Chris,

I don't nor am trying to start a flame war here, but with 8:1 he can run 10-12 lbs of boost on pump gas easily. I'm just adding heads to mine and I'm going to be running 8-10 lbs on pump gas with 9:1. Why do you say so low? With 8:1 unless he's making some big hp on motor, the thing want get out of it's own way.

The driver I worked for last season (2000) builds exotic race engines and tests them at his dyno and out at Willow Springs, and I've tagged along several times to learn what I could. What I learned from him regarding boost pretty much echoed what I was taught in my engines class at the local cc when I was preparing for my ASE Engine Repair and my ASE Engine Performance Certification exams, and that was that pump gas is not safe beyond 6 pounds of sustained boost in a stock small block Ford with 9:1 pistons. In fact, this is a widely accepted standard. There are different options available that can alter these numbers, but for a general guideline, 9:1 compression and pump gas can safely handle no more than 6 pounds of boost. That doesn't mean that you can't run more and not have a problem, it is only a recommendation of how much boost can be added without increasing the chances of detonation to an unacceptable level. For my customers, I will not install any combinations that will result in more than 6 pounds of boost if I know they have a stock bottom end with 9:1 or higher compression, without having them first sign a warranty release that becomes effective the first minute after they take possession of the vehicle. This hasn't caused a problem yet, as my customers trust my recommendations. I know I've got books around here that repeat what I'm saying, and if you like, when I have time, I'll dig through and find one or two for you if you would feel better about this by seeing it in writing.

I know that there are many people who regularly run 8-10 psi with 9:1 compression on pump gas, but they tend to be the ones that have the failures you hear about. I won't make a recommendation that has a high chance of failure. It doesn't help me and it doesn't help the customer, so why bother.

As far as an accurate method of measuring boost, the dyno was the best, but since it's difficult to carry one around in your trunk, a quality boost/vacuum gauge that is hardlined to an isolated manifold vacuum source is the best bet. Boost-only gauges, meaning those that do not also measure vacuum, are the worst, and almost always read higher boost than is actually being produced. The factory boost gauges are often off by 50% or more. Most forced air engines produce less boost than the owner thinks they do. I had a customer with a turbo T-Bird who claimed to have 10 pounds of boost "all the time". It actually never exceeded 5 psi, and even that was rare. He wasn't happy to hear that, and preferred the exagerated numbers, but then, he was that kind of guy. He liked exagerated HP numbers, too. ;)

Take care,
-Chris

Chris,

Are you talking non-intercooled or intercooled boost? I agree if someone trys to run more than 8 lbs on pump gas without the proper tune, problems can occur. Maybe not immediately, but overtime. But if the proper tune is applied, especially once compression is dropped below 9:1 and using intercooled boost, 10-12 lbs is easily attainable on pump gas without fear of problems.

I understand if this is something you do for customers, why you would try and be conservative, I probably would be too.

Thanks,
James

Non-intercooled. Like I said, there are alot of options that can change the recommendation, but with a stock set-up (9:1, 69cc heads, stock gaskets and bolts, and no intercooler), the fine line between safe and dangerous is 6psi on pump gas. Larger cc heads, o-ringed gaskets, ARP studs, heavy duty pistons, etc. all can contribute to higher safe boost levels, but it's still an octane game. If the pressure get's too high for the octane to control, the mixture will just start popping like pop corn.

Take care,
-Chris

Chris,

I assumed since the original topic delt with a turbo, we were talking intercooled boost. I don't think you ever specified in your posts that you were only talking non-intercooled boost. Now that we have that straight, I totally agree with you. If 429 builds a 331 with an intercooled turbo setup, he will easily be able to run 10 - 12 lbs on pump gas.

The twin turbo setup I'm going with from Turbo Driven Concepts is intercooled. As far as the rest of the motor, I'm thinking of going with AFR 185 heads, GT40 lower and Downs box upper, 331 stroker kit from D.S.S.(forged pistons,I-beam rods,race prepped crank)- I'm still not sure about the compression and camshaft, I'll probably go with a custom grind cam from Crane and keep the compression around 9-1, should I go with a smaller cam with 1.7 rockers or a little bigger with 1.6 rockers?

Dont get a stock Block If you are getting all the other good stuff **(I'm thinking of going with AFR 185 heads, GT40 lower and Downs box upper, 331 stroker kit from D.S.S.(forged pistons,I-beam rods,race prepped crank)- I'm still not sure about the compression and camshaft, I'll probably go with a custom grind cam from Crane and keep the compression around 9-1, should I go with a smaller cam with 1.7 rockers or a little bigger with 1.6 rockers?)
You want 600HP Get a new stronger Block and a EZ 700HP+ could be done With GOOD gas! I to Will be going to a Trubo When I get a New Short Block I Hope for Min of 600HP IF I do. I want alest 100% more Power over N/A And to get 600HP you will not have to come close to that Much with 100% more power that should make very Close to 400HP and a 50% incress right there will give you 600HP.

You should be a dignitary somewhere. No one has ever been so cordial with me when disagreeing.

Truthfully, I kept thinking he was talking about a blower, but the same rules apply. Yes, with an inter-cooler he will be able to sustain higher levels of boost, but I still don't see 12 pounds (sustained) on pump gas with 9:1 compression. Maybe, but I wouldn't recommend it. That's just me, though.

I have become a firm believer in lower compression for boosted engines. The lower your compression, the more boost you can safely achieve. Think about it... If you go with 8:1 and find some A/V gas, you could easily reach 15 pounds. At 15 pounds of boost, that 331 suddenly has over 670 cubic inches of displacement. That's what I like.

Make sure you get blower pistons. They're stronger, and are less likely to crack the skirts.

Take care,
-Chris