Cranking compression results are in

[F-BIRD'88]

F-Bird, this is a gen2 LT1. They start at 10.5:1 with god awful quench from the factory and a tiny cam and have no problem running on crap 87 octane fuel. Factory cranking compression is 185-200psi on these engines. With aluminum heads and reverse flow cooling these things break the rules of conventional carbureted, iron headed gen 1 small blocks as engineered. People regularly run these over 11:1 even on the factory cam. So 11.5:1 is high for sure, maybe the edge of streetable for this platform, but then there are modern engines that do the same without any super fancy technologies like variable cam timing or direct injection.

When you do run these stock LT1 engines with just 87 octane gas the computer dials back the spark timing @ WOT to avoid detonation. You will no make full power if on 87 octane gas.
Again do not get all carried away on compression on 92 octane gas. People do this an claim that. Lots of bragging.
Most will never tell you that the engine ate itself in a short time.
Or ring seal quality went away in a short time. A fresh engine suddenly needs a re ring job in a short life because it was detonating.
Some people who attempt to then hone and re ring it find that the "freshened up engine) has evern MORE Blowby now as the Piston ring lands are now DISTORTED from high speed detonation.


Yes these engines have a great reverse cooling system. and computer control. Keepte real compression ratio under 11:1
(With good quench) and run it on approriate fuel octane. Forget the 87 octane BS.
You can make a TON of reliable power @ 10.5:1 cr. Make the engine process MORE AIR AND FUEL.

This engine as it is builtr if the CR is 11.5:1 won't last very long on pump gas. Use 100 octane unleaded if you want:
A: Maximum performance. B: a decent service life.
There is no free lunch.

ALL the factory SHow czr classes that rin these high compression ratio factory cars includiing the LS engine cxars run them on 98 to 100 octane unldeaded.. These cars cost a fortune and cost a furtune to rebuild. No body races them on pump gas.

Have you ever tried this or do you just chatter? All kinds of vehicles run 11.5 to 1 comp on pump swill. We have 6 grain trucks from the 90s, BBC with that comp and 215 lbs cranking compression. When loaded we run the shit out of them. They are not short lived and have yrs and 1000s of miles on them. AGAIN do you build anything or parrot bs you read?

Yes iI have " Tried this" ran many very high compression ratio enginers on pump gas. ( all with reduced WOT spark advance)

[Lizardracing]

Cranking compression....
What happens if my starter only cranks the engine 125 rpm? What about if it's a super fast starter and cranks at 250rpm? What if I installed a custom starter at 48v and can crank it at at 700rpm?
What RPM would I use to build it? NONE
Cranking speeds and DCR has never made sense to me NOR have a EVER heard a legit cam guy use it either.

[SilverChicken]

Some say “YOU’LL DIE!”, others seem to say “no prob, gofer!” I had no way of knowing cranking RPM but I was doing this with a fully charged battery, gear reduction LT1 mini starter, with all the other plugs out and before the heads went on I cleaned and oiled the cylinders. Sounds like RPM was up and ring seal really good which could skew my readings higher than they might be otherwise possibly. Maybe? Hopefully? Just 10-20psi lower and I’d probably have never even asked.

[HQM383]

From a smart cookie here on ST:

SCR and DCR are often misunderstood and therefore misused.
Both are simply ratios that do not predict actual combustion events. To predict actual combustion events we
need real numbers like compression temperature and compression pressure. Neither SCR nor DCR provide this
information. SCR is incomplete since it only represents a ratio based on full stroke. DCR is based on stroke at IVC.

More than 2 numbers are needed to predict Octane tolerance for a given engine.

[Dave B]

FBird
I fully believe you been reading shit and being a parrot since you were 13. I also believe you have absolutely no real world experience in engine dynamics. We can either listen to f tards on inet or do as I prefer and actually build and race .Show me pics of what powerhouse you are building today and I will post pics of the mountain motor prostock induction a customer has me doing tday.

[cab0154]

Building for 11.5:1 on pump gas is a mistake for a street driven car. The operating conditions on the street in real world traffic are JUST TOO VARIABLE.
You can make it "run" as is but the timing will need to be compromized to keep it alive on the street on pump gas.
If you want to let the lobster loose at the track use 100 (or more) octane gasoline. ( max power ignition timing for best engine power)

interesting. i guess my 12:1 4.145 bore small block doesnt know that it isnt supposed to have pump gas in it. when i run it around on the street part/mid throttle, light load it is way safe on 93. some will, some wont. depends on your build, cam, chambers, cooling, timing curve, ect.

As long as you never open the throttle.

maybe, i usually run it on c12 because it makes the most power. It is over 110% VE on motor, so i havent really tried to dick with 93 at the track. but on the street i can open it far it enough on 93 to knock the drag radials off trying to get to hwy speed. and that is at peak tq, and it makes over 1.3 lb/ft per cube, at 416", so there is that. as far as WOT on pump gas, my wifes new toyota is 13:1 and runs on 87 running a 210* coolant temp, and I can go WOT on that one....weird. I guess you better let the toyota engineers know that 11.5:1 is a mistake in a street driven car.

[Dave B]

Building for 11.5:1 on pump gas is a mistake for a street driven car. The operating conditions on the street in real world traffic are JUST TOO VARIABLE.
You can make it "run" as is but the timing will need to be compromized to keep it alive on the street on pump gas.
If you want to let the lobster loose at the track use 100 (or more) octane gasoline. ( max power ignition timing for best engine power)

interesting. i guess my 12:1 4.145 bore small block doesnt know that it isnt supposed to have pump gas in it. when i run it around on the street part/mid throttle, light load it is way safe on 93. some will, some wont. depends on your build, cam, chambers, cooling, timing curve, ect.

