When calculating engine CFM should I figure CFM @ the point I want to make maximum power (6250RPM)? Or @ redline 660RPM?

Thanks
JET

 

If I'm not mistaken it should be around peak torque.

cid*rpm*0.5*E(v) / 1728

Saw your other post requesting comp. maps. Are you looking to upgrade turbo's?

 

I'm no one to disagree with you, but peak torque? Can that be right? Peak torque is like 4200 rpm if I remember correctly. The engine will be run up to 6250rpm so after 4200 the turbo will be out of it's effieciency range. Yes... I am looking for a replacment turbo for my single turbo kit.

JET

 

You should be looking at CFM for the entire operating range not just peak pressure. That formula is correct for CFM just make sure to convert to lb/min as most compressor charts read in lb/min. What you are basically looking for is the peak air flow you require the turbo to produce (although its a VERY rough estimate each 1 lb/min of compressor air flow can be translated to 10 WHP). Using a rough calculation (see prior note) if you want to make 450 RWHP you want to look at what efficiency the compressor is for 45 lb/min at the pressure ratio you plan to run. Not to talk **** at all but have you read the book Maximum Boost by Corkey Bell? It would help you better understand what to look for in a turbo (compressor and turbine that is).

 

You need something bigger than a gt35r if you want it to pull to redline, and like a .96 exhaust housing. www.majesticturbo.com call these guys and ask them what size turbo they would recomend. I came up with a gt35r to make over 500 hp at the wheels...

 

I don't think a GT35R turbine has enough flow potential for this motor at those power levels. With 3.5L of displacement and that much air flow on 15-20 PSI of boost its going to choke that turbine really bad... I don't know if any of the current T3 wheels on the market can even flow enough. If it has to be right now it may need a T4 turbine. Something like a P trim.. Not to mention there are other compressors out there that will operate more efficienctly at those power levels. And even if the GT35R did flow ok there he would have little to no room to improve...

 

Sorry JET I was thinking more along the lines of the engines peak volumetric efficiency which is usually at peak torque.

Now if you're looking to play around with plots on the turbo comp map to see how efficient the turbo will be I'd calculate RPM at the point you think it'll hit full boost. Then calculate in the RPM just shy of redline and you should have 2points from which to draw your line through the map.

Don't know if you already know the formula but I'll post it:

Pressure Ratio = 14.7 + boost / 14.7

Basic Engine CFM = (formula above in previous post)

Airflow Rate = Pressure Ratio * Basic Engine CFM

 

All good data except you still need to plot airflow and pressure ratio before boost threshold is reached. You have to factor in the surge line at some point to ensure you are using a compressor that will stay in front of it throughout the operating range...

 

Well if you read the book all i have to do is ask you!

My lb/min calculations are 33.7 lbs/min @ 15psi @ 6600rpm with a preesure ratio of 2.0

Sound about right?

JET

 

Mia thats why i said bigger than a gt35r. I know for sure a holset HX40 would make that power and pull to redline. It wouldnt be that laggy either, full boost would be around 3500-3700 rpms. You can make over 600hp on a 2.3L ford turbo with a holset hx35w, and the 40 is alot bigger than it. The calculation i did for a gt35r would maybe do 500-550 pushing. If you want a turbo that wouldnt peter out get a damn holset HX40.

 

Here are the Garrett GT3852 charts...





JET

 

They should be higher then that under boost.... At 15 PSI on a GT35 or 60-1 compressor the mass flow rate should be well above 45 lb/min

 

MIA,
I definitely agree but wouldn't this be more of an issue if you started playing around with different A/R's and turbine wheel trims??

 

Holset does make some nice units. But there are better options out there is my point. For instance the HKS series and the Turbonetics GT-K series are both using ported shrouds to help larger turbos spool up faster and prevent surging at lower engine speeds. This would allow you to run a larger A/R and turbine and keep decent boost threshold. Since you know Holset I assume you have worked with diesels and are well aware of the improvement ported shrouds can make on larger compressor and turbine units... In the end a lot of turbos out there can do the job but weren't designed with performance gasoline engines in mind. The mid frame and larger small frame turbos that are performance oriented are going to cost you some money but you are getting something designed with your type of vehicles design in mind..

 

Yes and no. You have to keep in mind that amount of mass flow you are asking the turbo to produce at a given pressure ratio. If you use too large of a compressor it may be capable of making those flow numbers but it might be right at the surge limit and would certainly surge in the bottom of the operating range. Besides this whole exercise was begun to find him something that could flow to his needs but did not have the issue he has now which is a turbine section that is being overwhelmed by exhaust gas. With 3.5L of displacement and 15 PSI the CFM output of the motor just goes up. The key is to find a balance of a GREAT flowing turbine wheel, adequately sized A/R and at the same time keep boost threshold < or = to current.

 

Mia holset has that its called Map Width Enhancement.It recirculates air at lower rpms to prevent surging, at high rpms it pulls more air into the back set of blades. this helps the turbo flow about 10-15% more air and prevents surging, im used to holsets on gas engines. They also use titanium wheels.... They will not surge if you use a proper size waste gate. These turbos also designed to spin backwards since diesels do not use a blow off valve, or bypass valve..

 

Sounds about right. I came up with 35.5 lbs/min (3.5 PR)

213.5 * 6600 * 0.5 * .87 / 1728

Guestimated on the E(v) at 87%

According to the map you posted that'll put you on the 79% efficiency island...

 

I double checked my calculations. I still get this....

My lb/min calculations are 33.7 lbs/min @ 15psi @ 6600rpm with a pressure ratio of 2.0

I calculated 80%.... so my calculations are in the ballpark of yours.

JET

 

Well I wasn't referring to surging due to throttle body closure. I was referring to operating at or near the surge limit with a larger compressor on a smaller displacement (and or mass flow rate) motor. Yeah I noticed Borg Warner calls it the same thing (ported shroud that is). The principle is the same as the Holset or Borg Warner uses but is done in a different fashion that is a bit more efficient for these particular applications. The specifics on efficiency of the porting I won't even try to explain as the port size, location, angle, and width is not something I even want to try to figure out. There are far more qualified people then me behind these port designs and they could burry you in engineering studies Suffice to say that the type of porting used on the HKS and GT-K units was made with specific principles of design in mind that the designers (at least of the GT-K units) hold to be far more efficient then traditional shroud porting. Turbonetics does the same kind of ported shrouds as Holset and Borg Warner but only on their diesel line. The type they used on the GT-K and HKS used on theirs was design was done with these types of applications in mind. As for surging from a wastegate, not sure what you mean there. I've never had a wastegate cause surge. Only an improperlly sized Blow Off/Bypass valve (relief valve). I'm sure thats what you meant.

 

Thats WITHOUT turbo being used. You have to factor in that CFM output will increase as boost increases. So at 2000 RPMs if you have 3 PSI of boost the added CFM from that needs to be factored in. And as an FYI these motors run 100% V(e)