BOOST! How much is too much?

BlackMark8 said:
I just think its crazy that you can have a supercharged mark viii with 400+ RWHP and driller has a naturally aspirated car with 250 RWHP and run a low to mid 13's. When the supercharger just barely gets you into 12's .

And who is that guy with that black 98 lsc with that big vortec and runs low 11's. Is that guy a member of LOD ?
Click to expand...

I don't think JP is running low 13s... but his N/A time is very impressive, he has quite a few modifications though, to the intake, exhaust, gears, wheels/tires etc.

Running 13s in a Mark is no small feat... and running 12s in even a supercharged Mark VIII is pretty difficult, don't forget guys that you start with cars that weigh 3800lbs+ and put down 210-225 to the wheels stock. It takes 100rwhp or xxrwhp and gear/converter changes just to get into the 13s. 12s are a whole nother ball park.
 
Normally I run upper 13s, my best is mid 13s.

My philosophy is simple. Maximize what you have before adding more, keep it's stock apperance and maintain the luxury status as much as possible. ;)
 
The Mark VIII has many things working against it to be a true track performer.
1 IRS Mushy bushings eat power
2 4000LB + with Driver
3 4R70W Power robing and poor shifting
4 Small CID low bottom end torque
5 Weak chassis to resist twist
6 Poor Intakes
7 Poor cams
The above 7 reasons are enough to make anyone rethink using a Mark ViII for a drag car. But they are neat cars so we keep trying.
I have been testing and breaking these with a Vortech since 98. Stock motor 400HP if maintained can get 160k miles http://user1358701.sites.myregisteredsite.com/sitebuildercontent/sitebuilderfiles/1280pass.mpg with regular daily beatings. 450hp every thing must be right or it will detonate or bend and break rods. We broke one new old stock motor on the dyno during a tuning session, one with detonation when a fuel pump quit when it was -18 degrees, and one to see how much boost was too much. 12 psi Intercooled is the answer to your question.
I can hardly wait for spring to go and try to break more parts.
 
All of above can be overcome. Boost is a measurement of restriction basically. The more restictive the intake/exhaust is plus the amount of displacement of the engine the more boost you will have. Take a completely stock Mark 8 with a SCer that measures an honest 10 lbs of boost. Now put a free flowing exhaust on it with no changes and you'll put out 8.5 - 9 lbs. Install a free flowing intake such as an extrude honed Gen 1 or a Cobra and your boost will then drop to 8 or less with no changes to the SCer. The same goes for a fully ported set of heads. This is why I keep saying it's not the boost but the actual hp you'er making. The limit I've found is around 450 rwhp with a very safe tune. But then I've also seen very safe tuned engines still break rods. I've also still have customer with a Cobra just under 500 rwhp that's been running fine over 9 years now.
 
You are correct Boost is not a a way to determine the limits of any engine. Put it on a dyno
and keep increasing power until it breaks then you know the the max power of any given engine. Do that enough times and you will get a feel for the capacity of any one combo.
So back to a stock motor with a Vortech S trim 400rwh drive any where, race every thing.
450rwh drive any where make shure every thing is working before you race.
If you go with ported heads and intake low restriction exaust and intake and didnt put in rods and pistons you wasted your money. The stock Mark VIII with 2 1/2" exaust and a Vortex S trim and intercooler and gears can put 450 to the rear wheels.
 
maxmk8 said:
If rods break how is it a safe tune :p?
Click to expand...

If a motor has a safe tune it should not go lean or be excessively rich.
Exceding the capacity of the rod is just making more power or more RPM's than the physical limits of thr part.
IFyou take a 2003 cobra and put in a 1996 cobra rod the rod will fail because the part is weak not because the tune is bad.
 
maxmk8 said:
If rods break how is it a safe tune :p?
Click to expand...

Easily, the engine's strength is based on it's weakest link. The rods are the weakest internal part. You can have an engine with a perfect tune, perfect a/f and have a rod let go. It has nothing to do with the tune when this type of failure happens. It's like rolling dice. You start stressing those rods with a lot of load and high rpms you will weaken them to the point they will break. The rods are only powdered metal, not forged.
 
I want to see if I can get someone to clarify something for me, if I can:

I have always read (and I am seeing this verified here) that the maximum "safe" horsepower for this engine is 400, and at 450 things begin to break.

But is that crank horsepower or rear wheel horsepower? Since the average Mark puts out ~215 horsepower rear wheel, and ~280 horsepower at the crank, that's an powertrain loss of ~23% (slightly higher than the accepted 18-20 percent for most automatic transmissions). Assuming that is a linear percentage, to get 400 rear wheel horsepower you would need to be producing 520 horsepower at the crank. To produce 450 rear wheel horsepower you would have to be cranking a whopping 586 horsepower at the motor.

There's quite a bit of difference between 450 horsepower and 586 horsepower. So does the danger point come at 450 horses at the wheel, or 450 horses at the motor?

(Methodology: simple ratios.
215/280 = x/100 : 215*100/280 = x : 77 = x [rounded].
215/280 = 400/x : 280*400/215 = x : 520 = x [rounded].
215/280 = 450/x : 280*450/215 = x : 586 = x [rounded].
I am sure that the actual function is more logarithmic than this but it should serve to get pretty close.

Furthermore just to be academic about it, assuming a standard of 14.8 psi normal air pressure and 12 pounds of boost out of a supercharger [ignoring parasitic drag since I am only concerned about the amount of horsepower that is actually abusing the rods] i am coming close to a number of 390 or so engine horsepower which to ME would seem to indicate that we are talking about a safety level of 400 horses on the motor and not on the rear wheel.)

