Help on 408 build

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PRH said:
The verification that the cranking compression is too high is the fact(from what’s being described) that it won’t even come close to running on pump gas, even with octane boost added.

Sure, double checking with another gage is fine....... but whatever the CR is...... it seems to be too high.

I’d plan on at least pulling one head for some measurements.
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Agreed.. but it could be something simple/silly like the cam advanced a tooth. Getting the numbers to hang together is worthwhile IMHO. Example: 240-260 psi is just not sensible at all for this setup; that should have been questioned earlier IMHO. So if we get some other odd numbers, then it is worth questioning. Working 'long distance', you need all the clues you can get.....
 
pittsburghracer said:
Just throwing my 2 cents in again. Some of you guys don't understand BUDGETS and sometimes the shortcuts needed to stay on one. Is it the right way? HECK NO!!!!!!! But sometimes we do things we gotta do to enjoy our rides. New pistons are not cheap and We are now pushing towards April and for those of us that live in cold areas is the start of car season, not rebuild your engine season. I personally would call and talk to Jim Dowel at Racer Brown (after 3pm eastern standard time) or any highly recommended cam manufacturer and tell him whats going on compression wise and see if he can recommend a cam to "bleed off" some compression and spec a head gasket from there to see if you can make it drive-able to enjoy the cruising season. I wish you nothing but the best of luck so you can enjoy your new found horsepower.
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If you go back and read my earlier post I suggested he just burn AV gas in it, the stuff is readily available at the local airport and does not cost much more than pump premium. Cost nothing in parts and I have personally ran engines with 220 psi compression on the stuff. Sit back enjoy your snappy high compression engine and the smell...
 
Ok so I got a couple numbers but haven’t had a chance to bring in the heads to get CCed. I’ll drop them off tomorrow.
Zero piston to deck clearance and gasket thickness if .049. But that’s not compressed thickness
 
Mopar204 said:
Ok so I got a couple numbers but haven’t had a chance to bring in the heads to get CCed. I’ll drop them off tomorrow.
Zero piston to deck clearance and gasket thickness if .049. But that’s not compressed thickness
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Who does your head CCing?
 
You could also try Brazier if competition engine doesn’t have time or wants to charge hundreds to do it...
 
Were the head gaskets those out of a standard Felpro gasket kit? And when you say uncompressed, do you mean these are the head gaskets' thickness after you pulled them out? That WOULD be very close to the compressed thickness.

Did you measure the piston height below deck with a caliper or feeler gauges referenced to a straight edge across the deck, or with a dial indicator? Checking by eyeball is not adequately accurate....Not trying to make your life hard but you want good numbers to get you as close as you can get.

If you have that SCAT kit number please post it. With the piston numbers, it will be possible to crosscheck your piston height measurement.
 
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I used both methods and both came up with 0 piston to deck clearance at tdc.
The gasket thickness was measured after taking the head off. The scat kit number is 1-98112bi
 
OK, thanks..... I was in the ICON catalog, and they list that kit as having dish pistons but I think that is a catalog mistake based on the piston volume of -5 cc (5 cc would be eyebrows only), and the compression ratios that they list. And I am sure you would not miss a dish LOL! So I am thinking these are probably ICON IC744 pistons. Those would sit .012" below a nominal LA deck height, so was this block decked milled for zero decking the pistons? It is not a massive difference.... about 1/4 point in the SCR and DCR numbers. But we just want to be as accurate as possible here to help get you out of the woods.

I would take that 'used' gasket thickness as removed as being very close to actual as installed.

The PRH and my numbers on SCR are looking to be close if the J heads were not milled any more than just a bit to true them up. Do you have any info on any head milling? They typically are at 72 cc's + or -. So that is encouraging and you may very well get by with thicker Cometic head gaskets and some better tuning on the timing.

Not to jump the gun too much, but if it is correct that you were in the low-mid 8's for DCR, then I am not at all surprised that you had consistent detonation with that timing. I've run iron heads at 8.3 DCR WITH quench and could not run the amount of timing you had in it. So with no quench, you are going to run into detonation even more readily with advanced timing.

I'd also suggest timing the cam (if it was not already done) and making sure your 0 mark on the timing is exact with the piston stop method. You are working in a compression territory where everything needs to be known and exact to get it to work well. And the suggestion to put in an AFR meter is also spot-on IMHO... again, you are pushing the dynamic compression up there and need to know things with accuracy.
 
Mopar204 said:
I used both methods and both came up with 0 piston to deck clearance at tdc.
The gasket thickness was measured after taking the head off. The scat kit number is 1-98112bi
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If you want I can get the head CCed for you. PM me and I will make arrangements to get it from you...no worries if you want to use a local shop instead.
 
I have the head and will try cc it tonight, I will post the results.
 
