I put a posting in the C3 Tech section and have had 20 people check out the post but no answers. Maybe someone here can help me out. Here is the link to the post.

http://forums.corvetteforum.com/show....php?t=1858863

Thanx,

shmoky

 

All the cam companies have a tech/help line that should get u fairly close with what they sell on-the-self. And most have good info right on thier w/s to help answer common questions. If u are choosing a cam for the first time then it might be better to use the mfr recommendations as they tend to be coservative enough to keep u out of trouble with your application.

But from what i read your c.r. is way too high for iron heads and now u are trying choose a cam with enough overlap to reduce the dynamic c.r. - not something for a beginner. Kind'a like if u knew your c.r. was going to be that hi then u should have known before hand how to figure a cam with enough overlap for this. But to prove the experts recommendation for <9.5 c.r. with iron heads wrong with your first cam choise is diffinatly a stretch. U need a reliable calculator to choose enough overlap for this and it looks to me u don't even know how calculate overlap from cam data.:o
To reduce c.r. u either need to open up the combustion chambers, find larger chamber heads or change the pistons. Sorry if the truth hurts but u need to know this now.
No i don't know what cam will reduce your c.r. enough but i do know enough not to go there myself.

Regards,
cardo0

 

:o Oooppssss, cylinder press is determined by when the intk vlv closes - not vlv overlap. My bad as i have posted this mistake before. But the rest of my advice stands as is only u would need a cam with a late closing intk vlv to reduce the dynamic c.r..

cardo0

 

Every big block in 1968, 1969 and 1970 Corvette ran 10.25 to 1 and up compression most with iron heads . There is a much wider selection of cams available now than then, All you have to do is find a cam that works in the RPM range you want, keep your quench around .040 and make sure your DCR is 7.5 or lower and your golden on pump gas.

 

In '68 '69 and '70 anyone could buy 103 octane leaded fuel at any gas station. To think u can run 10.3 c.r. with 91 octane unleaded now its obvious the above information bad and misleading. Now do u want to chase a 7.5 DCR with that kind of advice too?

Good luck,
cardo0

 

The information regarding the dynamic compression ratio of 7.5 is right on. That is the compression the engine actually sees during operation. Making statements about detonation problems with regard to a static compression ratio is also misleading. If you don't know the answer to his question why post?

 

I guess I have to repeat this. ALL corvette Big blocks built in 1968, 1969 and 1970 were 10.25 or higher compression from GM. They are also not running optimum quench in factory configuration. Think all these cars are running Race fuel??? There are a lot of these that are driven long distances to NCRS meets, on the Hot rod power tour and cross country cruises for pleasure. Most will run just fine on 93 octane which is available at almost ANY place that sells fuel in my area. Some of the 11 to 1 and higher cars will need to be detuned a little to run it, but the variety of cams available today, even custom grinds if needed, gives you a lot of options to have power in the RPM range you want and still run on pump gas. I have a New street 548 for sale on the C3 parts for sale section that pulls 778 HP on 91 octane (not 93 octane) with 10.25 to 1 compression, Iron heads and zero detonation issues! Still think it can't be done? You just have to know what you are doing with engine design. If you don't, dont't post . Proper quench. correct inlet valve closing point, making sure your car runs cool, and correct A/F ratio for your application is the key to avoiding detonation. Shmoky is running a 3.70 gear and 4 speed trans.That helps open up his options also.

 

I am trying to help a fellow forum member as are most others posting. If you are not willing to do the same, why belittle the OP and others that are trying to help him acheive his goal? My experience with the Tech line at the cam companies has been they hire mostly high school kids with little or no real knowledge of engine design that read you the information available on the website. If that is who you want to trust to design your build have at it. I think the OP is going the right direction asking those who have been there and done that. Anybody can install parts on their car. Designing a build from the bottom up to reach specific goals and be well matched throughout requires a lot of homework and attention to detail. Shmoky is doing his homework now.

