I have a matching numbers 67 400hp air car that is undergoing a frame off restoration. The objective is to make the car as NCRS correct as reasonably possible to keep the value and the originality of the car although it may or may not be judged. The car will be driven only occasionally on Sundays and to car shows. The engine has never been rebuilt but has just been torn down this week by the engine builder to start the rebuild process. It is now decision time. Should the engine be rebuilt to completely stock although it will need to be rebored .030, i.e., cast pistons, basic head work, cam with a profile as close to original cam as possible, etc. OR should I go for a little more modifications to get more performance, i.e., Comp No. 272 H or 274 H cam, forged pistions, some head porting with larger valves, and would modifications such as these be viewed as a positive or a negative in the NCRS camp. I am not looking for a radical engine in any case and want to end up with an engine sounding as close to the original as possible. I am new to NCRS and would appreciate any thoughts or suggestions you may have to help me make a decision.
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Re: Engine Rebuild
Use all OE or equivalent components. Find a good head guy to pocket port/port match and do a three angle vavle job with .040" wide inlet seats and .060" on the exhaust. Do not install oversized valves, but used the latest and greatest method to rebuild the valve guides.
Magnaflux the rods, and have the entire rotating reciprocating assembly precision balanced.
Have the machinist check for head warpage. If they are okay, don't mill them. Same with the block deck. Did the machinist measure the piston/crown deck clearance before the engine was disassebled so you can compute the compression ratio with various gasket thicknesses?
Do not remove metal from anything unless absolutely, positively REQUIRED to correct a problem. Watch out for the "we do them all that way" response. If that's the response collect your stuff and get the heck out of there.
It's your engine. You are the customer. Find someone to do it YOUR way.
Duke -
Re: Engine Rebuild
Paul - this is always a personal choice, and it depends on what you are really after with the car. First, I would not suggest going to forged pistons on an engine that originally had cast based on your assumption they are needed for strength. Better to go to a Keith Black hyperutectic cast piston instead, they are nearly as strong as forged, have excellent stability characteristics, and are fitted with tight cylinder to piston clearnances which add longevity and better oil control than forged. I would have to rods maganafluxed for cracks, and resized using ARP wavelock rod bolts for duranbility and peace of mind. In addition, I would absolutely balance the entire rotating assembly from harmonic balancer to clutch by a competent engine machine shop. In addition, if you can afford or find pistons with less overbore than 0.030, I would invest in the smallest overbore as possible. Granted, 0.030 over on a vintage BB is not a problem at all, plan for the least aggressive work as possible. Many machinsts just suggest 0.030 since it is very easy for them, they don't have to check as carefully and setup is easier for cleaning up the walls. This choice really depends on the pistons you choose, and what your wallet will bear. On the camshaft choice, without matching head modifications, you can easily just kill the low end torque, and not gain much. Changing the characteristics of the power band without gearing to suit will not make you a happy camper either. If you were ever to PV the car, the cam you mention would be audibly different at idle from your stock cam, as would the sound of forged pistons.....CraigComment
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Re: Engine Rebuild
Thanks Craig and Duke for the information. The engine builder I have selected is very conscientious and will work with me to give me what I want. I just need to decide and you are helping me do that. I will ask if he measured the piston/crown deck clearance before disassembly but he intends to achieve a 9:1 compression ratio. I know this is lower than the original compression but with the lower gas octane today does this lower compression seem reasonable or what compression ratio would you like to see? Also, to go with OE type componants, is there a specifc cam that you would recommend if I go with the cast pistons and pocket port& polish plus a 3 angle valve job on the head? Duke, what does PV the car mean? Thanks for the help!Comment
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Re: Engine Rebuild
I would go with the OE cam. If it's no longer available from GM you should be able to buy an exact duplicate from Federal Mogul. I'm not as much of an "expert" at BB cams as SB cams, but I can tell you that the old OE SB cams are impossible to beat IMO. The head work will pickup 5-10 percent more top end power without affecting low end torque, idle quality, fuel economy, or driveability.
A CR of 9:1 seems conservative. The engine will probably run fine on premium unleaded at 9.5:1 true CR. The advertised OE ratios are optimistic and were a quarter to a half point lower than advertised on average for an average production engine. Again, since my experience/knowledge is primarily with SBs you should get some input from other L36/68 owners on what true CR you can run on pump premium.
