compression vs. octane rating (for a street car) - Team Camaro Tech
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post #1 of 20 (permalink) Old Mar 15th, 01, 06:18 PM Thread Starter
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I have heard some people say that they have higher (10.5/11) compression and are okay on pump gas, but I am left wondering how my proposed buildup is going to do on 87 octane (if needed since we are on the verge of an energy crisis...).

I am building a 400 small block with 64cc vortec steel heads. I will have 5.7 rods and 30cc reverse dome (d-dish) pistons. After I get my decking / head gasket combo figured out as far as height goes, I will have about 9.2:1 compression. This isn't figuring in the lesser bleed off due to a really mild cam I will be running.

With this configuration, am I going to want to stay away from 87 octane??? Or will I be okay even if I really step into it? I am also worried about detonation, but with proper decking and head gasket I should be okay since the vortecs have a really efficient chamber and the reverse dome pistons also make it better.

I realize that it I were at 10:1 or more I would definitely not want to make a habit of running 87 octane, and that most factory engines come with about 8.1 or so(?). This leaves me wondering where I will stand with mine and driveability.

Also, are there any GM aluminum heads (probably pre-92 due to the new LT1 and LS1 heads incompatibility with standard sbc's) that I should look for used that will flow as good as the vortecs???? Running 5.7 rods in a 400 is much much better than the stubby stockers, but presents problems with streetable piston selection when using a 64cc head... Kieth Black makes about the only good piston for this combo, and youhave to really watch rod balance pad clearance with the bottoms of the dish in the piston on these...

Thanks,

Chuck
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post #2 of 20 (permalink) Old Mar 15th, 01, 06:48 PM
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IMHO
you will need at least the mid grad octane.

BUT . . . It also depends on altitude.

Also, on camshaft overlap.

A BB can handle a bit more compression with out detination.
I am running 10.5-1 right now in a 454 on 91 octane. . . no detination.

A small block will beat itself to death on pump gas with over 10-1 compression.

Once again, JMHO
Tim

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[This message has been edited by ratchetmaster (edited 03-15-2001).]
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post #3 of 20 (permalink) Old Mar 16th, 01, 05:41 AM Thread Starter
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Okay. I downloaded a compression calculating program, shareware version, so I can't use the cam options and a few others. But I arrived at this basic figure (various overbores, haven't machined block yet so who knows):
4.145 (.020 over) - 9.03:1 compression
4.155 (.030 over) - 9.06:1 compression
4.165 (.040 over) - 9.1:1 compression

The only things this leaves out is the extra volume around the piston above the top ring (very miniscule) and cam bleed off.

Here are my cam specs that I am proposing, one of these two wolverine blue racers:

266/266, 209/209, 109/105, .414"/.414" lift
(hydraulic cam - FLAT torque curve from 2000 to 3500rpm at 465 t and 339hp at 4500 rpm)

228/230 at .050", 110.5/108, .383/.383
(Duntov solid cam - 475 t from 2000 to 3500 rpm and 358hp at 4500rpm )

The hp estimates are from a desktop dyno program.

Since I will be running such a small cam, this is just going to mean less bleed off of compression, right? So the figure I came up with (9.1:1) would be reduced a lot basically if I had a longer duration (more overlap) cam pattern? Running a small cam can't increase my compression from this figure, or can it??? I woould be very happy with 9.1 with the efficient vortec comb. chambers and the reverse dome pistons. Should be very streetable. Oh yeah, if I use the Duntov cam I might use 1.6 rockers.

-Chuck
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post #4 of 20 (permalink) Old Mar 16th, 01, 08:13 AM
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those are very mild cams.

What are your goals with the engine?

the 1.6s will increase your lift.

Why not just get a cam with more lift?

The calculations of compression based on chamber CCs and piston dome usually don't include changes due to camshaft overlap.

At 9 to 1 you will be fine with a mid grade octane with that camshaft.

oh ya, those Hp ratings sound a little high too.

Maybe someone else and expand on my thoughts.


[This message has been edited by ratchetmaster (edited 03-16-2001).]
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post #5 of 20 (permalink) Old Mar 16th, 01, 08:30 AM
 
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Chuck, pdq67 here. Lets see what everybody has ta say. And tell them what your rearend gears and tire size are gonna be so that we don't over-cam.. Plus tranny type.

Heck, I might be all wet on using small cams for your tall gears!



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post #6 of 20 (permalink) Old Mar 16th, 01, 04:13 PM Thread Starter
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Okay... Basically my goals are as follows. My camaro will be my primary summer driver, and I do lots of highway miles. It now has a 2.41 posi in it, and I may step up to a 2.73 soon. No more ever than a 3.08. I want to have a really good highway cruising speed at low rpm's so I can do long trips. Also shooting for halfways decent mileage. I have a TH350 tranny as well, and my tires are 235/60/14's.
The plan was to go with a 400 for the monster torque figures that the mere cubic inches can produce, and that will help me a bit with accelleratiion off the line since I will have really steep gears. This way I can still have a moderately quick car without building a much less streetable 350 and still not quite coming up to the torque potential of the 400.
The idea was to run a small cam that will give me a whole lot of torque right off the line. hence the small cam specs.

