[tooski]

Without boring everyone with specifics, I basically want to know what compression ratio I can safely run. I have basic tuning skills, so nothing on the edge. 400 engine.
I have a 211°/221°/110 cam. Maybe going to a Summit 2801. I have a few options for adjusting compression. With 9.77 CR w/8 vr pistons it detonated. Maybe changing pistons but I don't want to for a mild DD. What would be a safe CR for my cam with the 8 vr pistons?

[Stan Weiss]

Where is the ICL set at and what is your cranking compression?

Stan

[Cliff R]

Difficult question. How far down in the holes at TDC are the pistons? How thick is the head gasket? How much timing are you trying to run and when does it ping, part throttle, heavy throttle, full throttle, etc?

110LSA works against you as far as detonation because it narrows up the power curve and the engine will make peak power earlier in the RPM range.

The engine should be tuned for no detonation at all. Some engines will not like or want a lot of timing anyplace and be fine when you take enough away to make them happy. Others will end up so far down on power you'll need to make other changes.

I've tuned a butt-load of engines here, many brought from great distances and have found many common denominators with the troubled ones. One of the biggest issues I see is folks trying to run some sort of super-quick timing curve to get all the advance in early. Very few engines need all the timing on them early in the RPM range and high compression engines on pump gas will often ping in protest........Cliff

[steve25]

Your stated 9.77 compression, what heads was that with and what Bore size is the 400?

[tooski]

Engine is a 400 + .040. Stock rods. Rebuilder cast 8 valve relief pistons. Originally built in '82. I had it zero decked and checked that when I changed heads. Started out with an HC-02 cam. (Going into another build.) The 9.77 CR is with the original 72 cc (unconfirmed) #48 heads with .039 head gaskets. Heavy part to full throttIe detonation. 91 octane. Mechanical timing 32°-36°. Vacuum advance unconfirmed. I changed to 6X-4 heads to reduce compression. I asked a shop to cut the heads to 90 cc, but I have not confirmed that. The engine now does not detonate anywhere with 87 octane. I was not displeased with the performance. The engine began to not run quite right, so I did a compression test. 7 cylinders at 140, 1 at 70. Since I have to take at least that head off, and because of the past detonation I thought I would freshen the engine with bearings/rings, etc. and maybe a cam with wider LSA. I am also considering changing the undesirable cast pistons.

[steve25]

Structural wise .090" is a lot to wack off those heads!

I think I would go back to number 48 heads and since your thinking about new Pistons then get some dished ones and rebalance the motor.

["QUICK-SILVER"]

Originally Posted by steve25

Structural wise .090" is a lot to wack off those heads!

Shouldn't take .090" to get 6X-4's down to 90cc's.

Clay

[tooski]

Originally Posted by steve25

Structural wise .090" is a lot to wack off those heads!

Originally Posted by "QUICK-SILVER"

Shouldn't take .090" to get 6X-4's down to 90cc's.

Clay

I asked the shop to cc the heads. He told me it came in at 95cc. I told him to take off .025 to get it to 90cc. I did not check to make sure he had done that. One of the jobs in this refresh.

[tom s]

2X Steve.Tom

[70GS455]

With a not-too-large cam I would stay around 10:1 static so your dynamic CR doesn't go over 8:1

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


info and calculator

Sent from my SM-T817V using Tapatalk

[Jay S]

I am not sure exactly which 8VR pistons your running, but if you had it zero decked with those and are wanting to replace them you might be somewhat limited on the replacements because many of the 8VR pistons had shorter compression heights, especially the generic cast pistons. IRC, I have had some that were 1.7 comp height. So some of the replacements will stick out the deck .014+. It might need some sort of forged like a DSS or an autotech versus a speed pro, ross or icon.

