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Chevy 186 head flow questions

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Chevy 186 head flow questions

Postby chevyracer8262 » Wed May 19, 2010 9:59 am

I'm currently building a low budget SBC 383 stroker to go into my truck. I've chosen to run a pair of Chevy 186 fulie heads with 2.02"/1.60" on this engine (I know vortecs/aftermarks better but right price FREE), how difficult is it to get in the 250cfm@28" out of these heads? I've started porting on them and when I get one port finished I'm going to take them to my friends shop and od a 3 angle valve job on them then flow them on his flowbench. How much flow can actually be gotten out of a pair of highly modified 186's? Also what grind cam would work well with these heads, here's my combo as of right now. Eagle rotating assy 10.4:1 comp, the ported 186 heads, tunnelram intake (modified for efi w/LS1 TB and injectors) 1 3/4" full length headers, 2.5" true dual exhaust. Looking for a powerband from 1500-5500rpms. The trucks a 93 GMC ext cab 4x4 with 33" tires and 3.73:1 gears with a modeified 4l60e tranny. Any suggestions?
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Re: Chevy 186 head flow questions

Postby steve25 » Wed May 19, 2010 11:43 am

If they started out as heads fitted with 1.94" intakes and the chambers where not unshrouded for the 2.02" valves they flow about 210 to 215 cfm@28".
I have ported these to 265 cfm@ 28" with the avaerage amount of rust and core shift they have after all these years of use.
One rare set I did topped 270, but this set was new old stock with only .010" worth of core shift!
For a first time porter to hit 250 cfm you will need some guidance.
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Re: Chevy 186 head flow questions

Postby chevyracer8262 » Wed May 19, 2010 11:57 am

Thank's for the response, these heads are factory 2.02"/1.60" valve heads. So far they've been machined for 1.44" dia springs, screw in studs, guideplates, and press in valve guides. My friend that has the shops been giving me some very good guidance. So far I've blended the intake bowls out to 89-90% of my valve dia, raised the roof .100", started working that area between the runner and bowl, along with the ssr. Any comments or suggestions?
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Re: Chevy 186 head flow questions

Postby chevyracer8262 » Wed May 19, 2010 1:25 pm

Oh yea i almost forgot how much flow can be gotten out of the exhaust side of these heads as well?
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Re: Chevy 186 head flow questions

Postby ap72 » Wed May 19, 2010 2:12 pm

chevyracer8262 wrote:10.4:1 comp
186 heads
tunnelram intake
powerband from 1500-5500rpms
93 GMC ext cab 4x4 with 33" tires and 3.73:1 gears with a modeified 4l60e tranny


What fuel are you planning on running?
What displacement do you have?
What intake?
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Re: Chevy 186 head flow questions

Postby chevyracer8262 » Wed May 19, 2010 2:32 pm

I plan on running 92 octane, here's my combo it's 383cuin, 10.4:1 comp 2v flattop pistons, I plan on running a GM LT4 Hot cam 112lsa 218/228@.050" dur .525"/.525" lift hyd roller cam, 1 5/8" headers w/full 2.5" dual exhaust, these ported 186 heads, crane aluminum roller rockers, as far my intake I have a modified Weiand tunnelram that's been converted to EFI using ev6 fuel injectors and LS style tb (I got all my dimensions for the plenum box and the injector angle from a Holley stealthram EFI tunnelram intake i just moded it to accept the ls1 tb instead of a tpi tb). It'll be run in MAF batchfire efi mode till I can modify it to sequential port efi. Pipe max estimates it to need 258cfm/184cfm, if we put everything in correct.
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Re: Chevy 186 head flow questions

Postby steve25 » Wed May 19, 2010 2:56 pm

The exh can be made to go over 200 cfm but it take alot of hard to come by detail work in conjuction with the needed valve job specs.

Note that with the 186 heads producing intake flow numbers over 265 takes you to the tiping point where you will start to drop off some of the lower lift flow range numbers, but the added port area and volume will help the motor hang on to its peak HP higher in the rpm range.

With the way you want to build your motor and its cam specs your calling for 250 to 255 cfm will work well
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Re: Chevy 186 head flow questions

Postby ap72 » Wed May 19, 2010 3:11 pm

I don't think you'll be able to run that on pump gas, not without pulling a lot of timing out of it at least.
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Re: Chevy 186 head flow questions

Postby chevyracer8262 » Wed May 19, 2010 3:16 pm

ok thanks, would the exhaust flow higher than the 184 cause any low or mid lift flow or power loss? The exhaust's already have a ton of work done to them and they look like they'll flow really well, I'll find out more when we bolt them on the flowbench though. My buddy that ports heads did the work on the exhaust a few years back and started porting the intake ports but never finished them.
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Re: Chevy 186 head flow questions

Postby steve25 » Thu May 20, 2010 5:38 am

A prime concern with exh port working well is to not loose any added low lift flow gained thru re-work (up to .300" lift) by shotting for the ulitmate in high lift flow.
Also a common mistake on the exh side is to exactly match the port outlet to the tube size used.
The floor of the port should be shy of matching the tube by and 1/8" and the rest of the outlet by 1/16".
Running 10.4 comp on pump gas with iron heads and that spec cam will be creating a crank buster!!
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Re: Chevy 186 head flow questions