 

Ok so Vizard, Lingenfelter, Smokey Y. are misleading others too - i don't think so. Don't take my word for it, read what they have said and published. And actualy 9.5 is the limit for iron heads (with good quench, cumbustion chambers, etc.) while 9.3 is more reasonable for the new enthusiest. I can say this all day long but to someone that blames holes in the pistons from detonation on the piston manufacture it does no good.

Who said anything 'bout race fuel? I said high octane leaded fuel was availible nearly everwhere back then and the higher c.r. factory engines could run on that. BTW what happened after '74? All the factory c.r. dropped to use the lower octane fuels until EFI emerged in the 90's. My '94 has 10.5 c.r. but with aluminum heads, reverse cooling, a knock sensor and electronic timing controls to retard the timing out of detonation.

OK, to help shmoky more i advise him to reduce the c.r. now before its too late. Don't use a long duration cam to chase a 7.5 DCR. Why? Because big cams just plane stink in a street car. In tight traffic and parking lots they drive like a 3 legged buffalo. Vacuum accessories choke and fuel milage goes form bad to terrible.

Oh and the next bandaid fix they give u is widen the LSA. Wider LSA brings back some vacuum and idle but not enough for the full fix and the power band now moves higher. Yes less mid-range torque and the engines makes its power at even higher rpm. Stomp and it and waite till revs up to high rpm - hope u packed a lunch. Oh it will make power above 4,000rpm but how often do u drive there?

The next fix they give u is to retard the timing. I had to retard an Olds one year to get to pass smog and it ran lousy. That car would stumble and even backfire on acceleration. Backfired when i backed it down too. My '94 Z28 with EFI and knock control will run on low octane regular but when i try to accelerate it surges and bucks and i have to back out of it. Around town only 91 octane premium goes in my car. Sorry folks i just can't imagine driving with retarded timing. It's a kind'a retarded idea.

Read up and good luck shmoky,
cardo0

 

Best way to decide on a cam is take a look at your DCR, which is calculated from your point of intake valve closure. Calculate the amount of volume above the piston at that point and put that into the CR equasion. On pump gas you should stay around 8-8.5 DCR.

Valve overlap (and thus LCA's) are mostly determined by the rpm you want to run. If you stay under 6000 is assume an LCA of 110 will be fine. Take a look at the catalogs and choose one that is close to what you will find after calculating.

 

Good # for aluminum heads. one point less for iron.

 

I gave the figure that David Vizard gave in his book. Of coarse that was with premium gas.

 

Since CardoO likes to misquote David Vizard here is some educational reading on static and dynamic compession ratios and ways to reduce detonation issues running higher compressions that contradict almost everything CardoO has stated above. http://www.popularhotrodding.com/tec...ech/index.html by David Vizard. This article has a DCR to fuel octane chart that does show 8 to 8.5 DCR for pump gas (The info Belgian1979vette was refering to.) Although I would still be more conservative with iron heads.

 

Good article.

 

That article was about the max c.r. for 93 octane gas in an absolute maximum effort engine - not even close to a street engine.

As far as what i said 'bout c.r. and iron heads still stands as that article did not even mention iron heads once. Not once did he mention iron heads. How anyone could claim that article contradicts what i have said and what Vizard himself has said has comprehensive problems and should seek proffessonal help.

Anyone can read in Vizard's books like: How to build Horsepower Vol I or Vol II, or How to build a High Performance small block Chevy on a buget. Vizard states 9.5 c.r. is max for iron heads. He is not explaining the theoretical limit in his "how to" books but constructing a street performance engine with vacuum operated accessories in daily driven traffic.

That article does show the extent of Vizards expertice and i have high regards for that expertice but no he is not contradicting me or himself. He shows the max possible limit of 93 octane (BTW only 91 octane is availible in CA) with a dedicated motor using Aluminum (not iron) heads, advanced combustion chambers, a 0.024" quench and using a cam with up to 300* duration . How that relates to what shmoky is using is just another misleading post by the same participant here. Poor shmoky is soo confused now that he hasn't replied to his own post.

