Your lucky you haven't detonated that thing to death. 8.6 is very high DCR for iron heads, even high for aluminum heads on 91 octane.

Im afraid maching pistons would cost more than buying new ones.

The sealed power 12.5cc dish hyper pistons would give you right at 9.1 with 76cc heads give or take a little depending on whether your heads have been milled, gasket thickness, etc. In fact, I have a brand new set I may sell pretty cheap, pm me if interested.

Or there are various TRW forged and KB pistons in the 15-22 cc dish range that would give you even lower than 9:1. Just be aware that they run more bore clearance than the hyper pistons.

 

I wouldn't think your compression would be that high. I've got a 406 with Sealed Power forged pistons with a .083 dish and 64cc iron heads. My static compression is 9.91:1. The same piston with a 76cc head is 8.90:1. (Source: Jeg's).

I believe the rule of thumb is about 10:1 is max for iron heads and pump gas.

 

Sounds like an engine I would run on pump gas.

 

Need to lower compression

firechicken383, the engine hasn't been installed in the car yet. I've just done a guesstamate on the dynamic compression. I can't find the actual measurement for the closing of the intake valve for the comp xe262.

MI2600, the pistons that I have are the flat-tops with 6 cc dish for the 4 valve reliefs. I believe the ones you have are a dished design. I believe the part number of the pistons I have are H400CP40. You pistons are the forged ones with a 14 cc dish that I am looking at.

So it is your opinions with the exception of Firechicken383 that I should be okay? If so I gonna drop it in tomorrow. It sucks if I have to get new pistons because the assembly was balanced. Here are the numbers I used to figure the static compression.
Stock Bore: 4.125
Over Bore: 0.040
Stock Stroke: 3.75
Stroke Change: 0
Chamber: 76 cc
Piston Dish: 6 cc
Deck Height: 0.020
Gasket Thickness: 0.041
Cylinders: 8
= 9.67:1 and 408.9 cubic inches

Part Number 12-238-2
Engine 1955-1998 Chevrolet
262ci-400ci
8cyl.
Grind Number CS XE262H-10
Description
Intake Exhaust
Valve Adjustment 0 0
Gross Valve Lift 0.462 0.469
Duration At 0.006 Tappet Lift 262 270
Valve Timing At 0.006
Open Close
Intake 25 57
Exhaust 69 21
These Specs Are For The Cam Installed At 106 Intake CL
Intake Exhaust
Duration At 0.05 218 224
Lobe Lift 0.308 0.313
Lobe Separation 110

 

You won`t have any problems, if you need to feel safer run a bottle of marvel mystery oil in with the fuel. Keep a close eye on the spark timing, if you hear it rattle back off 2 degree`s at a time until it stops. Keep the engine temps on the cool side, and the fuel mixture on the rich side, and you`ll come out fine.

 

You could scrub off some DCR by running more cam. That is a fairly mild cam for a 408 SBC, have you considered something more like the XE274? That would help get your DCR down and in a 408 you'll probably still have more low end torque than you can get to the ground. I ran an Erson TQ30 in my old 383 and you could barely tell it had a cam at all at an idle and it worked fine with a stock converter. It was 226 Dur @ .050.

 

Cranking compression will be one indicator as to what fuel this thing will live on. I've had quite a few smallblocks with iron heads that pumped 180-190ish compression and got by ok on 87 octane, of course they did make more power with higher octane and more timing but no problems with 87. If there's problems you can alsways retard your cam some or put a larger cam in like already suggested. I bet it'll be just fine. Get your spark and fuel curves dialed in before you make any rushed decisions. If you're on the edge finding a happy spark curve can take some time. I wouldn't use any vacume advance till you get your initial, total, and rate of mechanical figured out first. JMO, Bob

 

I wouldn't think your compression would be that high. I've got a 406 with Sealed Power forged pistons with a .083 dish and 64cc iron heads. My static compression is 9.91:1. The same piston with a 76cc head is 8.90:1. (Source: Jeg's).

I believe the rule of thumb is about 10:1 is max for iron heads and pump gas.

I've got the same basic set up and have no detonation on 91. My heads are Sportsman ll's. I run 34 deg total mechanical timing. Your set up will be fine. More cam wouldn't be a bad idea.

