: STSi heads availability



Purplewgn2000
09-17-03, 02:03 AM
I have seen the pics and read the reports on the Cadillac STSi pace car. I know the main difference is that the heads have been machined so the compression ratio is changed from 10.0:1 to 10.45:1 and that the intake and exhaust ports have been machined to open the flow up a bit. I know I can have this work done myself on a set of heads, but my question is if I can order these heads from somewhere and if I do this would this create a problem with the OBDII computer in my 1997 STS. I live in St. Louis where the Gateway Clean Air Program is killing any performance modding around here. Thanks.

JaxxMan
11-05-04, 02:25 PM
Not sure if they are street legal but I came accross this company, its worth a look.
http://www.chrfab.com/
They put the N* into sand rails.
They have complete engines, heads or just CAM & Spring options.
One day I plan on getting the CAM & Spring option.

BeelzeBob
11-05-04, 11:30 PM
I have seen the pics and read the reports on the Cadillac STSi pace car. I know the main difference is that the heads have been machined so the compression ratio is changed from 10.0:1 to 10.45:1 and that the intake and exhaust ports have been machined to open the flow up a bit. I know I can have this work done myself on a set of heads, but my question is if I can order these heads from somewhere and if I do this would this create a problem with the OBDII computer in my 1997 STS. I live in St. Louis where the Gateway Clean Air Program is killing any performance modding around here. Thanks.


Well....the STSi LeMans pace car engines were the 2000 and later generation of Northstar engines. Different heads, ports, valve train, etc... from the earlier 93-99 Northstar "first generation" engines. None of the work done to the LeMans pace car engines applies to the 99 or earlier engines.

The port work done on those engines did little or nothing for actual power. The ports flowed better on the flow bench, true, but the engines actually did not make any more power due to the porting in the car. The problem was that the work done to the ports, while improving flow, killed the in-cylinder mixture motion causing the engines to be very very sensitive to the delivered air/fuel ratio. They would make more power when leaned out severely but when richened to the level required to keep the pistons from melting would not make anymore power as they were on the verge of rich misfire at the correct airfuel ratios. Sounds like a good mod on paper an in the magazines but one that did not work in practive.

You can gain a little bit by doing the exhaust ports (I would recommend Extrudehoning the exhaust ports) but leaving the intakes alone. You can deck the head but 0.75 mm to gain some compression but you will need to buy very good gas to realize any gain from it as the extra compression causes detonation that results in spark retard causing loss in power unless very very good gas is used.

I saw the dyno work done on those engines and did some test driving of the LeMans pace car package and can tell you directly that the best running setup was with the stock air box and the unmodified ports with a 1 mm decking of the heads running on 95 octane track fuel. I would never recommend those mods for a daily driver. Especially do NOT mess with the intake ports unless you want some real driveablity headaches.

The issue at hand is the fact that port fuel injected engines have a very stratified charge. All the fuel is injected for that cylinder event onto the back of the intake valve so that the heat of the valve can help vaporize it. When the valve opens the first bit of the charge rushing into the cylinder contains ALL the fuel for that event and the remaining 90 percent of the "charge" rushing in is nothing but air. Compare this to a carbureted engine that has a relatively homogenous charge since the carb is constantly adding fuel to the incoming air. All the charge in the intake manifold and ports consists of fuel and air. With the port fuel injected engines all the fuel is injected in one shot of the injector and only a small part of the charge going into the cylinder has all the fuel. This system requires extensive incylinder motion to mix the charge inside the cylinder as it is compressed before the spark plug ignites it. The intake ports are designed for swirl and tumble which, along with the squish areas in the chamber, thoroughly mix the charge so that it is no longer stratified. Random grinding and porting of the intake ports, while looking good on the flow bench, will often cause a power loss as it almost always takes away the swirl and tumble features built into the head making the engine very A/F sensitive. As an example, the production 2000 model year engine can run happily at full power with ratios as rich as 10:1. No problem. The Pace Car mod'd heads would rich misfire if run rucher than 12.5:1 indicating severely non-homogeneous charge levels. Not good. You cannot run the engine that lean for more than a few seconds without risking preignition and/or piston dome overtemp. It works OK for the 15 second pull on the speed shop dyno but would be sure death to the engine if you did a late night top speed run. you do NOT want to go there.

speedyman_2
11-14-04, 02:15 AM
That well written paragraph which I'm glad I read entirely. :D

So, the Northstar is pretty much running at it's peak. I would love to get more power out of mine. But, I also wasnt to clarify to people that "hate" on my car that there is a lot of technology and "brain work" that went into this engine. And seeing how A/F sensative it is, will a Supercharger setup really be as reliable as people have said it to be??

