I had some engine work done over the winter by a shop and am questioning some parts of it and looking for some feedback.

This Grand Sport is not a daily driver, and built for HPDE/Track days. Aero, APs, Tractives.
I spent a good chunk of coin on this setup and have had the car out to the track twice now this season.
Lap times have dropped, however, I am not entirely sure it is due to the engine work based off the data from my events (rookie with data analysis).

Once the car returned home from a 3 month shop visit/upgrades over the winter, I had some low throttle % stuttering and somewhere between 25-3500rpm it would "click" then rev out as it should.
I sent some logs to the shop that did the work, had a couple revisions sent back and no improvements. Went to my first track day and met another vette tuner and mentioned the studder issues.
He drove the car, figured it was throttle map related and asked me some details of the build. A few points to note:

-The second tuner that tested the car, noticed the cam specs and mentioned this is not an ideal spec for road racing (shop that did the work mainly does drag builds)
-Not sure if this is relevant, but the car had a soler throttle controller prior to engine work, and the shop that did the work said it was causing issues with tuning on HPTuners and removed it
-The tuner that I met at the track, sent me a new map to try and the hesitation seems to almost be gone now so I now question the original work and cam choice a bit further...

At this point, I am absolutely stumped, mainly because upon pulling data, the car has barely gained any top speed from last year and the accel G's have actually decreased.
I would love and appreciate some feedback, opinions or advice. With the money I've spent, I am not against the idea of a cam swap and fresh tune if needed.

Cam specs and engine mods below:

Fuel lobe +32 - 229/239 .635/.635 - 112LSA



Dyno Chart - The shop was having issues with it not reading and dropping out at higher RPMs as seen below:
Blue - OEM/baseline
Green - Pump gas
Red - Flex fuel (E50)




Data points from 2022 with OEM power vs 2023 with mods at 2 different tracks.
Note: Grey rows (bottom 3 in each chart) are 2023 data points with current engine mods.

Track 1:




Track 2:




Thanks in advance!

 

The Z51 and Grand Sports are interesting. I have a GS as well. Normally I'd say that cam isn't a good fit for road racing but with the transmission gearing in these cars it's not bad. To wit, my cam is similar....226/246 (Int / Exh duration) and 639/638 Iint / Exh lift). I'm 570 rwhp and 493 tq. I have an RPS carbon street twin clutch and aluminum flywheel that's >40lbs lighter than stock. Your hp seems low so a second opinion on the tune may be warranted. I had to do the same thing with mine. Bad experience with the first shop. I went over everything and then for peace of mind had a second shop go over everything again. I also had them go through the tune and they picked up another 17hp while still being very safe with the air/fuel ratio.

If you haven't already, I'd look at PDR data in Pi Toolbox or the Cosworth software. From there you can more easily compare laps. Pi Toolbox is also made by Cosworth but has way more capability than the basic software package. None the less, the Cosworth package does allow for interesting comparisons including visuals...


Besides the visuals you can also see lap-by-lap data for a variety of parameters.

Looking at your data, it suggests you're on relatively short tracks (<2.0 miles) with 9 to 12 turns. If accurate, then with the GS gearing in 1st, 2nd, and 3rd it's not surprising what you're seeing. When you shift to 4th you're dropping a lot of rpms and then it takes a bit for the car to get back to the powerband. If you change cams you give up the 3rd gear gains and shift them to 4th. As such, IMO, it's all about the transmission gearing.

The track I run is 1.7 miles and 13 turns. I've dropped about 3 seconds off my lap times with the new setup so similar to your experience.

 

If you have stuttering and the shop cannot figure it out then you need to go to a different shop. Regardless of cam you shouldn't have that issue. Drivability is 90% of tuning and WOT is the other 10%.

This is why dragy is useful because it lets you see exactly how fast your car is. That point is moot now unless you compare your times with a stock car, but something like weather and DA could be the cause of you losing a little bit of speed, along with any aero or setup upgrades you may have done.

