Turbos, Turbo Selection 101

Turbo class 101

How to read Garrett Turbos:

GT Models explained

GT Models use a new naming system. The new system was introduced to permit an easier identification of the turbo's characteristics.

New models can have up to 10 digits, that specify its range, measurement of the compressor wheels and the rest of the turbo's characteristics. The old naming system is obsolete.

Example	G	T	3	2	7	1	B	F
Digits	1	2	3	4	5	6	7	8	9	10

Dígits Used

1-2	Should always be GT
3-4	Range denomination ( based on the size of the turbine wheels and the turbine housing)
5-6	Corresponds to the diameter of the compressor wheel in mm ( In the event that the wheel is bigger than 100mm only the last two figures are used )
7-10	These are used to designate the specific characteristics of each model, according to the following table:

A	Variable nozzle turbochargers (VAT)	N	Imitation shroud wheel
B	Compressor wheel without nuts	O
C	Ceramic turbine	P	Variable nozzle turbochargers (VNT OP)
D	A double hole in the turbine housing bypass	Q
E	Adapter integrated into the turbine housing	R	Ball bearing turbos
F	Carbon seal	S	A single hole in the turbine housing bypass
G	Recirculation Actuator	T	Titanium-aluminum turbine
H	Separate manifold adaptor	U
I	Manifold elbow and turbine housing integrated	V	Variable nozzle turbochargers (VNT)
J		W	Refrigerated turbine housing
K	Turbo assisted hydraulic	X
L	Body refrigerated by water.	Y
M	Manifold and turbine housing integrated	Z	Compact

Click for Complete Master List of Garrett Turbos

What is an A/R ratio and how is it calculated?:

The A/R in a relationship that is obtained when dividing the interior area of the turbine where the inner walls are found, through the turbine housing radio from the center to the tongue as the illustration indicates.

A/R values are expressed as .35, .47, .68, .84, 1.00, 1.15, etc.

A small A/R indicates a small interior volume in the small turbine and a large A/R indicates a greater volume.

At a minimum A/R the motor's response is produced at small revolutions per minute but at high revolutions we will not achieve an adequate caudal. We should always find a compromise between achieving the lowest response possible and have enough caudal at high revolutions. The picture below is for reference:

What is the Trim of a turbo and how is it calculated?

Each turbine wheel y compressor wheel model generally have the same turbine diameter (highest diameter), but different steps (lowest diameter). Each type of step (trim), has different blowing characteristics.

TRIM values are expressed as 45, 50, 55, etc... and can only go from 0 to 100. A value of 100 means Dp = Dg
A large TRIM indicates a large turbine diameter.
A TRIM of 55, gives 10% more caudal than a TRIM of 50.
TRIM is used in the same way for turbine wheels as for compressor wheels.
TRIM is calculated through the following formula.

TRIM = ( Dp / Dg )² x 100
Si Dg = 50 mm y Dp = 35 mm
TRIM = ( 35/50 )² x 100 = 49

What are the different flanges and what are the sizes?

All most all of your turbo head units come with the flanges described below. The T3 housing is the smallest and flows the least, with the T6/Thumper flange being the biggest and flowing the most. The flange plays a role in spool up, backpressure...etc. The rule of thumb here is use the largest flange you can possibly fit. Of course this will be limited by what headers you use, since most are pre-fabbed and come with a flange already, and under hood space will also be a limitation.

Basic T3

Basic T4

Basic T6

Selecting a Turbo for your Engine 101

Which turbo for me?

First you must select a Horsepower goal. This is the first parameter you need, to start doing the elimination process to reach your goal.
Second, what rpm range or max rpm range are you shooting for?
Third what boost pressure are you looking to run?
And lastly, how much room do I have?

Below we will explain what is needed to select a turbo and how to do it using a theoretical setup. Not all details are covered since their are millions of different configurations, such as cam selection, head selection...etc. Keep in mind the internal combustion engine is still nothing more then mechanical air pump. So these calculations will get you VERY close to what you will need. They will also teach you how to read a compressor map as well as understand the physics of choosing a turbo.

