New to the turbo world and trying to get info on what compression would be best for a twin turbo’d 460. I’ve got a 97’ F-250 that I’ve had for quite awhile and decided to turn it into a project. I’ve already got 2 65mm turbos mounted, new Holley EFI engine harness with ecm. Decided I’m gonna pull the engine and beef up the bottom end instead of just running it till she blowers and starting all over lol. I’m not looking to turn it into a drag truck, just something fun to drive and piss some people off with. What compression is recommended for a boosted big block ? I’ve heard some say lower some say higher, just not sure which to go with when I replace the rods and pistons
Thanks in advance
Guess I should also ask about cam’s as well. What’s a preferred cam anyone is using or would suggest?

 

Lower compression is turbo friendly. Factory turbo engines are often 8.5:1 or 9:1 range.

When you add compression ratio, the engine will develop higher peak pressures which may cause damage.
The lowest compression ratio has the potential to make the max power output safely, lowest peak pressure.

The only reason to raise compression is in search of economy when using gasoline fuel. If you want to drive the vehicle and get some good or decent miles per gallon when using 87 octane gasoline fuel or even 93 octane fuel. It depends on cost per gallon vs miles per gallon. There is no argument FOR using high compression with E85 because once you start using E85 or E50+ the cost of fuel and energy content is so low it isn't saving you any fuel mileage at all even with the higher compression. Might as well have just kept a lower compression and made more power on the same fuel quality.

 

Here is a sort of guideline I made up

7.5:1 or lower: Max effort drag racing, e.g. 1 mile race 200mph or boat steady state WOT application with thousands of hp
8.0:1 Too low for general economy, good for generic forced induction WOT racing and safety
8.5:1 Typical gasoline/turbo OEM Engine w/ daily driver reliability from the 90's, good for 29mpg at 3000lbs, 500rwhp/2.0L
9.0:1 Modern gasoline/turbo, needs more care & tuning but overall considered similar to 8.5:1
9.5:1 Can use gasoline but be careful with high output, high IAT, high boost pressure, high EGT, etc... monitoring and sensitivity
10.0:1 Won't tolerate much heat on gasoline, but it can be used off-boost or low boost gasoline and switch to E50+ Flex when available
10.5:1 Not suitable for high output forced induction without severe tuning care and modifications, good for low boost 7psi application on gasoline perhaps more if IAT is around ambient, very high pressure peak possible, even with alcohol it can damage pistons or head gasket

If I was building a turbo motor for gasoline and mostly run around on gasoline I would not push more than say 9.4 to 9.8:1 compression, which helps with economy and torque while out of boost. The better you are at tuning, and more familiar with the behavior of gasoline and alcohol and water combustion in modern chamber forced induction apps the higher compression you may get away with. Somebody wanting to make 1000hp with 3.5L but very new to tuning I would recommend they use a much lower compression ratio to help with tuning error, the engine will be more forgiving. 9.8:1 if I tune a flex 1200rwhp, but 8.5 or 9.0:1 if you are just learning starting out with less displacement and large power goal. With a very large displacement and low power goals you won't need much boost so the compression can be higher as power requirement is less and less. I have no idea how much power you are trying to make.

Keys to getting large engines achieve decent economy is limiting valve spring pressure, reduce cam lift, use factory bearings & service manual clearances, thin lightweight synthetic oil, and superb balancing with main girdle stability support. Use lightweight clutch/converter and minimize vehicle weight & rotating mass wherever possible. It also needs end of injection timing sufficiently moved near intake stroke and enough insulation/heat to raise efficiency. Big block can achieve similar economy to a 4-cylinder under the right circumstances because the amount of energy to move the vehicle constantly against friction is mostly based on vehicle weight and not displacement of the engine. A larger engine may have more friction but also can rotate more slowly so comparing their gear/mph cruise ratios is not sufficient to say whether economy will be better or not. Many of 4-cylinder and 6-cylinder engines are quite comfortable cruising near 3000rpm while a large V8 may desire 2200rpm or less for optimal economy.

A few pros to low compression
Low compression < 10:1 makes life easy on the starter, takes a load off the battery. Battery will last longer and engine will turn faster even if you get stranded while extended cranking the engine.
Low compression helps the engine tolerate bad gas when it happens
Low compression helps prevent detonation even if the engine goes lean
Low compression low peak pressure means larger plug gaps and more efficient spark energy

 

On my 460 I preferred 9.2:1 greatly over 7.8:1. I would build for 9.5:1 or so, more if you want to run E85 or race fuel.
Lunati 10340702k worked well for me with some home ported (RHP guide) C9 heads and performer RPM intake. 6200lb street truck, c6 trans, 3.55 gears and 33" tires.
Few build threads you can look at.
https://www.theturboforums.com/threads/1978-f150-coyote-swapped-turbos-coming.383603/
https://www.theturboforums.com/threads/73-mach-1-460-bbf-single-turbo-blow-thru.386110/
https://www.theturboforums.com/threads/nanner-the-f100-gets-an-ht60.385114/

As usual your full of shit. The entire ecoboost range is over 10:1, in fact almost all of the modern turbo engines are high compression. Your information dates back to the 90s when fuel management was crap and people didn't understand how to tune a turbo engine.

No need to respond, you are on ignore. I choose to view your messages periodically to prevent your bullshit from spreading across the forum.

