Okay, so im building a 400 and going with a forged bottom end. Well as im sure alot of you know, once you start changing items in a bottom end, of the shelf selection gets more and more thin. Well im going with 6" rods and wanted at least 10:1 compression. I decided on SRP 10.3:1 pistons.

Well this is my first engine build and my instructor first questioned why so much compression? Then gave me some story about a some drag car he built the engine for that runs 11's and only has 9:1 compression...but he's all about making power so after talking it through he decided I'd likely be okay.

Then I took my engine to the machine shop to be surfaced, align honed and rough cut for my pistons, then the machinsts questions which pistons I would be running and said he would try to match what I had planned on spending. Then once again gave me the "You plan on running pump gas? Aluminum heads, right? I wouldnt go much more than 9.5:1." I stated the fact that my iron headed 305 was running 9.5:1 from the factory, and that I *thought* I could get away with more compression on aluminum heads.

Now my friend got this same story when he built the 347 for his mustang which is running fine at 10.5:1 on 92 octane. Are these guys just cautious or old school?

Comfort me.

406
brodix track1 221cc 2.08/1.6 valves
comp pro mag RR
victor jr
holley 850DP
herbert 242/242 .555/.555
eagle 4340 crank
6" 5140 eagle rods
SRP 10.3:1

I think that they are being overly cautious. You should have no prob running pump gas with 10.5.

My 400 has ~11.25 compression and alum heads, runs fine on 93 octane.

There are many factors involved in the correct compression ratio.
Lack of hot spots (benefit of Aluminum heads), combustion chamber design, fuel mixture and atomization, quench, and cam specs are all part of the overall picture.
11:1 can be done on iron heads with pump gas. Not on your average engine, but it can be done.

If your heads are aluminum then I'd say you will be fine with 10.3:1 compression, but make sure you have the quench right to be sure.

dude Compression is your freind

dude Compression is your freind

That is almost "sig" worthy.

My 400 has ~11.25 compression and alum heads, runs fine on 93 octane.
ON my 406 I had flat top two valve relief zero decked with 64cc heads. it netted 11.25 compression and ran fine on 93. I did have dart pro1 aluminum heads. :crazy: So I would definitely have no problem telling you 10.3 is fine as long as your quench is right!!

i am running 11 to 1 with my 355 alum headed motor with no problems. runs like a charm

With aluminum heads, Vizard says you're good to 10.5:1 if your quench is between .035 and .045, as long as there are none of those nasty sharp edges.

stock vettes are 11:1
with that cam youll be fine, just do a compression test when its assembled and if your under 200psi youll be ok.
but not on 87

I ran 12.5:1 pistons in a 320 Chevy on the street. I love the power but hated the fact that it took more money for gas to run it. It also took a very definative tuneup to keep it steetable too. IMHO, this is why your machine shops and freinds are tring to talk you out of building a high(er) compression engine for the street.

10-10.5 to 1 rules!

350 4.060" bore. Iron heads "492's". Smoothed all sharp edges. 10.21 to 1 comp. Pump gas no problem.

With alum heads should be easy!

Theres alot more to this subject than meets the eye.

Best thing for you to do is look into all of this, do a search on here under my name and subject of "DCR"

I run a 10.18:1 iron headed 355 in a daily driven car on pump 93. The car as 212 to 218 psi cyl pressure.

Cyl pressure is what you really got to look at. For a newbie with basic tuning skills I rec. 190 to 200 psi cyl pressure max. The cam size plays a major role in your cyl pressure..

You can have a 13:1 engine with only 180 psi cyl pressure if you use a monster cam in it. Or you can have a 9.5:1 engine with 200 psi if a tiny cam is used.

When you get past 200 psi cyl pressure you can still run pump fuel but you really have to be on your game tuning wise. Timing has to be took out and set to right amount for combo, inlet air has to be kept cool, water temp kept cool, colder heat range plugs used, plug gaps closed up some to keep the higher cyl pressure from blowing flame out, carb jetted on the rich side, etc etc.

I'm a little surprised you guys are getting away with that kind of compression with today's crappo gas. I guess it must work, cause you guys are doing it. How do they run in hot weather though?

