Hey I have a timing question for anyone in Colorado that might have the answer to this. I have the new cam installed in my 86 IROC, the car is a L69 optioned car built in 1985 and registered as a 1986. The 305 block rolled off the assembly line with 335 hp according to the gm block casting numbers website. Right now with the heads ported and polished, the TPI polished in and out and a solid lift cam, polished crank and the block was line bored. I am running stock pistons and moly rings. My cam as I mentioned is a solid lift 472 grind and a little radical. I have always adjusted my timing around 10 to 12 degrees as a base and usually 28 to 30 degrees around 3000 rpm. The cam builder recommends a sea level base timing of 18 to 20 degrees of base timing and 36 to 38 degrees at 3000 rpm. I have had people tell me to set it at 26 degrees of base timing and let the upper end run near 44 degrees under full advance. Anyone with a experience with this would be appreciated as I am getting ready to set the timing and trying to figure out what will run best.
Thanks,
Shawn

 

TPI is an LB9 engine, not L69. L69 had a carb, and in the '86 model year there were only something like 74 produced - very rare, and they all had 5 speeds behind them.

The LB9 engine was rated at 190 HP, just like the L69. That's a net flywheel horsepower rating, which typically is about 20% lower than a gross flywheel horsepower rating would give you. You can't tell horsepower by the block casting #.

Using a solid lifter cam in a computer controlled, knock sensor equipped engine is not wise. I suspect you actually have hydraulic flat tappet lifters (what the engine came with) rather than hydraulic roller lifters that they started using in the '87 model year. "472" sounds like a lift-at-the-valve number, which if the case, it would be .472", rather than a duration #. Too radical of a cam will cause TPI fits. Do you have all of the specs on the cam?

As for timing, you aren't going to set the total, you're going to set the base timing. The total is in the ECM - base plus advance. 6 degrees BTDC is the stock base setting, a little more for the altitude, perhaps 8 to 10 degrees, is about all it will handle. That is set with the ECM disconnected and the engine at idle; when it is reconnected it'll jump up into the 20s at idle.

 

I might be confused about what model IROC I have but yes you can check and see what engine run GM built the motor series in and what horsepower they rated it in. This may be my first thrid gen TPI, but I have owned and raced 5 second gen and 1 first gen (a 75 two 73s a 70 1/2 a 79 and a 69) which I have built all the engines myself as I have done with this one. So yes I have a solid cam and lifters, it is no a cam out of an 87, but a erson cam, and yes I forget to mention it is a .472 lift I dont every worry much about duration. And runing on 1.6 rockers, which once the motor is broke in I will change to erson roller rockers. I do know the computer adjusts for timing and since I have been in the automotive industry since 1985 as a Mechanic til 94 and parts guy since then til currently.

I have to admit I dont know all the quirks of the TPI and cam timing thats why I was asking if anyone had information on timing for a radical cam. I am not running a knock sensor since I decided to for go that and the cold start valve since the car rarely runs in cold weather and starts up normally indoors and all the articles I have read said you can eliminate the cold start valve without any issues.

As far as the body itself goes, it was assembled in July 1985 and sold as an 1986 due to slower sales in 1985. It is a 4 wheel disc car with t-tops and every option you could get them with including a 700r4 transmission and all are still there except the stereo which was taken out years ago. I did a lot of research on this car before I did anything with it since has no thrid brake light and no external markings of being an IROC except the original dash that says in the Orange letters "IROC-Z" and below that "Z28" and all the external labels read "Z28" I dont spend time memorizing option numbers so it is my mistake I said it was a L69.

All of the emissions equipment is there including that MAF that has the funny little screen that burns off debris and I had to repair the relay for that. Only thing I have done without is the knock sensor. The car runs perfect and idles a little rough through the warm up stage and backfires a little after it warms up so I have been playing with timing settings and can't seem to find a good one. It also idles at 900 rpm and lopes a little from the cam, yet I don't know if any of that has to do with the motor not being totally broken in yet. Hence the reason I asked about timing.