BeelzeBob
11-15-04, 12:01 AM
Not to say that the engine is at it's absolute peak potential...but...most of the low hanging HP fruit was picked to get it to the power level that it is at and changing the state of tune to get more power may raise the peak power level but may also slow the car down in the process due to loss of low end torque.

Any mods to the engine are going to take a retuning or recalibration of the fuel injection system to optimise the setup. This has proven to be pretty difficult to do.

No engine that is making over 1 HP per cubic inch gives you much room for error with air/fuel distribution, fuel delivery, spark delivery, etc... Detonation, leaness, preignition will rapidly destroy the engine.

A supercharger setup would make more power and could (heavy on the "could") be reliable as long as the air fuel delivery is correct and cylinder to cylinder air and fuel distribution were correct. Also, spark delivery would have to be modified for the increased cylinder pressure. There will not be much room for error...in fact, there is NO room for error. Trial and error tuning with a supercharger on a NOrthstar is going to get frustrating, expensive and slow as you will have to rebuild the engine each time you make a mistake. Not a knock on the Northstar in particular....any engine making this power has to be correctly tuned so as to live and not self destruct.

The diffulty , as has been stated before, is that you have a relatively small engine (280 cubic inches) in a heavy luxury car with an automatic transmission. Final drive ratios are very limited for the 4T80E transaxle and the 3.71 is the lowest possible ratio available...that is what is in the STS cars with the 300 HP version of the Northstar. Applying the typical hot rod techniques of porting, more cam, etc... would gain more power but without higher RPM shift points and a 4.11 or 4.56 final drive to make use of the added RPM capability and offset the loss in low end torqe the car is going to slow down overall.

An engine is an air pump. The more air it pumps the more fuel that can be burned and the more power will be made. More fuel without more air does not make more power. Physics dictate what you can do to the engine/air pump to make it pump more air and thus burn more fuel to make more power. You either have to rev the engine higher, increase it's existing volumetric efficiency, increase the displacement (make a bigger air pump) or put a supercharging device on the engine to pressurize the inlet so that it pumps more air. That is it...other than the bottle...LOL. The Northstar volumetric efficiency is already pretty good so you are talking small gains there. Revving the engine higher is really not in the cards due to the limited gear ratio selection. Increasing the displacement is a dead end with a Northstar. The supercharger is really the only way to make a significant improvement.

The use of the Northstar at higher power levels in sand cars, dune buggies, rock crawlers, street rods, etc...works as those applications are far lighter and have manual transmissions with low final drive rear axles. The manual lets the driver drive around the lack of lower end unlike the 4T80E. Also, those applications involve changing the engine installation by turning it 90 degrees so that it is a longitudinal engine thus making exhaust headers application possible and easier. It is going to be hard to improve the performance of a TFWD Cadillac with a Northstar simply by making more HP. It is much easier to take the existing balance of torque and HP with the production engine and make it move the car faster by using a higher stall torque converter to raise the operating RPM of the engine and effctively lower the final drive ratio. More traction, a locked diff, etc... will also improve the performance once the converter stall speed is raised. Trying to brute force it with more HP is going to be tough.

speedyman_2
11-15-04, 04:22 AM
Cool. Thanks for that. So, you have to give a little to get a little in this case. Yeah you get more peak power but not necessarily a faster car. I understand that now. But, since you mentioned the weight of the car....I wonder if it's possible to shave off a good amount and see the results with a bone stock STS :D Say, a loss of 500lbs??? Wonder if there's that much stuff to strip without losing the quiet interior and such. Meanwhile I have my 75 shot ;) Oh well. Thanks again :thumbsup: