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dwd3726
02-06-2012, 07:21 PM
Hello,
I have been lurking here a bit while I have been in the process of putting a sbc stroker together. The motor is a 1978 Merc 260, it has original tags on the block and valve cover and oddly enough is a 4 bolt block. Long story short, promised a few items by the machine shop doing the short block and received something else.

I received the short block and it has 5cc valve relief flat tops in it. I want to put 64cc vortec heads on it. Every time I manually calculate the CR or use an online calculator I get around 10.7:1. He claims it will be 9.8:1 (despite me specifying I wanted it to be 9:1). I disagree with his math. He is accounting for the increase in the chamber size at TDC, but not the increase in the volume of the cylinder with the piston at the bottom as a function of the overbore. Fortunately, he is a very reasonable guy and told me if I want them changed he'd do it.

I think a piston with about 19cc of dish would be better suited along with an appropriately chosen HG. What are your thoughts? Thank you!

Don


Specs:
4.060 bore
3.75 stroke
6in. rods
pistons 0.020 down in the hole at TDC

Hemidoc
02-06-2012, 07:50 PM
Save the money on the vortecs and get better heads.
The Vortecs are fine but are limited by their smaller cc intake runners.
Look for a head with at least 185cc intake runner for a 383. Look at a 70cc chamber to get ya into 10-1 comp.
If ya need iron heads look at RHS Summit sells them for a reasonable cost(about 450-550 each head)
The 383 I built had flat-tops and I used Edelbrock performer RPM heads 70cc chambers. That with the LT4 hot cam some comp 1.6 roller rockers a pro products cross flow intake and an Edelbrock 750 carb. That thing really runs well. Went with the boat when I sold it.
Edit to add: A 4.060 bore 3.75 stroke makes that more than 383. 388?

turbo2256
02-06-2012, 10:52 PM
Still have a set of Brodix IK 180s with 70cc chambers. New, MIL spec hard anodized for sale.

dwd3726
02-07-2012, 11:28 AM
I was debating a set of dart iron eagles or rhs heads. I shouldn't have used the term 'vortec' in place of small cc head. I know it's actually a 388, but put 383 so this thread is more searchable.
My concern was putting the compression well over 9:1, it's beat into your head that in a boat you should stay around that number. The boat weighs about 3700lbs fully loaded and lives in Reefpoint Marina (Lake Michigan) all summer. Because of this I'm stuck with 89 fuel, if it was a trailer boat I'd have no problem putting premium in. The machine shop owner vows it will not blow up at near 10:1.
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Hemidoc
02-07-2012, 03:04 PM
RHS has iron heads with 72cc chambers. That extra 5 cubes does bump the static compression up by .11
A 8 cyl engine with a 4.060 bore a 3.75 stroke with a 5 cc valve relief .020 deck clearance felpro 1700 gaskets compressed thickness of .048 and 72cc heads will give ya a static compression ratio of 9.7 to 1.
ETA link http://www.summitracing.com/parts/RHS-12303-01/

dwd3726
02-07-2012, 04:33 PM
I have contemplated going with those heads (72cc) and keeping the flat tops. The machine shop owner claims around the number you obtained, but with a 64cc head and slightly thinner HG. I think he is wrong, he is adamant he is right. What are your thoughts on running 9.7:1 on 89 marina gas?
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Hemidoc
02-07-2012, 05:33 PM
What are your thoughts on running 9.7:1 on 89 marina gas?
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I think it's doable but close. Would be alot easier if ya could get some better gas. Your cam choice, ign timing, Air/fuel ratio and engine temps will all be factors as well. If ya run it too lean, too hot, too much advance it's gonna detonate.
If ya feel better with 9.1 to 1 I'd say have him change the pistons. Even if ya change the pistons to an 18cc dish the 64cc heads are gonna put ya at 9.25 to 1

dbkski
02-07-2012, 06:08 PM
So your boat is going after a world speed record? No I didn't think so. Then why
push your luck? Your initial thought is spot on. Build your engine for max compression
to run 87 octane. Since we run the same waters I can attest to the fact that although
there are a lot of marinas in the area, the chance of stopping at one that is out of
a particular grade of gas when you need it is high. Plan for the best. Be prepared for
the worst.

Blue Thunder
02-07-2012, 06:43 PM
Hello,
I have been lurking here a bit while I have been in the process of putting a sbc stroker together. The motor is a 1978 Merc 260, it has original tags on the block and valve cover and oddly enough is a 4 bolt block. Long story short, promised a few items by the machine shop doing the short block and received something else.