Postby chevyracer8262 » Thu May 20, 2010 5:47 am

What grind cam do you think would do best with that fuel and comp? I'm trying to lower the comp as much as I can with thicker head gaskets and laying back the chamber as much as i can.
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Re: Chevy 186 head flow questions

Postby ap72 » Thu May 20, 2010 8:06 am

chevyracer8262 wrote:What grind cam do you think would do best with that fuel and comp? I'm trying to lower the comp as much as I can with thicker head gaskets and laying back the chamber as much as i can.



bad idea... Instead just run the proper piston with the proper piston to head clearance and use an optimal chamber shape for flow and flame propagation, not for lowering compression. Running thicker gaskets and hacking up the chamber will make a bad problem worse. You need to change the pistons. You need to get a piston with about 10cc's more volume.
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Re: Chevy 186 head flow questions

Postby chevyracer8262 » Thu May 20, 2010 11:27 am

Ok since I got these replies I've been rethinking my combo, I have to use the rotating assy that I've bought, can't afford not too. so i'm run some math and please correct me if I'm wrong this is the first engine I've ever put this much research, planning and math into. I've been reading about static ans dynamic comp ratios and came across some info of past engines that GM built that ran high comp ratios and still was able to run off of pump gas. The one that hit me was the 11:1 comp 375hp 350 lt1. Now here's the calculations I came up with, using a compression calc I found on the KB-Silvolite website. Using the 4.030" bore block, 64cc heads, 4.125 bore .039"thk head gasket, 0 piston to deck height, 5cc piston, 3.75" stroke and 5.7"lg rods It calculated a static comp ratio of 11.11:1 and using the specs from an old school L79 cam advanced 4degs 113lsa 222/222@.050" dur .447"/.447" lift (.476"/.476" lift with 1.6's) IVO 40 IVC 86 EVO 92 EVC 34 (before advance) it calculated a dynamic comp ratio of 7.56:1. I've read several articles and posts stating that an engine running on 91 octane of greater needs between 7.5-8.5:1 dynamic comp ratio to keep cyl pressure within the 170-200psi range, is this true? Now would this engine at these specs with that cam run dependably and without issues with that cam and a 2000rpm stall converter? I'm really looking for dependability because this engine is going into my truck I had other plans using a set of RHS Pro Action 180 w/72cc chamber heads when I bought the rotating assy but financial reasons and necessity made me have to change direction, the engine in my truck is shot so I can't wait till I can afford the other parts. My buddy gave me these 186 heads and the new L79 repro cam, so if I can use them that'll be great, I also have a set of 1.6:1 roller rockers to give the cam a little more lift.
Last edited by chevyracer8262 on Thu May 20, 2010 11:38 am, edited 1 time in total.
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Re: Chevy 186 head flow questions

Postby ap72 » Thu May 20, 2010 11:34 am

chevyracer8262 wrote:Ok since I got these replies I've been rethinking my combo, I have to use the rotating assy that I've bought, can't afford not too. so i'm run some math and please correct me if I'm wrong this is the first engine I've ever put this much research, planning and math into. I've been reading about static ans dynamic comp ratios and came across some info of past engines that GM built that ran high comp ratios and still was able to run off of pump gas. The one that hit me was the 11:1 comp 375hp 350 lt1. Now here's the calculations I came up with, using a compression calc I found on the KB-Silvolite website. Using the 4.030" bore block, 64cc heads, 4.125 bore .039"thk head gasket, 0 piston to deck height, 3.75" stroke and 5.7"lg rods It calculated a static comp ratio of 11.113:1 and using the specs from an old school L79 cam advanced 4degs it calculated a dynamic comp ratio of 7.559:1. I've read several articles and posts stating that an engine running on 91 octane of greater needs a 7.5-8.5:1 dynamic comp ratio to run safely without any detonation issues. Now would this engine at these specs with that cam run dependably and strong with that cam and a 2000rpm stall converter? I'm really looking for dependability because this engine is going into my truck i had other plans for different heads and cam combo when I bought the rotating assy but financial reasons have made me have to change direction, and the engine in my truck is shot so I can't wait till I can afford the other parts. My buddy gave me these 186 heads and the new L79 repro cam, so if I can use them that'll be great, I also have a set of 1.6:1 roller rockers to give the cam a little more lift.



That will not work. Your choices are- spend the money for new pistons, spend the money for new heads, spend the money for a new cam, converter and gears, spend the money for race fuel, or don't spend any money right now put it together and then spend the money to rebuild what you destroyed.

Buying pistons is the cheapest easiest answer to your problem. You can run 9.5:1 compression if you want to run premium 93 octane fuel.
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Re: Chevy 186 head flow questions

Postby chevyracer8262 » Thu May 20, 2010 11:44 am

ok what if I ran a set of 441's or 993's instead of the 186's? 76cc chambers, That'll drop the comp and they can still be made to flow well. That'd give me around 9.75:1 would that work? I have another friend that has a set of 993's and have found a set of 441's on craigslist for $100.
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