As long as you never open the throttle.

This is how out of touch you are! I'm working on 17.45 to 1 x 820 plus inch engine. I could put pump gas in it and Rev it to 8000 and not hurt it. Under load would be different. That's why a stupid dcr formula means nothing. We don't race engs off the starter, at least I don't. Now bird,show me what Briggs carb you are cleaning from your moms basement .

[SilverChicken]

C’mon y’all, I don’t know F-Bird from Adam but just respect that he’s got his way and y’all have yours. I’m able to use discernment in what information I use as gospel truth. I understand his point of view and I get he’s probably blown up some engines in what he’s tried. What none of us know is his ability to build, his method, or the components he used on what has blown up vs what has worked. The LT1 cannot have everything he said so unless everything stays 100% as the factory made it, it will immediately be outside the parameters he specified. I’m able to see that and so I know that where he’s coming from can’t apply here. Here’s how an LT1 starts life. Piston, .025” in the hole just like every other SBC. Crazy .050” head gasket for .075” quench. Chambers are 54-56cc and the cam is puny, sub 200* @.050. If you want good quench you’re at the SCR im at out the gate. Being 350cid even a 230* camshaft is big considering these things are RPM limited by the bottom end construction. Around where I live I have access to 93 octane fuel easily and as the cooling system sits now, in traffic, in the stupidly hot summers here (100-110F), my coolant temps top around 210. I have no problems running some TR6’s or colder in the engine but that’s what I’m looking for more than these arguments. Yes run the engine as it sits, keep the temps under this, run this heat range plug, run it a tad rich to help it keep out of det, or no, no crutch will let you enjoy this engine like you wish to short of race gas which is more difficult to source. Thanks fellas, just let people be how they’re gonna be. We get old, we get set and get fussy when someone else counters the wisdom we gain from those experiences. We’re all like that at some point.

[cab0154]

you need to get the coolant temp down around 170*. thatll help a bunch. even back in the 90s, when i had my turbo kit on my fox, in 110* heat and FW traffic, it would hang out around 175. so it can be done. as far as f-bird, a lot of us have had to see his BS for well over a decade. it gets old, especially to those of us who build our own stuff/or build real engines using technology that isnt stuck in the 80s.

[Dave B]

To the peeps that think dcr matters? Maybe you should get a test engine and some pump gas. Now get all fuel data, color, smell ,taste etc. Then order 9 starters. The one that makes the most cyl pressure I want to buy for my race programs.

[Stan Weiss]

Actually RPM has no bearing. Trapped mass / VE% does.

Stan

ab-cgp-ve-st.gif

[SilverChicken]

you need to get the coolant temp down around 170*. thatll help a bunch. even back in the 90s, when i had my turbo kit on my fox, in 110* heat and FW traffic, it would hang out around 175. so it can be done. as far as f-bird, a lot of us have had to see his BS for well over a decade. it gets old, especially to those of us who build our own stuff/or build real engines using technology that isnt stuck in the 80s.

That 210* (if my memory is serving me well. I may also be mixing it up with the Firebird was a once or twice occurrence in the worst of situations when I’m running a 180* t-stat and the fans come on at a set point someone else decided. It normally only ran around 195* at the highest, probably when the fan was set to kick on. That’s rocking a factory replacement radiator and stock fans. If it’s a must I can look into running an aftermarket 2 row with big tubes.

[Art]

C’mon y’all, I don’t know F-Bird from Adam but just respect that he’s got his way and y’all have yours. I’m able to use discernment in what information I use as gospel truth. I understand his point of view and I get he’s probably blown up some engines in what he’s tried. What none of us know is his ability to build, his method, or the components he used on what has blown up vs what has worked. The LT1 cannot have everything he said so unless everything stays 100% as the factory made it, it will immediately be outside the parameters he specified. I’m able to see that and so I know that where he’s coming from can’t apply here. Here’s how an LT1 starts life. Piston, .025” in the hole just like every other SBC. Crazy .050” head gasket for .075” quench. Chambers are 54-56cc and the cam is puny, sub 200* @.050. If you want good quench you’re at the SCR im at out the gate. Being 350cid even a 230* camshaft is big considering these things are RPM limited by the bottom end construction. Around where I live I have access to 93 octane fuel easily and as the cooling system sits now, in traffic, in the stupidly hot summers here (100-110F), my coolant temps top around 210. I have no problems running some TR6’s or colder in the engine but that’s what I’m looking for more than these arguments. Yes run the engine as it sits, keep the temps under this, run this heat range plug, run it a tad rich to help it keep out of det, or no, no crutch will let you enjoy this engine like you wish to short of race gas which is more difficult to source. Thanks fellas, just let people be how they’re gonna be. We get old, we get set and get fussy when someone else counters the wisdom we gain from those experiences. We’re all like that at some point.

It's a "my dad can whip your dad" contest, or "my Johnson is bigger than yours". It's a schoolyard thing.

[cab0154]

It's a "my dad can whip your dad" contest, or "my Johnson is bigger than yours". It's a schoolyard thing.

Or its a guy who just parrots the same stuff magazines published 25 years ago, vs people who actually work at engine shops with a real dyno/flowbench and know how to get more NA power out of stuff that was already making 3hp/ci without increasing the rpm. a lot of very knowledgeable guys have stopped posting over the years because of the ones who still think the earth is flat.

[Tom68]

Actually RPM has no bearing. Trapped mass / VE% does.

Stan

ab-cgp-ve-st.gif

Depends how much leakage you've got in a cranking test, used to advantage by cranking comp tested racers.