 

marvel mystery oil

You won`t have any problems, if you need to feel safer run a bottle of marvel mystery oil in with the fuel. Keep a close eye on the spark timing, if you hear it rattle back off 2 degree`s at a time until it stops. Keep the engine temps on the cool side, and the fuel mixture on the rich side, and you`ll come out fine.

ANY oil product will lower the octane rating of the gasoline...... PERIOD

also dialing in the advance curve and vacuum advance will be of utmost importance. Conservative is the word; you won't get all that advance in at 2500 rpm and not ping. This is not a time to guess what will work, you need a distributor expert. Running less timing to crutch a low grade fuel problem will cause your engine to run hotter coolant temperatures, maybe even make red headers.

Here's somethiing to consider: Gas has gone up almost $ 1 per gallon in 2 years. Gas $$$ is NOT going to go down in price. You can build a 400 hp engine with a fuel problem, or a 380 hp engine that runs on 87 octane. How willing are you to bite the bullet for a few hp? There is a great variation in gasoline ACTUAL octane from station to station, and day to day. If you are borderline, you will find days of crappy gas that will ping like h.... once you get a tank of it, your only hope is drain in out or add octane booster to crutch it til you burn it all out.

Stick with Dave Vizard's advice, he is an expert. After you drive it and try everything else, change those pistons and use a little more cam, but be conservative/cam with the lesser compression. Ask the experts CompCams. Shoot for 160 psi cranking compression.

 

Joe Mondello speaks on the subject here:
http://www.babcox.com/editorial/ar/ar60135.htm

While there have been many arguments on it, I`ve used marvel a long time, on race engines to daily drivers, never a problem.
I don`t think one bottle of MMO in a tank of fuel will lower the octane enough to worry about.

 

Need to lower compression

Ok, my mind is a little more at ease. However I am still stuck on this dynamic compression thing. If I shoot for say 7.5:1 just playing with some numbers. BTW, I am using the calculators found here:
http://wallaceracing.com/dynamic-cr.php
According to this I need like 9:1 comp and 58 ABDC inlet valve closing. Problem is the Comp Cam spec card gives the closing at .006 whereas this calculator is using .050. The other thing I wondered is if I buy new pistons by the time I buy the rings and have a machine shop press the old off and the new on. Now the money I paid to have the rotating assembly balanced goes out the window. So that whole thing could cost me $400-500. That money could have gone towards aluminum heads. If I can find a set for less then $1000. I'd probably just install the engine as is. Then if it does tend to detonate, install aluminum heads with a 72 cc or larger chamber. That should then put me in the safe zone to run on pump gas and will my dynamic compression work with this cam? Or maybe not I just did the calculator and it shows 10.1:1 with 72cc.

 

Dcr

OK, I found a DCR calculating in a post here that measures events at .006. According to this program my DCR is 8.239. Now if I were to swap in an XE274 this number drops to 7.96 or if I chose a crane 268 cam (218/218@.050) it drops to 8.13. So do I need to look for a cam with a later closing point? How is the drivability of the XE274? Will I be able to have 15" of vacuum in gear?

 

I have a 400 with a 7cc flat top piston. I have 64cc Sportsman II cast iron heads and only use 91 octane. My cam is 234o duration @ .050". I run the optimum ignition curve with no detonation.

 

I also have a 400 with a 7cc flat-top with 69cc heads and use 91-92 octane. I got a 282HR and pulls a little over 14" vacuum. It has no detonation running 34 degrees of timing. It is set though to run a little rich and has a very ample cooling system.

 

I was on line with one of the Techs at Summit Racing asking some questions on building a short block 350 with 10.5 compression, iron heads with 64cc chambers and they said I was borderline with pump gas



http://www.cardomain.com/ride/1633712

 

I am always amazed when I see people saying you can run 9.xx compression with 87 or 10.xx with 93 but you can't just look at the Static compression ratio. I always use Dynamic Compression Ratio as a guideline as well as quench height (the more quench the more likely you will encounter detonation), octane ratings, timing. etc.

Here is a DCR calculator-

http://www.kb-silvolite.com/calc.php?action=comp2

Here is Pat Kelleys write up on DCR-

http://members.uia.net/pkelley2/DynamicCR.html


Here is a quote from it -

Caveats: Running an engine at the upper limit of the DCR range requires that the engine be well built, with the correct quench distance, and kept cool (170º). Hot intake air and hot coolant are an inducement to detonation. If you anticipate hot conditions, pulling some timing out might be needed. A good cooling system is wise. Staying below 8.25 DCR is probably best for trouble free motoring.