I've got the modern LT1 in one of my daily drivers and it is 10.25 to 1, BUT these engines are reverse cooled. They run way cooler than the old blocks. My electric fans don't even have to run most of the time.

Theres alot more to this subject than meets the eye.

Best thing for you to do is look into all of this, do a search on here under my name and subject of "DCR"

I run a 10.18:1 iron headed 355 in a daily driven car on pump 93. The car as 212 to 218 psi cyl pressure.

Cyl pressure is what you really got to look at. For a newbie with basic tuning skills I rec. 190 to 200 psi cyl pressure max. The cam size plays a major role in your cyl pressure..

You can have a 13:1 engine with only 180 psi cyl pressure if you use a monster cam in it. Or you can have a 9.5:1 engine with 200 psi if a tiny cam is used.

When you get past 200 psi cyl pressure you can still run pump fuel but you really have to be on your game tuning wise. Timing has to be took out and set to right amount for combo, inlet air has to be kept cool, water temp kept cool, colder heat range plugs used, plug gaps closed up some to keep the higher cyl pressure from blowing flame out, carb jetted on the rich side, etc etc.

I'm not a real big believer in "DCR" being the final answer on whether or not you can run pump gas.
DCR to me is just another "gimmick" or mental masturbation that still doesn't take everything into account. What about gearing and vehicle weight...?

If you cam a motor so big that you drop the DCR and cranking compression down, then you have a pig with no bottom end power, that might eventually make power like it should way up the rpm scale, and then if it does, it WILL build up the cylinder pressure and get into detonation. Then what did the DCR calculator do for you?

11-1 with aluminum heads on a SBC and premium pump gas simply isn't an issue unless you get too lean or too much timing.
You could do it with a baby cam and huge cranking compression or you could do it with a huge cam and less cranking compression. Either way at some point on the rpm scale, the volumetric efficiency will become great enough that the dynamic compression will "act" like it's 11-1.

302 was built and run in the early 70 when Sunoco had the 260 ht gas. The 302 wouldn't run on anything else without baulking and pinging.

302 was built and run in the early 70 when Sunoco had the 260 ht gas. The 302 wouldn't run on anything else without baulking and pinging.


12.5-1 and iron heads isn't a good idea on pump gas... No matter how big the cam or what the DCR is.

I wouldn't even do it with aluminum heads unless I had a state of the art EFI and controller (and someone who knew how to tune it better than I..)
Even then I think the knock sensors and timing retard would be very active.

Sort of veering topic, but higher compression nets better mpg... Any idea what a guess would be in the difference in mpg would be per point in compression? Ie, 9:1 vs 10:1, etc?

As Jake mentioned, the cam selection plays a big role in the whole pump gas equasion (sp?) What cam do you plan on using?
Also as stated It would be a good Idea to polish the combustion chambers on the heads or at leats break the machined edges. Polishing or having the pistons coated isn't a bad idea either.
Aluminum heads allow greater comp ratio's cause they disapeate heat faster, and heat makes detionation.
Also like Jake said the weight and gearing of the car are also added into the mix. I believe we need more info about your complete combo to make a definitive decision. That said I have ran a Iron headed SB 355 with 11.5-1 on 93 oct and had no problems, but I made sure everything was compatable.

i agree with the guys above,,, tuning and controlling all the temps must be on the higher you get. it's safer for a machinist to recommend that it be kept within reason or to a slightly lower "proven",, easier to control ratio,, so the average tuner can get it close without burning things down first.
but mwilson is right,, compression can be your friend when controlled well. and like night rider said,, controlling all the temps and timing is important. quench as well.