Below is a consumption chart for a 414 C.I. LS1

PSI	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20
rpm\PR	1.00	1.07	1.14	1.20	1.27	1.34	1.41	1.48	1.54	1.61	1.68	1.75	1.82	1.88	1.95	2.02	2.09	2.16	2.22	2.29	2.36
3400	23	24	26	27	29	30	32	33	35	37	38	40	41	43	44	46	47	49	50	52	54
3500	23	25	27	28	30	31	33	34	36	38	39	41	42	44	46	47	49	50	52	54	55
3600	24	26	27	29	31	32	34	35	37	39	40	42	44	45	47	49	50	52	53	55	57
3700	25	26	28	30	31	33	35	36	38	40	41	43	45	47	48	50	52	53	55	57	58
3800	25	27	29	31	32	34	36	37	39	41	43	44	46	48	49	51	53	55	56	58	60
3900	26	28	30	31	33	35	37	38	40	42	44	45	47	49	51	53	54	56	58	60	61
4000	27	28	30	32	34	36	38	39	41	43	45	47	48	50	52	54	56	58	59	61	63
4100	27	29	31	33	35	37	39	40	42	44	46	48	50	52	53	55	57	59	61	63	65
4200	28	30	32	34	36	38	39	41	43	45	47	49	51	53	55	57	59	60	62	64	66
4300	29	31	33	35	36	38	40	42	44	46	48	50	52	54	56	58	60	62	64	66	68
4400	29	31	33	35	37	39	41	43	45	47	49	51	53	55	57	59	61	63	65	67	69
4500	30	32	34	36	38	40	42	44	46	48	50	52	55	57	59	61	63	65	67	69	71
4600	31	33	35	37	39	41	43	45	47	49	52	54	56	58	60	62	64	66	68	70	72
4700	31	33	36	38	40	42	44	46	48	51	53	55	57	59	61	63	65	68	70	72	74
4800	32	34	36	39	41	43	45	47	49	52	54	56	58	60	63	65	67	69	71	73	76
4900	33	35	37	39	42	44	46	48	50	53	55	57	59	62	64	66	68	70	73	75	77
5000	33	36	38	40	42	45	47	49	52	54	56	58	61	63	65	67	70	72	74	76	79
5100	34	36	39	41	43	46	48	50	53	55	57	59	62	64	66	69	71	73	76	78	80
5200	35	37	39	42	44	46	49	51	54	56	58	61	63	65	68	70	72	75	77	80	82
5300	35	38	40	43	45	47	50	52	55	57	59	62	64	67	69	71	74	76	79	81	83
5400	36	38	41	43	46	48	51	53	56	58	61	63	65	68	70	73	75	78	80	83	85
5500	37	39	42	44	47	49	52	54	57	59	62	64	67	69	72	74	77	79	82	84	87
5600	37	40	42	45	48	50	53	55	58	60	63	65	68	70	73	75	78	81	83	86	88
5700	38	41	43	46	48	51	54	56	59	61	64	66	69	72	74	77	79	82	85	87	90
5800	39	41	44	47	49	52	54	57	60	62	65	68	70	73	76	78	81	83	86	89	91
5900	39	42	45	47	50	53	55	58	61	63	66	69	71	74	77	80	82	85	88	90	93
6000	40	43	45	48	51	54	56	59	62	65	67	70	73	75	78	81	84	86	89	92	94
6100	41	43	46	49	52	55	57	60	63	66	68	71	74	77	79	82	85	88	91	93	96
6200	41	44	47	50	53	55	58	61	64	67	69	72	75	78	81	84	86	89	92	95	98
6300	42	45	48	51	53	56	59	62	65	68	71	73	76	79	82	85	88	91	93	96	99
6400	43	46	48	51	54	57	60	63	66	69	72	75	78	80	83	86	89	92	95	98	101
6500	43	46	49	52	55	58	61	64	67	70	73	76	79	82	85	88	91	93	96	99	102

This chart is derived using a Constant EV of .87.
Intake air temperature of 120 deg F.

Now taking 2 compressor maps we can compare what would be a good turbo to use for this motor. We will just randomly take 3 turbos that would be a popular choice for this engine combination. A T66, T76 and T88.

Now you can take some reference points from the above air consumption chart and see where the engine would fall in the efficiency of the turbo.

4000rpms at 10psi, would give you a PR of 1.68, and the engine would consume 45 lbs/min of air. Now looking at the compressor maps above for reference.

T66 = 72% efficiency

T76 = 78% efficiency

T88 = 74% efficiency, but very close to the surge limit

5500 rpms at 14psi, would give you a PR of 1.95, and the engine would consume 72 lbs/min of air. Now looking at the compressor maps above for reference.

T66 = Into the Choke line, which is the max volume flow the inlet can reach for the turbo. Beyond this is what you se see above of the compressor maps denoted by a steep descend in the compressor speed lines.

T76 = 70% efficiency

T88 = 80% efficiency

6500 rpms at 19psi, would give you a PR of 2.29, and the engine would consume 99 lbs/min of air. Now looking at the compressor maps above for reference.

T66 = Flow would be beyond what the 66 could achieve.

T76 = Flow would be beyond what the 76 could achieve.

T88 = 74% efficiency

Now you can see from above how a turbo selection can greatly effect your setup depending on your wants for the setup. Below I have linked several charts for some combos that are used quite often. Remember though this is just a map to get you going in the right direction. Parameters like large frame and small frame are not put into the function, nor is the flange type. For any added help in your selection contact us. To make it easier for some click on the links below to pull the chart up and being with your selection process. You can cross-reference the consumption charts with our compressor maps here. If you wish to have a chart added for your configuration/engine please e-mail us.

Turbo Cam Selection 101

How to select a turbo cam

Duration:

Duration is critical to a turbo setup since its probably the single most important event of a turbo motor (i.e. time valve sits open and closed). Since the air is being forced instead of drawn into and out of the combustion chamber, duration will be your largest variable on how that incoming/outgoing air is managed.