Any thoughts? I'm not gonna race it, but I would expect low 14's (eventually..., maybe after I switch to a 2.73 or 3.08). This way I will have decent mileage and good idle, excellent highway cruising speed/rpm's, and still have something that isn't a total dog like my 170hp 305 with the 2.41 gears!

ALL INPUT WELCOMED...

Thanks,

Chuck
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post #7 of 20 (permalink) Old Mar 16th, 01, 10:21 PM
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Let me throw my opinions at ya

I think what you are planning will work wonderfully...the vortecs are great, and 9.2-1 compression will work just fine on 89 octane with a mild cam...87 may be just a touch low if you use such a small cam though. 400's are a bit more likely to ping than a 350 due to the fact that they run a tad hotter (siamesed bores) and because of the larger bore size. With some really sharp tuning 87 will work...but it would take fine tuning of the advance curve and a top notch cooling system, plus you will have to pay very close attention to carb jetting. Your cam specs will also play as much of a role in this as the mechanical compression ratio. I run a 350 in my 78 chevy 1/2 ton truck that has 9.4-1 compression, vortec heads, headers, duals, performer rpm intake, 600 edelbrock carb, and a comp cams XE268 cam and kit (224/[email protected], .477/.480 lift on 110 lca/106 ica). I also have a 2000 rpm convertor and 3.08 gears. Vehicle weighs 4500 pounds. I can run 87 easily with no ping, but economy suffers slightly over 89 octane. I have gotten a best of 15.5 mpg at a 2600-2700 rpm cruise (65-68mph on the highway) and it runs [email protected] I have way more wind resistance than you do. The cam may be slightly too big for my application...but it works. I pull my 23' camping trailer with ease, and from a rolling start I can beat a great many stock and mildly modified cars out there. I believe that with the tuning I have done since last year I can get very high 14's now (wont know for sure until I can get back to the track)...I really need 3.73 gears but I occasionally take long camping trips. The mph is a good indication of how well this combo runs...with 3.73's and traction I could have mid 14's easily. The vortec heads combustion chamber shape is so much more efficient than the older style heads...it allows more compression without detonation, and doesnt require as much spark advance either for max power. If you plan on keeping the 2.41's, then the 209/209 .414/.414 cam will work fine and be very torquey, but will run out of steam pretty early on a 400...but even 4000 rpm with 2.41's will be flying. For what you want, 2.73's coupled with the short tires you have would work great for economy and performance, but I would use a little bigger cam...something along the lines of the comp cams XE262. I would also use a dual pattern grind...its a proven fact that a mild dual pattern cam will get better economy with a street exhaust system and it also helps compensate for the slightly weak vortec exhaust port. Plus, this cam will work great with the stock vortec springs. The vortec springs are a little light but this also contributes to fuel economy...and since you wont be making power beyond about 5000 rpms then they will work great. Use a vortec pattern performer intake and a tuned q-jet, and I think with a stock convertor, 2.73 gears, small tube headers and duals, you could easily have a mid 14 second car that gets 16-18 mpg on the highway...traction will be a big problem with this much low end grunt though.

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post #8 of 20 (permalink) Old Mar 18th, 01, 10:25 AM
 
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Thx, Travis!!

I've been helping Chuck on the side through many, many posts and am very happy with his heart felt desire to figure out how , why, etc things do the way they do. Heck, I'm learning everyday.

Chuck wants to walk that oh, so fine line of having a powerful daily driver that gets good gas mileage and can still run hard. He has delved into all sorts of topics that can be and are used to max out an engine as its being put together. You know, min. quench, piston types, rod lengths, heads, exhausts, cams etc...

I was after max grunt low rpm cam spec's because of his 2.42 gears but haven't really figured out how cams can be allowed to "grow" as you install them in different engines starting from a little '56, 265 then up throught all of the small block family to a 406.

I do know/(think I know??) that the cam needs to be sized such that its max torque peak should be close to the average cruise rpm range with the tire and rear gears being run so that it will operate at max volumetric efficiency (Or something like that!!???)