I received the short block and it has 5cc valve relief flat tops in it. I want to put 64cc vortec heads on it. Every time I manually calculate the CR or use an online calculator I get around 10.7:1. He claims it will be 9.8:1 (despite me specifying I wanted it to be 9:1). I disagree with his math. He is accounting for the increase in the chamber size at TDC, but not the increase in the volume of the cylinder with the piston at the bottom as a function of the overbore. Fortunately, he is a very reasonable guy and told me if I want them changed he'd do it.

I think a piston with about 19cc of dish would be better suited along with an appropriately chosen HG. What are your thoughts? Thank you!

Don


Specs:
4.060 bore
3.75 stroke
6in. rods
pistons 0.020 down in the hole at TDC



Your numbers will make 10.72 static compression with the proper quench and diameter head gasket. You should get all the pertinent numbers from your engine guy that he thinks add to 9.7:1. He must be using a thick head gasket or something else undesireable. On if it will work, are the heads alum or iron?

Hemidoc
02-07-2012, 07:11 PM
Your numbers will make 10.72 static compression with the proper quench and diameter head gasket. You should get all the pertinent numbers from your engine guy that he thinks add to 9.7:1. He must be using a thick head gasket or something else undesireable. On if it will work, are the heads alum or iron?

The 9.7 was with a 72cc head. The 10.7+ was with a 64cc head. The FelPro 1700 has a compressed thickness of .048
If it were me I'd go for the 9.7 route and find a way to get gas on land. Chit I'd carry 93 in cans to the boat before I'd pay on the water prices for 89. The guy I bought my Libby from would take it out of the water to go fill it up rather than pay an extra $1 a gal for gas on the water. With 100 gallon tank I can see why he would do that.

Blue Thunder
02-07-2012, 07:57 PM
[QUOTE=Hemidoc;978440]The 9.7 was with a 72cc head. The 10.7+ was with a 64cc head. The FelPro 1700 has a compressed thickness of .048

Why would his engine builder be calculating using a 72cc head when the head being use is a 64cc head?

Hemidoc
02-07-2012, 08:16 PM
[QUOTE=Hemidoc;978440]The 9.7 was with a 72cc head. The 10.7+ was with a 64cc head. The FelPro 1700 has a compressed thickness of .048

Why would his engine builder be calculating using a 72cc head when the head being use is a 64cc head?

I took it he had not bought heads yet.

dwd3726
02-07-2012, 09:55 PM
I would probably get tossed out of the harbor if I carried gas cans in or would at least be threatened with it.

He did not calculate using 72cc heads, he calculated it with the 64cc heads i plan on putting on. The heads will be iron.

Engine builder says in its current configuration it will have ~9.8:1 with 64cc heads and a .048 compressed HG, part of our disagreement involves using 7cc reliefs to calculate instead of 5cc. We are going to cc them on saturday (I looked up part number and called company). He got his 7cc figure from a catalog. He is basing his number in a table from the piston manufacturer and adjusting it because the table uses 0 deck and 30 over as a baseline. My shortblock is 60 over and has a .020 deck height, he decreased the value in the table by using the increase in the volume of the chamber at TDC because of these two things.

I think he is overlooking that fact that the volume of the piston sweep from TDC to the bottom also increases because the block is 60 over opposed to 30 over. I calculated the CR using the larger volume of the chamber at TDC and the volume of the piston sweep of a 4.030 bore and arrived at a number dangerously close to his value of 9.8:1 (the 2cc discrepancy was also factored).

That's it in a nutshell. I am most likely going to request the pistons are switched barring a result I don't expect on Saturday after we cc the top of the piston in the hole.

Hemidoc
02-08-2012, 03:34 PM
I would probably get tossed out of the harbor if I carried gas cans in or would at least be threatened with it.

He did not calculate using 72cc heads, he calculated it with the 64cc heads i plan on putting on. The heads will be iron.

Engine builder says in its current configuration it will have ~9.8:1 with 64cc heads and a .048 compressed HG, part of our disagreement involves using 7cc reliefs to calculate instead of 5cc. We are going to cc them on saturday (I looked up part number and called company). He got his 7cc figure from a catalog. He is basing his number in a table from the piston manufacturer and adjusting it because the table uses 0 deck and 30 over as a baseline. My shortblock is 60 over and has a .020 deck height, he decreased the value in the table by using the increase in the volume of the chamber at TDC because of these two things.

I think he is overlooking that fact that the volume of the piston sweep from TDC to the bottom also increases because the block is 60 over opposed to 30 over. I calculated the CR using the larger volume of the chamber at TDC and the volume of the piston sweep of a 4.030 bore and arrived at a number dangerously close to his value of 9.8:1 (the 2cc discrepancy was also factored).