In niether of the Vizard books i mentioned does Vizard say that anyone needs to calculate the DCR to choose a camshaft. But i guess someone that thinks he's smarter than the Vizard himself needs to make it even more complicated than needed.

Again my advise to shmoky is to read up on what the true professionals - Vizard, Lingenfelter, Smokey Y. - have to say 'bout building a street engine and u will avoid holes in your pistons - while your power brakes and headlights will work too.

cardo0

 

Dude, you and I must be reading two totally different articles. Here is a quote from the first paragraph of the article: don't be fooled into thinking that the use of 93 octane pump fuel in any way diminishes the importance of the CR used. Understanding how the CR affects your street or race engine can easily net as much as a 50hp benefit. Also there is no mention of aluminum heads in the above article. And the .024 quench reference says "Don't try this at home" That is why I mentioned "misquoting David Vizard".
Talk about comprehension problems!! I think Shmoky has all the input he needs from me as far as what I can do to help him with his selection although CardoO says the best advice on which cam to buy will come from the Tech line at the cam companies, OK??? And you don't need to check your DCR when selecting a camshaft in a 10.3 to 1 iron head motor??? Here is CardoO's biography, Hot Rodder Wanna Be. Attended too many car shows and races as an spectator and too few as paticipant." That explains a lot. I would suggest you read the article through a couple times to try and comprehend what is actually being said and refrain from giving advice when you obviously have no clue what you are talking about. Have you ever even built a high performance street engine from the ground up? Reading a book that you are unable to comprehend does not make you an engine designer. I see you are running a Goodwrench with a few bolt ons.

 

And in tonights MAIN Event we have, in this corner...

~Maverick

 

My old motor had iron 292 turbo heads with regular quench and domed pistons that provided a solid 10.5/1. Not detonation there either.

Just to let you guys know.

Anyway, this article was about cam selection. And with cam selection one needs to look at dcr. I have Davids book about cylinder heads here and when he is talking about iron heads he consideres 8-8.5 max in that book, 9 for race fuel.

 

Another forum member that has achieved the impossible! And that is on European fuel which I believe is rated different than ours
With cam selection one needs to look at DCR. I would regard this, quench, A/F ratio and lowered water temperature as the most important items to address to avoid detonation issues. 8.0 DCR with iron heads is dangerous territory unless everything is dialed in absolutly perfect.

 

Again i get misquoted, i said if c.r. is matained <9.5 there is no need to calculate the DCR. Only those that think they are to smart to use that advise from D. Vizard, Lingenfelter, Smokey need to make it more difficult than it is. And then the original post goes on to bad-mouth the information i provide in my profile - even my occupation - like thats going to make his mis-information correct (then modifies his post to hide it). I guess he doesn't have to worry about off-idle cruise in parking lots or heavy street traffic. Nor vaccuum accesorries like power brakes and headlights. See folks when u just bad-mouth enough u won't have to answer questions like overlap and actual street drivability.

Some needs to read his own article again: paragraph 20, second sentance - "Here, aluminum heads without water between the intake and exhaust valve conduct a great deal of heat from the exhaust side to the intake."


Just because someone claims he was able to run high c.r. with his iron heads dosen't mean it was a success - too many unanswered questions. What was idle vacuum? What was idle rpm. Vaccuum headlights work? Can car idle with A/C on? How car is driven and where (just for fun and fwy cruise or in real traffic)?

I did read Davids book on cyl heads too and am aware of how he explains DCR. But again for any street driven engine with normal vacuum requirements a long duration camshaft will kill vacuum and thats what happens when u try to reduce high static c.r. to a low DCR. But why doesn't someone quote from D. Vizards "How to build Horsepower Vol I or Vol II" or Lingenfelters book? I'm too busy with the holiday preps right now but i will drag out books again when i get the chance.

More later have to run,
cardo0