[Nmolina9239]

Evening All,

I’ve been trying to research the relationship between compression ratios and forced induction. I have struggled to find a good video or article that doesn’t lead me to ask myself more questions.

I’m looking to get an education on their relationship to one another. Pros and cons of increasing CR and boost or any other combination. Anyone have time to educate someone trying to learn

Thanks ahead of time.

[oldman]

DI engines can take more compression because the charge is cooled during the compression stroke due to phase change of liquid to vapor of the fuel. Also the cooler the boosted air the more you can boost you can run, last a engine with smaller bores can and 2 valves can take a bit more boost because of flame front and tumble. LT1 with good cool boost 10 to 11 PSI on 11.5 CR and 93 octane seems to be a safe limit. But one man's safe is another man's just getting started, and is another man's that crazy.

On stock cast piston and stock ring gaps I'd probably stick to 8 or 9 PSI, for most normal applications. Road racing and endurance events I would always go with forged, the piston is cooled so that is a good thing, still there is lots of expansion and I would not trust factory economy smog ring gaps.

Also things like towing, lugging the engine, and a small cam can cause midrange detonation, nut shell there is no black / white to what works. Keep in mind many times the detonation point is reached in the mid RPM range (near torque peak where VE is at the max) at WOT in higher gears. not at max boost near HP peak (the piston is moving too fast, as well as VE of the engine is not as high).

[95 imp]

Alright, I'll take a stab at it....

.

11.5/1 from BDC to TDC NA. You're limiting factor is how much air you can get into the cylinder. Theoretically, you could be 2/1 with the throttle closed because very little air is able to go into the cylinders due to the throttle blades causing a massive restriction.

Supercharging (mechanical or turbo) CRAMS the air in. It adds air above atmospheric, which increases the volume of air that is squeezed. The more CFM (not necessarily PSI)
forced in the cylinders increases the compression ratio.

Top Fuel motors actually run on the verge of hydro lock because they are cramming so much air and fuel in the cylinders.

The limiting factor is detonation. The higher the CR the less boost you can run because of fuel octane limitations.

Lower static compression allows more boost to get to the same final CR. That leads to more power because of the extra air/fuel in the cylinder. This is best for track/drag racing

Leaving a higher static CR and a lower boost allows you to have more power when not on the blower. This is generally better on the street.

Again, you need the proper octane fuel to keep detonation away.

[KingLT1]

I would base targeted compression off of octane used. E85 will support high compression and higher boost and power levels because of its superior ability to remove heat from the cylinder. 93 octane, high compression, and boost generally is a bad mix. However with Direct Injection it allows about another 1 point higher compression compared to PI based on a explanation above. So it's not much different then adding boost to a gen 4 LS motor. If the DCR wasn't so high you could probably push the boost and power level up a little further. That is why you see most cam and supercharged LT1's get into the 700whp range where a stock cam LT1 would be limited to 650whp or so SAE on a Dynojet even if it had the fuel system to get there. The octane wall with the stock cam is 650 with a cam you can get up into the 720 range on 93. So it depends on your goals. If you have access to E85 stock compression is fine especially with a cam. If you are trying to make big power north of 750-800 on 93 then you will likely want to drop the compression a point.

[Joshinator99]

Originally Posted by KingLT1

I would base targeted compression off of octane used. E85 will support high compression and higher boost and power levels because of its superior ability to remove heat from the cylinder. 93 octane, high compression, and boost generally is a bad mix. However with Direct Injection it allows about another 1 point higher compression compared to PI based on a explanation above. So it's not much different then adding boost to a gen 4 LS motor. If the DCR wasn't so high you could probably push the boost and power level up a little further. That is why you see most cam and supercharged LT1's get into the 700whp range where a stock cam LT1 would be limited to 650whp or so SAE on a Dynojet even if it had the fuel system to get there. The octane wall with the stock cam is 650 with a cam you can get up into the 720 range on 93. So it depends on your goals. If you have access to E85 stock compression is fine especially with a cam. If you are trying to make big power north of 750-800 on 93 then you will likely want to drop the compression a point.

