Tuning out dips in torque (with a big cam)

[Kevin Doe]

I have noticed that looking at many big cam dyno sheets that there is almost always a dip in torque from ~2000 rpm to ~3200 rpm. There is evidence that this is a physical characteristic of the engine. I just got done mapping my VE table, which is accurate to 0.5%. As many of you already know, VE is correlated to torque. If you view the VE table as VE vs. RPM with lines of constant MAP you'll see the general shape of your torque curve.

I have an MS4 cam in my car. Here are some specs: 239/242, .649"/.609", 111 LSA.

Here is my VE table both as a map, and as table data.
Map
http://i93.photobucket.com/albums/l55/Kevin_Doe/RX-7/VEmap.jpg

Table Data - I have circled in red the dip in the VE table, which will also likely be a dip in the torque curve. This characteristic seems to be fairly normal for a cam of this size.
http://i93.photobucket.com/albums/l55/Kevin_Doe/RX-7/VEtable.jpg


Then I started looking at stock tunes. I have done a comparison of a 2002 Z06 tune. I noticed similar behavior, and it looks like GM took some measures to reduce this behavior. I'll show what I think they did in a minute.

The Z06 VE map:
http://i93.photobucket.com/albums/l55/Kevin_Doe/RX-7/VEStockMap.jpg

The Z06 table - Note the circled red flat. The Z06 cam is smaller, so it looks like this shows up as a flat as oppposed to a dip, but its still there.
http://i93.photobucket.com/albums/l55/Kevin_Doe/RX-7/VEStockTable.jpg

To make the torque curve smoother it looks like they did two things in the region of of the flat. First it looks like they leaned out the commaned AFR curve in the PE curve.

Z06 PE curve - Note the area circled in red. You can see they leaned it out in the range of the VE/torque flat.
http://i93.photobucket.com/albums/l55/Kevin_Doe/RX-7/AFRcommandStock.jpg

It also looks like GM added some timing in that same range.
Z06 Timing Map - Note the hump at the top of the "hill"



How does this relate to people with big cams.

Here is my PE table, which I think is plenty "lean" in that location of the dip, so I don't think I'll change anything there.

[Kevin Doe]

My PE curve:
http://i93.photobucket.com/albums/l55/Kevin_Doe/RX-7/AFRcommand.jpg

My timing table - Note there is no hump at the top of the hill.
http://i93.photobucket.com/albums/l55/Kevin_Doe/RX-7/Timing.jpg


I was considering adding a hump to the top of the hill much like GM did on the Z06 tune. Hopefully this will minimize the dip in torque. Here is my proposed timing table (I have yet to try this out yet).
http://i93.photobucket.com/albums/l55/Kevin_Doe/RX-7/TimingModified.jpg

Your thoughts are highly appreciated!

[Kevin Doe]

Well for some reason it won't let me edit my post. I left out the stock Z06 timing hump. It can be seen here.

http://i93.photobucket.com/albums/l55/Kevin_Doe/RX-7/TimingStock.jpg

[cmitchell17]

Ive seen this a lot were there is a dip in ve/torque right before 3600rpm especially in gen IIIs. Becuase the torque curve is flat right before about 3600 then it goes up a lot from there.

[TAQuickness]

Good eye Kevin. I was told once "welcome to big cams".

[Kevin Doe]

Good eye Kevin. I was told once "welcome to big cams".

LOL. Yeah, I'm still trying to completely rid the bucking but I'm thinking that I'm gonna be stuck saying, "welcome to big cams," when its lightly bucking at low rpm low load.

Do you agree with my idea on minimizing the dip in torque? Do you think a ~2 degree hump is a good place to start?

[TAQuickness]

I think it's a great idea and something I'd like to try myself (but the opportunity won't be present for some time). 2* doesn't sound unreasonable. Personally, I like small changes and would probably start with .5* to 1* while leaving the CAFR unchanges just to weigh the effects.

[Kevin Doe]

Ideallly I'd like to try this on a dyno to really see what happens, but I'm cheap, and I'll likely just do my tuning on the street like I've been doing. I guess I could always just compare to other dyno charts. It seems like they all have that dip. If I don't get much of one, I'll assume it helped.

I really put my race pads and rotors to the test hitting some of those high rpm, high load cells for ~5 seconds each.

[SSpdDmon]

The only true answer is to make a pull on a dyno (Mustang dyno if possibe) to see what your torque actually looks like. Just because there is a hump in that portion of your VE doesn't mean there's a hump in your power/torque curve. You need to follow the process....measure>change>measure>change>measure>etc.

[Kevin Doe]

True, I've actually got some tentative dyno time scheduled. I was just gonna do a few power pulls, but I might as well put this theory to the test as well.

[Black02SS]

Part of that dip is due to header design IMO. I have tried several different methods with various cars on the dyno and the only thing that seems to help is header design.

[Gelf VXR]

Interesting, a very smooth / flat spark map produces a nice smooth VE map and 0.005% BEN's, very nice

[Gelf VXR]

My timing table - Note there is no hump at the top of the hill.
http://i93.photobucket.com/albums/l55/Kevin_Doe/RX-7/Timing.jpg

Can I ask how you derived this timing table?

[SSpdDmon]

Can I ask how you derived this timing table?
I sent him that timing table and then he pulled a little in the part throttle region as needed. I'd suggest to him that if he pulled timing in those RPM regions, that he carries that reduction all of the way across to the higher grams/cyl. cells.

The idea for this table was based off of the concept of how a distributor works. At low manifold vacuums (high grams/cyl), it ramps up to a flat timing number. As manifold vacuum increases (grams/cyl decrease), timing is advanced. For Kevin, his H/C car will most likely make best power with a timing value of 26~27* @ WOT from 2800rpm on up. Applications that run in higher altitudes may require a different timing figure. This value needs to be determined on a dyno in both scenarios.

[Kevin Doe]

He is right on. I only pulled timing from those low throttle and low load area as needed. I havnt' seen any issues with the higher loads at the same rpms, which is why I havn't pulled any timing there.

Actually, my knock wasn't a function of the load/rpm timing. For some reason it only happened when these criteria was met.
1. Low rpm, low load (~0.2 g/cyl, and 2400 rpms)
2. About 15 mph, but only if I just came from a dead stop.

I pulled a total of 5 degrees from that area in attempt to stop the knocking. No real effect. Some guys actually said they had a similar problem adn it was cured by ADDING timing there. I'm livin with ~1 to ~4 degrees of KR at that low rpm, low load point. No audible knock at all.