How modern engine design creates Turbo lag.

[xctasy]

I have been reading screeds of data from recent magazines on Ford Falcon XR6 Turbo verses Ford Falcon F6 Typhoon turbo lag. One has a nominal 4 to 6 pound boost, restrictive intercooler, GT40 turbo, and no lag. The other has the same turbo but a free flowing intercooler and 9 pounds boost. People really notice the turbo coming on with the F6, but not the XR6 Turbo.

There is technically very little difference in peak power, a conservative 330 hp verses 365, but there is a huge difference in torque, with something like 450 units on the XR6, but 540 on the Typhoon.


I have two observations. One, engines with restrictive intakes have less lag. Mainly,I'm guessing as pressure can build up quickly in engines with lower volumetric efficiency. Two, improved primary induction (ie better volumetric efficiency) increases lag, and creates a more noticeable progression between on and off turbo.


Can someone explain the likely changes in turbo lag given a controlled test on the same T3 turboed engine if one has , say, a restrictive 200 with log head and the other a 2V head is added? Take the turbo as the same trim, and assume the peak boost is the same at about 9 pounds. Assume a nice 264 cam, and about 200 hp, and about 250 hp with the 2V head.

How will low speed lag out of corners be influenced when a better breathing head is bolted on?

 

[StrangeRanger]

I think the answer is that the higher VE engine needs to run proportionally more boost to offset its ability to digest more air

 

[rbohm]

8) first understand that boost pressure is actually back pressure. when you install better flowing heads, and dont change the wastegate settings, you actually lose boost pressure, but still gain performance because air is flowing much better through the system. you are then seeing turbo lag, only because it isnt trying to force air through a less efficient system.

 

[xctasy]

Thanks, SR and rbohm.

Am I right in concluding that we had better stop looking out for bigger turbos for street applications where the breathing is improved? It looks to me that boost modulation and anit-lag devices are of more benefit than a bigger huffer.

I had one guy tell me three years ago that to create more usable boost, you just have to use an undersized turbo. As long as your out of the overspeeding zone, and its not so huge to create surge at low speed, then its better to undersize. Problem is, when you do an exhast or head upgrade, or race it at 125 mph, it then overspeeds and blows the turbo. It looks to me as if Ford is hooked into oversizing turbos again, and when it is tuned for performance, and has the potential to be modified, they lag big time.

One Aussie guy who worked for NormalAir Garret said the cutom had been for the Japs and Germans to undersize turbos. So the Japanese in 1982 started using undersized turbos on the Nissan EXA's. A 2.2 liter Lotus used the T3/ 60 on the Espirt Turbo, they boost modulated it to suit the 1.3 liter the Metro Turbo, the Datsun 280 ZXT's. The first Falcon Turbo, the 1983 AIT, also used the T3 60. One version of the early T-bird 2300 Turbo had a tiny IHI turbo.

The idea was to ensure that the steady state condition was running out of the turbo, sort of ambling a long in lean cruise. When there was a hard out acceleration from any speed, there was as little inertia or spool time as possible.

 

[addo]

I wouldn't put a bit of "window dressing" out of consideration, either.

By that, I mean engineering the response so a jump in power is noticed, rather than seamless. Purely for marketing reasons.

 

[rbohm]

8) undersized turbos are useful in creating low speed boost(in fact just off idle when sized right). the problem come in at higher speeds where overboosteing becomes a problem, thus you need a wastegate to control the boost. and as long as you keep the compressor flow with in the turbo map, you are good to go.

 

[lyonsy]

if ford have over sized the turbo one reason whould be to prevent turbo failure and warrenty claims from people who have blocked the wastegate up and played with the computer or even leting air cleaners get to restrictive.
as this is a big problem with truck engines where always relacing turbo's due to lack of maintence or clamped wastegate.
drift

 

[Does10s]

Can someone explain the likely changes in turbo lag given a controlled test on the same T3 turboed engine if one has , say, a restrictive 200 with log head and the other a 2V head is added? Take the turbo as the same trim, and assume the peak boost is the same at about 9 pounds. Assume a nice 264 cam, and about 200 hp, and about 250 hp with the 2V head.

How will low speed lag out of corners be influenced when a better breathing head is bolted on?

We'll be doing this exact thing soon on Kelly's car. As you know we're currently running the log head, 274 cam, 500cfm Holley.
We'll be bolting on an Oz head sometime after this weekends race. We'll be using a 4bbl 450cfm Holley on a modified Oz intake.
Probably a few weeks after that we'll bolt on the new FSPP Aluminum head with the same carb, intake, cam and see what that does.
We'll be doing all of the testing on a chassis dyno here in Tucson.
Later,
Will

 

[xctasy]

Great stuff!

I'm not knockin Ford Progress, just woundering what will happen to the low speed characteristics when the head can potenitally breath twice as much air at any given rpm.

(I know it will be gooood!)

 

[rbohm]

Great stuff!

I'm not knockin Ford Progress, just woundering what will happen to the low speed characteristics when the head can potenitally breath twice as much air at any given rpm.

(I know it will be gooood!)

8) that always depends on how good the original head was in the first place., and how much improvement there is in the low speed, and low lift ranges. going from say a stock sbf head to a world products windsor jr head, you gain in all rpm ranges. step up to the larger windsor head, and you lose a little low end power due to the larger ports.

 

[xctasy]

I'm interested in mixture motion benefits of the alloy FSSP head.

A loss in bulk air fuel mix speed may be balanced by more complete combustion of the in coming mix.

I am of the opinion that a more efficient head will need special modulation of the turbo to get the best results. Without any added turbo intelligence, it'll be interesting to see how the charactersitics rate.

This is really lots of fun doing the prediction first.

 

[rbohm]

A loss in bulk air fuel mix speed may be balanced by more complete combustion of the in coming mix..

it is possible that the quality of combustion will overcome poor air flow to a point.

.[/quote]I am of the opinion that a more efficient head will need special modulation of the turbo to get the best results. Without any added turbo intelligence, it'll be interesting to see how the charactersitics rate.

.[/quote]

not really, as long as the compressor flow stays with in its proper map, added attention to the turbo isnt required.

 

[Stubby]

A loss in bulk air fuel mix speed may be balanced by more complete combustion of the in coming mix.

If I am correct, the new head will flow a lot more air but not nessesarily slower from the bigger ports. I would like to see some comparisons of intake port volume. If I understand what they are after, it is velocity and quality. When Chevrolet built the Vortec heads the ports were smaller than the cast iron Bowties and flowed more air.

I suspect the 250 2v head might suffer more than the new head design. If the new head suffers from anything, it will probably be over scavanging from the older cam designs. Now that I think about it, I don't know if a turbo motor can suffer from this. I am more familiar with blown motors.

I can tell you that good things will happen with the proper turbo. 8) I can only imagine the nervous energy flowing thru Mikes shop! 8)