cam / turbo suggestions....

[turbocliff]

Currently I have a 236/232 solid roller from DLS. I suspect this cam is a bit on the mild side for my combo, and am looking for suggestions for cam specs that would be more appropriate.

Combo:
4.1 on center 153 block
3.6 stroke crank
6.5 carillo rods
9.7:1 JE pistons
Alum S2 heads (2.10 / 1.60 valves) ported
was running modified carb intake, but switching to custom sheet metal.
90mm TB
160# injectors
have ATR headers, but going to custom turbo kit for large frame turbo
PT88 was on car, but looking to go with GT47-88 or similiar....

The car is a stock suspension 10" tire car that will be raced on 28X10.5s as well as 275-60-15 radials.

Also, what kind of rpm range should I be seeing peak HP? With the current set up, it made peak HP around 6500. I suspect this was due to possibly worn valve springs, and backpressure issues with the PT88 and a full exhaust, but didnt read too far into it as I knew I would be making changes to the cam and turbo anyways.

As far as target for what I plan on running, lets just say TSO competitive.

 

[turbocliff]

Beuller?

 

[Dusty Bradford]

I don't think your far off with that cam. It should definately go more than 6500 with the right springs. We tried the large cam in Todd's car and it just lost a butt load of low end torque, made it feel like the converter was 600rpm too tight.

 

[turbocliff]

Thanks for the input Dusty.

I expect that the valve springs were shot, as power dropped off at 6500 even at low (10psi) boost.

I am talking to a couple people about turbos, and depending on which route I decide to go I might just swap springs for now, or may still try something a little more aggressive.

 

[Dusty Bradford]

Thanks for the input Dusty.

I expect that the valve springs were shot, as power dropped off at 6500 even at low (10psi) boost.

I am talking to a couple people about turbos, and depending on which route I decide to go I might just swap springs for now, or may still try something a little more aggressive.

Same S2 heads and all. He's got 375 on the seat and .080 from bind. Even with the small cam it will go 7400 at 30psi.

But remember, if you have enough spring pressure, the converter can hold the motor at 6500 if your not making enough power to accelerate the engine past the converter coupling point. You can turn the boost down and the engine can stay at 6500 rpm due to the converter coupling effect. It will eventually climb once it's fully coupled. Todd's car will sit at 6400 rpm until we get the boost up enough to accelerate the engine past this point. That's around 20-24psi.

 

[turbocliff]

Same S2 heads and all. He's got 375 on the seat and .080 from bind. Even with the small cam it will go 7400 at 30psi.

But remember, if you have enough spring pressure, the converter can hold the motor at 6500 if your not making enough power to accelerate the engine past the converter coupling point. You can turn the boost down and the engine can stay at 6500 rpm due to the converter coupling effect. It will eventually climb once it's fully coupled. Todd's car will sit at 6400 rpm until we get the boost up enough to accelerate the engine past this point. That's around 20-24psi.

Interesting. My car peaked at 6400rpm, and power fell off at 6500 both at 10, 15, 25, and 30psi. Not making enough power I don't think was the issue once it was turned up. It made 900rwhp on a mustang dyno. (At 30psi).

At the moment, I am still running the TSM converter I got from you a while back, and it is probably a little tight, but I'll be calling to work that out once I know exactly what changes I will be making to the combo.

 

[Chris Hogeland]

Cam

Trust me thats all the Cam you need.

 

[Dusty Bradford]

Interesting. My car peaked at 6400rpm, and power fell off at 6500 both at 10, 15, 25, and 30psi. Not making enough power I don't think was the issue once it was turned up. It made 900rwhp on a mustang dyno. (At 30psi).

At the moment, I am still running the TSM converter I got from you a while back, and it is probably a little tight, but I'll be calling to work that out once I know exactly what changes I will be making to the combo.

Chris set Todd's heads up.

What kind of seat psi do you have? How far are they from coil bind at max lift? Your right, with 30psi it should go over 6500 unless it is a spring issue. At 10-15psi I bet the converter was holding it back but once you increased boost and backpressure the issue turned to the springs instead.

The converter may be 200-400 rpm tighter than other TSO cars but I bet it's close. Once you get it to rpm we'll see what the rpm drop is and we can go from there.

 

[turbocliff]

I cant seem to find my cheat sheet with the motor specs from reassembly by my engine builder. I know he checked the springs, and noted the seat pressure of each, but don't recall what they were at, I just remember them all being within 10% of each other. I will give him a call tomorrow and see if he still has that info as well or recalls.

Off the top of my head I want to say they were in the 225 range, but could be way off..

 

[Dusty Bradford]

I cant seem to find my cheat sheet with the motor specs from reassembly by my engine builder. I know he checked the springs, and noted the seat pressure of each, but don't recall what they were at, I just remember them all being within 10% of each other. I will give him a call tomorrow and see if he still has that info as well or recalls.

Off the top of my head I want to say they were in the 225 range, but could be way off..

If they are 225 they are way off. That would be the issue.

 

[BlownV6]

Off the top of my head I want to say they were in the 225 range, but could be way off..

