Is anyone successfully running this CR on pump gas as a street/strip application? I know a friend of mine is running 11.0 with no problem, but I'm curious if if the .6 is going to make a huge difference.

Do you plan on racing it, or just driving around town?

Is anyone successfully running this CR on pump gas as a street/strip application? I know a friend of mine is running 11.0 with no problem, but I'm curious if if the .6 is going to make a huge difference.

I get away with 11:1 in my '69 Z because of the horrendous overlap in the cam shaft. I would guess that would have a big impact on the increased CR. Type of head would seem to be a major factor also.

I plan on racing anytime I can, but I'm going to cruise it too. It'll get about equal amounts of race and drive time. I'm running camel hump, 2.02, 1.6 full port and polish job. 64 cc

I forgot to mention that the pistons will be SRP's 4.165 bore flat top with a -5 cc. This is a 409 that I'm thinking about building but didn't want to overdo the CR. The block will be decked to 9.0, using the pistons mentioned, running the above mentioned heads. I don't know what the gasket thickness will be or should be really.

i'm also at 11.1:1 comp and running the exact cam as flowjoe is and no issues with 91 octane pump gas

With a zero deck block and flat tops with 64cc heads you will be above 13:1 with a .039 head gasket.

Not going to happen IF you really do have 11.6. If you just think you do and it's really 9.5 you should be fine. I run just a touch over that with some of the best heads and piston design money can buy and with full EFI control it is very difficult. I think a lot of folks would be surprised what their compression really was if it was actually tech inspected.

So no, if true, you won't be able to run pump gas.

Overlap is a myth as well. Once you get into the operating range of the camshaft and the VE goes up you are going to detonate.

I forgot to mention that the pistons will be SRP's 4.165 bore flat top with a -5 cc. This is a 409 that I'm thinking about building but didn't want to overdo the CR. The block will be decked to 9.0, using the pistons mentioned, running the above mentioned heads. I don't know what the gasket thickness will be or should be really.

I ran a very similar 406 in a full size truck years back and had to mix my gas. It was a flat top motor with cast iron dart heads and a 292 magnum comp cam.

With a zero deck block and flat tops with 64cc heads you will be above 13:1 with a .039 head gasket.

How did you figure that?

Not going to happen IF you really do have 11.6. If you just think you do and it's really 9.5 you should be fine. I run just a touch over that with some of the best heads and piston design money can buy and with full EFI control it is very difficult. I think a lot of folks would be surprised what their compression really was if it was actually tech inspected.

So no, if true, you won't be able to run pump gas.

Overlap is a myth as well. Once you get into the operating range of the camshaft and the VE goes up you are going to detonate.

I entirely agree.

How did you figure that?

Calculates to 13.36:1
http://www.wallaceracing.com/cr_test2.php


Edit: Punched it in wrong. Comes to 11.77:1

no way on pump gas.........

my 404 is 11.3 to one ( I have a burret and measured ), pro1 heads fast efi runs fine on 94. SRP flat tops, blended all sharp edges in the combustion chamber.



for kicks I ran your numbers on the Wallace calculator and it said

Your displacement is 408.73 CID
The Compression Ratio is: 11.71

Calculates to 13.36:1
http://www.wallaceracing.com/cr_test2.php


Edit: Punched it in wrong. Comes to 11.77:1

I used a different calculator

I used a + instead of a - for the piston cc

I stand corrected, you are right (approx. 11.6:1)

Ok. Well ill go a different route then. Thanks for all the input.

Don't forget the pin height and the effect it has on where the top of the piston ends up in you calculations as well.

Doug,I know from the years of power tour gas varies greatly State to State. Wwe both know we can whip out the lap top go into our map and compensate. With some gas in So Illinois even that doesn't help! With the specs the OP posted my experience would be it won't work on the typical 93 pump gas around our area.

Rick, I agree with you especially with that combination I wouldn't want to attempt it with old school camel hump heads. My local station dropped the 94 and now has 93. I didnt mean to stir the pot, I was just saying it can be done but I see your points
time to get some sleep here

later
DJ

Ok. Well ill go a different route then. Thanks for all the input.

One option would be to run it on E85 (105 octane)

One option would be to run it on E85 (105 octane)

Unfortunately, we down here don't have it yet. Closest place is the Houston area. Back to the drawing board.:crazy:

Any time you're getting in to the 11-1 and higher range, todays petrol gas isn't going to cut it. I figured if you were just going to drive it around, you could go conservative on the timing and probably be fine driving around town, but then I thought who the heck does that ;)

Not going to happen IF you really do have 11.6. If you just think you do and it's really 9.5 you should be fine. I run just a touch over that with some of the best heads and piston design money can buy and with full EFI control it is very difficult. I think a lot of folks would be surprised what their compression really was if it was actually tech inspected.