and like jakeshoe said,, if you're using a good aftermarket head, cam, and intake combo that can potentially achieve more than 100% v/e with a camshaft/intake package that allows that,,,,,,(starting arround 230@.050 or so,, give or take a few degrees depending on lsa) then it's entirely possible when using a slightly the bigger camshaft,, that you'll detonate even a 10.0 ratio,, if it's ram tuning more than the cylinder holds at a 200lbs cranking compression check. the cranking compression is not the end all,, but can give a very good idea on where you are with dynamic v/e if you can't get the tune nailed down at max v/e without detonation on the fuel octane used. the cylinder could actually end up arround 230-240lbs (or more than 200lbs so to speak)(actual dynamic compression pressure) when actually scavenging properly at the torque peak,, at peak v/e,, and it possibly could be more cylinder pressure than the pump fuel can tolerate with the timing not pulled out some or controlled early in the rpm range,, or with inlet temps, engine temps, engine loads not controlled. a bigger stall converter has a fairly big effect on making that easier also. it gets the engine into the meat of the torque curve and atomizing the fuel alot better,, before the engine loads are applied. engine loads have a huge effect.
jmho.

i've watched it happen with a guy here that insisted on tunning 11.8 compression with trick flow aluminum heads,, a mid 250@.050 solid camshaft,, with a victor jr intake and 850 holley,, in a 351W setup.

he says,, but i only have 180lbs of cranking compression,, and still,, it's lifting the top ring land off these hyper pistons and it's tearing up the rest of the engine.

and i said,, it's got more than 180lbs at the torque peak at peak v/e. his timing was locked out at 36* with no vacuum advance,, with a 4500 converter and 4.56 gears on premium pump fuel,, so the low end engine load wasn't really a problem. but he wouldn't try pulling back the timing either. it was achieving more than 100% v/e cylinder pressure at peak v/e. the cam can choke off the v/e even with a good head if it's a small enough cam with huge cranking compression and allow that huge cranking compression. just an example from jake's mention of it. a hands on view.

i'm basically running a 70 LT-1 360hp motor with the stock 11.5:1 comp but with GM #140 solid cam and re-worked #186 cylinder heads . with all the mods i've done to this engine it's putting 425hp to the flywheel and can easily use 91 octane super .the camshaft is helping me bleed off cylinder pressure so i can run this lower octane fuel than what it needed back in the 70's

I'm not a real big believer in "DCR" being the final answer on whether or not you can run pump gas.
DCR to me is just another "gimmick" or mental masturbation that still doesn't take everything into account. What about gearing and vehicle weight...?


I'm glad somebody said it before me! I think DCR is one of those things that too many people hang their hat on. A DCR calculation might tell you you can run 13:1 if you shove a big enough cam in the motor. NOT the case. It's a helpful calculation but not any more useful than knowing your engine's static compression is if you don't apply a little "sanity" to it.

Yes, big cams can tolerate (and, in fact, require) more compression to run well. But don't go overboard here. Static compression STILL matters. DCR is good at helping you know about your engine's average cylinder pressures but it doesn't address the PEAK pressures of combustion. And the peak pressures are where detonation happens. Peak pressures are still correlated with your static compression ratio. Now, I'm no expert on combustion theory and much of this is beyond my experience and knowlege level, but I can tell you that trying to use a big cam exclusively to get away with compression that's clearly too high is often an exercise in frustration. You give away torque on the bottom end but when you get up into an RPM range where the cam starts to sing you often still get detonation.

Now, in your case, with a little over 10:1 squeeze, aluminum heads and a fairly alrge cam I think you're in safe waters. I don't think you'll have problems. I've built similar combos and not had detonation problems.

i've watched it happen with a guy here that insisted on tunning 11.8 compression with trick flow aluminum heads,, a mid 250@.050 solid camshaft,, with a victor jr intake and 850 holley,, in a 351W setup.

he says,, but i only have 180lbs of cranking compression,, and still,, it's lifting the top ring land off these hyper pistons and it's tearing up the rest of the engine.

and i said,, it's got more than 180lbs at the torque peak at peak v/e. his timing was locked out at 36* with no vacuum advance,, with a 4500 converter and 4.56 gears on premium pump fuel,, so the low end engine load wasn't really a problem. but he wouldn't try pulling back the timing either. it was achieving more than 100% v/e cylinder pressure at peak v/e. the cam can choke off the v/e even with a good head if it's a small enough cam with huge cranking compression and allow that huge cranking compression. just an example from jake's mention of it. a hands on view.