Probably might also want to keep in mind the Summit 2801 comes installed at 107 on the ICL, 5 degrees advanced. That 211/221 @ 110 lsa was probably 106 ICL. IMHO, if it had preignition issues with the 211/221 110lsa at 106, the 214/224 112LSA at 107 won’t be much better with the same set up. With the 8VR personally I would stop a 9 SCR with the 2801. If you could run a better piston Probably 9.5 and degree the cam in at 110ICL. I still have a couple engines with that basic cam grind in it, but have used a bunch of them. The most compression I have done was a 10:1 455 Olds with the ICL set at 112. I personally never thought the power gains were worth it to go past 9.5 on any of the engines we have done with that particular cam. Below 9 SCR I leave the cam advance in. My two cents anyway, good luck. Jay

["QUICK-SILVER"]

Originally Posted by Jay S

if it had preignition issues

He said no det at all with 87 octane and the 6X-4.

Clay

[Jay S]

Originally Posted by "QUICK-SILVER"

He said no det at all with 87 octane and the 6X-4.

Clay

I can’t remember how many cc’s the 8vr pistons releifs are, isn’t it something like 14cc’s? No issues at all with 87 octane mid to low 8s compression, good quench and the little cam. But at 9.77ish he had issues. Just to clarify, when it was ran with 9.77 SCR that was always ran with the HC-02 then but with a poor quench? Jay

[pastry_chef]

Originally Posted by Jay S

if it had preignition issues with the 211/221 110lsa at 106, the 214/224 112LSA at 107 won’t be much better with the same set up.

Yes, exactly.

The intake valve close event is by far the largest factor. Second would be the exhaust valve open event.

[tooski]

The info I have read on the forums indicate that no way a cam with the specs of the HC-02 should be causing detonation at 9.77. Unless it was poorly installed. Which it was. By me. At the time (mid 80's) I had no idea about cam degreeing. Turns out it ended up 4° retarded. I am thinking that would significantly affect the DCR leading to detonation.
I can see the engine not being happy with the smaller cam even if it was installed correctly. Hence the head change to lower the compression. I think 8.1 to 8.4 is a bit low.
As Jay posted I believe I will need to go to a custom piston (Autotec) because of compression distance. Why put those nice new forged pistons on old cast rods? So, new full floating forged rods would also be in order.

["QUICK-SILVER"]

Originally Posted by tooski

The info I have read on the forums indicate that no way a cam with the specs of the HC-02 should be causing detonation at 9.77. Unless it was poorly installed. Which it was. By me. At the time (mid 80's) I had no idea about cam degreeing. Turns out it ended up 4° retarded. I am thinking that would significantly affect the DCR leading to detonation.

Backed up 4° would have less effective stroke. Advancing is what builds higher cylinder pressure.

Need to get it opened up and see what's going on.

I suspect the detonation problem you had was from the little ridges on them 8VR's heating up and acting like a glow plug. Higher cylinder pressure means higher cylinder temperature. And your 9.77 SCR combo was just right for heating them 8VR's up.

Clay

[Jay S]

I would also agree with you that the 9.77 should have been fine with the HC-02. But there are a couple wild cards there. The 8VR replacement pistons are really hard to predict. Sometimes they have a terrible quench and can be .040 down in the deck. Then I think either the crown of the piston runs hot between the inner and outer valve reliefs and/or the extra contours create hot spots and end gas issues. Some engines will do ok with those pistons with good compression, most seem to be with OEM pistons that had a decent quench and a OEM cam. The replacement ones with the shorter height seem to have all kinds of problems unless the deck is cut like your engine.

Another thing that might have worked against it was the cam being retarded that much. Whenever I degree a cam in I assume it is running 2 degrees retarded from were it is set at. So 4 retarded is getting closer to 6 dynamically. That is not a big deal on a little cam with a small amount of overlap. But I think the bigger cams start heating up the air charge significantly when they are ran retarded. IMO, that is one of the reasons that bigger cams loose performance faster when timing chains stretch. But your timing chain could have easily stretched those 4 degrees to if you drove it much, it could have started out dead on. I would still tend to believe it was more to do with the 8vr pistons.