Duration when using a manifold or log design on most turbo cams is usually about 6 degrees more intake duration than exhaust duration (226/220, 240/234). This is mainly because a manifold/log design will typically see higher then a 2:1 pressure ratio in the exhaust ( as high as 4:1 with some logs). By using a reverse split duration this will somewhat help prevent from getting exhaust gas reversion.

Duration when using an efficient header setup with most turbo cams will usually be (230/230, 224/224) or better known as a dual pattern cam. The thinking is with the exhaust backpressure being only 2:1 you can leave the exhaust valve open a little longer then if the exhaust backpressure was 3:1 or higher. Also some of the new turbo designs produce a much lower backpressure with the advent of better flowing turbine wheels and housings which further decrease the total amount of backpressure created by the system.

Overlap:

Overlap definition, is the time period when both the exhaust valve and the intake valve are open at the same time. The exhaust valve needs to stay open after the piston passes TDC in order to use the vacuum created of the exiting exhaust gases to maximize the amount of exhaust gas drawn out of the cylinder. The intake valve opens before TDC in order to use the vacuum created by the exiting exhaust gases to start drawing the intake charge into the cylinder.

This sequence of events above are controlled by the duration and LS (Lobe separation) of the cam. On a typical N/A motor this is essential since you have no pressure being developed on the intake side to push the charge into the combustion chamber. The problem with this event is a turbocharged motor will create a larger amount of backpressure on the exhaust side. Due to this event the above definition will not apply. Reason being is, when the intake valve opens at BTDC, the burned gasses in the chamber will exit out the intake since the pressure is lower than the exhaust. Since this is true you would not want to open the intake valve until the piston has started going down, ATDC. This will lower the combustion chamber pressure till it's below the intake manifold pressure.

To calculate the overlap of your cam simply follow these steps below:

**Example turbo cam:**

Duration @ .006 218/212

Lift .544/.544 lift

Lobe Separation (LS) 114

Add the intake and exhaust durations
Divide the results by 4
Subtract the LSA
Multiply the results by 2

Overlap is -6.5 Degrees of overlap

**Example N/A cam :**

Duration 236/242

Lift .568/.576

Lobe Separation (LS) 112

Add the intake and exhaust durations
Divide the results by 4
Subtract the LSA
Multiply the results by 2

Overlap is 15 Degrees of overlap

Above was the process on how to calculate your cams overlap. As you can see, the overlap in the 2 cams differ greatly. Running the N/A cam example on a manifold setup would be a horribly in-efficient setup and the engine would be operating well below its potential output. While running the example turbo cam would work well even with the most in-efficient of the header systems out there.
Typically a overlap spread of -8 degrees to +2 is a safe bet. Of course this will differ with whatever combination header, turbo and exhaust is used, so those #'s could be higher or lower.

Lift:

How much lift should I get in my cam? Well that will depend on your heads' flow characteristics. To choose the correct turbo camshaft, you really need to know how your cylinder heads flow. Reason is if your cylinder head flows X amount of air at X amount of lift, choosing a cam that has a lift much greater then that will gain you nothing except extra heat and premature wear of the valve spring. Airflow through a head reaches a peak as the valve is opened, then starts to drop off as the valve is lifted beyond that peak. Most of this of this will hold true to definition, but with a forced induction motor, valve lift is not as critical since the incoming air is pressurized.

A good rule of thumb is to select a cam that will lift the valve 20-25% past its peak flow point.

So be the definition above if your head flows best at 0.500" of lift, use a cam that will lift the valve between 0.600" and 0.625". The reasoning behind this is, if you lift the valve only to its peak flow point, then the valve only flows best when it's wide open. The cycle is brief and would only happen once per stroke. So to benefit from you peak flow the most, you want to lift the valve past its peak. That way the valve will pass its peak flow twice in the cycle. The result is more flow during the opening and closing event of the valve. You do not want to raise the valve much past the peak flow though, or you lose total flow by going too high.

Calculating the best lift:

0.500 X 1.20 = 0.600

0.500 X 1.25 = .0625

Conclusion:

There are way too many factors to just say XX cam will make XX power with your combo. Things like "114LS is best, or 117LS, or ..etc", are just blanket statements. Backpressure, RPM range, boost level, target horsepower, A/R of turbo, turbo frame (T3, T4, T6/Thumper), head flow, cubic inches, and even location of turbo...etc. All of these factors are extremely important in determining the cam that best suits your needs. There is no rule of thumb with a turbo cam. There are too many variables and the only way to get the right cam is to take all of those your parameters into consideration, and only then can a proper cam be selected. All of the points of reference above are just to get you on your way to building the best and most powerful turbo system for you. Study your design and ask questions along the way and you will be smiling the next time your opponent lines up next to you. Feel free to contact us for your needs. Also once you have read this and want to know the theory behind turbo charging, check out our advanced look at the engineering behind turbochargers.