[oldman]

Originally Posted by KingLT1

I would base targeted compression off of octane used. E85 will support high compression and higher boost and power levels because of its superior ability to remove heat from the cylinder. 93 octane, high compression, and boost generally is a bad mix. However with Direct Injection it allows about another 1 point higher compression compared to PI based on a explanation above. So it's not much different then adding boost to a gen 4 LS motor. If the DCR wasn't so high you could probably push the boost and power level up a little further. That is why you see most cam and supercharged LT1's get into the 700whp range where a stock cam LT1 would be limited to 650whp or so SAE on a Dynojet even if it had the fuel system to get there. The octane wall with the stock cam is 650 with a cam you can get up into the 720 range on 93. So it depends on your goals. If you have access to E85 stock compression is fine especially with a cam. If you are trying to make big power north of 750-800 on 93 then you will likely want to drop the compression a point.

Agree here

[Umbriel]

My tuner said he wanted 10.5:1 on my motor when it was getting built, shooting for 1200 rwhp on E85.

[oldman]

way out of my league, but yeah the question is how are you going to hook on the street. You going twin turbo right?

[Umbriel]

Originally Posted by oldman

way out of my league, but yeah the question is how are you going to hook on the street. You going twin turbo right?

If you are asking me I doubt I will, only from decently higher rolls probably or peddle the hell out of it. Yes twin turbo on a 416 stroker and Crawford Racing port injection.

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[Nmolina9239]

So if I wanted to stick to 93 octane then its better to drop compression and up boost?

[Joshinator99]

Originally Posted by Nmolina9239

So if I wanted to stick to 93 octane then its better to drop compression and up boost?

Yep

[oldman]

Originally Posted by Nmolina9239

So if I wanted to stick to 93 octane then its better to drop compression and up boost?

IMO u should run the highest CR u can and the least boost for your projected HP. CR gives u throttle response and MPG, so it I did not drop my CR, works fine up to 10.5 psi, 7200 rpm, and 93. If any parameter changed I’d run 10.5. Also, IMO, boosted DD cars are more prone to detonation when lugging in higher gears, or WOT in midrange with small cam and roots. here turbo or centri, maybe even large roots will work better as the boost comes in later and an engine has less likelyhood of detonation at higher RPM. As a counter 11.5 cr OEM supercharger I’d say 8 PSI is the limit on stock cam, the engine is building huge amounts of midrange torque and that means cylinder pressure and that means detonation potential on a slowish spinning engine, less to no problem with the huge centri out now. Nutshell boost likes RPM as there s LESS cylinder filling per outputted HP, the piston is moving faster, and the valve stays open less long so less heat load and more quench of the valve / valve head. IMO, a 4v engine in thIs regard, is in they end a better design for medium to high boost / hp numbers given pump fuel.

[Choda]

I dropped from 11.5 to 10.5 for my power goal and sticking w 93 and meth.

[EDFHOBBIES]

There was a time when the original LT4 debut in the 90s and boosted cars ran 8.5 to 9.5 like myself with a vortec Trim, then the LS came out they wanted boosted cars to run 10.5 or more shops told customers if you lowered compression to much it would be a dog off the line, And now with the new LT line who knows but I have noticed this, I've seen a lot people do the LT4 blower swap on an LT1 with the LT4 injectors, LT4 pump, Lt4 fuel line, and Aux dsx pump.. Why do they fall short of LT4 numbers.. It's not the lack of fuel, or a different blower.. could it be the compression is more and you can't run as much timing hmmm hmmmm!

And why would shops like LPE use cometic head gaskets on there LT4 builds that are thicker.. Well Brandon told me when I duplicating there 1000 hp build they like to drop compression a tad..

But build how you want! A lot people will tell you you can add meth or E if you need more octane.. For me these cars have plenty of tq, When I start messing with the wifes LT1 I'm getting bigger CC chambers, lower compression pistons, and the cometic head gasket..