I finished up my 355 rebuild late last year doing all my research and feeling like I did a decent job. I had the heads ported by AI, and decided to run the HOT cam. My compression ratio ended up right around 10.7 with a DCR at 8.5. So far this combo to me seemed like it should work fine, and although my quenched distance was .054", it was improved by .010" over stock.

So with all my homework done and the motor built it started right up and I was feeling good until I drove it for the first time, and it was pinging like hell, to the point that the computer could not compensateenough. So I drove it home and parked it untill I have the pcmforless send me another program with the timing pulled back. THe second program ran better but I still was seeing the computer retarding the timing. The 3rd program he sent me the timming was pulled back further than stock and finally it got rid of the knock retard. I would normally be happy with the fact that it is at least running, but the truth is I am upset that I can not even break the tires loose with this motor the way it is.

So I am toying with the option of either tearing the entire motor down and dropping the DCR to around 8.0 with new pistons, Or simply chaning out the cam and head gaskets to get to around .041" quench. I believe that changing the head gaskets alone will not produce the results I am looking for because that would raise my DCR to 8.9 or so. (The KB website states 8.0 DCR is a good target, which is confusing when I see 9.5 on this site?)

Keeping in mind that I have an A4 with stock TQ converter, stock exhuast manifolds, and a 3.23 rear end, does anyone have any sugestions or comments on any of this I have bored you with?

 

i dont think the compression ratio is your problem. maybe you have some bad gas? have you done any data logging to see what your computer is doing?

 

Why are you running the stock exhaust manifolds on a built up motor like that? I

 

.054 quench is pretty high for a DCR of 8.5:1

How did you measure the DCR?

9.5:1 is WAY to high for a DCR target, unless you really know what you are doing and have the money for the pistons and chamber needed to pull that off.

Bret

 

Honestly I don't know... I was planning on throwing a set of Long tubes on it this year, but ran into these problems. Could the stock manifolds be making the exhuast valves run hotter?

Also I have run through about 5 tanks of gas through it and tried running the 2nd tune program again with more timming, and it started to retard timming again (basicly with the 2nd program the car sees knock under heavier accelerations as seen by a data logger)

I calculated DCR using the following.
CR-10.7:1
IVC-65 BTDC (HOT CAM value at .006")
Rod length-5.7

The problem I have here is that if I change just the head gaskets to give me a .041" quench, my DCR just shot up to 8.9 or so. To me it seems that this alone will not fix the problem. (Am I wrong here?)

Also maybe someone can tell me why a motor calculated at 8.5 DCR could ever result in a physical compression check of 175 psi (11.9:1)? Why aren't these values the same?

Thanks All

 

Wouldn't the IVC be 65º ABDC?

 

Give us all of the info you have for calculating the Compression Ratio. I have a feeling that 10.7:1 SCR isn't quite accurate.

My 396 SCR was 11.7:1, but the DCR was a little over 8:1. The DCR values are equally based valve timing/overlap and static compression, so that's what you have to figure out.

Mike

 

Also, you want to measure your intake closing degrees ABDC, not BTDC.

Mike

 

Here are the numbers I used. My calculations came out to be 10.78:1 & if I use .026in for the gasket thinckness, my CR would change to 11.17.

Head Volume: 55.8cc
Piston Dish Volume: 7cc
Gasket Thickness: .039in
Gasket Bore: 4.100in
Deck Clearance: .015in
Stroke: 3.48in
Cylinder Bore: 4.030in
Rod Length: 5.7in

Here are the values from my cam card @ .006in lift.

Lobe_Sep: 112
Lobe_CL: 109 Int 115 Exh
Duration: 272 Int 281 Exh

Using these number I cam up with 65 deg ABDC. Sorry about the typo before, I type without thinking sometimes... Using this info I came up with a DCR of 8.58:1 for the current setup and 8.88:1 using .026in for the gasket thickness. If my calculations are correct, is this combined with my quenched value the problem?

Jeff

 

Honestly, I don't see anything wrong with your calculations, nor your setup. Are you positive you're hearing detonation and not something misfiring? (Opti)

Mike

 

It sounded like I was running a deisel... Kinda scarry hearing it, knowing that these head gaskets blow so darn easy.

On a side note, is anyone that is running the hot cam loosing low end torque and power? Seems that this cam isn't what I expected. It felt like the stock cam had more guts than this cam. (Maybe it all has to do with the timming being retarded) Or is this becuase of the stock converter and 3.23 rear end?

I'm tring to figure out what to do here... Any sugestions? I really think I should be able to run power programing without a hitch and to do that I think I have to change out the pistons to Something with a lower CR. Or should I just find somewhere to buy turbo-blue and go to town. Maybe it will cost about the same just to get racing fuel...

 

Race gas isn't going to fix the problem at all. You can run upwards of 12:1 static on 93 octane with these motors without detonation.

Something else is wrong. Check ignition/fuel stuff: Optispark, coil, wires, injectors, fuel pump, etc. The "Hot Cam" should honestly run decently on a stock tune and shouldn't have any of the problems which you described... and for God's sakes... get some headers on that thing...

Mike

 

Jeff,

SCR is 10.78

DCR is 8.47

IVC is 67 with the 109 ICL

One easy fix is to try the cam at a 112 ICL and see what that does since it will lower the DCR some (8.26).

BTW Mike Overlap doesn't have anything to do with the DCR.... How can the exhaust valve be open when you are tring to compress the air fuel charge?

I do agree that it could be something else here other than compression....

Bret

 

So are you saying to retard the cam by 3 degrees to get the IVC at 70 deg? I do have a Double roller timing chain that will allow me to advance and retard the cam by 3 degrees either way. How would this effect the rest of the motor? Would it shift the torque band up to a higher RPM? Also if I would retard the cam what would I need to do about the timming since it will be 3 deg retarded also?

 

TransAm-Z

This is Bret on the old mans account.....

Yeah you would retard the timing 3degs which would install the cam straight up... have to degree the thing in to do this. It will help the upper RPM range of the motor and also lower the DCR.

As for the timing.... on a LT yeah you will have to change the timing tables accordingly.

Bret