That's it in a nutshell. I am most likely going to request the pistons are switched barring a result I don't expect on Saturday after we cc the top of the piston in the hole.

http://www.summitracing.com/expertadvice/storydetail/calcsandtools/Compression-Calculator/
Type in the facts.
Given the fact you are pretty much stuck on buying gas on the water. I would agree with another poster and build it to where you could run 87 if ya had to. Ya might give up a few ponies but not alot.

aquaforce
02-08-2012, 06:20 PM
I would probably get tossed out of the harbor if I carried gas cans in or would at least be threatened with it.

He did not calculate using 72cc heads, he calculated it with the 64cc heads i plan on putting on. The heads will be iron.

Engine builder says in its current configuration it will have ~9.8:1 with 64cc heads and a .048 compressed HG, part of our disagreement involves using 7cc reliefs to calculate instead of 5cc. We are going to cc them on saturday (I looked up part number and called company). He got his 7cc figure from a catalog. He is basing his number in a table from the piston manufacturer and adjusting it because the table uses 0 deck and 30 over as a baseline. My shortblock is 60 over and has a .020 deck height, he decreased the value in the table by using the increase in the volume of the chamber at TDC because of these two things.

I think he is overlooking that fact that the volume of the piston sweep from TDC to the bottom also increases because the block is 60 over opposed to 30 over. I calculated the CR using the larger volume of the chamber at TDC and the volume of the piston sweep of a 4.030 bore and arrived at a number dangerously close to his value of 9.8:1 (the 2cc discrepancy was also factored).

That's it in a nutshell. I am most likely going to request the pistons are switched barring a result I don't expect on Saturday after we cc the top of the piston in the hole.


Definately do your own homework and calculations to make sure of everything.

I listened to a very experienced engine builder and a seasoned race supplier who both know their stuff very well. They didn't see what I was working with but they ran the numbers and had me thinking one way. Once I got my buret out and did some measuring of my own I found out I was so far off that my piston purchase would not work. :mad: That cost me several hundred dollars to correct all on my own nickel because I listened to someone else.

Good luck.

Blue Thunder
02-08-2012, 07:37 PM
He did not calculate using 72cc heads, he calculated it with the 64cc heads i plan on putting on. The heads will be iron.

That's it in a nutshell. I am most likely going to request the pistons are switched barring a result I don't expect on Saturday after we cc the top of the piston in the hole.

You are correct on your conclusions and need to change the pistons to get down below 9.5:1 for sure with those heads. I might also consider changing my engine guy in the process as well. If you were not dilengent you would have had a pile of worthless parts in short order thanks to him.

Another consideration is because you are .020 in the hole the head gasket needs to be pretty thin to get the quench right. I'm not sure on a sbc, but if this was a bbc you'd want the quench at .040. SBC should be the same but you should confirm. If so, you'd want a .020 thick head gasket to get the quench where it should be, or as close to that as is available. Then adjust the piston to get the comp correct. Your current ..020 down, plus a .048 head gasket would give a quench of .068 which is no good in marine app and can cause detonation.

dwd3726
03-01-2012, 11:14 AM
Here is an update. I went to the shop and we CCed the top of the pistons and deck height. The owner was not there, but the builder was. He said I was correct and that the owner failed to deliver what was promised. We calculated compression numbers that matched my own as well as yours. They took it back and redid the job correctly. I have the short block in my garage and am getting ready to finish putting it together.

Though I do not think highly of the owner, the guy that did the work was honest and seemed to really care. I am crossing my fingers this thing holds up, it helps that I trust the man that actually did the work. I will not be going back there, ever.

If any of you out there are seeking machine work around the 60081 zip code, request my email address. I can give you the specifics of my dealings and hopefully steer you in your decision on where to go.

Thank you all for the advice and sanity check.
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TomZ
03-01-2012, 05:01 PM
Your current ..020 down, plus a .048 head gasket would give a quench of .068 which is no good in marine app and can cause detonation.

That's no good period!

I was hoping someone would chime in on the quench factor. Excessive quench is an easy way to ruin the longevity and output of a perfectly good combo.

dwd3726
03-01-2012, 05:07 PM
I won't be using a 0.048 head gasket.
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Blue Thunder
03-01-2012, 07:09 PM
Cool beans dwd. Too many of these builds go wrong because people trust the "experts". A good measure of critical thinking can go a long way. Not to mention a little internet work.

sprint18
03-02-2012, 11:24 PM
shop around before using any machine shop. look at their equip and talk to staff. the guy doing the work will tell you what he really thinks of the place.