Anybody else want to chime in here because they certainly are welcome. pdq67



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post #9 of 20 (permalink) Old Mar 18th, 01, 07:38 PM
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Just to add to what I posted the other day...When I bought my current truck it had no engine in it...I built the shortblock I am running now but I started out with lightly ported 882 heads, a weiand 8004 dual plane intake, and a cam dynamics (now crane) 266 energizer cam (210/[email protected], .440/.440 lift, 110 lca/105 ica)...my shortblock (.030 over with cast flat top's), 600 edelbrock carb, headers and duals, K&N filter, and HEI was used on the original buildup and on the upgraded engine. I originally built it for truck use...strong low end torque and fuel economy...it ran pretty well and pulled well, and it got a best of 16 mpg on the highway. I quickly got tired of the performance though...best time was [email protected] mph...the engine had very little power over 4500 rpm. The mph was ok for what I had but I never could get into the 15's. When I upgraded to the vortec heads, XE268 cam, and performer rpm vortec intake (following the chevy hi-performance goodwrench buildup) I wasnt really concerned about economy since I dont drive this very much anymore. I was pleasantly suprised though...I lost 1/2 mpg and gained 8 tenths/8 mph in the 1/4 mile. Not too bad of a tradeoff! I suspect that the whole quench thing is a little over rated though...my pistons (trw cast replacement flat tops) ended up a whopping .040 in the hole (yuk!)...my compression with the 882's was 8.4-1 and with the vortecs 9.4-1...yet the 8.4-1 combo pinged easier than the new higher compression setup. I suspect that quench is more important with the old style, poor open chamber design that most 76cc sbc heads use. I like the way Chuck thinks...nearly all engines that I build use the same idea...strong low and midrange with good performance and economy with relatively high gears. It makes for a wonderful daily driver!

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post #10 of 20 (permalink) Old Mar 18th, 01, 09:36 PM Thread Starter
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Travis and Paul (pdq67),

Thanks for the encouragement and advice! Glad to know that someone thinks that I am on the right path! I don't know what performance to really expect, or how much tire frying can be expected out of any engine with a 2.41 rear end... That is at a minumum right now with a stock 78 low powered 305. I think it had 170hp and 220tq new... I will notice a huge difference now...

The Edelbrock 1406 seems to be the carb that is right for me for a daily driver. And the vortec heads seem to be top notch as far as effieiency and power go (without compromising each other), and the price is right - the cheapest set of heads that I can buy rebuilt...

The d-dish pistons and good quench height will also help me out in allowing me to run up to maybe 9.5:1 without any worry of problems.

I still need to learn what LCA/ICA combinations are best for low end torque., throttle response, high end hp. etc... I guess the new issue of Chevy High Performance has a big article in it. The whole thing sems to be coming together with the 400 small block really well. If I play my cards right I will have the perfect engine for my desires and application for less than $2600 from carb to pan to exhaust...

I know I could have done a 350 for probably $1900 complete, but the 400/406 sounded much more alluring and would enable me to run a higher gear (2.73 ideally) for mileage and still have the grunt to get it moving off the line.

I am also building my car up to go around corners really well,so I'll have a really well rounded daily driver.

THANKS!
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post #11 of 20 (permalink) Old Mar 19th, 01, 04:47 AM
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there was an interesting article in one of my car mags awhile back called "the engine chevy should have built." They took a 400 block, put in a 3.25'' stroke crank with 6.25 inch rods. Aluminum heads topped it off with a mild (215/215?) tight lobe separation roller cam as the brains of the operation. By using the super long rod/stroke ratio, they made 415 horses and similar torque while using 87 octane... with 11:1 compression. Just thought y'all might be interested.
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post #12 of 20 (permalink) Old Mar 19th, 01, 03:26 PM
 
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boodle,

This is the stuff that I've been harping on until I realized that the guys weren't interested in it, so I just shut up.

It goes right along with the Fueling big block head stuff and the POP.HOTROD. article awhile back about doing the same thing to both the SB and BB engines (as well as F--ds)but by only using small cc heads that have been cut further to get the CR. up to 11 to 1+.

All this is supposed to run on 87 octane gas!!! I know we gain 2 to 4 percent power with every one point increase in CR., so if we can run this crappy gas we got now, then we can still make power the old-fashioned way by using "squeeze". AND I don't mean NOS!!

I wished I had the money to build a 454 this way to find out just for the h-ll of it!! pdq67



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post #13 of 20 (permalink) Old Mar 19th, 01, 03:41 PM
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Right on Pdq67.
I don't have the money to build such a cool beast, (not even to build a budget 383 at the moment) but someday when I do I will try to build a pseudo-exotic piece like that. It would be cool. If anyone wants to read the article, go back to those lovely AFR article archives at www.airflowresearch.com click on article archives and look for "the small block chevy should have built" or something like that.

Oh, wait... did I mention that they built it FOR JUST $3000!!! Maybe I will be able to afford it before I thought...
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post #14 of 20 (permalink) Old Mar 19th, 01, 04:23 PM
 
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Hey i saw everyone talking about octane ratings and was wondering what I should put in my car. Its a 350 with an edelbrock intake, stock heads, stock cam, headers, edelbrock carb and K & N filter. There are 3.31 posi gears and I am running a T-56 take out from a 94 Z-28. thanks guys
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post #15 of 20 (permalink) Old Mar 20th, 01, 03:16 PM
 
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USDred67RSSS,

Are you asking about CR.??

If so, go w/ 9-9.5 to 1 w/ cast-iron heads and 9.5-10 to 1 w/ aluminum ones and you should be fine. Also try to set the quench up at about .038"-.046" or so give or take a thou. to try to make the combustion chamber a little more efficient.

If you are asking about engines, the sky is the limit!! pdq67



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