To put this in perspective , if you have a 2.100" intake valve and do the math , thats nearly 3.5 square inches. At 30# of boost x 3.5 square inches of valve area there is nearly 105# of force pushing the valve open from the backside So that puts you at around 125# of working valve spring. Take that a bit farther and you are trying to close the valve against air that is moving say 300' per second and has heavy resistance to closing because of the inertia of the air/fuel charge. Ok Im done - just something to think about!! Mike

 

[Dusty Bradford]

To put this in perspective , if you have a 2.100" intake valve and do the math , thats nearly 3.5 square inches. At 30# of boost x 3.5 square inches of valve area there is nearly 105# of force pushing the valve open from the backside So that puts you at around 125# of working valve spring. Take that a bit farther and you are trying to close the valve against air that is moving say 300' per second and has heavy resistance to closing because of the inertia of the air/fuel charge. Ok Im done - just something to think about!! Mike

The back pressure is more scary to me.

Intake valve is open, piston moving down in the hole to draw in the air but you have 50# of backpressure at 30# of boost. It's pushing exhaust back into the intake if both valves are open??? Not sure if it's possible but strange things happen when spring pressure is that low.

 

[turbobitt]

The back pressure is more scary to me.

Intake valve is open, piston moving down in the hole to draw in the air but you have 50# of backpressure at 30# of boost. It's pushing exhaust back into the intake if both valves are open??? Not sure if it's possible but strange things happen when spring pressure is that low.

There is never that much pressure differencial accross the valve unless the turbine is way undersized. If the turbine is sized correctly, the pressure is equal on both sides of the valve. With this condition, the engine thinks its normally asperated. The cylinder pressure will never drop below exhaust back pressure and in some cases, may help the intake valve if it is more than boost pressure. At least in theory....
I have run spring pressures as low as 180# at the seat and pulled to 7500+ with no issues. Cam lobe profile, seating velocities, and overlap play a big factor in required spring load.
Allan G.

 

[XXQUICK6XX]

To put this in perspective , if you have a 2.100" intake valve and do the math , thats nearly 3.5 square inches. At 30# of boost x 3.5 square inches of valve area there is nearly 105# of force pushing the valve open from the backside So that puts you at around 125# of working valve spring. Take that a bit farther and you are trying to close the valve against air that is moving say 300' per second and has heavy resistance to closing because of the inertia of the air/fuel charge. Ok Im done - just something to think about!! Mike

x2 and it sounds like the same cam I'm going to be running.

Steve

 

[turbobitt]

The back pressure is more scary to me.

Intake valve is open, piston moving down in the hole to draw in the air but you have 50# of backpressure at 30# of boost. It's pushing exhaust back into the intake if both valves are open??? Not sure if it's possible but strange things happen when spring pressure is that low.

I have done extensive dyno testing on Heavy duty diesel engines and monitored valve motion vs. crank angle during dyno pulls. One interesting thing to note is header tube size played an important roll in valve float. This is why -
The pressure is not constant in the header tubes. The opening and closing can produce pressure pulses sometimes 1.5 times the average gage pressure in the header and happen way to fast to pick up with a gage. These pulses can get dampened with larger tube headers, acting as a big accumulator. I have actually witnessed this by changing out manifolds during one of our dyno tests. These test showed significant reduction in exhaust valve float with increase in tube size with the average pressure remaining constant.

In other words, you may see 50# back presssure but may have spikes as high as +75# acting against the exhaust valves at certain crank angle points.

Allan G.

 

[turbocliff]

Thank you very much for the input thus far guys. I am definitely thinking about things more clearly thanks you your help. This is some interesting information. I think for the time being I will be sticking with the cam that I have, but taking a better look at the valve springs and seeing where I am at.

I know back pressure was an issue as well, but with a different turbo and turbo kit in the works, I expect to get to that resolved also.

 

[Dusty Bradford]

There is never that much pressure differencial accross the valve unless the turbine is way undersized. If the turbine is sized correctly, the pressure is equal on both sides of the valve. With this condition, the engine thinks its normally asperated. The cylinder pressure will never drop below exhaust back pressure and in some cases, may help the intake valve if it is more than boost pressure. At least in theory....
I have run spring pressures as low as 180# at the seat and pulled to 7500+ with no issues. Cam lobe profile, seating velocities, and overlap play a big factor in required spring load.
Allan G.

All good info.

Your talking backpressure in the headers correct. I've never seen a turbo system in a race application where the backpressure isn't higher than the boost pressure though. Sure at lower boost level the ratio is 1:1, but start pushing the system and maxing the turbo out and the exhaust pressure always will nearly double over the intake pressure. I guarantee that in Cliff's situation with the PT88 and 30psi of boost, his backpressure is well over intake pressure.

 

[turbocliff]

All good info.

Your talking backpressure in the headers correct. I've never seen a turbo system in a race application where the backpressure isn't higher than the boost pressure though. Sure at lower boost level the ratio is 1:1, but start pushing the system and maxing the turbo out and the exhaust pressure always will nearly double over the intake pressure. I guarantee that in Cliff's situation with the PT88 and 30psi of boost, his backpressure is well over intake pressure.

Correct. At least as far as I could tell with CO2 pressure to the WG... at 30psi of intake pressure, I needed almost 50psi of CO2 pressure on the gate. While at 10psi intake pressure, I only needed 6-7psi of CO2 pressure on the gate. (The spring in my gate was trimmed so "base pressure" is only about 4psi)