So no, if true, you won't be able to run pump gas.

Overlap is a myth as well. Once you get into the operating range of the camshaft and the VE goes up you are going to detonate.

What can I say? I don't race so have never been tech inspected, I can only go off the published info. Built the 302 to GM specs but at 30 over (TRW domed pistons (L2210AF .030), non-modified "186" casting heads (64cc), etc) and 11:1 is what is called out. You may very well be correct that it doesn't reach that number.

I can only refer to my own experience with regards to detonation. I've never experienced detonation on a regular basis (there have been occurrences as one might expect but it is not chronic).

As to overlap, the cam specs are as follows: lift: .493"/.512", duration at .50: 257/269, lobe separation: 112. It is the mechanical, off road GM cam. I know that below ~2500 RPMs I throw a lot of raw fuel out of the combustion chamber and out the back of the car. That has to be due to overlap - which would "bleed off" compression - right?

Overlap bleeds of cylinder pressure but doesn't change static compression calc. Raw fuel is low VE and poor combustion. Low VE and poor combustion has a profound affect on cylinder pressure. Neither low VE or poor combustion is good for performace or flame front. Because you potentially have poor slow combustion, and low power output versus ideal conditions, thiscontributes to it tolerating pump gas most of the time, no matter what the actual compression is.

Overlap bleeds of cylinder pressure but doesn't change static compression calc. Raw fuel is low VE and poor combustion. Low VE and poor combustion has a profound affect on cylinder pressure. Neither low VE or poor combustion is good for performace or flame front. Because you potentially have poor slow combustion, and low power output versus ideal conditions, thiscontributes to it tolerating pump gas most of the time, no matter what the actual compression is.

Yeah, that cam is really meant for 3K+ and is the product of 42+ year old engineering philosophies (of course I suppose that's OK since I'm running it in a 40+ year old car;)).

So I guess the real lesson here is that CR is not the only factor in determining an engine's ability to run on pump gas (as you just laid out above). That's really what I was trying to get at in my 1st post although perhaps not as technically detailed as your postings and relying upon my personal situation too much.

I think I'll just be on the safe side and run the dished -16 pistons instead, to reach a more comfortable and less problematic 10.5 or so. Thanks for all the info.

Overlap is a myth as well. Once you get into the operating range of the camshaft and the VE goes up you are going to detonate.

that's the fact. a head that is very restrictive will allow the extra at and beyond peak torque. you could always throw lots of gear at it, lighten it up to about 2000 lbs and never load it hard. :)
andy

And never drive it above 65 degrees........as temp is a huge factor as well.

I am at 12.1:1 and we set cable slackers motor up at 11.7:1 you need the right cam & converter set your timing at 34-36 or whatever and forget about vac advance. there is more to it but thats the basics

Not sure what wet dream you guys are having but I want in. Unless your dizzy is tricked up, there's no pump gas today that will run a 11:1 CR motor (60's-70's motors). When I was running with 11:1 in the good ol days we had Sunoco 260 which if I recall was 110 octane. The only way I was able to use pump gas in the 80's with 11:1 was with my inital timing set at 2-4* with no balls (LT-1). So if you guys are running today on 91 octane then you should have told Sunoco to stop making 110 octane if 91 was plenty. We have Sunoco 94 in Canada but there's so much corn in it, my Z28 and Grand National don't like it, so I use Shell 91 which has 0% ethanol. New cars and trucks computers are set to run on so much ethanol and when people use Sunoco 94, they wonder why their vehicle run like crap and cough.:eek:

I've always wondered how so many get away with the high CR and pump gas. I was on my home today in the 2006 TL, and I stepped on it coming up a hill in 2nd gear, and got a little knock for just a second. This is an all-aluminum 11:1 V6 with VERY TIGHT fuel and spark control from an ECM, and guys here are doing with iron heads, carbs and points distributors. It makes me wonder if you're just not hearing it...

Could not have said it better myself COPO. Wet dream. I loved that.

http://ultrastreet.net/engines/454_lsx.asp

No one ever said it would be cheap. ;)

I like this one
http://ultrastreet.net/engines/427_realstreet.asp

Especially at that price. Good value shopping z. :)

man if i had 9k.......... i bet that would be some runner, imagine opening that up on a desolate section of highway maybe say at 3am, of course with no coppers around...... i would have to install straps on my back window :)....