Timing locked at 36* is part of his problem KB rec's no more than 32* total (last time I spoke with them) for the hyper's for that exact reason, top ring land failure!
Don

So basically, if you give up 1/2 a point in compression and stay around 9.5:1 you will likely avoid most all of the above mentiond issues. You'll find the engine is much more forgiving if your quench isn't a perfect .040, your timing is a couple degrees out, you get cheap and run regular gas, or put some spray into it.

Compression by itself is only worth about 4% in hp. If your 383 is putting out 400hp you'll be giving up around 8hp. There's easier and more forgiving ways to get that power back than in compression.

I'm suprised people overlook altitude as much as they do. I have a 355 just over 10:1 and I live at 3100'. TFS heads, timing at 36*, small hyd cam, and run it on 87 octane all day. When I go down to the coast, at about 1500-1800' it'll start to rattle under wot. By the time I'm at 500' there's an egg under my foot until I get some 91 in it. What might work all day long for me or Gregh, will likely be instant disaster if Dirt Reynolds tried it, just due to the air dencity difference.

Cardinal,

Were you running the old Chevy 2nd design, -140 solid lifter cam in your 12.5 to 1 CR. engine??

I think that's like SY1's combination too..

pdq67

Timing locked at 36* is part of his problem KB rec's no more than 32* total (last time I spoke with them) for the hyper's for that exact reason, top ring land failure!
Don

exactly chopped!! told him of it as well. goes to show that it's a wee bit much on the timing and compression ratio even showing 180lbs cranking compression with aluminum heads and a "bigger with narrow lsa to lower dynamic compression" camshaft. just saying that everything depends on everything in a round-a-bout way. he didn't hear it pinging,, it just let go.

the bigger cam gets the flowtract more efficient like jake said. might be fine at low engine speeds and light loads,, but when you get it up to the torque peak at peak v/e it can come back to bite you.

"bigger with narrow lsa to lower dynamic compression" camshaft. .


Ummm that's two of that ole boys probs. He's stubborn and mis imformed.

narrow LSA does not lower the DCR it makes it higher.

Lets start with an engine that has a 114 LSA cam and has 8.122 DCR, just only changing the LSA and intake center line to keep 4* of advance in the cam.. Not changing duration or any other specs.

112 LSA 8.249 DCR
110 LSA 8.372 DCR
108 LSA 8.492 DCR
106 LSA 8.608 DCR

Then locking the timing to 36* and not testing/changing it, and not checking plugs for signs of detonation wasn't the brightest thing to do.

I have to run 34* total on my 10.18:1 355. At 35* I get some very light detonation (not heard with ear, but seen on plugs as a few tiny black specs. Heck can't even see them good with out a lighted magnifing glass)


If you want to push the SCR/DCR/cyl pressure limits on pump fuel you have alot of things that you have to do as your building the engine and alot of testing and tuning after it's built.

Quench height is a big thing .035" to .045" max.
The right cam for the combo
deburring the pistions and chambers
Polishing the chambers and piston crowns
Keeping the carb inlet air temps low
Keeping coolant and oil temps down
Colder spark plugs (I'm having to run plugs 2 heat rangers colder on mine)
The right plug gap
The right amount of total timing which is gonna be less than you think or less than enough for great part throttle driving so playing with vac advance is a good tuning tool aswell
The right A/R and it will end up being richer than you think it should be


Is it worth all the extra tuning, and prep work???? Some will say yes, some will say no. Myself I 100% fully say yes it's worth it.

night,, i agree. also,, with the way that i'm saying or wording dynamic compression(which changes),, i'm referring to the running pressures at engine speed while pumping. not the static compression measured from the intake valve closing point. guess i should say the BMEP at the torque peak or at peak v/e.

i heard someone here use the term kenemetic (sp?) compression once,, in describing static compression measured from the intake valve closing point,, or what pat kelly's program measures for instance. i'm not really reffering to that in how i'm wording it. but,, i think we understand each other. on the same page and all in that he didn't start low and tune up on the timing, etc. and the hyper pistons will tell you very quickly if it's detonating. that's a for sure. not really fond of them. and i don't think i'd try 11.8 either. just a bit much to tune arround. ran 10.8 with my iron headed rect bbc setup though. wasn't a problem at this altitude. but i creeped up on the timing watching plugs/temps/etc... it's all good.