I am all for more compression, a good piston, rods and the summit 2801 or 2802. I have never ran the HC-02, but always though it looked like a great performance street cam, unusual to see issues with it. Jay

[Formulajones]

There are 2 ways of thinking when it comes to cam install.

Most think that retarding the camshaft is a good idea to ward off detonation, and advancing increases detonation.

It's actually more involved than that, and I think Jay eluded to it. Advancing the cam can in fact help to fight off detonation by allowing some of the hot gasses to escape the combustion process. I've been experimenting with this theory thanks to Paul on the last few cam installs I've done and it works exceptionally well. The key is to make sure the intake valve is open about .030" higher than the exhaust on the overlap stroke at TDC. That's when you know you have enough advance. The Voodoo cams especially respond to this install position. Keeping in mind it's more difficult to accomplish this with cams that have a large amount of LSA.

[gtofreek]

Originally Posted by pastry_chef

Yes, exactly.

The intake valve close event is by far the largest factor. Second would be the exhaust valve open event.

I would disagree with that statement. I've made many engines quit detonating by advancing their cams.



Instead, I would restate that comment as the biggest factor in detonation would be the valve opening difference between the intake valve and the exhaust valve at TDC during overlap. Retarding the cam causes the exhaust valve to be open more, and the intake open less at TDC so when the piston starts down the hole, for the first few degrees it is sucking harder on the exhaust port and trying to suck hot exhaust gasses back in which I feel are the main ingredient for detonating. I like to see the intake valve open between .035 and .040" more than the exhaust at TDC during overlap on 110° LSA cams. Wider lobe seps will bring these numbers closer so with 112 LSA the difference will be more like .025"-.035". If your exhaust valve is open close to the same amount as the intake at TDC during overlap, then your engine will almost certainly detonate without low compression. Looking at it this way, I have built quite a few 10:1+ iron headed engines that ran fine on 91.

[pastry_chef]

Agree if an engine has issue with residual exhaust, advancing the cam can help reversion. Advancing also opens the exhaust valve earlier.

Originally Posted by gtofreek

I like to see the intake valve open between .035 and .040" more than the exhaust at TDC during overlap on 110° LSA cams. Wider lobe seps will bring these numbers closer so with 112 LSA the difference will be more like .025"-.035". If your exhaust valve is open close to the same amount as the intake at TDC during overlap, then your engine will almost certainly detonate without low compression

Some real numbers:

Magnum
intake lobe 5241 / exhaust lobe 5204
236 / 244

112 LSA - 108 IC / 116 EC
1.65 rocker.

lift at TDC overlap.
intake .137
exhaust .114

--------------------------

intake lobe 5241 / exhaust lobe 5204
108 LSA - 104 IC / 112 EC
1.65 rocker

lift at TDC overlap.
intake .161
exhaust .129


With 112 LSA and 111 intake centerline the lifts at TDC are close to the same. Straight-up at 112 IC the exhaust has more lift.


UDHarold - Dec 19, 2005

Use my 'Quick-n-Dirty' method to check if the cam is in correct.
The only disassembly you have to do is take off a valve cover.
I use a Philips screwdriver in the #1 sparkplug hole to find TDC, turning in normal rotation until I feel it touch the piston, marking the degree wheel, then turning in opposite rotation until it hits again. I then find the average of the 2 readings, and that's TDC. Do it several times if you're unsure.
I then turn the #1 piston to TDC, and, using a 6" Machinist's Steel Rule, I check from the top of the INTAKE retainer to the spring seat, writing down the measurement. I then check from the top of the EXHAUST retainer to the spring seat, writing down that measurement.
The intake valve should be at least .025" to .050" closer to the head than the exhaust valve, if the cam is in the engine advanced. If the exhaust is closer, particularly a lot, then the cam is retarded