63.5cc

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I’m really curious now if there is something I could do? New piston and keep the cam. Or new pistons heads and cam. Or pistons and heads. I really would love to hear what you guys think. It’s not looking like a gasket would work and honestly I don’t feel comfortable even if it would work.
 
Mopar204 said:
I’m really curious now if there is something I could do? New piston and keep the cam. Or new pistons heads and cam. Or pistons and heads. I really would love to hear what you guys think. It’s not looking like a gasket would work and honestly I don’t feel comfortable even if it would work.
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A gasket will not work all that well unless you are really on top of the tune. Most aren't. Swap to a different piston and keep your cam, it is a nice cam. Fix the root problem not the symptom. J.Rob
 
Kudos for your quick help, 67ac! OK, yes, those HAVE been milled a bunch... .040 or .050" from stock. The OP's original number of 11:1 SCR is close; I get 11.4 with the head gasket, chambers, pistons, and true .000" deck height). And DCR is around 9.4 so that explains it. DCR of 8 or under is a reasonably safe number for iron head, and open chambers.

Options:
- Head gasket; would have to be in the range of .125" thick to get this under control. Cometic does make them that thick.
- Cam: Even with an advertised duration of 300 degrees and wide LSA, the DCR is still in the mid 8's so unless this is a pretty serious race-only engine with an even bigger cam, that is not going to do it.
- Pistons: the probable choice at this point

OP, what is you planned use for this car/engine? That will drive the general cam selection which will drive the piston selection and SCR/DCR.
 
My suggestion(to be added to what I’m sure will be many) is, pistons and a little bigger cam.

You can sell the ones you have to help with the cost of new parts.
If you wanted to stick with the Lunati cams, just get the next bigger one.
It’s a better size for your combo really.

The heads may not have been milled that much.
Most aftermarket valves have less of a tulip shape than the OE valves, and will reduce the chamber volume.
 
I’m not a professional engine builder but what are your thoughts on him getting away with a set of aluminum heads? His combo would run better and with the efficiency of the aluminum head you can always get away with more compression. Heck I ran 11.4-1 in my 408 on BP93 pump gas and a Racer Brown cam. That combo easily ran high 9’s at the track with a small .520 lift cam. Lots of cheaper aluminum heads out the to pick from but I would have a set of ProMaxx heads on it if it was mine.
 
nm9stheham said:
Kudos for your quick help, 67ac! OK, yes, those HAVE been milled a bunch... .040 or .050" from stock. The OP's original number of 11:1 SCR is close; I get 11.4 with the head gasket, chambers, pistons, and true .000" deck height). And DCR is around 9.4 so that explains it. DCR of 8 or under is a reasonably safe number for iron head, and open chambers.

Options:
- Head gasket; would have to be in the range of .125" thick to get this under control. Cometic does make them that thick.
- Cam: Even with an advertised duration of 300 degrees and wide LSA, the DCR is still in the mid 8's so unless this is a pretty serious race-only engine with an even bigger cam, that is not going to do it.
- Pistons: the probable choice at this point

OP, what is you planned use for this car/engine? That will drive the general cam selection which will drive the piston selection and SCR/DCR.
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Well it turned out mopar204 works about a 2 minute drive from my office and had the head with him... I will drop it back off with him at lunch today
 
It would still need a cam change with AL heads. The 65 cc chambers are the maximum chamber size that I know of in typical SBM heads (Promaxx, Edelbrock, SpeedPro) and so the DCR with his cam and present pistons would still be well above 9 and that is still too high. At least he would get closed chambers and quench which would help, but he would still need to get the DCR in the mid 8's or lower at most to make an easy tune and get some margin. That would drive to bigger cam, which may or may not suit his intended engine use.

If you ran 9's, then you had a quite big duration cam, and that lowered the DCR to probably the low 8's or less, which put it in a safe zone for pump premium.
 
Anyone know what you can get for dished pistons for this combo? I would assume something like -20cc would be about it.
 
Somewhere around there.... Icon lists an IC745 which is -20.5 dish+valve reliefs. Same compression height, same ring pack, 10 grams more weight. That gets the OP down to 9.75 SCR and just under 8:1 DCR with his cam. With the small extra weight, you either live with it being a bit out of balance, or balance this by adding a bit more weight to the crank, or taking weight off. The rods are probably the standard SCAT I beams so that is another area to work for balance.

They also list some with more dish volume but a lower CH so the numbers need to get re-worked for those. But 7.95 DCR is runnable with fair to decent tuning and the cam can be retarded a few degrees to help too.

I have not looked at other piston vendors.
 
I would think he is getting close with a -20cc piston and he should be able to keep his cam. It’s too bad his heads are milled so much, if they were 70cc he would probably be fine.
I wonder if he has the balance sheet, he might be fine with the heavy pistons...
 
Can the current flat top pistons have a dish lathe turned? Trading labor for parts, but should be quick once the first one is set up.
 
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