Keeping in mind that he has less than an ideal quench with an iron heads on 91 octane, and he wants lower 87 octane gas, I really think 8.6 DCR is very high. Im not saying this to try to sell him my dish pistons, i was gonna put em on ebay. Now, if he doesn't want to tear into the shortblock, then he could put a bigger cam in there to bleed off some of the cylinder pressure and give him a more acceptable DCR. (But IMO, he would have to run a much larger cam to give him a low enough DCR)

And IMO, having to add Marvel Oil to the gas or retard the timing or mix race gas or anything like that is just simply a band-aid fix to a problem. ( Hope I didn't offend anyone by this)

In fact I have a truck that has a 350 with flat tops with a bad quench height, and 64cc heads. I have darn near 10:1, iron heads, small cam, bad quench, and it rattles like all hell. I have to retard the timing by several degrees and run 93 octane just to keep from damaging my motor. It doesn't have much power due to the reduced timing. Needless to say the ony fix for this is to pull it and swap in the correct pistons and run about 9:1. with a decent quench (.040-.045).

 

In fact I have a truck that has a 350 with flat tops with a bad quench height, and 64cc heads. I have darn near 10:1, iron heads, small cam, bad quench, and it rattles like all hell. I have to retard the timing by several degrees and run 93 octane just to keep from damaging my motor. It doesn't have much power due to the reduced timing. Needless to say the ony fix for this is to pull it and swap in the correct pistons and run about 9:1. with a decent quench (.040-.045).

Are you sure your timing is right? Have you verified TDC? Have you double checked your timing light?

 

Are you sure your timing is right? Have you verified TDC? Have you double checked your timing light?

He's absolutely right..... you might be chasing rabbits.

please check out www.readershotrods.com drag cars/georges

 

Are you sure your timing is right? Have you verified TDC? Have you double checked your timing light?

Yes I have. I've checked and verified all of that. Timing light is used on many different engines, both my on and others and has recently been checked with a brand new $260+ Matco digital light, so Im quite sure it is accurate.

Seriously, what makes you think that a 9.8:1 350 with iron heads, a baby cam, and crappy quench SHOULD run on 87 octane without retarding the timing?

Can I run 14:1 domes on 93 octane?

Im sorry, he has a 9.7:1 iron headed engine with a aggressive small cam that builds cylinder, less than ideal quench. runs on 91 octane, wants 87 octane, Im sorry, thats not right.

If pat kelleys DCR write up says that 8.25 DCR is about max on pump gas, he has 8.6 DCR, then uch is the limit. 8.6 DCR is OK on mid grade.

 

Newly calculated DCR

I appreciate all the responses. If you read one of my posts on the first page when I calculated the DCR at first I was guessing because the only DCR calculators I could find wanted the intake closing value at .050. Which I didn't have that value as my cam card only listed it at .006 valve timing. After finding a post here about correctly calculating the DCR and a link to a free downloadable program to do so I recalculated. If this program is correct (it measures at .006 timing) then my actualy DCR is 8.23. I can live with running on 91 if need be. Others have mentioned trying the comp cam XE274 but that has an intake closing of 61 versus the XE262 at 58. It does drop the DCR to 7.9 something. But that .3 something difference may not be that much of an issue and is throwing $100 out the window. For now I will install the engine and monitor my knock sensor and hook up my air/fuel gauge. I may beef up the cooling system just to give myself a safety margin. I'll probably limit total timing to 32* to start and slowly work up to 36*. It may be a week before I am able to get the new engine in. The old one was really greasy and I have a big mess to clean.

 

Seriously, what makes you think that a 9.8:1 350 with iron heads, a baby cam, and crappy quench SHOULD run on 87 octane without retarding the timing?

Can I run 14:1 domes on 93 octane?

I ran 10:1 no problem with small cam.

Then run 91 octane, put in a bigger cam, or run a thinner head gasket to improve the quench.

 

I appreciate all the responses. If you read one of my posts ......... I may beef up the cooling system just to give myself a safety margin. I'll probably limit total timing to 32* to start and slowly work up to 36*.
QUOTE]

Consider: Maybe it is not your total timing.... maybe your advance is coming in too quickly.

check out www.readershotrods.com drag cars/georges