SHPE Ignition Timing

[Ron F.]

Hi, I'm putting a '68 SHP Engine in my '64. Running an LT1 cam, solids, roller rockers and headers. My question is where do I set the timing. I assume it should be advanced from the stock setting and I prefer not to experiment..

Thanks,

Ron

[Edward J.]

Re: SHPE Iggnition Timing

Ron, I will let Duke give you the Low down on what spec.s to set up you engine. I am thinking the Roller tip rockers might not be a good choice with solid lifters.

[Ron F.]

Re: SHPE Iggnition Timing

This is my first experience with roller rockers so I am curious as to why they might be a bad idea.

Ron

[Edward J.]

Re: SHPE Iggnition Timing

Ron, Roller tip rockers work ok with zero lash hyd. lifters, but the constant hammering of roller tip rockers against valve tip will cause wear problems. solid lifters have a 8 to 10 intake and 14 to 16 exhaust lash adjustment. GM rockers work best. Duke will this post and answer better than I can, from a engineering stand point.

[Ron F.]

Re: SHPE Iggnition Timing

Interesting, thanks. Things you never consider.

[Duke W.]

Re: SHPE Iggnition Timing

Roller tip rockers are heavier than the OE precision stamped rockers and will reduce valve float speed. You might as well roll up a a wad of $20 bills and smoke 'em, but Davidoffs and Cohibas are a much better smoke. So called roller rockers (rollers at the tip only) are just a way for aftermarket outfits to relieve you of $150-200... pure marketing BS. They are totally worthless and useless. Modern Corvettes have roller trunnion rockers with a curved tip that opens and closes the valves - just like vintage rockers.

Headers really won't do too much with any kind of reasonable exhaust system, even the low restriction OE system, and they radiate a lot of heat to the engine compartment and components and have other common issues like bolts coming lose, and weak alternator mounts. A few years ago a test of a "327 LT-1" with massaged heads on a lab dyno showed about eight percent greater peak gross torque with headers, but only about three percent more peak gross power, and a reasonable road legal exhaust system will reduce those improvements by about half. If you have OE manifolds and associated hardware, especially the 2.5" outlet version I recommend you use them.

If you massaged the heads properly it will peak at 6500 and make useable power to about 7200 with the OE 3911068/Sealed Power VS 677 valve springs when set up for .090/.100" coil bind margin, all with OE valvetrain reliability.

The '68 OE rods are okay as they are basically 350 rods which are stronger than the second design 327 rods.

The ideal spark advance map is the same as the OE 365/375 HP engines, You can download the '65 AMA specs from the GM Heritage website to see what they are.

Then the question becomes how is the distributor you plan to use setup. If it's from the same '68 L-79 engine, you can look up the OE map in the '68 AMA specs and then determine how you can get to the '65 SHP/FI map, but it may have been modified, so you'll have to wait until you get the engine running to check it with a dial back light. Or if you know someone with a Sun distributor testing machine you can check it there.

For sure you will need an OE 236 16 or a replacement B28 VAC, which should still be available under the Airtex brand, 4V1053. Be sure it's connected to a full time manifold vacuum source.

The nice thing about the '68 big bearing block is that you can drop a 3.75" stroke crank in at relatively low cost with the OE rods and suitable pistons. Useable revs will drop to somewhere between 6000 and 6500 depending on head flow. Peak power will be about the same as an otherwise identically configured 3.25" stroke engine, but it will have significantly more torque/power from off idle to peak useable revs.

And since OE hydraulic lifter cams with proper OE spring setup will rev to 6500 you can install a L-46/82 cam and not have to deal with valve adjustments. The L-46/82 cam makes very similar torque/power as the LT-1 cam. It just won't rev as high, but on a long stroke engine you run out of head flow before you run out of valvetrain speed.

Allocating the cost of those hokey rocker arms and any other hot rod parts you are contemplating to a stroker crank is a much wiser way to go that will pay real SOTP rewards.


Duke

[Patrick H.]

Re: SHPE Iggnition Timing

Ron mentions using roller rockers in the original post, yet the thread takes an immediate tangent regarding the use of roller-TIPPED rockers. These are two very different designs.

However, for a street motor, neither one is likely needed.

[Timothy B.]

Re: SHPE Iggnition Timing

I would use 14-16* initial timing with 15* manifold vacuum advance. Get all the centrifugal in before 3000 rpm. I would use all stock rockers and a stock oil pump and stock engine breathing set up(PCV etc.).

Are you using the stock Holley and aluminum manifold?

[Ron F.]

Re: SHPE Iggnition Timing

WOW!! Words fail me, thank you very much.

[Ron F.]

Re: SHPE Iggnition Timing

These are roller tipped. And, I've definitely got the idea that I don't need them. This is my first foray into non-stock and the only reason I went there was because my '64 did not have the original engine. This allowed me to take liberties, thus the Hooker headers and sidepipes. My goal was to have a good looking, COOL sounding driver. Thus the cam and solid lifters. I even had 3 dueces on for awhile but I couldn't stand having Ford carbs on my vette. The rockers were done only on the intake valves, I figured the exhaust side of the breathing was good enough.

[Ron F.]

Re: SHPE Iggnition Timing

Thanks to everyone for their input. I'll try 15 first.

Yes, original '68 carb and manifold.

[Duke W.]

Re: SHPE Iggnition Timing

I would use 14-16* initial timing with 15* manifold vacuum advance. Get all the centrifugal in before 3000 rpm. I would use all stock rockers and a stock oil pump and stock engine breathing set up(PCV etc.).

Are you using the stock Holley and aluminum manifold?

Typical idle behavior for the LT-1 cam is 12" @ 900, so a 15" VAC will not pass the Two-Inch Rule.

If the distributor is unmodified from a '68 L-79 the max centrifugal is 30 @ 4700, so 14-16 initial is way too much.

The OE '68 L-79 manifold is the same cast iron item as the base 300 HP engine with a differently calibrated Q-jet.

As I said earlier the ideal spark advance map for the LT-1 cam is the same as OE on '65 SHP/FI engines.

I don't know if the OP is listening to me, but if you are search the Web - my name, San Diego, Corvette - and download and read my tuning seminar.

Duke

[Ron F.]

Re: SHPE Iggnition Timing

12 it is then and thanks again.

Ron

[Gene M.]

Re: SHPE Iggnition Timing

The 68 L79 is a good package right out of the box. Sure the Q-jet and cast iron intake don't look cool compared to the 67 L79. But with head pocketing the 68 is a good street engine. And with the Q-jet is more responsive than the Holley from a stop light. I have both 67 Holley and the 68 Q-jet is way under rated. LT1 cam would destroy bottom end as the old "302" Z28 was.

[Duke W.]

Re: SHPE Iggnition Timing

A LT-1 cam with the '65 SHP/FI spark advance map makes 80 percent peak torque at 2000 - about the same as the Duntov. The L-79 cam with the OE lazy spark advance map is about the same, but better with the '65 SHP/FI map. The 30-30 with the same map needs close to 3000 to make 80 percent peak torque.

Most guys who knock the LT-1 cam have have never optimized the tune or dyno tested an engine with one and have probably never even driven a car with a LT-1 cam. I've done all three.

Duke