Sorry i know the question is very general, but im wondering if anyone can lend some hints or tips on how to determine a timing curve for my car. I have been told many, many different #s for where to start to keep it safe.. Is there a better way than guessing where to start?

388 LT4 fairly mild cam (solid roller) with heavily ported heads. CR is 11.5

If there's any more info that may help please let me know !! Im tuning with a Gen 7 DFI. Im getting a wideband in this week or early next so it will definitely help but i still have so many blanks i need to fill !

TIA

 

the amount of timing will depend on a few factors. a few of the more important ones are dynamic compression ratio, intake runner length, and combustion chamber design/efficiency.

i'm assuming you have LT4 heads, and LT4 intake too, right? are you positive the compression is really 11.5:1? if so, then what are the cam specs and the rod length? (you need those to calculate dynamic compression.) for the cam specs, you need to know the advertised (overall) duration, and intake centerline.

knowing these numbers for sure makes all the difference. my 396 had a difficult time getting more than 400rwhp. i had to tear it down due to a problem with the cam sprocket, but once apart and carefully measured i discovered that the compression was only 10.4:1, and that the cam was installed a few degrees retarded from where it was supposed to be (due to stacking of tolerances). so my dynamic compression was much lower than stock, even.

-michael

 

MSR thanks for the reply. Im basically starting from scratch, this is something I've never learned or read much about so anything helps. The Heads are LT4 heads and the intake too. Im fairly certain the compression is 11.5:1 BUT... just as you stated about dynamic compression that's exactly what the builder told me so I'm assuming the dynamic compression is lower.

I have tried to ask about cam specs etc because im beggining to learn to tune my car myself with my gen 7 dfi. He refuses to give me the #s. As far as im concerned that's a complete load of crap. I could probably go on about everything that he has done and how I'm not happy with the end product but i wont since that isn't what this thread is about. What i CAN say is that I have the old cam we were going to use, and i do know those specs. I had asked him to get a more aggressive cam since i spent my 2 years of my income on this project i would at least like to make a good amount of power.

The old cam is a custom grind comp cams solid roller ( for sale btw new never installed! ).
Here's what the card says:

INTAKE EXHAUST
Valve adjustment .018 .020
Gross valve lift .526 .528
duration at .015 tappet lift 268 276

valve timing OPEN CLOSE
at .050 INT: 1 BTDC 43 ABDC
EXH: 55 BBDC 3- ATDC

These specs are for cam installed at 111 Intake center line
INTAKE EXHAUST
duration at .050 224 232
Lobe lift .3510 .3520
Lobe seperation 115.0


The Motor is a 388 ci bored .060 over. 5.85 rods. Hope this helps at all. What Im wondering is that is it safe to start with stock LT1/4 timing #s or are these always generated based on the combination. I keep hearing things like 'these motors like xxx amoutn of timing" and LT1s like xxx amount of timing". Frankly to me this is like explaining the skeletal structure of a beetle to a 2 year old. how about we start with.. does it even have a skeletal structure...

 

Why does the engine builder refuse to give you the cam specs?? That is way beyond me, in fact that pisses me off just thinking about it! Does he think that he has some secret mojo cam or something that eveyone may copy??? Tell him that you are taking the cam out and having it mastered, then you will post the numbers in every public forum! I have done that with a lot of so called secret grinds and many I have found to be nothing special.

As for your timing, bear in mind that with higher compression the engine doesn't require as much total timing as a engine with lower compression. Also if it has a high efficiency chamber less timing is also required, this is in the case of the new Fast Burn style chambers th at are found in the Vortec heads and the GMPP Fast burn heads. If I were you I would start out with timing similar to a LT4 base table. You are still using the LT4 heads so that isn't an issue, what worked for the LT4 will work for you know. That is unless you are blown, which you are not so nothing to worry about there. This stuff isn't rocket science, its more common sense than anything else. All the same rules apply for any internal combustion chamber, only the EFI part throws a kink in the formula.

 

I feel the same way. it completely pisses me off. From what i know the am specs are more aggressive than those posted. If i remember right the lift is slightly over .600 on the new cam.

With regards to timing, I definitely will start with a base LT4 map. But how do you determine if it needs more or less ? thats what im confused about. does it need dyno pulls and you pull and remove to determine if it makes more power? surely not everyone spends thousands of hours on a dyno to figure it out to the T trying different timing curves. Sorry if that sounds dumb just i don't have any clue where to start and which way to even go.

Im fairly certain the a/f ratio will be affected by timing as well right? What i was worried about was if my timing was way off.. and here I am adjusting fuel curves to lean/richen the motor out when the combustion isn't even happening at the right time. My headers show signs of the ceramic coating burning off starting about 2.5-3 inches from the head. A suggestion was made to me that the timing is off and combustion is happening way too late. Im thinking it may be because it is running rich because my cats glow orange after a minute of idle.