I've always wondered how so many get away with the high CR and pump gas. I was on my home today in the 2006 TL, and I stepped on it coming up a hill in 2nd gear, and got a little knock for just a second. This is an all-aluminum 11:1 V6 with VERY TIGHT fuel and spark control from an ECM, and guys here are doing with iron heads, carbs and points distributors. It makes me wonder if you're just not hearing it...

you cant load a high compression motor at low rpm, Im assuming you have a stock cam, tight converter factory gearing. All those things equal bad situation.

Running high compression is very easy if you build the rest of the engine for it. stick a 268XE in a 355 with 12.1 and and a 1,500 stall with 3.08 gears and its a recipe for dissaster. I wouldnt reccomend anyone just go build a 12.1 motor for 93 unless its gonna be geared, have the proper stall and a cam that comes in above 3,000. Not exactly a daily driver but if youe already looking at compression that high I think your more concearned with power anyway.

It's so easy even a cave man can do it.

You are correct Rick IMHO it is easy but it has it application you dont go halfway, if you do it right no worries.

I am over 11.5 -1 with no problem runs like a monster and guess what i running 93 with no problems, and yes it can be done. Was at 11.7 -1 with camel hump heads with no problems also . No wet dream about cars just slutty women!

I have to say I agree with Rick's assertions.

Mike I agree with 'some' of what you mentioned, but that part about the cam "coming in above 3000 RPM" I don't agree with. The reason I say that is because once in the cams' operating range you'll still have to deal with the high compression problem. This is the one major reason I don't believe in all that DCR crap people talk about. Trying to crutch a high-compression street engine with a big cam and playing with the LSA will still have the same problems a smaller cam will, but just at a slightly higher RPM range.

Some guys like to live on the edge of what pump premium can stand, and maybe they get away with it, at least according to them. I've gone down that road with 12:1+ compression street engines on 94 octane pump gas and broken pistons in the process.

Some of you guys also have to understand that DENSITY ALTITUDE plays a HUGE role in what a particular engine can stand. Living in North Carolina you might get away with it, but in the Pacific Northwest you will not. The lower the DA -- and it can get VERY low up here -- the more octane a given engine will require.

I wasn't agreeing I was mocking. I think if someone advises to run 12:1 compression on pump gas or says its no problem they should be held accountable for the consequences to a poster like a builder is to a customer.

I'm done with the thread as hopefully the OP has the info he needs.

And never drive it above 65 degrees........as temp is a huge factor as well.
Altitude is also a big player. Here at ~~1500' I can run 87 and 38* and vacuum advance.
When I go down to sea level I have to run premium 94 or it will rattle at 1/2 throttle.

I wasn't agreeing I was mocking. I think if someone advises to run 12:1 compression on pump gas or says its no problem they should be held accountable for the consequences to a poster like a builder is to a customer..

I agree to be accountable and I clearly stated running High compression is a purpose built enging/car. It can be done easily but dont expect to idle like a 99suburban. Most "purpose built" cars dont get subjected to everything the average daily driver would and people that drive them know what they have and what they are doing with it. It is a choice that should be made by an individual but it would be wrong to say it is "impossible" that would be a lie and I believe in telling the truth and letting people make thier own choices,

I have to say I agree with Rick's assertions.

Mike I agree with 'some' of what you mentioned, but that part about the cam "coming in above 3000 RPM" I don't agree with. The reason I say that is because once in the cams' operating range you'll still have to deal with the high compression problem. Some of you guys also have to understand that DENSITY ALTITUDE plays a HUGE role in what a particular engine can stand. Living in North Carolina you might get away with it, but in the Pacific Northwest you will not. The lower the DA -- and it can get VERY low up here -- the more octane a given engine will require.

Now I agree with the density but seems everytime I hit the track its around 3,000' so thats probrably a good average? I also cant explain why but if you get the motor up in RPMs before loading it it seems happy, If you run a stock converter and high compression it will be bad. I have read several srticals on it and I am gonna agree that getting out of low RPM's quick is good for high compression.

Altitude is also a big player. Here at ~~1500' I can run 87 and 38* and vacuum advance.
When I go down to sea level I have to run premium 94 or it will rattle at 1/2 throttle.

Exactly my point. Here in Vancouver 94 octane is readily available everywhere, for a reason. The DA here can get incredibly low -- at Mission one night it hit -200ft. That is the primary reason so many Stock Eliminator and Super Stock records have been set here over the years. Bobby DeArmond ran the first-ever 9 second run in Stock Eliminator and John Gallina ran the first ever 7-second pass in Super Stock here at Mission.


Now I agree with the density but seems everytime I hit the track its around 3,000' so thats probrably a good average? I also cant explain why but if you get the motor up in RPMs before loading it it seems happy, If you run a stock converter and high compression it will be bad. I have read several srticals on it and I am gonna agree that getting out of low RPM's quick is good for high compression.

