.S. My wife's 454 with well ported oval port heads and a high lift short duration hydraulic cam pulls even harder through 5000 RPM .

Mark