That said if your current tuner was unable to resolve your drivability issues I would have doubts on whether or not they have even tuned the car efficiently elsewhere. Your first step IMO should be to find a new tuner. Only if they can definitively resolve your issues would I start looking into mechanically altering the car like swapping the cam.

 

GS/Z51 gearing is incredibly bad. I'm the only one with a Z06 gearbox in my GS, it's night and day. I'm same or slightly pulling on Z06 in Straights (I tested with X25 Monster Z06).

3-4 is a dream with Z06 gearbox = 6800rpm-5600rpm ! Instead of 6800-4600 with Z51 shitty gearing

 

It's simply was never intended for use on a road course. The Z51 gearing traces waaaaaaaaaaaaay back to 2005 and at the time it was advertised for Autocross. Then Chevy continued with the same ratios when the C6 GS was introduced in 2010 and they carried it over for the C7 Z51 (and ultimately GS) albeit with the addition of 7th gear. Why they never changed I have no idea but I'll wager it was $$ and/or they wanted the 0-60 time.

It's really the only thing I don't like about the Grand Sport (and ditto when I had my 2013 Grand Sport). One of these days I may change like you did. I followed your thread when you made the swap. To your point, the Z06 nicely accelerates in 4th gear. The GS gearing is a dog.

Back to the OP.....without knowing what track he's on, it's hard to say what he may/may not be experiencing. There are tracks out there that are barely over a mile...1.1, 1.2 sort of thing....so he may never make it out of 3rd gear. Seems the OP disappeared so we may never know.

 

@Nabush other than swapping the gearbox did you have to perform any other work to install the Z06 gearbox? Computer stuff?

Is it possible to change just certain parts inside the gearbox? Has anyone ever made a list?

 

Yeah some PCM programming needed. I tune myself, I had to trick some sensors to make it work...Some tables are not working how they should in HP tuner...

Changing the gears is possible but a lot more work involved...Need to remove gearbox and disassemble, find correct gears, assemble correctly...

 

Thanks for the feedback everyone!

@96GS#007 Nice to hear that you were in a similar situation and had a second shop check things out and saw some gains. I have some new parts arriving this week which I plan to install then re-align and corner balance (AMT eccentrics, toe links, LG sways and tractives) then I plan to head to the second shop to get on the rollers and check things out. My car unfortunately does not have PDR (1LT). I run the Garmin Catalyst to review and compare data currently. Tracks vary for me but they are all longer lengths. The 3 I normally run are Canadian Tire Motorsports Park/CTMP/Mosport @ 3.95km/2.45miles, Calabogie Motorsports Park @ 5.05km/3.13miles and Shannonville Motorsports Park @ 4.03km/2.5miles.

@JasonVette I agree 100%, the car needs to be looked at again.

@Nabush Now this is where a bit of frustration begins after reading your post, then finding the install thread and reading further. First off, I should have came to the forums and/or done more research.. Prior to me sending my car to this shop for the work over the winter, I talked to another guy in Canada with a GS with similar mods who did the Z06 trans swap as well and spoke extremely highly of it. I had asked the shop that had my car about doing this trans swap and I was 100% on board and had budget available to do the swap and was told it was a terrible idea, so I trusted them and took it off the to-do list... live and learn I guess! Also, thanks for the write up and info, now I am obviously reconvinced this needs to happen and it is on the to-do list. Did you end up getting Rev Match and cruise working? Looking at those new RPM 3-4 drops and my dyno chart explains why the car is a bit of a dog and not a whole lot quicker, if any, in certain situations.

To sum it up - Again, thanks for the replies and help! Dyno visit I plan to make happen in the next couple weeks and time to start looking for a new trans

 

Nope still no rev match nor cruise control. It does not bother me too much to be honest...

 

Pretty incredible gains with what is effectively a cam and bolt-on breather mods. Awesome.