Hopefull I don't come off as offensive, but I have been a Camaro guy since I had my liscence in 1983.
Thanks,
Shawn

 

This isn't intended as a criticism, but I HAVE to ask - why no knock sensor?? Is it because of the solid cam?

And what did you do with the wiring for the KS? It kinda sounds as though you want the equivalent of a carbed engine with the TPI intake...

 

Not with the casting #.

The lift of the cam isn't nearly as important as the duration and LSA. Too much duration and too little LSA and the O2 sensor gets confused at low RPMs.

The ignition timing advance has more to do with chamber design than with cam. It's the flame travel that necessitates timing advance.

 

"This isn't intended as a criticism, but I HAVE to ask - why no knock sensor?? Is it because of the solid cam?

And what did you do with the wiring for the KS? It kinda sounds as though you want the equivalent of a carbed engine with the TPI intake"

I wont take it as critism. I went to a class in 1996 taught by Standard Electroincs which they talked about certain times when you dont want to run a knock sensor. One of those occasiosn is when you are running higher then stock compression heads, since the milling my heads are running an estimated 10 to 1 compression. The knock sensor seems to be casuting predetention problems, so it is still in the block and the wire tied out of the way. If Standard who built the knock sensor said to do it that way then I will trust their advise. No I am not trying to make it into a carberated car by any means.

5kid7 I dont want to argue about horsepower or who knows what, that is not why I signed up to the message boards. I will let you know my horespower after the car is dyno'ed in the end of march considering it is all ready by then.

As far as the cam, I was going by memory, so I pulled the cam card. The cam is an Erson E119830 grind r270-1 advertised duration 270 intake, 278 exhasut; duration 230 intake, 238 exhaust; lift .555 intake and exhaust; center lobe 112 degrees and 4 degree advanced timing. The car is built as a street legal dragster so I wanted to go wild but not too wild. It is built according to the NHRA rulebook in every aspect.

I have done consulting for racing teams running oval track cars the last couple of years and also doing consulting for a couple of drag teams running newer GM's but I have to admit I have not played with the old tuned ports that much. As me about one of the newer corvette and I can tell you almost anything, but the early TPI still give me issues this is why I am asking about timing. I have been out of the racing scene as a driver for about 9 years and consulting for several people the last few years.

Any help with the timing is appreciated since last weekend I played with it again and cant seem to get the right degree of timing.

 

My point, in case someone came across this thread sometime in the future, was to clarify that casting numbers do not tell you anything about the HP of the engine. The same block casting number, for instance, could be in a truck, passenger car, or performance car. It's what's on and in the block that determine horsepower.

And, getting the engine dyno'd is the only "real" way to know what you've got.

 

Anyway since that is settled. Hey 57kid....you have seen my cam specs and realize I have planed or decked which ever you prefer heads...where would you try to set your base timing for a street legal dragster (which for anyone who sees this I do not steet race only on the strip.) Noting that I will change my timing from street to strip. I dont want to have an even harder time with the min-spool on the street then I have to...besides this being my third car, not a daily driver and my wife has a lead foot.

This is open to anyone who might have a suggestion.

Thanks,
Shawn

 

For the strip, set the total mechanical in the 36 degree range.

For the street, you just want it to not kick back in the starter, and you do want functioning vacuum advance.

It's very possible to have both, with proper timing curving.

 

I will try that and see how that works. As far as vacum advance my HEI does not have one, it is the original distrtubitor and uses a centrifcal advance stock to the car. I will let you know how it works out after this weekend. I am haiving some exhaust problems ATM and until I fix them this weekend end I am not going any further on the timing...dont want to cook a valve on my fresh motor.

 

All factory HEI units were either vacuum/mechanical (centrifugal) or electronic advance.

I keep getting lost in the maze of posts and forget which combo somebody is talking about. TPI, right? Okay, set the base timing at around 6 BTDC, adjust from there. You might be able to tolerate 10, but try 6, then 8, then 10, see which one works best. You shouldn't have to change it at the track.