I would recommend the OE type cast pistons (Federal Mogul again) or an aftermarket hypereutectic from a reputable vendor.
SHP SBs can run a true 10.5 on unleaded premium with, maybe, a little tuning of the igntion map. Mild cams such as that used on the L-68 can't run as much static CR because their relatively early closing inlet valve increases dynamic CR. Also, BB dynamic CR should probably be about a quarter point below an equivalent SB because their larger bore size increases their "octane appetite".
"Duke, what does PV the car mean?"
Performance Verification as done by NCRS? I'm not sure what you mean by the question as I do not think I used that term.
Regarding NCRS performance verifications, one sure way to fail before you even get off the field is to have an aftermarket cam that does not duplicate the idle characteristics of the OE cam.
DukeComment
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Re: Engine Rebuild
A true compression ratio of 9:1 will run well on any pump premium you can buy. I have that ratio in my 1970 454 AC car. If you can get 9.5:1 so much the better! My 1969 L89 was rebuilt as a 489 (4.25 stroker motor) and has a true compression ratio of 10.2:1 with the aluminum heads. It runs like a bat out of hell with 4 degrees initial advance, 34 degrees total at 3000 RPM with the factory vacuum advance also being used, but if I run it on pump gas it diesels for about 1 second whenever I shut it off. I have had more than several different motors in this compression range and have found the 10:1 range to be a bit cranky on pump gas!
As for the cam, if you're going to run a lower compression, DON'T go to a more radical cam. The milder cams will help build more cylinder pressure and will give you much more kick in the seat when you hit the gas. Be honest with yourself, you're looking for the highest average torque reading from idle to 5500 RPM, not high RPM power.
Depending upon the condition of the heads and how much you plan to drive the car, I would consider adding hardened exhaust valve seats. If you're going to buy new valves, consider some stainless steel ones - Chevy now puts them in all the 454's as stock. I wouldn't bother going up to a larger valve size in your original heads as the chambers are quite tight and you won't gain a whole lot unless you plan to do porting and chamber work too (and you're not running headers anyway). I've had lots of heads flow bench tested (that's how I pick my cams) and just changing the valves alone never did much on the older closed chamber heads.
As noted in a post above, the Kieth Black hypereutectic pistons are fantastic for any street or street/strip car and allow tight piston to wall clearances for longer ring life. They're pretty cheap too! Make sure you follow the top ring end gap recommendations - the hypereutectic pistons like a little more gap. If you still have extra money, consider a "gapless" second ring set like those from Total Seal, you'll be surprised how much it cuts back on the blowby.
I used the hypereutectic pistons in another 489 stroker motor and abused it to no end shifting at 6000 to 6500 RPM and they held up GREAT.
The important thing to remember with internal modifications is that you want to go for the most reliability. Generally the closer you stick to stock, the better off you are. Always keep in mind what the true use of the motor will be (the real use RPM range), that you won't have headers or an open exhaust, that you'll have the AC on, etc. and build your engine as a matched package to that goal.
If for some reason you desperately want to change the cam, you can add a little lift but keep the duration at 0.050" the same and the lobe centers the same as stock so you don't change the idle quality. I would not recommend just swapping to a 1.8:1 rocker arm as it does increase the valvetrain loading and you MIGHT gain a few HP at most - that stuff is best left to those who are dyno testing their engines to squeeze out every last HP.
Good luck, e-mail if you want to talk.
MarkComment
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Re: Engine Rebuild
My SB research has shown that these are very well engineered engines with excellent torque bandwidth. If you want to increase top end power without any deleterious effects on low end torque, driveability, throttle response, and idle quality, the biggest payback is from cylinder head work! Everything else is fine, (except early SB small bearing rods) and by using OE or equivalent parts the OE reliability is there.
Pay top dollar for the best cylinder head work you can get, but don't spend a dime on hotrod parts. Stuff like high ratio rocker arms just increase the load on the valvetrain and decrease reliability with VERY LITTLE improvement in top end power.
The Chevrolet engineers really know their stuff when they designed these old V-8s.
DukeComment
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