Looking for opinions on SBF boost level.

Stsfxn

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I'm restoring a 94 GT and plan on building a Windsor based 427 with an aftermarket block upon completing the resto. But the engine will take some months to a year even to do it the right way the first time and I have a factory 5.0 (hypereutectic pistons) that I am using to test a side project before fitting it to another engine in another car. My current plan is to replace the rod and crank bearings, lap the valves, replace the pistons with forged units from summit (and new rings with wider gap), and copper head gaskets with ARP studs. The cylinder walls were good 3 months ago (maybe 4 hours run time since then) so I plan on getting by with rehoning. Everything else stock with Megasquirt MS3X engine management (leftover from the 342 stroker I had in the car prior to teardown) and appropriate injectors.

The car is not and will not be a daily driver, I don't rely on it for transportation to and from work, I will not be heartbroken if I push too far and bend a rod, split the block, insert favorite catastrophic failure here. My question is at what boost level does failure become likely? Not looking for 100% safe levels as that has been covered to exhaustion. Just max levels that I will "probably" be okay at for 6 months to a year with an intercooled turbo setup. Stock bottom end, forged pistons, gapped rings, studs & copper head gaskets, and probably a high volume oil pump and a pyro. Thanks for any input.
 
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jrichker

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Stock 5.0 roller blocks start to have problems when run at power levels of 450 RWHP or higher and run at high RPM's. Girdles and fancy tricks don't help much if any at all. The blocks typically spit starting in the lifter valley and part right down to the crankshaft main journals.
 
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FastDriver

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There are more variables involved than you have supplied, and even if you did there are very few who know every combination, every intercooler, the quality of your tune, your gas, your engine as a system.

The goal of lasting 6 months to a year provides nothing that allows someone to alter an edgy, non-safe recommendation. Further, you could be driving a stock engine and without knowing the mileage, treatment/condition no one could tell you if that motor would go 6 months to a year.

Having said that, with a turbo car on a stock block, 500 rwhp is my "safe" number. Hyper pistons have a well-know reputation to be quick fails in any detonation, hence the need for a conservative tune. I've actually known guys making over 700 rwhp on the stock block. I don't think that would last. 500? Sure. But, if you go into detonation or exceed the limits of an already compromised engine even briefly, it could be done on your next drive.

While it is not a matter of a specific boost level, boost does raise temperatures before ignition and raises the likelihood of detonation. Theoretically, if you could get ignition temperatures cold enough and still get it to ignite, there is no boost limit. The limit is torque and RPM. If I were you, and I tune myself, I'd have fun with it. I'd approximate n/a power to the tire, let's call it "p," and take (500/p * 14.7 - 14.7) as my initial boost level through an intercooler and perhaps with torco if I was worried about octane issues. Then, as my other project neared completion, I'd bump boost progressively and hit the track until the motor tapped out.
 

John Dirks Jr

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Even though the back up engine is basically stock, pay a few extra bucks and have everything balanced. Pistons, rods, crank including flywheel and harmonic. This will let it handle more abuse before it goes and also net some extra power. Keep the compression down. For any milling of the heads and block, use thicker head gaskets to prevent the compression from being bumped up.

Based on my boost levels and the power my engine makes, properly tuned I’d say you can survive 12psi on a regular basis is thing are put together right.
 

Stsfxn

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Thanks for the input everyone!

Fastdriver, I'm really glad you chimed in. I just signed up for an account but I've been a long time lurker and I've gotten a lot of value from your posts in the past. I know there are other factors I can't control or prevent without a complete check up of the block and engine components but it's a risk I'm willing to take. Since I am running MS I will be burning E85 with a flex fuel sensor so hopefully that will mitigate any fuel issues. I plan on putting in a set of $300 summit forged pistons with a re-hone. Probably not the best ones out there but they oughtta be better than the stock hypers. And on keeping rpms low. From what I've gathered (correct me if I'm wrong) high revs is one of the largest factors to engine failure. So I'm going to set the limiter at 5800 on this application.

John Dirks Jr, as good an idea as it is I had not even thought about getting the rotating assembly balanced. But that's why I posted. Thank you for the advice!

Srtthis, I am aware. I have other plans for the 427. Thank you for your input.
 

a91what

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Keep timing conservative 16* above 14psi let the boost make the power. Do you tune in lambda or the gasoline scale? The switch to e85 can he tricky the first time, dont rescale anything or change the output on the O2.... just tune it in the gas scale as it's just multiplying lambda anyway. Shoot for 11.3 on the wb with e85 in the gas scale for safe fueling and some cylinder cooling.
With an EBC ramp boost in with mph at the track, start with a 7# spring for a good launch and by 50mph or so have the boost target around 14#
Enjoy. Let me know if you need help setting it up
 

FastDriver

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I think you're on the right track, buddy. Your RPM limit and the forged pistons are good ideas for a boosted motor. Are the pistons going in your build or your 302? I'd just be putting that money in the build, personally, since the 302 seems like a throw-away motor. @a91what is stangnet's local tuning guru. You're in great hands listening to him about tuning.
 

Stsfxn

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A91what, your tuning knowledge definitely exceeds mine but... if by gasoline scale you mean AFR then yes, thats what I used on the stroker. I never dabbled with the lamba scale. Although I only had MS on there for about 3 months before tearing it down for resto and the car was not a daily driver so my tuning experience is VERY limited. On tuning for E85, with the flex fuel sensor do you gradually tune it back based on alcohol content or do you just bring it back to a gasoline tune once it gets out of a preset specification? I have zero tuning experience with E85, or with an EBC for that matter but I definitely want to go that route when the time comes. Thank you for the offer on helping with the setup, I will definitely be taking you up on that!

Fastdriver, I appreciate the concern and I definitely considered it but for under $500 for a set of pistons and rings I would much rather put them in in hopes of increasing the longevity of the engine (especially with the chance of mistakes due to my limited tuning experience) than end up shelling an engine 2 months after install and being in a hurry to finish the 427. This was my first car (hence the restoration on a 94 GT) so I want to get it set up the way I want it without cutting any corners. This is all probably several months out as the car is stripped bare and in the sanding process in the evenings but I have the engine in question apart checking it out before I mount it on a test stand for some trials on a couple other ideas I have, so now is the time to stud it and change out the pistons and bearings.

Thanks for the input so far, you guys are great!
 

a91what

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You can set the flex sensor up a few different ways let's walk before we run though.
All fuel scales are AFR.... its air to fuel ratio...
14.7:1 gas
9.8:1 ethonal (depending on blend)
Ect...
All these scales are based on lambda... lambda is the ratio on air to fuel for all fuels. Lambda 1 is a perfect burn
All ratios are multiplied off lambda

Anyway I'm sure you will do fine
 
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