3000' DA will allow you to get away with a lot compression-wise vs. pump gas. The higher the DA, the less octane you need. The lower the DA, the more octane you need. However, the lower the DA the more power the engine will make, the "bridge" between the two -- more power, lower DA -- is higher octane.

I also cant explain why but if you get the motor up in RPMs before loading it it seems happy, If you run a stock converter and high compression it will be bad. I have read several srticals on it and I am gonna agree that getting out of low RPM's quick is good for high compression.

You're right on the tight converter and not loading up the engine down low part. The part I was referring to "above 3000 RPM" regarded cam profiles, ie use a 'bigger cam to allow the engine to run more compression'. Once that big cam gets up into its' working RPM range the problems of the high-compression engine on pump gas will come back to haunt. At least it does here with the aforementioned low DA issues.

You're right on the tight converter and not loading up the engine down low part. The part I was referring to "above 3000 RPM" regarded cam profiles, ie use a 'bigger cam to allow the engine to run more compression'. Once that big cam gets up into its' working RPM range the problems of the high-compression engine on pump gas will come back to haunt. At least it does here with the aforementioned low DA issues.

well I didnt quiet word that properly and that was a mistake but what I meant to say was get the motor out of low rpm IE high stall and if your gonna do it use a cam that matches your compression / stall. IE race car with tags & insurance
Actually when my car was a 4spd I could hit it at any rp, in any gear with zero detonation and it pulled hard at 1,500 on up so All I can say is Im still learning but high compression can be done.

everytime I hit the track its around 3,000'
That makes it possible. Your car here, will probably have its pistons in the pan by the 60' cone. It certainly can be done, in Denver as well, GregH's home track in Calgary is way up there too. But along the coast and in cooler climates, even 10:1 can take some tuning and 91 gas to get full timing.

That makes it possible. Your car here, will probably have its pistons in the pan by the 60' cone. It certainly can be done, in Denver as well, GregH's home track in Calgary is way up there too. But along the coast and in cooler climates, even 10:1 can take some tuning and 91 gas to get full timing.

I actually live 100miles from the coast but the DA is always higher, I vavent figured that one out either.

we see some 7000-10000+ foot d/a's here in the heat of summer. and at 3000 feet of altitude,, i've started building with 1/2 point more compression than at sea level where i came from in houston. here it tolerates it even on 91 octane.

absolute air pressure is 13.1-13.2 psia at this current elevation. that's absolute as i repeat. absolute pressure is one of the factors in figuring d/a, along with heat/temp, humidity, etc....
wow,, looked outside and it's snowing here now!!
andy

wow,, looked outside and it's snowing here now!!
andy

Wow it 80* here!

pretty seasonal here. like 35* now and barely sticking to the ground. guess it comes with the altitude. 1 month of spring, one month of fall, ~2 months of winter,, the rest is HOT and dry!!

I do miss the power from sea level run engines. like yanking plug wires here. the more hp you make,, the more it takes away here due to less absolute pressure getting mixture through the carb and into the engine.

can also run about 50cfm less carb here to maintain a good booster signal due to less pressure differential above and below the boosters. engine still trys to move the same amount at a specific rpm no matter what. go round and round with guys here about that. i'm still outrunning them with less carb,, one day they'll get it about the mixture quality. the extra compression does help bring up the vacuum a tad though.
andy

Remember compression is your friend!!

The bottom-line is building a high-compression pump gas engine is most definitely dependant on where you live. Density altitude seems to be one of those 'overlooked' important details few talk about but can mean the difference between success and a wrecked engine.

i'm running a .030 over 350sbc basically a 70's LT1 with 11.1:1 pop ups , 186 cyl heads with port work especially in the pocket area and the GM #140 solid flat tappet cam 257/269 @ .050 493/512 lift 112 LSA and timing is at 38 deg total .were i live at is 1769 ft elev and right now corrected altitude is 3119 ft and during the summer will see corrected elevation well above 5500ft

Overlap bleeds of cylinder pressure but doesn't change static compression calc. Raw fuel is low VE and poor combustion. Low VE and poor combustion has a profound affect on cylinder pressure. Neither low VE or poor combustion is good for performace or flame front. Because you potentially have poor slow combustion, and low power output versus ideal conditions, thiscontributes to it tolerating pump gas most of the time, no matter what the actual compression is.


why dont you go to the this website it will tell you your dynamic compression ratio safe is 8.5 but i run 8.8 with 93 but im using afr heads wich have a great combustion chamber with your heads i would lean to the 8.5 will be close. have a good cold air intake setup would be fine.
http://www.kb-silvolite.com/test/calc.php?action=comp2
good luck