Hello fellow 'Vette owners. While this is long winded, it will eventually be a tech question. I just bought a buddies clean '98 C5 auto and am pretty pleased with it. However when shifting it, short shifts from second to third below 5000rpm at wot.

The car has a level 5 RMP trans, 3200 stall, ported polished 853 heads with a "thumpr" cam (why he chose this I don't know, but its coming out) 36lb injectors, long tube headers, test pipes, and B&B exhaust, custom tune and CAI. With all this said, when shifted manually its pretty awesome, but left to its own volition, the shifts are garbage.

I simply would like to know what programmer I can use to firm up the shifts and make each gear hold to red line, at wot? The build sheet for the trans says it come with a trans-go shift kit, otherwise this would be my first swap.

Glad to be apart of the group and looking forward to the feed back.

 

Are your rear tires STOCK ?? Has the rear diff ratio been changed ?? This would explain the weird shift points--
With your mods I'm sure at one point your ECM was profesionally tuned--However most tuners just do "dyno tuning" and just make adjustemnts for the WOT settings and nothing else--On an automatic it is also critical to also do a "street driven tune to address all your PT issues--Especially with a higher stall--
Good news it all can be easily fixed--BUT it wil require a tune from a full tuner like HP or EFILIVE--No handheld will address all the appropriate tables---If you're not willing to spring for another full tune--Any of the forum " mailorder tuners" can fix it as well for a 1/3 of the cost--Just give them all your details and mods---what you want fixed-a call on the phone is much better than email----

PS : GM short shifts Corvettes in stock trim to keep the load and RPM's down by making it shift into a higher gear so it will pass smog easier--

 

Thanks for the response Blu. The tires are 19's all the way around, 285 and 305 respectively. Rear diff is stock, might be a 2.73 or 3.15, though I could not locate the info in the glove box.

The Ecm was indeed profesionally tuned in Columbia, and put out 364whp...without this lv5RPM trans bein installed. Maybe a retune from the same tuner wil make a difference once the new trans is take into consideration.

Your mentioning the street tune just shed some light on one of the causes of the PT issues. The stall is a 3200 rpm unit and I can't say whether that is good or bad until I get this thing sorted.

If the last tuner seems prohibitively expensive then a mail in deal may have to work.

On average what do these mail order tunes run?

 

Changing the shift points is easy to do for any tuner. But you might wanna call RPM before messing with the line pressures for the shift firmness, because I think they want the line pressures left alone in there transmissions...

 

I will give them a call Doss, and see what works best. I simply want a good combination that makes for a quick car, building on a strong NA platform, so when I get boost...well it all falls in place.

 

1998's had the codes listed under one of rear compartment storage bin lids. Check there for stock codes.

 

OK well that sorta explain it---You need to measure the sitting ride height of your rear tires---shoild be between 24-27"
A speedo correction must be preformed for a tire change
A rear diff ratio change the speedo correction is NOT required on any
C5
A "shift point correction" is required however on both

A Street driven tune is exactly what it implies---The tuner hooks his laptop to your ECM and drives the car under several different conditions
freeway--in town---stopped WOT launches--- mid throttle WOT launches ( like passing a car on the frwy)--idle---A/C on -off etc---
He records all the data and uses that, and his experience and "feel " for adjusting the shift points--Trying to hit every possible driving senario--Spending the most time on PT Upshifting and Downshifting--
Then doing the WOT upshift and downshift--
Secondly he would do some engine adjusting for spark and fuel--
A dyno tuner will just strap it to the dyno and run it up to 6000 RPM's in 3rd gear where they tune it there only for maximun HP

The Rear ratio can be found ON the rear end itself
A 3200 stall is a great mid level stall--Should really make your car a beast with 364 RWHP !!--keeping tires on it and keeping it from spinning with the TC off will become an issue--
Every tuner tunes just slightly different way to achieve the same end results---
If I were to tune it for the 1st time I will mention a few things that i always do for an engine modded automatic with a higher stall
1. max out misfire tables
2. delete all tq mgmt ( leave about 1/2 of it in the 1-2 upsfhift)
3. return all line pressures to STOCK
4. Set all shift times to .22
5. Delete 3rd gear TCC apply and releases
6. Set the TCC PWM entire table to 95% min and 99% max
7. Subtract 6% to the "force motor current" positive table from 6 PSI on
8. Copy the positive force motor current into negative table
9. Set the Max engine TQto 450
10. Delete the "abuse mgmt"
start with that----

Your main cause for the short shifting is because the PT shift points were not adjusted for your new rear tire size
Also as I mentined GM short shifts Corvette's to get them into a higher gear when they are on the smog rollers to aid them in passing
A high stall converter reports "false misfires" ( a misfire wether real or false will unlock your converter)

 

Wow Blu,

You have given me a ton of useful information. Since I am not sure when I will be able to take it to the tuner, I was thinking of buying a used ECU and doing the mail order tune with the points 1-10 that you listed. What do you think?

 

I wouldn'tbuy another ECM 97-98 C5's are different then all other C5's and can be pricey and hard to find- Just get your's flashed--they can always fix something you don't like and anyone with a TECH II can refalsh back in the stock tune--Even the dealer---