There is no reason to build under 9.5:1, even on the 460 with its massive bore diameter. All you are doing is killing off boost response and making it a pig to drive.

 

Me: target 9.4 to 9.8:1
Him: Hes full of shit use 9.5:1


I lol'd


Also
1. Aren't all ecoboost engines direct injection which is why they are able to utilize a higher compression ratio than usual
2. Ecoboost engines aren't using turbos in a performance sense, its not a 1000hp setup. Its an economy setup tailored to downsizing displacement where efficiency is prioritized.
3. In the 90's factory 3.0L Supra engine 1000rwhp just fine 8.5:1 compression, no alcohol
Toyota factory ECU piggyback using HKS F-conV or SAFC = Toyota knows how to tune a turbo engine in the 90's.

There is no comparing overhead injection 3.5bar performance setups to economy downsized low power direct injection turbo applications.
Performance engines from OEM of the 90's are 8.5 to 9.0:1 and today we have added about a half point to that for 9.0 to 9.5:1 ranges thanks to all the incredible modern advancements, just a half a point of compression when using gasoline overhead injection.

 

I say ~ 9:1 at or near sea level, possibly more at elevation on pump gas. The huge bore, and less than perfect combustion chamber doesn't help with detonation resistance. Cam selection will make a bit of difference as well.

 

If someone is a novice/beginner 9:1 or 8.5:1 is safer no matter how old the combustion chamber

pro tuner can get away with 9.9:1 compression, E73, 1170rwhp stock bottom end 5.3 for 200,000 miles of reliability provided adequate filtration

9.9 is basically 10:1

and that is the world famous Aluminum block -120lbs off the front of the car version
Just like with 2jz-gte 3.0L, compare what it can do instead of how big or how modified

 

Huh? I'm not even sure what you're trying to say. WTF does a 5.3, and 2jz have to do with an old 460? I gave my opinion about cr, nothing about max hp on E73, or how much of a tuning god you are.

 

fyi, the bad mama jamas of the race world run 11.0 ish compression with 40-80 psi of boost on methanol.
10.5 compression is not a low boost limit.

 

As usual, @F4K gets involved in a thread and it becomes a troll fest filled with misinformation. Sorry your thread is getting filled up with garbage OP.

 

All of this information is what I’ve been scouring the internet for honestly. I kinda figured it would start a debate eventually (not my intention, but it’s gonna happen) and sorry for that lol. I plan on running just 93 pump gas and shooting for 700-900 range hp. I’m sure eventually I’ll want more but as a beginner with no experience on tuning, I don’t want to totally f it up lol. What cam would y’all recommend for this range/build? Also should mention the average elevation around me is about 900-1,000. I’m in north Texas just north of Dallas Fort Worth

 

Getting to the 600 HP range should be fairly doable on an EFI era 460, with boost. You will need a good cam, some basic head porting, cut the valve guides then 13-15 psi. Or skip the porting and just rock 20= psi.

Getting to 700+ is going get trickier, you are going to be fighting the intake manifold and heads by that point. The stock efi castings can be made a lot better, but your intake manifold choices are limited/non existent due to the way ford setup the late model (E or F) heads. The stock EFI manifold isn't great. You can certainly make 900hp on them with 55psi of boost, it's just going to take a lot of effort and a lot of octane.

In other words, the EFI heads are not a great answer. You would be much better off with a set of junkyard/used pre-EFI castings (c8, c9, d0, d3) and a bunch of porting. RHP will sell you a lifetime site "subscription" that has a great head porting guide for $20, people have pushed the early castings over 700 N/A on extreme builds with solid rollers and high compression. If you could get your base engine up over 500 it would help the turbo's make the power you are looking for.

Aftermarket heads could be a really good deal for your goals, but ones of any quality are over $2k a set new, and that is tough to swallow. But you get a modern combustion chamber, much better exhaust ports, and all the advantages of aluminum heads.

 

Yeah I’m def trying to keep it on the cheaper end of things do to funds available. Will a pre-EFI head work on my set up with the intake manifold I have now?

 

Is it the stock EFI 2 piece manifold?

 

Yes sir

 

If that's what you want to use just use the late model efi heads.

RHP has a write up on porting the EFI heads, focus on the exhaust side. Cut the guides to get 0.550 lift and soften the chambers. 600hp on the stock bottom end should be fine. Keep it under 6000RPM and you will be alright.

 

CR will depend on what head you go with. If you're willing to run a set of aftermarket AL heads, I would target 9.6-10.1:1. If you are forced to use iron heads, 9.0-9.5:1.

 

I’m kinda stuck with the iron heads till I can save up for a decent set of AL heads. Thanks, this all helps a ton

 

Hopefully my last question on this topic for awhile, I’ve had a few people from work telling me that they run double felpro head gaskets under each head. Is that b.s. or is there some truth behind it?

 

On gasoline, it is. I was very specific to using gasoline. VERY SPECIFIC and you bring up methanol lol
It could 13:1 with methanol, why stop at 11? Oh 80psi of boost is nothing when the fuel is very expensive and the engine has an O-ring.

Everything I write is accurate and there is no misinformation. I am talking to beginners, we are discussing gasoline fuel and economy. Not O-rings 80psi and methanol racing engines. Way to achieve context