Anything anyone can say to either clear that up or tell me my math is fatally flawed would be appreciated.
 
Last edited:
It's RWHP - I've seen it on dyno sheets(400-450). What the engine HP is I do not know, but I suspect that is a non-linear relationship as well as variables due to gearing and torque converter lockup. There is also the HP difference in the type of supercharger to achieve the same boost levels and on top of that the efficiency of the supercharger and intercooler.

I'm sure others can elaborate more. :)
 
driller said:
It's RWHP - I've seen it on dyno sheets(400-450). What the engine HP is I do not know, but I suspect that is a non-linear relationship as well as variables due to gearing and torque converter lockup. There is also the HP difference in the type of supercharger to achieve the same boost levels and on top of that the efficiency of the supercharger and intercooler.

I'm sure others can elaborate more. :)
Click to expand...

JP is correct - it is non-linear.

Ponyfreak's 1000+hp Viper doesn't lose anywhere near 23% through the driveline. Yes, it's a manual, but you get my point.
 
Stpro said:
The Mark VIII has many things working against it to be a true track performer.
1 IRS Mushy bushings eat power
2 4000LB + with Driver
3 4R70W Power robing and poor shifting
4 Small CID low bottom end torque
5 Weak chassis to resist twist
6 Poor Intakes
7 Poor cams
The above 7 reasons are enough to make anyone rethink using a Mark ViII for a drag car. But they are neat cars so we keep trying.
Click to expand...

Like Lonnie said, not much money needed to shoot most of those down.

1. tccoa.com sells poly IRS bushings. I put them in and they make a huge difference. ~$300.
2. Not much can be done with weight.
3. A stage IV 4R70W from Lonnie will compete with anything out there. And his carbon fiber lock up converter eliminates power loss. ~$3000
4. A KB blower or turbo setup and high stall converter eliminates bottom end weakness. $5K and up.
5. Bill at supercoupeperformance.com sells all the parts needed to stiffen the frame for a good price. ~$700
6. Blowers eliminate the weak intake issue.
7. I assume cams would be one reason for the weak bottom end referenced in item #4. See #4 for answers.

So for ~$1000, one can eliminate the suspension and chassis issues. $3000 for the tranny and TC, and $5K for a blower assuming you do the work yourself.

You've now got a drag car that can give a Z06 a hard time...for less than $10k...and for $70k less than a Z06.
 
It's rear wheel hp. The reason the Mark 8 looses so much in the drive train is that it has to deal with the automatic and IRS.

Another thing to look at is how the boost is supplied. A positive displacement blower will put more strain on the rotating assembly than a cent. type would. Where as the turbo put the least amount. With a properly tuned engine you shouldn't have any detonation problems. But with a PD blower you are increasing the down force on the piston at much lower rpms. This causes incredible forces that the piston and rod were not designed for. Where as the cent type doesn't build up any boost until the rpms are up so it's a gradual increase. The same with the turbo but it doesn't put any strain on the crank or pulley assembly either since its exhaust driven. Your typical 8 lbs boost driven blower will rob anywhere from 20 - 30 hp just to be driven remember. The same when using nitrous, you are slamming the rotating assembly all at once with a force that wasn't designed into the rotating assembly. Something has to give sooner or later.
 
Nitrous is certainly a sledgehammer, because it happens all at once.

OK, so given that 400 rear wheel horsepower is safe, what does that translate to on the crank? Even the most liberal automatic loss estimates work out to 18%, and that's still 472 horsepower at the motor.
 
I use 21% for these cars and the 89 - 97 Tbird/Cougars which works out pretty good. Most of the engine builders I have worked or talked with will tell you just under 600 fly wheel hp is the max for the stock rods and pistons on the 93 - 98 DOHCs. The 03/04 Cobra is of course a different animal.
 
Lonnie, thank you for clearing that up for me! That has been something that has been confusing to me for some time.
 
logres said:
Like Lonnie said, not much money needed to shoot most of those down.

1. tccoa.com sells poly IRS bushings. I put them in and they make a huge difference. ~$300.
2. Not much can be done with weight.
3. A stage IV 4R70W from Lonnie will compete with anything out there. And his carbon fiber lock up converter eliminates power loss. ~$3000
4. A KB blower or turbo setup and high stall converter eliminates bottom end weakness. $5K and up.
5. Bill at supercoupeperformance.com sells all the parts needed to stiffen the frame for a good price. ~$700
6. Blowers eliminate the weak intake issue.
7. I assume cams would be one reason for the weak bottom end referenced in item #4. See #4 for answers.

So for ~$1000, one can eliminate the suspension and chassis issues. $3000 for the tranny and TC, and $5K for a blower assuming you do the work yourself.

You've now got a drag car that can give a Z06 a hard time...for less than $10k...and for $70k less than a Z06.
Click to expand...

You forgot the price of a Mint Mark VIII, New forged motor,and labor to install all of above. I think another 30-40 K$ would be close
 
Stpro said:
You forgot the price of a Mint Mark VIII, New forged motor,and labor to install all of above. I think another 30-40 K$ would be close
Click to expand...

Why start with a "mint" Mark VIII if you are going to do all that to it? Start with a 500 dollar wonder. And who is doing your installs for 40K? The Prince of Wales?

Ok, let's say you do all that. Worst case scenario you are still "only" 30,000 dollars less than a Z06.
 
You must log in or register to reply here.
Back
Top