Pro Stock John

iTrader: (34)

Anyone care to discuss, how a cam spec'd for pump gas but with big boost would be designed and why?

Let's throw out some parameters and then if you have any thoughts post 'em up!

Target: 700-800rwhp on 93 pump, no meth
Engine: 348ci, 8.2:1
Turbo: 88mm T6
A/A intercooled
Max rpm 7000

How does cylinder pressure influence the cam specs?

SStrokerAce

iTrader: (2)

Minimum RPM, Stall Speed, way more motor specs.... the turbo RPM and the boost that is sees will also play a part in this since the higher the boost pressure (meaning more restriction in the intake tract) the higher the temps and more problems you will run into the more you try to get it on the edge.

Bret

SStrokerAce

iTrader: (2)

More cylinder pressure the more octane you need, cam specs on a NA motor need the same thing looked at. There it's VE and DCR that will get you to the edge.

Bret

white2001s10

iTrader: (4)

I'm going to guess it would have to peak torque way up in the RPM range when there is little change of detonation.
Your 7000 RPM limit sort of precludes that a bit. I suppose you could peak torque near 6000 RPM and have 1000 to play with. Hello Liberty 5-speed.

J-Rod

The important thing to keep in mind is that for big turbo there are other factors to keep in mind.

From the camshaft discussion thread....

Turbos are limited in exhaust duration. Let me clarify... For a STREET driven turbo car where low-end is important, the exhaust lobe is limited.

Because of the pressure in the exhaust manifold, turbo motors have a longer 'power stroke' then N/A, SC, & Nitrous cars. These motor's power stroke is from 0 to 90 degrees. Turbo motors continue to push for another 45 degrees of crank rotation. Hard one to follow, but true.

Stock cams work so well with turbos becuase of the lack of overlap & small lobes. Aftermarket cams are setup for N/A & SC's for the most part & open the exhaust inside the power stroke of a turbo motor. Average spot to open the exhaust valve is ~45 degrees BBDC @ .050 lift, which is ~ 70-80 BBDC @ .006 lift (depending on ramp-rate). So, the exhaust lobe on a turbo motor *should* be opened late to avoid bleeding off power. At roughly 60 degrees BBDC is were you can open the exhaust valve. Add this to the lack of overlap on a turbo motor, because exhaust manifold pressure is ALWAYS higher then the incoming intake charge (by 1.5-2 times depending on compressor efficiency) & you can see why large cams don't work so well with these motors.
So, if you're following me, a street turbo motor is limited to ~ 260-270 total duration on the exhaust side. & that's WITH ~40 degrees of overlap @ .006. (nominal stock overlap)

Track turbo motors use more traditional N/A grinds as they stay in the upper RPM band were overlap will actually help the turbo make power. But it KILLS the low-end.

Turbos make full boost by ~3000rpm & therefore make tons of torque, so most don't notice that the N/A cam they're using is bleeding off power.

I'm going for the most efficient combo as possible.... This new cam will open the exhaust @ 60.5 degrees BBDC. As opose to 69 BBDC w/the stock cam in there now.

SStrokerAce

iTrader: (2)

Good N/A and Blower motors do the same thing....

Turbo and Blower motors need different cam specs but they make the power the same way, remember that.

FWIW the turbo has more to do with it on the setup in question than the camshaft will.

Bret

Pro Stock John

iTrader: (34)

Thanks, this corroborates a few phone calls I had this week.

Steve Bryant

iTrader: (1)

I'd like to find some reference that correlates dynamic compression ratio NA to DCR with boost at a certain level and the approximate octane number required.

Steve

SStrokerAce

iTrader: (2)

Steve,

The problem is there are so many other factors in this... temps and RPM being two biggies not to mention the chamber design. I don't see there being a sted fast easy rule there.

Bret

white2001s10

iTrader: (4)

To use a generalization, I'd start out just applying it by a %.
If the density in the intake tract is increased by say 40% by the boost, then I'd just add 40% to the DCR to get in the ballpark.
Nothing is ever that easy, but I figured you were looking for something very general.

J-Rod

But, you have to cam around the turbo, especially if its a big turbo. I agree that forced induction motors do the same thing, but since a turbo is exhaust driven you optimally have to take that into consideration.

As a stated boost is a measurement of restriction. I had a similar discussion with someone the other day who had reached x psi. They made changes to the motor which should increased airflow. But, they saw no gains. So, they reported the item as being of zero value. In fact it was of a gain. They found in later testing there were restrictions in the intake tract which meant once a certain flow was reached, it hit a plateau. Once that restriction was removed, the dynamics changed.

Point being, if your motor makes X HP at Y ammount of boost if you decrease restriction, you icrease CFM, and boost drops (all else being equal), and HP stays the same.

Have you mapped out your compressor, etc... for your requirements Or did you just buy "a big ole turbo"? Not tha thre is anyhting wrong with that. But, if you do the clacs, and have the map handy, you can see what the compressor is going to do.

Pro Stock John

iTrader: (34)

For example, trying to hit 700rwhp on pump gas, no other adders or injection, with say a cap of 7200 max engine rpms, and max of 125F IAT's, means that I may want to cam the engine for the least amount of boost it will take to get there. A 348ci boosting 22 psi will run hotter than the same engine hitting 15 psi.

So running a bigger cam to reduce boost (restriction) would be the way to go. I am going to a set of cnc ported All Pro heads and a single plane intake at the same time.

As one shop guy put it, hitting 700rwhp spinning to 8000rpms should work. But I am not going to run a solid roller.

J-Rod

Pete was making 733 RWHP on pump gas with a small cam and one of Martin's kits on his Ute on his 347. I don't see it being a big deal. He actually used a less than ideal cam ( a blower cam). It had lots of overlap, it bled off boost, but it made nice power. Also, a turbo will kill a lot of lope off a bigger cam. I think Pete was running a 244/244 110 on his... Again, less than ideal for a turbo. But I think he is going to do a cam swap, as the overlap was killing his gas mileage.

Ben R

You guys might be supprised what a modern turbo cam looks like. Its a lot different than you might expect.

Pro Stock John

iTrader: (34)

Too bad Ben took all the time to post, but not post anything specific.

Seems like making the engine the least restrictive as possible and keeping the boost down is the way to go. I'll keep you guys posted. I'm only impressed with dyno number backed up by track numbers.

Ben R

The information I have is proprietary, sorry. I can say this. If I gave you the specs you'd think I was spraying it.

Pro Stock John

iTrader: (34)

-10 more points, why bother posting then. I know a guy who did a _ and then the _ bleh.

Back topic... I'm kinda thinking that a high revving combo, say going to 7300, with a loose converter, turbocharged, should be able to run 8.99 @ 3300 with 93 pump gas.

Ben R

I basically told you the types of cams people are using in turbo cars these days. Geeeez.

Where do you come up with these combo's John? Why would high revving mean anything? So you can put a big gear in it? LOL. A loose converter? WTF.

SStrokerAce

iTrader: (2)

John, Ben is trying to tell you it's a wide LSA large exhaust split camshaft in turbo motors.

Ben, I can see where you are coming from with not saying anything.

Bret

Pro Stock John

iTrader: (34)

1) I am opposed to traditional split camshafts due to the research I have done.

2) I was thinking higher rpms and some revs, since I didn't think a 8.2:1 348ci LS1 on 93 pump gas would make 700rwhp by 6200 rpms. I guess we will see. Conversely a car making 900rwtq at low rpms might be a monster to hook too....

Ben and Bret go get a room already you make a cute couple!