Wish there was a shoppe up here that i could trust to tune my car . I so should have sold this thing and bought a new 02 Z this project has been a 4 year dissapointment... so far..

 

the base LT4 map might be a good start, but if you have a smallish solid roller my guess is that it's too much advance. soild rollers tend to have shorter overall durations, so your intake closing point might be 65 degrees, or less. LT4 timing with 11.5:1 on that early a closing point is probably a bit much.

be careful with dyno tuning timing changes for power. what i have found is that max power on the dyno will detonate on a road car at higher speeds, where the rate of engine acceleration is lower, and where the engine temps might be higher. for a roadrace car, for instance, i use the dyno brake software to make sure the engine doesn't detonate at constant rpm/speed and wot, and then i do 3 pulls in a row to make sure it doesn't detonate even when really hot. doing that usually results in about 3-6 degrees less timing than most dyno tunes. on some engines i can't keep it from detonating using that routine without a drastic decrease in power from the lower timing, and those are cases where the cam/dynamic compression isn't well matched to the combination.

it's interesting to note, also, how many drag cars will blow themselves up if tested using my usual dyno routine.

do you know if the dfi timing table is actual? with the gm ecm, for instance, the timing reported through the obd-ii port is 4 degrees higher than what's in the table. i never figured out which one was correct. regardless, my engine made best power (within the guidelines above) with 33 degrees advance in the tables. (stock LT4 table is 35 degrees.)

timing changes do affect afr readings using a wideband. this shouldn't be confused with the effective afr in the chamber, since the differences in exhaust reading might have more to do with the results of the burn rather than the atmosphere of the burn. it's not that big a deal or difference, but this at least partly explains why different engines seem to like different wideband-measured air-fuel ratios for best power. (there are plenty of other factors, so that's just one.)

bottom line, don't get too hung up on the afr, ballpark is good. one good method is to get in the ballpark by looking for close to 13:1, then finetune on a dyno. lean it until it stops gaining power, then go a touch richer.

your cats shouldn't be glowing at idle. depending on the overlap of the cam, you might nead to lean out the idle. a cam with tons of overlap needs to see leaner than stoicheometric at idle, sometimes, in order to be "right."

also, make sure your idle timing is at least 20 degrees of advance. even if it in the tables, make sure it really is, there might be a constant or offset somewhere that is changing actual timing. best to check it with a light and making a mark somewhere to show true tdc. if you're heating the cats enough to glow and the timing is 20 or more, you definitely have a fueling problem... or, actually, this could also be a cam timing problem, or a firing order problem.

 

Mike I would start with around 24 degs at closed throttle or in your case with teh DFI system. Make the cells in the idle high MAP regions around 24 degs across the board, and for total timing set it to 34 degrees to start with. Then datalog it, and listen for any knock at WOT or otherwise. The DFI system can use a knock sensor, but you need one for a earlier L98 car with the proper knock module. That way you can datalog Knock as well. Accel has a kit for the DFI system or you can get the parts and wire it up.

Timing does affect AFR but not as much as one may think. Maybe a couple tenths at best. At least that is what I have seen on my chassis dyno during test sessions. Get your WBO2 and get it into the system. Then you can more accurately tune the car.

 

question i have is I looked at a alightly modified LT4 map and found the timing was 1-2 degrees numerically higher across the board. My builder said because of the incresed compression you should 'take out' timing. now does that mean numerically lower #s ??

I'm just confused with the terms and the relation to the direction of change in #s. When you increase compression etc and have a slightly faster burn rate.. wouldn't you want to have the timing happen 'sooner' ??

My question is what does that mean if indeed it requires it sooner. I would think 'advancing' would mean sooner.. and i believe that would be numerically higher #s ??

...really confused

 

I don't have any specifics like those guys, but maybe I can help with this part.

taking out timing would mean lessening the advance, which is a lower number. The timing is before TDC, while the piston is still travelling up. So "sooner" in real time would mean more advance.

On a faster burning head, you'd want to wait until later, not sooner, as it burns faster. I.e. you'd want to start it closer to when the piston hits TDC and then begins travelling down on the power stroke. So you'd want less advance, a lower numerical number.

FWIW, I found this article interesting: http://members.aol.com/aurora402002/...ine_basics.pdf

It's not going to help you with any specifics like where to start with the timing, but it's a nice general overview of things.

 

Thank you for helping clarify !!


If i might ask one more question.. what effect does spark plug gap have with regards to timing ??? I was going to stay with a .050 gap but with some reccomendation i put in the plugs at .044. Should i remove them and gap them at 50 or leave it at 44 ??