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Looking for advice on whether the parts I’m using will go well together and give an a decent increase in power over standard.
Motor is the original 308 for my trimatic HG. Keeping the original converter, intake and quadrajet, air cleaner, exhaust manifolds and distributor as I want to keep the stock appearance. Original style dual exhaust.
Block 0.040” over, decked -0.020”.
Original HQ type heads, machined face -0.010” L34 style valves, throated to suit bigger valves, hardened exhaust seats.
Later A9L rods.
Mild crow 5666 ski flat tappet hydraulic cam with springs to suit.
Flat top pistons come up 0.020” below deck height.
Using overpriced 0.027” stainless head gaskets to give 0.047” quench height.
A Google search suggest the HQ heads combustion chamber standard would be around 56cc. Hoping this is correct.
Using a compression calculator with all the info above, it should be around 10.8-1 compression ratio .
Have not rebuilt a Holden V8 since the 1983 and totally in the dark how much power this combination will produce. Feeling a bit unsure of myself and if these parts will work well together
Any tips appreciated.
Jim
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Hey Jim,
Might be a touch too many compressions for such a small cam. Do you have to use the 0.027" gaskets? What does the comp come out at with a 0.040"
Dynamic compression is what matters.
I probably wouldn't want more than 9.5:1 or so but I'm not an engine builder.
Who specced the engine?
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Thanks The cam was recommended by crow cams with all the info above. I also have a 0.040” gasket which would give it 10.5-1 compression. The advice I got was to reduce the top of the piston to head clearance from 0.060” to 0.047” by using the thinner head gasket. He said something about quench and that 0.060” not being ideal. A lot of it went over my head but he did say 0.060” will be more prone to detonation and reduce potential power. Not sure on all the technical side myself. Edited by user Wednesday, 23 April 2025 7:47:51 PM(UTC)
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Crow are spot on with the head gasket advice, but I agree that 10.8 static is a bit too high with iron heads and work out the dynamic compression. You should cc the heads too, they are specced as 55.6 +/-1cc by GMH. WIth the L34 valves hopefully they are opened up a bit for de-shrouding and they are closer to 62cc. |
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Thanks Yes the heads have been machine de-shrouded for the bigger valves. I guess I should have a go at measuring the cc of the chambers myself and recalculate the static compression. I noticed today that the inlet valves taper down toward the middle around 0.100” which might increase the cc’s of the chambers a little. As it is, if I’m doing it right, using ABDC close at 32 for this cam, the 55cc chamber, the dynamic compression comes out at 10.25:1 which I assume is too high. Edited by user Thursday, 24 April 2025 3:50:58 PM(UTC)
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I reckon your chambers might be closer to 60cc. The L34’s were 9.8:1 static with (I believe) flat top pistons and steel shim gaskets. The HJ engines were 9.7:1 with standard heads and a slight piston dish. |
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I think you’re right, had a look earlier and the de-shrouding has taken a fair bit of material out. I attempt to measure it tomorrow.
Using a dynamic compression calculator which is all new to me, it looks like I would have been better off with 12cc to 18cc dished pistons.
If the ideal is 8:5-1 dynamic, I need to get rid of at least 10cc off each chambers or the middle 2 1/2” of the pistons. |
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Or buy a cam that reduces the dynamic compression. The guys at Crow will give you good advice on what will be OK. Get yourself a burette or pippete, flat piece of Lexan and some fluid. Have a read online, example: https://help.summitracin...e/article/SR-05226/en-us |
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Thanks for the link. I will give it go. |
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I just found GMH's claimed combustion chamber size for L34. 59.5cc +/- 2cc. Pistons are quoted as "solid skirt slipper type with flat top", basically the same as SBC spec. |
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Probably similar to mine except my block has been decked 0.020”.
Just got a different dynamic compression result at 8.3-1 and 10.3-1 static using the Port City calculator. It goes into a lot more detail, crown diameter, how far rings are down from the crown. It also has the 5666 crow cam specs on there to help.
Feeling a bit more confident but not still not 100% confident.
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Remember though that most aftermarket pistons sit further down the bore than originals, to allow for block squaring/decking. Factory designed quench height would have been around 40 thou give or take. Manufacturing tolerances would see variations though but mattered less when the engine is 9:1 or thereabouts. Steel shim gaskets are about 20 thou from memory, and with pistons down the hole 15-20 thou all is good. Most aftermarket composite gaskets are about 40 thou, hence why they zero deck the block. Given your pistons are down 20 thou, the 27 thou gaskets are your friend, 47 is way better than 60 thou. My engine builder told me he’d deck to set pistons 10 thou down to allow further decking of 5 or 10 thou if required. With 10 thou I’ll use the 27 thou Cometic gaskets like yours. If it has to ever be decked another 5 thou it’d use the 30 thou versions. If decked to zero use the 36 or 40 thou gaskets. This is a 4.040” bore, 3.75” stroke and 64cc alloy heads with static about 11:1, but cam setup to bring dynamic down to be suitable for 98. |
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Had a go at cc’ing the chambers. They’re around 59cc give or take. I guess machining 0.010” of the head face would not of reduced it much.
This is the 308’s 1st rebuild. I noticed the original GMH head gaskets I took off looked very thin. |
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I built both of my 308s with 10.5 to 1 being the aim. They run great on 98 and give good increase on standard hp with more performance oriented cams but still hydraulic.
Compression was achieved with flat top pistons, decked to zero block, minor decking of the heads, can't remember the cc of the heads but in the early 60s rings a bell. That was was also with L34 size valves.
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Originally Posted by: HGV8  Had a go at cc’ing the chambers. They’re around 59cc give or take. I guess machining 0.010” of the head face would not of reduced it much.
This is the 308’s 1st rebuild. I noticed the original GMH head gaskets I took off looked very thin. Rerun your compression calculator using 59cc. Make sure you account for valve reliefs in the pistons if there are any. I just ran it on the Summit calculator: https://www.summitracing...VPAKZS5TTqOXHcVAFVCERcC
Using: 4.040 bore. 3.0625 stroke. 59cc chambers. 0.020 deck clearance. 0.027 gaskets. Flat top pistons (0 dome volume). 8 cyl. Got 314ci 10.34:1 compression. Edited by user Friday, 25 April 2025 6:00:56 PM(UTC)
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Thanks No valve reliefs. I also got 10.3 also using the Port City engines calculator below. It has a lot more details/specs you have to input to calculate the dynamic compression ratio. It has selection of crane cams, including my 5666 cam details which match my cam card at the bottom of their page. Using my 270 camshaft intake duration and 112 cam lobe separation angle it calculated an 8.3-1 dynamic. Hoping this is reasonably accurate as others were way above this value. https://pcengines.com.au...VKWHHWkKNntv4Bi9zYU8-tEVEdited by user Friday, 25 April 2025 6:44:12 PM(UTC)
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Looks like my DCRatio is too high after all. Ran it through multiple calculators and it’s around 9:7.1. I’ll give Crow a call. Interesting looking at this genuine separated L34 engine (link below). The chambers and valves look the same as mine, flat top pistons also the same. I assume the static compression must of been similar. Surely they must of used a pretty aggressive cam grind to get around this DCRatio issue? https://www.my105.com/se...8-4a7b-b90c-cfc6e8c8eef5 |
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L34 used the stock 253 cam retarded, same as HT to HQ 308. I’ll get you the cam specs. |
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